PID alternatives

Tom Jenkins said:

The programmer had initially used PID. ("That's our standard.") We diddled around for half a day and couldn't get it tuned for reasonable response and stability at both high and low flows. The blower response to valve changes is very non-linear, but fast compared to the process- a couple of seconds

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Cool, thats one of the reasons I put several variables on my trainer... you can control the fan speed, a manual damper on the top and also a electrical damper on the bottom, I found its was fairly easy to tune something with a single action, say heating a platen but once you introduce another variable it gets a lot harder fast, I have been lucky to be involved in several projects that you are trying to stabilize your process variable with a fluctuating control variable and that taught me a lot.

I had one system that was heating air (incinerating) via natural gas to 1450 deg and that would normally not be a big deal but the air that we were incinerating was ranging in temp from 100-600 deg and then it also had a LEL from 0-30 and it had a huge variable speed fan that was keeping the other system negative so your SCFM would change also... forgot the size of the fan but it was over 6ft tall, we ended up making two PID's depending how far away it was from the SP was depending what PID it used.... sounds like I could of used floating control back then

Thanks again

Tom Jenkins said:

Here is an example of floating control on an inlet throttled blower. Amps are proportional to air flow.The application was dissolved oxygen (DO) control of a variable level sequencing batch reactor (SBR). The big step was a shift from a pre-aeration set air flow mode to controlling air flow to maintain DO.

The programmer had initially used PID. ("That's our standard.") We diddled around for half a day and couldn't get it tuned for reasonable response and stability at both high and low flows. The blower response to valve changes is very non-linear, but fast compared to the process- a couple of seconds.

I finally convinced him to abandon PID and program floating control. Within half an hour we got both good response and stability.

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There is no set point to compare the process variable to.

This video compares our advanced control algorithm to a normal PID with velocity and acceleration feed forward. This system was designed to be uncontrollable using a PID with velocity and accelerations feed forward.
Notice the auto tuning makes tuning easy using our advanced algorithm.


We have 4 hydraulic systems that we use to teach hydraulic control. They are all very different to emphasize different techniques.
We also have one servo motor lab.



http://deltamotion.com/peter/Videos/NF-FOA.mp4

Ceramic firing process, on the ascending side there are not 1 but 14 lines of the separate temperature regulation zones and on top of all the setpoint (TEMP. PROG.)

Using my ID algorithm.

Peter Nachtwey said:

There is no set point to compare the process variable to.

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True enough. This happened to be a plot I found while looking for something else.

I guess you'll just have to take my word for the fact that the amps/flow were controlled to the set tolerance, which is demonstrated by the width of the trend plot fluctuations. The programmer and the owner were happy, anyway.