Mechatronics Design of a Mecanum Wheeled Mobile Robot

Abstract

The Mecanum wheel was designed in Sweden in 1975. Using four of these wheels provides omni-directional movement for a vehicle without needing a conventional steering system (Muir & Neumann, 1990; Dickerson & Lapin, 1991; Braunl, 1999; Navy, USA, 2002 and Lunze & Schmid, 2002). The wheel itself consists of a hub carrying a number of free moving rollers angled at 45° about the hub's circumference. The rollers are shaped such that the overall side profile of the wheel is circular. However, wheel slip is a common problem with the Mecanum wheel, particularly when the robot moves sidewise, as it has only one roller with a single point of ground contact at any one time. This severe slippage prevents the most popular dead-reckoning method, using rotary shaft encoders (Everett, 1995 and Borenstein et al, 1996), from being performed well on the Mecanum robot. To cope with the problem, visual dead-reckoning was used as a slip-resilient sensor (Giachetti et al, 1998; Nagatani et al, 2000 and Kraut, 2002). This technique, also used in optical mice, makes use of an on-board video-camera continuously capturing frames of the ground beneath and image processing hardware on the robot determining the speed and direction in which the current frame has moved relative to the previous frame thus allowing the speed and direction of that point of reference to be calculated. However, visual dead-reckoning using a single camera or optical mouse can not provide all threedegree-of-freedom positional information for robot navigation and motion control. Fixed line following is the simplest and most reliable solution, yet is also the most limiting. A physical line is marked on the ground along the path which the robot is to follow (Everett, 1995 and Borenstein et al, 1996). For a robot that is set up in a fixed location for a set task this system is effective but for a research robot with omni-directional capability this approach is seen to be a primitive, though still viable, option. This chapter presents a research project recently completed at Massey University, New Zealand. The research started upon an existing omni-directional platform built of a boxlike aluminium chassis, four electric window winder motors and four Mecanum wheels (Phillips, 2000). The aim of this project was to provide the platform with motion control that could be programmed to accommodate various robotic behaviours specified. With respect to the path following behaviour, two optical mice were attached to give positional feedback for closed-loop control and dead-reckoning for navigation and a Mitsubishi M16C/62 microcontroller was interfaced and programmed to implement robotic behaviours. A closed-loop control in Cartesian space was proposed to control x and ymovement and rotation motions of the robot.