36
8
3.5.2 Valve Setup (Boundless Types) – Fig. 3.4
A ‘boundless’ process controller provides an output that is effectively the time derivative of the
required regulator position, i.e. the COMMANDER 350 signals the regulator, not where to go to
(position derivative), but in which direction to travel and how far to move, by a series of integral action
pulses. Thus, the COMMANDER 350 does not need to know the absolute regulator position and is
unaffected when regulator reaches the upper or lower limit, as determined by the regulator’s limit
switches (giving rise to the term ‘boundless’).
When a deviation from set point is introduced the regulator is driven, for a length of time equivalent
to the proportional step. The regulator is then driven by integral action pulses until the deviation is
within the deadband setting.
…3 SET UP MODE
Proportional
Step
Time
Control
Deviation
Raise
Lower
Time
Integral
Action Pulses
Proportional
Step
Proportional
Step
Integral
Action Pulses
+
–
Fig. 3.4 Boundless Control Action
Calculation for Control Pulses (Boundless Control)
The following calculations are shown for guidance when setting deadband, proportional and integral
values. They can be used to check the suitability of boundless control for a particular actuator/
application.
Minimum 'ON' time of integral action pulses (for a fixed control deviation).
=
Travel Time x Deadband %
% Proportional Band
(in seconds)
Minimum (approximate) time between integral action pulses (for a fixed control deviation)
=
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