Open Source Stepper Motor

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Overview

The Global Village Construction Set (GVCS) has a need for stepper motors for linear and rotational motion control. The CNC Multi-machine, torch table, circuit board miller, and others all call for precise motion controlled by a computer or dedicated control device. While the early prototyping stages of these projects will use off-the-shelf stepper motors like the Xylotex Stepper Motor or NEMA. Eventually, we'd like to be able to fabricate stepper motors using the GVCS. This page is used to collect information on the fabrication of stepper motors.

Theory

A stepper motor uses controlled electrical pulses applied to a winding that generates a temporary magnetic field that causes a toothed rotor to move a small amount. Pulses applied sequentially to a set of winding poles is used to create motion. There is a very good animation of this from the | wikipedia article on stepper motors. This article also covers all of the basic theory, more characteristics, types of motors, waveforms, forces, etc.

A | micro-stepping tutorial at Zaber Wiki goes into the electrical aspects stepper motors.

Types of Stepper Motors

  1. Permanent Magnet
    • The rotor is a permanent magnet moved by electro magnet stators.
  2. Variable Reluctance Stepper
    • A plain iron rotor and operate on the principle that minimum reluctance occurs with minimum gap, hence the rotor points are attracted toward the stator magnet poles.
    • These stepper motors have soft iron multipole rotors and a wound stator. The number of teeth on the rotor and stator, as well as the number of winding phases, determines the step angle. Variable-reluctance stepper motors are generally medium step-angle devices (5 to 15°) which operate at high step speeds. The stepper motor's torque is generally low. Rotor inertia and, thus, inertial load capacity are extremely low. Motors of this type operate at maximum pulse rates from 300 to 1,000 steps/sec and have a maximum load inertia capacity of about two-thirds of rotor inertia. When excited in an overlap mode, these stepper motors can move at half step angles and double pulse rates. These stepper motors produce a net output velocity, which remains the same. [1]
  3. Hybrid Synchronous Stepper
    • User a combination of permanent magnets and variable reluctance to gain maximum performance.
  4. Lavet type stepping motor
    • A single phase motor with a permanent magnet rotor. Often used in wrist watches.

Modern stepper motors tend to be hybrid synchronous steppers.

Parts

Some of the following is taken from the Engineer's Garage.

Depending on the kind of stepper motor being built, the following parts need to be created:

  1. Toothed rotor
  2. Shaft
  3. Shaft bearings
  4. Stator poles (also toothed)
  5. Stator cores
  6. Windings
  7. Case

metallurgy

metals used and sources

Fabrication

Part Technique
Rotor Lathed and index milled
Shaft Lathed
Shaft Bearings (separate fab)
Stator Poles Milled
Stator Cores Milled ?
Windings Winder (multi-machine?)
Case Cast? Milled?

Most of the fabrication needed can be accomplished by a multi-machine (lathe/mill). Bearings present a special problem that should be considered in a separate fabrication process. Roller bearings may be sufficient for this purposes. Winding the cores is also a special task since insulated wire (lacquer?) must be carefully wound around a core. This might be accomplished by a source of rotary motion (the multi-machine) and a linear feeder that moves back and forth across the rotating pole.

The case will also require some thought. Milling it would be quite wasteful. It might be cast out of aluminum, then bored and milled inside to receive the stator poles. This view shows the inside of a commercial motor. Note the inner case to which the stator poles are mounted. Separate end pieces would aid assembly, disassembly and repair.

Assembly

Assembling the motor might consist of (roughly) the following steps:

  1. Mount the rotor on the shaft.
  2. Slip bearings over shaft.
  3. Wire up the stators.
  4. Mount stators inside the case.
  5. Slide the shaft assembly into the enter of the mounted stators.
  6. Attache front and back pieces of the case.

Other Sites

These are links to other sites that discuss how to build a stepper motor:

  • Aero Design - [2]
  • Kylmatec - [3]
  • Fly Electric - [4]
  • Radio Control Zone - [5]

Discussion

Open designs are out there already. I am open to suggestions if you think we should actually build a sample. Better yet - inform me with a full lowdown of what it would take to make a serious stepper - I'll blog it - and we can probably find somebody on the net to volunteer to build a prototype. At the larger scale (2000 in oz, etc) these things are hundreds of dollars - in which case it's very worthwhile to build ourselves, since parts will only be like 20 bucks or so.

We should open source the various jigs required to build these, and go to town. Then we could build industrial robots for thousands instead of hundreds of thousands.

Marcin

From my reading of various sites, it seems to be in scope. With the right tools, machining the parts seems to be straight forward (thought out of my current skill range).

For the purposes of the torch table, I think you should continue with off-the-self motors. While expensive, these motors are necessary to bootstrap several important additions to the GVCS. Delays at this point to build the motors might impact momentum on important projects.

I can probably add more detail to the page that's there now. Some digging might turn up the spec's needed to create actual design plans. If I were attempting this (and the future, I might), I would take it in stages. First, build a very, very simple motor with 4 or 8 stators and a soft-steel or magnetic rotor. Use off-the-self bearings and wire it up on an open mount of some kind. They do some tests using a hand-operated controller (a rotary switch for 1amp current to windings might do it).

Inevitably problems will arise that will give a better understanding. Plan to ramp up to more sophisticated stepper motors in stages. I'd expect 3 or 4 protoyping phases.

I don't think that all that many jigs are needed. There is the indexer to created the toothed rotor and the coil winder. Those are the unusual parts, I think.

- Mark J Norton

See Also

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