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Stepper Motors: Care & Feeding

About once a week (it seems), I get a press release talking about some company's newest stepper motor or stepper motor driver IC. Rather than discuss just one manufacturer's product to the exclusion of others (don't want to look like I'm shilling), let's just consider some general comments.

I will say I've used stepper motors before, and they are pretty cool. Like their name says, they can do incremental step movements rather than the more common smooth rotation of the other motors we've talked about here. (See: Variable Speed Motor Drives: Boon or Bane? and Going Green: The Trend Toward High-Efficiency Brushless DC Motors.)

Nothing wrong with smooth continuous rotation, but sometimes you want to move/rotate just a very small, repeatable amount, measured in fractions of a degree, and maybe without the need for an encoder to verify the position.

Of course, sometimes (in mission critical applications), you darned well better use an encoder or a precision pot to make sure that you're moving stuff to where you intended. I did a design for an X-ray collimator used in a CT scanner. A collimator focuses and shapes the X-ray beam coming from the X-ray tube before it passes through the patient on its way to the detector array. This is clearly important stuff, so I used stepper motors and some very precision linear pots to confirm absolute position of the collimator blades.

However, in less critical applications, the “steppines” of the motor can be all you need. These motors use permanent magnet rotors shaped like a gear (with teeth). The pole pieces on the stator coils also have a similar-toothed structure. Combined, this arrangement will make the rotor move. So, this makes steppers seem vaguely like a BLDC (brushless DC) motor. But, there is no means of switching or commutating these field coils integral to the motor.

If one coil is energized, the rotor will rotate slightly so that its teeth line up with that pole piece's teeth. In the stepper, the next coil is set slightly ahead of the first one (“ahead” being a relative term). If the first coil is de-energized and the second one is energized, the rotor will quickly rotate and park in alignment with the teeth on the second pole piece.

This is what that would look like with a four-coil stepper motor. Note that with the teeth as shown and with these pole pieces, each step is 3.6°:

StepperMotor

This nice GIF comes from Wikipedia: Stepper motor.

Important considerations: To keep the rotor locked in a specific position, you keep power on the appropriate coil. To make the motor spin rather like a more conventional motor, you need a sufficiently sophisticated controller that can sequence power to the coils in the correct order and even change the coils polarity as needed. To put the rotor where you want it quickly and accurately, your controller needs to be smart enough to minimize overshoot of the rotor — so this starts to sound like a closed loop servo-system.

In more sophisticated controllers, more than one coil at a time can be energized with pulse-width modulated (PWM) power levels. That would permit “half-stepping,” or even “micro-stepping,” for extra-precision applications.

There is plenty of information available that can provide good stepper motor control applications help. Here are two easy to read, useful examples:

Related posts:

8 comments on “Stepper Motors: Care & Feeding

  1. eafpres
    July 9, 2013

    Hi Brad–stepper motors always seemed cool to me.  For some reason, I believe some older hard drives or floppy drives actually used steppers for the spindle, but I may be just too old and fuzzy to remember right.

    I know there are a lot of project folks on Planet Analog, and Arduino is a popular platform.  I thought you might like to see this board (they call them shields–unfortunate use of term for somebody like me who was involved with EMI shielding!) that includes both servo and stepper controls:

    adafruit motor shield (i.e. control board)

  2. Brad Albing
    July 9, 2013

    Interesting little board – may have to get me one of those to play with.

    Hmm… disc drives… maybe to move the read/write head in and out? That seems more likely.

  3. eafpres
    July 9, 2013

    Hi Brad–old HDDs definitely used steppers to move the heads, all replaced by voice coil and closed-loop servoing.  I worked on an odd floppy drive once–144 MB 3.5 inch FDD (used special media from 3M/Imation) and for some reason think it had a stepper, but probably not.

  4. RedDerek
    July 10, 2013

    I worked for one company, Schaeffer Magnetics, out of Chatsworth, CA. They built steppers for space flight to control flight data  recorders, solar array drives, and even was the motor for the teathered satellite program that was used on the space shuttle a few times. They did some neat stuff in order to set up redundancy without increasing the length of the motor as well.

    Steppers have been around quite a while and are quite useful in a miryad of products.

    As for the hard drives, they were used to accurately spin the platter and drive the arm of the read-write head. Later, they used piezos to make fine movements of the head tip in order to go to higher density. This was years ago and I am sure better techniques are used.

  5. Brad Albing
    July 10, 2013

    That application qualifies as one of the “mission-critical” applications to which I referred.

  6. Dirceu
    July 11, 2013

        One of most widespread advantages of stepper motors from the beginning was to avoid complicated control theory when precise positioning is required.

  7. SunitaT
    July 29, 2013

    Once stepper motor is out of control or under inappropriate control, resonance is apt to be produced.  Comparative high speed and high torque is not easy to be achieved either. Stepper motor has no advantages in features of size and weight. Energy use ratio is comparatively low. When overloading, the synchronization will be broken. Vibration and noise occur when running at high speed.

  8. Brad_Albing
    July 30, 2013

    Well, yes, but it's the responsibility of the design engineer to avoid using the stepper motor in applications for which it is not intended; or to include methods and techniques to minimize vibration/resonances.

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