Driving a SSR with a PID Loop ???

Hi guys, I am having sleepless nights over this problem!

I am reading a temperature value and using a PID routine to make sure that the temperature stays at the process value. My problem is this:

The PID loop gives me a variable from 0 - 4000 depending on where the temperature is.

How can i manipulate this so that as the variable changes, the pulse of the output is driven directly proportional to the value???

Eg, 4000 = ON
2000 = pulse 50% ie. 10ms
etc

????
Cheers,

Brett

PID_Output_Value/PID_Max_Output_Value * Time_Period.

Thus, if your PID output is 25% and the time period is 5 seconds, then 1000/4000 * 5 = 1.25 seconds. Your SSR is on for 1.25 seconds and off for 3.75 seconds.

In your example above you stipulated a value of 10mS for 50%, meaning you have an overall time proportioned output time of 20 mS. This is realistically way too short of a time period as your PLC scan time will be a signifcant percentage of the time proportioning period. And thats not even accounting for your IO response time and the .016 second response time of your SSR.

Thanks Alaric, that has racked my brains into gear!!!

I will ammend the programme and try it now!

Will let you know if it works!

Thanks

There is a simpler way IFF the PID is in a interrupt routine.

If the PID is called at fixed intervals then one can just use the following code to turn the SSR on and off.

This works well if the update time to the PID is too fast for a timer.