(The official version of this article was made available in May, 2016.)
A Method to Inhibit Roller Action on Mecanum Wheel.
Timothy D. Greer
Disclosed is a method to inhibit roller action on mecanum wheels. The method stops, or reduces, rotation of the rollers relative to the main wheel, while allowing the main wheel to continue to rotate.
Mecanum wheels are distinguished by the rollers set at an angle (almost always, 45 degrees) around the circumference of the main wheel. The rollers rotate freely with respect to the main wheel. The advantage is that by driving each wheel independently, the vehicle can be made to move (translate) in any direction, rotate, or translate and rotate simultaneously. But they have a disadvantage when interacting with other robots.
A mecanum-wheeled robot is easily pushed around by a robot with more conventional wheels or treads. If the rollers are made to not roll in such circumstances, the mecanum wheel behaves more like a conventional wheel and the robot is less susceptible to external forces. Disclosed is method to stop, or reduce, rotation of the rollers relative to the main wheel, while allowing the main wheel to continue to rotate.
In accordance with the method, a second wheel with a vertical axis and aligned near ground level is used with the mecanum wheel. The relative orientation of the two wheels is like that of bevel gears. The second wheel is made to press against the mecanum wheel, so that relative motion between the rollers and the body of the mecanum wheel is prevented. But by allowing the pressing wheel to rotate freely, turning of the mecanum wheel itself is not inhibited. While pressing is engaged, the mecanum wheel is effectively transformed into an ordinary wheel.
In accordance with the method the rollers are only partially stopped, this might be made adjustable depending on how hard the pressing wheel pressed. So the transformation between a mecanum and regular wheel could be a continuum rather than all or nothing. This aspect would make failure less catastrophic: when forced, the rollers could slip against the pressing wheel rather than for example breaking a pin that is holding them fixed.
The method disclosed herein might give rotational energy efficiency, resulting from superior power transmission through a solid wheel as compared with the mecanum design. The impeded mecanum's greater rotational energy efficiency relative to a mecanum with unimpeded rollers is cited in alternate designs that stop all the rollers at once by means of mechanisms built into the wheel itself. There would be some energy loss due to the spinning of the vertical-axis wheel, but since that wheel is intended to freely rotate, such losses are minimal and likely less than the losses due to the odd mechanisms of the alternate solutions.
The intent of this mechanism is to provide means to stop a mecanum wheel's rollers from turning, while continuing to allow the wheel as a whole free rotation. This has the effect of making the wheel into a solid wheel rather than one with rollers around the circumference. The actual need is to prevent the rollers that are in contact with the ground from rotating relative to the rest of the wheel. Relative rotation can be stopped by having something press against both the roller and the wheel body. Since the wheel may be rotating, the item doing the pressing must be able to rotate along with the wheel.
In accordance with the method, an arrangement analogous to a bevel gear is used. The pressing mechanism is a wheel, positioned just above ground level, with its axis vertical. This pressing wheel is covered in a flexible material which provides friction against both rollers and the wheel body. The material needs to be somewhat flexible and compressible because the wheel body and roller presents an irregular face. It is possible to design the mecanum and pressing wheels such that the mecanum's face presented to the pressing wheel is regular and the two could mesh much like a bevel gear, and such a design is more effective in stopping the rollers. However, if the pressing wheel is covered with flexible and compressible material, e.g. packing foam or the fuzz that velcro sticks to, commonly available mecanum wheels can be used. Since mecanum wheel bodies are typically metal, improved friction with the pressing wheel can be obtained by covering some of the metal surface with some higher-friction surface, e.g. the same material being used to cover the pressing wheel.
The pressing wheel is attached to a vertical shaft, and the shaft/wheel combination is intended to be freely rotating. Actuators move the shaft and wheel parallel to the ground, so that the pressing wheel can be brought into contact with the mecanum wheel or pulled free from contact. If the pressing wheel can freely rotate, it does not impede the rotation of the mecanum wheel as a whole even when in contact with it. It however in that position prevents rotation of the rollers relative to the body of the mecanum wheel. By positioning the pressing wheel just above ground level, the rollers it stops will be those in contact with the ground. The pressing force does not really have to be parallel to the ground -- and the force direction matters at all only when the two wheels come into contact. While not in contact, the motion of the pressing wheel due to the pressing action need not be parallel to the ground, although that would be the simplest design. In order to allow more ground clearance during normal operation (that is, when the pressing wheel is not in contact with the mecanum wheel), the pressing wheel could slide up to a higher position, e.g. by using a guide slot for a collar encircling the axle of the pressing wheel.
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