ABB C505 User Manual page 49

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 500 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 500 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.
+
Control
Deviation
–
Raise
Lower
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
Minimum (approximate) time between integral action pulses (for a fixed control deviation)
Integral Action Time x Deadband %
=
2 x % Control Deviation
Duration of the proportional step
% Control Deviation
= 2 x
% Proportional Band
% Control Deviation
Set Point – Process Variable
=
Eng Hi – Eng Lo
% Deadband
Deadband (eng units)
=
Eng Hi – Eng Lo
Integral
Action Pulses
Proportional
Step
Fig. 3.4 Boundless Control Action
(in seconds)
(in seconds)
x Travel Time in Seconds
x 100%
x 100%
Proportional
Integral
Step
Action Pulses
3 SET UP MODE...
Time
Proportional
Step
Time
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