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EDM’s

EDM’s. Electronic Distance Measurement Indirect Measurement Uses Electromagnetic Waves. Classes of EDM. Infrared Microwave Requires transmitter at each end Communicate through them Laser Total Station. Improvements Over Time. Decreased Size Decreased Power Usage Smaller Batteries

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EDM’s

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  1. EDM’s • Electronic Distance Measurement • Indirect Measurement • Uses Electromagnetic Waves

  2. Classes of EDM • Infrared • Microwave • Requires transmitter at each end • Communicate through them • Laser • Total Station

  3. Improvements Over Time • Decreased Size • Decreased Power Usage • Smaller Batteries • On-board Microprocessor

  4. Method of Measurement • Not time-based • Speed of light = 186,000 mi/sec • We measure to 0.01’ • Need exact time to 1/1011 sec • Light “Waves” • Intensity of light varies • At any point, find intensity, rise or fall

  5. Phase Angle • Sine wave • 0°<x<360° • -1<sin(x)<1 • Observe wave intensity, I • Scale so Imax = 1, Imin = -1 • If I = 0.5 and is falling, sin-1(I) = 150°

  6. Infrared Operation • FG creates infrared waves • Signal corrected, transmitted • Splitter sends two waves, same phase • Mirrors, target carry one out and back • Phase difference circuit determines distance

  7. Decade Modulation • EDM’s generate light waves at different frequencies • Frequencies selected to yield practical values • Choose f1 so that 1 = 100’/cycle • Choose f2 = f1/10, 2 = 1000’/cycle • Phase detector determines change in the cycle

  8. Decade Modulation Example • Set 1 = 10,000’/cycle • Phase detector reads  = 40.2° out of 360°/cycle • Phase distance = 10,000*40.2/360 = 1117’ • Set 2 = 1000’/cycle • Phase detector reads  = 42.1° out of 360°/cycle • Phase distance = 1000*42.1/360 = 116.9’ • Set 3 = 100’/cycle,  = 60.7°, D =16.86’ • Wave traveled 1116.86’ out and back • Measured distance = 558.43’

  9. Corrections • Light waves affected by • Humidity • Atmospheric Pressure • Temperature • Reflector Constant • Vertical Angle

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