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Electronic Distance Measurement

Electronic Distance Measurement. - Physics and Application to Structural Engineering Research – 11 March 2014 J. M. Lynch CEE 398 KUC. Evolution of EDM Instruments. Got its start in 1948 with Swiss device using visible light (range=40km only at night) http://www.gmat.unsw.edu.au

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Electronic Distance Measurement

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  1. Electronic Distance Measurement - Physics and Application to Structural Engineering Research – 11 March 2014 J. M. Lynch CEE 398 KUC.

  2. Evolution of EDM Instruments • Got its start in 1948 with Swiss device using visible light (range=40km only at night) http://www.gmat.unsw.edu.au • Microwaves used in 1957 (range=80km) http://www.gmat.unsw.edu.au • Infra-red, IR, devices are common today http://www.pentaxcanada.ca

  3. EDM Physics Lesson • Electro-magnetic energy travels through the atmosphere according to the following: • Where: • c = speed of light in vacuum • n = atmospheric index of refraction, 1.003 for STP • f = frequency of the electro-magnetic energy • l = wavelength of the energy http://131.251.21.249/squ1/

  4. l l l l EDM P l l l EDM Physics Lesson (continued) • The propagation of electro-magnetic radiation may be used to measure distances as shown below • If the round-trip distance is not an integer number of wavelengths, the partial wavelength may be measured by noting the phase shift of the returning signal.

  5. EDM Physics Lesson (continued) • The distance between EDM and reflector is expressed • Counting the number of complete wavelengths “n” would require very accurate measurement of time • Instead, longer wavelength signals are sent as demonstrated below Therefore, 2L = 252.806 m

  6. EDM Physics Lesson (continued) • Modern instruments use Amplitude Modulation (AM) to generate multiple wavelengths from the same radiation source. http://www.tm.agilent.com/data/static/eng/tmo/Notes/interactive/an-150-1/classes/liveAM.html • This is done for economics reasons and also to make the units smaller, lighter, and less power hungry. • The “carrier wave” used for most modern EDM instruments is generated by a Gallium-Arsenide-Diode Laser at l = 900 nm.

  7. Current EDM Instruments • So called “total-station” instruments package a digital theodolite (for measuring azimuth and altitude) with an EDM, data storage device, and often a modem for transmitting data from the field. • The most common instruments use a “pulse” of 2 to 4 AM frequencies ranging from 150kHz to 15MHz. • This range of frequencies have corresponding “half-wavelengths” of 1.0km to 10m, respectively. http://www.nikon.co.jp/survey-e

  8. Application to Structural Engineering • Use of EDM on the scale of m-strain measurements is postulated as follows: • Approx. cracking strain of concrete = 40me • Use a gage length of 100mm • Distance accuracy measurement needs to be on the order of • Therefore, l = 1mm or 0.1mm would be needed to obtain the specified distance resolution • This wavelength corresponds to the IR and RADAR regions of the electro-magnetic spectrum • (NOTE: AM is not likely to be appropriate given that the carrier wave would have a wavelength on the order of the desired AM signal)

  9. EDM EDM Reflector Test Specimen Application to Structural Engineering • Assuming that displacement measurements of the required accuracy can be obtained . . . • An array of reflectors and two radiation sources are positioned as shown

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