Determining Motor Speed?

[Grim Tuesday]

We had ours at about that speed, and we realized we really don't need it. You should be at something around 20 Feet/sec to be making half court shots. Having them too fast makes them draw more power, and raises the spin up time for repeating shots. Furthermore, it makes your realm for adjustment considerably lower. I would suggest changing your gearing, if you can.

[45Auto]

Our dual 775's running through cim-u-laters freespin our 8" wheel up to just over 3600 RPM, as measured with a photo-tach. Works out to about 125 fps, or 85 MPH. We use around half power to shoot from the key, but since we're using a single axle we only get half the wheel velocity as ball velocity (the rest is backspin).

At full power it's max range was right around 30 feet. We're using a high angle fixed hood, so it's not optimized for max range.

[jwakeman]

Originally Posted by 45Auto

Our dual 775's running through cim-u-laters freespin our 8" wheel up to just over 3600 RPM, as measured with a photo-tach. Works out to about 125 fps, or 85 MPH.

Am I doing my calculation incorrectly? For 3600 rpm with 8" wheel I got 40 fps.

(3600 / 60) * (8/12) = 40

Originally Posted by 45Auto

We're using a high angle fixed hood

We are also using a hood with a fixed angle of 65 degrees.

[Ether]

Originally Posted by jwakeman

Am I doing my calculation incorrectly? For 3600 rpm with 8" wheel I got 40 fps.

(3600 / 60) * (8/12) = 40

3600 rpm / 60 = 60 rps

8 inches * pi = 25.13 inches per rev

60 rps * 25.13 ipr = 1508 inches/sec

1508 inches/sec /12 = 126 feet/sec

[Ether]

Originally Posted by 45Auto

Our dual 775's running through cim-u-laters freespin our 8" wheel up to just over 3600 RPM, as measured with a photo-tach.

free speed of 775-12 at 12 volts is 7300 rpm. CIM-U-LATOR gear reduction is 2.7:1

7300/2.7 = 2704 rpm So you can't be using 775-12.



free speed of 775-18 at 12 volts is 13000 rpm

13000/2.7 = 4800 rpm

3600/4800 = 75%

775-18 at 12 volts at 75% speed is drawing 23 amps, and cranking out over 200 watts. And you say you have 2 of them. Something doesn't add up.

[jwakeman]

Originally Posted by Ether

3600 rpm / 60 = 60 rps

8 inches * pi = 25.13 inches per rev

60 rps * 25.13 ipr = 1508 inches/sec

1508 inches/sec /12 = 126 feet/sec

Oops that's right, circumference not diameter. I did have it right in our code so we max out at 112 feet/sec which is about 3200 rpm.

[45Auto]

We're using 775-18's.

Something doesn't add up.

What doesn't seem to add up? Running the two motors at 100% power through two Victors gives us the 3600 RPM as measured at the wheel. Looks like we're losing about 25% of the potential motor free-spin RPM through the speed controllers, gear train, shooter axle bearing misalignment, etc.

[jwakeman]

Originally Posted by Ether

free speed of 775-12 at 12 volts is 7300 rpm. CIM-U-LATOR gear reduction is 2.7:1

7300/2.7 = 2704 rpm So you can't be using 775-12.



free speed of 775-18 at 12 volts is 13000 rpm

13000/2.7 = 4800 rpm

3600/4800 = 75%

775-18 at 12 volts at 75% speed is drawing 23 amps, and cranking out over 200 watts. And you say you have 2 of them. Something doesn't add up.

We are using two 775's (I guess the -18's) and we max out at 3200 rpm as mentioned above. We are using chain drive with sprockets. They look like 1" sprockets on the motor shafts and 6" sprockets at the wheels (I'm not sure, I just help with the software ). It takes something like 10 or 12 amps on each motor just to get the thing moving and I think it was drawing 20 or 30 amps each at full speed.

[Ether]

Originally Posted by jwakeman

We are using chain drive with sprockets. They look like 1" sprockets on the motor shafts and 6" sprockets at the wheels

That's the part you left out.

[Ether]

Originally Posted by 45Auto

What doesn't seem to add up? Running the two motors at 100% power through two Victors gives us the 3600 RPM as measured at the wheel. Looks like we're losing about 25% of the potential motor free-spin RPM through the speed controllers, gear train, shooter axle bearing misalignment, etc.

775-18 motor at 12 volts and 75% of free speed is generating over 200 watts of output power. Times two is 400 watts. If it's taking well over one-half horsepower to drive your unloaded free-spinning shooter wheel and you're happy with that then go for it.

[45Auto]

If it's taking well over one-half horsepower to drive your unloaded free-spinning shooter wheel and you're happy with that then go for it.

That's what makes the "real world" vs paper theory interesting!

[Ether]

Originally Posted by 45Auto

That's what makes the "real world" vs paper theory interesting!

What's "paper theory" ?

[45Auto]

What's "paper theory" ?

Calculating theoretical system performance without testing as validation.

Is the battery delivering the full 12 volts? We hope so, we haven't bothered to measure it under load.

Are the Victors really delivering the full battery voltage? They're off our 2007 robot, who knows? There's a definite audible difference between running the motors hooked up directly to a battery and running them through the FIRST control system (there's probably some reason that there's a fan on the Victors).

Are the 775's performing to spec? They're the same ones we used on our forklift last year, so they already have one full season plus a year of demo's on them. They aren't even getting warm after 3 or 4 hours of practice, so we plan to keep running them.

Lots of system variables in there that aren't accounted for if you're using a nominal P=IR^2.

It's currently scoring well over 90% from the key. Due to the ball variables, the best we could hope for with a lot of additional effort is a couple of percent improvement if we're lucky. Plenty of other things higher on the priority list before we worry about that!

[theprgramerdude]

Originally Posted by Ether

You're not suggesting that they're gearing down 2.7:1 and then gearing back up 1:3 are you? What would be the point of that?


But suppose they were.

775-18 free speed is 13000 rpm.

13000/2.7*3 = 14444 rpm.

So if the output is 10000 rpm then the 775-18 would be spinning at 69% of its free speed. It would be pulling 28 amps and burning over a hundred watts of waste heat. (I'm assuming here that the OP meant 10000 rpm at 12 volts; maybe that's not what he meant).

In any event, the tangential speed of an 8" diameter wheel at 10000 rpm is 238 miles per hour.

I'm pretty sure the photosensors for counting shots will still register individual chunks of ball landing in the nets, even if the rest of the ball is obliterated on launch

[Ether]

Originally Posted by 45Auto

It's currently scoring well over 90% from the key. Due to the ball variables, the best we could hope for with a lot of additional effort is a couple of percent improvement if we're lucky. Plenty of other things higher on the priority list before we worry about that!

That's pretty darn good. Don't mess with success!

Originally Posted by Ether

What's "paper theory" ?

Originally Posted by 45Auto

Calculating theoretical system performance without testing as validation.

Ah. Never heard that phrase before. If that's what "paper theory" means I would not rely on it. Always validate simulations before trusting their predictions. That's what real-world engineers do.

they aren't even getting warm after 3 or 4 hours of practice

Does anyone know how long a 775-18 will run at 12volts & 9750rpm before it gets warm?