Shooter consistency
 

At our recent regional in Lubbock, we had an issue with our autonomous shooting not being consistent despite our measuring and adjustments to positioning. One of the ideas that came up was that a 13V charged battery vs a 12V battery may give the shooter more power causing it to shoot harder/higher. our shooter is a fixed position. Any ideas on what we can do to correct that or what may be wrong?

Re: Shooter consistency
 

Get a sensor to measure your shooter speed such as a halleffect sensor. By using the feedback of the sensor to determine shooter RPM you can determine if the shooter is actually at the correct speed. There are numerous ChiefDelphi threads about this.

Re: Shooter consistency
 

A standard encoder might not be able to handle a wheel spinning that fast. What we used was a photoswitch from the 2011 KoP to count the spokes on the wheel.

Re: Shooter consistency
 

How fast is "that fast"? The OP did not say what speed they are running their shooter at.

A "standard" US Digital E4P 250CPR encoder, if setup properly*, should be able to easily handle speeds up to 5000 rpm.

If the spokes are not wide enough, that detector will have difficulty keeping up at high speeds.


* connect only Channel A. Leave Channel B disconnected. Instantiate it as an up/down counter in the Counter class, and set it to count up only. Set the FPGA sample averaging ring buffer to 125. Use the getPeriod() method in the counter class, and compute rpm = 60/(250*period). See attachment.

Re: Shooter consistency
 

What kind of motor, gearbox/drive train, shooter wheel, material on the "fixed" shoe, any other disc guides?

Re: Shooter consistency
 

I can vouch for hall effect latch sensors on shooters and a well designed feedback/pid implementation. My team is able to pinpoint our desired speeds in about a second. We use a US1881 and custom pcb. Feedback from a sensor is key as others have stated, battery charge/quality can add a lot of variation otherwise.

Re: Shooter consistency
 

We had great luck putting a piece of reflective tape on our wheel and having the sensor count rotations of that instead.

Re: Shooter consistency
 

Good to hear.

Would you mind answering a few questions to make this useful for other teams?

1) What optical sensor are you using?

2) What angle of arc is subtended by the tape at the radius where the sensing is taking place? In other words, what is the width of the tape (in the circumferencial direction) where it passes under the sensor, and how from the center of rotation is the sensing element located?

3) What wheel speeds were you measuring?

4) Were you indeed "having the sensor count rotations" or were you using the FPGA to measure the period? (That's an important distinction).

5) What speed control algorithm did you use?

Re: Shooter consistency
 

I can definitely get an answer by tomorrow. I wasn't the one who worked with the optical sensor.

Re: Shooter consistency
 

Would voltage control mode on a CAN Jaguar be consistent enough to get away without a feedback loop? The way our shooter was done, I can't really see an easy way to add one. Transmission used belts, and the wheels we were given are Aluminum (so no magnetic sensor), and an encoder mount is impossible without redesigning the entire gearbox. We also used pneumatic tires with a custom machined rim (so no holes to track with an optical sensor). Unless we attached a magnet to the wheel and used a hall effect sensor below it... but that would destabilize the already fairly shaky wheel unless we used a *tiny* magnet.

Re: Shooter consistency
 

Betcha you could put a piece of reflective tape on your belt and use a IR proximity sensor on that. Don't get too attached on putting the sensor on some rotating thing, yeah?

As for the voltage control on a CAN Jaguar, that's better than nothing, not as good as actual feedback control. It will compensate somewhat for state of charge differences in your battery. It won't compensate for the spin down from firing a frisbee and it's going to deal poorly with rapid voltage changes from, say, your drivetrain or some other system making voltage fluctuate. Feedback is really the best way to go for getting fast, consistent shooting.

Re: Shooter consistency
 

Paint the wheel black and then attach a piece of reflective tape and use an optical sensor.

Re: Shooter consistency
 

Would the line tracking photoswitch from 2011 work, or is there another sensor that would be a better choice?

Re: Shooter consistency
 

We've had near perfect accuracy with our shooter, and it's essentially a 2-wheel linear shooter using the AM 8-inch pneumatics without the pneumatic inner tubes inside the wheels on AM Spinboxes powered by a CIM and a MiniCIM. No feedback whatsoever, constant 100% power on the wheels when shooting.

Re: Shooter consistency
 

Yes, though it's a little slow. You typically need a pretty wide stripe to get it to work. Too narrow a stripe and it won't register. Practically speaking, you can make the stripe take up half of whatever surface you're sensing and it'll work fine.