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.
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.
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.
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.
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.
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 [strike]motor[/strike] 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)
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.
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?
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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.
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!
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).
@Clark: would you please review your original post and clarify if that’s what you meant? I assumed you meant you wanted to lower the speed (although as Nick says your wording suggests otherwise).
If your team is indeed pushing for increasing the speed up from 19300, there’s no way you want to do that.
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Sorry for the confusion everyone. I’ll clarify (This is what i get for typing while i’m tired :o)
The team insisted we want to gear the 550s UP rather than DOWN, and that didn’t sound right to even me, who has only taken a conceptual physics class I never paid attention in. I looked around CD, and need a little more information and help to prove to my team we want to gear down, and to get some sort of range of gearing ratios. Had our design not get changed again, the information in this thread would be ample to get us going.
However, the team says they now want to switch to a launch system drive by one side of wheels (underneath). So, now I’m lost completely in the physics (again), and stressing out about deadlines (again).
Any additional help would be appreciated. More gearbox recommendations would be appreciated as well; seeing as ship day is too close for us to get things ordered (Especially with the way our school processes orders), we will be having to add gearboxes at the competition. The plan is to bring it all as-is when shipped to the competition and add it there, since they could be considered a COTS part.
If you want to go with a single-wheel shooter with a hood, just think about what the ball is doing. In that case, it rolls between the hood and the shooter, which cuts its speed in half, when compared to a two-wheel shooter design. So, where my team has a 7:1 reduction for a two wheel shooter, we would want a 3.5:1 reduction for a single wheel shooter. That would double the speed of the shooter wheel, which would keep the speed of the ball the same as we have now.
Ha, I screwed up. When we cut the launch angle in the guide I measured it from the normal plane rather than the parallel plane. So it’s inverted with a flatter trajectory than we want. Theoretical distance is the same, yet we need the extra height for the high goal. We also comepletely re-fabbed the shooter shaft since there was a slight wobble, and relocated one of its bearing blocks since it was slightly out of alignment. All told, I think we’re back up to 30 feet-ish (ball barely peaked before it hit the wall at 15 feet) with the flat trajectory. Once we finish sensor wiring tonight I’ll know more.
There would be significant magnus effect if the ball’s trajectory went further or higher – we hit the wall at probably > 70 degrees angle of incidence (20 degrees from parallel with the floor) and the ball came almost straight down due to the backspin. May make for some interesting layup shots.