what is control engineering 1

8/3/2019 What is Control Engineering 1

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What is Control Engineering?

engineering discipline that focuses on themathematical modeling systems of a diversenature, analyzing their dynamic behavior,and using control theory to make a controller

that will cause the systems to behave in adesired manner.

Control engineering is closely related to

electrical engineering, as electronic circuitscan often be easily described using controltheory techniques.

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What is meant by Control?

Control – the process in a system in whichone or several input variables influenceother output variables as a result of the lawspertaining to the system. Controlling ischaracterized by the open-loop sequence ofactions via the single transfer element or the

control chain. (according to DIN 19226)



As shown in the Fig.1, the input variables xe …..acting on

this system are linked in a self- contained box andissued as output variables xa ….. and these variables

now act on the energy flow or mass flow to be controlled.Fig 1.1

x e1

x e2 x a1

x e3 x a2

In general: xa = f ( xe )

The term “control” is often applied to the complete systemin which controlling takes place, not only to the controloperation itself.



What is meant by Control?

The field of control within chemicalengineering is often known as processcontrol. It deals primarily with the control ofvariables in a chemical process in a plant. Itemploys many of the principles in controlengineering, and is a well-established field

in its own right.

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http://a/wiki/Process_control








Process Control (process controlengineering):

an engineering discipline that deals witharchitecture, mechanisms, and algorithms

for controlling the output of a specificprocess.

uses analog sensors to monitor real-world

signals and digital computers to do theanalysis and controlling; makes extensiveuse of analog/digital and digital/analogconversion.



In practice, process control systems can becharacterized as one or more of the following forms:

Batch – Some applications require thatspecific quantities of raw materials becombined in specific ways for particularduration to produce an intermediate or endresult. One example is the production ofadhesives and glues, which normally require

the mixing of raw materials in a heatedvessel for a period of time to form a quantityof end product.



In practice, process control systems can becharacterized as one or more of the following forms:

Continuous – Often, a physical system isrepresented though variables that aresmooth and uninterrupted in time. Thecontrol of the water temperature in a heating jacket, for example, is an example ofcontinuous process control.

Hybrid - applications having elements ofdiscrete, batch and continuous process

control



What is a Controller?

a component of a system that makes it operate within desired limits.

a device that attempts to control the states or outputs of a dynamic system . Generally, itaccomplishes this using feedback to correctdisturbances to the system; known asclosed-loop control.

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Example 1.1: If the output of an air compressor is controlled by the quantity

drawn in, then: The opening and closing of the valve is the control operation

The valve, whose setting affects the quantity drawn in, is thecontrol element

The opening provided by the valve is the controlled variable

y. The handwheel with which the valve is actuated is the

control device .

The varying load on the compressed air system caused by

the users that affects the control system is the disturbance z . This also applies to speed fluctuations or variations in thedegree of efficiency caused by the compressor. On theaccount of the open action loop of the control system, it isnot possible to compensate for such disturbance variables.



Types of control loops:

open-loop controller does not use feedback tocontrol states or outputs of a dynamic system. Open-loop control is used for systems that are sufficientlywell characterized to predict what inputs are

necessary to achieve the desired states or outputs.E.g. the velocity of a motor may be well characterizedfor the voltage fed into it, in which case feedback maynot be necessary.

closed-loop controller uses feedback to controlstates or outputs of a dynamic system.

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Fig. 1.2 shows the block diagram representing an open-loop control itself together with the system to be controlled.

Fig. 1.2

Disturbance z1

Energy/Mass

Flow

Controller Output

y Sequence of Actions

(Action Loop)

Disturbance z2

Controlled

System

Controller



Controller

Sequenceof Actions

Controlled System

Controller Output(Error) y

Energy/ Mass Flow

Command Variable w

Disturbance z2

Disturbance z1 ControlledVariable x

Controller

Fig. 1.3 Closed-Loop Controller



Automatic control

Process in which the controlled variable iscontinuously measured and compared with another

variable, the command variable, the process beinginfluenced according to the result of this comparisonby modifying to match the command variable.

The sequence of actions resulting from this takesplace in a closed loop, the control loop . The purposeof the closed loop control is to match the value of thecontrolled variable to the value specified by the

command variable even if perfect equalization is notattained under the prevailing circumstances. (accordingto DIN 19226)



Terms and Definitions: Controlled System – the part of the total system to be

influenced. Actuator – element that acts on the mass flow or energy

flow to be controlled and is located at the input to thecontrolled system.

Actuating path –

path along which the actions determininga control operation are transmitted.

Controller – part of the actuating path causing thecontrolled system to be influenced by the actuator; the

control or automatic control proper whose elements link theinput signals in accordance with the respective laws.

Disturbance point - point at which a factor acts that is notinfluenced by the system and which disturbs the conditionto be maintained.



Variables and their ranges in the actuatingpath:

Controller output y – output from the controller andat the same time input variable to the controlsystem.

Controller output range y h – range within which theoutput maybe adjusted.

Desired value x A – value to be acted upon by thecontrol

Control range x Ah – range within which the desiredvalue may be when the control is operatedproperly.



Variables and their ranges in the actuatingpath:

Command variable w – value introduced from theoutside to the control chain or to the control loopwhose output value is to follow in a predeterminedmanner (ie. setpoint device in close loop control,

input signal in open loop control.)w h – range of command variable

Disturbance variable z – variable acting from theoutside that influences the intended action of thecontrol.

z h – range within which the disturbance variable maybe allowed without adversely affecting the

operability of the control.



What is feedback?

In cybernetics and control theory, feedback isa process whereby some proportion or ingeneral, function, of the output signal of asystem is passed (fed back) to the input.

Often this is done intentionally, in order tocontrol the dynamic behavior of the system.

Feedback may be:

negative , which tends to reduce output, or

positive , which tends to increase output.

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Example 3 Process: cooling a room

Desired outcome: reach/ maintain a definedtemperature constant overtime, say 20 o C

Controlled variable : temperature

Input variable: temperature, since it is measuredby a thermometer and is used todecide whether to cool or not

Setpoint: 20o

CManipulated variable: state of the cooler (the setting

of the valve allowing chilled water to flow throughit)



Types of Signals

Analog – information is assigned continuouslypoint by point to a range of values.

Digital – the range to be considered is divided

into a finite number of separate value ranges,and one specific item of information is assignedto each range of values.

The digital group includes the binary signal,also known as an on-off signal, representingtwo items of information.



Types of Signals

Digital signals are used more frequently incontrol engineering and the digital signals aremainly in the form of binary signals.

These binary signals are of considerablesignificance for information processing becausethey can easily be produced by equipment (e.g.switches) and can also be processed simply.

In practice, it is essential to clearly define therelationship between range of values and signalin the case of binary signals



Analog/Digital Signals illustrated

If a continuously changeable pressure from 0to 600kPa is considered, each intermediatevalue of the range maybe assigned a specific

signal. If the pressure is indicated on a Bourdon

pressure gauge, each intermediate value

corresponds to a specific position of thepointer. The position of the pointer representsan analog signal.



Analog/Digital Signals illustrated If the dial is now divided into separate value

ranges, say in pressure steps of 50 kPa andif each range is assigned a specific item ofinformation:

50 . . . 100 kPa, value =1100 . . . 150 kPa, value = 1.5

150 . . . 200kPa, value =2,

Then, we are dealing with digitalsignals!



Representation of a closed loop in the signal flow diagram

Xe1

Xe2 = y

y

X

W

Xd = W - X

_

+



Breakdown of the Control Chain

In the preceding sections, the controller has

been represented as a self-contained blockwhich can be broken down even further. Acontrol can always be broken down by the

same method to show the arrangement of theindividual components; at the same timeshowing the signal flow.

The control chain is thus characterized by asignal flow from signal input via signalprocessing to signal output/execution of

instruction.



Breakdown of the control chain:

Actuating Device

Processing Element

Input Element

Signal output/ executionof instruction

Signal Processing

Signal input

Hardware breakdown Signal Flow

H d t



Hardware terms:

Actuating mechanism – element that has direct effect on a

controlled system, moves the final control element whenmechanically actuated.

Actuating device – consists of actuating mechanism and finalcontrol element.

Signal transducer – device transform an input signal as clearlyas possible into an associated output signal, where necessaryusing auxiliary energy. Among others, this group of devicesincludes amplifiers and signal converters.

Signal amplifier – device using auxiliary energy for poweramplification.

Signal Converter – devices in which input and output signalshave different structure

Examples of Hardware Elements



Examples of Hardware Elements Signal elements: limit switch with cam and roller

operation, proximity switches,light barriers, reflex sensors,push buttons, manual switches,etc.

Processing elements: Electronic logic elements,contactors, relays, valvesreleased by pneumatic logic, etc.

Final control elements: Power contactors, pneumatic

and hydraulic (directional control)valves, etc.

Drive elements: Electric motors, pneumatic/

hydraulic motors, cylinders, etc.



Types of controls vis a vis power requirement:

Control without auxiliary – power requirement to

adjust the final control element is provided by theinput element of the control.

Control with auxiliary energy – power required toadjust the final control element is supplied entirely or

in part through a source of auxiliary energy.

It is possible to operate with different levels ofenergy within the control chain, thus it is necessary

to distinguish the working energy – the energyrequired to operate the actuating device, from thecontrol energy that supplies the signal input andsignal processing.

B d th id ti t d d t l



Based on these considerations, an extended controlchain can be drawn up as follows:

Input Element

Processing element

Transducer

ActuatingDevice

Controlled System

Execution ofInstruction

Signal Output

Processing element

Signal element

Controller

Operative part



Types of energy for operative and control part

By means of suitable devices (signal transformers/

transducers) it is possible to convert one type ofenergy into signals of another type of energy – incontrol engineering, one can work within the controlledsystem with different types of energy.

In practice however, it is not always easy to select the“right control system”. Apart from the immediate

requirements of the problem, the auxiliary

requirements in particular (place of installation,environmental influences, etc.) determine the solution.These auxiliary often conflict with the simple solutionto the problem that can make project engineering

more difficult.



Types of energy for operative and control part

If a system uses different types of energy for the

operative and control parts, one refers to a mixedtechnology – which is being used to an increasingextent in control design.

Working Media:-Mechanical-Electrical-Hydraulics

-Pneumatics

Criteria for system selection:-Force-Displacement-Type of motion

-Speed-Physical size-Life-Sensitivity-Working safety

Ch i i f ki di



Characteristics of working media:

Electrical:

Energy storage difficult, transmission fast, costs low.

Creation of straight line motion complex andexpensive, as it is necessary either to convert bymechanical means or short displacements possible

with lifting magnets and only small forces possiblewith linear motors.

Creation of rotary motion at very high efficiency,

large physical size, speed limited, speed torqueregulation difficult and elaborate

Elements not overload-proof, not intrinsicallyexplosion-proof.



P

S1



Logic Controllers

Logic controllers usually respond toswitches or photoelectric cells, and causethe machinery to perform some operation.

Logic systems are great for sequencingmechanical operations in places likeelevators and factories, but notably poor at

managing continuous process controls insuch places as oil refineries and steel mills.



Logic Controllers

Logic systems are quite easy to design, andcan handle very complex operations. Logicsystems may be designed with a system

similar to Boolean logic . ( Logic gates thatare primarily electronically-controlled butcan also be constructed from

electromagnetic relays , electronic diodes ,fluidics , optical or even mechanical elements, are commonly employed.

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Linear or feedback controls

Linear controls use negative feedback tokeep some desired process within anacceptable range. For example, athermostat is a simple negative feedbackcontrol; when the temperature goes below athreshold, control starts. Systems thatinclude feedback are prone to hunting ,

which is oscillation of output resulting fromimproperly tuned inputs of first positive thennegative feedback.


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In the furnace example, the valve may open andshut indefinitely in a cycle as the furnace heats, andthen overruns the target temperature. This is bad

because it stresses the system. In a furnace, theconstantly turning valve will quickly wear out. Moreexpensively, the fluctuating temperature causesexpansion and contraction all through the furnace,

causing unnecessary, very expensive mechanicalwear. Most systems have similar problems.




Often, if the response of the system isslowed down enough to prevent oscillation,the system doesn't respond fast enough to

work in normal situations. To resolve theproblems, the most common feedback loopscheme has mathematical extensions tocope with the future and the past. This typeof loop is called a Proportional-Integral- Derivative Loop , or PID loop .

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what is control engineering 1

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