TE Technology TC-48-20 - Manual (Page 25)

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3.0 Controller Tuning
This tuning method follows the Ziegler‐Nichols closed‐loop tuning principals. Briefly, the controller will first
be set to a high proportional bandwidth setting with no integral or derivative function (integral gain and
derivative gain = 0). Then, the bandwidth is gradually decreased until the temperature approaches set point
and a small, sustained oscillation in temperature is observed. The other tuning parameters are then
readjusted based on the time period of the temperature oscillation (natural period) and the proportional
bandwidth needed to cause this oscillation.
Improper tuning of this temperature controller can lead to excessive thermal cycling and/or
overheating of the thermoelectric device, either of which are known to reduce the lifetime of any
thermoelectric device. Care should be taken to prevent the temperature of the thermoelectric
device from going beyond the range specified by the device manufacturer. Care should also be
taken so that any thermal cycling of the thermoelectric device is a result of changes in the
controller’s set‐point temperature and not instability at a given set point due to improper selection
of the tuning variables.
3.1
Set the desired control temperature in the SET TEMPERATURE menu.
3.2 Set the PROPORTIONAL BW to 20. This is just a starting value; the goal is to determine the proportional
bandwidth at which the temperature of the TE device will first begin to oscillate. The bandwidth needs to
be adjusted in incremental steps and allow the controller to reach steady state between each adjustment.
You might be able to start with a smaller bandwidth, depending on the system, thereby reducing the time it
takes to determine the proper bandwidth for good control stability.
3.3 Set the INTEGRAL GAIN and DERIVATIVE GAIN to zero.
3.4 Decrease the PROPORTIONAL BW incrementally, allowing the TE device to reach steady state at each
increment, until the temperature of the TE device begins to oscillate. The system temperature will usually
begin to oscillate before it actually reaches the set point temperature; this is normal. You can decrease the
bandwidth in initial increments of 5 °C or perhaps even more. Then, once an initial oscillation is observed,
increase the bandwidth in small increments until the oscillations are barely detectable.
Measure the time period of oscillation in minutes. This will be used to determine the INTEGRAL GAIN setting
below.
3.5 Multiply the current PROPORTIONAL BW setting by 2.2 and enter this as the new bandwidth. The system
should now maintain a steady state temperature near the set point.
3.6 The integral gain is calculated as follows: I = 0.54/T where I is the integral gain and T is the time period, in
minutes, determined in section 3.4 above. Enter this value in the INTEGRAL GAIN setting.
3.7 The derivative gain is often times difficult to use and might cause more trouble than it is worth. If you are
not experience with process control, you might be better off leaving the DERIVATIVE GAIN set to zero.
If you decide to use derivative gain, the other control parameters should be adjusted first.
a) Instead of multiplying the initial proportional bandwidth setting by 2.2, multiply it by 1.7, and
enter this as the new PROPORTIONAL BW setting.
b) Calculate the integral gain as follows: I = 1.2/T, and enter this into the INTEGRAL GAIN setting.
c) Calculate the derivative gain as follows: D =0.075 x T, and enter this into the DERIVATIVE GAIN
setting.
3.8 The control parameters are approximate settings. Further adjustments might be needed.
Tuning Example using proportional bandwidth and integral gain:
A. Suppose that smallest PROPORTIONAL BW setting that causes oscillation was determined to be 2.1 °C.
B. It was then observed that the natural period of this oscillation was 2 minutes.