Question
Why does it take so long to settle to the setpoint temperature?
Answer
Long settling times arise from a thermal system being either over or underdamped. If your system is underdamped (it cycles several times before settling) then increase the integrator time constant. If your system is overdamped (it never overshoots, just takes a long time to reach the setpoint) then decrease the integrator time constant and the proportional gain.
Another reason the settling time may be too long is the location of the sensor. If it is far from the thermoelectric (or resistive heater), during the time the thermal change takes to propagate to the sensor, the controller is still driving hard. When the sensor starts to feed back the thermal change, the system has already overshot the setpoint. The actual temperature will oscillate around the setpoint.
If current limit is set too low, the system may take awhile to settle, too. Initially, taking the system from ambient to setpoint, if the system has large mass, the controller will see a large feedback error signal and drive to the current limit. If the limit is set quite a bit lower than the capacity of the thermoelectric, the change induced by the thermoelectric may be small and the large feedback error signal continues. Increase the current limit (but stay below the damage threshold of the thermoelectric) to see if that brings the system to temperature more quickly.
Another limiting factor may be the resistance of the thermoelectric or resistive heater. Wavelength temperature controllers are current sources. They will drive output current as dictated by the difference between the setpoint and feedback signal. If the resistance of the actuator is too high, the system may come up against the compliance voltage limit which limits the output drive current. Calculate the resistance of the thermoelectric from the V – I curve produced by the thermoelectric manufacturer. Multiply the current limit setting by this resistance to see what voltage will be developing over the thermoelectric. If this exceeds the maximum compliance voltage of the controller (listed in the Electrical Specifications table), the power delivered to the thermoelectric could be inadequate to accomplish a fast time to temperature.
If the thermoelectric or resistive heater is too small for the load, the system will take time to achieve a stable temperature. Evaluate the thermal mass of the load for the desired temperature differential and choose a thermoelectric that meets or exceeds this requirement.
Another reason for long settling times when using a resistive heater could be the ambient temperature. The setpoint should be at least 10°C above ambient because the unipolar controller can only heat the load. When setpoint is exceeded, the controller turns off and relies on ambient temperature to cool the load. Load cooling can be slow with less than 10°C separation between setpoint and ambient temperatures.