PID vs Bang-Bang for Shooter Consistency

[lori558]

Here is our slightly modified code. With the minimum value being at 0, this caused a more inconsistent shooter since the wheel would turn off and on with too much of a difference between the two speeds. Therefore we changed 0 to -0.3.
(Keep in mind, our values are negative simply because this is how our code is written. This can be fixed in the Begin.vi if needed but this is working for us.)

By changing the value to -0.3 it makes it impossible (based off of this code) to shoot below 30%. However, the Frisbee will barely go anywhere at that percentage anyway, therefore that number is perfect for us.

Since the minimum value was changed to -0.3, this caused the wheel to spin at a constant speed of 30% if the desired RPMs were ever lower than that. Thus we added the =0 select value into the code to make sure that when we do not want the wheel to move, it doesn't.

As for the rest of the code, it is the same as using a tachometer and the more basic code so graciously provided by billbo911.

[Ether]

Originally Posted by lori558

With the minimum value being at 0, this caused a more inconsistent shooter since the wheel would turn off and on with too much of a difference between the two speeds.

[edit] First things first: check to make sure the motor controller brake/coast jumper is in the coast position. If it's not, put it there, change that "-0.3" back to zero, and try it. (hat tip to Billbo911) [/edit]

If that doesn't fix the problem, continue:

The code is not running fast enough.

Take that waveform chart out of there and change the loop timing to 5ms and see if it fixes the problem.

Therefore we changed 0 to -0.3

Hmm. What motor controller are you using?

[billbo911]

I'm thinking the Break/Coast jumper is in the break is position.

[Ether]

Originally Posted by billbo911

I'm thinking the Break/Coast jumper is in the break is position.

Yikes! You could be right!

That wasn't even on my radar - I just assumed folks knew it should be coast.

[lori558]

The jumper is in the coast position. We made sure as we were wiring the robot that it was put there.
Before we modified the code we had tried changing the 10ms to 1ms and it was way better, but still not as consistent as it is with the modification. It could just be the way that our bot is designed and programmed, but it works for us.

[Ether]

Originally Posted by lori558

Before we modified the code we had tried changing the 10ms to 1ms and it was way better, but still not as consistent as it is with the modification.

Did you try it without the waveform chart ?

It could just be the way that our bot is designed and programmed, but it works for us.

Understood. But here's a chance for you to help those who helped you. We'd like to understand what's going on; to contribute to the community knowledge base.

[Kevin Sevcik]

So turn the robot on and leave it in disabled mode. Try turning your wheel by hand, then unplug one of the motor wires and try turning it by hand again. If it doesn't get easier to turn when you unplug a wire, then it's definitely in coast.

Also, if it's fairly hard to turn even with a wire unplugged, then you just have a lot of mechanical resistance in your system. That would also explain why you're seeing better stability with a -0.3 instead of 0. The extra mechanical resistance is slowing your wheel down more rapidly, so giving it a little push instead of letting it coast makes it ramp down slower. So you're having to bump it less often to keep it in tolerance, which I assume is what you're counting as better stability.

[billbo911]

Originally Posted by Kevin Sevcik

...

Also, if it's fairly hard to turn even with a wire unplugged, then you just have a lot of mechanical resistance in your system. That would also explain why you're seeing better stability with a -0.3 instead of 0. The extra mechanical resistance is slowing your wheel down more rapidly, so giving it a little push instead of letting it coast makes it ramp down slower. So you're having to bump it less often to keep it in tolerance, which I assume is what you're counting as better stability.

Excellent point!
An ideal system for Bang Bang have very little resistance to rotation and a considerable amont of rotational inertia.
Maybe Lori's solution is something to consider when using Bang Bang on a less than optimal system. This is deffinately something I would like to invetigate a bit more.