34
4.2
Motorized Valve Control
Information.
This page is not displayed if position proportioning or boundless control is not enabled on either of the controllers.
• Motorized valve control with or without feedback – position-proportioning (with feedback) or boundless (without
feedback).
• Ratio and bias settings – can be applied to adjust the range of valve travel (position-proportioning only).
• Deadband setting – adjustable to minimize hunting of the motorized valve.
4.2.1
Motorized Valve with Feedback (Position-Proportioning) – Fig. 4.3
Proportional
Step
Time
Control
Deviation
Raise
Lower
Time
Integral Action
Pulses
Proportional
Step
Proportional
Step
Integral Action
Pulses
+
–
P.I.D. Output
Desired Valve Position
Engineering Range
Ratio Setting
(0.01 to 10.00)
Bias Setting
(within Eng. range)
100
50
0
1.00
0.50
–25%
0%
Ratio
Bias
Proportional
Bandwidth*
80
20
0%
100%
0%
Valve Travel
Limits
75%
25%
Valve Travel
Limits
50%
0%
Valve Travel
Limits
Valve
Open
Valve
Closed
4.2.2
Motorized Valve Control without Feedback (Boundless) – Fig. 4.4
A ‘boundless’ process controller provides an output that is effectively the time derivative of the required regulator position, i.e. the
controller 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 controller does not need to know the absolute regulator position and is unaffected when
the regulator reaches the upper or lower limit, as determined by the regulator’s limit switches (giving rise to the term ‘boundless’).
In this system, the final regulator must act as an integrator, integrating both the raise and lower pulses in direction and duration
so that the final position of the regulator reproduces the required 2 or 3 term control function, and must remain stationary
indefinitely in the absence of raise or lower commands.
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.
…4
CONTROL CONFIGURATION LEVEL
Fig. 4.3 Position-Proportioning Schematic Diagram
Fig. 4.4 Boundless Control Action