What would cause an unstable reading on a temperature controller?

[CuteQ]

I have a temperature controller connecting to a heater mat, it takes the reading of the heater mat temperature and also controls steady state relay to switch on or off the heater mat.

The controller’s set point is 90 degreeC, when the temperature reaches around 90 the reading starts to jump up and down a lot, one second it reads 90.6degreeC the next it reads 98degreeC. I doubt the heater mat will heat up or cool down 8 degree in just a couple of seconds?

The temperature controller is set to auto-tune. What I expect to see is the displayed temperature changing slowly and showing the true temperature of the mat.

Can you please help?

Thanks

Michelle

 

[KiwiMace]

If this is an on/off control, your temperature sensor might be reading the heating wire temp and jumping up and down at set point as the controller switches to maintain set point.

The auto tune function does not work well where the heat capacity is low (fast changing) and on/off control is used. Look up previous threads on auto-tune.

 

[waross]

Try a second temperature sensor connected to a second instrument to see what is happening. Alternately, put an appropriate meter on the sensor leads to see if the sensor is indicating abrupt temperature swings or if the problem may be in the controller.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[CuteQ]

Thanks.

It's a PID controller, that's why it's set to auto-tune.

 

[Compositepro]

If it only happens near the set temperature, I would suspect electrical interference between the heater current and the temperature sensor. Perhaps some leakage through the insulation. Is the grounding working properly?

 

[waross]

Pid controllers have probably been around since before you were born. Auto-tune is a recent feature. PID controllers can and do work with auto-tune turned off.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Turn off the integral and derivative and see how it works as a proportional controller. If that helps, add in a little reset (integral).

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

It is electrical interference between the power and the sensor. There is a voltage offset being created in the sensor when power is applied. This makes it unnoticeable while the controller is calling for 100% but once PWM has started you're seeing that offset difference between the ON and OFF states.

This has nothing to do with the auto-tune as auto-tune doesn't have anything to do with the display of the temperature, i.e. the reading of the temperature value.

What technology is the sensor?

How exactly is the sensor wire routed with respect to the heater power cable?



Keith Cress
kcress -

http://www.flaminsystems.com

 

[CuteQ]

Thanks!
I will try everything that has been suggested. I checked controllers (with same configuration) on other plants, they were pretty stable.

I can’t do anything just yet, because the plant is running.

They are K type thermocouples. The heater mat came with thermocouple cables and power cable. Normally we use helaglass sleeving to keep thermocouple cables and power cable together. That’s how it’s done on all the other plants, it seems I’m only having problems with this one. I might try to use a different heater mat.

 

[waross]

Consider that you may have a bad controller.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

The controller may well be coming into its proportional band as it nears setpoint and this change in mode from 100% duty or 0% duty to some modulated intermediate value is the source of the interference.

Is the supply voltage stable or is it being modulated by the controller? You will probably need a scope to see this although a fast meter might pick up fluctuation caused by an integral cycle controller. We recently had some serious problems with burner controls caused by the I.C. controllers in our gas chromatographs modulating the supply at the DB. Simple to fix once we knew what we were chasing but took a bit of finding.


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