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Home Brew Forums > Home Brewing Beer > Automated Brewing Forum > Manually tuning PID
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Old 09-27-2012, 06:42 PM   #11
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Shifting the bulk of my coefficient magnification to the integral component seems to have done the trick. I still need to keep P relatively high, somewhere near 80 or so, to get the thing to settle in the first place. Thanks for all the help!

...

Well, what do you guys mean by "slow changing"? I gain or lose 5 psi in less than 15 seconds.
Glad to hear you were able to make improvements to your control.

However, I don't really agree with Poobah58's statement regarding slow changing process variables. Typically, derivative gain is useful in a process that has a lot of dead time. Dead time is the length of time is takes to see ANY response in the process due to a change in your control output. All that being said, it sounds like you probably don't have a lot (if any) dead time in your system.
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Old 09-27-2012, 10:15 PM   #12
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Glad to hear you were able to make improvements to your control.

However, I don't really agree with Poobah58's statement regarding slow changing process variables. Typically, derivative gain is useful in a process that has a lot of dead time. Dead time is the length of time is takes to see ANY response in the process due to a change in your control output. All that being said, it sounds like you probably don't have a lot (if any) dead time in your system.
This is probably not the time to debate this but you are wrong. Dead time is the amount of time it takes for a process to start changing after a disturbance in the system. If there is dead time your gain is probably too low. By definition with derivative action the controller output is proportional to the rate of change of the process variable or error. Thus, if it's a slow moving process, derivative won't give you much of a control action. Slow moving PV's are helped more by the integral control action. With integral action the controller output is proportional to the amount and duration of the error signal. I've been tuning PID control loops in boiler room enviroments for 25+ years and have used derivative on 1 job and it was due to incorrect scaling of a transmitter. I apologize to the OP for the rant.
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Old 09-28-2012, 04:29 PM   #13
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I can't really evaluate the theory on this stuff, but I will say that in playing around with my variables I was never able to get the system to hold steady without a significant derivative component.

Thanks for the help everyone!

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Old 10-25-2012, 05:17 PM   #14
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This is probably not the time to debate this but you are wrong.
Yet, you debate it anyway. So bold.

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Dead time is the amount of time it takes for a process to start changing after a disturbance in the system. If there is dead time your gain is probably too low.
I disagree with your definition of deadtime. Deadtime is the amount of time that it takes to see a change in the process variable after making a change in your control variable. You cannot reduce deadtime of a system simply by changing gains. Deadtime is an inherent property of the system. A common cause of deadtime is transmission delay.

This guy has a good explanation of deadtime.
http://www.controlguru.com/wp/p51.html

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By definition with derivative action the controller output is proportional to the rate of change of the process variable or error. Thus, if it's a slow moving process, derivative won't give you much of a control action.
The amount of derivative control action is dependent on the derivative gain. A small rate of change can still make a significant impact on the controller output. I've tuned loops with derivative gains in the 1000's.

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Slow moving PV's are helped more by the integral control action. With integral action the controller output is proportional to the amount and duration of the error signal.
A slow moving PV can cause a lot of integral wind-up.

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I've been tuning PID control loops in boiler room enviroments for 25+ years and have used derivative on 1 job and it was due to incorrect scaling of a transmitter. I apologize to the OP for the rant.
I haven't been tuning loops for 25 years, but I've tuned a wide varity of loops. Flow control, level control, pressure control, density control, cascaded loops, etc. I agree that derivative isn't necessary in all applications, but the majority of times that I have used it involve integrating heating applications, like steam super-heaters that have a lot of thermal mass.
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Old 10-26-2012, 02:53 AM   #15
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Hm I attended a seminar on PID tuning recently, and the presenter mentioned something I don't remember hearing before. First that temperature control is usually not linear; i.e. if you double the power from 10 watts to 20 watts the steady state temperature reached (proportionately) is usually not the same as when you then double from 20 watts to 40 watts.

The other was whether the system is self regulating. He indicated that whether a change in CV would result in a leveling out of the PV. So for a level control usually that does not occur, like opening a tank drain 10% does not normally result in a steady state of tank level if that were your

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