3 fundamental type of controller

3 components

of Process Control

Measurement

Decision -Controller

Action

Type of controller - controller

strategies

Regulatory and servo control

• For process that the controlled variable deviates from set

point because of disturbance – regulatory control

• For process that the most important disturbance is the set

point itself – servo control

Terminology

Controlled

variable

Sym: c, PV

“The parameters

that indicate

product quality or

the operating

condition of the

process.” (L.M.

Gordon)

Manipulated

variable

Sym: m, CO, MV

A process being

adjusted (controlled)

by a final control

element (such as

control valve,

damper, motor

speed).

Load variable

Sym:

“All other

affecting

variable, other

than the one

being

manipulated.”

(L.M. Gordon)

Set point

Sym: r, SP, SV

The desired

operating

condition.

TC

TT

controlled variable

(temperature of

sterilised saline)Load variable

Manipulated variable

(saturated steam)

(Flow, temperature

of saline solution)Uncontrolled variable

(Saturated steam)

Heat exchanger

30°C

100 L/min

95°C

80°C

I/P

100°C

Terminology

© Abdul Aziz Ishak, Universiti Teknologi MARA Malaysia (2009)

• Load variable = set of all other variables that affect the CV to deviate from SP. Any one of the load variables may be made into MV.

= DV (Seborg page 263)

• Process Load = the set of all load variables that cause a change or deviations of CV from SP. It excludes CV

• Do you still remember the sentence :

“Change of process loading of load variable”?

- you are actually changing MV

Control strategies

TC

TT

controlled variable

(temperature of

sterilised saline)

What is feedback control?A method of control that the action taken is

based on past result.

Merit?Simple design. Easy to tune (for linear, fast &

short deadtime processes: flow, level &

pressure).

Demerits?Slow recovery for slow process such as

temperature control upon load disturbance or

change in setpoint. A process with long deadtime

may exhibit oscillatory process response with

long settling time.

Load variable


(saturated steam)

(Flow of saline solution)


(Saturated steam)

Heat exchanger

30°C

100 L/min

95°C

80°C

The ProcessSaline solution at 30°C is sterilized by

heating the solution in a heat-exchanger

system to 80°C. The temperature of the

sterilized saline solution is maintained by

regulating the control valve of steam input

to the heat-exchanger.

Feedback control I/P

100°C


Explanation:

1. With feedback control, a process deviation occurring near

the beginning of the process will not be recognized until the

process output.

2.The feedback control will then have to adjust the process

inputs in order to correct this deviation.

3.This results in the possibility of substantial deviation

throughout the entire process.

Demerits?Slow recovery for slow process such as temperature control

upon load disturbance or change in setpoint. A process with

long dead time may exhibit oscillatory process response with

long settling time.

4.The system could possibly miss process output disturbance

and the error could continue without adjustment.

5.Generally, feedback controllers only take input from one

sensor.

6.This may be inefficient if there is a more direct way to control

a system using multiple sensors.

7.Operator intervention is generally required when a feedback

controller proves unable to maintain stable closed-loop control.

8.Because the control responds to the perturbation after its

occurrence, perfect control of the system is theoretically

impossible.

MERITS

• Corrective action occurs as soon as the controlled

variable deviates from the set point, regardless of

the source and type of disturbance.

• reduces the sensitivity of the controlled variable to

unmeasured disturbances and process changes.

• Simple design

- do not require detailed knowledge of the system and, in

particular,

- do not require a mathematical model of the process,

- can be easily duplicated from one system to another.

DEMERITS

• no corrective action is taken until after the disturbance has

upset the process, that is, until after the controlled variable

deviates from the set point.

DEMERITS

• It does not provide predictive control action to

compensate for the effects of known or measurable

disturbances.

• It may not be satisfactory for processes with large

time constants and/or long time delays. If large

and frequent disturbances occur, the process may

operate continuously in a transient state and

never attain the desired steady state.

• In some situations, the controlled variable cannot be

measured on-line, so feedback control is not feasible

• Feedback control

Feedback controller

Process

RC

D

BLOCK DIAGRAM

M

R = set point D = disturbanceM= manipulated variable C = controlled variable

Control strategies

Feedforward control

30°C

100 L/min

95°C

80°C

I/P FFC

What is feedforward control?A method of control based on process model:

mass and energy balances.

Merit?Faster recovery than cascade or SISO.

Compensate for disturbances before they effect

the controlled variable.

Demerits? Cannot work if any of the sensors fail esp. load

variable.

Requires process model in order to design

feedforward system ---that’s why PV@CV is not

measured

Characteristic of feedforward

control:• Measurements of load variables online

(disturbance variable is measured, but the

controlled variable is not.)

• Utilization of computational blocks.

Heat exchangerTT

12 TTWH

CW p

s

p

s


controlled variable

(temperature of

sterilised saline)

Load variable


(saturated steam)



(Saturated steam)

FT

Demerits? Cannot work if any of the sensors fail esp. load variable.

Explanation:

In the house example, a feed-forward system may measure

the fact that the door is opened and automatically turn on the

heater before the house can get too cold.

For instance, if a window was opened that was not being

measured, the feed-forward-controlled thermostat might still

let the house cool down.

MERITS

• corrective action is taken before the controlled variable

deviates from the set point.

- Ideally, the corrective action will cancel the effects of the

disturbance so that the is not affected by the disturbance.

DEMERITS

• the disturbance variable must be measured (or accurately

estimated). In many application, this is not feasible.

• no corrective action is taken for unmeasured disturbances

• Ideal feedforward controllers that are theoretically capable

of achieving perfect control may not be physically

realizable. Fortunately, practical approximations of these

ideal controllers can provide very effective control

• Feedforward control

Feedforward controller

ProcessR C

D

M

R = set point D = disturbanceM= manipulated variable C = controlled variable

Control strategies

Cascade control

30°C

100 L/min

95°C

80°C

I/P FC

100°C

What is cascade control?A method of control of which the inner loop

controller obtains its setpoint from an outer loop

controller.

Merit?Faster recovery time as compared to SISO

feedback control.

Demerits?Controllability will be worst than SISO if both

controllers were not properly tuned.

Characteristic of cacade control:Two transmitters, two controllers and one

control valve (2-level cascade).

Application: Transform from SISO to

cascade strategy if faster recovery time is

desired.

Heat exchangerFT

TC

TT


controlled variable

(temperature of

sterilised saline)

Load variable


(saturated steam)



(Saturated steam)

TSP

FSP

The cascade control loop structure has two distinguishing features:

1. The output signal of the master controller serves

as the set point for the slave controller.

2. The two feedback control loops are nested, with

the secondary control loop (for the slave controller)

located inside the primary control loop (for the

master controller)Master controller

Slave

controller

Primary @Outer@

@Master Control Loop

Secondary @

Inner@Slave

Control Loop

•Cascade control

Primary controller

Process II

Y(s)C

Process ISecondarycontroller

Control strategies

Feedforward/Feedback control

30°C

100 L/min

95°C

80°C

I/P

Heat exchangerTT

12 TTWH

CW p

s

p

s

TC

TT


controlled variable

(temperature of sterilised saline)

Load variable


(saturated steam)



(Saturated steam)

FT

What is feedforward/feedback control?Feedback compensation added to feedforwardcontrol

Merit?Feedforward control compensates for majordisturbances, while feedback control compensatesfor all other disturbances.

FFC+

+

Process

R C

DFeedforward

controller

Feedbackcontroller

M++

+

++-

Control strategies

Ratio control

Wild flow 30°C

100 L/min

95°C

80°C

I/P FC

What is ratio control?Keeps a flowrate in proportion to another

flowrate.

Merit?Keep two flow in proportion.

Demerits?Inherits PID characteristics

Heat exchanger

FT

FT

X


controlled variable

(temperature of

sterilised saline)


(saturated steam)



(Saturated steam)

Ratio Control

• Type of feedforward control

• The objective is to maintain the ratio of two

variables at a specified value

• One variable is manipulated to keep it as ratio of

another.

• Implemented in two basic schemes.

multiplier

divider

• Required to blend 2 liquid streams, A and B in some

ratio

R=FB/FA

•Measuring the flow in steam A

•Multiplying it with the desired ratio

to obtain the required flow rate in

stream B (set point to the flow

controller of stream B)

•FBset = R x FA

•If the flow of stream A varies, the

set point of the controller of stream

B will vary.

Multiplier : set the ratio

The flow of stream B must vary, as the flow rate of

stream A is varies

(Wild flow)

•Measuring both stream

•Divide them in FY16 to obtain the

actual ratio flowing through the

system

•R=FB/FA

•The calculated ratio is the sent to

the controller which manipulated

the flow B to maintain set point.

•The set point to this controller is

the required ratio.

Divider

• Feedback control

Feedback

controllerProcess

Y(s)Y

D

•Feedforward control

Feedforward

controllerProcess

Y(s) Y

D

•Cascade control

Primary

controllerProcess II

Y(s)Y

Process ISecondary

controller

BLOCK DIAGRAM

• Self – exercise

- How to ensure that the level in the tank remains at or near the set

point? Propose the following control strategy:

- Feedback

- Feedforward

- Feedforward/Feedback

Self - exercise

• The objective of the following figure is to control the

temperature in the tank. Draw the block diagram for the

following feedfoward control.

• Draw the block diagram for the following cascade control.

3 fundamental type of controller

Engineering

Transcript of 3 fundamental type of controller