Why does the heater in my Tempe sensor overshoot the desired temperature?
Heater-overshoot may be caused by a delay between the response of the temperature of the sensor body and the PID controlling the heater. The PID is controlling the heater to heat up the sensor head; however, the temperature of the head requires time to respond. PID settings can be adjusted to minimize this overshoot.
A fitting example is using an electric stove to heat a pot of water. The stove can be turned to its highest heat setting but time is still required for the water in the pot to match the temperature of the heating coils. Once the water in the pot reaches the desired temperature and if the stove is suddenly shut off, the pot of water continues to absorb heat from the heating coils on the stove and continues to rise in temperature, before it begins cooling. This is a common issue with regulating heaters with digital controllers.
The stove and water pot scenario is the same effect that is occurring in the sensor head. Generally once the heater reaches the correct temperature, the PID can effectively maintain the temperature of the head. Further, environment can also influence how sensor reacts to the heater. Various environments – e.g., vacuum, radiation, etc. – can cause unpredictable behavior. This is why Colnatec offers the user the ability to tune the heater to specific environments.
The following settings can help prevent heater overshoot:
1) Reducing the P parameter:
The Proportional (P) parameter in the PID controller determines how quickly Eon™ makes adjustments to the heater to compensate for changes in sensor head temperature. Using a higher P value for the PID causes the heater to increase or decrease its temperature more quickly, and can increase overshoot. Reducing this value helps reduce heater reaction time – increasing the time required for the sensor head to react and reducing overshoot. It should be noted that this approach will increase the time required by the heater to reach the predetermined temperature.
2) The max power setting [%]
To determine the max power of the heater, a quick, one-time test can be performed in the same environment where deposition is to occur. In Eon™ software, enter “Manual Mode”. Select “Heater” from the yellow drop-down menu. Increase the power percentage in increments, and allow the heater to stabilize for 3-5 minute each increment. Do this until you reach your desired temperature set point and record the power percentage being applied. Also, instead of starting a 0%, activate the heater and record the %power of the heater when the temperature becomes stable or close to stable. Otherwise, use a general starting point such as 15-20% depending on the set point temperature.
Eon automatically logs the temperature and the power being applied to the heater in the “…\Public Documents\EON_LOGS\HEATER” folder. As an alternative to finding the logs manually, click the “Open Log Folder” button in the “Settings Screen” in the “General Tab” and open the heater folder.
Once a power that allows the heater to reach the specified temperature is achieved, set this as “Max Power” value in Heater Settings. This prevents the heater PID from applying more power than that value. Increase the temperature 5-15 degrees to compensate for any cooling effects that might occur in the chamber.
Remember, although preventing overshoot and stabilizing the heater, these adjustments will increase the time it takes for the heater to achieve a preset temperature.
3) Soak 2 Power [%]/ Soak 2 Time [s]
If the above change does not achieve your desired results, you can adjust the “Soak 2 Power” and “Soak 2 Time” once the target power is known by the prior test. However, I suggest doing the previous first before applying this setting. Pre-deposition power, specifies that you want the heater to reach a specific amount of power, for the specified amount of time, before it goes into PID mode (To maintain its temperature) this process happens before the heater enters PID mode, so you can set a higher Power level during this period of time to increase the speed at which the heater gets to the set point, then it will go into PID mode and regulate the temperature for the set point. However, setting Soak 2 Power to a power percentage too high, will also cause overshoot. However, this should be a onetime occurrence when the heater is activated, then the PID should activate and regulate the temperature.