Fundamentals of Control Engineering

1 InstrumentationHeiligenhausFundamentals of Control EngineeringData terminology in Control Engineering .. General .. Controlled systems .. Actuating signals .. 32 Selecting the controller .. Controller or automation system .. Selection according to actuating signal .. Selection according to time response .. Selection according to input signal connection .. Analog or digital Control .. Additional functions .. Remote Control and logical operation .. Computing functions .. according to constructive characteristics.

2 According to safety Engineering requirements .. 7 Page3 Matching the controller to the controlled system . determination and setting of Control parameters .. controllers .. in Hartmann & Braun controllers .. 94 Serial interfaces .. 95 Computer applications .. 96 Explanation of terms .. 117 Symbols .. 118 Bibliography .. 121 Basic terminology in Control GeneralThe terms automation, closed-loop Control and open-loop controlare frequently used synonymously, although there are consider-able differences between Control is a subset of tasks are encountered in numerous situations indaily life.

3 The object is to cause the status of a system to approachthat of a predefined value. In Control technology [5] this task isdescribed as follows:The controlled variable (actual value) x of a controlled systemis measured continuously and fed as a feedback variable r to acomparing element where it is compared to a reference variable(set point) w. A controller output variable YR is calculated fromthe resultant Control deviation xw = x - w or the error signal e =w - x. This output variable YR is then used as a manipulatedvariable y to bring about a reduction in Control deviation in thecontrolled system.

4 This yields a closed Control loop. Disturbance variables z act on this Control loop to repeatedlyelicit a Control controlled variable x may be replaced by an object variablexA if the variable to be controlled cannot be measured directly fortechnical controlling the composition of a mixture it may prove im-possible to measure that composition (the object variable) , a representative property (pH, density, turbidity) may beadopted as the controlled 1 The Control loopThe Control precision or Control quality achievable in thecontrol loop is very dependent on how well the controller ismatched to the particular controlled system.

5 The choice of thecorrect controller and its adaptation to the controlled system aretherefore of primary importance. Open-loop Control also acts on a system to change it in thedesired manner. The difference between this and closed-loopcontrol is that the success of the intervention cannot be (directly)monitored. For instance, open-loop Control opens a valve withoutchecking whether, and if so, how much water flows through theopened valve. In other words, the output signal of the system(water quantity) is not fed back to the input of the Controlled systemsControlled systems are characterized by their time are a number of procedures suitable to determine timeresponse.

6 These are based on the controlled system being acti-vated by a suitable test signal. The reaction of the system providesinformation on its dynamics. A very simple method and thereforecommonly adopted approach is to activate the system through anabrupt change in the actuating step response/transfer function is then evaluated to deter-mine the setting values of the 2 Self-regulating controlled system, step response/transferfunctionFig. 2 shows a self-regulating controlled system in which a newsteady-state condition results upon an abrupt change in the inputvariable. This state is reached when the input to the system equalsits output.

7 Controlled systems which are not self-regulating are also ofimportance. These are generally containers in which the fillinglevel is to be following data, of importance to the evaluation of the controlledsystem, can be derived from the step response:Controlled system gain: KS = x/y1 Delay time: TuRecovery time: TgIn process Engineering and similar industrial processes, the valueof Tg will vary from a few seconds to several addition to the absolute value of these variables, the ratio Tu/Tgis also very in which Tu/Tg << are easily controlled. Those in whichTu/Tg can still be controlled.

8 If this value is exceeded thenthe results obtained with a controller will deteriorate in inverseproportion to the increase. In such cases the possibility of inter-rupting the Control loop should be investigated so that cascadecontrol can be tested. Controller Controlled systemZ-15454 ENFundamentals of Control EngineeringData ENPage 2 of Actuating signals and final controlling elementsActuating signals constitute the input signals of controlled sys-tems. They allow changes in power supply, flow cross-sectionsand the , contactors, semiconductor switches or continuously-vari-able thyristor actuators can all be used to vary the power supplyto electrically-heated systems.

9 Relays and contactors are inex-pensive and their performance can be easily checked. A disad-vantage, however, is that their mechanical life is limited, possiblyresulting in down-time of plant. By contrast, semiconductor swit-ches and thyristor sets are more expensive, but free of or pneumatically-driven valves and flaps areusually used for the temperature Control of gas or oil-heated plantand for the Control of flow rates, pressure and the H&B product range includes electrical actuators (Catalogue68), electropneumatic and intelligent positioners and electropneu-matic signal converters TEIP (Catalog 18).

10 2 Selecting the Controller or automation systemVery different types of controller are used in industrial controllers are preferred for the Control of individualcontrol loops. These are characterised by having all the necessaryfunctions contained in a single this sense, controllers constitute the least expensive stage ofprocess automation. They are provided with direct inputs andoutputs for process variables as well as a front panel for operationand monitoring of the process. In addition, controllers fulfil therequirements for use under adverse environmental conditions. Digital controllers with serial interfaces are used for centralizedoperating and monitoring of a few Control loops in small andmedium size process plants.