Questions about motors, gearing and control. Making a dozen individually rotating display pedestals.

[lucky lazer luke]

Greetings.

I am making about a dozen rotating pedestals for a large store display
window. Still working though some of the engineering issues. They
are going to run though a series of states during the day and at
certain times doing different routines. ( Lighting is also going to be
automated.) The turntables all need to be individually controllable.
So was thinking of using the HB-25 controller from Parallax (available
from Solorbotics as well). http://www.parallax.com/tabid/768/ProductID/64/Default.aspx
and an Arduino Mega 2560. With a PC running Max MSP as the "brain".

The pedestals are going to range from 12 inches to 7 feet (So I am
going to need a variety of motor sizes) and I need to control the
speed and direction. Ideally I would like to have them range from
very slow to something in the neighborhood of a vinyl record speed. So
0-1RPM to approximately 45 RPM.

Was wondering if anyone has had experience with controlling DC motors
in this way and or making this kind of mechanism. Want to make a
good estimate on the necessary motor strength and speed control ranges
when choosing the motors. Would like to order parts for the first
tests tomorrow. Pedestals are going to be on swivel bearing plates
and could use a combination of cable, belt or gear driven mechanisms.
Some gearhead motors I have found might work great for the smaller
ones. http://www.allelectronics.com/make-a-store/item/DCM-351/24-VDC-GEAR-MOTOR-W/TURNTABLE/1.html
Was thinking that the larger (7 foot) might need to have a steel cable
drive mechanism and larger motor?. Hoping the HB-25 can handle that.
It can do .5 HP at 25A with a 6 - 16 volt motor. Was thinking that
should be enough as long as the bearing spinning mechanism is smooth
and has low resistance.

Any one have any ideas or insight?

Best,

Luke

[Rob Graham]

You're going to need to do a little experimenting, but there are two different setups you could use:

Regular DC motors controlled by PWM at the controller or stepper motors,

It'd be a little more difficult to use AC powered motors because the 110v drive electronics are beefier.

If you use cheaper DC motors, you'd want to control the speed using  a pulse width modulated signal. That's where you send the motor a square wave signal and you vary the "on" and "off" times of the drive voltage.  you might think about jst using the arduino to directly control a power transistor, it'll be cheaper per channel.  Or you can use a R/C speed controller - look here http://www.hobbypartz.com/esc.html.  you're probably using brushed motors, so make sure you get a brushed motor speed control.  It's fairly easy to control, but you won;t be able to determine exactly how fast the motor will go unless you test it out first.  The nicer motors are rated at a rpm per volt, but you sound like you need slow and reliable speeds.

A stepper motor lets you precisely control the speed of a motor because you have to send a signal to the motor to advance a quarter turn (depends on the motor).  The control programming's a pain though because it takes 4 wires for each motor.

You're going to need to seriously gear down your motors to get the speeds you're looking for.  One suggestion for driving something like what you're doing comes from Battlebots - instead of a direct gear or chan drive, use a rubber wheel on your motor, and use a spring loaded bracket to press the rubber wheel against the underside of your turntable surface.  Moving the drive wheel closer or father from the axis also lets you gear and/or control the speed without to much trouble.

---

From: lucky lazer luke <luke.s...@gmail.com>
To: NYCResistor:Microcontrollers <nycresistormi...@googlegroups.com>
Sent: Tuesday, June 28, 2011 9:53 AM
Subject: [NYCR:Microcontrollers] Questions about motors, gearing and control. Making a dozen individually rotating display pedestals.

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[b.zerk]

Hi Luke,

Driving that motor with The HB-25 (at up to 16v, obviously) should be
fine. A missing and important part of this equation is the mass of the
pedestal and the object upon it. You could do some moment of inertia
calculations based on the gearbox and the torque ratings of the
RS-380SH-12300, but I'd recommend just buying one each of the motor
and controller and seeing what you can do with them. It'll be fine for
one of the smaller pedestals at the very least, and will give you a
point of reference when speccing out what you'll need to drive the
larger platforms. I, for one, can't make specific suggestions on the
heftier motors and controllers without doing a little research, but I
hope that helps a little.

- Noah

[Luke Schantz]

Thanks Noah!

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[Nick Vermeer]

If you'd like to come in to NYCResistor on a Thursday craft night with your parts and hack on it, I'm sure we could help you out with some of the details. 

The HB-25 can probably handle the motors you would use on the smaller turntable, and I would recommend looking at a stepper motor and driver for the larger table.  Steppers move a small fraction of a rotation per step (often 200 steps per rotation) and this can add some noise and vibration to the project.  If this is not acceptable, then a beefy DC motor with a high gear ratio should solve your problems. 

If this is a project you are doing for a client then I highly recommend sticking with professionally made motor drivers. Units designed for industrial automation and homebrew CNC are good sources for high power drivers. (Gecko drives are common in the CNC arena.)  These units are less sensitive to abuse and errors that might occur during the development due to the additional protection circuitry that has been added to the design.

Driving all of this from an arduino should be pretty easy.  As others noted you would just send PWM signals to the drivers to control the speed of the motors.  The HB-25 is interesting as it uses servo pulses to set speed and direction. Other drivers will require a PWM signal for speed and another line to set the direction.

Do you need to have indexing capability? That is, do you need to turn to a specific position with some degree of accuracy?

[Luke Schantz]

Thanks for all the feed-back.  I will not have the pieces in hand for this Thursday but will stop by next Thursday.

For the larger turntable what about using multiples of the smaller motors (@ 3 6 9 and 12 o'clock) with the "battle bot" technique of positioning them under the turntable?

As for the indexing.  It is not required but would be a nice touch.  Especially for the larger turntable as it is going to have a divider on separating it into two scenes.   Could I put some mark or a small magnet on the bottom of the turntable and a optical or hall sensor as a reference point.  And OR maybe use an rotary encoder?

[Nick Vermeer]

I won't be able to make it next Thursday, but there are usually several people there with the time and experience to assist. 

On Wed, Jun 29, 2011 at 1:31 PM, Luke Schantz <lu...@lukeschantz.com> wrote:

Thanks for all the feed-back.  I will not have the pieces in hand for this Thursday but will stop by next Thursday.

For the larger turntable what about using multiples of the smaller motors (@ 3 6 9 and 12 o'clock) with the "battle bot" technique of positioning them under the turntable?

Not quite sure how you mean to connect the multiple motors here.  If you mean to just use a traction drive with multiple motors then yes, this would work,  Also you can check into using a timing belt and pulleys to drive the large turntable.  If the table is supported correctly on a large bearing then the torque needed to drive it should be fairly low as long as the acceleration requirements are moderate.
 

As for the indexing.  It is not required but would be a nice touch.  Especially for the larger turntable as it is going to have a divider on separating it into two scenes.   Could I put some mark or a small magnet on the bottom of the turntable and a optical or hall sensor as a reference point.  And OR maybe use an rotary encoder?

yes, yes add yes.  You could use any one of these or a mix of the options to provide a feedback system to control the position.  Some of the methods may work better than others for your application.