Error Averaging Effect in Parallel Mechanism Coordinate Measuring Machine
Error averaging effect is one of the advantages of a parallel mechanism when individual errors are relatively large. However, further investigation is necessary to clarify the evidence with mathematical analysis and experiment. In the developed parallel coordinate measuring machine (PCMM), which is based on three pairs of prismatic-universal-universal joints (3-PUU), error averaging mechanism was investigated and is analyzed in this report. Firstly, the error transfer coefficients of various errors in the PCMM were studied based on the established error transfer model. It can be shown how the various original errors in the parallel mechanism are averaged and reduced. Secondly, experimental measurements were carried out, including angular errors and straightness errors of three moving sliders. Lastly, solving the inverse kinematics by numerical method of iteration, it can be seen that the final measuring errors of the moving platform of PCMM can be reduced by the error averaging effect in comparison with the attributed geometric errors of three moving slides. This study reveals the significance of the error averaging effect for a PCMM.
A Lens-array-based Measurement Technique For Spatial Angle
The accurate measurement of spatial angle is the key technology for the plane flatness and guide rail straightness detections. This paper presents a lens-array-based optical system and an analytical model for micro spatial angle measurement. In this system, a collimated light beam passes through a four-array-lenses arranged in pyramid shape and forms a regular array spots on CCD (Charge Coupled Device) sensor. The angels with respect to the axes of X, Y are calculated by analyzing the distance of these spots on the CCD, the distance between adjacent apertures on the lens array and the inclination angle between the lens array and the CCD. Using the Coordinate value of array spots on CCD, the angle around Z axis also can be calculated simultaneously. Finally, the accuracy of the proposed method is verified by comparing the measured results with the autocollimator, and it is shown that the proposed approach enabled to achieve RMS≤0.1″. Additionally, this measurement system which is consists of a laser source, lens arrays and CCD, compared with other methods, is smaller in volume and more convenient to carry.
Accuracy Improvement of Parallel Kinematic Machine-Error Compensation System for Joints , Links and Machine Frame-Takaaki OIWA
This paper describes compensation methods for deformations of both the parallel kinematic mechanism and the frame supporting the mechanism. In general, these deformations are caused by the heat, the internal forces and the external forces during the processes. First, runouts and deformations of spherical joints and revolutionary joints are measured and compensated by nine displacement sensors built in above joints or by linear scale units built in the prismatic joints. Second, the longitudinal deformation of the prismatic joints is mechanically compensated by Super-Invar rod connected to the joints and the linear scale units. Moreover, nine displacement sensors with nine SuperInvar rods measure the variation of distances between the surface plate and three spherical joint supports. Consequently, the forward kinematics of Hexapod mechanism calculates the displacement and the attitude variations of the mechanism during operating from measured data. Last, the coordinates of the end effector are compensated by these displacements and attitude of the mechanism. Experimental results show reduction of the influence of the temperature fluctuation and variation of measured values.