Design and Control of an Omnidirect

Design and Control of an Omnidirectional
Mobile Robot with Steerable
Omnidirectional Wheels
Jae-Bok Song*, Kyung-Seok Byun**
*Korea University, ** Mokpo National University
Republic of Korea
1. Introduction
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