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Process control instructions
30.1 Explanation of the operation of the PID instuctions
parameter
(Kp, Ti, Td, Ts)
+
Control
set
A/D
input
value
-
(SP)
measurement value (PV)
POU body
The above POU body represents the standard control loop. The control input is determined by the user (e.g.
desired room temperature of 22°C). After the A/D conversion the set point value (SP) is entered as the input
value for the PID processing instruction. The measured process value (PV) (e.g. current room temperature) is
normally transmitted via a sensor and entered as the input value for the PID processor. F355_PID calculates the
standard tolerance e from the set point value and the process value (e = set value - measured value). With the
parameters given (proportional gain Kp, integral time Ti, ...) a new output value (MV) is calculated in increments
set by the sampling time Ts. This result is then applied to the actuator (e.g. a fan that regulates room
temperature) after the D/A conversion. The analog section represents the system's actuator, e.g. heater and
temperature regulation of a room.
A PID operation consists of three components:
1. Proportional part (P part)
A proportional part generates an output that is proportional to the input. The proportional gain Kp determines by
how much the input value is increased or decreased.
A proportional part can be a simple electric resistor or a linear amplifier, for example.
The P part displays a relatively large maximum overshot, a long settling time and a constant standard
tolerance.
+
Set value (SP)
-
e
1
0
2. Integral part (I part)
938
F355_PID
e
PID
output
D/A
calculation
value (MV)
A/D
Standard
tolerance e
Proportional
gain (Kp)
Measurement
signal (PV)
MVp
2
1
0.5
0
t
output
quantity
analogue
section
Output (MVp)
Kp=2
Kp=1
Kp=0.5
t
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