CNT-SVX08F-EN
33
PID Control
An alternative way to calculate the proportional and integral gains is by using the following
calculations:
Proportional gain
0.80
output
range
throttling range
-----------------------------------------------------
=
Integral gain
0.20
output
range
throttling range
------------------------------------------------
=
The proportional gain is scaled by a factor of 0.80, so it contributes 80% of the final output. The
integral gain contributes 20% of the final output.
Example
In a duct static pressure system, an actuator can move the inlet guide vanes of an air handler from
0–100%, so the output range is 100— a throttling range of 2.0 in. of water column (wc) is desirable.
A change in pressure of 2.0 in. wc. or more will drive the output from 0–100% or vice versa. The
calculations look like this:
Proportional gain
0.80 output range
throttling range
-----------------------------------------------------------
0.80 100
2.0 in. wc.
-------------------------------
40
=
=
=
Integral gain
0.20 output range
throttling range
-----------------------------------------------------------
0.20 100
2.0 in. wc.
-------------------------------
10
=
=
=
Therefore, based on the desired throttling range of 2.0 in. wc., the starting proportional gain is 40
and the starting integral gain is 10.
Duct static pressure
Inlet guide vane or variable frequency
drive (VFD) position
0–100%
2.0 in. wc.
(0.5 kPa)
40.0 (160)
10.0 (40.0)
Discharge air cooling
Electric/pneumatic
5.0–15.0 psi (34–103 kPa)
20.0°F (11.1°C)
0.4 (4.0)
0.1 (1.0)
Table 7.
Starting gain values for applications
Loop
Output
Throttling
Range
Proportional
Gain
Integral
Gain
