Re: Shooting Motors and wheels
 

Tell us what language you plan to use to code your robot and you may get more detailed advice and help.

Re: Shooting Motors and wheels
 

The first thing I would suggest is to take advantage of the Virtual KOP. In it you received a code for an encoder from Armabot that will attach directly to a 775 Pro. Use that with a Talon SRX, and you can have very stable, reliable control of your RPM in one afternoon.

Re: Shooting Motors and wheels
 

Ye, last year we fried a couple of 775 pros last year and broke a fan blade on one of them, but other than that I love 775's

Re: Shooting Motors and wheels
 

We r using c++ most likely

Re: Shooting Motors and wheels
 

For quick prototyping we wire up a motor controller in between the battery and the motor (and an in-line automotive fuse) and then use the thrifty throttle from Andymark to control the motor controller. This allows students to vary the speed of the motor to try various experiments. At $29 it's a lot cheaper than tying up a robo-rio for this and it's a lot quicker to set up.

Re: Shooting Motors and wheels
 

Want to drop a tip for everyone. If you choose to run Colson Performa wheels from vex for your shooter attempt to keep them under 6000 rpm. We proto typed with some and regret it. The rubber melted and got al sorts of hard and nasty. The 60A Andymark compliant wheels are way more consistent and hold up at higher speeds. They are also rated for over 9000 RPMs.

Re: Shooting Motors and wheels
 

There have been a lot of good suggestions so far, especially on the control side of things. But as a note on the actual shooter design, it's hard to say whether 5:1 is a good speed for your wheels without knowing what size wheels you're using. There are probably four main variables in designing a shooter in FRC*: wheel surface speed, contact distance, compression profile, and wheel/hood material (durometer, coefficient of friction). Of course, wheel surface speed and contact distance are both functions of wheel diameter. So if you use larger wheels, you can have a higher reduction from the motor, but for small wheels, you'll want them spinning faster.

In an ideal world, you would have plenty of traction and distance to get the balls up to speed and have no slip between the balls and your wheel(s)/hood. Then the exit velocity of the balls could be calculated quite simply by equating the surface speed of the wheel(s) and balls.**

This, put simply, is not going to happen in real life. The balls will slip on the wheels, but how much they do so depends on how well the wheels can grip the balls. The more distance you can keep the balls in contact with the wheels (this means larger diameter wheels or a larger angle in contact), the closer you can get to this ideal. Softer wheels are also sometimes advantageous for this, at the expense of faster wear.

Additionally, how much you compress the ball/wheels how that compression is distributed over the path of the ball has a major, if somewhat complicated, impact on your shots. In general, more compression can allow for more energy transfer, but only up to a certain point. After that point, you're just excessively deforming the balls and bogging down the shooter.

And finally, a general rule of thumb that I've come to follow is that softer balls should take harder wheels and vice versa. This year, the prototypes I've seen online and with my old team are suggesting that hard wheels will probably be fine—These balls can compress a bit. But all these are just guidelines, and your mileage may vary with any of them.

*Actually, I'll add a fifth, but it's not as much about geometry and it's important this year, so I'll keep it separate: motor power. Shooters this year should have a pretty high throughput, so be prepared to use your newfound closed-loop control knowledge that you got from the fine folks here to keep your shooter at its proper speed as you're shooting. With that in mind, consider whether one motor will have enough power to keep the wheels spinning at their proper speed as balls go through. If not, you can add another motor to increase your available power and help your speed rebound between shots.

**If you use a single-wheel shooter with a hood, the exit speed of the ball would be half the surface speed of the wheel, and for a double-wheel shooter like a pitching machine, the ball speed would equal the average of the two wheel surface speeds.

Re: Shooting Motors and wheels
 

We r using a 4 by 2 wheel and have about .04 inches of compression

Re: Shooting Motors and wheels
 

We also changed the wheel from the kit of parts andymark ones to colson performa wheels which are shooting the balls much better

Thanks with all the help, I will post a video soon of our shooter in action

Re: Shooting Motors and wheels
 

I would like to add to this great explanation.

Every time you shoot a ball you impart a portion of the shooters energy into your shoot robbing the shooter of the speed. At the same time motors are trying to "charge" the shooter back up with energy.

Unfortunately the motor(s) have a limit to how much energy they are able to put into the mechanism. This is correlated to the moment of inertia of the sum of you spinning parts and the free speed of the motors.

This equation is (1/2)Iw^2 or half the moment of inertia times angular velocity squared.

Your motors angular velocity is limited by its free speed RPM so the only way to add more energy is to increase the moment of inertia. For the sake of simplicity if we assume a solid relatively uniform mass in the form of a drum shooter then moment of inertia would be:

I=(1/2)m*r^2

For all wheel shapes this equation remains almost the same with the coefficient in front of the mass being the only change.

Using this knowledge increasing the mass and moving the mass farther out increases our energy storage meaning the that the energy imparted onto the ball has less effects on the overall speed allowing more consistent shots.

This concept is called flywheels and is a form of mechanical battery.

Sent from a sleep deprived highschooler at 11:42pm on there phone. Please go easy on me for typos and the sort.

Re: Shooting Motors and wheels
 

+1

Re: Shooting Motors and wheels
 

We've prototyped with a direct-drive CIM motor. Comes up to speed very quickly. 1st ball robs some speed, several more after that are consistent. Today we hope to add an encoder.

Re: Shooting Motors and wheels
 

Thanks for providing the math. I probably would have done it, but I'm here on the east coast and it was even later for me, so I just wanted to get it done and get to bed.

Re: Shooting Motors and wheels
 

So what compression are teams finding works well? 0.04 sounds very light - did you mean 0.4 inches?

We currently have 0.18 inch compression, but are concerned that it wont be enough to overcome variations from fuel buildup on wheels.