Moeller DF51 - PID control; EMC compliance

10/05 AWB8230-1541GB
Braking
25
PID control
In closed-loop control systems – in contrast to open-loop control –
the actual value of the controlled variable is automatically fed back
to the controller. This control circuit consists of a controller and a
controlled system. The controller’s task is to adjust the actual
value to the reference value in as short a time as possible, so that
the difference between reference value and actual value (the
system deviation) is as close to zero as possible.
The PID control is a combination of proportional, integral and
differential components. The control parameters are the
proportional-action coefficient K
P
, the reset time T
N
and the
derivative action time T
V
. Using all three control methods, PID
control meets all basic requirements, such as:
•speed,
•stability,
• static accuracy,
• sufficient damping.
In practice, PI control is often used next to PID control.
P: proportional component
Proportional control responds without delay and proportional to
any system deviation but can not fully match the reference value
so that a deviation always remains. Small values of K (normalizing
constant) dampen the control action (i.e. reduce the rate of
change), while large values result in instability.
I: integral component
In integral control, the rate of change is proportional to the system
deviation. This avoids fluctuations in the actual value due to
overshoot but can not cope with rapid changes. With I-control,
zero deviation (actual value = reference value) can be achieved.
Figure 15: Block diagram of a closed-loop control system
w: Reference input variable (reference value)
e: System deviation (difference between actual value and reference value)
u: Manipulated variable for the frequency inverter
y: Controlled variable (measured or actual value)
Control device: here a frequency inverter with PID controller
Controlled system: here a motor with controlled variable
y
M
3
~
w
eu
+
–
PID
Figure 16: PID control
e: System deviation (difference between actual value and reference
value)
u: Manipulated variable for the frequency inverter
K
P
: Proportional-action coefficient (gain)
T
N
: Reset time
T
V
: Derivative action time
x
d
: Deviation
u
+
+
K
P
T
N
x
d
I
K
P
P
K
P
T
V
x
d
D
e
+
Figure 17: P-control
Figure 18: I-control
K
P
T
V