industrial control

Contents Hong Kong IGDS 2000 Industrial Control Table of Contents ● Chapter 0 - Introductions ● Chapter 1 - Sensors ● Chapter 2 - Actuators ● Chapter 3 - Industrial Electronic ● Chapter 4 - Motor Drives ● Chapter 5 - Control Components ● Chapter 6 - Sequential Logic ● Chapter 7 - Programmable Logic Controllers ● Chapter 8 - Computer Control Systems ● Chapter 9 - Computer Interface ● Chapter 10 - Real Time Control Platform ● Chapter 11 - Sstems Modelling ● Chapter 12 - Servo Control Time Table End Chapter 0. Introduction Chapter 0. INTRODUCTION 0.1 AUTOMATE, EMIGRATE, LEGISLATE, OR EVAPORATE "Automate, emigrate, legislate or evaporate." This was a choice many manufacturers. Some manufacturers tried to lower prices by reducing manufacturing costs. They either automated or emigrated. Many countries legislated trade barriers to keep high quality, low cost products out. Manufacturers who did nothing disappeared, often despite their own government's protective trade barriers. Many consumers still choose imports over domestic products, but some North American manufacturers are now trying more thoughtful measures to meet the challenge. Automation is a technique that can be used to reduce costs and/or to improve quality. Automation can increase manufacturing speed, while reducing cost. Automation can lead to products having consistent quality, perhaps even consistently good quality. Some manufacturers who automated survived. Others didn't. The ones who survived were those who used automation to improve quality. It often happened that improving quality led to reduced costs. 0.2 THE ENVIRONMENT FOR AUTOMATION Automation, the subject of this textbook, is not a magic solution to financial problems. It is, however, a valuable tool that can be used to improve product quality. Improving product quality, in turn, results in lower costs. Producing inexpensive, high quality products is a good policy for any company. But where do you start? Simply considering an automation program can force an organization to face problems it might not otherwise face: ● What automation and control technology is available? ● Are employees ready and willing to use new technology? ● What technology should we use? ● Should the current manufacturing process be improved before automation? ● Should the product be improved before spending millions of dollars acquiring equipment to build it? Automating before answering the above questions would be foolish. The following chapters describe the available technology so that the reader will be prepared to select appropriate automation technology. The answers to the last two questions above are usually "yes," and this book introduces techniques to improve processes and products, but each individual organization must find its own improvements. Chapter 0. Introduction 0.2.1 Automated Manufacturing, an Overview Automating of individual manufacturing cells should be the second step in a three step evolution to a different manufacturing environment. These steps are: 1. Simplification of the manufacturing process. If this step is properly managed, the other two steps might not even be necessary. The "Just In Time" (JIT) manufacturing concept includes procedures that lead to a simplified manufacturing process. 2. Automation of individual processes. This step, the primary subject of this text, leads to the existence of "islands of automation" on the plant floor. The learning that an organization does at this step is valuable. An organization embarking on an automation program should be prepared to accept some mistakes in the early stage of this phase. The cost of those mistakes is the cost of training employees. 3. Integration of the islands of automation and other computerized processes into a total manufacturing and business system. While this text does not discuss the details of integrated manufacturing, it is discussed in general in this chapter and again. Technical specialists should be aware of the potential future need to integrate, even while they embark on that first "simplification" step. The large, completely automated and integrated environment shown in figure 0.1 is a Computer Integrated Manufacturing (CIM) operation. The CIM operation includes: ● Computers, including: i one or more "host" computers ii several cell controller computers iii a variety of personal computers iv Programmable Controllers (PLC) v computer controllers built into other equipment ● Manufacturing Equipment, including: i robots ii numerical control machining equipment (NC, CNC, or DNC) iii conntrolled continuous process equipment (e.g., for turning wood pulp into paper) iv assorted individual actuators under computer control (e.g., motorized conveyor systems) v assorted individual computer-monitored sensors (e.g., conveyor speed sensors) vi pre-existing "hard" automation equipment, not properly computer-controllable, but monitored by retro-fitted sensors ● Computer Peripherals, such as: i printers, FAX machines, terminals, paper-tape printers, etc. Chapter 0. Introduction ● Local Area Networks (LANs) interconnecting computers, manufacturing equipment, and shared peripherals. "Gateways" may interconnect incompatible LANs and incompatible computers. ● Databases (DB), stored in the memories of several computers, and accessed through a Database Management System (DBMS) ● Software Packages essential to the running of the computer system. Essential software would include: the operating system(s) (e.g., UNIX, WINDOWS NT, OS/2 or SNA) at each computer; communications software (e.g., the programs which allow the LANs to move data from computer to computer and which allow some controllers to control other controllers); and the database management system (DBMS) programs ● assorted "processes" which use the computers. These computer- users might include: i humans, at computer terminals, typing commands or receiving information from the computers ii Computer Aided Design (CAD) programs iii Computer Aided Engineering (CAE) programs iv Computer Aided Manufacturing (CAM) programs, including those that do production scheduling (MRP), process planning (CAPP), and monitoring of shop floor feedback and control of manufacturing processes (SEC) v miscellaneous other programs for uses such as word processing or financial accounting As of this writing, very few completely computer integrated manufacturing systems are in use. There are lots of partially integrated manufacturing systems. Before building large computer integrated systems, we must first understand the components and what each component contributes to the control of a simple process. Chapter 0. Introduction Fig. 0.1 A computer integrated manufacturing environment 0.3 CONTROL OF AUTOMATION/PROCESS CONTROL Automated processes can be controlled by humans operators, by computers, or by a combination of the two. If a human operator is available to monitor and control a manufacturing process, open loop control may be acceptable. If a manufacturing process is automated, then it requires closed loop control. Chapter 0. Introduction Figure 0.2 shows examples of open loop control and closed loop control. One major difference is the presence of the sensor in the closed loop control system. The motor speed controller uses the feedback it receives from this sensor to verify that the speed is correct, and drives the actuator harder or softer until the correct speed is achieved. In the open loop control system, the operator uses his/her built-in sensors (eyes, ears, etc.) and adjusts the actuator (via dials, switches, etc.) until the output is correct. Since the operator provides the sensors and the intelligent control functions, these elements do not need to be built into an open loop manufacturing system. Human operators are more inconsistent than properly programmed computers. Anybody who has ever shared the road with other drivers is familiar with the disadvantages of human control. Computerized controls, however, can also make mistakes, when programmed to do so. Programming a computer to control a complex process is very difficult (which is why human automobile operators have not yet been replaced by computers). The recent development of affordable digital computers has made automation control possible. Process control has been around a little longer. The difference in the meanings of these two terms is rapidly disappearing. Process control usually implies that the product is produced in a continuous stream. Often, it is a liquid that is being processed. Early process control systems consisted of specially-designed analog control circuitry that measured a system's output (e.g., the temperature of liquid leaving a tank), and changed that output (e.g., changing the amount of cool liquid mixed in) to force the output to stay at a preset value. Fig. 0.2 Open loop and closed loop speed control Automation control usually implies a sequence of mechanical steps. A camshaft is an automation controller because it mechanically sequences the steps in the operation of an internal combustion engine. Manufacturing processes are often sequenced by special digital computers, known as programmable logic controllers (PLCs), which can detect and can switch electrical signals on and off. Digital computers are ideally suited for "automation control" type tasks, because they consist of circuits each of which can only he either "on" or "off." Chapter 0. Introduction Process control is now usually accomplished using digital computers. Digital controllers may he built into cases with dials and displays which make them look like their analog ancestors. PLCs can also be programmed to operate as analog process controllers. They offer features which allow them to measure and change analog values. Robots and NC equipment use digital computers and a mixture of analog and digital circuit components to control "continuous" variables such as position and speed. Advances in automation and process control have been rapid since the start of the silicon revolution. Before modern silicon devices, controllers were built for specific purposes and could not be altered easily. A camshaft sequencer would have to have its camshaft replaced to change its control "program." Early analog process controllers had to be rewired to be reprogrammed. Automation systems of these types are called hard automation. They do what they are designed and built to do, quickly and precisely perhaps, but with little adaptability for change (beyond minor adjustments). Modification of hard automation is time-consuming and expensive, since modifications can only be performed while the equipment sits idle. As digital computers and software improve, they are replacing hard automation. Digital computer control gives us soft automation. Modem digital computers are reprogrammable. It is even possible to reprogram them and test the changes while they work! Even if hardware changes are required to a soft automation system, the lost time during changeover is less than for hard automation. Even a soft automation system has to be stopped to be retro-fitted with additional sensors or actuators, but the controller doesn't have to be rebuilt to use these added pieces. Digital computers are cheap, powerful, fast and compact. They offer several new advantages to the automation user. A single digital controller can control several manufacturing processes. The designer only needs to ensure the computer can monitor and control all processes quickly enough, and has some excess capacity for future changes. Using digital computers, the equipment that is to be controlled can be built to be more "flexible." A computer controlled milling machine, for example, can come equipped with several milling cutters and a device to change them. The computer controller can include programs that exchange cutters between machining operations. Soft automation systems can be programmed to detect and to adapt to changes in the work environment or to changes in demand. An NC lathe can. For example, modify its own cutting speed if it detects a sudden change in the hardness of a raw material being cut. It may also change its own programming in response to a signal from another automated machine requesting a modification in a machined dimension. 0.4 COMPONENTS IN AUTOMATION When we discuss automation in this text, we will always mean controlled automation. A circuit that allows you to control a heater is a labor-saving device, but without components to ensure that the room temperature remains comfortable, it is not an automated system. Figure 0.3 illustrates the essential components in a controlled automated system: ● the actuator (which does the work) ● the controller (which "tells" the actuator to do work) ● and the sensor (which provides feedback to the controller so that it knows the actuator is doing work) 0.4.1 The Controller as an Automation Component Chapter 0. Introduction A controlled system may be a simple digital system. An example is shown in figure 0.4, in which the actuator consists of a pneumatic valve and a pneumatic cylinder that must be either fully extended or retracted. The controller is a PLC that has been programmed to extend the cylinder during some more complicated process, and to go on to the next step in the process only after the cylinder extends. Fig. 0.3 Components of a simple controlled automation system When it is time to extend the cylinder, the PLC supplies voltage to the valve, which should open to provide air to the cylinder, which should then extend. If all goes well, after a short time the PLC will receive a change in voltage level from the limit switch, allowing it to execute the next step in the process. If the voltage from the switch does not change for any reason (faulty valve or cylinder or switch, break in a wire, obstruction preventing full cylinder extension, etc.), the PLC will not execute the next step. The PLC may even be programmed to turn on a "fault" light when such a delay occurs. A controlled system might be an analog system, as illustrated in figure 0.5. In this system, the actuator is an hydraulic servovalve, and a fluid motor. The servovalve opens proportionally with the voltage it receives from the controller, and the fluid motor rotates faster if it receives more hydraulic fluid. There is a speed sensor connected to the motor shaft, which outputs a voltage signal proportional to the shaft speed. The controller is programmed to move the output shaft at a given speed until a load is at a given position. When the program requires the move to take place, the controller outputs an approximately correct voltage to the servovalve, then monitors the sensor's feedback signal. If the speed sensor's output is different than expected (indicating wrong motor speed), the controller increases or decreases the voltage supplied to the servovalve until the correct feedback voltage is achieved. The motor speed is controlled until the move finishes. As with the digital control example, the program may include a function to notify a human operator if speed control isn't working. Digital and analog controllers are available "off the shelf" so that systems can be constructed inexpensively (depending on your definition of "inexpensive"), and with little specialized knowledge required. Chapter 0. Introduction Fig. 0.4 A digital controlled system Fig. 0.5 An analog controlled system Most controllers include communication ports so that they can send or receive signals and instructions from other computers. This allows individual controllers to be used as parts of distributed control systems, in which several controllers are interconnected. In distributed control, individual controllers are often slaves of other controllers, and may control slave controllers of their own. Chapter 0. Introduction 0.4.2 Sensors as Automation Components Obviously, controlled automation requires devices to sense system output. Sensors also can be used so that a controller can detect (and respond to) changing conditions in its working environment. Figure 0.6 shows some conditions that a fluid flow control system might have to monitor. Sensors are required to sense three settings for values that have to be controlled (flow rate, system pressure, and tank level) and to measure the actual values of those three variables. Another sensor measures the uncontrolled input pressure, so that the valve opening can be adjusted to compensate. A wide range of sensors exists. Some sensors, known as switches, detect when a measured condition exceeds a pre- set level (e.g., closes when a workpiece is close enough to work on). Other sensors, called transducers, can describe a measured condition (e.g., output increased voltage as a workpiece approaches the working zone). Sensors exist that can be used to measure such variables as: ● presence or nearness of an object ● speed, acceleration, or rate of flow of an object ● force or pressure acting on an object ● temperature of an object ● size, shape, or mass of an object ● optical properties of an object ● electrical or magnetic properties of an object Fig. 0.6 Sensing in an automated system [...]... able to measure and control its output "Closed loop control" is the term used for a self-controlled automated process A complete closed loop control system requires an actuator (perhaps several working together), which responds to signals from a controller, and at least one sensor that the controller uses to ensure that the manufacturing process is proceeding as it should Most controlled automation... a complete closed-loop position control system The internal control system ensures that the valve opens proportionally with the DC signal it receives The three-stage actuator we have been discussing as a component of a closed loop control system, therefore, includes another closed-loop control system Some actuators can only be turned on or off A heater fan helps to control temperature when it is turned... Digital computers are used in modem soft automation systems, replacing hard automation controllers Custom-built analog controllers, traditionally used for process control applications, are examples of "hard" automation The main advantages to be reaped by using digital control are in the area of increased flexibility Digital controllers, including PLCs, can be programmed to respond to more conditions, in... temperature control system Pneumatic cylinders are usually either fully extended ("on") or fully retracted ("off") Other actuators respond proportionally with the signal they receive from a controller A variable speed motor is an actuator of this type Hydraulic cylinders can be controlled so that they move to positions between fully extended and fully retracted Fig 0.7 Hydraulic actuated position control. .. of standardization was the problem NC equipment showed up as the first real application of digital control The suppliers of NC equipment built totally enclosed systems, evolving away from analog control toward digital control With little need for interconnection of the NC equipment to other computer controlled devices, the suppliers did not have to worry about lack of standards in communication Early... because off-the-shelf components can be used Numerical controlled (NC) machining components, analog process controllers, and mechanical and electrical sequencers have been available for quite a while, but the development of programmable controllers (PLCs), robots, and open-architecture computers really made automation accessible to the average industrial user Signal conditioning is still required to... 0.9 shows a PLC as it might be connected for a position -control application: reading digital input sensors, controlling AC motors, and exchanging information with the operator Another advance which made automation possible was the development of the robot A variation on NC equipment, the robot is a self-enclosed system of actuators, sensors and controller Compatibility of robot components is the robot... Now computerization of the factory floor could proceed Fig 0.8 (a) A programmable controller; (b) installation of I/O module on bus unit (Photographs by permission, Westinghouse Electric Corporation/ Electrical Components Division, Pittsburgh, Pennsylvania.) Chapter 0 Introducation 0.5.1 Interfacing of Controllers to Controllers Standards for what form the signals should take for communication between... inconvenience Signal conditioning is often required so that incompatible components and controllers can be interconnected The size of the problem can be reduced by the user by selecting components with similar power requirements and control signal characteristics, if possible Another option is using a PLC as the controller, and selecting a PLC which offers I/O modules for all the different sensors and... to a 0 to 24 volt DC signal which controls how much fluid the valve actually allows through As with sensors, there may be incompatibilities between the signal requirement of an actuator and a controller's inherent output signals Signal conditioning circuitry may be required 0.5 INTERFACING AND SIGNAL CONDITIONING There was quite a long delay before digital computer control of manufacturing processes . individual controllers to be used as parts of distributed control systems, in which several controllers are interconnected. In distributed control, individual controllers are often slaves of other controllers,. shows examples of open loop control and closed loop control. One major difference is the presence of the sensor in the closed loop control system. The motor speed controller uses the feedback. the control of a simple process. Chapter 0. Introduction Fig. 0.1 A computer integrated manufacturing environment 0.3 CONTROL OF AUTOMATION/PROCESS CONTROL Automated processes can be controlled