Design and Control of an Omnidirectional Mobile Robot with Steerable Omnidirectional Wheels

Applications of wheeled mobile robots have recently extended to service robots for the handicapped or the aged and industrial mobile robots working in various environments. The most popular wheeled mobile robots are equipped with two independent driving wheels. Since these robots possess 2 degrees-of-freedom (DOFs), they can rotate about any point, but cannot perform holonomic motion including sideways motion. To overcome this type of motion limitation, omnidirectional mobile robots (OMRs) were proposed. They can move in an arbitrary direction without changing the direction of the wheels, because they can achieve 3 DOF motion on a 2-dimensional plane. Various types of omnidirectional mobile robots have been proposed so far; universal wheels (Blumrich, 1974) (Ilou, 1975), ball wheels (West & Asada, 1997), off-centered wheels (Wada & Mory, 1996) are popular among them. The omnidirectional mobile robots using omnidirectional wheels composed of passive rollers or balls usually have 3 or 4 wheels. The three-wheeled omnidirectional mobile robots are capable of achieving 3 DOF motions by driving 3 independent actuators (Carlisle, 1983) (Pin & Killough, 1999), but they may have stability problem due to the triangular contact area with the ground, especially when traveling on a ramp with the high center of gravity owing to the payload they carry. It is desirable, therefore, that four-wheeled vehicles be used when stability is of great concern (Muir & Neuman, 1987). However, independent drive of four wheels creates one extra DOF. To cope with such a redundancy problem, the mechanism capable of driving four omnidirectional wheels using three actuators was suggested (Asama et al., 1995). Another approach to a redundant DOF is to devise some mechanism which uses this redundancy to change wheel arrangements (Wada & Asada, 1999) (Tahboub & Asada, 2000). It is called a variable footprint mechanism (VFM). Since the relationship between the robot velocity and the wheel velocities depends on wheel arrangement, varying wheel arrangement can function as a transmission. Furthermore, it can be considered as a continuously-variable transmission (CVT), because the robot velocity can change continuously by adjustment of wheel arrangements without employing a gear train. The CVT is useful to most mobile robots which have electric motors as actuators and a battery as a power source. Energy efficiency is of great importance in mobile robots because it is directly related to the operating time without