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The measurement of inner diameter is a traditional and important measurement task in machining. Methods to measure the inner diameter are divided into contact and non-contact measurement. A contact measurement method with multiple inductive transducers is presented in this paper. In this paper, the constitution and structure of the system are described in order to establish the mathematical measurement model and analyze the measurement error emerged by inaccurate parameters. Moreover, the outputs measured by sensors are transformed into corresponding coordinates, and then the least square method is adopted to fit the coordinates to calculate the diameter, details will be presented in the paper. The measuring accuracy of the system is verified by experiment. The results indicate that this method could reduce the error caused by sensors deviated from the center of measuring circle and improve the measurement accuracy.

Double Parallel Joints coordinate measuring machine (CMM) is a new type coordinate measuring device with 9 structural parameters with accuracy difficult to be ensured just by processing and assembly. Calibration is needed to identify the structural parameters of the measuring machine. This article describes the calibration process of such a Double Parallel Joints CMM, and simulation was employed to analyze how the number of samples and angle measurement accuracy influence the measurement precision and calibration result during calibrating process. The conclusions are very instructive to precision design or error allocation and calibration process of the Double Parallel Joints CMM. Key words o CMM, Double Parallel Joints, Parameter Identification, Simulation INTRODUCTION The focus of CMM research is on orthogonal coordinate measuring system with over 40 years development. Conventional CMMs have a very wide range of applications for its wide measurement range, high accuracy, connecting with flexible manufacturing systems and computer integrated manufacturing system. With the maturity of CMM and the requirements of large-scale engineering measurement, increasing studies on non-orthogonal CMM were carried out. Though Articulated Arm Coordinate Measuring Machine (AACMM) involving kinematic model, error model and structural parameters identify or calibration and so on has been studied in many domestic colleges, the accuracy of AACMM is limited because of its structur e and exorbitant expense on six high precision angle measuring system. W. Lotze proposed a novel low-cost CMM named ScanMax