2012: Tuning the Jaguar's Speed Control PID Loop

[techhelpbb]

Originally Posted by NotInControl

Thanks for the info. I figured it wouldn't be as easy as splitting the encoder signals. But like I said I know there are solutions available, but at a risk. I'd rather be done with it on the CRIO than debugging the problem through the last week of build on the Jag.

As for the method I use to determine the gains:
I prefer the more modern approach of simulating the plant and designing a controller to a model and then implementing in on the real system and tweaking from there. I create a plant model in matlab or simulink and use the control system toolbox to determine the gains (within a close range because my model is a linear approximation of the system and doesn't account for all physical effects of the system.)

Guess and check methods like Ziegler-Nicholos methods do work but I feel that Guess and Check methods only work if you really have a background in control systems or have enough experience with controllers to know exactly how to react when you see certain behavior. I'd always recommend the modern approach, because at a minimum it will give you a starting point for your gains, where you can then tweak by hand. I'll always recommend the simulation approach over guess and check.

The gains for my test on the Jag were using the same process I documented in the video I posted earlier.


Regards,
Kevin

I posted a design I have used to cleanly split the encoders in the past early last year. Someone else posted a decent schematic of the basic idea. I still have no idea if it would pass field inspection even after 'open sourcing' it on Chief Delphi so I won't assure anyone that if you use it you'll be legal and the COTS rules might not help you. I can only say that it does electrically isolate both Jaguars from each other with regards to the encoder. It won't help you if you overload the Jaguars. It won't help you if you have wiring problems (extremely common), it won't help you if you busy out the CAN, it won't help you if the PID loop misbehaves, it won't help you if you set the system improperly. We abandoned CAN and went straight to PWM and eventually removed the Jaguars from our control system during 2011 (I currently have them mounted on my test frame). We had a few Jaguars fail and we had some wiring problems and since we were using PWM anyway we felt the additional space and ruggedness (off hand observation) the Victors have offered us were a good trade.

The way Jaguars in particular often end up getting tuned isn't a very clean solution because it often requires some familiarity with the system being tuned that ends up making the results somewhat arbitrary. Turning a value up till you get oscillation, for example, would imply that the person looking for the oscillation really understands what they are looking for. Having experience in the matter helps greatly, but, sometimes even that can fail you if there are many overlapping issues.

Add to this the chance that the PID loops available in the Jaguar might be ill suited to the system loads and you're starting to get into the sort of territory that in a 6 week period might be overwhelming.

I agree that modelling the system is advisable, and after attempting this pre-season in 2011, I personally can't imagine anyone jumping into this during season without some serious commitments of time in preparation. I have seen it pay off for people to just hit the 50% mark and not further detail their situation but then have that fall apart during competition when they experience wear and tear. As detailed in other posts certain modes seem much less prone to issues than others.

Having worked with a team to write some very complex synchronized PID loops in the cRIO in Java in the past, I have to agree with many of the other teams and mentors. There is much to be said for implementing the PID in the cRIO even if it pushes the limits of the cRIO (a matter that everyday seems less an issue of concern). I hope that we someday deploy a Jaguar based solution on one of our robots but it seems certain it won't be this year for us.

I do want to make it clear that I have invested much time into the Jaguars as far as making them work and I see the merit of their designs. It is just a unique challenge within the parameters of this competition.