Sestos PID slow to respond

[floydmeddler]

Howdy,

I've been using a Sestos PID controller for a few years now and haven't been able to get to the bottom of a really annoying problem. Here's a typical scenario. I simply want to bring the temp down from say 67c to 65c. I set it to 65c and instead of immediately knocking off the element, it decides to take its merry time and carry on at 67c for what feels like a minute sometimes. Any idea why it just won't kick in with the new desired temp straight away? Why this delay?

Here is the instruction manual. Any help in pointing me towards a solution would be fab.

http://www.sestos-hk.com/english/download/D1S-EN.pdf

Cheers!
Floyd

[rpruen]

Did you tune the parameters for the application? Or use the auto tune feature?

That's about all I can think of to improve things....

[Jocky]

You're going to need to run an autotune and then do some manual adjustment of the P/I/D values.

A PID is designed to hold a specific temperature and keep it stable without oscillation once tuned. It's not like a thermostat that turns the power fully on when the temperature drops and only turns it off again once it hits the set point. It aims for one temperature and tries to keep you there, so it's pretty cautious about changing anything.

When you change the set point by only a couple of degrees you're not far away from the current temperature so it only starts to make small adjustments, which it gradually increases over time until the temperature creeps towards the set point again.

The P/I/D values have different effects - I think you'll probably be wanting to reduce the Derivative value (for anything but RIMS you can use a very low value) which might give you a little bit of oscillation but should improve response time.

[floydmeddler]

Thanks guys. I used auto and when its doing its thing, it's super. But when I want to say reduce the temp, i'll tap in the new desired temp and it won't acknowledge my change straight away. Will keep going as it was for a while. It gets annoying when I override the PID if say I want to crank the temp up. When it gets to where I want, I knock off the override and set it back to PID. However, override will stay on for around 30 seconds and i'll over shoot. It doesn't react immediately to my inputs. Hope that makes sense!

[Jocky]

I know what you mean, you’d think that a PID would know to kill the power at that point. It sounds like the PID algorithm is taking a while to calculate how much power it needs to apply.

Not entirely surprising - it only has some rough guidelines from the tuning of how much power to apply for your system and unlike you or I doesn’t remember what happened when you last changed the temperature.

[Jocky]

By the way, are you using RIMS or HERMS?

[floydmeddler]

Thanks Jocky. something I just have to live with I think. First world problems eh?

I use a RIMs system.