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CONTI?OL .sY..srb~ OSJt::C-il ve::. oF Fe.dfol'I.' '" Feedback loop t Feedforward contribulion _ Feedback contributiOn\ Feedback i + controller Measured value dangerous levels. Safety systems are usually separate from the normal control system and can be treated independently in terms of strategy and implementation, Safety systems usually involve detection of unsafe or potentially unsafe conditions using on/off or switching sensors. Standby equij:iment may need to be started if there is a failure or the plant may need to be shut down safely. Safety systems involve the same problems of binary logic as sequentiaI control systems. ' Quite often control systems can be designed to fail safe. With the correct design, failures of instrumentation and control equipment can result in safe rather than unsafe situations. For example consider the level control problem examined above, It may be that the liquid held in the tank is corrosive or otherwise dangerous. lt would therefore be unsafe if the tank were to overflow due to a control system failure. The fail safe philosophy is to design the control system and associated instrumentation such that a failure in any one element causes the control errort to act in a safe way (i.e. the tank inlet valve should dos e )~ For a fail safe measurement, a failure in the measuring element should produce the same signal as exists in the dangerous condition, In the case of the level control system a level transmitter failure should look like a high level in the tank. This can be achieved by using a law signal to represent a high level (a reverse acting transmitter). Loss of the measurement signal would thus look ]ike a high level in the tank and so the controller would shut the valve preventing tank overflow. The fail safe philosophy can alsa be applied to the control system actumor. The argument here is simpler, the actuator should move to the safe pasition in the event of an actuatar power failure. For thetank inlet control valve this means that the vaIve should dose if the actuator signal is lost. This can be, accomplished by making the actuatar push against a spring to open the valve (i.e. a spring return actuator), The spring would then close the valve in the absence of actuator power, Figure 1.3 General black diagram of a control system ~ Th :il .~. ~ t ,~ ~. ~ ~ ~ i ~ { } -- Modulating Feedback Control Rather than switch the inlet valve open and shut, a more subtIe appro'ach is to inch the valve by an amount which depends on the difference between the actual Ievel and the desired level. This strategy can be termed moduZating feedback control. Modulating control implies a more elaborate level measurement and valve actuator. In the first place it requires a signal related to the actuaIlevel (Le, a level transmitter), Secondly the valve actuatar must be able to open and dose the inIet valve gradually (moduIate the valve opening). Furthermore the valve itself must have a smooth characteristic so that its resistance to Dow is infinitely variable. The control law can profoundly effect the way in which a feedback control system behaves. SimpIe on/off control can sametimes give acceptable performance; most domestic heating systems are controlled this way. The important characteristic of modulating control is that it is capable of providing a range of control effort and can produce smail, as well as large, conections. With a well designed control law, feedback control can provide go ad regulation and trajectory following. One of the primary functions of a control engineer is to design or select an appropriate control law for the plant which gives acceptable performance. Figure 1.3 shows a general black diagram of acontrol system with both feedforward and feedback control. The box labelled 'feedback controller' is where the feedback contribution to the control effort is produced. if there is a feedforward component, it is added to the feedback contribution to make up the total control effart signal. 1.5 SAFETY -- \x)8 CONTROL SYSTEM DESIGN AND SIMULATION INTRODUCTION TO CONTROL SYSTEMS 9 Temperature transmitter The above arguments are based on the premise that the tank should not be allawed to overllow. if the Iiquid held in the tank is essential to the downstream process (i.e. lubricating oil or coolant) then the tank should not be allawed to mn dry in the event of a faiIure. The situation is now reversed and the fail safe argument implies that a direct acting level transmitter should be used. Similarly the control valve must be designed to open, should the actuator power fai!. Safety considerations are very dependent on the process or system, and notwithstanding their importance, can only be touched upon ina general text of this nature. Experience and a thorough knowledge of the system operation are required before sensible decisions on safety aspects can be made. it must be emphasized that most control engineers spend far rnore time considering safety aspects than in designing the control system for normaloperation. Disturbances Temperature controller i---------~-------i ! i i Control i+Di1 i'__ \_:~~~~ ] \ 3!im it Controller Relerence Errar output temperature Measured temperature '\ Disturbances Fuel Ilow \ . i Process lurnace Process outlet temperature \ 1.6 EXAMPLES OF CONTROL SYSTEMS Figure 1.5 Biock diagram for temperature control of a process (urnace Furuace Control Control of a Robotic Arm Robats are increasingly used for materials handling, automatic assembly and fabrication. Robotic arms normally have several joints or axes each fitted with an actuator enabljng the arm to move in a variety of ways to pasition and orientate the gripper. The actuators are controlled by a dedicated computer so that the conect sequence o[ motions is carried out. The computer normally does this by replaying a stored sequence of desired motions Sametimes more sophisticated systems use televisian cameras or toueh sensors to heIp decide on the required gripper motion, value of the process outlet ternperature (the reference ternperature) can be set. The controller compares the measured temperature with the reference temperature and the fuel flow is increased or decreased accordingly. The exact relatianship between the error in temperature and the fuel valve mavement is determined by the control law. A black diagram of the control system is shown in Fig. 1.5. The main source of disturbances are changes in the process fluid flowrate. Changes in throughput are inevitable for nuqierous reasons. For example, if there is unsufficient feedstock or if there is a sudden increase in demand because of a weather change or even fluctuations in the stock market. Feedforward control would be relatively easy to implement by monitoring the flow of process fluid and adding in a component to the feedback control ef[ort. Turning to the safety of this [urnace control system, [ail safe precautions can be taken to ensure that a control equipment failure results in the burner shutting down. However there are many other safety considerations to be considered in fired equipment such as this. For example, during normaloperation disturbances in the process fluid could cause the furnace outlet temperature to rise. If the correcting control effort is excessive, the fuel valve may be shut of[ causing a bumer flarne failure. When the fuel flow is restored by the controller opening the valve, the potential for an explosion will exist because o[ the unignited fuel entering the not fumace. Solutions to this problem can Include placing limits on the [uel control valve travel, use of name detectors and automatic re-ignition of the extinguished flame. Heated process-Iluid to treatment plant Temperature transmitter To other consumers i i.---Measured temperature i __ _+-----ç__ Desired/reference ControJ,.. V temperature valve Temperature contrailer Fueloil \ Pump Furnaces are used in the process industry for heating feedstocks prior to further treatment. For example, in an oII refinery, cmd e oil is heated before it enters the cmde distillation column where it is split up into fractions which eventually produce marketabIe products such as aviatIon [uel or road bItumen. The initial heating is carefully controlled as the subsequent fractionation is quite dependent on the degree of vaporization of the crude oil [eed. Figure lA shows a simplified diagram of a process furnace. The temperature of the process fluid is measured at the outlet of the fumace by a temperature transmitter which sen ds a signal to the temperature controller. The controller has a dial so that the desired Figure 1.4 Process furnace ls>W -8 LO CONTROL SYSTEM DESIGN AND SIMULATION INTRODUCTION TO CONTROL SYSTEMS ]] Figure 1.7 Black diagrain of robotic arin control system (one axis only) Digrtallyencoded arm position r r r r r r i Control computer i ______________________ 1 The response of any natural system to stimuli is not instantaneous, it takes time for the system to respond. We are all aware that before we can make a cup of instant coffee, it tak es time for the kettle to boil after we have switched it on. When acar hits a bump in the road the suspensian bounces or oscillates for a while before it settles back to an equilibrium position. When we hit the accelerator pedal of our car, it takes time for it to accelerate to a new speed, evep. if we happen to own the latest Lamborghini! The way the response of a system evolves as function of time to a particular stimulus is termed the dynamic response. The dynamic behaviour ol systems can be modelled by differential equations. Models can be obtained by applying the basic equations of physics and mechanics to the system. This approach can beapplied to systems where the underlying principles are dear and where the system is sufficiently simple, or can be broken dow n into simple subsystems, to use a 'first principles' approach. Anather approach is to observe the behaviour of the system in its normal working enyironment or introduce test signals. A model can then be proposed from the observations of the input/oiitput behaviour. The solution of differentiaJ equations by hand methods is difficult and so a range of numerical methods have been developed which can be implemented on digita! computers. Many computer techniques and packages exist for solvlng differentia] equations and simulating the behaviour of dynamic systems. Control system design involves simulating the dynamic behaviour-of feedback systems, but it alsa involves other techniques which require the use of computers. Dynamic simulation a!one is not enough. Design method s for control systems have evolyed since the 1930s. At first methods were devloloped which were based on short-cut hand calculations. Analogue computers were used to do the final simulation and verify the design. This state of affairs did not change significantly unli! the early 1950s when the first digital computers came int o commercia] and scientific use. Computer programs In practice however, the control law is usually chosen to give adequate performance with a range of loads. 1.7 CONTROL SYSTEM i;>YNAMICS, MODELLING, SIMULATION ANDDESIGN Reference pasitlon i j ~. :~ -j:! .~ i i i .~ i ~ ] i , j Gripper Servomotor and gearbox Figure 1.6 Robotic ann The control computer must control all the robotjoints simuItaneously. Sametimes each axis has its own separate microprocessor which receives commancls from the main control computer. However implemented, the scheme for the PQsitioning of each axis is similar, so the controlaf just one arm joint will be examined as shown in Fig. 1.6. In this example, the joint is driven by a de electric motor (servomotor) through a gear box. A de motor is a flexible actuatorwhich can be driven in either directian at various speeds by altering the directian and magnitude of the motorcurrent. The current is supplied to the motor by a power amplifier which in turn receives its input from the computer or microprocessor. The computer deals with digital data and so the interface to the power amplifier requires a digital to analogue converter. The angle of the arm joint is monitored by an encoder. An en cad er is a transducer which ppoduces a digital output representing the measiired angle which can be directly interfaced tb the computer. As the computer retrieves the sequence of desired robot motions, the required positions for the axis are passed to a part of the program which implements the control law. The control cakiilation involves subtracting the measured arm angle from the required arm angle to find the angular pasition error. The computer then uses this error to detennine the magnitude and directian of the control effort. This calculated control errort is converted to an analogue signal which Is applied to the inpcit of the power amplifier to diive the servomotor and hence rediice the error. Figure 1.7 shows the arrangement in black diagram form. The main requirement in this control system is that of fast and acciirate trajectory following. The controller can use the rate of change of measured pasition to calculate the yelocity of the joint and so reduce the motor current if the arm is maving tf1ti fast. The control law in a high-performance robot may be q uite compiex and will be tailored to the way in which the arm is expected to respond. A difficulty arises because the arm may respond differently when carrying a load than when empty-handed. The motor will require more current to accelerate and decelerate with the load than wlthout. The control computer may know whether the gripper is holding a load and in theory could then compensate by using more current. ..:::.... CJ -12 CONTROL SYSTEM DESIGN AND SIMULATION INTRODUCTION TO CONTROL SYSTEMS 13 Figure P1.3 Oil cooler temperature control system '"' """" were wrjtten to solve sets of differential equations and results produced on listing paper. Graphic displays were yirtuaiiy nonexistent and the interaction between the designer and the computer was yery poor. Even with the development .of minicomputers in the mid 1960s the situation was not significantly better. Graphics display terminals first became widely available in theearly 1970s. Their use for control system design was generally restricted to larger universities and industria! companies. User interaction was still poor, and often the design suites were not well integrated. The 1980s saw the arrival of the 'personal computer' (PC) and the availability of graphics terminals of high quality. Many software houses adapted existingprograms to the PC, but really did not tak e advantage of its potential as a design tool. The ancestry of the programs was dearly vIsible. In addition to the improyements and cost reduction of the hardware, the second half of the 1980s saw an increasing awareness of well design ed (user friendly) software. Business sodftware with programs such as word processors, spreadsheets and databases was the first area in which this development occmred. Later, as graphics capabilities grew, computer-aided design paekages for draughting, three-dimensional modelling, printed eireuit design, ete., appeared. Computer-Aided Control System Design (CACSD) software was slow to respand to this trend sinee a fairly large investment was required with a more restricted market. Taday there are several good quality, low cost packages available. PROBLEMS 1.1 Classify the following into sequential or quanlitative control systems and identify whether feedback is presenI: (a) The thermosta!;c temperalure control in a central heating boiler. (b) The programmer controlling a typical central heating system. (c) A cruise control system, as Iltted to a modern car. (d) The over-speed cutout Iltted to acar engine. 1.2 Examine lhe fail safe requirements for the inslrumentaiion and actuators in the furnace temperature control syslem shown in Fig. 1.4. You may assume that it is unsafe for the Process fluid to overheat. 1.3 Consider the oil cooling system in Fig. P1.3. The oil temperature is controlled by throttling the cooler bypass valve. Safety considerations indicate thai control equipment failure should not cause lass of the cooling function. Determine the required aclion for the temperature transducer and the control valve. What action will the canIraller take if the signal from the temperature transducer reduces in value? 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