Cryo-con Model 24C
Appendix D: Tuning Control Loops
Various methods of manually tuning the controller are described below.
Manual Tuning Procedures
Manually tuning a PID control loop is relatively simple. It is greatly assisted by use of a
data-logging program, such as the Cryo-con utility software package described in the
Cryo-con Utility Software
section.
Ziegler-Nichols Frequency Response Method
This method is based on the assumption that a critically damped system is optimal
and the fact that stability and noise must be traded for response time. It requires
driving your system into temperature oscillation. Care should be taken so that this
oscillation does not cause damage.
Enable the Over Temperature Disconnect feature of the controller if there is concern
over possible damage from overheating.
1. Enter a setpoint value that is a typical for the envisaged use of the system.
Select a heater range that is safe for the equipment. Set initial PID values of
Pgain=0.1, Igain=0 and Dgain=0.
2. Engage the control loops by pressing the
Control
key.
3. Increase the Pgain term until the system is just oscillating. Note the Pgain
setting as the Ultimate Gain,
Kc
, and the period of oscillation as the Ultimate
Period,
Tc
.
4. Set the PID values according to the following table:
Control Type
Pgain
Igain
Dgain
P only
0.5*
Kc
0
0
PI only
0.4*
Kc
0.8*
Tc
0
PID
0.6*
Kc
0.5*
Tc
0.85*
Tc
5. Wait for the system to stabilize. If the resultant heater power output reading is
less than 10% of full scale, select the next lower heater range setting. A range
change will not require re-tuning.
i
Note:
In systems where there is high thermal noise, including
cryocoolers, a Dgain value of zero is often used. The Dterm is a
derivative action, which can introduce additional noise into the
control process.
Alternate Methods
There are various other methods to manually tune PID loops. Most are based on
graphical techniques and all use a stimulus-response technique.
For further reading:
Automatic Tuning of PID controllers
Instrument Society of America
67 Alexander Dr
PO Box 12277
Research Triangle Park, NC 27709
158
