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1. Introduction
The term automation is used to describe the automatic operation or control of a process. In modem manufacturing there is an ever increasing use of automation, e.g. automatically operating machinery, perhaps in a production line with robots, which can be used to produce components with virtually no human intervention. Also, in appliances around the home and in the office there is an ever increasing use of automation. Automation involves carrying out operations in the required sequence and controlling outputs to required values.
The following are some of the key historical points in the development of automation, the first three being concerned with developments in the organization of manufacturing which permitted the development of automated production:
1. Modem manufacturing began in England in the 18th century when the use of water wheels and steam engines meant that it became more efficient to organize work to take place in factories, rather than it occurring in the home of a multitude of small workshops. The impetus was thus provided for the development of machinery.
2. The development of powered machinery in the early 1900s meant improved accuracy in the production of components so that instead of making each individual component to fit a particular product, components were fabricated in identical batches with an accuracy which ensured that they could fit any one of a batch of a product.
Think of the problem of a nut and bolt if each nut has to be individually made so that it fitted the bolt and the advantages that are gained by the accuracy of manufacturing nuts and bolts being high enough for any of a batch of nuts to fit a bolt.
3. The idea of production lines followed from this with Henry Ford, in 1909, developing them for the production of motor cars. In such a line, the production process is broken up into a sequence of set tasks with the potential for automating tasks and so developing an automated production line.
4. In the 1920s developments occurred in the theoretical principles of control systems and the use of feedback for exercising control. A particular task of concern was the development of control systems to steer ships and aircraft automatically.
5. In the 1940s, during the Second World War, developments occurred in the application of control systems to military tasks, e.g. radar tracking and gun control.
6. The development of the analysis and design of feedback amplifiers, e.g. the paper by Bode in 1945 on Network Analysis and Feedback Amplifier design, was instrumental in further developing control system theory.
7. Numerical control was developed in 1952 whereby tool positioning was achieved by a sequence of instructions provided by a program of punched paper tape, these directing the motion of the motors driving the axes of the machine tool. There was no feedback of positional data in these early control systems to indicate whether the tool was in the correct position, the system being open-loop control.
8. The invention of the transistor in 1948 in the United States led to the development of integrated circuits, and, in the 1970s, microprocessors and computers which enabled control systems to be developed which were cheap and able to be used to control a wide range of processes. As a consequence, automation has spread to common everyday processes such as the domestic washing machine and the automatic focusing, automatic exposure, camera.
The automatic control of machines and processes is now a vital part of modem industry. The benefits of such control systems include greater consistency of product, reduced operating costs due to improved utilization of plant and materials and a reduction in manpower, and greater safety for operating personnel.
This section is an introduction to the basic idea of a control system and the elements used.
2. Control systems
As an illustration of what control systems can do, consider the following:
Control a variable to obtain the required value
1. You set the required temperature for a room by setting to the required temperature the room thermostat of a central heating system. This is an example of a control system with the variable being controlled being the room temperature.
2. In a bottling plant the bottles are automatically filled to the required level. The variable being controlled is the liquid level in a bottle and control is exercised to ensure no difference between the required level and that which occurs.
3. A computer-numerical-control (CNC) machine tool is used to automatically machine a workpiece to the required shape, the control system ensuring that there is no difference between the required dimensions and that which occurs.
4. Packets of biscuits moving along a conveyor belt have their weights checked and those that are below the required minimum weight limit are automatically rejected. Control is being exercised over the weight.
Control the sequence of events
5. A belt is used to feed blanks to a pressing machine. As a blank reaches the machine, the belt is stopped, the blank positioned in the machine, the press activated to press the required shape, then the pressed item is ejected from the machine and the entire process repeated. A sequence of operations is being controlled with some operations controlled to occur only if certain conditions are met, e.g. activation of the press if there is a blank in place.
6. You set the dials on the automatic clothes washing machine to indicate that 'whites' are being washed and the machine then goes through the complete washing cycle appropriate to that type of clothing. This is an example of a control system where a controlled sequence of events occurs.
Control whether an event occurs or not
7. The automatic clothes washing machine has a safety lock on the door so that the machine will not operate if the power is off and the door open. The control is of the condition which allows the machine to operate.
A control system can be thought of as a system which for some particular input or inputs is used to control its output to some particular value (FIG. 1(a)), give a particular sequence of events (FIG. 1(b)) or give an event if certain conditions are met (FIG. 1(c)). As an example of the type of control system described by FIG. 1(a), a central heating control system has as its input the temperature required in the house and as its output the house at that temperature (FIG. 2). The required temperature is set on the thermostat and the control system adjusts the heating furnace to produce that temperature.
The control system is used to control a variable to some set value.
As an example of the type of control system described by FIG. 1(b), a clothes washing machine has as its input a set of instructions as to the sequence of events required to wash the clothes, e.g. fill the drum with cold water, heat the water to 40''C, tumble the clothes for a period of time, empty the drum of water, etc. The manufacturers of the machine have arranged a number of possible sequences which are selected by pressing a button or rotating a dial to select the appropriate sequence for the type of wash required. Thus the input is the information determining the required sequence and the output is the required sequence of events (FIG. 3). The control system is used to control a sequence of events.
FIG. 1 Control systems (a) to control a variable, (b) to control a
sequence of events, (c) control -whether an event is to be allowed
FIG. 2 Central heating system
FIG. 3 Clothes washing machine system
2.1 Open- and closed-loop control
Consider two alternative ways of heating a room to some required temperature. In the first instance there is an electric fire which has a selection switch which allows a 1 kW or a 2 kW heating element to be selected.
The decision might be made that to obtain the required temperature it is only necessary to switch on the 1 kW element. The room will heat up and reach a temperature which is determined by the fact the 1 kW element is switched on. The temperature of the room is thus controlled by an initial decision and no further adjustments are made. This is an example of open-loop control. FIG. 4 illustrates this. If there are changes in the conditions, perhaps someone opening a window, no adjustments are made to the heat output from the fire to compensate for the change. There is no information yet/ back to the fire to adjust it and maintain a constant temperature.
Now consider the electric fire heating system with a difference. To obtain the required temperature, a person stands in the room with a thermometer and switches the 1 kW and 2 kW elements on or off, according to the difference between the actual room temperature and the required temperature in order to maintain the temperature of the room at the required temperature. There is a constant comparison of the actual and required temperatures. In this situation there is feedback, information being fed back from the output to modify the input to the system. Thus if a window is opened and there is a sudden cold blast of air, the feedback signal changes because the room temperature changes and so is fed back to modify the input to the system. This type of system is called closed-loop. The input to the heating process depends on the deviation of the actual temperature fed back from the output of the system from the required temperature initially set, the difference between them being determined by a comparison element. In this example, the person with the thermometer is the comparison element. FIG. 5 illustrates this type of system.
FIG. 5 The electric fire closed-loop system
FIG. 6 Ball valve in a cistern
Note that the comparison element in the closed-loop control system is represented by a circular symbol with a -• - opposite the set value input and a - opposite the feedback signal. The circle represents a summing unit and what we have is the sum
+ set value - feedback value = error
This difference between the set value and feedback value, the so-called error, is the signal used to control the process. If there is a difference between the signals then the actual output is not the same as the desired output. When the actual output is the same as the required output then there is zero error. Because the feedback signal is subtracted from the set value signal, the system is said to have negative feedback.
Consider an example of a ball valve in a cistern used to control the height of the water (FIG. 6). The set value for the height of the water in the cistern is determined by the initial setting of the pivot point of the lever and ball float to cut the water off in the valve. When the water level is below that required, the ball moves to a lower level and so the lever opens the valve to allow water into the tank. When the level is at the required level the ball moves the lever to a position which operates the valve to cut off the flow of water into the cistern. FIG. 7 shows the system when represented as a block diagram.
FIG. 7 Ball valve used to control water level in a cistern.
In an open-loop control system the output from the system has no effect on the input signal to the plant or process. The output is determined solely by the initial setting. In a closed-loop control system the output does have an effect on the input signal, modifying it to maintain an output signal at the required value.
Open-loop systems have the advantage of being relatively simple and consequently cheap with generally good reliability. However, they are often inaccurate since there is no correction for errors in the output which might result from extraneous disturbances. Closed-loop systems have the advantage of being relatively accurate in matching the actual to the required values. They are, however, more complex and so more costly with a greater chance of breakdown as a consequence of the greater number of components.
3. Basic elements
FIG. 8 shows the basic elements of an open-loop control system. The system has three basic elements: control, correction and the process of which a variable is being controlled.
FIG. 8 Basic elements of an open-loop control system
1. Control element
This determines the action to be taken as a result of the input of the required value signal to the system.
2. Correction element
This has an input from the controller and gives an output of some action designed to change the variable being controlled.
3. Process
This is the process of which a variable is being controlled.
There is no changing of the control action to account for any disturbances which change the output variable.
3.1 Basic elements of a closed-loop system
FIG. 9 shows the general form of a basic closed-loop system.
FIG. 9 Basic elements of a closed-loop control system
The following are the functions of the constituent elements:
1. Comparison element
This element compares the required value of the variable being con trolled with the measured value of what is being achieved and produces an error signal:
error = required value signal - measured actual value signal
Thus if the output is the required value then there is no error and so no signal is fed to initiate control. Only when there is a difference between the required value and the actual values of the variable will there be an error signal and so control action initiated.
2. Control law implementation element
The control law element determines what action to take when an error signal is received. The control law used by the element may be just to supply a signal which switches on or off when there is an error, as in a room thermostat, or perhaps a signal which is proportional to the size of the error so that if the error is small a small control signal is produced and if the error is large a large proportional control signal is produced. Other control laws include integral mode where the control signal continues to increase as long as there is an error and derivative mode where the control signal is proportional to the rate at which the error is changing.
The term control unit or controller is often used for the combination of the comparison element, i.e. the error detector, and the control law implementation element. An example of such an element is a differential amplifier which has two inputs, one the set value and one the feedback signal, and any difference between the two is amplified to give the error signal. When there is no difference there is no resulting error signal.
3. Correction element
The correction element or, as it is often called, the final control element, produces a change in the process which aims to correct or change the controlled condition. The term actuator is used for the element of a correction unit that provides the power to carry out the control action. Examples of correction elements are directional control valves which are used to switch the direction of flow of a fluid and so control the movement of an actuator such as the movement of a piston in a cylinder. Another example is an electric motor where a signal is used to control the speed of rotation of the motor shaft.
4. Process
The process is the system in which there is a variable that is being controlled, e.g. it might be a room in a house with the variable of its temperature being controlled.
5. Measurement element
The measurement element produces a signal related to the variable condition of the process that is being controlled. For example, it might be a temperature sensor with suitable signal processing.
The following are terms used to describe the various paths through the system taken by signals:
1. Feedback path
Feedback is a means whereby a signal related to the actual condition being achieved is fed back to modify the input signal to a process.
The feedback is said to be negative when the signal which is fed back subtracts from the input value. It is negative feedback that is required to control a system. Positive feedback occurs when the signal fed back adds to the input signal.
2. Forward path
The term forward path is used for the path from the error signal to the output. In FIG. 9 these forward path elements are the control law element, the correction element and the process element.
The term process control is often used to describe the control of variables, e.g. liquid level or the flow of fluids, associated with a process in order to maintain them at some value. Note also that the term regulator is sometimes used for a control system for maintaining a plant output constant in the presence of external disturbances. Hence the term regulator is sometimes applied to the correction unit.
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