Select a motor

[brewcore]

We need to select a motor first; we'll need to know the power requirements and specs of the motor before we can decide on other parts of the system.

[jd-boyd]

Emanuel mentioned https://www.servocity.com/

It looks like everything there wants something in 4.0 - 8.0 volt range (or some smaller range inside that) and a 5V (3.3V tolerant) PWM control signal.

It looks like torque is measured in oz-in, but I have no idea how much is needed for this.

Also, some of the smaller ones there are still 0.86” x 0.43” x 0.91”, and 18 of those seem to be pretty large.

[brewcore]

So, first problem to solve: how much torque do we need per motor? I'll try to get ahold of Emanuel, see if he can provide some insight.
I've thought about it a bit, and a related question will probably be: how much force will it take for our hand to grasp an object?

[jd-boyd]

http://msis.jsc.nasa.gov/sections/section04.htm#_4.9_STRENGTH

That page seems to suggest that a 5th percentile U.S. Navy personnel male has a thumb finger tip grip force to hold 8lb. I think it is close enough in this case to say that the thumb and finger each contribute 4lb, which would mean in joint might contribute 2lb?

From: https://www.servocity.com/html/what_servo_do_you_need_.html#.VP9l883d-J8

To convert in-oz. to pounds of force, divide this rating by 16.

That suggests motors to do 32 oz-in of torque. I think we could do with less, considering that a child can hold a full 16oz can and they have to be considerably weaker than a 5th percentile naval male.

This:
https://www.servocity.com/html/s3117_servo.html#.VP9o683d-J8
is a 23.6 oz-in server for $20. That is 2/3rd of my estimate derived from that chart, but while less power than a $18 unit (the cheapest one), it has the advantage of also being significantly smaller. Plus, that is the power at 6V. At 4.8V, it is 20.8 oz-in, suggesting that we could get away with running it off a cheaper/easier to get 5V power supply.

Come to think of it, a 16oz can is only about a pound so arguably just about anything sold on that site should be strong enough, so it looks like it is a mix of size and cost to decide.