Small Drive

Hi One & All,

This is my first time on this forum and I need some help with designing a small drive for a wagon. I need very steady and precise speed. With what I have gathered from the Internet that a STEPPER MOTOR would best, cheap and easies to do the job but not how to calculate correct size is not real there.

Specifications

1. The “Target Speed” of the wagon is about a half an inch per second.
2.Need a “HOLD” “RUN” control.
3. Need to vary the speed, about + 10% of “Target Speed” and still keep steady and precise speed.
4. Need to reverse the unit direction from time to time.
5. I would like to keep the total added weight (Stepper Motor & Driver) to two pounds or less.
6. The wheels on the wagon are four inches in diameters
7. The total weight of everything on the wagon and the wagon is about forty pounds.
8. The unit needed to come up to target speed fast, stay at that speed and STOP as fast as it can.
9. The driver wheel on the stepper motor was thinking of using a skate wheel which comes in diameters from 50 mm to 100 mm.
10. Wagon will be travelling level surface.
11. Drive and controller will be powered by BATTERIES.

The Ramsey Electronic controller has the features that we were look for and I believe it can drive NEMA 23 & 27 stepper motors but I am not sure?

www.ramseyelectronics.com/downloads/manuals/SMD1.pdf

What I need to know the following:

A. The NEMA type stepper motor?
B. The gear ratio on the stepper motor. 1:5, OR 1:10. OR 1:50, OR 1:100?
C. Power requirements of the stepper motor?
D. Size of the wheel on the stepper motor to drive the wagon wheel?
E. The frequency to drive the stepper motor plus pulse shape?
F. If possible I would like to use the Ramsey Electronics stepper motor controller!

I think I covered everything? Thank you for any help that you can give me in this project.

BEN

If it is battery powered then a stepper is a poor choice. An ordinary electric motor will take a big swallow of power to get you up to speed, draw peanuts to hold speed, draw nothing to slow you down if you use it as a brake. A stepper will draw the same regardless and eat your battery.

When you use a bog standard DC motor for positional control it gets called a "servo motor". The best DC motors, ie: most efficient, are the rare earth motors from Maxon. Less efficient motors are lots cheaper but you will need more battery and they make crummy brakes.

Speed is directly controlled by the voltage applied, positioning requires an optical "chopper" disc on a shaft somewhere.

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...

Hi One & ALL,

Thank you for the information and ideas. I will be go over everything and if I have question will post it.

Again thank you very much.

BEN