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2071 |
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50 m Linear Measuring Interferometer for Calibration of Surveying Tape |
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Metrology: Form & Geometry |
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A 50 m linear measuring interferometer for the calibration of survey tape is recently developed. It consists of a precision laser interferometer, a guide rail, a moving carriage, and optical microscope with CCD camera and an image processor. The entire instrument is housed in a 56 m tunnel where temperature is controlled. The base of the system is a reinforced concrete pier, 51 m long, cast in 33 segments on the concrete floor. 32 steel plates, which cemented on the concrete pier, supports the steel profile of length 50 m which consists of rigidly bolted steel channel boxes, 3 m long, 0.1 m high and 0.35 m wide. The profile is constrained laterally but is free to move axially to accommodate the thermal expansion difference between concrete and steel. The track, on which the carriage moves and the surveying tape seats, consists of two rails which are made up of ground rods of diameter 60 mm and length 3 m. 80 sleepers, rigidly fixed on steel profile at 0.65 m intervals, support the rods and provides the lateral and vertical adjustments of rails for straight motion of the carriage
The carriage, supported by 3 bearings, is driven by a DC geared motor along the guide rail with rack gear. The line image of tape is captured by the CCD camera of the optical microscope and wireless-transferred to the frame grabber of the computer to calculate the deviation between the center of the line and the center of the field of view of the CDD camera. This deviation is used as error signal to control the motor. This signal is also wireless-transferred to control unit at carriage to serve the carriage until it reaches nearly zero. The distance between lines is calculated by the displacement of the carriage measured by laser interferometer and the deviation of the line center from the center of CDD. The rechargeable batteries at carriage supply the electric power for motor, CCD camera and lamp of microscope and the computer links are performed through wireless communication. This feature doesn’t need the ducting for electrical control signal and leads to a simple driving mechanism.
The temperature of tape is measured by 6 thermister probes. For calculating the refractive index of air, air pressure, humidity and air temperatures at 3 different positions were measured. The main contribution to measurement uncertainty appears to be due to the quality of the lines and the temperature measurement of the tape. The system can be adapted to several other tasks. For example, ultrasonic distance meter and laser distance meter with short measuring range can be calibrated in this system. And also the short-range performance of electromagnetic distance meter is tested in the instrument. The system is fully automatically controlled. Now we are modifying the system for calibrating the laser interferometer comparing the master laser interferometer in this 50 m rail track
1. P. E. Ciddor, K.H. Edensor, K.J. Loughry and H.M.P. Stock, The Australian Surveyor, 33 (6), 493 (1987).
2.Chao-Jung Chen, Sheng-Hua Lu, Huang-Chi Huang and Ren-Huei Hsu, , Proc. SPIE, 966, 316 (1988).
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