Stepper Motor Driver

Another task with the stepper motor setup is how to control it.  A driver is used to convert input (step and direction) to the correct signal sequence in the coils to turn the motor.  It also provides power which in this case is 12 volts.  I ordered an EasyDriver and hooked it up in a breadboard to the motor on one side, 12V power on another, and input signal from the Arduino microcontroller on the third side.  In turn the Arduino is connected by a USB cable to my laptop to upload programs.

I uploaded a simple program to turn 10 steps in one direction, then 10 steps back, etc.  There was a problem.  The motor turned one way but with a click, then jiggled and wouldn't go back.  I am sure my freezing, heating, hammering, etc. trying to work out a coupling system ruined it.  So now I know exactly what not to do.  At this point there was no reason not to open it up and take a look.

The center has magnets and a toothed wheel that lines up with teeth in the electromagnet coils around the outside.  It is these teeth that have very precise tolerances and can get chipped or bent.  I tried moving things around a bit to try to fix it but it doesn't seem promising.  So the stepper motor project is on hold for the time being.

Motor Coupling

Earlier I posted about making a weak stepper motor.  I took a break from that and thought I would try working with a manufactured one first then returning to making a stronger one.  I got a NEMA17 motor and the first thing for me to figure out was how to connect the shaft to get it to turn something.

The motor shaft is 5mm in diameter.  I wanted to connect it to a 10-24 threaded rod as a worm gear.

I also picked up some 10-24 coupling nuts.

The the 5mm motor shaft is too large to go into the coupler, so I drilled halfway into the coupler from one side with a 3/16 inch bit.  3/16" is 4.76mm plus a bit more because of wobble from the drill.  Essentially this removed the threads and it looked like it could almost fit. 

In fact it was maddeningly close; less than 0.24mm is a tiny space.  I tried and tried to get it on but couldn't.  Then I had an idea!  I tossed the motor in the freezer for a while then heated the coupling on the eye of the stove with some pliers.  The kids already think I am nuts and are used to this kind of thing.  My hope was the cold would shrink the metal in the motor and the heat would expand the coupling enough to fit them together.  At first it wouldn't work; as soon as the metals touched they would equalize temperatures, but they kept just barely grabbing hold of each other and sticking for a moment before coming off.  I tried one last time and gave a light tap with a hammer (through a piece of wood--although you are not supposed to do this with stepper motors; the tolerances are very tight and can be ruined if it is bent or twisted at all) and it slipped on just a bit and held! 

For a few minutes, then popped off again.  This wasn't going to work.  My next size drill bit is 1/4" which is too large but I drilled it out anyway.  I the process I made some zinc wool.

Above is the nut drilled out to 1/4 inch on the near side.  It is still hot from drilling so I am still holding it with pliers. 

Now the hole is too large and I need a way to hold the motor shaft.  I drilled into the side with a 7/64" bit. 

Then I used a 6-32 tap to cut threads on the sides. 

And here it is with a 6-32 size nylon screw.

I drilled and tapped two holes, one for the motor and one for the rod. 

This works but the hole is so large the axis of the motor is shifted quite a bit from the axis of the threaded rod.  In the future I might try to get a drill bit in between 1/4" and 3/16", perhaps a 7/32".  7/32 inch is 5.56mm so that might work well.  A 13/64" would be even closer at 5.16mm. Also, I can cut the screws shorter so they don't stick out so much.

Brushless Motor

I redesigned the motor we made earlier.  I figured the brushes were creating a lot of friction and keeping its top speed down.  I wound four new coils onto spent wire spools, 1,000 wraps each of 32 AWG enamel wire, and mounted the new coils around the bottle with the magnets taped to it. 

The magnets are mounted 90 degrees apart (or as close as I could get them to 90 degrees using the pattern printed on the bottle).  If the electromagnets were small enough I could put them 22.5 degrees apart within a 90 segment and pulse them in a row.

 
However, they wont fit this close together, but I can place them around the full 360 degrees in multiples of 22.5 degrees +/- 180 or 90 degrees--in other words 67.5 degrees--and if I get the right placing it will work the same way (with, in this case, lots of tweaking and taping).

(Originally I tried to do this with three coils in multiples of 30 degrees, which in theory should work, but it was too sensitive to the placement.)  In this second case the magnets are pulsed in the opposite direction from the way the rotor turns.  I hooked up a controller and wrote a short program to drive the motor.  The LEDs on the breadbord indicate which coil is powered and to what degree.  It worked better to move adjacent coils up and down in voltage in a triangle wave pattern to trade off the rotor.

Here is a code snippet to give an idea.

All I had at the time were some PNP transistors to hook it up so everything is "drawkcab".  A "HIGH" signal means "LOW" voltage, etc.  I need to clean up the code by putting the pins in an array and having a single loop.  I am also thinking of using a smoother sine wave function to trade the rotor between coils and increase the voltage to the coils (currently it is running on 5 volts off the controller board, but the control of the rotor is very loose).  One surprising side effect, as the power is adjusted along the triangular wave the board can not do a true analog transition, but approximates this by varying the pulse of an on/off signal very rapidly, this causes the magnets on the bottle to vibrate and produce sound.  The motor "sings" as it turns.  Also, contrary to my original goal, I can not get to high speeds with this setup, but I can turn the motor as slow as I like, pause for as long as necessary, and easily reverse direction, kind of like a step motor, which gives me some ideas for applicaiton if I can build a more rigid structure, run it with more power, and gear down for a mechanical advantage without too much friction.

A more powerful motor

We made another motor from four 3/4" x 1/4" NdFeB magnets taped to a used water bottle. Between the magnets are strips of aluminum foil to make contact between the brushes.  The brushes are held onto the cardboard box frame by clothes pins.  Also, an axle running through holes in the bottle was made from a coat hanger wire.  At the base is an angled electromagnet (enamel coated 32 AWG wire wound onto a glued together paper spool and mounted onto a cut plastic cup).  The setup is timed so that as a magnet just passes the coil the circuit is connected and the magnets push apart, then the circuit is broken as the bottle rotates and the next magnet comes into place. 

 

It can run on only six volts but it is more fun to hook up twelve volts in series to run it faster (top picture).  After running for a while the aluminum foil was almost completely worn through so we replaced them with stripped wires, which worked a lot better.  

And we needed one further modification.  Only using the magnet to push the bottle around means it is turned off most of the time.  I added a second set of brushes to make contact at a different point and hooked it up to reverse the flow of electricity through the coil.  The results in a pull of the next magnet toward the coil, then it switches off briefly as the magnet swings by, then kicks back on with a push of the magnet away. 

The pull-push configuration spins the bottle much much faster than before.  It will run on a minimum of 3 volts.  With 9 volts it will self start (not requiring a push to start spinning).  We hooked up all the batteries we could find in series and got it up to 33 volts, at which point it seemed like the motor was about to fly apart.  It was getting dark at this point and with the lights off we could see lots of little sparks flashing between the brushes and contacts as it spun.  

The wiring was trickier for a pull-push setup.  At least two batteries are needed because the current is set up to flow in both directions, so if one battery were used for everything it would short circuit. 

Simple Motor

I have been trying to start doing projects with the kids each weekend. Last weekend we tried to make a simple speaker from scratch, no success on that one so far. So this weekend I went for something easier and we made a simple motor out of wire, paperclips, magnets, tape, an AA bettery and two alligator clips. All taped to a plastic plate base. Above is a picture of it unhooked (I used small ceramic magnets to hold the wires to the battery). The coil is about 35 wraps of 26 AWG enamel coated wire. The connections to the clips have the enamel scraped off one side, so each revolution the electromagnet turns on and off repelling it from the magnets below and it spins. Below is a picture in action.

T made a motor like this from a kit a few years ago, but this one is completely from common off the shelf parts.