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I got the task to make a control system for a air conditioning unit which I guess needs 4 PID loops. One each for heating, cooling, humidification and dehumidification.

Only two loops will be active at the same time.

The product overview for Samba SM35-J-TA22 say "Auto-tune PID, 2 independent loops ". Does this mean that it is not possible to have 4 loops in the program?

I would appreciate a quick reply!

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  • MVP 2023

To answer your question - yes, the Samba only does two loops.

You may want to reconsider whether you need PID at all for an HVAC system.  Often people think they need PID control when on/off control will do and is much simpler.  

You're probably trying to control to 1% humidity, which is actually impossible because most humidity sensors are accurate to +/- 2% RH.  In addition, it probably takes a while for the environment to react to the humidifier or dehumidifier being turned on, so if you turn it off when the setpoint is reached (on/off control) it probably won't overshoot.

Ditto on the temperature.  There are a lot of "probably"s in my comment.  Let me know if I've made incorrect assumptions about what you're doing.

Joe T.


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Thanks Joe,

It will be for a system to control temperature and humidity in archives. 18 +/- 2 deg C and 35% +/- 5% RH are the requirement.

after some thinking, I do believe I won't require more PID loops than two in most applications. I have earlier used Siemens PLC systems with a lot more loops which is why I asked.

Heating and cooling use two AO's. I was thinking to use two PID for this, or maybe some kind of linear function instead works just as fine.

For humidity I think I will use some compare blocks to trigger the DO's.


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