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Help me think about controlling 5 heaters with 3 RTDs
KT88:
Rethinking the control scheme...
- The containers have to have a defined temperature +/-0.5K.
- The power of the heaters H1...H3 is not sufficient.
- H4, H4 have no sensor...
- The issue is the insufficient power of H1...H3.
- Heaters H4, H5 can contribute suffient power to offload H1...H3.
- A loop to keep the power of H1..H3 constantly within their capability (~50% DC) would solve the problem.
- The power of H1...H3 is proportional to their duty cycle.
- That points towards the suggestion of NiHaoMike.
ejeffrey:
--- Quote from: NiHaoMike on September 04, 2020, 12:39:42 am ---Control the outer heaters based on the duty cycles of the inner 3, trying to keep those inner 3 at close to a 50% duty cycle. You'll probably have to use a much slower loop to control them compared to the inner heaters.
--- End quote ---
I think this is the best option. You can then tweak the 50% threshold or the max power of the inner heaters as low as possible without getting too much overshoot. That will keep the outer heaters just below the inner ones.
KT88:
--- Quote ---You can then tweak the 50% threshold or the max power of the inner heaters as low as possible without getting too much overshoot.
--- End quote ---
For (very) low duty cycles the steps (of the ratio) of the PWM can get pretty coarse. To stay away from the extremes you could reduce the power of the inner heaters, if nescesary...
ejeffrey:
--- Quote from: KT88 on September 07, 2020, 03:53:23 pm ---
--- Quote ---You can then tweak the 50% threshold or the max power of the inner heaters as low as possible without getting too much overshoot.
--- End quote ---
For (very) low duty cycles the steps (of the ratio) of the PWM can get pretty coarse. To stay away from the extremes you could reduce the power of the inner heaters, if nescesary...
--- End quote ---
That is a good point, although reducing the max power will increase the time needed to initially reach the setpoint. If this is a problem you could also use a simple delta-sigma modulation of your PWM to get a bit more effective resolution at the low end. This is effectively what PID loop will do: if the ideal setpoint is between two PWM codes, the feedback loop will hop back and forth between two duty cycles. Delta-sigma just does this with a faster digital loop that doesn't have to wait for the slow thermal response.
jbb:
I had two thoughts. First: do you have access to the person who designed the heater assembly? They may have already done some work on this, eg their calculations for heater resistance / power.
Second - and I may get peanuts thrown at me - but I also wonder if we can help this situation with MOAR MATHS!
Can you add some more RTDs for engineering purposes on a small number of development units? Maybe two more on each ‘outer’ heater? This would allow you to characterise a bit more about the heaters.
Then you could do a larger system model which includes your 3 inner zones and 2 (or maybe more...) outer zones. At this point you could do some formal control system analysis to see if the system is observable for the temporary ‘engineering’ sensors. If these are observable, you could then build a mathematical observer into your control system, which would output temperature observations for the missing 2 sensors.
If successful, you would have 5 temperatures for 5 heaters.
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