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Comment by modeless | Hacker News Reader

I've wondered for a while why we don't use electrostatic forces more. Electrostatic motors and electrostatic speakers do exist but they are very rare compared to the ubiquitous magnetic types. It seems like it should be possible to make a really good electrostatic muscle for robots. I know that there has been some work in this direction like https://www.artimusrobotics.com/technology, but what makes it so much harder and less developed than regular magnetic electric motor technology?

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In air, a relatively small electric field will cause electric discharges.

A magnetic field can have very large intensities without creating problems, so that is the reason why we use electromagnetic motors and not electrostatic motors.

The maximum magnetic field in a motor is normally limited by the saturation of the ferromagnetic materials at values where the density of energy in the space between the stator and the rotor is many times higher than the density of energy at which an electric field would cause electrical discharges.

Electrostatic motors can be useful (and they are actually used in certain devices) only at microscopic sizes.

Hmm, so I guess you would have to fabricate an array of millions of microscopic electrostatic motors. Which is kind of like what biological muscles are in a way, right?

On top of the arcing issue already mentioned, it's a lot easier to control a current than to control a charge distribution. So electromagnets win.

Constraining the electrons apart would require a very serious insulator. The muscle would just arc between its two ends.

To make it work you have to distribute electrical potential along all the "motor". Just like biological muscles do with protein configurations. We just aren't technogically advanced enough to build such metamaterials ourselves.

If you charge two halves of a muscle oppositely so they attract, you would get arcing between them without a strong insulator in between. But if you charge two halves both negative or both positive there would be no arcing between them. They would still strongly repel and it seems like you could use that to do work.

Electrostatics generally require high voltage (the HASEL muscles you linked to require 2-20kV [0]) to generate usable force.

This creates lots of engineering and regulatory challenges for product development because, as the esteemed William Osman says, "Welcome to the world of high voltage, where everything's a wire and you're probably going to die" [1].

[0] https://onlinelibrary.wiley.com/doi/full/10.1002/advs.201900...

[1] https://youtu.be/IiJAq53knwc?t=288

Even if you do not have a motor but just a single charged sphere, at its surface there will be an electric field increasing with the charge.

At a certain charge value, and at a not very large one, the surface electric field will exceed the breakdown field of air and a corona discharge will start.

See St. Elmo's fire.

https://en.wikipedia.org/wiki/St._Elmo%27s_fire

So unfortunately the idea of using the repulsion forces does not work.

To prevent the discharges, the entire environment around the 2 charged moving parts would have to also be charged with the same sign, but then the repulsion forces between your parts would be balanced by the repulsion forces from the surrounding medium.

Thanks for the patient explanations! You could immerse the thing in insulating liquid, maybe inside a rubber pouch, would that not be enough?

High-voltage equipment, e.g. high-voltage transformers, is indeed immersed in insulating fluids, either in special insulating oils or in the gas sulfur hexafluoride, which is more convenient than liquid oil.

This increases several times the breakdown field compared to air but it is not enough to reach similar energy densities like with magnetic fields.

To give some numbers, the air breakdown field is around 3 MV/m, while the maximum magnetic field in a motor might be up to 2 Tesla.

The ratio of the energy densities is the square of the ratio between the product of the magnetic field with the speed of light and the electric field, i.e. the square of (2 x 3 x 10^8) / (3 x 10^6), so 200 squared, i.e. 40 thousands.

Even if you increase the breakdown field 10 times, which is quite hard to achieve with fluids, the magnetic field would give forces much, much higher at a given size.

For a bit, but if you keep raising the voltage you find the electric breakdown happing on the outside surface of the robot (or whatever’s using the electric field muscle) instead of where you put the insulation.

You can't create a monopole, either electrostatic or magnetic. If you impress a negative charge on something, there must be an equally strong positive charge somewhere nearby that you'll have to insulate somehow.

It would also want to arc to other parts of the mechanism, it's not just the tiny subsystem of the actuator here we have to consider.

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