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Flow temperature ap-
proaches the
setpoint only slowly
Actuator running time
P-band Xp
Example
Basic rule
Integral action time Tn
Electrothermal
actuators
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Setpoint
Flow
temp.
A
B
A Difference between the first pulse and the following pulses is small:
Measure the effective actuator running time (0...100 % stroke) and enter it. If the
control behavior does not considerably improve: Increase P-band Xp
B Long starting pulse followed by many short pulses: Decrease integral action time Tn
The actuator running time must be matched to the type of actuator used.
This setting is important for both 3-position and DC 0...10 V actuators.
If in doubt with 3-position actuators, the setting is to be increased since otherwise the
actuator will not optimally operate in the range between 0 % and 100 % stroke (also
refer to synchronization pulse in subsection 5.7.3 "Control signal").
The P-band Xp is given in K (Kelvin).
If, after a setpoint step, the control deviation equals the P-band, the valve will be read-
justed by 100 %.
With a P-band of 40 K and a setpoint change of 5 K, the valve will be readjusted by
5 / 40 = 12.5 %. Having an actuator with a running time of 150 seconds, this means that
it takes the actuator 18.75 seconds to fully open or close.
If the P-band is increased, the controller will respond less intensely to the same control
deviation. With a P-band of 60 K, for example, the actuator will only take 12.5 seconds
to travel to the fully open or fully closed position.
Increasing the P-band Xp means: The control responds more slowly and the tendency
to oscillate is smaller.
This means:
• The control action is too slow.
Decrease P-band Xp in steps of about 25 %
• The control action is too fast.
Increase P-band Xp in steps of about 25 %
The integral action time Tn is given in seconds.
It indicates how long it takes the controller in the event of a constant temperature devia-
tion to cover the same valve travel as this would be the case with the P-part.
For example, an integral action time of 120 seconds means that in the event of a con-
trol deviation of 5 K in the above example (Xp = 40 K), it takes the mixing valve
120 seconds to travel 2 × 12.5 % toward the fully open or fully closed position (12.5 %
due to the P-part and 12.5 % due to the I-part).
If the integral action time is increased, the control system will respond more slowly.
5.7.3
Control signal
Since the control algorithm uses a stroke model which does not provide control at 0 %
and 100 % respectively, the use of electrothermal actuators is no longer possible as
this was the case with the RVL47...
Boiler Sequence Controller RMK770
5 General functions, fundamentals
Open pulse
Close pulse
Open pulse
Close pulse
CE1P3132en
01.03.2005
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