PID and PLCs

[MikeHolm]

I picked up a cheap PLC but unlike DDC controls (TAC, KMC, etc) it does not have an embedded PI or PID algorithm on board. Does anyone know how to easily use PID with a PLC? Any thoughts greatly accepted. Please, pretty please? With sugar:D

[HVACRsaurus]

Can't really help you - I just refer any mods to our regular PLC contractor, but I think they generally have some sort of function or function block to do that kind of thing. I know they have timers, counters, latching flag points & similar.

[MikeHolm]

Yea, I asked the manufacturer but they don't have a function block for PID. I haven't had to do this before so I wondered how hard it was to do ourselves.

[HVACRsaurus]

Hi Mike,*

Can you tell me a little more about the application?

PID is probably something that you know more about than me but there is likely a calculation that can be made to achieve a similar result.

P - Deviation from setpoint multiplied by a Factor.
I - Value from P multiplied each (specified) time duration.
D - bit trickier, offset for Rate of Change (not always used).

*I have used other (glorified) step controllers*using PLC timers & counters to achieve suitable results.

[MikeHolm]

I certainly know what a PID "block" is in many controls and I know many of the differences between between DDC (direct digital controls) and PLCs. There are differences and one of them is that there seems to be no preinstalled register to place the PI values. DDC controls are more HVAC specific and they have this and are used for pump controls, outdoor compensation etc. which is what I am using this for.

My controls are to control the complete heating system including pumps, mixing valves and compressors.