I’m disappointed

power output problem in Autotune

I was using the Autotune to get a range of PID values for different set points, and reduced the maximum power output from 100% to 25% for the lower set temperatures as I found this gave more consistent results.

However the device now no longer gives any more than 25% power output when in Autotune. It moved between 0% and 100% in Autotune before, but now it only moves between 0% and 25%. It no longer works properly.

I changed the maximum power output back from 25% to 100% to tune at higher temperatures.Now, although I can use 100% power output in Manual mode, and up to 100% power output is given when the process temperature is lower than the set temperature, when tuning the output only moves between 0 and 25%.

The device is a 3216, and the first display after the screen test is 'U 2.13' on top and 3216 under that. I presume the first is a software number.

Any ideas? How can I get this device working again?
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  • The 3216 has a set of tuning power limits - this is important in some situations particularly in a well lagged system where otherwise there can be problems calculating a relative gain between heating and cooling. Normally they're just set to 0 and 100%.

    What has happened is that in reducing the general power limits you've caused the tune power limits to be clipped into the same range, because the general power limits are the master override for the amount of power the instrument is allowed to apply - the tune high limit has been pushed inwards as you reduce the output high limit. Having then relaxed the limits back out again, the tune limits have just stayed where they are, so you need to find them and set them back to 100%. It's a bit of a gotcha if you don't know about it.

    I don't have an instrument here (Sunday morning breakfast table!) and I don't remember exactly the list structure, but you'll find the limits if you go into configuration mode - they will either be in the output list or the control list and will be fairly obvious as set to 25%. Itools will also find them if you can connect that.

    It's interesting that you felt the need to reduce the OP limit - that may indicate a problem somewhere else. Reducing the power limit really just allows you to use less of your heating (like having a four bar heater but only using one of them), but the controller itself can work out how to use the heaters for best effect via tuning. I guess with the higher limit you're seeing overshoot and oscillations? May be worth talking to the helpdesk
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  • I’m concerned for your Sundays
    Thank you Tim, you have solved the problem from your breakfast table. Impressive, and I am very grateful to you.
    It wasn't oscillations is the control that I was trying to avoid, but variations in the PID values generated by the Autotune process.
    Cheers
    Phli
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