Terminology and Symbols in Control Engineering

1 Part 1 FundamentalsTerminology and Symbolsin Control EngineeringTechnical Information1 Part 1:FundamentalsPart 2:Self-operated RegulatorsPart 3: Control ValvesPart 4:CommunicationPart 5:Building AutomationPart 6:Process AutomationShould you have any further questions or suggestions, pleasedo not hesitate to contact us:SAMSON AGPhone (+49 69) 4 00 94 67V74 / SchulungTelefax (+49 69) 4 00 97 16 Weism llerstrafle 3E-Mail: FrankfurtInternet: InformationTerminology and Symbols inControl EngineeringIntroduction .. 5 Terminology in Control Engineering .

2 6 Open loop Control .. 6 Closed loop Control .. 7 Process .. 8 Control loop .. 9 Abbreviations of variables relating to closed loop Control .. 10 Symbols in Control Engineering .. 12 Signal flow diagrams.. 12 Blocks and lines of action .. 12 Device-related representation .. 15 Instrumentation and Control tags .. 19 Control Systems and Structures .. 22 Fixed set point Control .. 23 Follow-up Control .. 23 Cascade Control .. 24 Ratio Control .. 24 Appendix A1: Additional Literature .. 263 Part 1 L101 ENSAMSON AG 00/03 CONTENTSP refaceThe technical informations presented in this document are based on defini-tions according to DIN, the German organization of standardization (Deut-sches Institut f r Normung).

3 Continuous efforts are being made to determineinternational definitions in order to achieve an increasing similarity in the ter-minology used. Nevertheless, differences in designations and representa-tions do exist in international use. Literature presented at the end of thisdocument includes international standards and publications relating to DINstandards, or being derived from and text sections referring to DIN are often cited in shortform, summarizing the contents. The precise facts must always be read - alsobecause of possible extensions or amendments - in the current edition of therespective Terminology and Symbols in Control EngineeringSAMSON AG V74/ DKEI ntroductionPlanning, design and start-up of process Control systems require clear andunambiguous communication between all parts involved.

4 To ensure this, weneed a clear definition of the terms used and as far as the documentation isconcerned standardized graphical Symbols . These Symbols help usrepresent Control systems or measurement and Control tasks as well as theirdevice-related solution in a simple and clear 1 L101 ENSAMSON AG 00/03 Terminology in Control EngineeringTo maintain a physical quantity, such as pressure, flow or temperature at adesired level during a technical process, this quantity can be controlled eitherby means of open loop Control or closed loop loop controlIn an open loop Control system, one or more input variables of a system acton a process variable.

5 The actual value of the process variable is not beingchecked, with the result that possible deviations caused by disturban-ces are not compensated for in the open loop Control process. Thus, the cha-racteristic feature of open loop Control is an open action task of the operator illustrated in Fig. 1 is to adjust the pressure (p2)inapipeline by means of a Control valve. For this purpose, he utilizes an as-signment specification that determines a certain Control signal (y) issued bythe remote adjuster for each set point (w). Since this method of Control doesnot consider possible fluctuations in the flow, it is recommended to use openloop Control only in systems where disturbances do not affect the controlledvariable in an undesired Terminology and Symbols in Control EngineeringSAMSON AG V74/ DKEp1yp2 Fig.

6 1: Operator controls the process variable p2via remote adjusterAssignment:wa=> ya=> p2awb=> yb=> action flowdisturbances arenot recognizedClosed loop controlIn a closed loop Control system, the variable to be controlled (controlledvariable x) is continuously measured and then compared with apredetermined value (reference variable w). If there is a difference betweenthese two variables (error e or system deviation xw), adjustments are beingmade until the measured difference is eliminated and the controlled variableequals the reference variable.

7 Hence, the characteristic feature of closedloop Control is a closed action operator depicted in Fig. 2 monitors the pressure p2in the pipeline towhich different consumers are connected. When the consumption increases,the pressure in the pipeline decreases. The operator recognizes the pressuredrop and changes the Control pressure of the pneumatic Control valve untilthe desired pressure p2is indicated again. Through continuous monitoring ofthe pressure indicator and immediate reaction, the operator ensures that thepressure is maintained at the desired level.

8 The visual feedback of the pro-cess variable p2from the pressure indicator to the operator characterizes theclosed action 1 L101 ENSAMSON AG 00/03p1p2 Fig. 2: Operator controls the process variable p2an a closed loopclosed action flowdisturbances areeliminatedThe German standard DIN 19226 defines closed loop Control as follows:Closed loop Control is a process whereby one variable, namely the variableto be controlled (controlled variable) is continuously monitored, comparedwith another variable, namely the reference variable and, depending on theoutcome of this comparison, influenced in such a manner as to bring aboutadaptation to the reference variable.

9 The characteristic feature of closedloop Control is the closed action flow in which the controlled variable continu-ously influences itself in the action path of the Control Control process can also be regarded as continuous if it is composed of asufficiently frequent repetition of identical individual processes. The cyclicprogram sequence of digital sampling Control would be such a : In English literature we only find one term, that is Control , being usedfor actually two different concepts known as steuern and regeln in the Ger-man language.

10 When translating into German, we therefore come acrossthe problem whether Control means steuern or regeln . If both methodsare involved, Control often is translated as automatisieren or leiten (con-trol station). An exact distinction can be made if the German term Regelung is made obvious by using the English term closed loop Control .ProcessA process comprises the totality of actions effecting each other in a system inwhich matter, energy, or information are converted, transported or setting of boundaries helps determine sub-processes or Terminology and Symbols in Control EngineeringSAMSON AG V74/ DKEdefinition ofclosed loop Control :DIN 19 226difficulties with theEnglish term Control Examples.