Pid Control - IMO HD2 IP Series Instruction Manual

HD2 IP55 High-ingress Protection Series

PID control

5.5.15
PID control, a common mode for process control, is mainly used to adjust the inverter output
frequency or output voltage by performing scale-division, integral and differential operations on
the difference between feedback signal of controlled variables and signal of the target, thus
forming a negative feedback system to keep the controlled variables above the target. It is
applicable to flow control, pressure control, temperature control, and so on. The following is the
basic schematic block diagram for output frequency regulation.
Introduction to the working principles and control methods for PID control
Proportional control (Kp):
When the feedback is different from the reference, the output will be proportional to the
difference. If such a difference is constant, the regulating variable will also be constant.
Proportional control can respond to feedback changes rapidly, however, it cannot eliminate the
difference by itself. A larger the proportional gain indicates a faster regulating speed, but a too
large gain will result in oscillation. To solve this problem, set the integral time to a large value
and the differential time to 0, run the system only with proportional control, and then change the
reference to observe the difference (that is, static difference) between the feedback signal and
reference. If the static difference occurs in the direction of reference change (such as reference
increase, where the feedback is always less than the reference after system stabilizes), continue
increasing the proportional gain; otherwise, decrease the proportional gain. Repeat this process
until the static difference becomes small.
Integral time (Ti):
When feedback deviates from reference, the output regulating variable accumulates
continuously, if the deviation persists, the regulating variable will increase continuously until
deviation disappears. Integral regulator can be used to eliminate static difference; however, too
large regulation may lead to repetitive overshoot, which will cause system instability and
oscillation. The feature of oscillation caused by strong integral effect is that the feedback signal
fluctuates up and down based on the reference variable, and fluctuation range increases
gradually until oscillation occurred. Integral time parameter is generally regulated gradually from
large to small until the stabilized system speed fulfills the requirement.
Derivative time (Td):
When the deviation between feedback and reference changes, output the regulating variable
which is proportional to the deviation variation rate, and this regulating variable is only related
to the direction and magnitude of the deviation variation rather than the direction and magnitude
of the deviation itself. Differential control is used to control the feedback signal variation based
on the variation trend. Differential regulator should be used with caution as it may easily enlarge
the system interferences, especially those with high variation frequency.
When frequency command selection (P00.06, P00. 07) is 7, or channel of voltage setup
(P04.27) is 6, the running mode of inverter is process PID control.
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Basic Operation Instructions