Instruction Manual

Control Primer
7-18
Output
Time
A process loop with a loop gain of one would oscillate continuously. The oscillation can be stopped
by reducing the gain of the loop. Reducing the gain also increases the damping. The most common
damping used in the process industry is 1/4 amplitude damping, refer to Figure 7-16. A proportional
type controller with a gain of 0.5 would be 1/4 amplitude damped.
Output
Time
These are straight forward examples of various types of control choices - there are many more. The
best choice will depend on the process characteristics and the control objectives (eg. hold a level very
close to setpoint or let it drift up and down while holding the flow out of the tank as consistent as
possible).
Damping
Response Curve
for a ¼ Amplitude
Damping
Response Curve
for an Over-
Damped
Response
Figure 7-15
Figure 7-16