Ballistics (gr. ba'llein, "throw") is the science that deals with the motion, behavior, and effects of projectiles, especially bullets, gravity bombs, rockets, or the like; the science or art of designing and hurling projectiles so as to achieve a desired performance.
A ballistic body is a body which is free to move, behave, and be modified in appearance, contour, or texture by ambient conditions, substances, or forces, as by the pressure of gases in a gun, by rifling in a barrel, by gravity, by temperature, or by air particles.
Firearm ballistics information can also be used in forensic science. Separately from ballistics information, firearm and tool mark examinations involve analyzing firearm, ammunition, and tool mark evidence in order to establish whether a certain firearm or tool was used in the commission of a crime.
Ballistics is sometimes subdivided into:
* Internal ballistics, the study of the processes originally accelerating the projectile, for example the passage of a bullet through the barrel of a rifle;
* Transition ballistics, the study of the projectile's behavior when it leaves the barrel and the pressure behind the projectile is equalized.
* External ballistics, the study of the passage of the projectile through space or the air; and
* Terminal ballistics, the study of the interaction of a projectile with its target, whether that be flesh (for a hunting bullet), steel (for an anti-tank round), or even furnace slag (for an industrial slag disruptor).
A ballistic missile is a missile designed to operate primarily in accordance with the laws of ballistics.
The term ballistics is also sometimes used to refer to acceleration curves applied to the motion of a computer mouse.
A ballistic missile is a missile that follows an elliptical, sub-orbital, ballistic
flightpath with the objective of delivering a warhead to a predetermined target. The missile
is only guided during the powered phase of flight; its course governed by the laws of
orbital mechanics and ballistics.
The first ballistic missile was the A-4, commonly known as the V-2 rocket, developed by Nazi
Germany in the 1930s and 1940s under direction of Walter Dornberger. The first successful
launch of a V-2 was on October 3, 1942 and began operation on September 6, 1944 against
Paris, followed by an attack on London two days later. By the end of the war in May 1945
over 3000 V-2's had been launched.
A ballistic missile trajectory consists of three parts: The powered flight portion, the
free-flight portion which constitutes most of the flight time, and the re-entry phase where
the missile re-enters the earths atmosphere.
Ballistic missiles can be launched from fixed sites or mobile launchers, including vehicles
(Transporter Erector Launchers, TELs), aircraft, ships and submarines. The powered flight
portion can last from a few tens of seconds to several minutes and can consist of up to
three rocket stages.
When in space and no more thrust is provided, the missile enters free-flight. In order to
cover large distances, ballistic missiles are usually launched into a high sub-orbital
spaceflight; for intercontinental missiles the highest altitude (apogee) reached during
free-flight is about 1200 km.
The re-entry stage begins at an altitude where atmospheric drag plays a significant part in
missile trajectory, and lasts until missile impact.
Missile types
Ballistic missiles can vary widely in range and use, and are often divided into categories
based on range. The U.S. distinguishes:[1]
* Short-range ballistic missile (SRBM): range less than 1000 km; the three types of
ballistic missile ever used in an attack were all in this category and had conventional
explosives only:
o V-2 rocket
o Scud
o SS-21 Scarab
* Medium-range ballistic missile (MRBM): range between 1000 and 2500 km
* Intermediate-range ballistic missile (IRBM): range between 2500 and 3500 km
* Sub-continental ballistic missile (SCBM): range between ? and ? km
* Intercontinental ballistic missile (ICBM): range greater than 3500 km, broken down
into:
o Limited range intercontinental ballistic missile (LRICBM): range between 3500
and 8000 km
o Full range intercontinental ballistic missile (FRICBM): range between 8000 and
12,000 km
* Submarine-launched ballistic missile
Medium to short range missiles are often called tactical or theatre ballistic missiles
(TBM). Long and medium range ballistic missiles are generally designed to deliver nuclear
warheads because their payload is too limited for conventional explosives to be efficient
(though the US may be evaluating the idea of a conventionally-armed ICBM for near-instant
global air strike capability despite the high costs[2]).
Using a missile with a considerably longer range than the distance from launch site to
target can make sense: it can reach a higher altitude and come down with a higher speed,
making defense more difficult. For example, a missile with a range of 3000 km fired at a
target that is only 500 km away could arrive at its target after having reached an altitude
of about 1200 km —roughly the height reached by ICBMs. Like them, it would arrive at a speed
of typically more than 6 km/s (Mach 17).
The flight phases are like those for ICBMs, except that for a range less than ca. 350 km
there is no exoatmospheric phase.
Specific missiles
Specific types of ballistic missiles include:
* Abdali-I
* Agni
* Condor
* CSS-2 missile
* Ghauri-I
* Ghauri-II
* Ghauri
* Ghaznavi
* Hadès
* Hatf-I
* Jericho
* M5
* M45
* M51
* Minuteman
* Nodong-1
* Peacekeeper
* Pluton
* Polaris
* Poseidon
* Prithvi
* Scud
* Shahab-3
* Shahab-4
* Shahab-5
* Shaheen
* Skybolt Air-Launched Ballistic Missile (ALBM)
* SS-18 missile
* SS-24 missile
* SS-N-23
* Surya ICBM
* Taepodong-1
* Taepodong-2
* Trident
* V-2
Ballistic missile submarines
Specific types of ballistic missile submarines include:
* Vanguard class
* Resolution class
* Benjamin Franklin class
* Ohio class
* Triomphant class
* Redoutable class
* Xia class
* Additional ballistic missile submarines
Hydrogen Vehicle
A hydrogen vehicle is an automobile which uses hydrogen as its primary source of power for locomotion. These cars generally use the hydrogen in one of two methods: combustion or fuel-cell conversion. In combustion, the hydrogen is "burned" in engines in fundamentally the same method as traditional gasoline cars. In fuel-cell conversion, the hydrogen is turned into electricity through fuel cells which then powers electric motors. With either method, the major byproduct from the spent hydrogen is water that can move also a micro-turbine.
Hydrogen can be obtained from decomposition of methane (natural gas), coal (by a process known as coal gasification), liquid petroleum products, biomass (biomass gasification), high heat sources (by a process called thermolysis), or from water using electricity (electrolysis). A primary benefit of using pure hydrogen as a power source is that it uses oxygen from the air to produce water vapor as exhaust (and very little nitrogen oxides from the nitrogen in the air when burning at high temperatures). We could move the source of atmospheric pollution from many cars, back to a single power plant, where it can be more easily dealt with. That would mean that we would accept the transmission line loses and inefficiencies of separating our own hydrogen and compressing it ourselves. We could do better now by using a Plug-in hybrid electric vehicle, that is far more feasible. There are no hydrogen cars current, or anticipated, which use the fusion of hydrogen as a power source.
The major challenges in using hydrogen in cars, are the very high costs and the low energy efficiencies, with low probabilities so far, for successful solutions for the several challenges.
Hydrogen can be obtained from decomposition of methane (natural gas), coal (by a process known as coal gasification), liquid petroleum products, biomass (biomass gasification), high heat sources (by a process called thermolysis), or from water using electricity (electrolysis). A primary benefit of using pure hydrogen as a power source is that it uses oxygen from the air to produce water vapor as exhaust (and very little nitrogen oxides from the nitrogen in the air when burning at high temperatures). We could move the source of atmospheric pollution from many cars, back to a single power plant, where it can be more easily dealt with. That would mean that we would accept the transmission line loses and inefficiencies of separating our own hydrogen and compressing it ourselves. We could do better now by using a Plug-in hybrid electric vehicle, that is far more feasible. There are no hydrogen cars current, or anticipated, which use the fusion of hydrogen as a power source.
The major challenges in using hydrogen in cars, are the very high costs and the low energy efficiencies, with low probabilities so far, for successful solutions for the several challenges.
Electron-Beam Machining (EBM)
In electron-beam machining (EBM), electrons are accelerated to a velocity nearly three-fourths that of light. The process is performed in a vacuum chamber to reduce the scattering of electrons by gas molecules in the atmosphere. The stream of electrons is directed against a precisely limited area of the workpiece; on impact, the kinetic energy of the electrons is converted into thermal energy that melts and vaporizes the material to be removed, forming holes or cuts. EBM equipment is commonly used by the electronics industry to aid in the etching of circuits in microprocessors.
Ultrasonic Welding
Utilizing ultrasonic vibrations, Gulton Industries, Metuchen, N.J., have produced what is believed to be the first automatically operated continuous-seam ultrasonic welder. Through high-frequency sound alone, two pieces of metal are joined as a result of a molecular transference or plastic flow at the interfaces of the two metals and below the melting point of either metal.
The continuous-seam ultrasonic welder joins aluminum to aluminum, aluminum to stainless steel, or any two similar or dissimilar metals.
The continuous-seam ultrasonic welder joins aluminum to aluminum, aluminum to stainless steel, or any two similar or dissimilar metals.
Stealth Technology
Stealth Aircraft, military aircraft, fighters, and bombers designed to elude detection and tracking systems, such as radar and infrared monitoring . Stealth technology is also used to mask unmanned objects such as cruise missiles .
Stealth technology includes a variety of design features that affect an aircraft’s signal, also called its signature, on tracking systems. These features include an aircraft’s shape and the materials used to build it. For example, it is harder for a radar to detect an aircraft that has smooth, rounded curves. Special composite materials or coatings on the surface of an aircraft can absorb or deflect radar signals. Engines placed within the body of the aircraft and exhaust vents may be arranged to mask the heat emitted from engines and help hide an aircraft from heat-seeking infrared sensors. Reducing the noise and vibration produced by a stealth aircraft may also minimize its acoustic signature. In addition, stealth aircraft are equipped with special electronics for suppressing or confusing enemy monitoring systems
Stealth technology includes a variety of design features that affect an aircraft’s signal, also called its signature, on tracking systems. These features include an aircraft’s shape and the materials used to build it. For example, it is harder for a radar to detect an aircraft that has smooth, rounded curves. Special composite materials or coatings on the surface of an aircraft can absorb or deflect radar signals. Engines placed within the body of the aircraft and exhaust vents may be arranged to mask the heat emitted from engines and help hide an aircraft from heat-seeking infrared sensors. Reducing the noise and vibration produced by a stealth aircraft may also minimize its acoustic signature. In addition, stealth aircraft are equipped with special electronics for suppressing or confusing enemy monitoring systems
A.B.S.
n anti-lock braking system (commonly known as ABS, from the German name "Antiblockiersystem" given to it by its inventors at Bosch) is a system on motor vehicles which prevents the wheels from locking while braking. The purpose of this is twofold: to allow the driver to maintain steering control under heavy braking and, in most situations, to shorten braking distances (by allowing the driver to hit the brake fully without the fear of skidding or loss of control).Anti-lock braking systems were first developed for aircraft
On high-traction surfaces such as bitumen, whether wet or dry, most ABS-equipped cars are able to attain braking distances better (i.e. shorter) than those that would be easily possible without the benefit of ABS. An alert skilled driver without ABS should be able, through the use of techniques like cadence braking or threshold braking, to match or improve on the performance of a typical driver with an ABS-equipped vehicle
On high-traction surfaces such as bitumen, whether wet or dry, most ABS-equipped cars are able to attain braking distances better (i.e. shorter) than those that would be easily possible without the benefit of ABS. An alert skilled driver without ABS should be able, through the use of techniques like cadence braking or threshold braking, to match or improve on the performance of a typical driver with an ABS-equipped vehicle
Electric Car
An electric Car is an automobile with an electric motor powered by a system of rechargeable batteries. Electric cars are mechanically simpler and more durable than gasoline-powered cars, and they do not have to exploit nonrenewable resources (natural resources that cannot be replaced). They also produce less pollution than do gasoline-powered cars.
An electric car has a battery, a charger for replenishing the battery's power from an electrical outlet, and a controller. The Controller is connected to the accelerator pedal, for directing the flow of electricity between the battery and motor. Most electric cars use lead-acid batteries, but new types of batteries, including zinc-chlorine, nickel metal hydride, and sodium-sulfur, are being developed. To recharge the batteries, operators plug the car into a 120-volt or 240-volt outlet. The motor of an electric car harnesses the battery's electrical energy by converting it to kinetic energy, or energy that makes the car move. The driver simply switches on the power, selects "Forward" or "Reverse" with another switch, and steps on the accelerator pedal.
An electric car has a battery, a charger for replenishing the battery's power from an electrical outlet, and a controller. The Controller is connected to the accelerator pedal, for directing the flow of electricity between the battery and motor. Most electric cars use lead-acid batteries, but new types of batteries, including zinc-chlorine, nickel metal hydride, and sodium-sulfur, are being developed. To recharge the batteries, operators plug the car into a 120-volt or 240-volt outlet. The motor of an electric car harnesses the battery's electrical energy by converting it to kinetic energy, or energy that makes the car move. The driver simply switches on the power, selects "Forward" or "Reverse" with another switch, and steps on the accelerator pedal.
Airbags
Automobiles are being equipped with airbags that inflate on collision to protect the driver or passenger from injury. A car’s airbag is located on the steering wheel, this airbag is for the driver, and on the dashboard, this airbag is for the passenger. Automobile airbags are inflated by the electrical system and sensors in the car that triggers a chemical reaction to occur. When a car is involved in a car accident the airbag rapidly inflates and deflates to cushion the driver or passenger. In addition, for an airbag to successfully inflate, the propellant goes through a chemical reaction to produce the gas that inflates the airbag.
An airbag is successfully inflated when the automobile’s electrical system and sensor detect that the automobile has been involved in an accident. This whole process is part of a complex electrical system that a car is equipped with. The sensors consist of a tube containing a ball held in place by a spring. In a frontal impact, the ball is forced against the spring in proportion to the severity of the crash. Other systems use an accelerometer instead of crash sensors, frequently located within the steering column or in the airbag assembly itself.
An airbag is successfully inflated when the automobile’s electrical system and sensor detect that the automobile has been involved in an accident. This whole process is part of a complex electrical system that a car is equipped with. The sensors consist of a tube containing a ball held in place by a spring. In a frontal impact, the ball is forced against the spring in proportion to the severity of the crash. Other systems use an accelerometer instead of crash sensors, frequently located within the steering column or in the airbag assembly itself.
Electrochemical machining (ECM)
Electrochemical machining (ECM) also uses electrical energy to remove material. An electrolytic cell is created in an electrolyte medium, with the tool as the cathode and the workpiece as the anode. A high-amperage, low-voltage current is used to dissolve the metal and to remove it from the workpiece, which must be electrically conductive. A wide variety of operations can be performed by ECM; these operations include etching, marking, hole making, and milling.
Underwater welding
Underwater welding refers to a number of distinct welding processes that are performed underwater.
The two main categories of underwater welding techniques are wet underwater welding and dry underwater welding, or hyperbaric welding.
In wet underwater welding, a variation of shielded metal arc welding is commonly used, employing a waterproof electrode. Other processes that are used include flux-cored arc welding and friction welding. In each of these cases, the welding power supply is connected to the welding equipment through cables and hoses. The process is generally limited to low carbon equivalent steels, especially at greater depths, because of hydrogen-caused cracking.
In dry underwater welding the weld is performed at the prevailing pressure in a chamber filled with a gas mixture sealed around the structure being welded. For this process, gas tungsten arc welding is often used, and the resulting welds are generally of high integrity.
The applications of underwater welding are diverse—it is often used to repair and construct ships, offshore platforms, and pipelines. Steel is the most common material welded. For deep water welds and other applications where high strength is necessary, dry water welding is most commonly used. Research into using dry water welding at depths of up to 1000 m are ongoing. In general, assuring the integrity of underwater welds can be difficult, especially wet underwater welds, because defects are difficult to detect.
For the structures being welded by wet underwater welding, inspection following welding may be more difficult than for welds deposited in air. Assuring the integrity of such underwater welds may be more difficult, and there is a risk that defects may remain undetected.
The risks of underwater welding include the risk of electric shock to the welder. To prevent this, the welding equipment ought to be properly insulated, and the voltage of the welding equipment should be controlled. Underwater welders must also consider the safety issues that normal divers face; most notably, the risk of decompression sickness due to the increased pressure of inhaled breathing gases. Another risk, generally limited to wet underwater welding, is buildup of hydrogen and oxygen pockets in the weld, because these are potentially explosive
The two main categories of underwater welding techniques are wet underwater welding and dry underwater welding, or hyperbaric welding.
In wet underwater welding, a variation of shielded metal arc welding is commonly used, employing a waterproof electrode. Other processes that are used include flux-cored arc welding and friction welding. In each of these cases, the welding power supply is connected to the welding equipment through cables and hoses. The process is generally limited to low carbon equivalent steels, especially at greater depths, because of hydrogen-caused cracking.
In dry underwater welding the weld is performed at the prevailing pressure in a chamber filled with a gas mixture sealed around the structure being welded. For this process, gas tungsten arc welding is often used, and the resulting welds are generally of high integrity.
The applications of underwater welding are diverse—it is often used to repair and construct ships, offshore platforms, and pipelines. Steel is the most common material welded. For deep water welds and other applications where high strength is necessary, dry water welding is most commonly used. Research into using dry water welding at depths of up to 1000 m are ongoing. In general, assuring the integrity of underwater welds can be difficult, especially wet underwater welds, because defects are difficult to detect.
For the structures being welded by wet underwater welding, inspection following welding may be more difficult than for welds deposited in air. Assuring the integrity of such underwater welds may be more difficult, and there is a risk that defects may remain undetected.
The risks of underwater welding include the risk of electric shock to the welder. To prevent this, the welding equipment ought to be properly insulated, and the voltage of the welding equipment should be controlled. Underwater welders must also consider the safety issues that normal divers face; most notably, the risk of decompression sickness due to the increased pressure of inhaled breathing gases. Another risk, generally limited to wet underwater welding, is buildup of hydrogen and oxygen pockets in the weld, because these are potentially explosive
Robotics
A robot is a mechanical device that can perform preprogrammed physical tasks. A robot may act under the direct control of a human (eg. the robotic arm of the space shuttle) or autonomously under the control of a pre-programmed computer. Robots may be used to perform tasks that are too dangerous or difficult for humans to implement directly (e.g. the space shuttle arm) or may be used to automate repetitive tasks that can be performed more cheaply by a robot than by the employment of a human (e.g. automobile production).
The word robot is also used to describe an intelligent mechanical device in the form of a human. This form of robot (culturally referred to as androids) is common in science fiction stories. However, such robots are yet to become common-place in reality and much development is yet required in the field of artificial intelligence before they even begin to approach the robots of science fiction.
Finally, bots are sometimes referred to as robots, because they perform mundane, repetitive tasks.
The word robot is used to refer to a wide range of machines, the common feature of which is that they are all capable of movement and can be used to perform physical tasks. Robots take on many different forms, ranging from humanoid, which mimic the human form and way of moving, to industrial, whose appearance is dictated by the function they are to perform. Robots can be grouped generally as mobile robots (eg. autonomous vehicles), manipulator robots (eg. industrial robots) and Self reconfigurable robots, which can conform themselves to the task at hand.
Robots may be controlled directly by a human, such as remotely-controlled bomb-disposal robots, robotic arms, or shuttles, or may act according to their own decision making ability, provided by artificial intelligence. However, the majority of robots fall in-between these extremes, being controlled by pre-programmed computers. Such robots may include feedback loops such that they can interact with their environment, but do not display actual intelligence.
The word robot is also used in a general sense to mean any machine which mimics the actions of a human (biomimicry), in the physical sense or in the mental sense.
The word robot comes from the Czech word robota, industrial labor. The word has first appeared in Karel Capek's science fiction play R.U.R. (Rossum's Universal Robots) in 1921, and has probably been invented by author's brother, painter Josef Capek.
The word robot is also used to describe an intelligent mechanical device in the form of a human. This form of robot (culturally referred to as androids) is common in science fiction stories. However, such robots are yet to become common-place in reality and much development is yet required in the field of artificial intelligence before they even begin to approach the robots of science fiction.
Finally, bots are sometimes referred to as robots, because they perform mundane, repetitive tasks.
The word robot is used to refer to a wide range of machines, the common feature of which is that they are all capable of movement and can be used to perform physical tasks. Robots take on many different forms, ranging from humanoid, which mimic the human form and way of moving, to industrial, whose appearance is dictated by the function they are to perform. Robots can be grouped generally as mobile robots (eg. autonomous vehicles), manipulator robots (eg. industrial robots) and Self reconfigurable robots, which can conform themselves to the task at hand.
Robots may be controlled directly by a human, such as remotely-controlled bomb-disposal robots, robotic arms, or shuttles, or may act according to their own decision making ability, provided by artificial intelligence. However, the majority of robots fall in-between these extremes, being controlled by pre-programmed computers. Such robots may include feedback loops such that they can interact with their environment, but do not display actual intelligence.
The word robot is also used in a general sense to mean any machine which mimics the actions of a human (biomimicry), in the physical sense or in the mental sense.
The word robot comes from the Czech word robota, industrial labor. The word has first appeared in Karel Capek's science fiction play R.U.R. (Rossum's Universal Robots) in 1921, and has probably been invented by author's brother, painter Josef Capek.
Cell Integration into a Manufacturing System
Integrating the cell into a larger manufacturing system is basically a step up the system hierarchy of figure 3-5.Now the problems become ones concerning the cell as a whole (or, to use the earlier lexicon, a module). This purpose of this section is not to provide a detailed set of guidelines for the system design but to consider the most important features of the system and to discuss the relationship between the cells and the system.
5.1. System Definition
The system should be usable as a building block for a complete factory. Like the cells beneath it, the system will be a module with a defined input and output. It will depend on the next higher level of the hierarchy for certain services such as inventory, engineering support, and scheduling.
5.2. System Capability
Initially, the manufacturing system may consist of just one or two cells but with expansion and flexibility in mind the initial design should at least have provisions for incorporating the following features:
· The system will monitor material flow within its boundaries. Whether the parts are moved manually or automatically from one cell to the next, the system should be able to keep track them.
· Information as well as materials will flow from cell to cell. The system will be responsible for coordinating the information passage. For example, when a batch of parts travels from one cell to another it is accompanied by descriptive information. This is done so that the destination cell knows how many parts are in the batch, what their description is, what their orientation is, and so on. At first, information giving the part description and orientation may not be used since the cell program will assume a particular orientation for a particular part type. As cells become more sophisticated they will assume less. Instead, they will rely on their sensors, aided by the information accompanying the parts as they enter the cell.
· The manufacturing system will store and maintain programs associated with producing the families of parts. In particular:
o The system level computer- will be responsible for maintaining the repertoire of part programs used by CNC machines in the cells.
o Likewise, the cell hosts require a variety of instructions from the system. The sequence programs, for example, for a given part will come from the system level computer.
· The system will be responsible for maintaining the statistics for the cells and machine tools. These are the usage, maintenance, and history statistics that are necessary for autonomous operation.
· One of the more important functions of the system level supervisors should be the capability to gracefully degrade the system as individual components fail. The system should also be able to recover from failure when the components are repaired.
The system may be a very large and versatile collection of cells, but it is not factory. In particular, the following operations lie beyond the scope of the manufacturing system:
· The retrieval and initial processing of raw materials is done outside the system. For example, the bar stock used for the family of parts discussed in section 3 is usually delivered in 20 foot long sections. Handling these sections and cutting them to a more manageable size are operations that lie outside the manufacturing system.
· Inventory and its control are handled outside of the manufacturing system. In-process parts are controlled by the system, but storage of the finished parts and storage of the bar stock and bar stock sections are controlled at the factory level.
· Maintenance functions required in the factory are not part of the system. These include the maintenance of the machines, robots, and computers, and also of the software required to run them.
· The CAD/CAM (Computer Aided Design / Computer Aided Manufacturing) system mentioned in section 4.4.2 will reside in computers outside the manufacturing system. The manufacturing system computer will store parts programs and sequence programs but will not be used for designing parts and generating CNC programs. Figure 4-1 schematically shows the relationship between the manufacturing system and the operations listed above.
5.3. Modifications at the Cell Level
The modifications to a machining cell that transform it from a solitary, developmental cell into a member of a manufacturing system are not difficult. The design for the cell specifies input and output for the cell, and these are services that the manufacturing system provides to the cell. The cell has been designed so that virtually no modifications in the cell equipment are required. In fact, the performance demanded of individual machines in a cell may reduce as the number of cells in a manufacturing system increases. Redundancy makes each piece of equipment less crucial to the function of the system as a whole.
The main modification of the cell will be to the cell host. A communication channel between the manufacturing system and the cell host must be established. The communication channel will be via the local network of the factory and the cell host must be given the software and the hardware to use it. The cell host will receive its instructions and programs from the manufacturing system computer instead of its own on-line storage.
5.4. System Components
The most important components of the manufacturing system are the cells that it comprises, but some additional components will be needed to tie the cells together:
Inter - cell materials handling devices
The devices responsible for transporting components between the cells in a manufacturing system can take many forms. The traditional conveyor-based
systems are expensive and difficult to maintain. The newer developments based on robot trucks, for example, also require a large capital investment. The success of the system depends most on implementing the cells correctly. Achieving a completely unmanned material flow is less important, and initially we allow the handling of materials between the cells to be manual. To use robot trucks requires not only the very high initial expense of the carts and their guiding apparatus, but also the development of an intelligent, automated system for controlling them, Sensors are required to keep track of where the individual carts are at any given time, and what parts they are carrying. Batches of parts will enter or leave the cells only at occasional intervals, so the autonomous nature of the cell and manufacturing system is not too compromised.
The manufacturing system supervisory computer.
The supervisory computer for the manufacturing system will fill much the same role for the system that the cell host computer does for the cell. A few of the more important chores performed by the system computer are listed below:
· Schedule the flow of parts between the cells within the manufacturing system.
· Direct the flow of information concerning the parts.
· Store the CNC part programs (for both the CNC machine tools and the robots) in an on-line storage facility.
· Maintain a database that relates particular parts to the programs required to make them.
· Supply the part programs to the cell host when a new part is sent to a cell.
· The manufacturing system computer must also provide the cell host with its operating software. If the cell hosts need software support each time they are booted, then the system computer will be responsible for that too.
· The statistics arising from the individual cells will need to be compiled, compressed, and analyzed by the manufacturing system computer.
· The software for gracefully degrading the system as one or more cells experience problems will reside in the system computer.
The computer to accomplish these functions will need to be a fairly substantial machine. A Digital VAX 11/780, or comparable equipment, should be sufficient for most systems. The computer will be running some advanced software, so the same caveats apply to this system that apply to the cell host, ie. it should run a good operating system that can support program and system development.
If there is only one cell, a single computer could act as both the cell host and the manufacturing system supervisor but there are some arguments against doing this. First, the entire production floor will become dependent on a single device. Although reliable, today’s computer technology docs fail now and again. Such a failure could have serious effects on the system productivity. Second, future expansion will require a separation of computing power (one machine will not economically support the whole load). Many headaches can be avoided by separating the two supervisors (cell and manufacturing system) from the beginning. And finally, the separated supervisors will be more marketable as separate products. A good cell host with the proper software is a product not available on today’s market. It should be.
Network links.
The system computer will communicate to the cells over a factory wide network. The technology for Local Area Networks (LANS) is developing rapidly. Most computer manufacturers are building some sort of network or interface to an established network. The manufacturing system, the cells and the factory should capitalize on the new LAN technology.
The
The hardware and transmission protocols are only part of the networking system, though. The software to support the LAN at each computer must also exist. Typically, the software to support a complex system lags the hardware implementations (a truism of software engineering). The software for the Ethernet is well advanced and will probably be available by the time anyone wants to implement a system such as this.
Another possibility is to use one of the systems which is marketed by a single vendor. DECnet and IBM’s SNA (Systems Network Architecture) are two examples. The major problem with such an approach is the restriction to one vendor’s computers for the whole system, or the special development of interfacing software. Given the wide variety of computers within the manufacturing system and the even wider variety of tasks, there is much to be said for using a network system that will be a
The last factor for consideration here is the way the LAN will adapt to the growth of the manufacturing system. The small manufacturer will probably not invest in a full-blown CAD system until the price drops considerably. Nonetheless, the original choice for the LAN should not later restrict the choice of CAD systems. Similarly, machine tool makers will be implementing links to LANS at some point in the future.
When this occurs, the serial links in the manufacturing cells may be replaced with a faster LAN scheme, provided that the LAN originally chosen for the manufacturing system is compatible with what the machine tool builders offer. The adoption of a
Alternatives for the Future of Machine Cells
The functions and equipment for the cell described above are by no means exclusive. Various alternative configurations can be used to increase flexibility -or productivity. For example, the machining cell becomes considerably more versatile if a four or five axis machining center is used in place of a vertical axis CNC mill. The much greater expense of a machining center is not justified for machining parts from the family typified by figure 3-1 but it would allow the cell to tackle complex parts that require too many set-ups with a vertical axis mill or that require contour cutting. As a second example, the lathe could be replaced with a multi- spindle chucker which would considerably increase the number of turned parts per hour. For many small- batch precision parts, however, high production volumes are not an issue.
The way in which the cell locates and then acquires new parts is flexible, but slow. The vision system looks at each part, determines the coordinates of the part and sends them to the cell host. The cell host then sends them to the mill-loading robot which carefully grips the parts and transfers them, one by one, to one of the machine tools. A faster approach is to use a pallet with fixtures to hold the parts so that they arrive at the cell with a known orientation. The pallet and cart may be aligned with mechanical locating devices, or the pallet may be located using the vision system -just once. This approach makes less use of the vision system, and is less flexible since it presumes that parts are accurately located on the pallet when they arrive. However, if the parts have arrived from another cell they will have been put down in a systematic and accurate fashion. The only other requirement is to design the pallet so that the parts do not jostle while they are transported. The vision system is used only to count the parts, and check that they match the expected description. Even if the vision system is used to establish the position and orientation of parts, it is still advantageous to begin with a known approximate part orientation. This reduces the likelihood of ambiguous part orientations and can improve the speed and accuracy of the solution.
The sensors used in the cell could include pressure sensors, force sensors, vibration sensors, and optical measurement equipment. These devices would allow the cell to check for problems and do some rudimentary troubleshooting. For example, proportional sensors such as LVDTs or pneumatic gages on the fixtures could display not only whether parts were aligned but how well they were aligned compared to previous parts in the batch. Once random noise is filtered out of such measurements, they become very useful in indicating trends such as a gradual deterioration in robot accuracy or fixture performance. The goal is to discover such trends before they become a serious problem.
Sensors can also be used to monitor the processes within the cell in an effort to optimize them. For example, in the machining process one might emulate some of the senses that an experienced machinist uses when running his machine. Thus, in an attempt to automate a machinist's use of tactile feedback, accelerometers could be mounted in strategic locations on the machine tool to measure the vibration of the system composed of the machine tool, fixture and work piece. This information would be used to identify speeds. feeds, and fixture configurations which maximize chip removal while minimizing vibration. While a machinist might judge the rate of power consumption of a machine tool by listening to the spindle drive motor, a volt meter in combination with an ammeter could measure the drive motor's power consumption. Checking work piece dimensions, a simple matter of reading a value from a caliper or micrometer for a machinist, might be accomplished with displacement transducers mounted in specially designed tool holders. This, however, has the drawback of being limited by the accuracy of the machine tool itself and occupies the machine with measurement tasks when it could be cutting metal. The list can be extended to increasingly obscure parameters that are normally monitored by a machinist operating a milling machine but until better theoretical models of the machining process are available there is little point in monitoring any but the most basic variables.
The devices mentioned above are more delicate than the simple sensors specified in section 4.3 and they often require expensive signal processing electronics; but the real reason for not including them in the first implementation of the flexible cell is that they require sophisticated software support for control and diagnosis. A considerable amount of research and development would be required to produce such software.
Most of the alternatives to the near-term cell, including those mentioned above in this section, would increase the first cost of the cell. There are very few alternatives that will decrease the cost of the cell because a minimum cell configuration has been selected. The one exception to this rule is to wait. We can wait until machine tools become more flexible, until controllers become more powerful and versatile, until the software required for flexible cells is less novel, and then the cost of the cell will go down. Unfortunately if we do wait, we are no longer building the "Factory of the Future." Also, many of the development costs pertain to the first cost of the first cell. Once a couple of cells have been built, the development costs become minor.
Problems of Machine cells
Flexible manufacturing cells may be plagued by a number of common problems. Some of these stem from design errors and are largely avoidable. Others are errors that occur during cell operation and require the design and implementation of contingency plans.
4.6.1. Design problems
It is unlikely that we can anticipate all of the problems associated with designing a cell but we list a few of the most common design pitfalls below:
· Flexibility versus Productivity - The components of the cell can be designed to maximize either of these criteria but not both. It is therefore important to decide from a production standpoint how flexible the cell should be and what its productivity should be. All components of the cell should then reflect this compromise. There is no point in designing extremely adaptable fixturing if the supervisory control system for the cell is not flexible, and vice-versa.
· Failures - Occasional failures must be anticipated. A machine may fail to operate correctly or a 32 procedure may fail to take place as expected. Failures can result from many causes including equipment breakage, faulty incoming parts and robot drift. It is probably acceptable to scrap a part (or in a severe case, a whole batch of parts) but the equipment in the cell should not be allowed to seriously damage itself.
· Critical Items - Evaluate how crucial each component is to the partial operation of the cell. If the lathe is out of commission then the cell can still do parts that require milling. If one of the robots is out of commission then parts will have to be fed by hand. If the supervisory computer is out of commission then the machine tools and robots can no longer function as a cell. In the case of very critical components, such as the host computer, there should always be spare parts on hand. Sensors, such as limit switches used to determine the presence of a part, should be made redundant so that if one fails then the desired information will still be available.
· Cell Host Extensibility -- The designers of the cell host will certainly not think of everything the cell "should" do. They will probably avoid doing things that are just plain wrong, but experience in working with the cell will illuminate areas that need additional support. The programs running on the cell host must be easily changed. Their integration into what has been called the cell supervisor must be clear and well structured so individual programs can be easily modified and extended. In particular:
o There must be a straight-forward procedure for changing the way the cell supervisor sequences the machines in the cell. More than any other operational feature, the sequencing of the machines in the cell will need to be adjusted.
o The understanding of run-time errors will grow (probably exponentially, at first) with experience. The software to handle the errors should be designed to make it straightforward to add new routines and modify old ones. In addition, the hardware must allow for expansion of the software. A cell host that seems just adequate for the cell at design time will probably soon be bogged down with un-anticipated sensor handling routines.
o The type of data the Supervisor keeps track of will be fairly well understood at design time, but the individual pieces of data almost certainly won't. This is particularly true for the statistical data which is sent to the manufacturing system computer. Additional sampling of random run-time data should be easy to implement.
4.6.2. Runtime problems
Runtime errors will occur in any cell, although they can be reduced by proper design. The cell must be able to recover from minor errors and must at least be able to avoid damaging itself when confronted with serious errors. Runtime errors include broken cutting tools, dropped parts, misaligned parts, machine tool failures and computer crashes. The near-term cell described above is a minimum configuration cell and its response to most of these errors will be to allow one or more parts to be scrapped. One problem with a minimum configuration is that the consequences of any given error are more severe for the cell as a whole. Thus, it is especially important to design durable and perhaps redundant components for the cell.
Skills needed to Develop and Maintain the Cell
A flexible manufacturing cell requires an amalgam of conventional manufacturing skills and computer expertise. The mixture of equipment and techniques in the cell impose demands that are best solved by an eclectic group. The skills described below will be needed most while the cell is developed. After the cell is running smoothly they will occasionally be in demand for trouble-shooting and modifying the cell.
The mechanical equipment can be treated in much the same way as high-volume automated machinery is debugged and maintained today. Instead of compiling an exhaustive list of all the mechanical tasks involved in setting up, debugging, and maintaining a cell we focus on those that are not found in more conventional forms of production. The main features that distinguish equipment in a cell from other pieces of automated machinery are their complexity, their flexibility, and their physical interaction. For example, installing and maintaining robots requires some knowledge of the controller, the servo system, and the mechanical components. If something goes wrong it is important to know which of these systems is most likely at fault. Furthermore, the robot in this cell is not a stand-alone machine; it communicates with a host computer and it interacts physically with other pieces of equipment. It is important to have a global view of the robot as, well as a robot repair technician’s view, when working on the robot.
The cell we have described produces precision parts, and this calls for special attention to how errors accumulate as a part moves from one process to the next. For instance, when the perpendicularity of two surfaces on a part starts to deteriorate there are many possible contributors to the problem. The axis feedback on the milling machine might be in error, or the end mill might be deflecting. The fault may lie with the fixturing on the mill. The fixturing can slightly shift with respect to the mill, or the sensors on the fixture can indicate that the part is aligned when, in fact, it is not. If the sensors on the fixturing do indicate a problem then the robot, or its gripper, may be at fault. The part may be poorly aligned during its acquisition by the robot. The gripper is equipped with sensors to detect this condition, but they can fail. The possibilities continue beyond this list. The problem is to find the things which can be re-adjusted to improve the final accuracy of the part. Its solution is greatly aided by a thorough understanding of how the processes and equipment interact. Much of this Understanding will have to be acquired during the initial operation of the cell.
When the cell is first set up there will be a large amount of mechanical de-bugging to do. The design of fixtures is expected to be an ongoing and iterative process.
The development of the software and hardware to control the cell will require skills that are needed in any computer environment. The exceptional requirements for an application such as this are centered on the problem of integrating the computer and the machining worlds. For example, there will be development problems getting from the cell host into the machine controllers. The creation of the protocols and programs to achieve this will entail a great deal of initial effort. The effort can be eased by people with skills involving low-level interfacing of computers. The cell host will have the responsibility of assuring that no un-toward physical interaction occurs (like the robot running into the mill table). The software to support this duty will not be simple and will require the skills of a high-level program designer. The amount of programming to be done in the cell is large and will require several programmers. The managers for this project will be required to understand and communicate with people from two very different backgrounds: computers and machining.
Machine Cell Operation
The discussion of how the cell operates is divided into two distinct categories of cell-level tasks: steady state operations and periodic operations. Error handling is treated separately in section 4.6.2.
The cell operates in a steady state during a parts run. The parts move through the cell under the guidance of the cell host. Each machine tool and robot runs a CNC parts program or movement program at the command of the cell host, During the parts run the cell host needs little or no correspondence with any factory level management for decisions regarding cell operation. Management data concerning cell status is, as always, available from the cell host.
The temptation to only view the cell as a sequence of part movements and individual operations is strong, but such a view is incomplete. At any time, the cell is working on a variety of parts, each at a different point in the process. To realize the full potential of the cell the actions must occur asynchronously. The cell host must be able to manage the actions of the mill, lathe, robots, and vision system so they function in parallel. This turns out to be a difficult problem, for many of the differing actions are related. Managing a cell so that the machines can function in parallel is a topic of current investigation. One technique for coordinating the different activities is to use a rule based production system. The machine inter-relations within the cell are delineated in the set of rules and their parallel execution is managed by the production system. The production system is somewhat computationally expensive, though. It would also require considerable development work to implement. Other schemes may prove to be appropriate for managing the parallel operations within the cell, but most of the currently available systems are difficult to change and do not clearly describe how the cell, as a system, works. There is no panacea for this problem. Providing a mechanism for managing the parallel execution of machine operations, under the supervision of the cell host, will require an extended development effort.
The tasks associated with changing the cell for a different part family include:
· Down-loading new parts programs to the machine tools.
· Modifying fixtures. If the parts in the new batch are of the same family as the parts from the last batch, little or no modification is needed. On the other hand, if the new parts are substantially different then manual labor may be necessary for the modification.
· Installing different cutting tools. Just as modifications to fixtures are necessary if the part style is significantly different from the previous one, it may also be necessary to install different tools in the machine tool magazines.
· Providing statistical information, such as how long the machines have been running, how many incoming parts were processed and how many parts were rejected, to the factory management system.
These tasks are discussed in more detail in section 4.4.2.
The operations required for a cold start (after a power failure, for example) form a special category. A cold start involves substantial manual labor. People will be starting machine tools, down-loading parts programs, and boot-strapping computers.
Steady State
A number of scenarios are developed below to describe the operation of the proposed cell. Each scenario is specific to some locale within the cell, but some of the scenarios may occur simultaneously.
Locating a part when it enters the cell
New parts will arrive at the cell sitting on a pallet that rides on a cart. The parts may be nothing more than lumps of metal bar stock or they may have undergone some preliminary machining, perhaps in another cell. The cart is moved beneath a vision system camera. This camera is equipped with a relatively wide angle lens and has a panoramic view of the parts sitting on the pallet The parts will lie flat on the pallet, thereby reducing the problem of determining their orientation to a two dimensional one; which is something that a vision system is well equipped to handle. The ability of the vision system to determine the position and orientation of an object is embedded in the low level binary image processing algorithms shared by virtually all commercially available systems. The low resolution image of an entire pallet full of raw parts provides sufficient information for the mill-loading robot to grasp the parts one at a time.
In figure 3-2 the robot places the parts upon a linear table. At this point a second camera may be used. The second camera is equipped with a magnifying lens, giving 5 to 10 times the resolution of the wide-angle camera. This magnification can improve the resolution of the image enough to determine the position and orientation of the raw part to within about 2.015 inch. Because the image seen by this camera is magnified, the field of view is reduced to an area on the linear table approximately 6 inches square. This reduction in the field of view prevents the camera from looking at more than one part at a time.
In figures 3-3 and 3-4 the position and orientation of parts can be established by placing the second camera over the parts-cart or by equipping the first camera with a zoom lens so that it can take both panoramic and close-up pictures. In either case, the camera must be able to move to different locations over the cart for the close-up pictures. The equipment required to move the camera to different locations over the parts would be about as expensive and complex as a linear table.
A third possibility is to establish the position and orientation of parts while they are held by the mill- loading robot. The robot grasps a part and holds it directly under the camera for a closer look. In this case there is no need to move the camera about. However, the scheme will not work unless grippers can be designed which do not obscure the part. It will also be difficult to load parts into precision fixtures on the machine tools if the parts are not precisely seated in the robot gripper. Merely knowing the position and orientation of the parts may be inadequate (see Loading the mill).
Grasping new parts.
Once the position and orientation of a new part are established to within +.015 or -.015" inches, it can be picked by the 5 or 6 axis mill-loading robot and loaded into a machine tool. In figure 3-2 the mill-loading robot may alternatively reposition the part more precisely on the linear table to accommodate later acquisition by the lathe-loading robot. The mill-loading robot must accurately locate the part in its gripper to do either of these tasks. Basically, we are relying upon the grippers and the fixtures in this cell to improve upon the +.015" or -.015" positional accuracy by more than an order of magnitude.
The gripper will be of the type that tends to center a part as it closes. In this way the uncertainty in the part's position is slightly reduced. If the positional error is too large, the robot will have difficulty placing the part in fixturing on the mill or loading it accurately on the linear table. Several microswitches mounted in the gripper indicate whether the part is sitting squarely in it. If not, the robot releases the piece and grips it again. If this is unsuccessful then the vision system should take another look at the piece or the cell should request assistance. The process of accurately gripping the parts, one by one, and either loading them into the mill or repositioning them on the linear table is not a particularly fast one. For a higher volume cell some alternatives become worthwhile and these are discussed in section 4.7.
Grasping parts in-process
The requirements for grasping parts once they have been machined by one of the tools in the cell are somewhat different than those for new parts. In this case the parts may be sitting on the linear table (figure 3-2) or on the cart (figures 3-3 and 3-4). In either case, the position and orientation of the parts are now well defined because a robot has put them there. Consequently there is no need to use the vision system when picking them up again. At the same time, it is important to grasp the parts in a very precise and repeatable way. Once parts have been turned on the lathe or machined on the mill they become precision parts. They must be handled so as not to loose the precision invested in them.
The goal in grasping these parts is to ensure that the accuracy of the alignment between the part and the robot at least matches the working accuracy of the robot itself. If we know the greatest expected positional error of the parts we are able to design fixtures and program the robot accordingly. The smaller the error is, the easier these tasks will be. A number of micro switches will tell the robot whether a part is adequately positioned in its gripper. If not, the robot puts the part down and tries to grip it again.
Loading the mill
When the sensors on the robot gripper indicate that it has gripped the part the robot proceeds to the mill. The robot guides or locates the part against fixtures so that the part is brought into nearly perfect alignment on the mill. The robot, with its accuracy of + or - 0.008 inch, can only do this if there is some compliance in the mechanical system composed of the robot, the part, the fixturing, and the mill. It is helpful if the compliance is not accompanied by substantial hysterisis and if contact forces between the part and the fixtures produce deflections only in the corresponding directions, with a minimum of side-effects. One way to accomplish this, and thereby reduce the likelihood of jamming or galling parts, is to use a Remote Center Compliance unit mounted just behind the gripper (see clamps and fixtures, section 4.3). The Remote Center Compliance is a passive device, originally developed to assist robots in assembling precision parts. It will soak up small angular and radial errors made by the robot that could otherwise produce high contact forces between a part and the fixture it slides into.
The reason for taking such pains to load parts accurately into fixtures is that unless they are positioned to within + or - 0.0003 inch, there is no way to hold the required tolerances on most of them. The fixturing on the mill may need an expanding collet to hold the very close tolerances with respect to the internal diameters of parts such as the one shown in figure 3-1. It will probably be necessary to equip the clamps and fixtures on the mill with air nozzles. These would help to keep the clamping surfaces clean, and could easily be instrumented to detect the back pressure produced as the clamps pressed against the workpiece. Low back pressure would indicate imperfect contact due to dirt, metal chips, or misalignment of the part. The clamps should also be fitted with automatic brushes or air jets so that they can clean themselves when chips or dirt get in the way. In any event, the process of locating parts against fixtures and clamping them will require some trial and error experimentation before positioning tolerances of +0.0003 inch are achieved.
When the microswitches and pressure readings from the air nozzles indicate the part is properly clamped in place, the milling machine can begin cutting. If the sensors indicate that the part was not correctly loaded and clamped, the robot removes the part, lets the air jets or brushes try again to clean the surfaces, and loads the part again. If a part fails to load correctly after a few tries, the part is discarded. If several parts in succession, or a high percentage of parts over a period of time, fail to load correctly then something is wrong and the cell host requests assistance.
Machining
When a work piece is machined manually the machinist monitors several machining parameters and adjusts the operation accordingly. The monitoring of any particular parameter is not continuous, but is characterized by the collection of discrete samples at a frequency high enough to reveal general trends. The machinist seldom reacts immediately to this information, but strives instead to extrapolate from these trends the actions that must occur in the near future and to perform them before the need becomes critical. Consider, for example, a milling operation in which the machinist is making successive cuts in a work piece at constant spindle speed, feed rate, and depth of cut. The machinist observes that an increasing amount of effort is required to turn the hand crank which translates the tool through the work piece. Based upon this observation, the machinist extrapolates the need to reduce the forces acting on the tool before it fails. At a time determined by estimating the rate of change of this trend and through experience regarding how high these forces may become before the tool fails, the machinist acts to avoid tool failure. This action might be to increase spindle speed, reduce the feed rate, reduce the depth of cut, increase coolant flow, replace the tool with a newer and sharper tool, or some combination thereof. The desired result is to avoid scrapping the part and to eliminate a cause of unscheduled machine down-time.
The same monitoring and adjustment of machining parameters can, in a very limited way, be incorporated into a semi-autonomous, sensor-intensive machining cell. The key phrase here is sensor-intensive; what the machinist does with eyes and ears and touch, we emulate with sensors. To use the previously cited example, several techniques exist for measuring the gross forces experienced by the tool. Some techniques consist of instrumenting a part of the machine tool or the work-holding fixture with strain gages or piezoelectric force transducers. A less accurate approach is to monitor the (filtered) armature current of the machine tool motor.
Other sensor/machining-parameter combinations may be used in emulating the monitoring activities performed by a machinist but they are beyond the scope of a near-term cell. Sensors, with their related support equipment (amplifiers, filters, analogue-to-digital converters, additional processor capacity, etc), are expensive. Furthermore, every sensor added to a machining cell adds to the complexity of the system.
We have not yet discussed how machining-parameter adjustments would be made in response to the various monitoring activities. It must be possible to make appropriate adjustments under computer control. If such variables as spindle speed, feed rate, and depth of cut are not software selectable, there is little point in monitoring indicators such as force, vibration, and temperature. A machinist does not bother to gather information that cannot be used and neither should our automated machine tools. An additional problem lies in deciding what is an "appropriate adjustment". The machining programs will have to use a mixture of empirical and simple analytic formulae. To date, very little software has been developed in this field.
Eventually it should be possible to monitor and respond to changes in a variety of machining parameters so as to optimize the unmanned operation of a machine tool. The most promising possibilities are discussed in section 4.7. A realistic approach is to begin by ignoring most of these parameters and assuming, for example, that a new tool will last some fixed number of parts. The goal is to automate the actions of our hypothetical machinist incrementally, as the cell evolves and matures.
Unloading the mill
After a workpiece has been machined it is removed by the robot attending the mill. If the part requires further machining on the lathe then the robot in figure 3-2 places the part onto the linear table. In figures 3-3 and 3-4 the robot may take the part directly to the lathe or, more often, will place it upon the cart, waiting for the lathe to become available. The accuracy required for unloading the mill is not high but the robot should nonetheless be programmed to grasp the part and to set it down as accurately as possible. This is because we wish to preserve the accurate orientation of the part that we worked so hard to achieve earlier. Whether part proceeds to the lathe or to another cell, it will have to be picked up by another piece of automated equipment and loaded into another machine for precision processing. The better the orientation of the piece is known, the easier this task will be.
Loading the lathe.
For the cell arrangements shown in figures 3-3 and 3-4 the 5 or 6 axis mill-loading robot is also used to load the lathe. In this case the mechanics of loading a part into the lathe are essentially the same as those described earlier for the mill.
For the cell arrangement given in figure 3-2 things are different. When parts are ready to be turned on the lathe, the linear table slides over to a position beneath the lathe-loading robot. The lathe-loading robot is only able to pick parts from one position. This means that the linear table must index to present sequential rows of parts to it. It also means that when parts of a new style are loaded they must be placed so that their center lines match those of the previous parts. The alternative is to manually reset the fixed reach of the robot.
The lathe loading robot uses sensors to verify that it has gripped a part correctly. Like the mill loading robot, it needs to grip the part accurately. Next, the robot arm retracts, and the carriage travels to a position above the lathe chuck. The amount of carriage travel is programmable, and can be automatically changed to suit different part lengths. The arm extends, stopping when the centerline of the part and chuck coincide. The arm extension is not programmable and therefore, if the robot is to load different parts without manual readjustment, the gripper must grip different cross sections without producing offsets. The need for such a gripper has been met in other applications with a couple of simple designs. The robot carriage travels a short distance until the part bottoms out in the chuck on the lathe. The jaws of the chuck then close upon the part. As the above discussion reveals, the lathe-loading robot while inexpensive and rugged, is less suited to flexible operation than the mill-loading robot.
Loading the lathe with raw bar stock does not require extreme accuracy since the bar stock is oversize. Much more difficult tasks are: Reversing a part in the chuck of the lathe and loading a part that has already had some precision milling done on it. In either of these cases the accuracy of the finished part is a direct function of the position of the part in the lathe. As with the fixtures on the mill, it becomes vital that the jaws all meet the surface of the part squarely and that no chips or dirt get between the mating surfaces. Again as on the mill, it is useful to detect misalignment by monitoring the back-pressure of air jets at the clamping surfaces.
Reversing a part in the lathe poses a special problem for items such as the part shown in figure 3-1. These items arc too short to be gripped from the side and must be held by one end or the other. For the arrangements in figures 3-3 and 3-4 this means that the parts must be removed from the lathe, set into some holding fixture, re-gripped from the other side and loaded back into the lathe. For the arrangement in figure 3-2, one arm of the lathe-loading robot can remove the part from the chuck, swivel 180 deg. and present the part to the second arm of the robot which grips the part from the other end. The second arm then loads the part back into the lathe chuck. This is one advantage to the lathe-loading robot. Whichever cell arrangement is used, the part is either gripped or clamped three times in this process and it will be very difficult to maintain orientation.
Transferring a part between the lathe and the mill
In figure 3-2 parts travel between the lathe and the mill on a linear table carrying a pallet. The pallet will have some very simple locating fixtures to help the robots put parts down in a repeatable way. The linear table is a simple, but precise. device that travels in a straight line (see section 4.3). We take advantage of its precision, indexing it so that sequential rows of parts on the pallet are placed directly beneath the lathe-loading robot. The mill loading robot is more sophisticated and could be programmed to pick up parts from different rows on the pallet, but it is simpler and more precise to let the linear table do the indexing. Mechanical or optical switches indicate the position of the linear table, unless it has a continuous linear readout of position. Additional optical switches are placed where they will detect the presence or absence of parts on the linear table. These switches are a safety measure, verifying that a part really is present when it is presumed to be present and that an ostensibly vacant spot on the pallet really is vacant.
In figures 3-3 and 3-4 the mill-loading robot is responsible for transferring parts between the mill and the lathe. In figure 3-3 the robot rides on linear track. Ideally, the track should be treated as an extra axis for the robot, and equipped with a continuous linear readout of position.
Inspection of the completed part.
Upon completion of the machining a part, it is necessary to measure the machined dimensions to ensure that none fall outside some acceptable range for assembly into a finished product. In a typical job or batch production environment this is accomplished through manual inspection of the work piece by an individual using such measurement tools as micrometers, calipers, height gages, and so on. In our pursuit of an autonomous machining cell we would like to automate as much of this process as possible.
In addition to determining the fitness of a particular work piece for inclusion in a final product, the inspection of parts can be used to diagnose the state of the manufacturing process. Some clues are provided by the sensors located within the machining cell. Even a great many sensors, however, cannot monitor all of the variables that might cause failures. It is therefore ideal to extend the inspection of the finished part to the detection of symptoms of manufacturing failures. An example of this can be seen in the way a worn end mill in the milling machine produces a final dimension that is larger than the same dimension on an earlier work piece machined when the tool was not worn. If the cell host has been storing the dimensional data measured from previous workpieces, it not only knows that a particular dimension is oversize on the current workpiece, but also that previous work pieces have been exhibiting a trend towards this state and the tool may therefore be wearing out.
As mentioned earlier in section 4.1, final part inspection within this cell is restricted to automated visual inspection of those features which can be discerned from a profile view of the work piece. It is possible to compare these features with a previously developed data base which describes the desired or theoretical appearance of the object.
In section 3.3 it was pointed out that the resolution, or fineness of detail which can be discerned by a vision system is dependent upon the pixel density of the imaging element in the video camera. In addition, the system's resolution will be affected by magnification and by the presence of any aberrations or constructional defects in the optics used with the video camera. The resolution of the vision system is particularly important when one is attempting to use it to make dimensional measurements. The +.015" or -.015"resolution of the second camera described in Locating a part when it enters the cell is clearly too coarse for inspecting the machined dimensions on precision parts. A camera with a higher magnification would have a finer resolution but with a corresponding reduction in the field of view.
Between Part Runs
In the ideal flexible cell, the only task between part runs would be to load new programs into the machine controllers and cell host: and this would happen automatically. In a practical cell there are also a few manual tasks. The less flexible the cell, the longer they take. For the cell we describe, the tasks are as follows:
· Modify the cell fixturing to align and clamp the new part shape. If the new parts are very similar to the parts in the last batch then the fixturing is not modified at all. On the other hand, if the new parts are quite different, people will have to readjust or reconfigure all the fixtures. In the extreme case, new grippers will be bolted to all the robots and new clamps and fixtures placed on the machine tools.
· Run a few parts through the cell, step by step, with people watching the process and inspecting the parts. This precaution is most important when the parts in the new batch are not very similar to those of the last batch. Running a few parts through, slowly, allows us to verify that the programs are all good, and that the readjusted fixturing performs successfully. If the cell were more sophisticated, it could be asked to run a few trial parts by itself and could rely on its sensors and inspection equipment to check that all went well.
Transfer of parts programs form the host to the controllers
The cell host will provide the controllers with their programs. The cell host won’t store those programs, but will receive them from the a higher level computer. The heaviest traffic in machine program transfers (whether CNC part programs or robot movement programs) will occur between batch runs. Most of the controllers are capable of storing several programs at one time, so they will not need to be refreshed during the parts run.
The machine controllers will be able to support file transfers between themselves and the host. They will be able to do this without operator intervention. When the time comes for a changeover of controller programs, the cell host will tell each controller to accept the new program and then the host will ship the program. As is the case with file transfer protocols between full-fledged computers, the transfer software will guard against transmission errors. If an error is detected the two processors, host and controller, will correspond and reship the parts program.
The ensemble of programs that the controllers in the cell require is defined by the particular part being machined. A computer at the manufacturing system level will be responsible for maintaining the database of programs and for relating parts to the programs required to make them.
Replacement of tools. ,
An automated cell should treat cutting tools much like parts. New tools will be automatically loaded between part runs and tools that are worn, or that will not be used for the next part run, will be removed. To some extent, the vision system will be able to identify and locate new tools in the same way that it does for parts. The utility of the vision system will be limited, however, because cutting tools do not lend themselves to two dimensional visual locations as easily as parts like the one shown in figure 3-1. Another problem is that the robot grippers will have been carefully designed to handle a limited family of precision parts, and not tools. A way to get around this problem is to mount the tools in adaptors so that the robots can handle them. It would be easy enough to design the adaptors to also help the vision system locate the incoming tools.
Tools loaded into the mill will probably be inserted into a loading position on the tool changer. At this point it will clearly be necessary for the tool adaptor to release the tool. One simple solution is for the tool adaptor to be spring loaded. Similarly, tools for the lathe will be loaded into the turret. For the arrangement in figure 3-2 this requires positioning the turret so that one tool holder location is on the same centerline as the chuck. Otherwise the lathe loading robot would have to be manually readjusted. Here again, we see that the choice of a dedicated lathe-loading robot results in a less flexible system. In any event, it may be easiest to resort to manual tool replacement while the cell is in its earliest stages.
If a tool fails during a part run (and if there are no spares in the tool changer) we have a less tractable problem. It is difficult to introduce a new tool while the machines in the cell and the area beneath the vision system are full of parts. In this case it is easiest to inform the cell host that a new cutting tool will be manually introduced. The cell host halts the cell equipment, if it has not already done so, and waits for a signal that the tool exchange is complete.
Creation of new parts programs.
Modification and testing of parts programs will typically occur between part runs. The cell is not responsible for creating or storing parts programs, but it is useful to establish the programming requirements for the machines in this cell and to look at the tasks involved in testing new programs. A variety of commercial packages are available to generate parts programs but program generation represents only a portion of the required effort. The verification of new CNC programs is currently done on the actual machine tools and the time spent testing programs is lost production time. Generally, the techniques for program creation and verification differ for each tool, depending on the complexity of the controller. For the tools discussed in section 4.3 the procedures are listed below:
· For the CNC machine tools.
A large number of software tools are available to create parts programs from some user input The parts programs can then be down-loaded to the machine controllers. The systems that create parts programs vary in complexity from full blown CAD systems to simple, stand-alone programming aids. Even the latter are worthwhile in terms of parts programmer productivity. The generation of parts programs for CNC machine tools is a straightforward problem since most CNC machine tools are programmed in a derivative of the APT language. The structure of APT programs is simple and very amenable to automatic creation. The verification of newly created MT-like programs is not as well defined as their creation. Some sophisticated geometric modeling techniques exist for checking tool paths and such, but the final program is still checked on the machine tool.
· For the multi-axis robot.
The state of the art in program generation and verification in robots is more primitive than that for CNC machine tools. As above, machine time will be lost during verification of a new program, but more time will be lost during the creation of the program. The current method for creating robot programs is to manually teach them the motions desired in their work space. The programs can be tested only on the same robot. Some work is being done on the problem of robot program generation, but the efforts are still largely in the research arena. For the near-term cell we must anticipate cell down-time when new robot programs are being written, de-bugged, and tested.
· For the transportation device in figure 3-2.
The linear table in figure 3-2 will probably be controlled by a commercially available programmable controller (PC ). The programs for such devices are simple ladder diagrams input through something typically called a "program loader." The program loader is a terminal paired with software to convert the user input into machine language. The program loader is designed to send its machine code directly to the programmable controller but the output can also be re- directed to some storage device and later down-loaded to the linear table controller. The general issue of programming the linear table is not critical in the same way as that for the milling machine, lathe, or mill-loading robot. The programmable table will take its orders from the cell host. Those orders will be commands to move to some pre-defined location. The program residing in the programmable controller is. therefore, a simple program that interprets the host’s command into proper signal levels and times for the UC servo motors that drive the table. There will be no pre-programmed sequence of motions in the table controller. Those sequences must come from the cell host.
· For the lathe-loading robot
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