Black Lives Matter - Action and Equality. ... Adafruit is open and shipping.
0

Motor mechanism for ping-pong ball shooter
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

Motor mechanism for ping-pong ball shooter

by MikeyTheHutt on Mon Oct 11, 2010 10:21 am

Hi all-

I'm working on an Arduino-controlled ping-pong ball shooter. I want to add some pan and tilt and some logic to it later, but for right now, I'm just working on the mechanism to launch the ball.

I'm using a "pitching machine" design, with two wheels spinning in opposing directions at the end of a feed tube that launch the ball forward (this type of mechanism- http://www.youtube.com/watch?v=UJjsCS4e370). I'd like to have pretty precise control of the velocity of the ball as well.

I'm not experienced in working with motors- any recommendation on what types of motors would work well for this, and have the RPM's to get the ball up to a decent velocity (30-40 mph)? Should gears be used here?

Many thanks for your sage advice.
MikeyTheHutt
 
Posts: 6
Joined: Thu Sep 30, 2010 10:41 am

Re: Motor mechanism for ping-pong ball shooter

by cstratton on Mon Oct 11, 2010 1:41 pm

You probably don't want gear motors, just some squishy wheels.

Control may be interesting. You have freewheeling motors where rpm will depend primarily on voltage, but then you hit them with a load (depending a lot on spacing, how much the wheels squish grabbing the ball) at which point they will draw a lot more current. Three possible ideas of approach:

1) Just build it simply and take data and see if there's a trend you can exploit

2) Use higher quality servo grade motors, put encoders on them, and try to control the rpm during the loaded phase - would need a very fast loop as that's only a fraction of a turn

3) Put a flywheel on each motor so that you are launching the ball with the energy in the flywheel rather than the electrical input during the period of launching itself. Adjusting the free running rpm before it grabs the ball would then be your means of control.

My guess is #3 is most practical, though #1 may work if you don't have too much tire compression, since the armature of the motor itself and the tire may store quite a bit of energy compared to the mass of the ball.

cstratton
 
Posts: 294
Joined: Wed Sep 29, 2010 3:52 pm

Re: Motor mechanism for ping-pong ball shooter

by MikeyTheHutt on Mon Oct 11, 2010 2:56 pm

Thanks for your suggestions! I think I would probably lean towards option 1, at least to start. Do you think a couple of inexpensive DC brushed motors will do the trick? What's the drawback of using a geared motor here?

I'm also considering using squishy nerf-like foam balls instead of ping pong balls.

My crude calculations are telling me I need to get the wheels (3" seems like a good size?) spinning at around 3500 RPM to achieve the kind of distance I'm looking for.
MikeyTheHutt
 
Posts: 6
Joined: Thu Sep 30, 2010 10:41 am

Re: Motor mechanism for ping-pong ball shooter

by cstratton on Mon Oct 11, 2010 3:14 pm

A gear motor would be slow and noisy.

I'd look at cheap battery powered toy type motors, and if that is insufficient then smallish R/C car motors (which aren't going to draw anywhere near their rated current unloaded).

I'd be inclined to go with 2" or so wheels - if you want to do 3" you probably need some real flywheel effect or a large motor. Or yes you could try gears, but why?

It would be very interesting if you could get a relatively low resolution encoder on the motor and record the change in pulse rate during a launch event.

Presumably, there's an oversized solution where the rpm is scarcely changed during the launch event, and there's an underpowered one where the motor slows dramatically so that very little energy is imparted, and some kind of optimal one - like as a wild guess dumping half of your flywheel energy into the ball? Or a logarithmic kind of "half" such as a quarter.

cstratton
 
Posts: 294
Joined: Wed Sep 29, 2010 3:52 pm

Re: Motor mechanism for ping-pong ball shooter

by MikeyTheHutt on Tue Oct 12, 2010 1:38 pm

Thanks again for your suggestions. This is the motor I am looking at for starters... am I barking up the right tree?

http://www.robotshop.com/solarbotics-re ... tor-3.html

One other puzzling thing about these motors is that they don't seem to make wheels to fit the 1.5mm shaft... is there a trick to these, like using some sort of a sleeve around the shaft?
MikeyTheHutt
 
Posts: 6
Joined: Thu Sep 30, 2010 10:41 am

Re: Motor mechanism for ping-pong ball shooter

by cstratton on Tue Oct 12, 2010 1:59 pm

They don't sell wheel adapters for that motor because it needs a gearbox to drive a robot wheel. In contrast you would in effect be using a "temporal gearbox" by getting a flywheel up to the unloaded speed of the bare motor and then dumping that energy into the ping pong ball.

We used to take motors like that, put heat shrink on the shaft, and then shove the small lego pinion on. I remember one of the TA's was shooting something or other across a robot arena using two motors like that mounted with lego tires.

If you really need something solid, machined propeller adapters would be available for small R/C planes using motors in that range of shaft sizes.

If this motor proves too small, I'd go to someplace like tower hobbies website and just buy their cheapest house brand R/C car motors. (Though... an issue, there's this thing in some racing motors about brush timing being optimized for rotation in one direction, and you need a contra rotating pair unless you mount one above and one below)

cstratton
 
Posts: 294
Joined: Wed Sep 29, 2010 3:52 pm

Re: Motor mechanism for ping-pong ball shooter

by adafruit_support_bill on Tue Oct 12, 2010 2:49 pm

I've used the heat-shrink trick a couple of times. It works well for prototypes, but usually doesn't hold up to much abuse. You can find bushings and flywheels for that size. Here is one source: http://www.nwsl.com/hardware-parts/bearings-bushings-flywheels-shims

adafruit_support_bill
 
Posts: 77951
Joined: Sat Feb 07, 2009 10:11 am

Please be positive and constructive with your questions and comments.