Using Output for PID Control with MV

[brettshelley 0]

Well, here is an interesting one! If my PID gives me a variable MV, how is it possible to drive a pulsed output which changes according to the MV. I.e. Temperature value has been read, MV changes accordingly but instead of transfer to analogue output, must output to SSR. Any ideas Edited 30 Dec 2007 by brettshelley

[Steve Bailey 118]

Set up a timer that resets itself at convenient interval. Use the PID output to compare to the timer's accumulated value. If the PID output percent is 25% of full scale, turn on your SSR for 25% of the timer's cycle. If the PID is 50% of full scale, turn on your SSR for 50% of the timer's cycle.

[brettshelley 0]

Not quite understanding you here Steve?!? Sorry

[Steve Bailey 118]

What didn't you understand? Let's say the timer resets itself every second. When the PID output is 50% of full scale, turn on your SSR 500 milliseconds, and off for 500 milliseconds. When the PID output is 25% of full scale, turn on your SSR for 250 milliseconds, and off for 750 milliseconds. When the PID output is 75% of full scale, turn on your SSR for 750 milliseconds, and off for 250 milliseconds. How do you know when 250 or 500 or 750 milliseconds have elapsed? Use the timer's accumulated value. It will range between 0 and 999 milliseconds. In the case where the PID output is 25%, if the timer's accumulated value is less than 250 milliseconds, turn on the SSR, otherwise turn it off

[brettshelley 0]

Thanks Steve, so basically i would set up 3 sets of timers with 3 sets of compares so when the MV value is read by the compare routine it activates the timers driving the SSR. I will try that now... Cheers, Brett

[panic mode 246]

why three? SM400 ---| |---------------------------[- MVmax MVmin MVdelta] T0 ---|/|----------------------------(T0 MVdelta) ---[<= T0 MV]-----------------------(Y0)