5 Function blocks
5.13 DC, PID controller
Control Relay easy800
11/11 MN04902001Z-EN
www.eaton.com
191
Y
I
(t) = K
p
x T
c
/ T
n
x [X
s
(t) - X
i
(t)] + Y
I
(t-1)
K
p
= Proportional gain
T
c
= Scan time
T
n
= Reset time (also known as integration time)
X
s
(t) = Setpoint with scan time t
X
i
(t)
= Actual value with scan time t
Y
I
(t-1) = Value of the integral component with scan time t - 1
Differential component
The differential component Y
D
is proportional to the change in the control
difference. So as to avoid step changes or jumps in the manipulated variable
caused by the differential behaviour when the setpoint is changed, the
change of the actual value (the process variable) is calculated and not the
change in the control difference. This is shown in the following equation:
Y
D
(t) = K
p
x T
v
/ T
c
x (X
i
(t-1) - X
i
(t))
K
p
= Proportional gain
T
c
= Scan time
T
v
= Rate time of the control system (also called the differential time)
X
i
(t)
= Actual value with scan time t
X
i
(t-1) = Actual value with scan time t - 1
Manual mode of the PID controller
A value must be present at the MV input in order to set the manipulated
variable directly. If the coil DC…SE is activated, the value at MV is
transferred directly as manipulated variable QV. This value is present for as
long as the DC…SE coil is activated or the value at the MV input is changed.
If the coil DC..SE is no longer triggered, the control algorithm is active again.
Memory
96 + per
4 Byte/Constant at function block input
Cycle time [
μ
s]
easy800 standard
easy800-SWD
P
84
30
I
96
30
PI
99
31
PD
96
31
PID
111
34
￫
Extreme changes in the manipulated variable can occur when
the manual manipulated variable is transferred or deactivated.
￫
If the function block is running in UNI (unipolar) mode, a
negative signed manipulated variable value will be output as the
value zero.
