Version 2.02 15.05.2007 Function of VS 3000
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3.2.5 Control algorithm with combined controller
Low pressure detected by an A/D converter is compared with the setpoint:
Control error = Actual value (t
0_Act
) − setpoint (t
0_Setp
)
Depending on the control error, a controller output is computed and used as a 0 to 10 V signal to control com -
pressor speed. Since the controller operates as a PI controller, both P and I actions are computed.
Computation of P action:
Paction=
Control error
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Computation of I action:
The P action acts immediately on compressor speed in response to change of pressure. The remaining control
error is minimized by stepless increase or decrease of the output signal (ramping). Ramp actuating speed (I ac-
tion of controller) is a function of the control error. If ramp speed is too high, it results in repeated overshoot of
low pressure.
If ramp speed is too low, it results in long time delay for the low-pressure setpoint to be attained.
The controller is then too slow. Ramp speed can be adjusted by a definable adjusting difference (Menu: 3-2-1 -
Adjust. Diff. to match the I action to the system.
A positive adjusting difference results in faster control correction, a negative adjusting difference results in
slower control action. The I action is attenuated in order to prevent hunting of the controller at low part load.
If only the variable-speed compressor is running, the output signal (0 to 10 V) is less than 50% and the control
error is smaller than the neutral zone, the compressor pack will operate at low part load.
The I action is then
Iaction=
Controlerror + Adjustingdifference
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The pressure change is taken into account when the control error is small. If the control error is positive, pres -
sure is falling and is less than the setpoint plus neutral z one in part-load operation or half the neutral zone at
load, the I action will not be changed.
The I action is also not changed when the control error is negative, pressure is rising and is greater than the set-
point less neutral zone in part-load operation or half the neutral zone at load. So as to prevent speed increasing
too quickly, the I action increase is limited to a maximum of 10% of the output signal (0 to 10 V).
With control errors greater or less than 1.5 times the neutral zone, the compressor is operated at maximum or
minimum speed. So as to avoid trouble, the speed setpoint is increased at a ramp speed limited to 2 V/s. Re-
duction of the speed setpoint is not ramped.
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