Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

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Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

A good closed loop controller should get your shooter up to speed much faster than an open loop method. After shooting a ball, our shooter is back to shooting speed in a little more than a half second. We spent some time tuning the controller to make it work well, and we have a simple gain scheduler to help out.

We found that the shooter is much more consistent with closed loop control. Some balls slowed the shooter down more than other balls, but the closed loop control reduced this variation quite a bit.

Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

I plan to implement PID on our withheld launcher tonight. We installed encoders at the last minute to simply measure rate, and the wheel speed is stable within 10% of the average when driving them with a constant percent of battery voltage with a Victor.

Teams that have compensated with system voltage, what sort of stability have you achieved, and what algorithm did you use? My naive approach would be something like 12.0/V_battery*percentage, but a voltage-squared system was mentioned earlier. Would this provide a more stable speed across voltages?

Teams with closed-loop control, what sort of stability have you achieved? I can't imagine that it would be easy to tune PID for a flywheel for stability and rapid convergence. Have you had luck with any sort of feed-forward?

Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

We're using a closed-loop system (encoder feeding a PID). The feed system is not allowed to advance a ball unless the wheel is within 1% of the target speed. For us, less than 1% makes for an exponentially increasing huge delay between shots, more than 1% shoots faster but has more variance in the impact point.

We're driving 2 8" old kit wheels with 2 775's on Banebot Cim-U-Laters (3.7 to 1).

The wheel spins up to speed and stabilizes from a stop within about 2 seconds. It takes about a second or so between shots for the wheel to re-stabilize at the target RPM and the feed to start advancing again.

It took us about a full weekend plus one full day we were out of school for Mardi-Gras to get it dialed in. The PID loop is VERY picky!

We've only tested with the KOP balls, we're expecting to have to tune it at the regional. Our driver panel is set up to allow tuning without having to go into the code.

Video of testing at the end of week 5:

http://www.youtube.com/watch?v=0hIPc...ature=youtu.be

Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

Unfortunately I don't know enough about it to give you any more info...and I haven't had a chance to talk to our programmer or programming mentor since the competition

Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

This is a good post with some nice pictures to help you tune: http://www.chiefdelphi.com/forums/sh...7&postcount=34

Feed forward is good for improving dynamic response when you expect the setpoint to change rapidly or if your external disturbance change significantly (and predictably) with a change in set point. For a shooter the setpoint is pretty constant or will change slowly, and the disturbance doesn't vastly change with setpoint (yes, the air resistance changes with the square of speed, but the disturbance doesn't change direction like an arm that moves through vertical). We didn't find any need for feed forward. We found with the P&I tuned well the dynamic response was really good.

Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

Just as an aside, I wish you would change the title of the thread. there is no such thing as AM balls. AndyMark is just the venue FIRST chose to distribute the FIRST game pieces this year. There is no difference between the balls at competition and the balls distributed through AM.

Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

Interesting, we're using exactly the same motors, wheels, and gear ratio for our launcher (although we manufactured our own gearboxes). However, when applying a constant voltage, it takes our launcher quite a few seconds (more than 5, at least) to spin up. Maybe our imprecisely machined gearboxes are the culprit here. Does your controller overshoot initially to start the wheels and then decrease to stabilize?

Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

Well I think part of the original question was if the balls we get from andymark are different when they are fresh out of the box than the fresh ones at the competition because some people have said that this might be the case. This is not meant to slander andymark or anything but instead to see if it is part of the issue teams are having.

Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

A constant voltage applied will spinup slower than a proper closed loop control, and respond slower as well.

Lets say a 6V output results in roughly the steady state speed you desire, when you spinup (both initially and after a shot), you're running the motor at 6V.

Under closed loop control, when you spin up and are below your setpoint, you will get a voltage output higher than 6V (and likely the full ~12V for an appreciable amount of time), resulting in faster response.

Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

Hmm, I tuned our drivetrain with P alone, and found that any amount of I caused significant oscillation. I suppose the I term would would cause an initial overshoot, which we'd want to spin a launcher up quickly, but we wouldn't want that in a drivetrain. I'll update with any progress tonight.

Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

If you're applying the constant voltage that it takes to maintain the wheels at speed, then it is going to take a long time to ramp up. It takes very little power to maintain the speed.

Our system has no audible overshoot. Shooting speed is about 50% of max speed, so there is a big reserve of power available to bring the wheels up to speed. I haven't looked at the plots, but I would guess the control loop is ramping down from very near full power as the wheels come up to speed.

Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

Feedforward can also be helpful when you are trying to control speed with a canned PID routine that was designed to control the position of a position-integrating plant.

Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

It seems like the design of the system (gearing / wheel size / motors / compression / shooting distance) has a larger impact on spin up time than the control system does. Our light, low inertia system spins up in <1 second and is back up to speed after a shot in ~0.5 sec, and shots are pretty consistent at the moment with just a straight PWM value to each Jag. Old and new balls seem just as consistent. Video here of shooter firing off a couple sets of 3 balls, mixed with old and new.

We're still looking into a closed loop control system for the shooter, encoders are already mounted and spitting out data.

It sure is neat to hear how everyone is tackling this problem, I'd like to hear more teams post their results!

Re: Problems with tuning shooters using AM basketballs on Practice Field?
 

What's your gear ratio (from motor to wheel) and what motor(s) are you using?