The multi-directional wheel robot
by Jasher Tan
The Omniwheel robot, or more accurately, the Mecanum wheel robot, is unique in its ability to move in all directions on a flat surface. In contrast, a car needs to apply a steering system to change its direction, and caterpillar tracks, while able to pivot on a single spot, are not able to slide sideways compared to the Mecanum wheel system pictured in Figure 1.
The Mecanum wheel is thus able to move in the customary four basic directions of moving forward, reversing, turning left and turning right. In addition, it is also able to pivot like a tank and slide sideways like a crab.
How does it perform all of these moves? The answer lies in the wheel. Like the wheels of the Cherubim in Ezekiel’s vision that never turn right or left but go straight forward in any direction, the Mecanum wheel takes its inspiration from this vision, in that the wheel system, when installed in at least two opposing pairs on a rectangular frame, have no need for a steering system, nor do they need to be connected by chain-link, but are rigidly held, save for the transmission shaft that may rotate in either direction.
The Mecanum wheel consists of several rollers mounted along the rim of a main wheel. Typically, they are positioned at an angle of 30 to 60 degrees to the main wheel, with 45 degrees being the most common angle of tilt for these peripheral rollers. This differentiates the Mecanum wheel from the omniwheel, where the peripheral rollers are mounted at 90 degrees.
The advantage of the Mecanum wheel over the omniwheel lies in its ability to traverse steeper slopes, where an omniwheel equipped robot could encounter a situation where the peripheral rollers, due to their angle of mounting, would cause the robot to slide down helplessly.
The Mecanum wheel addresses this problem by placing the rollers at an angle, and in opposing directions for forward and aft pairs of wheels. This distributes the motive action of sliding down an inclined plane in a V-shape should the Mecanum wheel robot be moving across the inclined plane of the slope, preventing the robot from sliding down due to the parallel nature of the rollers as encountered in the omniwheel.
This property exhibited by the angled rollers also allows the Mecanum wheel to perform sliding actions. The Mecanum wheel pairs on the left and right side of the robot are mirror images of each other, and at any given moment, each wheel, when rotated, generates a resultant force in the direction of the angle of the rollers. Each resultant force, however, is made up of components that act perpendicular to each other, that is, in the X and Y axis.
When the mirror-imaged wheels are combined together in the following pattern, shown in Figure 2 below, rotating the wheels causes specific components of the resultant force to combine together, creating a new, dominant resultant force, typically in the forward, reverse or sideways direction.
A trick that allows the robot to move diagonally involves rotating only two wheels in the same direction, for example, to move forwards and to the right at the same time, only the front, right-hand wheel and the rear, left-hand wheel are activated, the other two being free-spinning. This allows the resultant force produced to be dominant in the forward-right direction (or conversely the reverse-left, if both wheel rotations are reversed).
To move sideways, the forward two wheels rotate in the same direction, while the rear wheels rotate in the direction counter to that of the forward wheels. This has the effect of ‘corkscrewing‘, much like a land-based version of a ship’s propeller. The dominant resultant force can be either to the left or to the right, depending on the rotating direction of the wheels.
To pivot, the wheels on the left side rotate in one direction, while the right side rotates in another, the effect being a form of ‘skid-steering ‘, albeit smoothened out by the assisting rollers. By playing around with the direction of rotation of each wheel, the Mecanum wheel robot may be made to move in almost any direction, except upwards, skyward, or downwards, into the ground.
The robot’s frame is constructed from riveted aluminium bars. In addition, a sheet metal plate, about 2mm thick, is bolted to the bottom section of the frame. This plate holds four 12V motorcycle batteries powering the eight motors of the robot, with two motors being slaved to each wheel. The power from the batteries is routed through motor drivers before reaching the motors, and a limiter to protect the Arduino microcontroller controlling the drivers. A wireless Playstation module was also installed, allowing the Mecanum wheel robot depicted in Figure 3 to be remotely controlled.
Future plans for the Mecanum robot include upgrading it with a manipulator, or arm, as well as sensors and camera systems to allow for autonomous motion. The robot is indeed a platform for upgrades and research for students who are interested in the field of robotics, and it is also hoped that it fulfills these roles in a way that also brings fun to an otherwise austere field of study.
Figure 1: A Mecanum wheel.
Figure 2: Arrangement of the Mecanum wheels.
Figure 3: The completed Mecanum wheel robot.
Jasher Tan took up mechanical engineering but has a greater interest in mechatronics. He used to fancy himself being a pilot, but when spectacles had to be worn daily, he doggedly clung to the skies by deciding to become an aircraft engineer instead. However, Physics, Fluid Mechanics and Fluid Flow Modelling were formidable challenges, and unlike Dynamic Systems and Automatic Control, the fun factor wasn’t there at all. So began his journey down the path of the Machines. A member of the Institution of Electrical and Electronic Engineers Student Branch (IEEE) of Curtin Sarawak and the Curtin University Mechanical Engineering Club (CUMEC), he’s currently completing his final year in Mechanical Engineering.