Wheel shooter gearing

[Clark Pappas]

Hi CD,

My team insists that we want to gear the 550 motors for our launcher up. I've been lurking around other CD threads on shooters for awhile, and most teams seem to be gearing down. We are using a two wheel system, positioned horizontally in relation to the rest of the robot.

I'm using the equations mentioned in the first response to this thread: http://www.chiefdelphi.com/forums/sh...aunch+velocity

The only issue I'm having is that the free speed RPM on the 550 motors is not actually what we are getting (Obviously). How would I calculate a reasonable estimate of how fast our wheels are spinning?

One 550 motor on each wheel
Two 8" Lawn mower wheels (At least, that's what we are using as of right now. They weigh around 7 pounds)


P.S: Bear with my lacking knowledge of physics, I've only taken conceptual, and didn't pay much attention.

[tzjin]

Our team typically uses the free speed to do most of our calculations. Truth is, you will probably be spinning quite fast since the wheels are under no load, but when the ball comes through, the impact will be pretty much impossible to calculate. Just test it.

But yes. Don't gear up those motors. I hear good things about Andymark's 3.67:1 gearbox paired up with six-inch wheels, though 8-inch may work too. You will most likely burn out those motors even if you direct drive them, and the wheels will take a very long time to speed up, and regain speed once a ball passes through.

[R.C.]

Clark, I would highly suggest gearing down to anywhere between 5,000 to 7,000 rpms. Free speed is way to darn fast.

-RC

[sdcantrell56]

Originally Posted by R.C.

Clark, I would highly suggest gearing down to anywhere between 5,000 to 7,000 rpms. Free speed is way to darn fast.

-RC

To echo what RC is saying not only is it too fast but it is DANGEROUS!

[jwfoss]

With 8" wheels you definitely want to gear the motors down. How much to gear down will depend on your configuration and desired range. In our experience single axle/hood shooters need to be geared 1/2 as much as a dual axle shooter to achieve the same distance. In other words, a single axle system has to spin twice as fast to get the same exit velocity.

For reference our design uses two 6" wheels on a single shaft with a hood with a 4:1 reduction using 2 FisherPrice (0673). We can shoot from the key and forward without a problem.

[Jon Stratis]

Here's our setup:

BaneBot 4 7/8" wheels on top and bottom, each independently powered by an RS-550 with a 7:1 gear reduction.

We measured the current into the motors when it was running at "full speed" to be about 7A each. Plug that into Ether's Motor Calculator, and we see that the speed of the motors is 18007 RPM. That gives us a speed of 2572 RPM for our shooter wheels, which is quite sufficient (we can hit the center of the top backboard from midfield, when running at "full speed").

Now, given that we're using smaller wheels than you, you should take that difference into account - you need about 5/8 the speed we have to get the ball to go the same distance - or about an 11:1 gear reduction.

[Ether]

Originally Posted by Clark Pappas

How would I calculate a reasonable estimate of how fast our wheels are spinning?

Method1:
Measure the motor current (at full voltage). Then you can use the motor curves to get an estimate of speed (see Jon's post#6).

Method2:
Temporarily mount an encoder or tach and read the speed.

Method3:
Fasten a small zip-tie to the motor wheel, then mount a small piece of card stock so the zip-tie hits it and makes a clicking sound once per rev. Record some audio of the wheel spinning then digitize it and run frequency spectrum analysis software to locate the frequency of the clicking.

Method4:
Borrow a strobe light with a strobing frequency readout. Find the fastest strobing frequency that stops the action*.


* paint a dot or put a small piece of tape on the wheel (see Jon's post#9) so you don't get fooled by stopping the action on the spokes - if it's a spoked wheel)

[engunneer]

Originally Posted by Ether

Method3:
Fasten a small zip-tie to the motor wheel, then mount a small piece of card stock so the zip-tie hits it and makes a clicking sound once per rev. Record some audio of the wheel spinning then digitize it and run frequency spectrum analysis software to locate the frequency of the clicking.

Genius! I've been trying to get some kind of tach together, but this one we can do with parts on hand. Thanks!

[Jon Stratis]

Originally Posted by Ether

Method3:
Fasten a small zip-tie to the motor wheel, then mount a small piece of card stock so the zip-tie hits it and makes a clicking sound once per rev. Record some audio of the wheel spinning then digitize it and run frequency spectrum analysis software to locate the frequency of the clicking.

hmmm... this is the only method we haven't used so far this year for our shooter, and now I want to try it!

We used a strobe (borrowed from work) on our prototype to figure out how fast it was going (it's easiest to tape a small strip of white paper on the wheel to to help you know when the action is stopped), since we were powering it with drill motors, and everything else seemed to complicated/long to figure out.

We measured the current to make sure the motors weren't going to burn out, and once we had that number, it was a fun little exercise figuring out the exit velocity of the balls (assuming no slippage).

We have encoders on the shooter to allow full PID control for accurate shooting. While I haven't seen it yet (programming has only had access to the robot for a couple of days), we are supposed to have distance detection working, and should be able to automagically adjust the shooter speed for a perfect basket every time.

[Ether]

Originally Posted by Jon Stratis

we are supposed to have distance detection working, and should be able to automagically adjust the shooter speed for a perfect basket every time.

To do this, are you planning to use test data in a lookup table, or use trajectory equations? Or perhaps some other method such as a math model whose parameters you tuned using test data?

[Jon Stratis]

That's a very good question... Unfortunately, I haven't worked much with the programmers, so I don't know what their strategy is! I think they were going to take it in to our practice field tonight to start working on making baskets, so hopefully we'll know if it works (and exactly how!) by this weekend.

[JesseK]

We're using a single 8" wheel, offset flywheel with adjustable weights, and 2 BB550's through 4:1 transmissions and linked via chain. We're going to play with the belt cogs a bit, but right now they're something like a 16:36 reduction coming off the transmissions for about 2,000 RPM final speed.

It shoots very consistently from the key without the need for a spin-up time. We're working on range, range control while driving, and other minor tweaks though. We may also have to redo our shooter guide for less compression since the IFI 8" wheel has a very grippy tread. I suspect that's why we're falling short of our max theoretical range.

To control it (I think) we'll have an encoder with camera ranging and a lookup table for range:speed lookups. I'm not on the programming side, so that's just a guess.

Hope that gives you an idea of what to try. Good luck!

[Ether]

Originally Posted by JesseK

I suspect that's why we're falling short of our max theoretical range.

Does this agree with your numbers?

69.8 feet/sec wheel tangential velocity,

34.9 feet/sec ball exit velocity,

38.1 feet theoretical* max range

* no drag, no magnus

[JesseK]

It's at 55 degrees, so theoretical's a bit shorter than that. Plus the digital tach read anywhere from 1800-2000(ish) RPM's. I think I remember the theoretical being in the 30-34 foot range after adjusting with the tach's #'s. During our first tests we were more in the 24-25 foot range. Perfect for the key, but imperfect for our higher-order strategy (though even that strategy is still workable at the current range so we're not in panic mode).

[GW Kalrod]

Your team wants to gear a 19,000 rpm motor up?

I'd get a basic FIRST concepts/Physics concepts PowerPoint rigged up for your team.