Small Drive

[irving2008]

Notwithstanding Robin's thoughts, all valid, lets follow the stepper approach through...

ok... some basic physics... 4"dia wheel = 4*pi = 12.56" circumference, so 0.5ips = .5/12.56 = .04revs/sec = 2.38rpm

I'm going to swiitch to SI units cos the math is easier 40lb = 40/2.2kg = 18.2kg

lets say we want to be up to speed in 0.1sec so we need to accelerate from 0 to 12mm/sec in 0.1sec, using v = u + a t then 0.012 = 0 + a * 0.1 => a= 0.12m/s^2

Force = mass x acceleration = 18.2 * .12 = 2.184N and the radius of the wheel is 50mm so the torque = 2.184 * .05 = 0.1Nm this is well within range of a NEMA23 or even a NEMA17 motor and direct drive

Assuming direct drive and a standard 200step motor, the step rate will be 2.38*200/60 steps/sec or about 7.9steps/sec.

The Ramsey board will certainly do the job, though you may need to adjust the capacitances to get the low step rate needed. Alternately you could gear the motor up 5:1 to get a step rate of 40stesp/sec (and needing 5 times less torque from the motor) or drive it externally from a simple 555 pulse generator (if you need help on that one let me know).

A couple of thoughts on the driver board: The driver board chip (MC3749) is limited to 350mA/phase. Most NEMA23 and NEMA17 motors are rated higher than that. That in itself isnt an issue, the driver will limit the current anyway and the torque from the motor will be limited accordingly. Since you need 0.1Nm (assuming not geared) a 1.4A motor rated at 1Nm will actually give 0.1Nm on 200mA, so you can choose a motor that gives you the best current draw and battery life. However you do need to keep the current below 200mA/phase otherwise the driver chip will overheat - or else fit a heatsink. If the board is encased in an enclosure you will need to ensure adequate airflow even with a heatsink.

Hope that helps as a starter... feel free to ask further questions...