MercuryiTC
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Page 149
Derivative action is characterised by an action time. If the measured temperature is changing at
a rate of one proportional band per derivative action time, the derivative action contributes a
signal sufficient to reduce a maximum control output to zero.
Figure 10 Proportional control with integral and differential action
Some control systems may have sufficient control response without using derivative action.
A PID controller can be made to operate in this mode by setting the proportional band (P)
parameter to a positive temperature value, the integral (I) parameter to a non-zero action time,
and the derivative (D) parameter to a non-zero action time.
14.2.6 North American terminology
In North America, a different terminology exists for describing 3-term control:
• Proportional band is replaced by its reciprocal, Gain.
• Integral Action is replaced by Reset. Reset may be specified as a time (as for Integral
Action), or by its reciprocal Repeats Per Minute.
• Derivative Action is replaced by Rate. Rate may be specified as a time, or by its reciprocal
Repeats Per Minute.
14.2.7 PID tables
Two alternative methods can be used to provide PID parameters to a control loop in the
MercuryiTC cryogenic environment controller:
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