Terminology and symbols in control engineering, Manual de Controle de Processo. Instituto Politécnico de Lisboa
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Terminology and symbols in control engineering, Manual de Controle de Processo. Instituto Politécnico de Lisboa

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Terminology and Symbols in Control Engineering

Pa rt

1 Fu

nd am

en ta

ls

Terminology and Symbols in Control Engineering

Technical Information

1

Part 1: Fundamentals

Part 2: Self-operated Regulators

Part 3: Control Valves

Part 4: Communication

Part 5: Building Automation

Part 6: Process Automation

Should you have any further questions or suggestions, please do not hesitate to contact us:

SAMSON AG Phone (+49 69) 4 00 94 67 V74 / Schulung Telefax (+49 69) 4 00 97 16 Weismüllerstraße 3 E-Mail: schulung@samson.de D-60314 Frankfurt Internet: http://www.samson.de

Technical Information

Terminology and Symbols in Control Engineering

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Terminology in Control Engineering . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . 26

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Preface

The technical informations presented in this document are based on defini- tions according to DIN, the German organization of standardization (Deut- sches Institut für Normung). Continuous efforts are being made to determine international 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 this document includes international standards and publications relating to DIN standards, or being derived from them.

Representations and text sections referring to DIN are often cited in short form, summarizing the contents. The precise facts must always be read - also because of possible extensions or amendments - in the current edition of the respective standard.

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Introduction

Planning, design and start-up of process control systems require clear and unambiguous communication between all parts involved. To ensure this, we need a clear definition of the terms used and – as far as the documentation is concerned – standardized graphical symbols. These symbols help us represent control systems or measurement and control tasks as well as their device-related solution in a simple and clear manner.

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Terminology in Control Engineering

To maintain a physical quantity, such as pressure, flow or temperature at a desired level during a technical process, this quantity can be controlled either by means of open loop control or closed loop control.

Open loop control

In an open loop control system, one or more input variables of a system act on a process variable. The actual value of the process variable is not being checked, with the result that possible deviations – e.g. 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 flow.

The task of the operator illustrated in Fig. 1 is to adjust the pressure (p2) in a pipeline by means of a control valve. For this purpose, he utilizes an as- signment specification that determines a certain control signal (y) issued by the remote adjuster for each set point (w). Since this method of control does not consider possible fluctuations in the flow, it is recommended to use open loop control only in systems where disturbances do not affect the controlled variable in an undesired way.

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p1

y

p2

Fig. 1: Operator controls the process variable p2 via remote adjuster

Assignment: wa => ya => p2a wb => yb => p2b etc.

open action flow

disturbances are

not recognized

Closed loop control

In a closed loop control system, the variable to be controlled (controlled variable x) is continuously measured and then compared with a predetermined value (reference variable w). If there is a difference between these two variables (error e or system deviation xw), adjustments are being made until the measured difference is eliminated and the controlled variable equals the reference variable. Hence, the characteristic feature of closed loop control is a closed action flow.

The operator depicted in Fig. 2 monitors the pressure p2 in the pipeline to which different consumers are connected. When the consumption increases, the pressure in the pipeline decreases. The operator recognizes the pressure drop and changes the control pressure of the pneumatic control valve until the desired pressure p2 is indicated again. Through continuous monitoring of the pressure indicator and immediate reaction, the operator ensures that the pressure is maintained at the desired level. The visual feedback of the pro- cess variable p2 from the pressure indicator to the operator characterizes the closed action flow.

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p1 p2

Fig. 2: Operator controls the process variable p2 an a closed loop

closed action flow

disturbances are

eliminated

The German standard DIN 19226 defines closed loop control as follows:

Closed loop control is a process whereby one variable, namely the variable to be controlled (controlled variable) is continuously monitored, compared with another variable, namely the reference variable and, depending on the outcome of this comparison, influenced in such a manner as to bring about adaptation to the reference variable. The characteristic feature of closed loop control is the closed action flow in which the controlled variable continu- ously influences itself in the action path of the control loop.

A control process can also be regarded as ‘continuous’ if it is composed of a sufficiently frequent repetition of identical individual processes. The cyclic program sequence of digital sampling control would be such a process.

Note: In English literature we only find one term, that is ‘control’, being used for actually two different concepts known as ‘steuern’ and ‘regeln’ in the Ger- man language. When translating into German, we therefore come across the problem whether ‘control’ means ‘steuern’ or ‘regeln’. If both methods are 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’.

Process

A process comprises the totality of actions effecting each other in a system in which matter, energy, or information are converted, transported or stored. Adequate setting of boundaries helps determine sub-processes or complex processes.

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definition of

closed loop control:

DIN 19 226

difficulties with the

English term ´control´

• Examples:

4Generation of electricity in a power plant

4Distribution of energy in a building

4Production of pig iron in a blast furnace

4Transportation of goods

Control loop

The components of a control loop each have different tasks and are distingu- ished as follows:

The components of the final control equipment are part of the controlling sy stem as well as part of the controlled system.

The distinction made above results directly from the distribution of tasks. The actuator processes and amplifies the output signal of the controller, whereas the final control element – as part of the controlled system – manipulates the mass and energy flow.

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Controlling system Controller and acuator

+ Controlledsystem Final control element, pump, pipeline, heating system etc.

+ Measuringequipment Temperature sensor, pressure sensor, converter etc.

= Control loop

components of the

control loop

components of the

final control equipment

Actuator (controlling system) Actuating drive

+ Final control element (controlled system)

Closure member

= Final control equipment Control valve

Abbreviations of variables relating to closed loop control

The abbreviation of variables allows the determination of standardized sym- bols. The symbols used in German-speaking countries and specified in DIN 19221 correspond with the international reserve symbols approved by the publication IEC 27-2A. Aside from that, IEC also determines so-called chief symbols which considerably differ from those used in DIN in some important cases.

x (IEC chief symbol: y)

In a control loop, the process variable to be controlled is represented by x. In process engineering, usually a physical (e.g. temperature, pressure, flow) or a chemical (e.g. pH value, hardness) quantity is controlled.

w (IEC chief symbol: w)

This variable determines the value that must be reached (set point) by the process variable to be controlled. The physical value of the reference varia- ble – this may be a mechanical or electric quantity (force, pressure, current, voltage, etc.) – is compared with the controlled variable x in the closed con- trol loop.

r (IEC chief symbol: f)

This variable results from the measurement of the controlled variable and is fed back to the comparator.

e = w – x (IEC chief symbol: e)

The input variable e of the controlling element is the difference between refe- rence variable and controlled variable, calculated by the comparator. When the influence of the measuring equipment is included, the equation e = w – r applies.

xw = x – w

The equation above shows that the system deviation yields the same result as error, however, with an inverse sign. When the influence of the measuring equipment is included, xw = r – w applies.

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DIN or IEC

controlled variable,

actual value

reference variable

feedback variable

error

system deviation

y (IEC chief symbol: m)

The manipulated variable is the output variable of the controlling equipment and the input variable of the controlled system. It is generated by the control- ler, or in case an actuator is being used, by the actuator. This variable de- pends on the setting of the control parameters as well as on the magnitude of error.

yR

When dividing the controlling system into the controller and actuator, the va- riable yR stands for the output variable of the controller or the input variable of the actuator.

z (IEC chief symbol: v)

Disturbances act on the control loop and have an undesired effect on the controlled variable. Closed loop control is used to eliminate disturbance va- riables.

Yh

The manipulated variable y can be determined by the controller within Yh, the range of the manipulated variable :

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ymin ≤ y ≤ ymax

manipulated variable

controller output

variable

disturbance variable

range of the

manipulated variable

Symbols in Control Engineering

Signal flow diagrams

A signal flow diagram is the symbolic representation of the functional inter- actions in a system. The essential components of control systems are repre- sented by means of block diagrams. If required, the task represented by a block symbol can be further described by adding a written text.

However, block diagrams are not suitable for very detailed representations. The symbols described below are better suited to represent functional details clearly.

Blocks and lines of action

The functional relationship between an output signal and an input signal is symbolized by a rectangle (block). Input and output signals are represented by lines and their direction of action (input or output) is indicated by arrows.

• Example: Root-extracting a quantity (Fig. 3) (e.g. flow rate measurement via differential pressure sensors)

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xe xa

Fig. 3: Root-extracting a differential pressure signal

xe = differential pressure xa = root-extracted differential pressure

• Example: Representing dynamic behavior (Fig. 4) (e.g. liquid level in a tank with constant supply)

• Example: Summing point (Fig. 5)

The output signal is the algebraic sum of the input signals. This is symbolized by the summing point. Any number of inputs can be connected to one sum- ming point which is represented by a circle. Depending on their sign, the in- puts are added or subtracted.

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xe xa

Fig. 4: Development of a liquid level over time

xe = inflow xa = liquid level

xe1

xe2 + +

_

xe3

xa

xa = xe1 + xe2 – xe3

Fig. 5: Summing point

• Example: Branch point (Fig. 6)

A branch point is represented by a point. Here, a line of action splits up into two or more lines of action. The signal transmitted remains unchanged.

• Example: Signal flow diagram of open loop and closed loop control

The block diagram symbols described above help illustrate the difference between open loop and closed loop control processes clearly.

In the open action flow of open loop control (Fig. 7), the operator positions the remote adjuster only with regard to the reference variable w. Adjustment is carried out according to an assignment specification (e.g. a table: set point w1 = remote adjuster position v1; w2 = v2; etc.) determined earlier.

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x1

x2 x1 = x2 = x3

x3

Fig. 6: Branch point

xw

Fig. 7: Block diagram of manual open loop control

man

remote adjuster system

control valve

signal flow diagram

of open loop control

In the closed action flow of closed loop control (Fig. 8), the controlled varia- ble x is measured and fed back to the controller, in this case man. The con- troller determines whether this variable assumes the desired value of the reference variable w. When x and w differ from each other, the remote ad- juster is being adjusted until both variables are equal.

Device-related representation

Using the symbols and terminology defined above, Fig. 9 shows the typical action diagram of a closed loop control system (abbreviations see page 10).

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xw + _

Fig. 8: Block diagram of manual closed loop control

man

remote adjuster

control valve

system

z

ew +

yr y x

r

Fig. 9: Block diagram of a control loop

controlling element

measuring equipment

controller

final control element

system

actuator

signal flow diagram

of closed loop control

elements and signals

of a control loop

Whenever the technical solution of a process control system shall be pointed out, it is recommended to use graphical symbols in the signal flow diagram (Fig. 10). As this representation method concentrates on the devices used to perform certain tasks in a process control system, it is referred to as soluti- on-related representation. Such graphical representations make up an ess- ential part of the documentation when it comes to planning, assembling, testing, start-up and maintenance.

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graphical symbols

for detailed, solution-

related representations

Fig. 10: Graphical symbols for describing temperature control of a heat exchanger system 1 Sensor (temp.) 2 Transmitter 3 Signal converter 4 Controller 5 Pneumatic linear valve 6 Heat exchanger

1

23

4 5 6

Each unit has its own graphical symbol that is usually standardized. Equip- ment consisting of various units is often represented by several lined-up sym- bols.

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PI

Fig. 11: Graphical symbols for controllers, control valves and software-based functions according to DIN 19227 Part 2

hand-operated actuator

motor-driven actuator

diaphragm actuator

valve with diaphragm

actuator

motor-driven butterfly valve

valve

controllercontroller (former symbol)

valve with diaphragm actuator

and attached positioner

PI controller

root-extracting element,

software-based

software counter with limit switch

functions performed by

software are marked

with a flag

Graphical symbols used for process control are specified in DIN 19227, in- cluding symbols for sensors, adapters, controllers, control valves, operating equipment, generators, conduits and accessories (Figs. 11 and 12). Howe- ver, there are a number of other DIN standards covering graphical symbols, such as DIN 1946, DIN 2429, DIN2481, DIN 19239 and DIN 30600 (main standard containing approximately 3500 graphical symbols).

It is recommended to always use standardized graphical symbols. In case a standardized symbol does not exist, you may use your own.

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P

F

F

T Pt 100 DIN

P

P L

L

I

Fig. 12: Graphical symbols for sensors, transmitters, adjusters and indicators according to DIN 19227 Part 2

level sensor

temperature sensor

pressure sensor

adjusteranalog indicator

flow sensor

pressure transmitter with electric

standardized output signal

current transmitter with pneumatic

standardized output signal

i/p converter, electr. into pneum.

standardized signal

graphical symbols

for process control

Instrumentation and control tags

Apart from the solution-related representation, process control systems can also be represented by means of instrumentation and control tags (DIN 19227 Part 1) which describe the task to be done.

An instrumentation and control tag is represented by a circle. When the cir- cle is divided by an additional line, editing and operating procedures are not carried out on site, but in a centralized control station. In the bottom half of the circle, you will find the instrumentation and control tag number. The iden- tifying letters in the top half specify the measuring or input variable as well as the type of signal processing, organizational information and the signal flow path. If additional space is needed, the circle is elongated to form an oval (Fig. 13).

The typical use of identifying letters in an instrumentation and control tag is shown below:

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TI 106

TI 106

FRCA 302

Fig. 13: Instrumentation and control tags disignated according to DIN 19227 Part 1

Example: P D I C

First letter (pressure) Supplementary letter (differential) 1st succeeding letter (indication) 2nd succeeding letter (control)

instrumentation and

control tags

The meaning and the order of the identifying letters are listed in the following table.

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Group 1: Measuring or input variable Group 2: Processing

First letter Supplementary letter

Succeeding letter (order: I, R, C, ...any)

A Fault message, alarm

C Automatic control

D Density Differential

E Electric quantities Sensing function

F Flow rate, troughput Ratio

G Distance, length, position

H Hand (manually initiated) High limit

I Indication

K Time

L Level Low limit

O Visual signal, yes/no indication

P Pressure

Q Material properties Integral, sum

R Radiation Record or print

S Speed, rotational speed, frequency

Circuit arrangement, sequence control

T Temperature Transmitter function

U Multivariable

V Viscosity Control valve function

W Velocity, mass

Y Calculating function

Z Emergency interruption, safety device

for further details,

see DIN 19227

The two possible methods of graphical representation are compared with each other in the Figs. 14 and 15. The device-related representation accor- ding to DIN19227 Part 2 (Fig. 15) is in general easily understood. Whereas instrumentation and control tags according to DIN19227 Part 1 (Fig. 14) are more suitable for plotting complex systems.

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VL

RL

TI 2

TI 3 TIC

8

KS 2

TIC 7

TI 4

GOS 6

SOSA 1

5

Fig. 14: Representation of a control loop according to DIN 19227 Part 1

instrumentation and

control tags

ZLT

ZLT

ZLT

tAU

%

T

T

T

PI

VL

RL

0 1

Fig. 15: Representation of a control loop according to DIN 19227 Part 2

device-related

symbols

Control Systems and Structures

Depending on the job to be done, many different structures of control can be used. The main criterion of difference is the way the reference variable w is generated for a certain control loop. In setting the controller, it is also impor- tant to know whether the reference variable is principally subject to changes or disturbance variables need to be compensated for.

4To attain good disturbance reaction, the controller must restore the origi- nal equilibrium as soon as possible (Fig. 16).

4To attain good reference action, the controlled variable must reach a newly determined equilibrium fast and accurately (Fig. 17).

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x

z

t

t

Fig. 16: Disturbance reaction

w

t x

t

Fig. 17: Reference action

designed for good

disturbance reaction

or reference action

Fixed set point control

In fixed set point control, the reference variable w is set to a fixed value. Fixed set point controllers are used to eliminate disturbances and are therefore de- signed to show good disturbance reaction.

The temperature control system in Fig. 18 will serve as an example for fixed set point control. The temperature of the medium flowing out of the tank is to be kept at a constant level by controlling the heating circuit. This will provide satisfactory results as long as high fluctuations in pressure caused by distur- bances do not occur in the heating circuit.

Follow-up control

In contrast to fixed set point control, the reference variable in follow-up con- trol systems does not remain constant but changes over time. Usually, the re- ference variable is predetermined by the plant operator or by external equipment. A reference variable that changes fast requires a control loop with good reference action. If, additionally, considerable disturbances need to be eliminated, the disturbance reaction must also be taken into account when designing the controller.

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Fig. 18: Temperature control by means of fixed set point control

w

x

follow-up controllers

require good

reference action

fixed

reference variable

Cascade control

Cascade control systems require a minimum of two controllers, these are the master or primary and the follower or secondary controller. The characteri- stic feature of this control system is that the output variable of the master con- troller is the reference variable for the follower controller.

Employing cascade control, the temperature control of the heat exchanger (Fig. 19) provides good results also when several consumers are connected to the heating circuit. The fluctuations in pressure and flow are compensated for by the secondary flow controller (w2, x2) which acts as final control ele- ment to be positioned by the primary temperature controller.

In our example the outer (primary) control loop (w1, x1) must be designed to have good disturbance reaction, whereas the inner –secondary– control loop requires good reference action.

Ratio control

Ratio control is a special type of follow-up control and is used to maintain a fixed ratio between two quantities. This requires an arithmetic element (V). Its input variable is the measured value of the process variable 1 and its output variable manipulates the process variable 2 in the control loop.

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Fig. 19: Temperature control by means of cascade control

w1=wsoll x2

x1 w2

q

master and

follower controller for

high-quality control

Fig. 20 illustrates a mixer in which the flow rate q2 of one material is control- led in proportion to the flow rate q1 of another material.

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Fig. 20: Ratio control

V

q2

q1

q2 = V q1

w

x

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