Esoteric Actuators

December 10, 2015 by Nava Whiteford 35 Comments

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

Piezo_bending_principle — The flexing of a piezo buzzer from arcbotics

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!

legs — The PiezoMotor LEGS actuator “walks” along a rod, pushing it as it goes.

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.

picomotor1 — Motion caused by a stick-slip motor

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!

Earth Day: Electric Vehicles

April 22, 2015 by Brian Benchoff 38 Comments

Electric vehicles are the wave of the future, whether it’s from sucking too much oil out of the ground, or because of improved battery technology. Most internal combustion engines are unsustainable, and if you’re thinking about the environment – or working on an entry for The Hackaday Prize – an electric vehicle is the way to go.

Here are a few electric vehicle projects that are competing in The Hackaday Prize that show off the possibilities for the electric vehicles of the future.

An Electric Ninja

Motorcycles are extremely efficient already, but if you want a torquey ride with a lot of acceleration, electric is the way to go. [ErikL] is hard at work transforming a 2005 Ninja 250R into an electric vehicle, both to get away from gas-sipping engines and as a really, really cool ride. Interestingly, the battery technology in this bike isn’t that advanced – it’s a lead acid battery, basically, that reduces the complexity of the build.

And They Have Molds To Make Another

Motorcycles aren’t for everybody, but neither are normal, everyday, electronic conversion cars. [MW Motors] is building a car from scratch. The body, the chassis, and the power train are all hand built.

The amazing part of this build is how they created the body. It’s a fiberglass mold that was pulled off of a model carved out of a huge block of foam. There’s a lot of composite work in here, and a lot of work had to happen before digging into the foam; you actually need to choose your accessories, lights, and other bits and bobs before designing the body panels.

While the suspension and a lot of the mechanical parts were taken from a Mazda Miata, the power and drive system are completely custom. Most of the chassis is filled with LiFeMnPO4 batteries, powering four hub motors in each wheel. It’s going to be an amazing car.

Custom, 3D Printed Electric Motors

If you’re designing an electric car, the biggest decision you’re going to make is what motor you’re going to use. This is a simple process: open up a few catalogs and see what manufacturers are offering. There’s another option: building your own motor. [Solenoid] is working on a piece of software that will calculate the specifications of a motor given specific dimensions. It will also generate files for a 3D printed motor given the desired specs. Yes, you’ll still need to wind a few miles of copper onto these parts, but it’s the beginning of completely custom electronic motors.

[Mike] Shows Us How To Use An Armature Growler

February 2, 2015 by Adam Fabio 24 Comments

[Mike] has put up a great video on his [SmallEngineMechanic] YouTube Channel about a tool we don’t see very often these days. He’s using an armature growler (YouTube link) to test the armature from a generator. Armature growlers (or just growlers for short) were commonplace years ago. Back when cars had generators, just about every auto mechanic had one on hand. They perform three simple tests: Check armature windings for shorts to other windings, for open windings, and for shorts to the armature body. [Mike’s] particular growler came to him as a basket case. The wiring was shot, it was rusty, and generally needed quite a bit of TLC. He restored it to like new condition, and uses it to help with his antique engine and genset ~~addiction~~ hobby.

Growlers essentially are a transformer primary with a V-shaped frame. The primary coil is connected to A/C mains. The armature to be tested sits in the “V” and through the magic of induction, some of the windings become the secondary coils (more on this later). This means some pretty high voltage will be exposed on commutator of the armature under test, so care should be taken when using one!

Testing for shorts to the ground or the core of the armature is a simple continuity test. Instead of a piezo beep though, a short will trigger the growler to turn on, which means the armature will jump a bit and everything will emit a loud A/C hum. It certainly makes testing more interesting!

Checking for open windings is a matter of energizing the growler’s coil, then probing pairs of contacts on the commutator. Voltage induced in the windings is displayed on the growler’s meter. Open windings will show 0 volts. Not all the armature’s windings will be in the field of the growler at once – so fully testing the armature will mean rotating it several times, as [Mike] shows in his video.

The final test is for shorted coils. This is where things get pretty darn cool. The growler is switched on and a thin piece of ferrous metal – usually an old hacksaw blade, is run along the core of the armature. If a short exists, the hacksaw blade will vibrate against the core of the armature above the shorted windings. We’re not 100% clear on how the coupling between the growler’s primary and two windings causes the blade to vibrate, so feel free to chime in over in the comments to explain things.

Most commercial shops don’t troubleshoot armatures anymore, they just slap new parts in until everything works again. As such the growler isn’t as popular as it once was. Still, if you work with DC motors or generators, it’s a great tool to have around, and it’s operation is a pretty darn cool hack in itself.

Click past the break for [Mike’s] video!

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Quadcopters Go Inverted By Reversing Their Motors

November 26, 2013 by Adam Fabio 17 Comments

Inverted Quadcopter? That generally means a crash is soon to follow. Not so for a new crop of quadcopter fliers. These new quadcopters are capable of sustained inverted flight. We’ve seen inverted quadcopters before here on hackaday. However, previous inverted quadcopters always used collective pitch to control the thrust produced by the blades. Collective pitch on a quadcopter is much simpler than it is on the main rotor of a traditional helicopter. R/C and full-scale helicopters mix collective and cyclic pitch to articulate the main rotor blades. A quadcopter only needs the collective portion, which is similar to a traditional helicopters tail rotor mechanism, or a variable pitch prop on an airplane.

These new quadcopters are using a much simpler method of flying inverted: Spin the motors backwards. Quadcopters control their flight by quickly varying the speed of rotation of each motor. Why not completely reverse the motor then? Today’s brushless outrunner motors have more than enough power to quickly reverse direction. The problem becomes one of propellers. Standard propellers are designed to create thrust in one direction only. Every quadcopter uses two clockwise rotation and two counterclockwise rotation propellers. Propellers will generate reverse thrust if they are spun backwards, however they will not be as efficient as they would when spinning the direction they were designed for. The quad fliers have found a partial solution to this problem: Remove the curve from the blade. R/C propeller blades are sold by diameter and blade pitch. The pitch is a measure of the angle of attack of the blades. R/C blades also have an airfoil style curve molded into them. Removing this curve (but not changing the pitch) has helped the problem.

This final problem is control systems. Since quadcopters already are relying on computer control for basic flight, it’s simply a matter of loading custom firmware onto your flight board to support motor rotation reversal. Speed controls also have to be capable of reverse rotation, which means new firmware as well. We’re curious to see how the quadcopter community settles on the control systems for inverted flight. The R/C helicopter community went through several iterations of control systems over the years. At one point they were using “Invert switches” which reversed controls as well as handled the collective pitch changes. As time went on, these switches fell out of favor and are now known as “Crash switches” due to the result of accidentally hitting one while flying, or before engine start.

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