Chapter 14

PID Implementation Issues

Overall Course Objectives

• Develop the skills necessary to function as an industrial process control engineer.– Skills

• Tuning loops

• Control loop design

• Control loop troubleshooting

• Command of the terminology

– Fundamental understanding• Process dynamics

• Feedback control

Reset Windup for PID Controllers

• Windup results when the manipulated variable is not able to control to the setpoint resulting in sustained offset causing the integral of the error from setpoint to accumulate.

• When control returns, accumulated error causes an upset.

• Windup can occur when a control valve saturates or when a control loop is not being used (e.g., select control).

Reset Windup

Time

yysp

c

A

• Note that controller output saturates causing area “A” to accumulate by the integral action.

• After the disturbance returns to its normal level, the controller output remains saturated for a period of time causing an upset in y.

Anti-Reset Windup

Time

yysp

c

• When the manipulated variable saturates, the integral is not allowed to accumulate.

• When control returns, the controller takes immediate action and the process returns smoothly to the setpoint.

Methods for Anti-Reset Windup

• Turn off the integral when a valve saturates or a control loop is not in use.

• Clamp the controller output to be greater than 0% and less than 100%.

• Apply internal reset feedback

• Apply external reset feedback

Industrial Approach

• External reset feedback

• Controller output clamping

• Digitally turn-off integral calculation

Internal Reset Feedback

K c

1/( Is+1)

E(s) C(s)++

F(s)

ssEKsC

ss

sEK

s

sEKsC

sCs

sCsEK

Ic

I

I

c

I

c

Ic

11)()(

1

)(

11

1

)()(

)(1

)()(

Conventional PI Controller

K c 1/IsE(s) C(s)++

• Therefore, internal reset feedback is equivalent to a conventional PI controller.

• It still has windup, but controller output can be clamped.

ssEKsC

Ic

11)()(

External Reset Feedback

K cE(s) C(s)++

1/( Is+1) U meas (s)F(s)

• An extension of internal reset feedback, therefore, it is equivalent to a conventional PI controller.

• When u saturates, windup will cease preventing windup.

• Less windup than clamping, but requires umeas.

Bumpless Transfer

• When a control loop is turned on without bumpless transfer, the process can become unduly upset.

• With bumpless transfer, an internal setpoint is used for the controller and the internal setpoint is ramped at a slow rate from the initial conditions to the actual desired setpoint to order to provide a smooth startup of a control loop.

Comparison of True and Internal Setpoints

Time

Internal Setpoint

True Setpoint

Control Performance With and Without Bumpless Transfer

Time

w/o bumpless transfer

w/ bumpless transfer

Split Range Flow Control• In certain applications, a single flow control loop

cannot provide accurate flow metering over the full range of operation.

• Split range flow control uses two flow controllers (one with a small control valve and one with a large control valve) in parallel.

• At low flow rates, the large valve is closed and the small valve provides accurate flow control.

• At large flow rates, both valve are open.

Split Range Flow Controller

FT

FT

FC

FC

Coordination of Control Valves for Split Range Flow Control

Total Flow Rate

Sig

nal t

o C

ontr

ol V

alve

(%

)

Larger ControlValve

Smaller ControlValve

Example for Split Range Flow Control

AcidWastewater

NaOHSolution

Effluent

FTFT

FC

pHTpHC

RSP

Titration Curve for a Strong Acid-Strong Base System

02

468

10

1214

0 0.002 0.004 0.006 0.008 0.01Base to Acid Ratio

pH

• Therefore, for accurate pH control for a wide range of flow rates for acid wastewater, a split range flow controller for the NaOH is required.

Other Split-Range Flow Control Examples

• When the controlled flow rate has a turn down ratio greater than 9

• See value sizing examples in Chapter 2

Split Range Temperature Control

TT

CoolingWater

Steam

Split-RangeTemperature

Controller

TT TC

RSP

Split Range Temperature Control

0

20

40

60

80

100

Error from Setpoint for Jacket Temperature

Sig

nal t

o C

ontr

ol V

alve

(%

)

SteamCooling Water

Overview

• All controllers that employ integral action should have anti-reset windup applied.

• Bumpless transfer provides a means for smooth startup of a control loop.

• When accurate metering of a flow over a very wide flow rate range is called for, use split range flow control.