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Sections: Definitions | Time Constant Delay | Transfer Lag | Transport Lag | Relation

Relation of Process Characteristics vs. Mode of Control

Factors. According to the nature and difficulty of the process reactions, the following are the factors that affect the choice of control modes.
1. Accuracy of Control.
2. Process Characteristics., which include:
a. Time Constant Delay
b. Transfer Lag
c. Transportation Lag / Dead Time d. Size of Disturbance
e. Speed of Disturbance.

Hereunder is the matrix of the modes of control versus process characteristics

MODE OF CONTROL	PROCESS REACTION DELAY	TRANSFER LAG	DEAD TIME	SIZE OF DISTURBANCE	SPEED OF DISTURBANCE
ON-OFF	Long only not short	Very short	Very short	Small	Slow
PROPORTIONAL	long or medium not too short	Moderate	Moderate	Small	Slow
PROPORTIONAL INTEGRAL	Any	Moderate	Moderate	Any	Slow
PROPORTIONAL DERIVATIVE	Long.medium not short	Moderate	Moderate	Small	Any
P-I-D	Any	Any	Any	Any	Any

For On-Off, a short PRD will cause excessive overshoot and undershoot, while a long PRD would generate slow response.

For Proportional, a long or medium PRD will reposition the final correcting device as controlled variable approaches the setpoint after a disturbance. A long tranfer lag, however, will cause cycling. If the lag is too great, by the time the controller realizes that the controlled value is returning to the setpoint, it has already permitted too much energy to enter the process.

For Proportional-Integral, it can handle any PRD and size of disturbance because the integral part reportions the final correcting devide until the setpoint is attained, no matter how large the load change.

Here are the Proportional-Integral Mode of Control setting with respect to Proportional Bands and Reset Rates.

PROCESS CHARACTERISTICS		CONTROLLER ADJUSTMENT
PRD	TOTAL LAG	P.B.	RESET RATE
Slow	Slow	Narrow	Fast
Slow	Medium	Medium	Slow
Fast	Slow	Medium	Fast
Fast	Medium	Wide	Slow

For Proportional-Derivative, narrow proportional band equates to short PRD that it can cause overshoot and cycling. It can handle rapid load change because of the action of overcorrection. It can also handle moderate to long PRD using a narro proportional band to keep the offset small.

For Proportional-Integral-Derivative, for long Transfer lag and Dead Time, use wide proportional band so that only a small proportion of the corrective action after a disturbance is dur to the proportional part. When derivate control is added, the overshoot and recovery time are lessened after a disturbance in a system with Transfer and Transportation Lags.

Delays/Lags

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