Control System Nur Istianah-THP-FTP-UB-2014

Transcription

1 Control System

2 Control System Control is the process of causing a system variable to conform to some desired value. Manual control Automatic control (involving machines only). A control system is an interconnection of components forming a system configuration that will provide a desired system response. Input Signal Control System Output Signal Energy Source 1 BMT 437

3 Three basic operations: SP (1) Measurement(M). Measuring the variable to be controlled (2) Decision(D). Based on the measurement, the controller decides what to do to maintain the variable at its desired value. Feed Ti(f) Final control element Controler TC TT Cooling water Transmitter Sensor; T(f) Fig. 1-2 Bioreactor control loop (3) Action(A). As a controller s decision, the system must take an action. This is usually accomplished by the final control element. 2

4 Control System Components i. Process To be controlled ii. Sensors Provides measurement of the system output iii. Actuators / transmitter Converts the control signal (measurement) to a power signal iv. Controller - The brain of the control system. - It decides what to do to maintain the variable at its desired value v. Final control element - receive the decision from controller and do the action i. Reference input Represents the desired output after action was taken 3 BMT 437

5 Control System Classification Open-Loop Systems in which the output quantity has no effect upon the process input quantity Simple system Must be closely monitored Closed-Loop (feedback) Systems in which the output has an effect upon the process input quantity in such a manner as to maintain the desired output value Uses the output of the process to modify the process to produce the desired result Continually adjusts the process 4

6 Control System Classification An open-loop control system utilizes an actuating device to control the process directly without using feedback. Desired Output Response Actuating Device Process Output A closed-loop feedback control system uses a measurement of the output and feedback of the output signal to compare it with the desired output or reference. Desired Output Response Comparison Controller Measurement Process Single Input Single Output (SISO) System Output Desired Output Response Controller Process Output Variables Measurement Multi Input Multi Output (MIMO) System 5

7 Application: CD player, computer disk drive Requirement: Constant speed of rotation Open loop control system: Block diagram representation: 6

8 Closed-loop control system: Block diagram representation: 7

9 Types of control strategies Feedback (FB) control Feed Forward (FF) control Cascade (FB-FF) 8

10 Feedback control SP Final control element Controler TC Cooling water Feed TT Transmitter Ti(f) Sensor; T(f) Fig. 1-2 Bioreactor Feedback control loop # Measurement of controlled variable (output) is used to adjust the desired value of that variable. Disturbance variable is not measured 9

11 The advantage of feedback control: 1. Simple technique 2. Overcome all disturbances 3. The feedback loop does not know, nor does it care, which disturbance enters the process. It tries only to maintain the controlled variable at set point and in so doing compensates for all disturbances. 4. The controller changes its input in such a way as to return the controlled variable to the set point. 5. Feedback control is the most common control strategy in the process industries 10

12 The disadvantage of feedback control 1. It can resolve a disturbance only after the controlled variable has deviated from set point. 2. That is, the disturbance must enter to the process before the feedback control scheme can initiate action to resolve it. 11

13 Feed forward control SP TT Final control element Transmitter Controler TC Cooling water Feed Sensor; Ti(f) T(f) Fig. 1-2 Bioreactor Feed forwardcontrol loop # Measurement of disturbance variable (input) is used as a set point to adjust the desired value of that controlled variable. Controlled variable is not measured 12

14 The objective of feed forward control is to measure the disturbances and resolve for them before the controlled variable deviates from set point. Suppose that in bioreactor example the major disturbance is inlet temperature. To implement feed forward control the disturbance first (Ti) must be measured and then a decision is be made how to manipulate cooling water flow rate to resolve for this change. Criteria for the measured variable (disturbance) in FF control: it must indicate the occurrence of an important disturbance the control variable must not affect the disturbance 13

15 With Feed forward control The advantage of feed forward control: 1. Corrective action is taken before the controlled variable deviates (not same) with the desired value (set point) 2. Corrective action resolve the disturbance effects on the controlled variable so that controlled variable will be constant (not be disturb) 3. If applied correctly, the controlled variable deviation would be minimum. 14

16 The disadvantage of feed forward control: The disturbance variable must be measured No corrective action is taken for unmeasued disturbance variable A process model is requested 15

17 Cascade control system Cascade Control Systems are needed if a single loop control system (FF/FB) are not enough. Disturbance causes poor control of desired variable. It contain integrated sets of control loops Primary Loop: Monitors the controlled variable and uses deviation from its set point to provide an output to the secondary loop. FF control Secondary Loop: Receives its set point from the primary loop and controls the reference variable accordingly. FB control 16

18 Feedforward control with feedback control FF Controler SC SP FB Controler TC ST Transmitter Final control element TT Cooling water Transmitter Sensor; N (rpm) Sensor; T(f) Fig 1-6 Feed forward control with feedback control Feed forward control now resolve for the major disturbance; feedback control maintained controlled variable and resolve for all other disturbances. 17

19 The advantage of cascade control: Effectively accounts for external disturbances Reduces dead time in variable response Compatible with other Control Systems 18

20 The disadvantage of cascade control: Multiple control loops make physical and computational architecture more complex Additional controllers and sensors can be costly The disturbance dynamics must not be significantly faster than the main inputoutput dynamics 19

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