| With the development of science and technology, the chain of measurement transfer meets the challenge of national defence industry. In the field of length metrology, the artifacts with length from 2m to the order ofμm are easily traced to the SI unit according to the formal calibration criterions or verification regulations, but some industry metroloty instruments with measurement range up to the order of tens of meters have no suitable way to trace. The fact is it's very hard for calibration device to keep a high accuracy in long measured range. Problems caused by long rail straightness and Abbe error compensation must be considered in developeing calibration device or instrument.A novel calibration device was developed. The device has two systems. One is 'the high accuracy large-scale length calibration system (LLCS)', which was mainly used to verify the laser interferometer, steel type, base wire, abslute distance meter, etc. Another is 'a large planer coordinate calibration system (LPCCS)', which is used to assess the performance of portable coordinate measuring machines, like laser tracker and multi-theodolite system.The rail body of LLCS is 36 meter long, which consists of nine huge granite blocks supported by jacks on an isolated concrete ground base. A dual-frequency laser interferometer made by former HP company is used as standard for measuring the movement of a pneumatic platform. The platform is drived by a motor and moved alone the granite rail. Three interferometric light-pathes are designed to measure the same movement of the platform so as to compensate the Abbe errors which caused by the rail linearity. The system can be employed to calibrate any interferometer whose retroreflector was put on any position in the platform with the accuracy as they are on the same measuring axis in same direction.LPCCS was established on base of LLCS. The calibration strategies for two-theodolite system and laser tracker are proposed by analyzing their coordinate measurement error model. LPCCS utilizes a standard plane to assess portable CMMs' coordinate errors in lab. In the system, former granite rail is used as horizontal moving axis, a 1.4-meter-long metal rail is added on the pneumatic platform as vertical axis. On the platform, there is a target plate driven by a servo-motor. Two dual-frequency laser interferometers are employed as standard, one is the same as in LLCS and also used for measuring the movement of a pneumatic platform alone the horizontal axis, another is added for measuring the total movement of target plate both alone the horizontal axis and the vertical one via a pentaprism. These two interferometers measure the position of a target on the plane formed by the light path of new added interferometer. This light plane can utilizes to check the coordinate measuring errors of a portable CMM. Uncertainty assessments and validation tests of these two calibration systems were also carried out. The results of tests show the expanded uncertainty of 'large-scale length calibration system' is less than 0.32×10-6 and the expanded uncertainty of '2-D planer coordinate calibration system' is less than 1.0×10-6.These two systems solve the problems in traceability of large-scale metrology and provide a feasible way to calibrate industry metrology instruments, and they are especially suitable for portable large-scale metrology instruments.
|
PDF Full Text Request