Basic Control System unit2

E3145/2/1PRINCIPLES OF CONTROLLERS

PRINCIPLES OF CONTROLLERS

OBJECTIVES

General Objective : To apply the concept of principles of controllers.

Specific Objectives : At the end of the unit you will be able to :

Explain the basic concept of principles of controllers.

State the definition of basic controller components.

Draw the schematic circuits for controller action types.

Explain the circuits operations.

UNIT2


2.0 EXPLANATION OF PRINCIPLES OF BASIC CONTROLLERS

Controller is a device which receives input from two points :(i) a value which is sent by transmitter(ii) a value which is set by set point

The output from the controller is send to the valve controller.

Figure 2.0 : Block Diagram of controller(Source : Mansor Bin Laman (1996), Amalan Bengkel Peralatan)

INPUTINPUT

The main component of controller are :Comparator mechanismControllerFeedback mechanism

Comparator mechanism

Feedback mechanism

Relay

Sensor

MV

SP

Output

Supply


Figure 2.0 shows the input controller is a signal which is sent by transmitter. This signal is known as a transmitter signal (MV) and set point. If the output depends on the two inputs functions well and the process is in a stable condition, then the transmitter signal is similar to the set point. The comparator mechanism functions as comparator of both input signals. An error will exist if the input value is not the same. The detector will detect the error signal and determine if there is imbalance between error signal and feedback signals. If there is a difference, the detector will balance both of these signals. The feedback mechanism is a mechanism which balances the system. The feedback signal is always similar to the output signal.

The main components of controller are :(i) Comparator mechanism. It consists of two bellows which is for

transmitter signal and set point signal. Its function is to differentiate both the input signals.

(ii) The controller consists of a flapper and nozzle. Its function is to detect the error signal from the different output and the feedback signal.

(iii) The feedback mechanism consists of the feedback bellows. Its function is to balances and stable the system. It also has an effect towards multiple output of a controller.

2.1 EXPLANATION OF BASIC CONTROLLERS COMPONENTS

2.1.1 Bellows

The structure of a bellow is shown in Figure 2.1. It consists of a thin metal which is formed into a wave cylinder shape. Air pressure will depress a bellow. When air pressure is increased, bellow will extend and displacement exists. This displacement is linked to the convenient ‘lever’ for give the pressure increase reading. This displacement force include in mechanical force categories.


Figure 2.1 : Bellows(Source : Mansor Bin Laman (1996), Amalan Bengkel Peralatan)

2.1.2 Flapper Nozzle

Flapper nozzle is a displacement transducer which the displacement into a differential pressure parameter. Figure 2.2 shows a structure of flapper nozzle. Basically air is used as work liquid. Air will give a constant time about 0.1s. Flapper nozzle is used for measuring of displacement between load cell. This displacement is very small.

Unknown pressure Bellows movement


Figure 2.2 : Flapper Nozzle(Source : Mansor Bin Laman (1996), Amalan Bengkel Peralatan)

2.1.3 Restrictor

Accuracy of an instrument is guaranteed by manufacturers only for a certain limit. Normally it is stated in the form of a full scale percent of that particular instrument. Deflection from the specification is called restrictor error.

Fixed ResistanceVariable Resistance

x

Flapper Plat Measured element

Ρ0

Ρs


Activity 2A

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…!

2.1 Explain the basic controllers components below:(i) Bellows(ii) Flapper nozzle

2.2 Draw the diagram of bellow and flapper nozzle.


Feedback To Activity 2A

2.1. (i) Bellows consists of a thin metal which is formed into a wave cylinder shape. Air pressure will depress a bellow. When air pressure is increased, bellows will extend and displacement exists. This displacement is linked to convenient ‘ lever ’ for give the pressure increase reading. This displacement force include in mechanical force categories.

(ii) Flapper nozzle is a displacement transducer which the displacement into a differential pressure parameter. Basically air is used as work liquid. Air will give a constant time about 0.1s. Flapper nozzle is used for measuring of displacement between load cell. This displacement is very small.

2.2 (i) Bellows

Unknown pressure Bellows movement


(ii) Flapper nozzle

Fixed ResistanceVariable Resistance

x

Flapper Plat Measured element

Ρ0

Ρs


2.2DESIGN OF SCHEMATIC CIRCUIT FOR CONTROLLER ACTION TYPES

2.2.1 Types of Controller

There are a few types of controller used to control a process either in a form of Proportional output to the error, Proportional and Integral to the error or Proportional and Derivative output to the first error.

Controller can be used in the form of single mode of Proportional, Integral, or Derivative, two mode of Proportional and Integral (P+I) and Proportional and Derivative (P+D), and three mode of Proportional, Integral and Derivative (P+I+D).

The figures below show the design of schematic circuit for controller action types.

There are three types of controller :a) Proportional controllerb) Integral controllerc) Derivative controller

INPUTINPUT


(i) Proportional Controller (P)(Source : Mansor Bin Laman (1996), Amalan Bengkel Peralatan)


(iii) Integral Controller (I)(Source : Mansor Bin Laman (1996), Amalan Bengkel Peralatan)

(iv) Derivative Controller (D)(Source : Mansor Bin Laman (1996), Amalan Bengkel Peralatan)


(iv) Proportional + Integral Controller (P+I) (Source : Mansor Bin Laman (1996), Amalan Bengkel Peralatan)

(v) Proportional + Integral + Derivative Controller (P+I+D)(Source : Mansor Bin Laman (1996), Amalan Bengkel Peralatan)


Activity 2B


2.3 List the form of controllers used to control a process.

2.4 Design the schematic circuit for controller action types below.(i) Proportional controller (P)(ii) Integral controller (I)(iii) Derivative controller (D)


Feedback To Activity 2B

2.3 Controller can be used in the form of (a) Single mode of Proportional, Integral, or Derivative (b) Two mode of Proportional and Integral (P+I) and Proportional

Derivative (P+D) (c) Three mode of Proportional, Integral and Derivative (P+I+D).

2.4 (i)


(ii)

(iii)


2.3 EXPLANATION OF CIRCUITS OPERATION

2.3.1 Proportional Controller (P)

This controller can be found in Proportional controller where the output is always proportional to the error ( e ) signal. If the error signal is small, the control action will also be small and if the error signal is high, the control action will also be high. Once the controller detects an error, it will start to act. Output depends on controller gain, kc i.e.

controller output = controller gain x error

Figure 2.4 (see INPUT 2.2) shows, when measured signal equals to the set point, the system is stable. In other words, if the input process is equal to the output process, a stable system is obtained. If the Proportional controller is used there will be an offset where the measured signal will not reach the set point. Therefore if the control valve has to be 50% opened, we need to open the control valve more than 50% to achieve stability.

2.3.2 Integral Controller (I)

Integral controller responses to the integration of error signal to time. So, output is proportional to the area below the curve with time,

Controller output α ∫ t e dt

In Figure 2.6 (see INPUT 2.2), bellows and spring are in the reverse position compared to Proportional controller in Figure 2.5.

INPUTINPUT


2.3.3 Derivative Controller (D)

Derivative controller reacts to the rate of change but not to the change of magnitude. Output is proportional to the derivation error signal (e) to time,

Controller output α d e dt

If error is not changing or fixed there will be no output. Derivative controller is shown in Figure 2.7 (see INPUT 2.2). Derivative controller has one limit which is known as Derivative Limit. This limit distinguishes Proportional Controller and Derivative Controller. In a stable condition the derivative controller does not affect the output and it is always used in the process where temperature is involved.

As a conclusion, if we want a fast controller, then the Proportional controller is better but offset will occur. However, if we want to reduce offset, the Derivative controller can be used and we want to eliminate it, then the Integral controller should used.

2.3.4 Two Mode Control System (Proportional and Integral Controller)

The Figure 2.8 (see INPUT 2.2) shows a basic concept of a two mode control system. The comparator mechanism (which consists of two bellows) is place on one end of lever while on the other end there are two feedback bellows. A sensor is placed between the feedback mechanism and comparator. The operation of a two mode control system is different from a single mode control system because it combines the action of either both Proportional controller or Integral controller only.

Based on Figure 2.8, two things are added to the controller i.e. Integral/ Reset bellows and Integral limit (adjustable). When measured signal is increased, force impedance will come closer to the nozzle and thus will increase output. Increased pressure will make proportional bellow change the position of force impedance hence stabilize output pressure. Pressure will drop when it passes through integration unit. This pressure will pass integration bellow and push force impedance closer to the nozzle. So, output pressure can be increased further.


A stable system will be achieved when measured signal is equal to the set point and output pressure will be stable. Thus, offset can be eliminated. If a stable system cannot be achieved, the integration limit can be adjusted. If the integration limit is fully opened, the controller will act as an ON/OFF Controller. If integration limit is fully closed, the controller will act as a Proportional controller only.

2.3.5 Three Mode Controller System (P+I+D)

Figure 2.9 (see INPUT 2.2), shows three mode controller system. The operation is similar to P+I Controller but with the addition of a derivative limit. For this system, all controllers (Proportional, Integral and Derivative) will affect the responses. So, we need to adjust every controller to suit the process. Adjustments need to be done individually to stabilize process and eliminate offset. To achieve that, integral limit and derivative limit must be adjusted correctly.

2.3.6 Advantages and Disadvantages

Controllers have advantages and disadvantages. Table 2.1 below shows the advantages and disadvantages of a single and two mode controller.

CONTROLLER ADVANTAGES DISADVANTAGES

Proportional Faster response when load is changing

Offset exist

Integral Eliminate offset Longer recovery time

Derivative Reduce offset No output when no error

Proportional + Derivative Reduce recovery time and offset

Offset still occurs

Proportional + Integral Can eliminate offset Longer recovery time

Table 2.1

(Source : Mansor Bin Laman (1996), Amalan Bengkel Peralatan)


Activity 2C


2.5 Draw a suitable figure and explain a basic concept of a two mode control system (Proportional and Integral controller)

2.6 Give the advantages and disadvantages of Proportional, Integral, Derivative, Proportional + Derivative and Proportional + Integral.


Feedback To Activity 2c

2.5

Based on Figure 2.12, two things are added to the controller i.e. Integral/ Reset bellows and Integral limit (adjustable). When measured signal is increased, force impedance will come closer to the nozzle and thus will increase output. Increased pressure will make proportional bellow change the position of force impedance hence stabilize output pressure. Pressure will drop when it passes through integration unit. This pressure will pass integration bellow and push force impedance closer to the nozzle. So, output pressure can be increased further.


A stable system will be achieved when measured signal is equal to the set point and output pressure will be stable. Thus, offset can be eliminated. If a stable system cannot be achieved, the integration limit can be adjusted. If the integration limit is fully opened, the controller will act as an ON/OFF Controller. If integration limit is fully closed, the controller will act as a Proportional controller only.

2.6 Advantages and Disadvantages

CONTROLLER ADVANTAGES DISADVANTAGES

Proportional Faster response when load is changing

Offset exist

Integral Eliminate offset Longer recovery time

Derivative Reduce offset No output when no error

Proportional + Derivative

Reduce recovery time and offset

Offset still occurs

Proportional + Integral

Can eliminate offset Longer recovery time


KEY FACTS

1. Controller can be used in the form of single mode of Proportional, Integral, or Derivative, two mode of Proportional and Integral (P+I) and Proportional and Derivative (P+D), and three mode of Proportional, Integral and Derivative (P+I+D).

2. Proportional controller (P)

Output depends on controller gain, kc i.e.

Controller output = controller gain x error

3. Integral controller (I)

Output is proportional to the area below the curve with time,

Controller output α ∫ t e dt

4. Derivative controller (D)

Output is proportional to the derivation error signal (e) to time,

Controller output α d e dt


SELF-ASSESSMENT

You are approaching success. Try all the questions in this self-assessment section and check your answers with those given in the Feedback on Self-Assessment given on the next page. If you face any problems, discuss it with your lecturer. Good luck.

Q2-1 State the definition of restrictor.

Q2-2 Design the schematic circuit for controller action types below.(i) P+I(II) P+I+D

Q2-3 (a) State the difference between two mode control system and single

mode system.

(b) Explain the operation of three mode controller system (P+I+D)


Feedback To Self-Assessment

Have you tried the questions????? If “YES”, check your answers now.

Q2-1 Accuracy of an instrument is guaranteed by manufacturers only for a certain limit. Normally it is stated in the form of a full scale percent of that particular instrument. Deflection from the specification is called restrictor error.

Q2-2 (i) P+I


(ii) P+I+D

Q2-3

(a) The operation of two mode control system is different to single mode control system because it combine the action of either both Proportional controller or Integral controller only.

(b) The operation of P+I+D controller is similar to P+I Controller but with the addition of derivative limit. For this system, all controllers (Proportional, Integral and Derivative) will affect the response. So, we need to adjust every controller to suit the process. Adjustment need to be done individually to stabilize process and eliminate offset. To achieve that, integral limit and derivative limit must be adjusted correctly.


Education

Basic Control System unit2