01/02 AWB8230-1415GB
PID controller
135
The example in the following figure shows a fan control system:
P: Proportional component
This component ensures that the output frequency and the system
deviation are proportional to each other. With PNU A072, you can
specify the proportional gain (K
p
) in percent.
The following figure illustrates the relationship between system
deviation and output frequency. A large value of K
p
results in a
quick response to a change of the system deviation. If, however,
K
p
is too large, the system becomes unstable.
The maximum output frequency in Figure 135 is defined as 100 %.
In PNU A072, you can set K
p
to between 0.2 and 5.0.
I: Integral component
This component results in a correction of the output frequency by
integration of the system deviation. In the case of purely proporti-
onal control, a large system deviation causes a large change in the
output frequency. It follows, then, that if the system deviation is
very small, the change in the output frequency is also very small.
The problem is that the system deviation cannot be completely
eliminated. Hence the need for an integral component.
The integral component causes a continuous adding up of the
system deviation so that the deviation can be reduced to zero. The
reciprocal value of the integration gain is the integration time
T
i
=1/K
i
.
For the DV6 frequency inverters, set the integration time (T
i
). The
value can be between 0.5 s and 3600 s. To disable the integral
component, enter 0.0.
D: Differential component
This component causes a differentiation of the system deviation.
Because pure proportional control uses the current value of the
system deviation and pure integral control the values from
previous actions, a certain delay in the control process always
occurs. The D component compensates for this behaviour.
Differential control corrects the output frequency using the rate of
change of the system deviation. The output frequency can there-
fore be compensated very quickly.
K
d
can be set between 0 and 100 s.
PID control
PID control combines the P, I and D components described in the
previous sections. In order to achieve the optimum control charac-
teristics, each of the three PID parameters must be set. Uniform
control behaviour without large steps in the output frequency is
guaranteed by the proportional component; the integral compo-
nent minimizes the existing system deviation the steady-state and
the differential component ensures a quick response to a rapidly
changing actual value signal.
As differential control is based on the differentiation of the system
deviation, it is very sensitive and also responds to unwanted
signals – such as interference – which can result in system insta-
bility. Differential control is normally not required for flow, pres-
sure and temperature control.
Figure 134: Example of a fan control system
G1: DV6 series frequency inverters
w: Setpoint value
x: Actual value
P1: Controlled variable
B1: Measured value converter
a
Fan
Figure 135: Proportional gain K
p
x: System deviation
w
B1
P1
x
0... +10 V
H
; 4 – 20 mA
G1
a
M
3
~
100
100
50
75
25
0
75
50
25
[%]
f
x
[%]
Kp = 1
Kp = 0.75
Kp = 2
Kp = 0.5
Kp = 0.25
0.2
F
Kp
F
0.5
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