Electrostatic motors are now common in MEMS applications, but researchers at the University of Wisconsin and spinoff C-Motive Technologies have brought macroscale electrostatic motors back. [via MSN/WSJ]

While the first real application of an electric motor was Ben Franklin’s electrostatically-driven turkey rotisserie, electromagnetic type motors largely supplanted the technology due to the types of materials available to engineers of the time. Newer dielectric fluids and power electronics now allow electrostatic motors to be better at some applications than their electromagnetic peers.

The main advantage of electrostatic motors is their reduced critical materials use. In particular, electrostatic motors don’t require copper windings or any rare earth magnets which are getting more expensive as demand grows for electrically-powered machines. C-Motive is initially targeting direct drive industrial applications, and the “voltage driven nature of an electrostatic machine” means they require less cooling than an electromagnetic motor. They also don’t use much if any power when stalled.

Would you like a refresher on how to make static electricity or a deeper dive on how these motors work?

The average Hackaday user could probably piece together a rough model of a simple DC motor with what they’ve got kicking around the parts bin. We imagine some of you could even get a brushless one up and running without too much trouble. But what about an electrostatic corona motor? If your knowledge of turning high voltage into rotational energy is a bit rusty, let [Jay Bowles] show you the ropes in his latest Plasma Channel video.

Like many of his projects, this corona motor relies on a few sheets of acrylic, a handful of fasteners, and a healthy dose of physics. The actual construction and wiring of the motor is, if you’ll excuse the pun, shockingly simple. Of course part of that is due to the fact that the motor is only half the equation, you still need a high voltage source to get it running.

In this case, [Jay] is revisiting his earlier experiments with atmospheric electricity to provide the necessary jolt. One side of the motor is connected to a metallic mesh electrode that’s carried 100 m into the air by a DJI Mini2 drone, while the other side is hooked up to several large nails driven into the ground.

The potential between the two gets the motor spinning, and makes for an impressive demonstration, but it’s not exactly the most practical way to experiment with your new corona motor. If you’d rather get it running on the workbench, he also shows that a more traditional high voltage source like a Van de Graaff generator will do the job nicely. As an added bonus, it can even power the device wirelessly from a few feet away.

So what can you do with a corona motor? While [Jay] is quick to explain that these sort of devices aren’t exactly known for their torque, he does show that his motor is able to lift a 45 gram weight suspended from a string. That’s frankly more power than we expected, and makes us wonder if there is some quasi-practical application for this contraption. If there is we suspect it’ll be featured in a future Plasma Channel video, so stay tuned.

If you want something to move with electricity, odds are you’ll be using magnets. Deep inside every servo, every motor, and every linear actuator is a magnet and some coils of wire. There is another way of making things move, though: electrostatics. These are usually seen in tiny MEMS devices, and now we have tiny little electrostatic speakers making their way into phones and other miniature devices.

For [Nathann]’s Hackaday Prize entry, he’s building electrostatic actuators on the cheap, and not just tiny ones, either. He’s building ‘human’ scale electrostatic devices.

The reason electrostatic devices are usually very small is simple: the force of any actuator is dependent on the distance between the plates and the voltage. Moving the plates closer together is right out, or else they would be touching, so the solution to building bigger electrostatic actuators is increasing the voltage. [Nathann] is doing this with a cheap boost converter that’s actually sold as a taser module. These modules are small, output about 800kV, and cost around five bucks.

The prototype for this project is basically a 3D printed box with intersecting fins. These fins are covered in aluminum foil, and the box is filled with oil to prevent arcing. Will it work? That remains to be seen, but this project is a great example of what can be done with some creative part sourcing, a 3D printer, and a tiny bit of know-how. It’s some of the best work the Hackaday Prize has to offer, and we’re amazed that [Nathann] put in the work to make this happen.

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Most of the electric motors we see these days are of the electromagnetic variety, and for good reason: they’re powerful. But there’s a type of motor that was invented before the electromagnetic one, and of which there are many variations. Those are motors that run on high voltage, and the attraction and repulsion of charge, commonly known as electrostatic motors.

Ben Franklin — whose electric experiments are most frequently associated with flying a kite in a thunderstorm — built and tested one such high-voltage motor. It wasn’t very powerful, but was good enough for him to envision using it as a rotisserie hack. Food is a powerful motivator.

What follows is a walk through the development of various types of these motors, from the earliest ion propelled ones to the induction motors which most have never heard of before, even an HV hacker such as yours truly.

Franklin’s Electric Wheel

In 1748, Benjamin Franklin invented what he called the electric wheel. This is usually cited as the first electrostatic motor and sometimes as the first electric motor, depending on your definition. The rotor consisted of a wooden hub with 30 glass bars attached to the circumference. These bars were strips cut from window glass. Brass thimbles were attached to the end of each bar. To turn easily, the rotor had a central shaft with an iron point on the bottom and a strong wire at the top, the wire going through a hole to keep it all upright.

The power source consisted of two oppositely charged Leyden jars, the original name given to a cylindrical capacitor made using a jar of some sort. The jar, and therefore the dielectric, was glass in Franklin’s day. A wire extended up from the inner capacitor plate and this wire was placed near where the thimbles would pass by. As a thimble passed, a spark would occur between the thimble and the wire, charging the thimble with the same polarity as the inner plate. Being like-charged, the thimble would be repelled from that wire, turning the wheel. As the thimble reached the wire of the opposite Leyden jar, being of opposite charge, it would be attracted to the jar’s wire. As it passed, a spark would occur between the two, charging the thimble to the same polarity as that wire and causing it to be repelled, turning the wheel some more.

The initial direction of the wheel was set by giving it a push by hand. In a letter (page 29), Franklin wrote that it rotated at 12 to 15 RPM, bearing the weight of one hundred Spanish dollars. He also wrote that:

if a large fowl were spotted on the upright shaft, it would be carried round before a fire with a motion fit for roasting.

Electric Whirl

This motor is a bit older than Franklin’s and dates back to 1745. It was made by Andrew Gordon, a Scottish Benedictine monk. It goes by many names but the original name was the electric whirl. The basic idea is that an electric field at a sharp point will be stronger than at a smooth point. This strong electric field ionizes the air near the point, and depending on the polarity, may even emit electrons to the air. This ionized air is called a corona. In either case, the ionized air around the point has the same polarity as the material making up the sharp point. Since like-charges repel each other, the ionized air repels the sharp point. You’ve of course heard of Newton’s third law which basically states that for every action, there’s an equal and opposite reaction. So the air is repelled in one direction, and the sharp point reacts by being repelled in the other direction. Arrange the point on an arm which is free to move around an axis, and the arm will rotate around that axis. The moving ionized air is often referred to as ion wind and for that reason this is also often called an ion wind rotor, or ion wind spinner or pin wheel.

Usually there are two points, each on their own arms, pointing in opposite directions to make for a more stable system. There can of course be more. The one pictured here is sitting on top of a Van de Graaff generator’s dome, which is at high voltage with respect to ground. The electric whirl is in electrical contact with the dome through the supporting shaft. The other end of the electric field is the surrounding air, the bottom of the Van de Graaff generator and the room.

Poggendorff/Corona Motor

The 1860s was a time when many were experimenting with influence machines, machines that used electrostatic induction, called influence at the time, to produce high voltages. One such experimenter was Wilhelm Holtz. In 1867, in a stroke of brilliance,  he connected the output of a similar machine to one of his and found it would run as a motor. And so in 1869, J.C. Poggendorff had a simplified version of Holtz’s machine built that had only what it needed to run as a motor.

Poggendorff’s motor consisted of a glass disk with combs facing it on either side. The combs had sharp points on them arranged so that the points would be near the disk but not touching. The combs could be turned so that they were strictly radial with respect to the disk. In this case the disk was not self-starting and, once given a spin by hand, would continue to rotate in that direction. However, the combs could also be turned to an off-radial angle. In this case the disk would self-start and be unidirectional.

One difference between Franklin’s wheel and Poggendorff’s motor is that Franklin’s places charge onto the thimbles using a spark, whereas Poggendorff’s sprays charge onto the disk through ionized air, or a corona. This corona is the same we mentioned when talking about the electric whirl, and is formed in the same way — the sharp points result in a strong electric field which ionizes the air.

Similar to Franklin’s the area of the disk, approaching a comb is charged with the opposite polarity with respect to the comb, and so is attracted to the comb. And the area moving away from the comb is charged with the same polarity, and so is repelled.

Poggendorff’s motor also has some advantages over Franklin’s electric wheel. The combs are always spraying onto the disk, continuously producing rotation, whereas with Franklin’s, this happens only when a thimble passes a Leyden jar. Also, there is always the same amount of disk surface area facing the combs, resulting in a constant torque being produced.

Due to the corona being the mechanism for transferring the charge, these types of motors are generally called corona motors. And in many of them the disk is replaced with a cylinder. The combs then become either just the sharp ends of thin wires or long sharp blades as in the one shown here that was run off of atmospheric electricity.

Capacitor Motors

This next type of motor was invented in 1889 by Karl Zipernowsky, but an easier version to illustrate is a 1904 one by N.G. van Huffel^[1]. In it the rotor and stator are both made up of two partial cylinders, or curved plates. The rotor plates have a slightly smaller diameter to fit inside the stator plates.

The stator plates are charged oppositely and the rotor plates receive a charge from their adjacent stator plates via a sliding contact just as they are rotating away from those plates. That way they repel away from the adjacent stator plate and reach a position where they’re then attracted to the next stator plate.

The big difference between all these motors is in the way the charge is moved around, via sparks as in Franklin’s, via corona, or via sliding contacts. However, since sliding contacts are used here, these motors can run at a lower voltage than the others. These can also run on AC.

Induction Motors

The induction motor is the one that operates on perhaps the most interesting principle. The rotor is not electrically connected to any power supply and is made of a dielectric material. The surrounding stator plates are supplied with AC, and so there is a rotating electric field surrounding the rotor. That electric field induces polarization of the molecules on the surface of the rotor. From the stator’s point of view, this appears as a net charge.

The key is that as the stator plates’ fields change polarity, it takes time for the polarization of the nearby rotor to change in response. That creates a lag between the moving field and the polarized molecules. That lag is what causes the rotor to follow the field.

The illustration shown here is of one made by Rocardo Arno in 1892-1893^[1].

Where Have All The Motors Gone?

As we said, electromagnetic motors are far more powerful. But electrostatic motors do find use in MEMS motors of microscopic dimensions. And of course, they haven’t really gone anywhere if you consider electrostatic tinkerers who love resurrecting these old mechanical marvels. I’ve dabbled in electric whirls and corona motors and have thirty thimbles on order from China for something which the attentive reader can guess at. What electrostatic motors have you toyed with in your home laboratory?

Resources:

[1] Electrostatic Motors: Their History, Types and Principles of Operation by Oleg D. Jefimenko

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.

Magnetostrictive actuators

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 motion

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.

Electrostatic motors

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!

[Steven Dufresne] of Rimstar.org is at it again with another very functional science experiment. This week he’s showing us how he made a large electrostatic motor, also known as a Corona Motor.

A Corona motor makes use of a cool
phenomenon called the Corona discharge, which is the ionization of a fluid
(in this case, air) surrounding a conductor that is energized. He’s done other high voltage experiments that take advantage of this, like his Ion Wind propelled Star Trek Enterprise!

The motor works by using an even number of electrodes on the motor, each electrically charged; positive, negative, positive, negative, etc.

Because each electrode is the opposite charge, they want to repel each other — but since the cylinder is electrically insulated, the charges have no where to go — instead the cylinder begins to rotate as the charges attract back and forth — when a positive charge on the insulation meets a negatively charged electrode, the charge is removed by ionization (creating the corona effect), and the cycle continues. The direction of rotation is determined by the angle of the electrodes. The motor can get going pretty fast but doesn’t have that much torque or power.

For a full explanation of the project, check out [Steve’s] explanation in the following video:

And how to make it!

This is actually [Steve’s] second Corona motor, as he already designed a simpler one that is easier to build previously.