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Chapter 4
Chapter Contents 4.1 Basic Elements of an Automated System 4.1.1 Power to Accomplish the Automated Process 4.1.2 Program of Instructions 4.1.3 Control System 4.2 Advanced Automation Functions 4.2.1 Safety Monitoring 4.2.2 Maintenance and Repair Diagnostics 4.2.3 Error Detection and Recovery 4.3 Levels of Automation
Automation can be defined as the technology by which a process or procedure is ac- complished without human assistance. It is implemented using a program of instructions combined with a control system that executes the instructions. To automate a process, power is required, both to drive the process itself and to operate the program and control system. Although automation is applied in a wide variety of areas, it is most closely as- sociated with the manufacturing industries. It was in the context of manufacturing that the term was originally coined by an engineering manager at Ford Motor Company in 1946 to describe the variety of automatic transfer devices and feed mechanisms that had been installed in Ford’s production plants (Historical Note 4.1). It is ironic that nearly all modern applications of automation are controlled by computer technologies that were not available in 1946.
Introduction to Automation
PArt II
Automation and Control technologies
76 Chap. 4 / Introduction to Automation
historical note 4.1 History of Automation
The history of automation can be traced to the development of basic mechanical devices, such as the wheel (circa 3200 B.C.), lever, winch (circa 600 B.C.), cam (circa 1000), screw (1405), and gear in ancient and medieval times. These basic devices were refined and used to construct the mechanisms in waterwheels, windmills (circa 650), and steam engines (1765). These machines generated the power to operate other machinery of various kinds, such as flour mills (circa 85 B.C.), weaving machines (flying shuttle, 1733), machine tools (boring mill, 1775), steamboats (1787), and railroad locomotives (1803). Power, and the capacity to generate it and transmit it to operate a process, is one of the three basic elements of an automated system. After his first steam engine in 1765, James Watt and his partner, Matthew Boulton, made several improvements in the design. One of the improvements was the flying-ball governor (around 1785), which provided feedback to control the throttle of the engine. The governor consisted of a ball on the end of a hinged lever attached to the rotating shaft. The lever was connected to the throttle valve. As the speed of the rotating shaft increased, the ball was forced to move outward by centrifugal force; this in turn caused the lever to reduce the valve opening and slow the motor speed. As rotational speed decreased, the ball and lever relaxed, thus allowing the valve to open. The flying-ball governor was one of the first examples of feedback control—an important type of control system, which is the second basic element of an automated system. The third basic element of an automated system is the program of instructions that directs the actions of the system or machine. One of the first examples of machine program- ming was the Jacquard loom, invented around 1800. This loom was a machine for weav- ing cloth from yarn. The program of instructions that determined the weaving pattern of the cloth consisted of a metal plate containing holes. The hole pattern in the plate directed the shuttle motions of the loom, which in turn determined the weaving pattern. Different hole patterns yielded different cloth patterns. Thus, the Jacquard loom was a programmable machine, one of the first of its kind. By the early 1800s, the three basic elements of automated systems—power source, controls, and programmable machines—had been developed, although these elements were primitive by today’s standards. It took many years of refinement and many new inventions and developments, both in these basic elements and in the enabling infrastructure of the manufacturing industries, before fully automated systems became a common reality. Important examples of these inventions and developments include interchangeable parts (circa 1800, Historical Note 1.1); electrification (starting in 1881); the moving assembly line (1913, Historical Note 15.1); mechanized transfer lines for mass production, whose programs were fixed by their hardware configuration (1924, Historical Note 16.1); a mathematical theory of control systems (1930s and 1940s); and the MARK I electromechanical computer at Harvard University (1944). These inventions and developments had all been realized by the end of World War II. Since 1945, many new inventions and developments have contributed significantly to automation technology. Del Harder coined the word automation around 1946 in reference to the many automatic devices that the Ford Motor Company had developed for its production lines. The first electronic digital computer was developed at the University of Pennsylvania in 1946. The first numerical control machine tool was developed and demonstrated in 1952 at the Massachusetts Institute of Technology based on a concept proposed by John Parsons and Frank Stulen (Historical Note 7.1). By the late 1960s and early 1970s, digital computers were being connected to machine tools. In 1954, the first industrial robot was designed and in 1961 it was patented by George Devol (Historical Note 8.1). The first commercial robot was installed to unload parts in a die casting operation in 1961. In the late 1960s, the first flex- ible manufacturing system in the United States was installed at Ingersoll Rand Company to
78 Chap. 4 / Introduction to Automation
4.1 Basic ElEmEnts of an automatEd systEm
An automated system consists of three basic elements: (1) power to accomplish the process and operate the system, (2) a program of instructions to direct the process, and (3) a control system to actuate the instructions. The relationship among these elements is illustrated in Figure 4.2. All systems that qualify as being automated include these three basic elements in one form or another. They are present in the three basic types of auto- mated manufacturing systems: fixed automation, programmable automation, and flexible automation (Section 1.2.1).
4.1.1 power to accomplish the automated process
An automated system is used to operate some process, and power is required to drive the process as well as the controls. The principal source of power in automated systems is elec- tricity. Electric power has many advantages in automated as well as nonautomated processes:
- Electric power is widely available at moderate cost. It is an important part of the industrial infrastructure.
- Electric power can be readily converted to alternative energy forms: mechanical, thermal, light, acoustic, hydraulic, and pneumatic.
- Electric power at low levels can be used to accomplish functions such as signal transmission, information processing, and data storage and communication.
- Electric energy can be stored in long-life batteries for use in locations where an external source of electrical power is not conveniently available.
Alternative power sources include fossil fuels, atomic, solar, water, and wind. However, their exclusive use is rare in automated systems. In many cases when alternative power sources are used to drive the process itself, electrical power is used for the controls that automate the operation. For example, in casting or heat treatment, the furnace may be heated by fossil fuels, but the control system to regulate temperature and time cycle is electrical. In other cases, the energy from these alternative sources is converted to electric power to operate both the process and its automation. When solar energy is used as a power source for an automated system, it is generally converted in this way.
power for the process. In production, the term process refers to the manu- facturing operation that is performed on a work unit. In Table 4.1, a list of common
Program of instructions
(1)
(2) (3) Control system Process
Power
Figure 4.2 Elements of an automated system: (1) power, (2) program of instructions, and (3) control systems.
Sec. 4.1 / Basic Elements of an Automated System 79
table 4.1 Common Manufacturing Processes and Their Power Requirements
Process Power Form Action Accomplished Casting Thermal Melting the metal before pouring into a mold cavity where solidification occurs. Electric discharge machining
Electrical Metal removal is accomplished by a series of discrete electrical discharges between electrode (tool) and workpiece. The electric discharges cause very high localized temperatures that melt the metal. Forging Mechanical Metal work part is deformed by opposing dies. Work parts are often heated in advance of defor- mation, thus thermal power is also required. Heat-treating Thermal Metallic work unit is heated to temperature below melting point to effect microstructural changes. Injection molding
Thermal and mechanical
Heat is used to raise temperature of polymer to highly plastic consistency, and mechanical force is used to inject the polymer melt into a mold cavity. Laser beam cutting
Light and thermal
A highly coherent light beam is used to cut material by vaporization and melting. Machining Mechanical Cutting of metal is accomplished by relative motion between tool and workpiece. Sheet metal punching and blanking
Mechanical Mechanical power is used to shear metal sheets and plates.
Welding Thermal (maybe mechanical)
Most welding processes use heat to cause fusion and coalescence of two (or more) metal parts at their contacting surfaces. Some welding processes also apply mechanical pressure.
manufacturing processes is compiled along with the form of power required and the re- sulting action on the work unit. Most of the power in manufacturing plants is consumed by these kinds of operations. The “power form” indicated in the middle column of the table refers to the energy that is applied directly to the process. As indicated earlier, the power source for each operation is often converted from electricity. In addition to driving the manufacturing process itself, power is also required for the following material handling functions:
- Loading and unloading the work unit. All of the processes listed in Table 4.1 are accomplished on discrete parts. These parts must be moved into the proper posi- tion and orientation for the process to be performed, and power is required for this transport and placement function. At the conclusion of the process, the work unit must be removed. If the process is completely automated, then some form of mechanized power is used. If the process is manually operated or semiautomated, then human power may be used to position and locate the work unit.
- Material transport between operations. In addition to loading and unloading at a given operation, the work units must be moved between operations. The material handling technologies associated with this transport function are covered in Chapter 10.
Sec. 4.1 / Basic Elements of an Automated System 81
To change the program, the operator simply changes the dial setting. In an extension of this simple case, the one-step process is defined by more than one process param- eter, for example, a furnace in which both temperature and atmosphere are controlled. Because of dynamics in the way the process operates, the process variable is not always equal to the process parameter. For example, if the temperature setting suddenly were to be increased or decreased, it would take time for the furnace temperature to reach the new set-point value. (This is getting into control system issues, which is the topic of Section 4.1.3.) Work cycle programs are usually much more complicated than in the furnace example described. Following are five categories of work cycle programs, arranged in approximate order of increasing complexity and allowing for more than one process parameter in the program:
- Set-point control, in which the process parameter value is constant during the work cycle (as in the furnace example).
- Logic control , in which the process parameter value depends on the values of other variables in the process. Logic control is described in Section 9.1.1.
- Sequence control , in which the value of the process parameter changes as a function of time. The process parameter values can be either discrete (a sequence of step val- ues) or continuously variable. Sequence control, also called sequencing , is discussed in Section 9.1.2.
- Interactive program , in which interaction occurs between a human operator and the control system during the work cycle.
- Intelligent program , in which the control system exhibits aspects of human in- telligence (e.g., logic, decision making, cognition, learning) as a result of the work cycle program. Some capabilities of intelligent programs are discussed in Section 4.2.
Most processes involve a work cycle consisting of multiple steps that are repeated with no deviation from one cycle to the next. Most discrete part manufacturing opera- tions are in this category. A typical sequence of steps (simplified) is the following: (1) load the part into the production machine, (2) perform the process, and (3) unload the part. During each step, there are one or more activities that involve changes in one or more process parameters.
ExamplE 4.1 an automated Turning Operation Consider an automated turning operation that generates a cone-shaped prod- uct. The system is automated and a robot loads and unloads the work units. The work cycle consists of the following steps: (1) load starting workpiece, (2) position cutting tool prior to turning, (3) turn, (4) reposition tool to a safe location at end of turning, and (5) unload finished workpiece. Identify the activities and process parameters for each step of the operation.
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Many production operations consist of multiple steps, sometimes more compli- cated than in the turning example. Examples of these operations include automatic screw machine cycles, sheet metal stamping, plastic injection molding, and die casting. Each of these manufacturing processes has been used for many decades. In earlier ver- sions of these operations, work cycles were controlled by hardware components, such as limit switches, timers, cams, and electromechanical relays. In effect, the assemblage of hardware components served as the program of instructions that directed the se- quence of steps in the processing cycle. Although these devices were quite adequate in performing their logic and sequencing functions, they suffered from the following disadvantages: (1) They often required considerable time to design and fabricate, forc- ing the production equipment to be used for batch production only; (2) making even minor changes in the program was difficult and time consuming; and (3) the program was in a physical form that was not readily compatible with computer data processing and communication. Modern controllers used in automated systems are based on digital comput- ers. Instead of cams, timers, relays, and other hardware components, the programs for computer-controlled equipment are contained in compact disks (CD-ROMs), computer memory, and other modern storage technologies. Virtually all modern production equip- ment is designed with some form of computer controller to execute its respective process- ing cycles. The use of digital computers as the process controller allows improvements and upgrades to be made in the control programs, such as the addition of control func- tions not foreseen during initial equipment design. These kinds of control changes are often difficult to make with the hardware components mentioned earlier.
Solution: In step (1), the activities consist of the robot manipulator reaching for the raw work part, lifting and positioning the part into the chuck jaws of the lathe, then retreating to a safe position to await unloading. The process parameters for these activities are the axis values of the robot manipulator (which change continuously), the gripper value (open or closed), and the chuck jaw value (open or closed). In step (2), the activity is the movement of the cutting tool to a “ready” position. The process parameters associated with this activity are the x - and z -axis position of the tool. Step (3) is the turning operation. It requires the simultaneous control of three process parameters: rotational speed of the workpiece (rev/min), feed (mm/rev), and radial distance of the cutting tool from the axis of rotation. To cut the conical shape, radial distance must be changed continuously at a constant rate for each revolution of the workpiece. For a consistent finish on the surface, the rotational speed must be continuously adjusted to maintain a constant surface speed (m/min); and for equal feed marks on the surface, the feed must be set at a constant value. Depending on the angle of the cone, mul- tiple turning passes may be required to gradually generate the desired con- tour. Each pass represents an additional step in the sequence. Steps (4) and (5) are the reverse of steps (2) and (1), respectively, and the process parameters are the same.
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conditions that lie outside the normal routine. These measures are discussed later in the chapter in the context of advanced automation functions (Section 4.2). Various production situations and work cycle programs have been discussed here. The following summarizes the features of work cycle programs (part programs) used to direct the operations of an automated system:
- Process parameters. How many process parameters must be controlled during each step? Are the process parameters continuous or discrete? Do they change during the step, for example, a positioning system whose axis values change during the processing step?
- Number of steps in work cycle. How many distinct steps or work elements are included in the work cycle? A general sequence in discrete production operations is (1) load, (2), process, (3) unload, but the process may include multiple steps.
- Manual participation in the work cycle. Is a human worker required to perform certain steps in the work cycle, such as loading and unloading a production machine, or is the work cycle fully automated?
- Operator interaction. For example, is the operator required to enter processing data for each work cycle?
- Variations in part or product styles. Are the work units identical each cycle, as in mass production (fixed automation) or batch production (programmable automation), or are different part or product styles processed each cycle (flexible automation)?
- Variations in starting work units. Variations can occur in starting dimensions or materials. If the variations are significant, some adjustments may be required during the work cycle.
4.1.3 Control system
The control element of the automated system executes the program of instructions. The control system causes the process to accomplish its defined function, which is to perform some manufacturing operation. A brief introduction to control systems is provided here. The following chapter describes this technology in more detail. The controls in an automated system can be either closed loop or open loop. A closed- loop control system , also known as a feedback control system , is one in which the output variable is compared with an input parameter, and any difference between the two is used to drive the output into agreement with the input. As shown in Figure 4.3, a closed-loop control system consists of six basic elements: (1) input parameter, (2) process, (3) output
Input Controller parameter
(5)
Actuator
(6)
Feedback sensor
(4)
Process
(1) (2) Output variable
(3)
Figure 4.3 A feedback control system.
Sec. 4.1 / Basic Elements of an Automated System 85
variable, (4) feedback sensor, (5) controller, and (6) actuator. The input parameter (i.e., set point) represents the desired value of the output. In a home temperature control sys- tem, the set point is the desired thermostat setting. The process is the operation or function being controlled. In particular, it is the output variable that is being controlled in the loop. In the present discussion, the process of interest is usually a manufacturing operation, and the output variable is some process variable, perhaps a critical performance measure in the process, such as temperature or force or flow rate. A sensor is used to measure the output variable and close the loop between input and output. Sensors perform the feedback func- tion in a closed-loop control system. The controller compares the output with the input and makes the required adjustment in the process to reduce the difference between them. The adjustment is accomplished using one or more actuators, which are the hardware de- vices that physically carry out the control actions, such as electric motors or flow valves. It should be mentioned that Figure 4.3 shows only one loop. Most industrial processes require multiple loops, one for each process variable that must be controlled. In contrast to a closed-loop control system, an open-loop control system operates without the feedback loop, as in Figure 4.4. In this case, the controls operate without measuring the output variable, so no comparison is made between the actual value of the output and the desired input parameter. The controller relies on an accurate model of the effect of its actuator on the process variable. With an open-loop system, there is always the risk that the actuator will not have the intended effect on the process, and that is the disadvantage of an open-loop system. Its advantage is that it is generally simpler and less expensive than a closed-loop system. Open-loop systems are usually appropri- ate when the following conditions apply: (1) the actions performed by the control system are simple, (2) the actuating function is very reliable, and (3) any reaction forces opposing the actuator are small enough to have no effect on the actuation. If these characteristics are not applicable, then a closed-loop control system may be more appropriate. Consider the difference between a closed-loop and open-loop system for the case of a positioning system. Positioning systems are common in manufacturing to locate a work part relative to a tool or work head. Figure 4.5 illustrates the case of a closed-loop positioning system. In operation, the system is directed to move the worktable to a speci- fied location as defined by a coordinate value in a Cartesian (or other) coordinate system. Most positioning systems have at least two axes (e.g., an x – y positioning table) with a
Input Controller parameter Actuator Process Output variable
Figure 4.4 An open-loop control system.
Controller
Motor x -value input Motor Leadscrew Feedback signal to controller
Worktable (^) Actual x
Optical encoder
Figure 4.5 A (one-axis) positioning system consisting of a leadscrew driven by a dc servomotor.
Sec. 4.2 / Advanced Automation Functions 87
safety monitoring capability: (1) to protect human workers in the vicinity of the system, and (2) to protect the equipment comprising the system. Safety monitoring means more than the conventional safety measures taken in a manufacturing operation, such as protective shields around the operation or the kinds of manual devices that might be utilized by human workers, such as emergency stop buttons. Safety monitoring in an automated system involves the use of sensors to track the system’s operation and identify conditions and events that are unsafe or potentially unsafe. The safety monitoring system is programmed to respond to unsafe conditions in some appropriate way. Possible responses to various hazards include one or more of the following: (1) completely stopping the automated system, (2) sounding an alarm, (3) reducing the operating speed of the process, and (4) taking corrective actions to recover from the safety violation. This last response is the most sophisticated and is suggestive of an intelligent machine performing some advanced strategy. This kind of response is applicable to a variety of possible mishaps, not necessarily confined to safety issues, and is called error detection and recovery (Section 4.2.3). Sensors for safety monitoring range from very simple devices to highly sophisti- cated systems. Sensors are discussed in Section 6.1. The following list suggests some of the possible sensors and their applications for safety monitoring:
- Limit switches to detect proper positioning of a part in a workholding device so that the processing cycle can begin.
- Photoelectric sensors triggered by the interruption of a light beam; this could be used to indicate that a part is in the proper position or to detect the presence of a human intruder in the work cell.
- Temperature sensors to indicate that a metal work part is hot enough to proceed with a hot forging operation. If the work part is not sufficiently heated, then the metal’s ductility might be too low, and the forging dies might be damaged during the operation.
- Heat or smoke detectors to sense fire hazards.
- Pressure-sensitive floor pads to detect human intruders in the work cell.
- Machine vision systems to perform surveillance of the automated system and its surroundings.
It should be mentioned that a given safety monitoring system is limited in its ability to respond to hazardous conditions by the possible irregularities that have been foreseen by the system designer. If the designer has not anticipated a particular haz- ard, and consequently has not provided the system with the sensing capability to detect that hazard, then the safety monitoring system cannot recognize the event if and when it occurs.
4.2.2 Maintenance and repair Diagnostics
Modern automated production systems are becoming increasingly complex and sophisticated, complicating the problem of maintaining and repairing them. Maintenance and repair diagnostics refers to the capabilities of an automated system to assist in identifying the source of potential or actual malfunctions and failures of
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the system. Three modes of operation are typical of a modern maintenance and repair diagnostics subsystem:
- Status monitoring. In the status monitoring mode, the diagnostic subsystem monitors and records the status of key sensors and parameters of the system during normal oper- ation. On request, the diagnostics subsystem can display any of these values and provide an interpretation of current system status, perhaps warning of an imminent failure.
- Failure diagnostics. The failure diagnostics mode is invoked when a malfunction or fail- ure occurs. Its purpose is to interpret the current values of the monitored variables and to analyze the recorded values preceding the failure so that its cause can be identified.
- Recommendation of repair procedure. In the third mode of operation, the subsys- tem recommends to the repair crew the steps that should be taken to effect repairs. Methods for developing the recommendations are sometimes based on the use of expert systems in which the collective judgments of many repair experts are pooled and incorporated into a computer program that uses artificial intelligence techniques.
Status monitoring serves two important functions in machine diagnostics: (1) pro- viding information for diagnosing a current failure and (2) providing data to predict a future malfunction or failure. First, when a failure of the equipment has occurred, it is usually difficult for the repair crew to determine the reason for the failure and what steps should be taken to make repairs. It is often helpful to reconstruct the events leading up to the failure. The computer is programmed to monitor and record the variables and to draw logical inferences from their values about the reason for the malfunction. This diag- nosis helps the repair personnel make the necessary repairs and replace the appropriate components. This is especially helpful in electronic repairs where it is often difficult to determine on the basis of visual inspection which components have failed. The second function of status monitoring is to identify signs of an impending failure, so that the affected components can be replaced before failure actually causes the system to go down. These part replacements can be made during the night shift or another time when the process is not operating, so the system experiences no loss of regular operation.
4.2.3 error Detection and recovery
In the operation of any automated system, there are hardware malfunctions and unex- pected events. These events can result in costly delays and loss of production until the problem has been corrected and regular operation is restored. Traditionally, equipment malfunctions are corrected by human workers, perhaps with the aid of a maintenance and repair diagnostics subroutine. With the increased use of computer control for manufac- turing processes, there is a trend toward using the control computer not only to diagnose the malfunctions but also to automatically take the necessary corrective action to restore the system to normal operation. The term error detection and recovery is used when the computer performs these functions.
error Detection. The error detection step uses the automated system’s available sensors to determine when a deviation or malfunction has occurred, interpret the sensor signal(s), and classify the error. Design of the error detection subsystem must begin with a systematic enumeration of all possible errors that can occur during system operation. The errors in a manufacturing process tend to be very application-specific. They must be anticipated in advance in order to select sensors that will enable their detection.
90 Chap. 4 / Introduction to Automation
error recovery. Error recovery is concerned with applying the necessary cor- rective action to overcome the error and bring the system back to normal operation. The problem of designing an error recovery system focuses on devising appropri- ate strategies and procedures that will either correct or compensate for the errors that can occur in the process. Generally, a specific recovery strategy and procedure must be designed for each different error. The types of strategies can be classified as follows:
- Make adjustments at the end of the current work cycle. When the current work cycle is completed, the part program branches to a corrective action subroutine specifi- cally designed for the detected error, executes the subroutine, and then returns to the work cycle program. This action reflects a low level of urgency and is most com- monly associated with random errors in the process.
- Make adjustments during the current cycle. This generally indicates a higher level of urgency than the preceding type. In this case, the action to correct or compensate for the detected error is initiated as soon as it is detected. However, the designated corrective action must be possible to accomplish while the work cycle is still being executed. If that is not possible, then the process must be stopped.
- Stop the process to invoke corrective action. In this case, the deviation or mal- function requires that the work cycle be suspended during corrective action. It is assumed that the system is capable of automatically recovering from the error without human assistance. At the end of the corrective action, the regular work cycle is continued.
- Stop the process and call for help. In this case, the error cannot be resolved through automated recovery procedures. This situation arises because (1) the automated cell is not enabled to correct the problem or (2) the error cannot be classified into the predefined list of errors. In either case, human assistance is required to correct the problem and restore the system to fully automated operation.
Error detection and recovery requires an interrupt system (Section 5.3.2). When an error in the process is sensed and identified, an interrupt in the current program execution is invoked to branch to the appropriate recovery subroutine. This is done either at the end of the current cycle (type 1 above) or immediately (types 2, 3, and 4). At the completion of the recovery procedure, program execution reverts back to nor- mal operation.
ExamplE 4.3 Error Recovery in an automated machining Cell For the automated cell of Example 4.2, develop a list of possible corrective actions that might be taken by the system to address some of the errors. Solution: A list of possible corrective actions is presented in Table 4.3.
Sec. 4.3 / Levels of Automation 91
table 4.3 Error Recovery in an Automated Machining Cell: Possible Corrective Actions That Might Be Taken in Response to Errors Detected During the Operation
Error Detected Possible Corrective Action to Recover Part dimensions deviating due to thermal deflection of machine tool
Adjust coordinates in part program to compensate (category 1 corrective action)
Part dropped by robot during pickup
Reach for another part (category 2 corrective action)
Starting work part is oversized
Adjust part program to take a preliminary machining pass across the work surface (category 2 correc- tive action) Chatter (tool vibration) Increase or decrease cutting speed to change har- monic frequency (category 2 corrective action) Cutting temperature too high
Reduce cutting speed (category 2 corrective action)
Cutting tool failed Replace cutting tool with another sharp tool (category 3 corrective action). No more parts in parts storage unit
Call operator to resupply starting work parts (category 4 corrective action) Chips fouling machining operation
Call operator to clear chips from work area (category 4 corrective action)
4.3 lEvEls of automation
Automated systems can be applied to various levels of factory operations. One normally associates automation with the individual production machines. However, the production machine itself is made up of subsystems that may themselves be automated. For example, one of the important automation technologies discussed in this part of the book is com- puter numerical control (CNC, Chapter 7). A modern CNC machine tool is a highly au- tomated system that is composed of multiple control systems. Any CNC machine has at least two axes of motion, and some machines have more than five axes. Each of these axes operates as a positioning system, as described in Section 4.1.3., and is, in effect, an auto- mated system. Similarly, a CNC machine is often part of a larger manufacturing system, and the larger system may be automated. For example, two or three machine tools may be connected by an automated part handling system operating under computer control. The machine tools also receive instructions (e.g., part programs) from the computer. Thus three levels of automation and control are included here (the positioning system level, the machine tool level, and the manufacturing system level). For the purposes of this text, five levels of automation can be identified, and their hierarchy is depicted in Figure 4.6:
- Device level. This is the lowest level in the automation hierarchy. It includes the actua- tors, sensors, and other hardware components that comprise the machine level. The devices are combined into the individual control loops of the machine, for example, the feedback control loop for one axis of a CNC machine or one joint of an industrial robot.
References 93
Most of the technologies discussed in this part of the book are at levels 2 and 3 (machine level and cell level), although level 1 automation technologies (the devices that make up a control system) are discussed in Chapter 6. Level 2 technologies include the individual controllers (e.g., programmable logic controllers and digital computer control- lers), numerical control machines, and industrial robots. The material handling equip- ment discussed in Part III also represent technologies at level 2, although some pieces of handling equipment are themselves sophisticated automated systems. The automation and control issues at level 2 are concerned with the basic operation of the equipment and the physical processes they perform. Controllers, machines, and material handling equipment are combined into manu- facturing cells, production lines, or similar systems, which make up level 3, considered in Part IV. A manufacturing system is defined in this book as a collection of integrated equipment designed for some special mission, such as machining a defined part family or assembling a certain product. Manufacturing systems include people. Certain highly automated manufacturing systems can operate for extended periods of time without humans present to attend to their needs. But most manufacturing systems include work- ers as important participants in the system, for example, assembly workers on a conveyor- ized production line or part loaders/unloaders in a machining cell. Thus, manufacturing systems are designed with varying degrees of automation; some are highly automated, others are completely manual, and there is a wide range between the two. The manufacturing systems in a factory are components of a larger production system , which is defined as the people, equipment, and procedures that are organized for the combination of materials and processes that comprise a company’s manufactur- ing operations. Production systems are at level 4, the plant level, while manufacturing systems are at level 3 in the automation hierarchy. Production systems include not only the groups of machines and workstations in the factory but also the support procedures that make them work. These procedures include process planning, production control, in- ventory control, material requirements planning, shop floor control, and quality control, all of which are discussed in Parts V and VI. They are implemented not only at the plant level but also at the corporate level (level 5).
REfEREncEs
[1] Boucher, T. O., Computer Automation in Manufacturing , Chapman & Hall, London, UK,
[2] Groover, M. P., “Automation,” Encyclopaedia Britannica, Macropaedia , 15th ed., Chicago, IL, 1992. Vol. 14, pp. 548–557. [3] Groover, M. P., “Automation,” Handbook of Design, Manufacturing, and Automation , R. C. Dorf and A. Kusiak (eds.), John Wiley & Sons, Inc., New York, 1994, pp. 3–21. [4] Groover, M. P., “Industrial Control Systems,” Maynard’s Industrial Engineering Handbook , 5th ed., K. Zandin (ed.), McGraw-Hill Book Company, New York, 2001. [5] Platt, R., Smithsonian Visual Timeline of Inventions, Dorling Kindersley Ltd., London, UK, 1994. [6] “The Power of Invention,” Newsweek Special Issue , Winter 1997–98, pp. 6–79. [7] www.wikipedia.org/wiki/Automation
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REviEw QuEstions
4.1 What is automation? 4.2 Name the three basic elements of an automated system. 4.3 What is the difference between a process parameter and a process variable? 4.4 What are the five categories of work cycle programs, as listed in the text? Briefly describe each. 4.5 What are three reasons why decision making is required in a programmed work cycle? 4.6 What is the difference between a closed-loop control system and an open-loop control system? 4.7 What is safety monitoring in an automated system? 4.8 What is error detection and recovery in an automated system? 4.9 Name three of the four possible strategies in error recovery. 4.10 Identify the five levels of automation in a production plant.
