Heater Loop controls Problemgreenspun.com : LUSENET : PID Controller Tuning : One Thread
At this point I'm going to be pretty vague. I have not been able to access the equipment to find out exactly what we have. Hopefully I have enough information for you to recommend a controls strategy.
We have a piece of packaging equipment which uses a heated plate to seal two pieces of laminate together. The plate needs to be controlled to +/- 2C. The plate is heated by a cartridge heater which is turned off and on by a temperature controller. When the equipment is running continuously the controller works fine. Unfortunately, the equipment is not normally running continuously. When ever stops and starts occur the plate temperature deviates by more than +/- 10C and does not settle in very rapidly. This is fairly high speed equipment and rejecting out of spec material is not really an option.
It seems to me that I need to control during 2 discrete states, running (high energy) and stopped (low energy), and I do not want to wait for the system to re-establish equilibrium after a disturbance. Since the disturbance is always the same and known, is there a way to program it into the controller? I could take a signal from the start/stop switch (PLC).
Is it standard practice to use ON/OFF control on heaters for temperature control? Couldn't tighter control be maintained by varying the voltage to the heaters?
-- Carl J. Hixon (email@example.com), August 02, 1999
If you have a stop/start condition where the plates get 10deg out of spec,then you are going to have to deal with the response time whenever the system starts. Thats purely a function of the thermodynamics of the plate/heater. If you need faster response time to get withing the +/- 2 deg, then you either need a bigger heater or insert a lag at the equipment control so you dont start processing laminates until the plates heat up. You should be able to characterize the plate response with a temperature gauge and use that to adjust the start lag.
-- Jamie Ross (firstname.lastname@example.org), August 04, 1999.
If you are using an on/off controller it would be expected that your suystem would have some degree of over / under shoot. as you increase the heater power to improve its response these effects become more pronounced.
Varying the voltage in proportion to the demand is the right idea. You should consider a PID ("proportional-integral-derivative" mode) controller, which will supply energy to the heater in proportion to its delta from set point. It will taper off energy input as the sensor approaches set point. Such controllers may be tuned to match your process.
This will minimize the occurance of temperature excursions. Your reponse time will probably improve due to the elimination of the problem of settling, since 80 to 90 % of your excursion will be eliminated.
Operating the block at a higher temperature in order to store power between processing periods may very well cause over heating and degradation of the product on a re-start.
However, to eliminate time required for heat up when a load is presented to the heater block, you may want to consider modifying the heater block to emulate the processing load. You may want to try pumping air through the block at a rate that requires the same amount of heat as your sealing process. I recommend pumping air through the block to eliminate stray currents that may cool surrounding portions of the equipment. If this is not a factor, the cheap-and-dirty first cut would be to spray the sealing platen externally with compressed air. (This may be a good first cut as a means of prooving the concept). Your PLC could be programmed to kill air flow, as you've suggested.
-- John Churchill (email@example.com), April 22, 2001.
You could vary the voltage to the heater or you could vary the ON time and the OFF time of the heater. There are temperature controllers that do this and we use them on some of our machines. They work fine for us and I suggest you try them out. Another thing you could do, is have a master control of sorts that "remembers" the heater setting during the "running" stage and the "stopped" stage. When you start up, the machine puts the heater in manual mode and sets the heater power at the "running" level until the temperature is within the +/- 2 degree limit. Then it can switch over to automatic mode.
-- Donald Eric J. Marcelo (firstname.lastname@example.org), July 08, 2001.