Controlling a motor (stepper?) with the Propeller

tdg8934

I have started a new job where I am building test fixtures for new products. I have the fixture built that uses a compact screw jack rated for 500 lb lifting. Originally I used a custom handle on the side shaft of the jack but I had to turn in 20 turns to lift the jack 1 inch so I decided to go with an electric drill. I found an 8 Amp one online and it works well. On the side shaft of the jack, I have attached a corded 3/8" Dewalt DWD112 VSR drill (8A, 120vac, 650W, 0-2500RPM). It has more than enough torque and drive to lift up a few hundred pounds on my fixture. However, after going through a review, the other Engineers thought I should use a stepper motor more permanently mounted to the 80/20 aluminum framed fixture. I guess they were afraid of someone just taking the drill off of the jack side shaft and it never being seen again. I'm not a mechanical engineer but electrical engineering trained so working with this is new to me.

What I wanted to find out is how to "size" a stepper motor or some other kind of controllable motor (with control buttons for lifting the jack "Up" and "Down"). I did see that the screw jack has an input torque of 17 in.-lbs @ 500 max rpm. How do I convert the information I have on the screw jack and drill(?) to what will be sized right for a Propeller to control? Where do I begin? I would think that someone has done this before or something like it. I would like to use a Propeller as I have one already and a little familiar with how it works but never worked with stepper motors (only those Parallax smaller Servo motors with a Basic Stamp and SX28).

Can anyone help me here? Let me know any other information you need to help me with this.

Thank you

lardom

A DC motor has more torque than a stepper for it's size. A stepper can also lose torque the faster you drive it since the coils have less time to become fully energized.

Mickster

To get an understanding of motor-sizing and motion control in general, this is a great resource.

With all respect, if you were doing it on my time, I'd have you go out and purchase an off-the-shelf solution.

Mickster.

kwinn

My best guesstimate would be that you would require a NEMA 23 size stepper or something close to that size/torque rating to replace thie average drill. You will also need an appropriate stepper motor driver for the prop to drive it. If you want this to be automatic you will need some force or position feedback from the fixture as well.

Please note I said GUESStimate. Knowing the desired speed, applied force, and torque required to apply that force would allow you to calculate the minimum torque required from the stepper.

groggory

lardom wrote: »

A DC motor has more torque than a stepper for it's size. A stepper can also lose torque the faster you drive it since the coils have less time to become fully energized.

It sounds like you don't need super precision, so yeah..I'm with the DC motor thought.

Strong DC motor + H-Bridge w/ support circuit + Then add safety switches to the top and bottom of the rig so that a user can't shove it all the way up or all the way down. Or of course you could just monitor the current and assume that if the motor is maxing out the current drive, they probably jammed it up against the top or bottom.

Either way, seems like a pretty straight forward setup.

tdg8934

Thanks for the great input!

What I found from McMaster-Carr was the following:

I knew that the compact screw jack had an input torque of 17 in-lb @ 500 rpm max. They had a right angle DC Gearmotor that was 1/4 HP with a listing of 30 in-lb and 500 rpm for $469. I beleive this will cover it (Does this sound right?). However, it works on 90 vdc and they said I need an AC to DC Motor Speed Controller (reversible) so this is another $355. Then I need a 2 button handheld Pendant switch ($88) for up/down movements with 2 limit switches ($43 X 2 = $86) to stop the motor if it goes too high or low on the screw jack thread. Total cost here = $998. Plus wire, cable snaps, plug, etc. = $1064 spent.

What's ironic is that I did all this with a $70 drill on the shaft of the screw jack - but for a more permanent solution so that the drill doesn't get legs and get taken is to spend almost $1100. Hmm

Anyway...Is there anyway with these items, I can still control the DC gearmotor with a Propeller (or is that only possible on Stepper motors, servos, etc.)?

Thanks,

Tim

RS_Jim

Tim,
Perhaps some creative modification of the drill assembly like removal of the battery compartment and the chuck would discourage the drill from growing unwanted legs. Even with the drill modified I would put some limit switches on the system. Micro switches are cheap at Radio Shack.
Jim

Circuitsoft

You can effectively get 88vdc (close to 90) with a half wave rectifier and no filter cap off of 120v line. From there, if you need speed control you could use a large MOSFET or DC Solid State Relay to PWM the motor from a prop pin.

pedward

Honestly, sometimes stupid-simple gets trumped by $$$ because of things like common sense. Sure the drill worked, but it presents a hazard in 2 ways: a) someone takes it because lacking common sense b) OSHA throws a fit because it relies on common sense not to do something stupid.

Guarding, limit switches, and insulated/isolated controls are all things brought to us by dumbasses. Sure, I have no problem with an unguarded table saw or cutoff saw, but if some idiot horses around or is high, tired, or just stupid, then it spoils it for the rest of us.