It's the real deal, lots of challenges with high emf. Not surprisingly a very common failure mode is that if you induce currents in the coils of the brushless motors their controllers which are using back emf to set their waveform phase get it wrong and the motors stop spinning, spin backwards, and sometimes just go back and forth like tiny washing machine motors.
Shielding helps of course, adds expense and adds weight, the two things that cut into how many you can make for $X and how far they can fly.
Counter drone systems in battle are going to be a thing, things like the Danish 'bird' RADAR sees them easily enough[1], targeting them with EMF just needs an antenna, generator, and some clever electronics.
This becomes more important as the drones become more autonomous because if there is no operator to 'jam', electronic counter measures are not as effective.
> Not surprisingly a very common failure mode is that if you induce currents in the coils of the brushless motors
No, that doesn't happen. Currents can be induced in the wires to the motors, but not in the motors themselves. For one thing, the outside surface of the motors is the aluminum rotor which is an extremely effective faraday cage. For another, coils don't act like antennas. Loops of wire in an electric field have the exact same voltage difference as a straight wire.
> Shielding helps of course, adds expense and adds weight, the two things that cut into how many you can make for $X and how far they can fly.
Shielding adds virtually zero weight; carrying a spool of fiber optic cable adds a lot of weight. All the drones in Ukraine right now are fiber optic but most of them are unshielded... the reason why is not that shielding is heavy, it's just that there are lots of jammers but very few truck-sized weapons intended to totally disable drones.
That's also assuming it would even work on a drone without an antenna. If these weapons are not relatively broad-spectrum then they will be very sensitive to the particulars of the circuitry, and they won't always work.
Currents can be perfectly well induced in the motors themselves, by the variable magnetic field of an electromagnetic wave.
Any electromagnetic wave has both an electric field and a magnetic field, hence its name.
An antenna can be made from either a straight wire sensitive to the electric field or from a loop of wire sensitive to the magnetic field.
The only reasons why a motor is usually a bad antenna is that it should have a case with good shielding properties (i.e. the magnetic circuits have only small gaps) and the high inductances of its windings act as low-pass filters for high-frequency induced currents, like those of a microwave transmitter.
There exist electric motors with very low inertia of the moving parts (to enable high accelerations), where the rotor does not have any ferromagnetic material and the stator has large gaps for the rotor. Such motors can be much more efficient antennas than standard motors, but such motors are not used in drones.
All the cheap radios for under 30 MHz signals used antennas made of a ferrite bar with a coil on it, very similar to a motor winding, except that the magnetic circuit had a much greater gap than in a motor, because they were more sensitive at small sizes than antennas sensitive to the electric field.
Moreover, brushless motors do not have an aluminum rotor. You are thinking about AC induction motors. Induction motors do not have brushes, but nobody calls them brushless, because they never had brushes. Only DC motors are called brushless, because their classic variant had brushes, which are replaced by power transistors in brushless motors.
The aluminum rotor of induction motors is normally inside, not outside. The inverted construction is rare.
Both induction motors and brushless motors have windings only on the stator, which is the external part in the normal motor structure, and those are equally susceptible to variable magnetic fields, except that they are usually bad antennas for the reasons mentioned above, especially at microwave frequencies.
In an ideal motor, the stator is not an electrical conductor (which is actually the case for ferrite stators), so it has no shielding properties for electric fields, but it has shielding properties for the magnetic field, if the gaps in its magnetic circuit are small.
Almost all quadcopter brushless motors are constructed as “outrunners,” with a fairly thick, sturdy aluminum motor bell (rotor!) with strong permanent magnets glued inside of it and the stator in the center. I agree that they are not immune to RF but at high frequencies it will require a really comical amount of power to do anything to one.
Oh it will absolutely work on a drone without antenna: in microwave range every PCB trace becomes an antenna. If the field is strong enough it'll just blow the gates on IC inputs. If it's far away it can still do soft upset of any periodic signal, e.g. one of the numerous clocks on digital circuits.
But typically with a much smaller number of turns. A motor coil should have a decently high inductance and thus act as an antenna only for pretty low frequencies.
It is not the number of turns that matters to distinguish coil antennas from motors, though indeed a high number of turns in both motors and antennas leads to a high inductance, which ensures that any resonance frequencies will be low, so a received radio signal of high frequency will not be amplified by a resonance.
The magnetic circuit of a coil antenna has a very big air gap, because its ferromagnetic core usually has the form of a cylinder or of a prism and the magnetic circuit closes through the air between the opposite ends of the core.
The magnetic circuit of a motor has only small air gaps between stator and rotor, which are required to allow the rotor movements. Because of the small air gaps, the inductance of a motor winding is much higher than the inductance of a coil antenna with the same number of turns and using a ferromagnetic core made of the same material.
IIRC NEMP is assumed to be at 50kV/m for milspec certification purposes. However most exposed devices are much simpler than quadcopters. There are some class exemptions too, e.g. electronic sights are allowed a fraction of second blackout after upset. Not something that would work for an attack drone though.
What I think makes Leonidas more efficient is they likely operate in continuous wave bursts rather than pulses. Probably with a broad comb rather than one specific value too.
I don't think a fraction of a second interruption would be overly problematic. These drones are somewhat heavy and thus bring relevant amounts of momentum. But the start up process is probably far too long
Yes the reboot takes too long, even before you account the time to re-negotiate comms (even over fiber it is not instant). However in a terminal approach on target say a 1/4 second disrupt will likely be unrecoverable. Quadcopter drones are not ballistic or aerodynamic, and stability recovery once it gets tumbling is challenging.
Electronics is electronics. But the weight trade-offs for anything that walks or drives on the ground aren't as severe as for drones, so you could presumably provide EM shielding and light armor more easily.
At the same time, terrain is just harder and slower to navigate, it's easier to erect barriers, and humans are better at spotting eye-level threats. There's a reason why murder-drones are common on the battlefield, and murder-humanoids aren't.
>At Terrapin, we feel that our two main products, the Terrapin Turtle ®, and the Terrapin Logo Language for the Apple II, bring together the fields of robotics and AI to provide hours of entertainment for the whole family. We are sure that an enlightened application of our products can uniquely impact the electronic battlefield of the future. [...]
>Guidance
>The Terrapin Turtle ®, like many missile systems in use today, is wire-guided. It has the wire-guided missile's robustness with respect to ECM, and, unlike beam-riding missiles, or most active-homing systems, it has no radar signature to invite enemy missiles to home in on it or its launch platform. However, the Turtle does not suffer from that bugaboo of wire-guided missiles, i.e., the lack of a fire-and-forget capability.
>Often ground troops are reluctant to use wire-guided antitank weapons because of the need for line-of-sight contact with the target until interception is accomplished. The Turtle requires no such human guidance; once the computer controlling it has been programmed, the Turtle performs its mission without the need of human intervention. Ground troops are left free to scramble for cover. [...]
>Because the Terrapin Turtle ® is computer-controlled, military data processing technicians can write arbitrarily baroque programs that will cause it to do pretty much unpredictable things. Even if an enemy had access to the programs that guided a Turtle Task Team ® , it is quite likely that they would find them impossible to understand, especially if they were written in ADA. In addition, with judicious use of the Turtle's touch sensors, one could, theoretically, program a large group of turtles to simulate Brownian motion. The enemy would hardly attempt to predict the paths of some 10,000 turtles bumping into each other more or less randomly on their way to performing their mission. Furthermore, we believe that the spectacle would have a demoralizing effect on enemy ground troops. [...]
>Munitions
>The Terrapin Turtle ® does not currently incorporate any munitions, but even civilian versions have a downward-defense capability. The Turtle can be programmed to attempt to run over enemy forces on recognizing them, and by raising and lowering its pen at about 10 cycles per second, puncture them to death.
>Turtles can easily be programmed to push objects in a preferred direction. Given this capability, one can easily envision a Turtle discreetly nudging a hand grenade into an enemy camp, and then accelerating quickly away. With the development of ever smaller fission devices, it does not seem unlikely that the Turtle could be used for delivery of tactical nuclear weapons. [...]
And with the right shape, the turtles would bin pack when falling into a ditch and with enough turtles they would make their own bridge for the turtles that came behind :-)
>Seuss has stated that the titular character Yertle represented Adolf Hitler, with Yertle's despotic rule of the pond and takeover of the surrounding area parallel to Hitler's regime in Germany and invasion of various parts of Europe.[3][4] Though Seuss made a point of not beginning the writing of his stories with a moral in mind, stating that "kids can see a moral coming a mile off", he was not against writing about issues; he said "there's an inherent moral in any story" and remarked that he was "subversive as hell".[5][6] "Yertle the Turtle" has variously been described as "autocratic rule overturned",[7] "a reaction against the fascism of World War II",[8] and "subversive of authoritarian rule".[9]
When Dr. Seuss Made Hitler Into a Turtle:
A reading of Dr. Seuss’s “Yertle the Turtle” with a bit of history in mind.
Shielding helps of course, adds expense and adds weight, the two things that cut into how many you can make for $X and how far they can fly.
Counter drone systems in battle are going to be a thing, things like the Danish 'bird' RADAR sees them easily enough[1], targeting them with EMF just needs an antenna, generator, and some clever electronics.
This becomes more important as the drones become more autonomous because if there is no operator to 'jam', electronic counter measures are not as effective.
[1] https://www.weibelradars.com/drone-detection/