ECKELMANN VS 300, Operating Instructions Manual

Page: 36 / 172

Function of VS 300
28 Doku-Version 2.05 - Firmware 2.10 - 15. September 2017
4.3.7 Control algorithm with combined controller
Low-pressure detected by an A/D converter is compared with the setpoint (see section 4.3.10 Determining the
Low-Pressure Setpoint. The following relationship applies:
Control error
Depending on the control error, a controller output is calculated for control of compressor speed as a 0-10 V sig­
nal. Since the controller works as a PI controller, a P and I action is calculated.
‐ Calculation of P action:
P
Action
control
‐ Calculation of I action:
The P action acts immediately on the speed in response to a change of pressure. The remaining control error
is minimized by continuously increasing or decreasing the control signal (ramp function). Ramp speed (I action
of controller) is a function of the control error.
An excessively high ramp speed results in continuous overshoot of suction pressure. An excessively low ramp
speed results in the suction pressure setpoint only being attained after a lengthy time delay.
In that case the controller is too slow. Ramp speed can be adjusted by means of the Adjust Diff. parameter to
match the I action to the system.
I
Action
I
Action
c
A positive control differential results in steady state being obtained faster, a negative control differential results
in slower control action. The I action is damped as a means of avoiding control swing at low part load. The
compressor pack operates at lower part load if only the variable-speed compressor is running, the control sig­
nal (0-10 V) is less than 50% and the control error is less than the neutral zone.
I action is then
I
Action
I
Action
1
8
control error
2
adjust diff.
20
The pressure change is taken into account for small control errors. No change of I action takes place if the con­
trol error is positive, pressure is falling and is less than the setpoint plus neutral zone in part-load operation or
half the neutral zone in full-load operation. I action is also not changed when the control error is negative, pres­
sure is rising and is greater than the setpoint less the neutral zone in part-load operation or half the neutral zone
in full-load operation.
In order to avoid speed being increased too fast, increase of the I action is limited to a maximum of 10% of the
control signal (0-10 V). With control errors above or below 1.5 times the neutral zone, the compressor is run at
maximum or minimum speed.