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How Many Wheels Do Your Mobile Robots Need?

By: Shana McGuinn, Contributor

Autonomous mobile robots (AMRs) are a key part of the automation systems being embraced by many companies in the warehouse industry. While we marvel at convention floor demonstrations of these energetic high-tech workers, it’s easy to forget that what allows them to be mobile is pretty fundamental: legs or, more often, wheels.

Why wheels instead of legs or systems that use treads? As long as terrain is flat and free of obstacles, wheeled robots are able to deliver the goods more effectively and at a lower cost than their legged or tread-based counterparts, because of their relatively simple design and production process, low cost and the ease with which they can be programmed. They’re speedier, too. Wheels are useful for narrow aisles or other tight spaces because they can spin and turn around without taking up much room. Wheels have few moving parts, so they are less expensive to manufacture and require fewer repairs.

If you are introducing AMRs to your operations, plan on guiding them around your facility – probably by remote control – so that they can create an internal map of it. Once that is done, the AMR can be programmed for destinations and obstacle avoidance. With that information they’ll be able to function autonomously – per their name.

One, Two, Three, Four or…

One-wheeled robots are not optimal for heavy duty, because their single point of contact means they have a tendency to become unbalanced. Two-wheeled robots are a little better in the balance department, although they must stay in motion in order to remain upright. Mounting batteries below the body keeps the center of gravity below the axle. The robot’s tilt angle and platform position can be controlled by sensors.

Three-wheeled robots offer greater balance and stability, as long as the center of gravity is located inside of the triangle of the wheels. There are two versions: one that powers all three wheels and another that powers only two wheels and allows the third one to rotate freely, for enhanced balance. However, a three-wheeled robot will tip over if a too-heavy of a mass is mounted to the side of the freely rotating wheel.

Differential steering can make four-wheeled robots especially navigable, because it can compensate for the disparity between the different speeds and directions the two different pairs of wheels can travel in. Four-wheeled robots without this feature are likely to deviate from the desired direction and move slower than those with differential steering. Note: differential steering can be used in two- and three-wheeled robots as well.

Omni-directional wheels, which can roll freely in more than one direction, are coming increasingly into use because they do not need to rotate first in order to move along a straight path and can arrive at their destination at the desired angle. Omni-directional wheels can easily change directions and angles. That directionality is also a drawback; wheels that don’t all rotate in the same direction mean more friction and less efficiency. Omni-wheeled robots may have three wheels or four, with the three-wheeled type being less costly.

There are AMRs and AGVs with five or more wheels, but unless your operations require very large robots that have to travel over rough or unusual terrain – like, say, that found on Mars – robots with up to four wheels should be sufficient for your needs. The Mars Rovers have six wheels and special suspension systems designed to keep all six wheels on the ground at the same time and help them move over sand and…whatever substance and obstacle they encounter on the red planet.

Other Considerations

Beyond number of wheels, it is important to take into account the size and weight of loads the robotic vehicles in your facility will need to transport. Other selection factors include: the surface conditions, the desired speed, whether they will be used indoors or outdoors – in wet or dry conditions – and the space available for maneuvering. If noise levels are a concern, softer rather than harder wheels may be a better choice – although they are not optimal for high load requirements. Vibration damping can reduce noise.

In addition to selecting wheel systems that are well-designed and of high-quality materials, make sure that the wheels are manufactured to tight tolerance specifications, so they will perform well over a longer period of time. Tolerance issues can result in tires wearing quickly, and not being able to keep robots on track. WMHS

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