The portable three-coordinate measuring arm is a three-coordinate measuring system based on rotating joint and rotating arm, the measure of length is replaced by the measure of Angle. Compared with the traditional three-coordinate measuring machine, it has the advantages of simple mechanical structure, small volume, high efficiency, light weight, large measurement range, flexible and convenient, low cost and on-site measurement. It is widely used in cars and spare parts, mould, aerospace, shipbuilding, steam turbine, heavy machinery and other machining industries. But this portable three-coordinate measuring arm is a series structure, with a large error factor and large error transfer coefficient. In order to improve the measurement precision of the measuring arm, calibration and error compensation must be carried out before use to ensure that its measuring accuracy is within the precision of the design.
The portable three-coordinate measuring arm is connected by six rotating arms and one measuring head through six rotating joints, the angle encoder is installed in each joint, the measuring arm is fixed to the base, and the head can move in space which can constitute a closed sphere measurement space; By installed in each joint, and stem a internal angle of photoelectric encoder angle and bar for each joint angle, combining the mechanical parameters of each joint arm (the angle between the arm length, joint, etc.), using coordinate model by 3 D coordinates of measuring points is achieved.
By improving the Angle encoder of measuring arm, machining and assembling precision of mechanical parts which can improve measuring precision. However, the improved precision is limited, the cost is very high. In the condition that there’s error in the parameters of the organization, the error compensation method can greatly improve the measurement precision. Traditional three coordinate measuring machine has high accuracy of measurement, which is much higher than portable three coordinate measuring machine, thus three coordinate measuring machine can be used for measuring arm space geometry error calibration and error compensation. The error compensation method proposed here is the error model of measuring the measuring arm and the standard value of the three-coordinate measuring machine. Based on the error model, the error database is established to correct the position error of the measurement arm.
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