Hi
I have a problem I need to solve. The system consists of a hot plate type heater that is either on or off. The bag of fluid lays on top of it. There is a sensor that the bag of fluid rests on and checks it temperature. There is also a sensor that checks the hot plate temperature. The bag of fluid needs to be heated to and kept at 40 C. It cannot go above 41C. Fluid may start as low as 1C with the heater element off. The rise time from start (can be any temp) to set point (40 C) can be no longer than 1/2 hr. I was researching a pid controller for the heater, but am concerned about any overshoot. Some articles I've read about the MPC (Model Predictive Controller) makes it sound like it may be better suited to this application. If someone has worked on a problem like this, I would like to hear about your experiences.
Thanks...
I used the PID controller for a drying aplication. You can try it for +- 1 degree. Also, you can use PLC or MPC.
Dear Mr. Sousa,
In order to enhance the heat transfer between your hot plate and your bag, you may use a thermally conductive paste (such as silicone from Corning). Should the plate material have not a high thermally conductivity, I would put a copper plate over it. This would slightly increase the thermal inertia of the system, but would insure a uniform contact temperature.
Then, I suggest you use a high quality PID controller to fulfill your temperature requirements.
Regards and good luck
You would be better off if you brake the problem into separate requirements.
1. In the setup that you are proposing, in order to protect the fluid from overheating, you can add another temperature sensor into the bag, that sensor connected to an additional, ON/OFF controller, included in series into the heating current, and set to 41 oC. The fluid in the bag has to be stirred, e.g. by kneading.
2. However, Innovators often constrain themselves unnecessarily, by saying "but it has to be done in this particular way." Please revise the requirements, eliminating the unnecessary ones and leaving only the main goal. The requirement to use the plate would be the first suspect, but not the only one.
Your real need seems to be: "Warm up the fluid in the bag to 40 oC in less than 30 minutes, without overheating.". If so, I would drop that bag into a bucket of water pre-heated to 40 oC and kept at that temperature by an immersion heater, stirrer, thermocouple and controller, and would knead that bag, if necessary.
If you add current control, or PWM, a PID controller might be used.
If the fluid is used incrementally, a flow-through heater at the point of use may be more appropriate than heating the entire bag. A lot easer to control.
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