A question about stepper motors

[Cajonpassfan]

Some of the turntable powering kits out there utilize “stepper or step motors”.
I understand each impulse moves the motor one “step”, but with continuous current, the motor shaft rotates smoothly (?) until interrupted. Is that about correct?
Many of the 12V motors I’ve looked at online have 200 steps per rotation, or 1.8 degrees per step.
So if I were to use a stepper for my San Bernardino turntable and roundhouse with its 7.5 degree stalls, I’d need either 1.5 or 2.5 degree steps (or 144/240 steps per revolution) to make it work. Anything out there that fits these criteria? Also, the motor specs I’ve looked at don’t seem to include rpm information...
Advice appreciated,
Otto K.

[StarCruiser]

I'm not sure on this but, it's mostly 200 or 400 step that are available.

You may be able to use an indirect drive and use gears to match the final angle per step. 1.8 degrees doesn't divide into 7.5 degrees very well so..? You may have to compromise a bit.

[rodsup9000]

 I've worked with both 1.8 and .9 degree steppers. Most stepper drivers will allow you to set the micro steps in full, 1/2, 1/4, 1/8 and 1/16. This translates to 200 steps, 400 steps, 800 steps, 1600 steps and 3200 steps per revaluation with a 1.8 degree steppers. With .9 steppers, the steps would double those of the 1.8 degree steppers.
 

   

[wvgca]

steppers use gears to [mostly] get the resolution that you want ..
and speed is a function of how much current, and how frequently that you drive the motors ..

[peteski]

While stepper motor might appear to rotate smoothly at higher speeds, the movement is still in (very fast) steps.  That makes them rather noisy.  But it might be ok for driving a turntable.

There is no "continuous current" running.  Stepper motors have at least 2 windings (which means they have 3 or more leads, often 4 or 6 leads), and they require electronic circuitry that will provide the sequential power to the coils to produce the stepping motion.

Form more  info on how those operate, see https://en.wikipedia.org/wiki/Stepper_motor

To see some examples of how to run them, do a Google search for "stepper motor driver".  Often microcontrollers like Arduino are used for driving them.  Since those devices are programmable and expandable, they make a good TT  motor controller. In addition to the stepper motor driver routines, you could add indexing and human-interface, so you could have the entire controller running of that single board.

[railnerd]

Stepper motors are pretty neat, but they do usually require a driver or controller to make them work.

Pololu, a robotics hobbyist shop in the Las Vegas area makes some great little boards for interfacing to stepper motors.

Driver boards typically go between a microcontroller/Arduino and the motors:
https://www.pololu.com/category/120/stepper-motor-drivers

They also make some standalone controllers which include the controller:
https://www.pololu.com/category/212/tic-stepper-motor-controllers


If you do end up with a gear setup, you'll want to consider "backlash"— which is caused by the spacing in the gear system.

-Dave

[Cajonpassfan]

I'm not sure on this but, it's mostly 200 or 400 step that are available.

You may be able to use an indirect drive and use gears to match the final angle per step. 1.8 degrees doesn't divide into 7.5 degrees very well so..? You may have to compromise a bit.

Yes, compromise a bit... At 7.5 degrees (per prototype) and only 42’ between the pit and the roundhouse, it’s already extremely tight at the edge of the pit and at the roundhouse entrances. So compromising at 7.2 degrees is probably out. I could possibly do 9 degree segmentation and lose some stalls...that maybe okay. Got to think about that...

Thank you all for contributing to this thread, this has been very informative! Although, I find the electronics/driver side intimidating, and would likely need help if I were to pursue this. Perhaps the old eyeball method utilizing a slow RPM motor and a center-off toggle is an easier way to go... got to think about that, too.
Thanks much again,
Otto

[peteski]

Thank you all for contributing to this thread, this has been very informative! Although, I find the electronics/driver side intimidating, and would likely need help if I were to pursue this. Perhaps the old eyeball method utilizing a slow RPM motor and a center-off toggle is an easier way to go... got to think about that, too.
Thanks much again,
Otto

Well, the slow-RPM DC motor, with its speed even further adjustable by a variable DC supply (like an old DC throttle) and eyeballs, would certainly be the simplest solution.  But if you want to go hi-tech then a geared stepper with a  micro-controller would be in order. Like others said, gearing the stepper motor woudl result with much smaller steps, plus micro-stepping to further reduce the angle of the steps might be a viable solution.  Gear lash could be a problem, but most N scale turntables use gears (which I'm sure aren't very precise) and they work well.

[Cajonpassfan]

Well, I ordered  a 3rpm 12V motor from Amazon. Figured I can slow it down with one of my old now useless DC throttles. Don’t need much torque, found a radial bearing that’s pretty sweet at a local ACE hardware for like four bucks. More on my LA Division Engineering thread (coming). I can always revisit the stepper motor idea in the future.
Thanks for the feedback everyone, very helpful (but hey, that is the Railwire .
Otto

[wvgca]

three rpm may be a bit fast  .. most of the model train turntables speeds are about half an rpm  .. mind you there are always variances, lol

[peteski]

three rpm may be a bit fast  .. most of the model train turntables speeds are about half an rpm  .. mind you there are always variances, lol

3 rpms is at 12V, since it uses a standard permanent-magnet DC motor with a large ratio gearhead, if he runs the motor at a lower voltage, it will likely run much slower. Even at slower speeds there should be more than ample torque to drive the bridge.

[Cajonpassfan]

Well, exactly, that is my hope anyway, to slow it down. As to how much, I inquired about the speed of this turntable’s rotation on another (Santa Fe Cajon Group) site and the answer is about 24 seconds per quarter turn. At least that’s what a video of the ATSF LA 120’ turntable shows. That would work out to be 1.6 minutes per turn. The table is /will be riding a radial bearing, see below, so in theory, it’s should rotate smoothly without the need for a lot of torque. Time will tell...
More on my LA Division Engineering thread.
Thanks, Otto

[Cajonpassfan]

Oh, got the motor today. As advertised, and blissfully silent. I tested it with an old MRC power pack, got it down to below 1RPM with plenty of torque left. It draws like nothing...10 miliAmps, 15 stalled. I maybe able to use a basic loco decoder and run it off the DCC roundhouse/turntable feed. That would save me running separate DC power to the (removable) roundhouse module. Two wires, that’s it. And, no dedicated turntable panel required, just the “engineer throttle” already affixed to the fascia. Address 1, variable turntable speed, top end speed preset, direction. Hmmm, I like it I think...
There will be more on my ATSF Los Angeles Division thread, I’m sure.
Otto

[kondor]

I'm picking up a project I started a while ago, a stepper motor powered turntable.
I'm using an Arduino, stepper motor driver, stepper, and a toothed belt/pulley system. I have my initial mockup running a homing routine with nice slow acceleration and deceleration and I have been working on moving the code to the next step of functionality which will be simple push button selection of the tracks. Beyond that is a small LED interface for track and direction selection since I have 22 possible positions (11 tracks and the table can face 2 directions per track).

I like your DC motor approach, it sounds like you're most of the way there already! At my rate of progress I'll be done in five more years.

[MDW]

Oh, got the motor today. As advertised, and blissfully silent. I tested it with an old MRC power pack, got it down to below 1RPM with plenty of torque left. It draws like nothing...10 miliAmps, 15 stalled. I maybe able to use a basic loco decoder and run it off the DCC roundhouse/turntable feed. That would save me running separate DC power to the (removable) roundhouse module. Two wires, that’s it. And, no dedicated turntable panel required, just the “engineer throttle” already affixed to the fascia. Address 1, variable turntable speed, top end speed preset, direction. Hmmm, I like it I think...
There will be more on my ATSF Los Angeles Division thread, I’m sure.
Otto

You are definitely on the right path, Otto.
I picked up stepper motors and 3 turntable drivers from an Australian electronics supplier to make my life even simpler.   The combination works like a charm - slow & smooth!  Just need to figure out how to bash the peco pits to look wooden lined.....

Michel