The Coordinate Measuring Machine (CMM) is widely used for the 3D measurements of objects. Traditional CMM is based on a serial mechanism: the components from base unit to end-effecter i.e. base unit, x-axis, y-axis, z-axis and measuring probe are connected serially. But some drawbacks of this mechanism are its weakness against external force and the accumulation of errors. Therefore CMM tends to become large and heavy in order to avoid the influence of the bending and twisting of its components and to decrease measurement errors.
Because of its weight, it has been a very difficult problem to make large CMM capable of fast 3D measurement. We develop a new type of CMM based on a parallel mechanism where the base unit and end-effecter are connected by six links parallel. The advantages of this mechanism are its robustness against external force and error accumulation[1]. Therefore we will able to make larger measuring machine that can measure large objects quickly. On the other hand, there are some disadvantages by using parallel mechanism for CMM. One is the difficulty in the calibration of CMM. Because of its difficulty in solving its forward kinematics and large number of parameters its geometrical model includes, it is hard to calibrate parallel mechanism efficiently.
To make the calibration of parallel CMM easy, we make our prototype by using unique spherical joint consists steel balls and magnets. That allows higher repeatability of positioning and setting-up. At first, the prototype of parallel CMM that has been developed in our laboratory is introduces. Next we discuss about the parameters we should identify to calibrate parallel CMM and how to calibrate it. Lastly, we give the suggestion of the method to calibrate our prototype efficiently.
