Question: Sample Time about quickpid HOT 2 CLOSED

Thanks for your thoughts on this. It will be a bit before I have a physical representation of the environment, so I'm sure you understand I'm trying to get close enough to use as a starting point right now.

I just identified a need to use two loops for this application, the main control loop (area being controlled), and a loop controlling the max temperature output of the heating system in order to avoid damage. This should be fun!

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I haven't done any testing on a slow acting control system yet, but its good first of all to get the time parameters identified and set correctly. A good thing to estimate firstly is the time constant of your system. Let's say for example that it's 5-minutes (300 seconds). It's suggested elsewhere that the sample time (PID compute period) should be at least 5-10x less than the time constant, so keeping with the example, this could be set to 30 seconds. With sensor readings coming in at 1 reading per 10 seconds, you would get 3 new readings (that are averaged in your circular buffer) for each sample period. I think the sensor rate should be more than 2x the PID compute period so this seems OK.

I took a quick look at the DS18B20 datasheet and it says the resolution can be set to 12-bit, which means you can get about 0.08°F resolution. For 12-bit mode, the max conversion time is 0.75 seconds.

I think using the running average temperature is OK, but you might find better response by using the raw temperature data. I wouldn't set the PID rate so fast that you get less than 2.5 temperature readings per PID compute period. As per the example, you could use 30 seconds: SetSampleTimeUs(30000000)

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