Sensor Alarm; Setting The Pid Gains - THORLABS TC200 User Manual

Temperature Controller

If the temperature stabilizes to TEMP SET after having crossed the TMAX threshold less than three times,
the !TMAX! alarm will remain active in the Main Display screen to alert a user to the fact that the system
exceeded TMAX momentarily. The !TMAX! display can be cleared by pressing the SELECT keypad while
in the Main Display screen.

Pressing the SELECT keypad while in the Main Display screen will always clear the TMAX count and resets
the alarm.

Pressing the ENABLE keypad at any time with a TMAX alarm present will disable the heater and clear and
reset the TMAX alarm.

5.10. Sensor Alarm


The TC200 can determine whether a temperature sensing element is short circuited or not installed (open
circuit). If this occurs, the TEMP ACTUAL field in the Main LCD display screen will read !SENSOR! If the
sensor alarm occurs during normal operation, the output will immediately be disabled. The unit cannot
become enabled again until the error is resolved.
The TC200 can only determine shorted or open sensor connections; it cannot determine if the
installed sensor does not match the programmed sensor selection. Care must be taken to ensure that
the settings match the actual sensor used, or damage will occur to the heating system.

A common sense approach to verifying the proper sensor is selected is to simply connect a heating system
to a unit that has been sitting at room ambient temperature for a while. Observe the TEMP ACTUAL value
displayed on the TC200. If the sensor has been properly programmed, the display will show a value
corresponding to the room ambient temperature.

5.11. Setting the PID Gains


The PID gains determine the overall stability and accuracy of the entire heating system. Incorrect values,
especially for the Integral (I) gain, could result in undesired overshoots and instabilities.

A good approach to setting these values is to start with a mid-range value (125) for the P gain and to set
both I and D gains to zero (0).

Enable the heater and observe the response and settling times. Typically, the system will undershoot the
set-point and settle to a value below TEMP SET. Despite the offset, the temperature should remain fairly
stable.

If the temperature offset is too great (greater than 3 to 4°), an increase in P gain might be required.

The offset can now be adjusted out by setting the I gain to a suitable value. Always start with a low value
(typically less than 10). Observe any changes in the system response after each adjustment to I gain. If the
value is too large, you will observe overshoot and oscillations about the set-point. If it is too low, you will
not remove the offset.

After arriving at a value for I gain, apply a Step function to the system by increasing the TEMP SET value
by a few degrees. Observe the response and adjust the gains accordingly.

The D gain is the hardest to observe any influence on, as it generally affects the rate at which the system
responds to disturbances. For most small heater systems being controlled by the TC200, a value of zero
(0) for the D gain is sufficient for good operation.
Rev N, July 31, 2017
ATTENTION
Chapter 5: Stand-Alone Operation
Page 15