High Roller

March 17, 2005
A novel all-terrain vehicle rolls over obstacles but uses standard motors for power.

Ted Severn
MicroMo Electronics Inc.
Clearwater, Fla.

The Shrimp III, a new-design all-terrain vehicle, demonstrates its flexibility rolling over a pile of rocks.

Buried logs are just minor bumps in the road for the Shrimp III. The six-wheeled articulated vehicle rolls right over them.

The entire system was designed to operate with off-the-shelf components such as these Faulhaber dc micromotors from MicroMo Electronics Inc.

To walk up a step the Shrimp III must first lift its front center wheel to the top. Then the two front main wheels move up and over the ledge followed by the two rear main wheels. The final action brings the rear center wheel up and the Shrimp continues on its way. The current design permits going up a step equal in height to two tire diameters.

The flexible geometry of the Shrimp III permits the vehicle to roll over bumps and depressions while maintaining solid tire contact with the surface.

The Shrimp III climbs a stone stairway using its six wheels to lift its body one section at a time.

Remotely controlled "bots" and platforms increasingly make the news pages when they snake their way through inhospitable environments like furnaces, the inner workings of nuclear power stations, and collapsed buildings. Unfortunately, this technology suffers from one disadvantage — inherent immobility without a lot of help from humans.

Most such devices carry guidance systems not much different from those on remotely controlled model airplanes. These vehicles aren't capable of getting around many obstacles without navigational help from a human operator. Truly autonomous vehicles demand extremely maneuverable vehicles capable of dealing with difficult terrain by themselves or without much outside help. Applications such as investigating mine accidents, conducting construction site searches, even exploring other planets, call for a high degree of reliability, redundancy, and autonomy. And these are all features of a new vehicle concept called Shrimp III by Blue-Botics SA of Lausanne, Switzerland.

Concepts and designs abound for "all-terrain" vehicles. Close examination of all design options reveals they are all based on just a few practicable approaches. First there are the tracked, or "crawler," vehicles. They rely on the simple but well-established tractor-tread technology. Next are "walking" vehicles that step on or over obstacles using mechanical legs. They work better today than in the past thanks to modern control technology. Last but not least, "rolling" or wheeled vehicles are the most widely used. Each method offers advantages and disadvantages.

Tracked vehicles are easy to steer, operate well on difficult terrain, and can turn in a confined space. But they need more driving power for their tracks than other vehicles for the same amount of movement. They are also heavier and their chassis experiences high wear.

Walking machines require complex active positional control. Walking is a slow process today and into the foreseeable future. The speed differential between level surfaces and rough terrain is minimal. So even when conditions favor high-speed transit a walking machine cannot take advantage of it.

In contrast, wheeled vehicles are easy to steer and highly maneuverable. Moreover, they are relatively lightweight so they need less drive power than other approaches. Wheeled platforms can surmount large obstacles in rough terrain and can also drive at high speeds on flat surfaces.

It was for these reasons that the Autonomous Systems Lab (ASL) of the Ecole Poly-technique Federale de Lausanne (EPFL) in Lausanne, Switzerland, favored the wheeled option for its new vehicle concept. One problem with previous applications of wheeled vehicles is that they are designed for either rough terrain or level surfaces but couldn't handle both. In contrast the new Shrimp III is a universal vehicle designed for both rough and level conditions.

The Shrimp is an allwheel-drive vehicle. AWD is the only consistent way of ensuring optimum drive power to the wheels with the best traction. Rather than installing one drive motor and distributing its power via gears and clutches, the Shrimp allocates one motor per wheel. One positive side effect of this approach is that the Shrimp can keep moving if a few of its motors fail.

Sophisticated chassis kinematics optimize the contact between the ground and all wheels. An all-terrain concept is based primarily on ground clearance — why "climb" over something if you can "roll" over it? The vehicle body has two different wheel suspension systems to give this clearance.

The four side wheels use a special parallel architecture. This keeps the virtual center of rotation for the four-wheel intermediate chassis at the optimum point between the wheel axles. The chassis itself sits high above the wheel axle to gain optimum ground clearance. The vehicle comes equipped with a centerline front and rear wheel. Special lever kinematics ensure that the front wheel is optimally guided on the traction surface, while the rear wheel is fixed to the main body of the vehicle via an outrigger arm. The two wheels let the Shrimp "pull" itself up and over obstacles two tire diameters tall.

In purely mechanical terms this special design maintains optimized ground contact for all the wheels. Active wheel placement control is not necessary. The turning circumference of the Shrimp is in the range of a vehicle length or less thanks to the swiveling capability of the front and rear wheels.

Despite this maneuverability, the Shrimp still exploits the main advantage wheeled vehicles offer. Drive friction is extremely low, so practically all output drive power is available for propulsion. There is no problem using energy-saving drives with battery back-up.

It was important to have a drive system capable of dealing with every possible application for the universal vehicle. The drive is from the Faulhaber motor and transmission group. It's modular motion-components create mix-n-match systems to fit virtually any operational need or range. The Shrimp uses the 2224...SR Series dc micromotors with precious-metal commutation. The ironless bell-armature motors are available in different voltage variants and dissipate only 4.2 W. They start spinning at low voltages — an extremely important feature for planetary exploration trips relying on solar cells or battery operation with voltages that drop when temperatures fall.

The chosen motor has an integral magnetic pulse generator that produces 64 to 512 pulses per revolution depending on the model. The motorcontrol unit uses the pulses for speed control, acceleration, and distance information.

Gears are needed to match the torque qualities to the wheel geometry. The motor and planetary gear combination have reduction ratios from 3.71:1 to 1,526:1. Available torque of 0.5 Nm provides a large bandwidth for optimum drive adaptation.

Because each wheel reports its drive data to the control system via its own indicator, the entire drive of the vehicle can always be regulated according to the traction and slip data from the individual wheels. An electronic differential lock is just as feasible as assistance for turning on the spot. All drive components are standard production, so the price advantage over special-purpose drives is enormous.

The sophisticated concept of the Shrimp III lets it climb over steps up to a height of double the wheel diameter. This climbing ability outstrips all current concepts by far. The vehicle has a high level of stability over difficult terrain, mastering front/side inclinations up to 40°.

With this model the payload can also be picked up at the front or rear. The concept particularly suits applications where the emphasis is on high profitability or where losses can be expected such as agriculture, minesweeping robots, and exploration robots for industry.

Though it uses conventional components, the approach is reliable enough for sophisticated space-travel applications as well as those earth bound. Its high level of efficiency in forward motion and its exceptional climbing ability when encountering obstacles make it a candidate for applications on any terrain.


Bluebotics SA,
+41 21 693 83 14, bluebotics.com

MicroMo Electronics Inc., (800) 807-9166, micromo.com


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