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The idea for the PID to control multiple devices simultaneously

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(Sorry for my english)


I have to drive the desired temperature X. In general, the matter would be trivial, if I had one device that generates me the temperature, but unfortunately it is not.

I have available:

1 - unit # 1 heating device with analog input (high power - priority 2)

2 - unit # 2 in heating mode with analog input (low power - priority 1)

3 - unit # 3 in the heating mode of the digital input (on / off - priority 3)

4 - unit # 4 in the heating mode of the digital input (on / off - priority 4)

5 - unit # 2 in the cooling mode of the analog input (with low power, but under certain operating conditions - priority 1)

6 - unit # 3 in cooling high-power digital input (on / off - priority 2)

7 - unit # 4 in cooling high-power digital input (on / off - priority 3)

All these devices can be operated at the same time, of course, only in one mode. For each device there are also relationships that allow their activation. Devices do not have consistent performance - it depends on many factors, which can not measure nor even guess.

I wonder on how to control these devices depending on the value of PID CV (Control Value). The only idea that comes to my mind is to assign to some range of values ​​for the turn on of the devices. I just wonder if the PID will function OK and wonder what to do with the start and the level of "zero". If CV would -100% to 100% is rather negative control must be attached to cooling devices, and positive to heating devices. Zero disables all devices.

I'm worried about controlling devices # 3 and # 4 - I can not turn on / off frequently - they are large chillers.

Tonight I got the idea to control these problematic units. If the PID is saturated by X time I would turn on device #3. Where it will 2X - turn on device #3 and #4. Now I had to figure out a condition for the descent of these aggregates. I suspect also that I need to reset periodically PID saturation and add a sort of hysteresis to include these units.

What do you think? Any ideas?

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