# How does Hendo hoverboards achieve the self-propelling motion? What is the MFA?

From what I understand of the idea behind Hendo hoverboards, they use four disc shaped hover-engines to generate a self-propelling motion.

I also understand that it uses Lenz's law and eddy currents. And that the hendo-hoverboards only work in non-ferromagnetic material.

From what I re-call, Lenz's law states that eddy currents are created when magnets are moved relative to conductive materials. In particular, lenz's law states that eddy currents are formed when there is a change in magnetic flux. So they used magnetic field architecture.

Magnetic Field Architecture (MFA) is a design where there are 4 electromagnets on the base of the board that propels on non-ferromagnetic material.

However, I don't understand how that works? How do they calculate the gravitational pull such that the MFA exerts more force to over come that gravitational pull?

• Bonus points to whomever can show that this is a trick meant to make it look like Back to the Future will actually be right about 2015
– Jim
Oct 23, 2014 at 14:57
• @Luxii maybe add a little more detail on how you think you think Lenz's law and eddy currents are used (possibly why this means they only work in/on non-ferromagnetic material) to show that you have done research and put in effort to answering this question yourself. Oct 23, 2014 at 15:12
• @jim: indeed, and there are lots of ways to make things hover. However there are none based on negative energy. If Luxii wanted to change the question to ask how a Hendo hoverboard works then that would be a good question and I'd remove my close vote. Oct 23, 2014 at 15:15
• @JohnRennie Good point. I should specify the question to that product in particular. Thanks! Oct 23, 2014 at 15:19

As explained on this page, Earnshaw's theorem says it's impossible to have perfectly stable magnetic levitation where none of the fields are changing with time. But as is also discussed there, it is possible to have levitation that appears stable to the naked eye if the the currents that create the magnetic field continually adjust to small movements of the levitated magnet in such a way as to quickly damp out these movements. This can be done using artificial feedback with electromagnets at different positions that can adjust their fields in response to updates from sensors about the motion of the levitating magnet, as mentioned in the "feedback" section of that page, but there are also some examples of systems where the currents in the material naturally adjust in this way. One of these is diamagnetism, where the currents are just motions of electrons around atomic nuclei, and the electrons adjust to changes in the external field in such a way as to always create a magnetic field that's aligned opposite to the external field, and thus repels the magnet creating that field. A nice conceptual explanation of Earnshaw's theorem and how it doesn't rule out stable-appearing levitation with diamagnets can be found here. For most diamagnetic materials the response is very weak, but superconductors are an exception with a very strong diamagnetic response, which is why they are often used in dramatic examples of magnetic levitation like the superconducting hoverboard shown here, or the example of levitation with flux pinning shown in this video.

The Hendo hoverboard doesn't use superconductors or ordinary diamagnetism where the magnetic response is caused solely by the realigning of electrons in atoms though--instead it relies on the effect discussed in the "oscillating fields" section of the first page I linked to. In this case, a magnetic field that's designed to oscillate in the right way (which can apparently be achieved either using rotating permanent magnet or a varying current in a non-rotating electromagnet) will induce eddy currents in a nearby conductor, motions of large numbers of electrons that are not bound to particular atoms in the conductor. As mentioned on that page, and also in the oscillating electromagnetic fields section of the wikipedia article on magnetic levitation, this is really a large-scale version of diamagnetic levitation--apparently the swirling eddy currents adjust to continually repel the source of the oscillating field in the same way that individual electrons adjust in ordinary diamagnetic levitation. The wiki article links to this pdf article with a more technical discussion of how this works in the case of an oscillating field produced by varying current in an electromagnet, though apparently the Hendo "hover-engines" creates an oscillating field with rotating permanent magnets, as mentioned in the patent John Rennie pointed to in his answer. Here is a paper dealing with a numerical simulation of a rotating permanent magnet hovering over a conductive surface, although the configuration is a bit different (in the paper the axis of rotation is parallel to the surface like a wheel, whereas with the Hendo hoverboard the axis of rotation is perpendicular to the surface like a CD--apparently the type of rotation in the paper leads to forward thrust as well as vertical lift, which might be useful in future applications of this technology that need to be able to accelerate or brake).

I believe the "magnetic field architecture" refers to the way the oscillating magnets are designed in such a way as to produce an especially strong field below the board but a much weaker one above, using some type of Halbach array. This article mentions that the patent filed for Hendo's magnetic levitation system specifically referred to the use of a Halbach array.

As for "self-propelling", I don't know the details but this article may suggest it has something to do with a built-in system where pressure-sensitive pads are used to adjust the magnetic fields created by the different hover-engines, creating a bias in the strength of these fields which in turn should introduce a directional bias in the eddy currents pushing it from below:

Riding the contraption was a lot fun, but also quite the challenge: The Hendo hoverboard doesn't ride at all like McFly's flying skateboard. In fact, without a propulsion system, it tends to drift aimlessly. Arx Pax founder and Hendo inventor Greg Henderson says it's something the company is working on. "We can impart a bias," he tells me, pointing out pressure-sensitive pads on the hoverboard's deck that manipulate the engines. "We can turn on or off different axes of movement." Sure enough, leaning on one side of the board convinces it to rotate and drift in the desired direction.

Although on their kickstarter page they seem to contrast the hoverboard being "primarily ... self-propelled" with the idea of moving it by varying the magnetic field strength:

While our hoverboard is primarily intended to be self-propelled, the actions which stabilize it can also be used to drive it forward by altering the projected force on the surface below.

...so that suggests that they may be using the term "self-propelled" in the same way a skateboard could be said to be "self propelled", i.e. you gain speed by pushing against the ground with your foot.

• Their video shows one demonstration of their thin white box kit moving in a pattern without external force, so they do have some amount of propulsion. One level of the kickstarter has this option with smartphone control. Oct 23, 2014 at 20:23

First let me make it clear I don't on a Hendo engine and I've never seen the design for one, so what follows is based on what I've found by Googling and what seems intuitively obvious.

The Hendo engine uses a technique called electrodynamic suspension. This can get very complicated very quickly when you try to do the calculations, so I'll just describe it in general terms. Note that this technology is used in some Maglev trains, so it's a well established technology and indeed the first related patent is 109 years old (sadly the Google OCR has made a real hash of this patent).

We all learn at school that a changing magnetic field through a conductor will induce a current in the conductor. This process is described by Lenz's law, hence the reference on the Hendo site. However the current in turn creates a magnetic field. The principle of electrodynamic suspension is that a moving magnet induces currents in a nearby conductor, and those currents create their own magnetic field. If you get the geometry right the original and induced fields will repel each other and you get magnetic levitation.

A key feature of this process is that the magnets have to be moving, because only a changing magnetic field will induce a current. I believe, and this is the bit I'm least sure of, that the Hendo engine uses a rotating electromagnet i.e. the magnet spins like a propellor. It's this motion that induces the current in the conductor under the board and creates the opposing magnetic field that levitates the board. This allows the board to hover even when it's stationary.

Later:

Aha, I've found the patent for the Hendo engine. It does use rotating magnets, but they're permanent magnets not electromagnets.

• The need for the levitated magnet to be moving is a feature of the proposed Inductrack maglev design, but apparently with an oscillating field from an electromagnet you can have levitation without relative motion, as discussed in this pdf article that I linked to in my answer, where they used "a stationary coil carrying a time-varying current, levitated above a conducting sheet." Oct 23, 2014 at 16:45
• @Hypnosifl: possibly, but the Hendo engine uses rotating permanent magnets. Oct 23, 2014 at 16:58
• true, and thanks for pointing out that info from the patent--but as mentioned here the basic requirement for this method is to use an oscillating magnetic field over a conductor, and since Hendo's videos show the board levitating when not in motion, presumably the required type of oscillating field can be created by rotating permanent magnets as well as by an electromagnet with varying current. Oct 23, 2014 at 17:02

The hover board is a reverse capacitor or a clearer example is the filament in an electroscope. In a capacitor you have two stacked plates with a dielectric layer to keep one charged plate from touching the other. When a positive charge is applied to one plate it retains a charges because the adjacent negatively charged plate holds the electrons captive. In the electroscope the opposite holds true, there you have two plates (or leaves, foil, etc...) that are stacked but joined by a wire. when you apply a charge the two plates repel each other because they both have the same charge.

In the case of the hover board, the magnet's negative poles are likely facing downward and the positive poles upward. The floor plate is non-ferrous (and preferably a good conductor - hence CU or AL)so that the magnets don't stick to it, and so that the entire floor can be charged (in this case negative through a ground wire to earth). The floor has the negative charge of the entire earth, and the motors on the board have the rotational charge of a motor with similar magnetic fields. The flat armature can then lift weight based on it's wattage an the work required to lift the rider.

• I'm not sure that I see the connection with an electroscope. I've also never seen the magnets poles referred to as "negative" and "positive." Mar 17, 2015 at 1:22
• You're confused - about the difference between electrostatic repulsion and magnetic repulsion. They are not at all the same thing. The voltage necessary to produce this amount of electrostatic repulsion would be far more than any reasonable amount of insulator could resist, and the rider's hair would be standing on end, not to mention lightning. Mar 17, 2015 at 1:29