Temperature controller for Torrefaction

Torrefaction needs a constant temperature about 350 Celsius. So a good controller is needed to control the temperature. The oven which is used is heated by a 3 kW heater. It is very difficult to control the heat in this system because the thermal inertia is very high.

The weakness of a PID controller for this system

PID controller gives a higher output when the error (the difference between set point and reading) is high. Then it's reduced when the system comes close to the set point. If a PID was used for this system, even though the PID output is decreased, the temperature will continue to increase by the stored thermal energy in the heater and the wall. So, in this case the controller output should be turned off or reduced before coming close to the set point. This can be achieved by using a very low value for proportional gain in the PID controller. But it will not work well when dropping the temperature from the set point. Because the PID output will not be increased until a large error value is occurred. So there should be a developed algorithm other than PID.

The algorithm used to get the system to the set point 


A PID was used to control the system when the error value is small.But, to get the system to the set point, a separate algorithm was used.
At the beginning the system is in the room temperature. So the maximum power is given by the heater until the error is reduce by 30% from the maximum error. 
Ex: If the set point is 350℃ and room temperature is 25℃
The maximum error that can be occurred = Set point - Room temperature
                                                                  =350℃ - 25℃ = 325℃
30% maximum error = 325℃*0.3=97.5℃
Heater turns off at 25℃+97.5℃=122.5℃

So the heater will be giving maximum power until the system comes to 122.5℃. Then the heater is turned off. The the temperature will be continue increasing by the heat in the walls of the oven. After some time the thermal energy stored in the walls will be finished and the temperature will start to drop because of heat loss to the surrounding environment. When the temperature started to drop in this way the heater is turned on again and wait until the temperature is increased by 5℃ and the heater is turned off. Then also the temperature will continue to increased in the same way. This method is repeated until the set point comes close. When the temperature is less than 10℃, a the heater is controlled by a PID controller. For a small range of the error value, a PID can be used.



The algorithm is written with LabVIEW. The LabVIEW code can be downloaded here
https://drive.google.com/open?id=1Ebh4QIDKWT1qKAKsFlA8IvFdJn1s5KL8 

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