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DETAILED PARAMETER DESCRIPTION
6.3.5 Special Function Parameters - P500 ... P599
6.3.5.1 PID Introduction
CFW-08 is fitted with the PID regulator that can be used for closed
loop process control. This function acts as a proportional, integral and
derivative regulator, superimposed on the normal inverter speed control.
The speed will be changed in order to maintain the process variable
(the variable that should be controlled - for instance: water level of a
container) at the desired value, set in the setpoint.
This regulator can, for instance, control the flow in a piping system.
The setpoint (flow) can be given by the analog input AI2 or through
P525 (digital setpoint), and the flow feedback signal is given at analog
input AI1.
Other application examples: level control, temperature control,
dosing control, etc.
Figure 6.25 shows the block diagram of the PID regulator.
The feedback signal must be sent to the analog input AI1.
The setpoint is the value of the process variable at which the operation
is desired. This value is entered as a percentage and is defined by the
following equation:
6.3.5.2 Description
set
point (%) =
setpoint (UP)
x P234 x 100%
full scale os the used sensor (UP)
where both the setpoint and the full scale value of the sensor are given
by the process unit (°C, bar, etc.).
Example: a pressure transducer (sensor) with ouput 4 - 20mA
and full scale of 25bar (i.e., 4mA=0bar and 20mA=25bar)
and P234=2.00. If the control of 10bar is desired, you should enter the
following setpoint:
setpoint (%) =
10
x 2 x 100% = 80%
25
The setpoint can be defined via:
- Keypad: digital setpoint, parameter P525.
- Analog input AI2 (only available in the CFW-08 Plus): the
percentage value is determined by considering P238, P239 and
P240 (see description of these parameters).
The parameter P040 indicates the value of the process variable (feedback)
in the scale selected at P528, that is set according to the following
equation:
P528 =
full scale value of the used sensor
P234
Example: Consider the data of the example above (pressure sensor of
0-25bar and P234=2.00) . P528 must be set to 25/2=12.5.
The parameter P040 can be selected as the display default parameter
P205=6.
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