SRI LANKA INSTITUTE of ADVANCED TECHNOLOGICAL EDUCATION

Training Unit

Control Systems 1 Theory

No: EE 064

INDUSTRIETECHNIKINDUSTRIETECHNIK

ELECTRICAL and ELECTRONIC ENGINEERING

Instructor Manual

1

Training Unit

Control Systems 1

Theoretical Part

No.: EE 064

Edition: 2008 All Rights Reserved Editor: MCE Industrietechnik Linz GmbH & Co Education and Training Systems, DM-1 Lunzerstrasse 64 P.O.Box 36, A 4031 Linz / Austria Tel. (+ 43 / 732) 6987 – 3475 Fax (+ 43 / 732) 6980 – 4271 Website: www.mcelinz.com

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CONTROL SYSTEMS 1

CONTENTS Page

LEARNING OBJECTIVES...................................................................................................3

1 GENERAL ....................................................................................................................4

2 OPEN LOOP CONTROL - CLOSED LOOP CONTROL ..............................................5

2.1 Open loop control.................................................................................................5

2.1.1 Control chain ....................................................................................................5

2.1.2 Types of open loop control...............................................................................6

2.2 Closed loop control ..............................................................................................8

2.2.1 Types of closed loop control ............................................................................9

3 TERMS AND DEFINITIONS.......................................................................................10

3.1 Controlled member of system or process (s) ....................................................10

3.2 Controlling means (R) ........................................................................................10

3.3 Control loop........................................................................................................10

3.4 Final control element or correcting unit ..............................................................11

3.5 Regulating point .................................................................................................11

3.6 Correcting variable (y) or controller output.........................................................11

3.7 Correcting range (yn) .........................................................................................12

3.8 Regulating time (ty) ............................................................................................12

3.9 Regulating speed (vy) ........................................................................................12

3.10 Command variable (w) or desired value (Set value) ..........................................12

3.11 Command range (wh) ........................................................................................13

3.12 Controlled variable (x ) .......................................................................................13

3.13 Control range (xn) ..............................................................................................13

3.14 Disturbance variable (z) .....................................................................................13

3.15 Deviation (xw ) ...................................................................................................14

3.16 Control difference (xd ) or error..........................................................................15

4 CONSTRUCTION OF A CLOSED LOOP OF CONTROL..........................................16

4.1 Signal flow diagram...........................................................................................16

4.1.1 Elements of the signal flow diagram ..............................................................17

4.2 Signal circuit (general analysis) .........................................................................18

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CONTROL SYSTEMS 1

LEARNING OBJECTIVES

The trainee should ...

… explain the terms "open loop control", "closed loop control", "regulating range” and

"controlled variable".

… state the different types of open loop control.

… name the types of closed loop control.

… explain the representation of signals in the signal flow diagram.

… name three elements of the sign flow diagram.

•

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CONTROL SYSTEMS 1

1 GENERAL

In many areas of technology it is no longer humanly possible to process all the available

information and to make correct decisions within a short time. An automatic control for

reliable operation of working processes is indispensible here. Control technology can,

therefore, be termed as the foundation of automation.

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2 OPEN LOOP CONTROL - CLOSED LOOP CONTROL

2.1 Open loop control

This is the process of influencing a variable by one or several other variables.

In the above oven the gas flow is throttled by a valve. The temperature can be raised or

lowered (controlled) by operation of this valve.

The oven temperature can change, however, by various influences such as different gas

pressures, badly closing oven doors, fluctuating external temperatures etc.

Should these factors have to be considered, a closed loop control would be necessary to

maintain a constant temperature.

2.1.1 Control chain

The output variable of the first transfer element is the input variable of the next element. In

the block diagram of a control chain the individual elements are represented as rectangles

irrespective of their functions.

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Example:

The arrow direction indicates the system operating sequence direction. In a control chain

operating sequence the input variable is

independent.

2.1.2 Types of open loop control

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2.1.2.1 Command control

There is a direct connection between the command variable and the output variable of the

control system, for example; the brightness control of a light bulb via a regulating

transformer:

Command variable adjustment of the transformer

Output variable brightness in the area illuminated by the lamp

2.1.2.2 Holding element control

When the command variable is removed or withdrawn, the value of the output variable

obtained is preserved.

Only when an opposing signal or signal of a different kind, or when an opposing command

variable or a command variable of a different kind appears, is the output variable brought

back to the starting value.

2.1.2.3 Programme control

Time-schedule control:

The command variable is supplied by a time controlled programme transmitter

(programme register). For example; control of a process by means of a programme

transmitter with plate cams which rotate at a constant speed.

Path pattern control:

The command variable is supplied by a programme transmitter, the output variable of

which is dependent an the path covered by a moving part of the controlled arrangement.

For example; control from the cam shaft of a machine tool.

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Sequence control:

The programme is stored in a programme transmitter and progresses depending upon the

condition reached at the time. The next programme step is triggered when the proceeding

one ends. This is effected mostly by sensing devices or transmitters. For example; putting

a conveyer belt system into operation, when one belt must not start until another has been

put into operation. (The last belt is switched on initially, then the next to last and so on.

This prevents a conveyor belt from depositing material onto one that is still idle.)

2.2 Closed loop control

This is a process in which the variable to be controlled (controlled variable) is continuously

measured, compared with the set value and corrected.

The oven temperature is continuously measured, the measured signal is conducted to the

control unit where it is compared with the set value and adjusts the valve correspondingly.

The difference between open loop and Closed loop control systems is essentially in the

operating sequence.

Closed loop control ... closed operating sequence

Open loop control ..… open operating sequence

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2.2.1 Types of closed loop control

Set value control of fixed set-point control

Follower control or variable set-point control

Time-pattern control or time-schedule schedule control

2.2.1.1 Fixed set-point

Fixed set-point control refers to all closed loop control systems in which the specified set

value must be kept constant. In other words, the command variable remains adjusted at a

fixed value (set value). Set value control is applied in industry for controlling the level,

temperature, speed, flow rate and pressure.

2.2.1.2 Follower control or variable set-point control

The set-point follows the value of the variable (command variable); Variation of the set-

point causes the controller to change its output that the controlled variable pursues the

set-point value.

Familiar applications of this type of control system:

A radar apparatus following a moving object

Automatic course control systems for aeroplanes and ships etc.

2.2.1.3 Time-pattern control

This is a follower control system in which the value of the command value is controlled in

a time dependant manner.

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3 TERMS AND DEFINITIONS

3.1 Controlled member of system or process (s)

(open and closed loop. controlled members)

An open or closed loop controlled member is any part of an installation which needs to be

controlled.

3.2 Controlling means (R)

By this is meant all units which affect the control process at the controlled member of

system (sensor, controller, final control element).

3.3 Control loop

lt is a loop which consists of controlling means and the controlled member of system

(process).

Process + Controlling means = Closed loop

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3.4 Final control element or correcting unit

This is the input element to the controlled member. The correcting unit consists of the

actuator element and the correcting element. The actuator adjusts the correcting element

in response to the Signal receives from the controller. Final control elements are, for

example; globe valves, butterfly valves, sliders, etc.

3.5 Regulating point

The regulating point is that point at which the energy flow or mass flow of a controlled

member is altered.

Since this alternation is carried out via the final control element, the regulating point is

generally located at the input of the controlled member.

3.6 Correcting variable (y) or controller output

This is the name given to the output variable of the controller. The correcting variable has

a direct effect an the final control element and produces a change in the mass or energy

flow.

Example:

A pulse appears at the controller output; let us call this pulse "more water". This pulse

called "correcting variable (y)" causes the water valve (final control element) to open by

means of the appropriate facilities and to remain open as long as this pulse is present at

the controller output. In so doing the moving part of the water valve passes through a

certain range.

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3.7 Correcting range (yn)

The correcting range is that range in which the correcting variable is adjustable. In our

example, therefore, this refers to the range from the completely closed valve to the

completely open valve and vice versa.

3.8 Regulating time (ty)

This term refers to that time taken for the final control element to traverse the entire

correcting range.

3.9 Regulating speed (vy)

The regulating speed is the correcting element displacement divided by the regulating

time. In general, it depends on the construction of the correcting unit and it has a decisive

influence on in the quality of the control.

3.10 Command variable (w) or desired value (Set value)

The command variable is not influenced by open or closed loop control. The working

variable (open loop control) or controlled variable (closed loop control) must follow it at the

specified dependence. The set value (command variable) is therefore the value at the

output of the control loop, which is to be attained.

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3.11 Command range (wh)

This term refers to the range in which the set value is adjustable.

For example; Pressure = 5 to 10 bar

Temperature = 200 to 500 °C

Volume = 100 to 500 cu. m

3.12 Controlled variable (x )

This is the variable which is to be controlled. In the field of open loop control technology it

is designated as the "working variable".

The value which actually appears at the output of a control circuit or of a closed loop is

called the "measured value of the controlled variable".

3.13 Control range (xn)

The control range is the defined range of the automatic control within which the controlled

variable can be varied.

3.14 Disturbance variable (z)

A disturbance variable is a variable which has an undesirable influence an the stability of

a control system.

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Example:

Room temperature control -

disturbance variables = opening of doors and windows

3.15 Deviation (xw )

Deviation is the difference between the measured value and the set

value (command variable).

Example:

Voltage control: Set value (w) = 12 V

Measured value (x) = 10 V

xw = ?

xw= x - w = 10 - 12 = -2 V

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3.16 Control difference (xd ) or error

The control difference is that value which is to be further processed in the control unit. The

control difference is not distinguished from the deviation by its size, but only by the polarity

symbol.

xd = —xw

Example:

Voltage control: Set value (w) = 12 V

Instantaneous value (x) = 10 V

xd = ?

xd= w - x = 12 - 10 = 2 V

In practice this would mean that the voltage has to be re-adjusted by 2 V in the positive

direction. This is also apparent an a measuring instrument the indicator of which registers

a deviation (xw) of -2 V.

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4 CONSTRUCTION OF A CLOSED LOOP OF CONTROL

The closed loop consists of

- controlled member (the member to be controlled)

- controlling means (sensor, controller, final control element)

4.1 Signal flow diagram

The Signal flow diagram symbolically represents the relationships between the signals in

a system or in a number of separate inter-acting systems.

The conventional representation of signals in a diagram:

- Signals flow in the direction of the arrow-headed lines (lines of influence).

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4.1.1 Elements of the signal flow diagram

Block rectangle symbolizes the dependency of the output signals an the input signals.

Summing point

This is represented by a circle.

The circle symbolizes the fact that the output signal is the algebraic sum of the input

signals.

Branch point

This is emphasized by a dot with a diameter three times greater than the thickness of the

lines. The dot symbolizes the fact that, at this point, a line of influence branches out into

several continuing lines of influence.

Interaction of the elements

The interaction of the transfer elements is portrayed symbolically by connecting their

symbols with lines of influence.

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4.2 Signal circuit (general analysis)

First step

In the "closed action sequence" of a closed loop the controlled variable (x) is measured

continuously by an appropriate measuring device and the signal for the measured value is

produced by a transducer.

Second step

The signal for the set value is formed in a set value adjuster to a level which accords with

the command variable (w).

Third step

The set value is compared with the measured value at a suitable point of comparison and

the control difference (xd) thus created is passed an to the controller.

Fourth step

In the controller, the control difference (xd) is treated in such a way that a correcting

variable (y) is created, which corrects the controlled variable (x).

Fifth step

A signal amplified and formed in accordance with the correcting variable (y) brings about

the adjustment of the final control element at the Input of the controlled member.

Sixth step

This signal circuit from the output of the controlled member, via the measuring transducer,

the point of comparison and the control unit, to the final control element at the input of the

controlled member remains effective until the measured value is adapted to the set value

and the desired value is present in the controlled member.

Disturbance variables (z), interferences which result in changes in the measured value

may be expected in the controlled member itself.•

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EE 064

Control Systems 1

Theoretical Test

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EE 064

CONTROL SYSTEMS 1

TEST

1. Name the three types of open loop control.

2. Define the term "Closed loop control".

3. By what symbol are signals indicated in the Signal flow diagram?

4. Give an example for the application of a path pattern control.

5. State what is meant by the term "Follower control".

6. Name three types of closed loop control.

7. Give a possible application of follower control.

8. Define the term "Deviation".

9. By which symbol is a branching point indicated?

10. State what is meant by the term "Command range'.

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EE 064

CONTROL SYSTEMS 1

TEST

(Solution)

1. Command control, Holding element control, Programme control.

2. Closed loop control is a process in which the variable to be controlled (controlled

variable) is continuously measured, compared with and corrected to the set value.

3. By arrow heads, pointing in the direction of signal flow.

4. Cam control of a machine tool.

5. The set value follows the value of the variable (command variable).

6. Set value control, Follower control, Time-pattern control.

7. Radar following a moving object; automatic course control systems for airplanes

and ships etc.

8. This is the difference between the measured value of the controlled variable and

set value of the command variable.

9. By a dot with a diameter three times the thickness of the line.

10. This term refers to the range within which the set value is adjustable.

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KEY TO EVALUATION

PER CENT

MARK

88 – 100

1

75 – 87

2

62 – 74

3

50 – 61

4

0 – 49

5