Quote:
Originally Posted by Michael Hill
Ah, that makes a lot more sense. I was wondering how they would get by the 1.25 reference voltage of a LM317. I guess they COULD use LT3080s to get around that, but like I mentioned before, I haven't taken a Victor apart (mostly because they're not mine, and our team doesn't have the kind of resources to waste on me accidentally breaking a Victor).
The reason I mentioned that it would be nicer to have more resolution is well...put it this way. This year, the majority of teams (including 3138), had shooter wheels with a speed controller. If your max speed (that is full voltage to the motors/128 counts) is 4000 RPM, which is easily attainable, the finest you can change your motor speed by is ~30 RPM, which can be a heck of a difference. I'm sure there's got to be a way to use it as an unsigned value (that is 0-12V rather than -12-12V) to get 256 counts of resolution, but that would still only give you a 15 RPM resolution. While many teams figured out it didn't make a huge difference, it would be nice from a controls perspective to be able to control it down to a single RPM (which would be attainable with a 12-bit PWM signal)
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Chris tackled how to get in-between PWM values by playing with the output signal.
To go a step further, you are stuck thinking in purely electrical terms here. You need to start thinking about the system you are controlling mechanically, which is what ether was getting at by pointing out control with bang-bang. More specifically you need to think about the inertia of the system.
You can send minute increases and decreases to the motors powering the wheels MUCH faster than the intertia of the wheel system will allow it to react. That means through the use of PID, or Bang-Bang, you can achieve much better resolution that your electrical system analysis would suggest, because the intertia of your mechanical system serves to damp or average the response.
In conversations with other teams that I trust, I heard of some managing to control as tightly as +/- 5 RPM. Our final value was about +/-13 rpm by the time I got sick of playing with encoders and Jaguars and gave up.
Being able to control it to a 'single' rpm in the way that you suggest wouldn't work very well. It would take a very long time for the inertia of the system to respond to a tiny increase in voltage so that you could actually get to that rpm.