EDMs

1
EDMs

• 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
• 0ltxlt360
• -1ltsin(x)lt1

• 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

• EDMs 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 10,00040.2/360 1117
• Set ?2 1000/cycle
• Phase detector reads ? 42.1 out of 360/cycle
• Phase distance 100042.1/360 116.9
• 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