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Inception Drive: A Compact, Infinitely Variable Transmission for Robotics (ieee.org)
136 points by mcspecter on Sept 20, 2017 | hide | past | favorite | 52 comments

This is cute, but the use case in robotics is questionable. A variable ratio transmission for a robot seems to solve a problem nobody has. Electric motor control today is able to do that job over a wide torque and speed range. From drones to electric cars to Diesel-electric locomotives, nobody uses variable-speed gearboxes much any more. There are many robot gearboxes, but I've never heard of one with a shifter. Machine Design has ads for robot gearboxes, including ads from Harmonic Drive, but they're all fixed ratio.

This thing would need a second motor to adjust the speed change, with an encoder, plus encoders on the input and output ends of the transmission. Something has to control all that.

If you need sudden bursts of high torque, overload the motor while monitoring temperature. You can run electric motors far above their continuous rating for brief periods, and if temperature is monitored, this is safe. That's how Tesla's "launch mode" works.

Not seeing the use case for this thing in robotics.

How do you feel about his claim that this would be good for energy efficiency?

Or the claim that this would allow for easier human-safety by having the torque, if lowered, not be dependent on logic, but on the actual mechanics, making it possible to stop the entire system if a failure in the torque-change motor is detected?

How do you feel about his claim that this would be good for energy efficiency?

It's unproven. If you have to haul more mass around, that's worse for energy efficiency.

easier human-safety by having the torque (limited)

There are mechanical 'fuses' for torque limiting.[1] Having a transmission that can downshift into a very high torque mode is probably worse for safety. If torque is directly proportional to motor current, there are lots of standard safety devices for limiting motor current.

[1] https://www.mayr.com/en/products/torque-limiters

Interesting, thanks for expanding. If you don't mind another question:

> a transmission that can downshift into a very high torque mode

I'm missing something here i guess, how is this different from a motor that can deliver full torque just because the logic chip is driving it at full power? Is it because the motor wouldn't be able to reach such high torque in the first place? (Which might be a point in favor of the transmission.)

Actually the gear shifting would also be done by logic, so nothing gained by that.

Not quite. In order to lower the torque on a stepper motor you need to have the logic controlling the motor control the torque for every single step the motor takes and that can switch from low to high between two steps. You can ward against this either with continuously checking logic or the torque limiters Animats mentioned: https://www.mayr.com/en/products/torque-limiters

If the inception drive is set to low torque a second motor needs to actively set it to a higher torque, which you can guard against by cutting power to the second one while in low torque mode, and having mechanical interlocks that cut power entirely if the shape of the drive changes to a high torque configuration while it's supposed to be low torque. Neither of these require continuous logic.

How cannot you have a continous checking logic in the driver logic of a gearless drive?

Let me help: you can, and I have taken part in developing one.

Link to patent?


Electric servos for steering wheels use safety critical code, with lot of safety checks, and use direct drive. No need for silly patent for common sense out of the United States.

What's the difference between continous checking logic and safety critical code with lot of safety checks?

I'm not a mech engineer, i do code. I'm asking to learn and if you can prove me wrong by teaching something, yay.

Those safety checks need to be run continously, actually, or the software will not get the certifications necessary to be released on the roads. There are passive means: practices and coding guidelines, static checks, but also active measures: defensive coding, redundancy, continously active safety check logics.

So my original point was: it doesn't matter if you use direct drive or a transmission, as both will be controlled by software, and ultimately the safety of that software will determine whether the System is safe overall. The same design principles and safeguards will need to be implemented in both cases to provide the needed integrity.

But I doubt there are any patents on this. I guess it would be illogical to demand vendors to use patents by someone. But there are lots safety regulations on the topic.

Fair enough, and it makes sense. Thanks for explaining. :)

I think one of the very few remaining applications of variable-speed gearboxes are wind generators with no converter. But this in itself is rare, because it may be more reliable (?), but also less efficient than the power electronics... (afaik off-shore wind generators are going completely gearless).

Wind turbine gearboxes are all fixed ratio, as far as I can tell. The blade pitch is adjusted to maintain a constant speed in synchronous machines. Ones with AC-DC-AC converters don't need to do that. There's also something called a "doubly fed machine", which is a 3-phase generator where the field windings are actively driven to rotate the magnetic field to compensate for minor speed variations of the rotor.

Wind turbine gearboxes take a lot of pounding as the wind shifts. Wear is a big problem and not fully understood. The trend is toward gearless systems. Transmission repair atop a wind turbine is not fun. Power semiconductors are good enough that it's easier to deal with this electrically.

IIRC Voith developed these... I'm in no way an expert about these, but I'd guess that going gearless and using a slow-running generator would have the advantage that you get a much longer and fatter shaft to put bearings on.

It almost looks like they combined a CVT and a planetary gear.

For those saying it has too many moving parts, consider the number of parts found in an 11-speed internally geared hub on a bicycle [1]

I would be really curious to see if this inception drive could be an alternative to internally geared bike hubs.

[1] https://www.sheldonbrown.com/alfine-11.html#details

That is a gem of a website. Glad it hasn't been ruined.

It seems one of the "killer features" of this transmission is the ability to rotate in the same direction as the motor is spinning as well as the opposite direction the motor is spinning. I'm not sure if there is a lot of utility in "bicycling backwards while pedaling forwards". :-)

Still - I would imagine there are many uses outside of robotics for this transmission.

Like a lot of mechanical contrivences, if you come from a world of digital 'machines' you're tempted to say but F R I C T I O N.. The thing is that clutches, gears, back-chatter, teeth wear, this is all bread-and-butter to mechanical engineers, this stuff has been well known for years. If these people have managed to package things into an assembly which is tractable, repairable, can be scaled, and suits their purpose I say hats (and spare arms, and feet) off to them.

I suspect if you went to the room of clockwork/gears in Musee des arts et metiers in paris, you could find much of this there, but the point is, you might not find all of it, as one composition.

Thats what I love about mecha: its often like rotational lego, you add it in the right combinations and it does stuff.

(the one which gets me, is the use of diagonally mounted rotators as 'wheels' which can drive sideways: you see that on some forklifts. Its like michael jackson moonwalking for machinery...)

If anyone is curious, the wheels that the parent poster is referring to are often called Mecanum wheels[0].

[0] https://en.m.wikipedia.org/wiki/Mecanum_wheel

That is a place I never knew I needed to visit.

It's worth an hours-long trip! Make sure you take the Metro; Arts et Metiers is a gorgeous station.


This seems like the mechanism can be summarized as working by precession.

The inner pulley wobbles inside the flexible V-belt and is induced into rotating by precession.

Separation of the inner pulleys controls the effective precession cylinder's inner diameter, the variation of which is accomodated by flexing of the belt: how far it is "squeezed off" and displaced from its outer mount.


Note in the precession animation here how the fast spinning green arrow can be regarded as an input (the wiggle), and the slow spinning of the square-centered blue wheel as an output: there is a many-to-one reduction here, effectively forming a transmission.

Not to be confused with: https://en.wikipedia.org/wiki/Precession

Trying to devise a mechanism like this has consumed many many hours while mowing the grass in rural Ohio. I'm a bit too much of a perfectionist and probably would have discarded this idea even though it seems to be perfectly serviceable for a number of applications. The segmented belt is interesting.

Unfortunately my search has yielded nothing but the same topological(?) traps over and over again to the point where now I just listen to podcasts and entertain my wife with seemingly unprovoked laughter as I chase my grass mulcher around the yard.

I don't think the article mentions this, but being able to vary speed like this leads to some interesting possibilities around centralizing power plants— for example, being able to do a diff-drive or mecanum/omni platform with just a single large motor rather than multiple smaller ones.

The responsiveness probably wouldn't be there for this application, but I'm also curious about whether this could be made to work for a longer range gas/hybrid quadcopter.

Maybe the greatest feature is that it can have a very high ratio while avoiding Harmonic Drive related IP.

Perhaps, although Harmonic Drive has previously licensed SRI's Abacus tech from the same inventor (Alexander Kernbaum)[0]. Given the two groups are already partners in commercialization, it wouldn't be a surprise if they extend their collaboration to include this design. In any event, it will be interesting to see what happens -- especially if it remains independent of Harmonic Drive, to your point.

[0] https://spectrum.ieee.org/automaton/robotics/robotics-hardwa...

Does anyone know if this Inception Drive will be patented heavily as well? Or if it will be fairly easy to build products around this sort of design?

Given the work is being completed at SRI, I would assume the tech would be positioned in a patent or portfolio of some sort. However, this is purely speculation. Nevertheless, this suspicion is further supported by the fact the inventor has a history of patenting for commercialization. That's largely the SOP at SRI and central to their business model.

However, worth noting that just because a patent is filed doesn't mean it (or all of it) will be allowed. So, wrt your second inquiry -- assuming the tech was filed on -- whether or not it can be designed around will largely depend on 1) what was claimed in the patent (e.g., how broad and thorough the claims are) and 2) what is eventually allowed by the examiner. The latter is largely a product of the prior art in the space.

Pretty sure some of the Harmonic Drive patents have lapsed. Cone Drive is now making some very similar products.

How long before you can print the parts for a low strength one off of thingiverse?

Nice design, but I wonder about wear issues. With that many moving parts I don’t imagine it could be made durable with any cheap material.

Especially that main drive belt! It's not a normal belt. It looks like you'd need a flexible material that would accept making high-aspect, thin teeth.

It is hard to believe it is really efficient seeing the description. I have driven cars with CVT and not been impressed.

All else being equal, cars with CVTs are usually more fuel efficient than with other types of transmissions.

Because they're tuned to perform in a manner that delivers the maximum fuel economy in order to drag up the fleet average, driver experience be damned.

What's wrong with the driver experience? Mine seems fine. It's certainly better than the automatic transmission in my old car due to smooth and instantaneous power delivery with no "dead spots" for shifting between gears.

I've read there's inertia when changing ratio in automated CVT though. Maybe it's an old issue.

There's very little inertia in the belt and pulleys, no more than other types of transmissions.

I think this generally happens for two reasons: 1) CVT transmissions can generally handle less torque, so they get lower-powered engines to compensate 2) Some CVT's function with preprogrammed "shift points" for people familiar with conventional transmissions to not feel weird, ruining the main benefit of a CVT

FWIW, they're claiming the whole system (motor operating at optimal state point + IVT gearbox) is more efficient than the usual whole system (motor operating far from optimal state point due to size and weight constraints + fixed ratio gearbox). Which is not immediately implausible.

Anecdotally, we ended up with a Nissan Altima with CVT, the first automatic I had driven for years. I loved the CVT with its smooth transitions between gears and no loss of power. When we got rid of our other vehicle with a standard transmission and picked up a non-CVT automatic, I was stunned by how sloppy it felt. I hadn't realized or remembered how "normal" automatic transmissions behaved and it was very jarring.

Would you mind elaborating on the problem you have with CVT cars?


I own a '16 Impreza and it really drags at low speeds. It's my only complaint with the car however, and it is better with higher octane fuel.

Unless you're getting premature ignition in your cylinders from compression (knock) the higher octane shouldn't matter.

If its a turbo (i assume not?) the car’s ECU will compensate for lower octane. Source: me when, half asleep, I accidentally half-filled my 03 WRX tank with 87.

I am no fan of Subarus

I think I've seen this nested configuration on youtube a while back...

Still nice drive.

It's clever. But so many moving parts, and doesn't look very sturdy, nor efficient.

There was a device called an electric wheel, a few years ago, which did the continuously variable thing using a planetary gear (the sun and ring were driven by separate motors, and drive was taken from the planet carrier.)


This design is inherently more compact than that arrangement, and the parts are simpler. (I think)

This is exactly how the Prius (Gen 1) 'Hybrid Synergy Drive' works.

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