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Robot rover tackles uneven ground using screw drive


May 23, 2011

Engineer Tim Lexen has created a prototype ranger device that can move across rough terrain using an omnidirectional screw drive system (Photos: Tim Lexen)

Engineer Tim Lexen has created a prototype ranger device that can move across rough terrain using an omnidirectional screw drive system (Photos: Tim Lexen)

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Some day in the future, it's possible that an unmanned rover may go trundling across the Martian landscape not on wheels, but on three rotating steel coils. While able to traverse rugged terrain without getting stuck, it could also move sideways to get around obstacles, would be unlikely to malfunction as it would have very few moving parts, and could perhaps even remain mobile if it were to take a tumble and flip over. Although such a vehicle might not be exploring Mars any time particularly soon, a fully-functioning prototype does already exist here on Earth, and its design could find terrestrial applications.

The basic device

Wisconsin engineer Tim Lexen built and designed the rover. At its heart is a flat triangular PVC main body (measuring about 7 inches/17.8 cm per side) that houses three electric servo motors, battery packs, and electronics. These independently power its three 8-inch (20.3-cm) stainless steel helical coils, also known as screws, each one extending horizontally from one corner of its body. The screws incorporate a low-friction outer coating that keeps them from getting stuck against rough surfaces while their augering action allows them to push or pull against those surfaces to move the rover in any desired direction. For this reason, it works best on textured surfaces such as dirt, grass, sand, or gravel, as opposed to completely flat, smooth surfaces.

The proof-of-concept device is remotely-piloted by a human operator, via a standard hobbyist's radio-control system.


Screw-driven ground vehicles in and of themselves are certainly nothing new, having been around since at least the 1860s - one relatively well-known example is the Russian ZIL off-road vehicle. Almost all of these, however, have utilized pairs of longitudinally-mounted pontoons with sharp-edged augers coiling around their outer surfaces. While these did a decent job of moving the vehicles forward over rough or sloppy terrain, they required the vehicles to face whatever direction they were turning.

The Y-layout and rounded edges of the screws on Lexen's rover, by contrast, allows for complete omnidirectional (or holonomic) drive - this means it can move in any direction, without having to turn to face that direction itself. This could be particularly useful when trying to get around obstacles in tight quarters. While a typical wheeled vehicle would need room to move forward or backward in order to perform its arcing turns, the rover could simply stop at obstacles, then execute sideways right-angle "turns" while still facing the same direction.

It should be noted that some indoor wheeled vehicles are also able to achieve holonomic motion via independently-swiveling caster-like wheels, but Lexen believes that these would not work well on uneven terrain.

Staying versatile

Although the rover can maintain its orientation regardless of what direction it's moving, it can also change that orientation as needed. In order to avoid tipping forward when going down a steep incline, for instance, it could swivel around so that it had one screw extended directly in front. When approaching a small hole, however, it could be swiveled to straddle that hole with two outward-angled screws on either side, instead of nose-diving into it with just one.

"This design should be suitable for outdoor rover applications requiring high maneuverability, robustness, reliability, and resistance to getting stuck or trapped," Lexen told Gizmag. "It has an absolute minimum of moving parts, no suspension, no mechanisms; is it less complex, by roughly an order of magnitude, than any other outdoor rover chassis design, not to mention any outdoor holonomic design."

Below is a video that Tim posted for us, which shows his device in action:

Via New Scientist.

About the Author
Ben Coxworth An experienced freelance writer, videographer and television producer, Ben's interest in all forms of innovation is particularly fanatical when it comes to human-powered transportation, film-making gear, environmentally-friendly technologies and anything that's designed to go underwater. He lives in Edmonton, Alberta, where he spends a lot of time going over the handlebars of his mountain bike, hanging out in off-leash parks, and wishing the Pacific Ocean wasn't so far away. All articles by Ben Coxworth

Hmmmm mixed ideas on it.

My opinion.

Good, innovative, clever.

Slow, uses lots of power, inefficient.

Me thinks for the same amount of power, 4 or 3 independantly orientated wheels with fat flotation tyres will do the same thing 10 x as fast, 10 x further.

Mr Stiffy

I absolutely love it. maybe the most impressive and simple invention I have ever seen. Great job.

Michael Mantion

It\'s ridiculously slow. If you make a concept device, then make it properly, as the british guys say. (:

Renārs Grebežs

If they paint the \"legs\" in fluro-acid colors this fella would be a best guy on trance party!

Михаил Финогенов

I would like to pass on an idea to this wonderful engineer: why not have small \'teeth\' on the screws that operate using magnetic power? The teeth would protrude to give traction, but in a situation where they were a liability, a small electric current would pull them in, using magnetic force. The screws would have to be hollow perhaps, to house the teeth and magnets, but as an engineer, he could likely come up with some cunning design. Or maybe they lie flat on the screws like hairs until released from magnetic force and allowed to \'bristle\'. Just a thought:)


I am not saying it's stupid - the guy has obviously put in a lot of hard work and time to get the screws synchronising to drive it in any direction. I just can't think of a use for it, that it would do better, than say driving wheels on driven spinning castors (shopping trolley wheels). IF he could find solutions that this, and almost nothing else except this, can either do or do way better, than anything else - he might have a winner.... It does have a unique range of features, functions and combinations of functions - as well as drawbacks and impediments. I think the secret to this is to capitalise on both, what it can and cannot do... But compared to say something like an RC model car with say castorised wheels - and covering ground quickly and efficiently - it cannot compete. It has to be something different, or how can it travel or in what can it travel IF the coils where set at different angles or orientations, or if the coil were swapped with vertically steerable propellors...

Mr Stiffy

It would work much better if the screws were mounted straight out, and with an automatic proportional mixing speed control so none of the screws drags along or gets pushed.

It\'s obvious the operator is manually controlling the direction and speed of the three screws. Get an automatic control system on it so the operator only has to wiggle a single joystick and it\'d be much faster.

Gregg Eshelman

This device could be used for landmine detection, maybe?

Joseph Thomas

Small flaw. but great for mine clearing and moving in rough terrain.

To fix your design do not have ends, they need to loop back onto the frame work.


OK imagine this....

A BIG radio controlled toy car, a STEERABLE caster - holding a wheel at each corner.

Each of the wheels has an electric hub motor.

Thus the \"radio controlled car\", can more or less instantly change direction, and head off at great speed.

It can do everything that the \"tri-screw drive\" can - but WAY faster, and for all the hysterics about obstacles like non stop volcano ejecta type bolderous terrain - there is in fact very little of the more or less impassible terrain in the entire galaxy.

Perhaps 0.000001% of the surface is impassible.

So the caster wheeled car with it\'s nice soft wide balloon tyres with knobby tread, can go almost every where.


So what is it that the \"tri-screw drive\" can do, or what places can it go, that the caster wheeled car cannot do or won\'t go?

What is it\'s principle advantage? How can it be used?

There is something clever in it - but I just can\'t think of it.

Think electricity, copper wire coils, and magnets.. One solution is electric motors, another is voice coils in speakers, and another is on and or off relays and circuit breakers..

But the tri-screw drive - how can you use that?

I\'ll offer a prize for the best answer.

Mr Stiffy

Just above the Youtube video, in the article, there is a link to a New Scientist online article on the rover. That has a link to an abstract of a paper on the design, and if really interested you should be able to get a copy of the full paper via IEEE. It looks like this design needs to have the coils sliding on the ground, in any direction: there is no way to have more than one coil driving efficiently in the intended direction, so at least two screws would be sliding in an imperfect way. And it seems the design, paradoxically, requires the low-friction surface/coating on the screws (like a snake). Really good control would likely required gyro, accelerometers, electronic compass, etc.


For some reason I immidiatly had the same association as Joseph Thomas: This is a landmine detector! If the robot is as simple and sturdy as suggested, I could imagine loads of those cheap critters roving a given piece of land and finding or igniting all mines. The coils can be any desired lenghts, right? And with any desired properties. Like a flexible end, specific surface, etc. Anyway it is a provoking new idea and I complement the inventor!

Harry van der Velde

I would think there may be the following things this new rover could do better than an omnidirectional steerable-powered caster design: 1) use a compact body, less than or equal to the height of the drive elements (wheels, screws, tracks); 2) allow a flippable design configuration, to operate upside down (the prototype shown cannot do this, since the body top is too tall, but the body could be made slimmer and symmetrical); 3) do either of the first two while maintaining good ground clearance; 4) be a very simple design, for instance using only three motors instead of a least six; 5) get various real traction options and advantages with a change in body orientation; and 6) be able to access/contact a large ground area from a comparatively small body.

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