How can I generate a single timer event in a loop?

[stephencox]

I have a simple PID script running inside of a timed loop to control a heating process.  It works perfectly except that I cannot find a clean way to start the timer when it reaches the setpoint.

Right now, the loop just runs for a set amount of time (e.g., thirty minutes).  I want it to run for a set amount of time after the heating reaches the setpoint.  I can think of several ways to fudge this, for example with a couple of shift registers and a case structure or with a second loop, but these options seem very clunky.

I am sure there is a better way.  Can I somehow create an event within the loop that only happens once, for example, to give a timer value one time when a boolean value changes and never rewrite it?

Thanks in advance for your help.

[crossrulz]

You could keep the boolean value in a shift register or a feedback node.  You then need a little bit of logic to check to see if the boolean changed from false to true.  From there, you can put in your get timer inside of a case structure.

[stephencox]

I decided to just initialize a shift register at 0 and increment it every time the process value equals the setpoint. Then if it is greater than zero, it stops resetting the timer. Still a lot of unnecessary processing, but seems less messy and prone to errors than checking for a Boolean change each iteration.

[altenbach]

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@stephencox wrote:
I decided to just initialize a shift register at 0 and increment it every time the process value equals the setpoint.

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I probably would not use an equal comparison. What is the datatype of the value?

[stephencox]

Ok, it's not actually an equals comparison.  It increments the dummy value everytime the setpoint is within an acceptable range of the setpoint.  Anyway, datatype is integers, so equals would work--I'm using a cheapo USB thermocouple reader that outputs integer temperatures, which is fine because we can easily tolerate +/- 2 degrees during the run.  The important thing for us is to avoid overshoot, so we require a modified PID routine that heats very conservatively.