8/2/2019 BASICS of Process Control

1/31

Basics On Process Control and PIDs

8/2/2019 BASICS of Process Control

2/31

PROCESSInputs Outputs

Controller

ControlElement

Sensor

DesiredValue(SP)

CAUSE EFFECT

DISTURBANCES

8/2/2019 BASICS of Process Control

3/31

Seven Control Objectives

SAFETY

ENVIRONMENTAL PROTECTION

EQUIPMENT PROTECTION

SMOOTH OPERATION

PRODUCT QUALITY

PROFIT

MONITORING AND DIAGNOISIS

8/2/2019 BASICS of Process Control

4/31

SAFETY

8/2/2019 BASICS of Process Control

5/31

ENVIRONMENTAL PROTECTION

8/2/2019 BASICS of Process Control

6/31

EQUIPMENT PROTECTION

8/2/2019 BASICS of Process Control

7/31

SMOOTH OPERATION

8/2/2019 BASICS of Process Control

8/31

PRODUCT QUALITY

8/2/2019 BASICS of Process Control

9/31

PROFIT

8/2/2019 BASICS of Process Control

10/31

MONITORING AND DIAGNOISIS

Failing to achieve any of these objectives will lead to operation that will beunprofitable, unsafe or worse

8/2/2019 BASICS of Process Control

11/31

8/2/2019 BASICS of Process Control

12/31

Input Variables-Manipulated (or adjustable) Variables

if their values can be adjusted freely by the human operator or a controlmechanism

-Disturbancesif their values are not the result of adjustment by an operator or a control system

Output Variables-Measured O/P Variables

if their values are known by directly measuring them-Unmeasured O/P Variables

if they are not or cannot be measured directly

8/2/2019 BASICS of Process Control

13/31

Types Of Controllers

On-Off Controller

Proportional Controller

Proportional Integral Controller

Proportional Derivative Controller

Proportional Derivative Integral Controller

8/2/2019 BASICS of Process Control

14/31

On-Off Controller

8/2/2019 BASICS of Process Control

15/31

Proportional Controller

WherePout: Proportional outputKp: Proportional Gain, a tuning parametere: Error = SP PV

t: Time or instantaneous time (the present)

M V(t) = Pout

A high proportional gain results in a largechange in the output for a given change inthe error.

In contrast, a small gain results in a smalloutput response to a large input error, and aless responsive (or sensitive) controller.

If the proportional gain is too high, the

system can become unstable.If the proportional gain is too low, the controlaction may be too small when responding tosystem disturbances.

8/2/2019 BASICS of Process Control

16/31

Proportional Integral Controller

WhereIout: Integral outputKi: Integral Gain, a tuning parameter

e: Error = SP PV: Time in the past contributing to the integral

M V(t) = Pout + Iout

The integral term (when added to theproportional term) accelerates themovement of the process towards setpointand eliminates the residual steady-stateerror that occurs with a proportional onlycontroller.

However, since the integral term isresponding to accumulated errors from the

past, it can cause the present value toovershoot the setpoint value (cross overthe setpoint and then create a deviation inthe other direction).

8/2/2019 BASICS of Process Control

17/31

Proportional Derivative Controller

WhereDout: Derivative outputKd: Derivative Gain, a tuning parametere: Error = SP PVt: Time or instantaneous time (the present)

M V(t) = Pout + Dout

The derivative term slows the rate of change ofthe controller output and this effect is mostnoticeable close to the controller setpoint.Hence, derivative control is used to reduce themagnitude of the overshoot produced by theintegral/proportional component and to improvethe combined controller-process stability.

However, differentiation of a signal amplifiesnoise in the signal. Hence, this term in thecontroller is highly sensitive to noise in the errorterm. If the noise and the derivative gain aresufficiently large then the process becomesunstable.

8/2/2019 BASICS of Process Control

18/31

Proportional Derivative Integral Controller

M V(t) = Pout + Dout + Iout

8/2/2019 BASICS of Process Control

19/31

Proportional Gain - Larger Kp typically means faster response since the larger theerror, the larger the feedback to compensate. An excessively large proportional gain willlead to process instability.

Integral Gain - Larger Ki implies steady state errors are eliminated quicker. The trade-off is larger overshoot: any negative error integrated during transient response must beintegrated away by positive error before we reach steady state.

Derivative Gain - Larger Kddecreases overshoot, but slows down transient responseand may lead to instability.

8/2/2019 BASICS of Process Control

20/31

Different Types Of ControlSystemsFeedback Control System

Feedforward Control System

Cascade Control System

Ratio Control System

Split Control System

8/2/2019 BASICS of Process Control

21/31

Feedback Control SystemController acts after the effect of disturbance has been felt by the system

Advantages:

Corrective action occurs regardless of thesource and type of disturbances.

Requires little knowledge about the process (Forexample, a process model is not necessary).

Versatile and robust (Conditions change? May

have to re-tune controller).

Disadvantages:

FB control takes no corrective action until adeviation in the controlled variable occurs.

FB control is incapable of correcting a deviationfrom set point at the time of its detection.

Theoretically not capable of achieving perfect

control.

For frequent and severe disturbances, processmay not settle out.

8/2/2019 BASICS of Process Control

22/31

Feedforward Control SystemController acts even before the effect of disturbance has been felt by the system

Advantages:

Takes corrective action before the process isupset (cf. FB control.)

Theoretically capable of "perfect control"

Does not affect system stability

Disadvantages:

Disturbance must be measured (capital,operating costs)

Requires more knowledge of the process to becontrolled (process model)

Ideal controllers that result in "perfect control:may be physically unrealizable. Use practicalcontrollers such as lead-lag units

8/2/2019 BASICS of Process Control

23/31

Feedback Control

Feedforward Control

Comparison

8/2/2019 BASICS of Process Control

24/31

FF ControlAttempts to eliminate the effects of measurable disturbances.

FB Control

Corrects for unmeasurable disturbances, modeling errors, etc. (FB trim)

8/2/2019 BASICS of Process Control

25/31

Cascade Control System

Di ti i hi f t

8/2/2019 BASICS of Process Control

26/31

Distinguishing features:

-Two FB controllers but only a single control valve (or other -final control element).

-Output signal of the "master" controller is the set-point for slave" controller.

-Two FB control loops are "nested" with the "slave" (or "secondary") control loopinside the "master" (or "primary") control loop.

Terminology

-slave vs. master

-secondary vs. primary

-inner vs. outer

8/2/2019 BASICS of Process Control

27/31

Ratio Control System

8/2/2019 BASICS of Process Control

28/31

8/2/2019 BASICS of Process Control

29/31

8/2/2019 BASICS of Process Control

30/31

Split Control System

2 Manipulated Vars.:V1 and V2

1 Controlled Var.:Reactor pressure

While V1 opens, V2 should close

3 6 9 15

8/2/2019 BASICS of Process Control

31/31