Electronic Distance Measurement

1
Electronic Distance Measurement

• - Physics and Application to Structural
  Engineering Research
• 27 September 2013
• J. M. Lynch
• CEE 398 KUC.

2
Evolution of EDM Instruments

• Got its start in 1948 with Swiss device using
  visible light (range40km only at night)
• http//www.gmat.unsw.edu.au
• Microwaves used in 1957 (range80km)
• 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
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/Note
s/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-Dio
  de 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
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