When you go by a handle like [Simplifier], you’ve made a mission statement about your projects: that you’ll take complex processes and boil them down to their essence. So tackling the rebuilding of the humble speaker, a device he himself admits is “both simplified and optimized already,” would seem a bit off-topic. But as it turns out, the principle of magnetostriction can make the lowly speaker even simpler.
Most of us are familiar with the operation of a speaker. A powerful magnet sits at the center of a coil of wire, which is attached to a thin diaphragm. Current passing through the coil builds a magnetic field that moves the diaphragm, creating sound waves. Magnetostriction, on the other hand, is the phenomenon whereby ferromagnetic materials change shape in a magnetic field. To take advantage of this, [Simplifier] wound a coil of fine copper wire around a paper form, through which a nickel TIG electrode welding filler rod is passed. The nickel rod is anchored on one end and fixed to a thin brass disc on the other. Passing a current through the coil causes the rod to change length, vibrating the disc to make sound. Give it a listen in the video below; it sounds pretty good, and we love the old-time look of the turned oak handpiece and brass accouterments.
Motors are everywhere; DC motors, AC motors, steppers, and a host of others. In this article, I’m going to look beyond these common devices and search out more esoteric and unusual electronic actuators that might just find a place in one of your projects. In any case, their mechanisms are interesting in their own right! Join me after the break for a survey of piezo, magnetostrictive, magnetorheological, voice coils, galvonometers, and other devices. I’d love to hear about your favorite actuators and motors too, so please comment below!
Piezo actuators and motors
Piezoelectric materials sometimes seem magic. Apply a voltage to a piezoelectric material and it will move, as simple as that. The catch of course is that it doesn’t move very much. The piezoelectric device you’re probably most familiar with is the humble buzzer. You’d usually drive these with less than 10 volts. While a buzzer will produce a clearly audible sound you can’t really see it flexing (as it does shown above).
To gauge the motion of a buzzer I recently attempted to drive one with a 150 volt piezo driver, this resulted in a total deflection of around 0.1mm. Not very much by normal standards!
For some applications however resolution is of primary interest rather than range of travel. It is here that piezo actuators really shine. The poster-boy application of piezo actuators is perhaps the scanning probe microscope. These often require sub-nanometer accuracy (less than 1000th of 1000th of 1 millimeter) in order to visualize individual atoms. Piezo stacks are ideal here (though hackers have also used cheap buzzers!).
Sometimes though you need high precision over a larger range of travel. There are a number of piezo configurations that allow this. Notably Inchworm, “LEGS”, and slip-stick actuators.
The PiezoMotor LEGS actuator is shown to the above. As noted, Piezos only produce small (generally sub-millimeter) motion. Rather than using this motion directly, LEGS uses this motion to “walk” along a rod, pushing it back and forth. The rod is therefore moved, in tiny nanometer steps. However, piezos can move quickly (flexing thousands of times a second). And the LEGS (and similar Inchworm actuator) allows relatively quick, high force, and high resolution motion.
The tablecloth trick (yes this one’s fake, the kid is ok don’t worry. :))
Another type of long travel piezo actuator uses the “stick-slip phenomenon”. This is much like the tablecloth magic trick shown above. If you pull the cloth slowly there will be significant friction between the cloth and this crockery and they will be dragged along with the cloth. Pull it quickly and there will be less friction and the crockery will remain in place.
This difference between static and dynamic friction is exploited in stick-slip actuators. The basic mechanism is shown in the figure below.
When extending slowing a jaw rotates a screw, but if the piezo stack is compressed quickly the screw will not return. The screw can therefore be made to rotate. By inverting the process (extending quickly, then compressing slowly) the process is reversed and the screw is turned in the opposite direction. The neat thing about this configuration is that it retains much of the piezo’s original precision. Picomotors have resolutions of around 30 nanometer over a huge range of travel, typically 25mm, they’re typically used for optical focusing and alignment and can be picked up on eBay for 100 dollars or so. Oh and they can also be used to make music. Favorites include Stairway to Heaven, and not 1 but 2 versions of Still Alive (from Portal). Obligatory Imperial March demonstration is embedded here:
There are numerous other piezo configurations, but typically they are used to provide high force, high precision motion. I document a few more over on my blog.
Magnetostriction is the tendency of a material to change shape under a magnetic field. We’ve been talking about magnetostriction quite a lot lately. However much like piezos it can also be used for high precision motion. Unlike piezos they require relatively low voltages for operation and have found niche applications.
Magnetorheological (MR) fluids are pretty awesome! Much like ferrofluids, MR fluids respond to changes in magnetic field strength. However, unlike ferrofluids it’s their viscosity that changes.
This novel characteristic has found applications in a number of areas. In particularly the finishing of precise mirrors and lens used in semiconductor and astronomical applications. This method uses an electromagnet to change the viscosity of the slurry used to polish mirrors, removing imperfections. The Hubble telescope’s highly accurate mirrors were apparently finished using this technique (though hopefully not that mirror). You can purchase MR fluid in small quantities for a few hundred dollars.
While magnetic motors operate through the attraction and repulsion of magnetic fields, electrostatic motors exploit the attraction and repulsion of electric change to produce motion. Electrostatic forces are orders or magnitude smaller that magnetic ones. However they do have niche applications. One such application is MEMS motors, tiny (often less than 0.01mm) sized nanofabricated motors. At these scales electromagnetic coils would be too large and specific power (power per unit volume) is more important than the magnitude of the overall force.
Voice coils and Galvanometers
The voice coil is your basic electromagnet. They’re commonly used in speakers, where an electromagnet in the cone reacts against a fixed magnet to produce motion. However voice coil like configurations are used for precise motion control elsewhere (for example to focus the lens of an optical drive, or position the read head of a hard disc drive). One of the cooler applications however is the mirror galvanometer. As the name implies the device was originally used to measure small currents. A current through a coil moved a rod to which a mirror was attached. A beam of light reflect off the mirror and on to a wall effectively created a very long pointer, amplifying the signal.
These days ammeters are far more sensitive of course, but the mirror galvanometer has found more entertaining applications:
High speed laser “galvos” are used to position a laser beam producing awesome light shows. Modern systems can position a laser beam at kilohertz speeds, rendering startling images. These systems are effectively high speed vector graphic like line drawing systems, resulting in a number of interesting algorithmic challenges. Marcan’s OpenLase framework provides a host of tools for solving these challenges effectively, and is well worth checking out.
In this article I’ve tried to highlight some interesting and lesser known techniques for creating motion in electronic systems. Most of these have niche scientific, industrial or artistic applications. But I hope they also also offer inspiration as you work on your own hacks! If you have a favorite, lesser known actuator or motor please comment below!
[Ben] takes apart those rectangular plastic security tags that end up embarrassing everyone when the sales people forget to demagnetize them before you leave the store. Inside are two metal strips. One strip gets magnetized and demagnetized, and the other is magnetostrictive — meaning it changes length ever so slightly in the presence of a magnetic field.
A sender coil hits the magnetostrictive strip with a pulsed signal at the strip’s resonant frequency, around 58kHz. The strip expands and contracts along with the sender’s magnetic field. When the sender’s pulse stops, the strip keeps vibrating for a tiny bit of time, emitting an AC magnetic field that’s picked up by the detector. You’re busted.
The final wrinkle is the magnetizable metal strip inside the tag. When it’s not magnetized at all, or magnetized too strongly, the magnetostrictive strip doesn’t respond as much to the sender’s field. When the bias magnet is magnetized just right, the other strip rings like it’s supposed to. Which is why they “demagnetize” the strips at checkout.
We haven’t even spoiled [Ben]’s explanation. He does an amazing job of investigating all of this. He even measures these small strips changing their length by ten parts per million. It’s a great bit of low-tech measurement that ends up being right on the money and deserves the top spot in your “to watch” list.
And now that magenetostriction is in our collective unconscious, what’s the next place we’ll see it pop up?
“In the future, we’ll be generating a significant fraction of our electricity from harnessing the waves!” People have been saying this for decades, and wave-generated electricity is not a significant fraction of an ant’s poop. It’d be fantastic if this could change.
If you believe the owners of Oscilla Power, the main failing of traditional wave-power generators is that they’ve got too many moving parts. Literally. Metal mechanical parts and their seals and so on are beaten down by sun and salt and surf over time, so it’s expensive to maintain most of the generator designs, and they’re just not worth it.
Magnetostriction is the property that magnetic materials can shrink or expand just a little bit when put in a magnetic field. The Villari effect (which sounds much cooler than “inverse magnetostriction”) is the opposite: magnetic materials get more or less magnetic when they’re squeezed.
So to make a generator, you put two permanent magnets on either end, and wind coils around magnetostrictive metal bars that are inside the field of the permanent magnets. Squeeze and stretch the bars repeatedly and the net magnetic field inside the coils changes, and you’re generating electricity. Who knew?
Right now, according to The Economist Magazine’s writeup on Oscilla, the price per watt isn’t quite competitive with other renewable energy sources, but it’s looking close. With some more research, maybe we’ll be getting some of our renewable energy from squeezing ferrous bars.