Sliding Mode Control

Just to let you know i have posted an Excel Spreadsheet in the FTP area based on Peter Nachtweys' Notes on Sliding Mode Control. Feel free to download and have play around with it...

Here's the original thread
Sliding Mode Control Original Thread

Does SMC have application where a burner is being controled by a proportional control or only where you are turning on and off an electrical heating coil?

I have an application right now with three burners that I'm planing on implementing with PID but the burner control is proportional. Also, there is no way to start and stop the burners quickly because of gas purge requirements before starting the pilot and burner.

I am planning on creating a test system which will lend itself well to testing the performance of PID vs. SMC and hopefully will have a chance to play around with that.

Take Care

Peter will probably have the best answer for this but I'll just toss in my two cents.
First off I think it depends on your process. You will need some amount of thermal mass to keep the temperature stable. If you are directly heating a moving web, for example, it might be pretty tough to get SMC to work for you. However, if you are heating a container of material or a metal roll that a web runs over it is definitely do-able.
Instead of turning your burner off, can you run it at some very low level? Since you plan to use proportional control anyway I assume you can turn it back a ways. Use the low heat level as your 'off' condition. Depending on how low you can go you may need to use different system time constants for heating and cooling, but it should work.

Keith

Keith got it right.

The update rate must be fast relative to the process. Sleepy Wombat's spread sheet and my web page used Ron's Beaufort's 'hot rod' gain and time constants. These were in minutes so updating every second was fast enough. Update every 6 seconds was a little coarse. Updating faster that one second was probably a waste of CPU. However, you should play with Sleepy Wombat's spreadsheet to prove this to your self.

Norm, unless there is some reason to keep some ideal fuel/air ratio constant, I don't think you need to ensure the flame goes completely off or on with SMC. I bet the flame stays always on unless the flame can consume all the gas each second or the valve is off for many seconds. The flame burning the gas will just be another time constant. In any case the on-off control is adding energy to the system in the form gas which the flame converts to heat with a little lag so controlling the rate of gas or time is what I think would be important.

I can add another time constant to my SMC control web sheet, but I have no idea what kind of time constant to use. Basically I would need to know how long after the valve goes of the flame goes off. If I consider this time to be 4 time constants then I think I can do a pretty good simulation with 3 time constants for heater, sensor and load.

OT

Norm, I know you are in the Chicago area and are intested in hydraulics. I will be at the DesignEngineeringShow on the 25th of Feb. I will probably be there for a couple days as we have a booth there. I will also be giving a presentation ( #332.2 ) on tuning and the steps required to do autotuning. It will cover how one finds the gains and time constants. A little knowledge of FFTs, poles, zeros, s domain and z domain would be good. I will be using data from our hydraulic system as an example. There will be a lot of hard engineering stuff with little fluff.