Pitching Machine Motor Calculations

[Cem8301]

My team is testing our pitching machine styled shooter and we were trying to figure out which motor is most ideal.

My first thoughts were to choose a motor with the highest rpm. However, it concerns me that those motors have a smaller torque and could potentially end up failing and/or providing WAY too much speed to ever be useful. Is there a way to calculate the torque placed on motor in a pitching machine setup? Does this depend on how much 'squish' we place on the ball when it is fed through?

Or what are good motor specs to look for? Currently we are considering the FisherPrice motor (fc12-28), with a stall torque of 0.532 Nm and an rpm of 20770.

Any help would be greatly appreciated

[the man]

You definitely do not want the wheels trying to spin at 20,000 rpm. Gears, belts, or chain are your friend. They will multiply torque and reduce speed and reduce current draw. The fisher price motors are what we chose. some where around a 4 or 5 to one ratio. We tested ours and it spun up the wheels at only 5 volts.

[PAR_WIG1350]

You definitely do not want to base this decision purely on RPM, but, based on your torque concern, it would appear that you already know that. The crucial number here is max power. Power is what lets you move the balls. RPM and torque can be changed by gearing, but the power is fairly constant (friction can reduce it some). If you don't use a motor with enough power, no gear, chain, or belt configuration will be able to fix anything (springs might, but I won't bother you with the details, picking the right motor is really the better solution).

Originally Posted by the man

You definitely do not want the wheels trying to spin at 20,000 rpm. Gears, belts, or chain are your friend. They will multiply torque and reduce speed and reduce current draw. The fisher price motors are what we chose. some where around a 4 or 5 to one ratio. We tested ours and it spun up the wheels at only 5 volts.

Watch out, the jump from 5 to 12 volts is a big one.

[the man]

Originally Posted by PAR_WIG1350

Watch out, the jump from 5 to 12 volts is a big one.

Yeah the conditions were less than ideal but we will control the speed carefully until we get it right. We used a car charger that put out 12volts and 6 amps with a variable voltage input. But the motor drew to much and it could only supply 5. But it worked even with only 5. So take that what is worth.

[Cem8301]

Originally Posted by PAR_WIG1350

The crucial number here is max power. Power is what lets you move the balls. RPM and torque can be changed by gearing, but the power is fairly constant (friction can reduce it some).

Does it sound reasonable to hook up the fp motor I mentioned to a GEM single staged gear box? That gear box has a 3.67: 1 ratio. That would make the new rpm: 5449.59 and the new torque: 1.95.

The current at stall (the power?) is at 291. Is that reasonable? I don't know how to interpret that number.

[the man]

You have to remember that rpm or 20,000 is free speed. Once you put a load on it you will lose some speed. Also depending on what wheels you use, assuming a ball launcher using wheels, you may want a different out put speed. And like I said a 4 to 1 provided enough torque to spin up two 8" wheels.

[Cem8301]

We are planning on using three 8" wheels.. so awesome!

One other concern... Can we expect that we can get almost full rpm capability out of the motor? Like my team practices only using 1/2 of the torque capability.. It seems to me that this is a very different application. My calculations read that the exit tangential velocity will be around 63 ft/sec using the GEM gear box and the fp motor combination. Disregarding air resistance and other non motor related variables... can the motor preform like it is expected? Or are there other things to worry about?

[pfreivald]

It is worth noting that the motor doesn't shoot the balls -- the flywheels do. As long as the increase in load is sufficiently small (and/or of sufficiently short duration), you should be able to get away with running powerful motors at various duty cycles to achieve different speeds, and do so without massive current demands.

Note that this is inherently what Victors and Jags do to control motor speed.

[MrForbes]

Originally Posted by Cem8301

One other concern... Can we expect that we can get almost full rpm capability out of the motor? Like my team practices only using 1/2 of the torque capability.. It seems to me that this is a very different application. My calculations read that the exit tangential velocity will be around 63 ft/sec using the GEM gear box and the fp motor combination. Disregarding air resistance and other non motor related variables... can the motor preform like it is expected? Or are there other things to worry about?

The shooter motor is not loaded continously, so it will indeed be able to reach almost the full rated speed if, you give it full voltage. There is no torque available at maximum specified "no load" rpm, so the speed will actually be a bit lower, because of friction.

You'll depend on the inertia of the wheels to put energy into the ball, and the motor will then have to work to get the wheels back up to speed, which shouldn't take too long. So the wheels will slow down some each time you shoot a ball.

[wireties]

FIRST Team 1296 is thinking of a transmission that will host 2 FP motors geared 4:1 and 2 AndyMark motors geared 3:1. We'll run 2 wheels (probably), perhaps with one geared to run 5% slower (to create backspin). We are thinking such a device could easily shoot from the the top of the key on the opposite side. This is not necessary of course but would be a kewl demo after the competition season.

Does this sound crazy?

[pfreivald]

Originally Posted by wireties

FIRST Team 1296 is thinking of a transmission that will host 2 FP motors geared 4:1 and 2 AndyMark motors geared 3:1. We'll run 2 wheels (probably), perhaps with one geared to run 5% slower (to create backspin). We are thinking such a device could easily shoot from the the top of the key on the opposite side. This is not necessary of course but would be a kewl demo after the competition season.

Does this sound crazy?

Why would you gear it to run slower when you can just put it on a slower duty cycle with the speed controller?

[Cem8301]

Originally Posted by pfreivald

you should be able to get away with running powerful motors at various duty cycles to achieve different speeds, and do so without massive current demands.

Can you explain this? Is there an equation we can use to determine how the the duty cycle determines the speed? Or would we just have to read one of those motor graph charts?

[Ether]

Originally Posted by Cem8301

Can you explain this? Is there an equation we can use to determine how the the duty cycle determines the speed? Or would we just have to read one of those motor graph charts?

At constant load, the speed varies approximately linearly with duty cycle.

The windage of a spinning wheel varies with speed, so the load is not constant. So the speed vs duty cycle is less linear.

You could do this empirically. Run different duty cycles and observe the effect of motor speed on your launch distance. Put the data in a table in your software.

The problem with that approach is that the voltage delivered to the motor at a given duty cycle varies with the bus (supply) voltage. If you are using CAN, you can use the "voltage" mode rather than the "%Vbus" mode, and it will compensate for changes in bus voltage.

Finally, you could put an encoder on your spinner and use rate feedback to control the speed.

[wireties]

Originally Posted by pfreivald

Why would you gear it to run slower when you can just put it on a slower duty cycle with the speed controller?

For more torque so the launcher wheel does not slow as much after shooting each ball. And the 3:1 versus 4:1 will match the power curves of the 2 motors (kinda) so both are operating in their sweet spot/range when giving us the RPMs we need. The 2 FP motors will probably do the job though.