CMP Block
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Hi All,
I'm having some trouble with the CMP block, more than likely user error!
I'm trying to control a cold space using it, a set point of say 2°C (U2), and hys of 1.5. It seems the the hysteresis pulls it under by the same about (1.5). Should the block turn off once U1 and U2 are equal. Turning back on once the temp input has gone above the hys setting?
So on at 3.5°C and off at 2°C. Or does the hys work on both sides of the set point?
Or have I misunderstood something?
Cheers
Mike
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Hi Mike,
yes the CMP block is symmetrical, turning on when (u1-u2)>hys and turning off when (u1-u2)<(-hys).For your needs, you can:
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Use CMP:hys=0.75 and add an offset of 0.75 (using ADD block) to the measured temperature.
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Use SUB and RLY and RTOI blocks to implement the desired non-symmetrical on/off controller.
Hope this helps, let me know.
Best regards,
Jaroslav -
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Hi Jarolslav,
That helps a lot, thank you.
In regards to U1 and U2, it seems to make a difference as to which one the measured temperature and set point goes to. The difference being whether you are heating or cooling.
Is it wise to swap U1 and U2 over from the examples? or can you use a not block to switch the output?
Cheers
Mike
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Hi Mike,
for heating, the inputs are typically connected like this:
Because you typically want to switch the heating ON, when measured temperature is BELOW the setpoint (u1 is greater than u2).As I wrote, the CMP block is symmetrical, therefore swapping inputs has nearly the same effect as using the NOT block at the CMP output. The only difference is the behaviour at system startup in the case when the measured temperature is very close to the setpoint (within the hysteresis band).
Best regards,
Jaroslav -
Thanks Jaroslav,
So using a not block would be the preferred way for making it work for cooling.
Cheers
Mike
