Alternatives to proportional time control for output?

[Cracked]

#3644

Im using my ospid to control a nichrome element (via ssr) to heat a fast moving air stream.

My understanding of how the outpu control works is that at say, 20% output, the pin value will be high for the first 20% of the window, and then remain low for the remaining 80%.

I’ve noticed that at the default setting (windowsize = 5000), I see a noticable oscillation due to the way “on/off” are proportioned within the window, and the rapid response of my system.

I realise I can reduce the window size, but was wondering if instead there could be a way to break up the window into smaller increments, and then choose at each whether the pin should be on or off using proballistic methods?

Is this something that has been done before, or could it be done?

[Brett]

#4082

I haven’t heard of something like this being done. Aside from the coolness factor, what would be the advantage over just using a smaller window?

For the record, if you’re using an SSR you can safely make that window size FAR smaller. 500mS should be fine. When designing the firmware we had to assume that people would be using relays (onboard or otherwise.) The 5sec window was chosen to maximize relay life while still getting decent control in most situations.

I will also say that in my experience you can’t make the window TOO small with an SSR. It may have just been the design of the $5 SSR I was using, but it would latch on until the next 0-cross of the 60Hz power wave. I tried using the arduino PWM (50mS window I believe) and it was a disaster. the SSR just stayed full on the whole time.

[Cracked]

#4083

Thanks for the response Brett!

Well, apart from coolness factor, there probably isn’t a reason 🙂

If I can figure out how to implement it, I might have a go just for kicks.

[Author]

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