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[on:]

i've been meaning to get into arduino for a while but i suck at learning to do something unless i need something specific done. well, now i do! for the main part of this project, i need a motor to spin in one direction. the motor will be connected to two arms, each about 12-18in long with a 5 oz weight connected to the end. the speed of the motor would be controlled by another sensor in the circuit, but i'd like to have it be variable and have the arms on a hinge so that at a low speed they spin while still pointing down into the center, and then at max speed they spin out so they are between 45 degrees and parallel to the ground.

i was going to buy an uno and something like the Adafruit Motor/Stepper/Servo Shield, but i don't think that will be powerful enough to control the motor i need since it can only manage 1A. i was thinking a motor like this http://www.robotshop.com/banebots-first-cim-motor.html could probably do what i need. what kind of motor control would i need? i'm figuring i'll power it off a nimh battery pack, but i'm not opposed to running it off ac wall power if that's possible without killing myself.

[Reply #1 on:]

Mechanically, I would use a spring that could be tensioned for the part that spins outwards at a certain rpm, and the motor might need to be geared.

Electrically, find a logic level mosfet that can handle the draw of the motor, use a pwm signal to speed up or slow down the motor(drive the mosfet), and a power source strong enough to power the motor.

[Reply #2 on:]

There are some excellent 12V geared motors on eBay - you do need to find out the maximum RPM your project requires before ordering a motor, and also the torque required...

The motor you quote is 5280rpm, which with 12" long arms attached would be a lethal weapon (quite literally).  That motor's specs seem very extreme (not what you want at all?) with a stall current of 115A.  A motor controller for it will require over-current detection and shutdown.  Any thoughts of using a simple single logic MOSFET to control it are doomed to expensive failure I think!

For 45 degree outswing on 12" arms I reckon you want about 55rpm.  For 18" arms about 45rpm.  Formula for centripetal accelaration is rw^2
(r = radius in metres, w = angular velocity in rad/s).  For 45 degree outswing the centripetal acceleration is equal to that of gravity, ie about 10m/s^2

[Reply #3 on:]

two arms, each about 12-18in long with a 5 oz weight connected to the end.

Are you going to have some sort of containment system? Those weights are going to have a heck of a lot of energy and if anything goes out of balance or gets detached this could be very dangerous.

[Reply #4 on:]

I knew physics class would come in handy someday.  

I was looking at stall torque to choose that motor, the other ones on that site seemed to weak to get the arms up. I don't seem to see that specification on most of the other motors I looked at.

So it seems like I want something that can vary from 10-60rpm. Should I assume that a motor capable of 60rpm can vary that much? does the speed change depending on the voltage? I'm thinking something like http://www.allelectronics.com/make-a-store/item/DCM-518/PRECISION-GEARHEAD-MOTOR-USED/1.html or http://www.allelectronics.com/make-a-store/item/DCM-365/6VDC-GEAR-MOTOR/1.html would be good to start? Would those be small enough to work with the motor shield?

Thanks for the help.

[Reply #5 on:]

two arms, each about 12-18in long with a 5 oz weight connected to the end.

Are you going to have some sort of containment system? Those weights are going to have a heck of a lot of energy and if anything goes out of balance or gets detached this could be very dangerous.

I don't think it will be that dangerous at the speeds I'm imagining. I understand balance is going to be really important though.

[Reply #6 on:]

I knew physics class would come in handy someday.  

I was looking at stall torque to choose that motor, the other ones on that site seemed to weak to get the arms up. I don't seem to see that specification on most of the other motors I looked at.

Torque isn't important for the arms to raise (assuming low friction), only angular velocity.  To get up to speed quickly you do
need more torque - so how quickly do you want to change angular velocity?

Multiply that rate by the moment of inertia and you get the torque required (all SI units
of course).  Little maths refresher might be required for those MoI calculations of course

Oh and whatever routine torque you want the motor to supply should be quite a bit less than the
stall torque of the motor, perhaps a third or less, in order to prevent it struggling (or excess strain
on the gearing).

[Reply #7 on:]

I don't think it will be that dangerous at the speeds I'm imagining.

What speeds are you imagining?

[Reply #8 on:]

What speeds are you imagining?

What mark said about 45RPM seems about right to me. I don't want it spinning so fast that the arms are parallel to the ground, which I assume would happen before things started flying off.

Torque isn't important for the arms to raise (assuming low friction), only angular velocity.  To get up to speed quickly you do
need more torque - so how quickly do you want to change angular velocity?

I think 0-45PM in 3-5 seconds is about what I'm aiming for. I'm quite certain I will screw this up somehow so I am assuming I may try more than one motor. So my Arduino will send a PWM signal to the motor shield, and that controls the speed of the motor?

For my purposes, a geared motor just means more torque, right?

Are there any retailers you'd recommend for parts like this? The amount of specifications given for little DC motors seem all over the place.