5 Function blocks
5.13 DC, PID controller
190
Control Relay easy800
11/11 MN04902001Z-EN
www.eaton.com
Output
The output DC..QV supplies the current manipulated variable as an integer.
You can assign the following operands to output
…
QV:
•
Analog output QA01.
•
Markers MD, MW, MB.
Function of the function block
The controller operates on the basis of the equation of the PID algorithm.
According to this, the manipulated variable Y(t) is the result of the calculation
of the proportional component, an integral component and a differential
component.
PID controller equation:
Y(t) = Y
P
(t) + Y
I
(t) + Y
D
(t)
Y(t) = Calculated manipulated variable with scan time t
Y
P
(t) = Value of the proportional component of the manipulated variable with scan time t
Y
I
(t) = Value of the integral component of the manipulated variable with scan time t
Y
D
(t)= Value of the differential component of the manipulated variable with scan time t
Proportional component
The proportional component Y
P
is the product of the gain (K
p
) and the control
difference (e). The control difference is the difference between the setpoint
(X
s
) and the actual value (X
i
) at a specified scan time. The equation used by
the device for the proportional component is as follows:
Y
P
(t) = K
p
x
[X
s
(t) - X
i
(t)]
K
p
= Proportional gain
X
s
(t) = Setpoint with scan time t
X
i
(t) = Actual value with scan time t
Integral component
The integral component Y
I
is proportional to the sum of the control difference
over time. The equation used by the device for the integral component is as
follows:
￫
The PID controller must be enabled so that it can work, i.e. the
coil DC..EN must be active. If coil DC…EN is not active, the
entire PID controller is deactivated and reset. The manipulated
variable is set to zero.
The coils for calculating the P, I and/or D components must be
active.
Example: If only coils DC…EP and DC…EI are activated, the
controller operates as a PI controller.
￫
The device calculates the manipulated variable every time the
scan time T
c
has elapsed. If the scan time is zero, the
manipulated variable is calculated every cycle.
