| This paper describes error
compensation methods for parallel kinematics machines (PKMs). First, joint
errors and link deformations caused by external forces and heat are discussed.
Because a linear scale unit is connected to each spherical joint located
at both ends of an extensible limb through two rods, the scale unit can
measure not only displacement of the prismatic joint but also the limb's
deformation. Moreover, the scale unit also measures the joint's runout
and its deformation in the limb's direction because the rod end is
in contact with the sphere of the spherical joint. Consequently, the unit
accurately measures any change in the distance between the spherical joints
independent of the external force. Further, temperature fluctuation has
little effect on measured distance because the rods are made of a low expansion
alloy. This device was built using experimental limbs and tested for use
with coordinate measuring machines. Results showed that, without any additional
sensor, the device acquired the same effect; it improved the stiffness and
the thermal coefficient of expansion of the whole limb. Secondly, another
compensation method for elastic and thermal deformations of the machine
frame supporting the mechanism is proposed. Nine displacement sensors with
Super-Invar rods measure the distance between the surface place and three
joint supports or the base platform of the PKM. Then, the forward kinematics
of the hexapod mechanism calculates displacement and orientation variations
of the base platform during operation from these measured displacements.
Consequently, coordinates of the PKM's end-effector can be compensated by
the dimensions, the displacement, and the orientation of the base platform
during operation. This device was installed in the experimental coordinate
measuring machine. Coordinate measurements using a master ball mounted on
the surface plate showed that this device decreased the influences of temperature
fluctuation and external force on machine accuracy. |
