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Author Topic: 3D Laser Scanner Project  (Read 2141 times)

Kevin

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3D Laser Scanner Project
« on: April 22, 2012, 12:57:46 AM »
Hi All,

I'm working on a project where I'm using a couple of DC Stepper Motors and EasyDriver stepper motor drivers.  I'm trying to create an enclosure for a 3D laser scanner (eg., david-laserscanner.com) that has a turntable to rotate the object to be scanned, and a linear actuator to move a laser line module up and down.

Here is the motor I'm using:

http://www.amazon.com/CanaKit-Stepper-Motor-with-Cable/dp/B004G51AZ4/ref=sr_1_1?ie=UTF8&qid=1335070032&sr=8-1

And here is the EasyDriver Stepper Motor Driver I'm using:

http://www.amazon.com/SparkFun-EasyDriver-Stepper-Motor-Driver/dp/B004G4XR60/ref=pd_bxgy_e_img_b

I'm facing a few issues:

1) The motor rotates very accurately when I rotate it by large degrees (eg, above 30 degrees), so I'm good for the turntable, but when I try to rotate it by a small degree (eg., 1 degree), the motor seems to "stick" sometimes.  This is impeding my efforts to get the laser line to scan up and down the object slowly so the webcam can get enough points for the point cloud.

2) I'm using a 12V 300mA power source (which matches the ratings for the motor), but the EasyDriver is getting pretty hot.  I've tried turning down the amps, but if I turn it down too much, I am finding that the motor isn't rotating accurately.  Do I need to add a heat sink?

3) The stepper motor has a non-threaded rod.  I'm trying to create a linear actuator, and so I want to attach a 12" threaded rod to the motor, but I'm unable to find any shaft couplings in any local hardware stores.  Are there any affordable online vendors or local stores that would carry couplings?  I'm thinking about using a servo motor instead, and just mounting the laser on the servo, which would rotate the laser about 90 degrees.  The only problem with that, I think, is that it may not be as effective for objects that are smaller at the base than in the middle; it may not be able to reach the object.

Thanks in advance for any advice.

Cheers,
Kevin

f00bard

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Re: 3D Laser Scanner Project
« Reply #1 on: April 22, 2012, 12:38:49 PM »
Hi Kevin!

1) Do you have microstepping turned on?  It's defaulted "on" on that stepper driver.  It looks like they break out the pins but you have to wire them up.  I ask because microstepping can be less accurate, and (assuming a 1:1 drive) maybe full steps is good enough for your application.

2) Yes, you probably need a heatsink.  Because (I assume) your application is applying holding torque to the motor when it's not rotating, you're going to be sinking a lot of current all the time.  I've noticed this on my Makerbot, though mostly that was the motor getting hot.  But if you need a lot of current to get it to move accurately, it's going to be sinking almost as much current just holding the thing.

But herein lies the trick: if you minimize the amount of current you need to move the load, you minimize the holding current as well.  So make sure there isn't much resistance in your turntable.  The stepper is holding it still for you, so any unnecessary friction is only hurting you.  Also, consider a reduction drive (see RepRap and similar projects for examples using belt drives).

3) eBay is filthy with couplers for this exact application: http://bit.ly/J2aHWs.  Just search for "shaft coupler".  Their dirt cheap if you get them from China, though it mightwill take a week or two to get over here

Kevin

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Re: 3D Laser Scanner Project
« Reply #2 on: April 24, 2012, 10:05:49 PM »
Hi f00bard,

Thanks for your reply.  And thanks for the leads on Ebay, never thought of that.  I've also been looking on mcmaster.com, robotshop.com, and servocity.com.

In retrospect, I never should have bought a motor with a metric rod, though ... it's making it very difficult to find a similar sized threaded rod or even a 5mm-1/4in. coupling. 

Yes, I guess I do have microstepping turned on, if that is the default.  I thought that if the step angle was 1.8 degrees, I could make 200 1.8 degree rotations, and come full circle.

I think what I'll do is just slow it down and make bigger steps for the linear motion.  All I'm really concerned about there is making sure the webcam has time to process all the data points.

Another question: How would I cut the current to the motor?  Or maybe I should cut off the current in the circuit before the driver altogether?

Also, regarding the power source, if I wanted to have just one power source, how can I figure out how to split up the voltage/current?  I'm thinking of just using an ATX power supply, but if it's easy to achieve, maybe I can do something with less of a size footprint.

Thanks very much,
Kevin

ThantiK

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Re: 3D Laser Scanner Project
« Reply #3 on: April 26, 2012, 05:52:46 PM »
Seeing as your stepper motor is rated for 12v, that's where your problem is regarding stepping.  Turn off microstepping for that controller and put it into full step mode by tying MS1 and MS2 to ground.


As far as the rods go, our 3D printers might be able to print you out some couplers.  There are a lot of parametric designs on thingiverse which allow you to specify shaft sizes in order to customize that kind of thing.


If you have 1.8 degree steps, you're correct, 200 steps would be full circle.


As far as cutting current to the motor, that's what the "Enable" pin is for.  I believe if you tie the enable pin to ground, it shuts off the driver.


Also, Xbox power supplies, micro-ATX power supplies, laptop power supplies are all very good sources for DC power.  The driver you're using can take up to 30v, your steppers are fine running at that, you just have to turn down the current to compensate.  This is actually how microstepping is done.  For example on my 3D printer, my stepper motors are spec'd for 4v.  I run them @ 12v with current limiting, which allows the chopper driver to enable micro-stepping.  If you find something that's 24-28v to utilize, you'd likely be able to use half or quarter stepping to up the resolution of the stepper motor your using to 400, or 800 steps per revolution.