Temperature rise offset

Temperature Profiles
1

Hello,
I thought I setup a temperature rise successfully. Set to Beer mode. Setpoint enabled. Setpoint driven by profile.
I found the beer temperature raising significantly rising at higher rate than the profile. I does raise very steadily as I expected… but the following not the profile. “Beer setting” temperature is right… What setting I’m I missing. How come it isn’t following “beer setting” value?

Thanks

2

Could you please post graphs for the PID and the setpoint profile?

3

Hey Bob - I think I figured where I messed up. I had minimum value of 80%. Thinking that when ON the heating would go at minimum of 80% power… but always ran at 80% power! Duh…

That tells me I don’t understand the PWM block. Surprise :open_mouth:.!
I would’ve thought the temperature is controlled by turning ON(100%)/OFF the heater power but on fairly long stretch. Don’t the PWM do that at high-ish frequency? Maybe that isn’t that detrimental for a resistive heater but what about a fridge and it’s compressor…
My sure guess is that I just get how it actually work!
I would like to understand for when it’s time to use the cooling PID.
thx

4

The minimum value constraint on a PWM indeed means that the actuator is turned on at least 80% of the time. The PID can set it >80%, but not <80%.

The PWM has a configurable frequency (the Period setting).
If your period is 10s, and its setting is 80%, the digital actuator will be turned on for 8s, and then turned off for 2s.

For a fridge it is indeed harmful to be switched overly often. This is prevented in two ways:

  • The default period for fridge PWMs is very long
  • The digital actuator for a fridge has a Minimum OFF constraint that will enforce a minimum off time regardless of PWM settings.
5

Thanks Bob. Great clarifications