Suzhou Lily Tech Co., Ltd.
Page 8, Total 8 Pages
Dimension
Apendix: PID control
Auto Tuning is able to find PID
parameters (Kp/Ti/Td, U30/31/32) automatically for most control system
1. Set power rate during auto tuning (U93).
2. Set set-point during auto tuning (SP, T11).
When not in set mode, keep
and
depressed simultaneously for 5 seconds to
enter into self auto tuning mode: LCD will
display “Pid”. Press
to start tuning. Then
LCD will display “At”.
After the room temperature (PV) reached SP, it
will vibrate around SP for several times.
The controller will calculate the PID parameters
(Kp/Ti/Td, U30/31/32) according to these PV
data. And the “At” display disappears.
Note:
a.
PID control, or hysteresis control?
Try hysteresis control 1
st
(T10 = 1):
⚫
Now is full power rate heating. If PV is not able to reach SP within reasonable time, increase heater
power rate.
⚫
If PV overshoot to SP
≤ 0.2
℃
, or over falling to (SP
– T12) ≤ 0.2
℃
: auto tuning is not able to get the PID
parameters correctly.
⚫
If all are ok, try PID auto tuning.
Try auto tuning (T10 = 2, start auto tuning):
N
ow the heating power rate is U93 percent of the heater’s power rate. Default U93 = 50.
If PV is not able to reach SP within reasonable time, increase U93 value, or increase heater power rate.
If the overshoot or over falling to SP
≤ 0.2
℃
, auto tuning is not able to get the PID parameters correctly.
The following will also make auto tuning failure, or get wrong PID parameters:
⚫
The PV vibration amplitudes differ too much.
⚫
The PV vibration periods differ too much.
⚫
The PV changes not smoothly (there are temperature interference, like opened door at some moment).
b.
Try to get best parameters
If make 2 auto tuning, the tuned PID parameters are similar, we could use the average data for application.
If the 2 tuned data are not similar, auto tuning the 3rd times.
When the environment temperature, or object inside (empty box and box with goods inside are not same
system), or air flow speed inside changed greatly, it needs auto tuning again.
c.
About PTC heating element
PTC has a Curie point parameter. When PTC’s temperature is high than Curie point, its power rate will be
reduced to almost zero, the PID control will fail.
With higher Curie PTC heater, with good dissipator for heater, with high air flow speed around heater to avoid
the failure.
d.
Manual tuning principle
Manual tuning could revise the control result. Kp = U30, Ti = U31, Td = U32.
Increase Kp, increase the speed of control, but more overshoot and bigger vibration.
Increase Ti, get more accurate temperature, but need longer time to reach SP after stating heating.
Increase Td, fast reaction to control, but add more “noise” to the control result.
