Development of holonomic mobile platform for field robotic applications

Abstract

Most mobile platforms or vehicles used today are non holonomic. They only have one or two independent degrees of freedom. Due to that its maneuverability is limited and often requires much space to control functions like turning and parking. By improving degrees of freedom (improving the maneuverability) of a vehicle, it can follow many complex trajectories that are difficult or impossible by conventional non holonomic vehicles. Any mobile platform that has three independent degrees of freedom in a plane is a holonomic platform. Independent degrees of freedom means that it can change its orientation or position without effecting other motions unlike in car type vehicles that require turning or changing its orientation when need to move. Holonomic motion is very useful to acquire abilities such as, avoid any obstacle while keeping its orientation the same, capability to move in constrained spaces and track a target while moving in an arbitrary trajectories etc. Because of these advantages and capabilities some of the scientific and industrial researches are targeting to develop holonomic mobile platforms. Already robotics community have managed to build some working models and used in applications like robot soccer games and mobile robot manipulators. Many different mechanisms have been created to achieve the holonomic capability. These include various arrangements of Swedish wheels or omni wheels, chains of spherical or cylindrical wheels, ball wheels and powered caster wheels etc. While most of these designs are practical in indoor environments they are not suitable for outdoor operations in large scale versions. In this research project our goal was to develop a viable design to achieve holonomic capability that minimizes these problems and more suited to outdoor operations. The proposed design has a wheel arrangement similar to a car but with the capability of independent driving and steering capability of each of the four wheels. Car type rolling and steering mechanism avoid any uneven wear of the wheels and avoid lateral forces applied on the wheels. Wheel driving and steering mechanism was designed such a way that wheels can be steered 360 degrees continuously without interfering with the wheel drive system. This enables the platform to move in complex trajectories continuously without stopping for wheel resetting. The developed platform has increased ground clearance that is necessary for outdoor rough terrain operations like farming. Although these benefits exist, controlling the robot to acquire the desired motion is very complex and need innovative algorithm. Four independent wheels with eight degrees of freedoms to achieve three degree of freedom motion is a redundant control problem and require complex control system. Using inverse kinematics model of the platform and multiprocessor design with advance micro-controllers we have tried to solve these issues and were able to achieve successful performance.