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David L. Linville, Yongwoo Park, Nay Lin, and Yuanqun Lin

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CMSC

Automated Laser Rail for ADM Calibration

Overcoming the challenge of temperature control

Published: Thursday, May 23, 2019 - 20:22

Calibrating an absolute distance meter (ADM) laser tracker requires long linear distances. For such distances, the room temperature is a significant factor. Even though the calibration room’s temperature is controlled within ± 2° C, actual temperature and temperature variation in one end of the room can be different from another because of uneven airflow in the room.

Controlling the air temperature and air flow along the rail can be costly for a laboratory. Thermally compensating the ADM distance for a laser traveling over a range of 50 m is not easy with just one temperature sensor. On the other hand, multiple temperature sensors placed along the ADM beam path further complicates ADM distance error compensation.

Existing processes for calibrating ADM laser trackers involve time-consuming practices, manual redundancies, and large footprints that inhibit an efficient process for sending laser trackers back into the field where they can be utilized by the end user. Automated Precision Inc. (API) of Rockville, MD, has developed the new ADM calibration rail to overcome the challenge of temperature control along the ADM laser beam path using common-path1 theory to eliminate any drift from beams or data collections. The new method is automated to collect data for a long distance. An XD laser is used as a reference for ADM calibration for better accuracy.

We will start with identifying and breaking the entire system down into four major components and describing the purpose and use of each partial system in the following descriptions.

The automated laser rail system consists of four isolated sections, as seen in figure 1:
1. Unit under test (UUT), an ADM tracker
2. Reference 1D laser source and receiver
3. Interferometer for Reference 1D, SMR target for ADM tracker
4. Motorized carriage and 2.5-in. hollow retroreflector, which serves the dual purpose of being the retro target for the Reference 1D laser and a beam-folding mirror for the ADM tracker while it is tracking to the SMR on the interferometer stage

Because of this, the Reference 1D laser path is half that of the ADM tracker laser path. This reduces the equipment imprint on valuable floor space for the support rail, which only require a minimum of 25 m of linear rail for the 50-m ADM distance.

Figure 1: Top view of OT2 ADM tracker with Reference 1D configuration

Measurement method

The internal interferometer of the IC laser, or reference laser (as seen in area 2 of figure 1) has been externally mounted. It now serves as a shared assembly for the reference laser and the UUT. This combines the beam paths and removes the effect of any drift in either beam, because all drifts induced by temperature fluctuations along the shared beam path is shared by both beams. The thermal drifts in one beam cancels out the drifts in the other and their net effect on collected data is 0.

The UUT (as seen in area 1 of figure 1) is placed orthogonal (90º) to the path shared by the interferometer and the optics folding assembly that are purposely disconnected to allow both beams to move proportionally. The UUT is typically an ADM tracker, however, the instrument mount can be easily adapted to accommodate any laser instrument.

The reference laser is mounted on the stage in line with the rail.

Using the folding mirror, the output of the UUT is bent 90° so it is parallel with the reference laser. The interferometer for the reference laser is designed to adapt from any standard 1 in. to 2.5 in. retroreflectors. Two 1-in. retroreflectors shift the 1-in. beam path (as seen in the dotted line in figure 2) to a 2.5-in beam path (as seen in the solid line in figure 2). These beam paths never cross and move collinearly. Both the UUT and reference laser share a common weather station to further reduce the thermal effects by monitoring air temperature, material temperature, barometric pressure, and humidity. The material temperature is used to monitor the rail base temperature so that a CTE can be applied during measurements to account for any thermal expansion effects.

Figure 2: Reference laser, interferometer, and 2.5-in. retroreflector on 27-m rail (dotted line represents outgoing beam path and solid line is return beam path)

To overcome the room temperature variation along the laser path, the Reference 1D laser and UUT are put on the stage at the end of the rail. The two beams are parallel and travel through the same air temperature variation. Being collinear (illustrations of the beam paths can be seen in figures 3–5), or having a “Common-Path”eliminates errors due to the air temperature variation and target movement in a 3D space, because both beams move the same amount. This results in drift elimination.

Figure 3: Side view of beam path

Figure 4: Top view

Figure 5: Beam-tracking points on 2.5-in. retroreflector

The rail design is intended for raw data collection by use of software that is completely customizable for various sampling rates. In comparison to the more commonly used back-to-back method, this wireless battery driven carriage can be programed for any number of points both forwards and backwards to the exact same distances every time. The software also has built-in stabilization detection that waits for stable data before data collection, as seen in figure 6 and figure 7.

Figure 6: A laptop is connected to the running software to atomically control the Reference 1D laser, UUT, and a wirelessly controlled motorized carriage with a 2.5-in. retroreflector.

Figure 7: Software to synchronize and automatically control data collection on the 27-m rail with Reference 1D, UUT, and motorized carriage

Data accuracy

The ADM error before and after compensation for a UUT is mapped, as seen in figure 8 and figure 9, respectively.

Figure 8: ADM error before compensation

Figure 9: ADM error after compensation

Then compensation is created by a software program, applied to the UUT, and verified as seen. The UUT ADM can achieve accuracy of equal to or less than 10 µm throughout its 50-m range.

References

1 Common-Path Method for Laser Tracker Ranging Calibration https://www.nist.gov/publications/common-path-method-laser-tracker-ranging-calibration

2 Zhu, Fan & Tan, Jiubin & Cui, Jiwen. (2013). Common-path design criteria for laser datum based measurement of small angle deviations and laser autocollimation method in compliance with the criteria with high accuracy and stability. Optics express. 21. 11391-403. 10.1364/OE.21.011391.

• Dyson, J. (1957). "Common-Path Interferometer for Testing Purposes" . Journal of the Optical Society of America. 47 (5): 386–387. Bibcode : 1957JOSA...47..386D .

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About The Author

David L. Linville, Yongwoo Park, Nay Lin, and Yuanqun Lin’s default image

David L. Linville, Yongwoo Park, Nay Lin, and Yuanqun Lin

The authors all work for Automated Precision Inc. David L. Linville is product manager/global labs manager/OEM manager, Yongwoo Park is senior engineering manager, Nay Lin is an automation engineer, and Yuangun Liu is a mechanical engineer.