Electromagnetic Sensors

1
Electromagnetic Sensors

• Group 249
• Andy Kottsick
• Jeremy Purcell
• Jeremy Lee
• Michael Newell

2
Introduction

• Electromagnetic theory is difficult to apply to
  real world situations.
• Simple measurements often require complex
  equipment and understanding for the measurement
  of electromagnetic fields.

3
Project Goal

• Create a suite sensors to aid in the
  measurement and understanding of the following
• Electric Fields
• Magnetic Fields
• Charge

4
Requirements

• Cover four frequency ranges Static, 0 to 1 KHz,
  1KHz to 100MHz, and 100MHz to 1GHz
• Accurately measure the source for which the
  sensor is designed
• Minimal interference with surrounding electronic
  devices
• The sensors need to have test points for
  measurements
• Inexpensive
• Classroom and student usability
• Output resulting measurements to a graphical
  interface

5
Sensor Choices
Field Mill - Charge
Flux-Gate DC Magnetic
Log Periodic Dipole Array Magnetic and Electric
(.6-2.4 GHz)
Loop Coils Magnetic and Electric (1-200 MHz)
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Options Charge Measurement

• Field Mill
• J-FET Transistor
• Charged Plates

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Field Mill Sensor

• Measures charge
• Amount of charge determined in Coulombs
• Polarity of charge determined as positive or
  negative

• Commonly used to measure charge in clouds and
  the detection of lightning

8
Field Mill Theory

• Rotating vane creates a sinusoidal output
• Magnitude is used to find amount of charge
• Polarity is found from phase information

Case 1
Case 2
i
i


R
R
Block Plate
Block Plate
e -
- - - - - - - -
- - - - - - - -
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-
e-


Vout
Vout
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Field Mill Calibration
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Final Field Mill
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Options - Static Magnetic Field

• Giant Magnetoresistive
• SQUID
• Fluxgate

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Flux-Gate Sensor

• Measurement of static magnetic fields
• Field intensity determined in Gauss
• Ferromagnetic core driven in saturation

• Commonly used for the measurement of the earths
  magnetic field

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Theory of flux gate
Flux -
Magnetic field
No
Flux net
Net Flux has a value when external field is
present
Flux
Vo
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Flux-gate calibration
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Final Flux-Gate
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High Frequency Electric/Magnetic Field
consideration

• Half wave dipole
• Log Periodic Dipole Array
• PCB patch antenna
• Loop Coil

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Magnetic Loop Sensors

• Magnetic and electric fields from 1MHz to 200MHz
• Field magnitude can be determined from
  calibrated Antenna Factor

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Loop Antenna Theory

• Equivalent circuit found using Network Analyzer
• Capacitance or inductance added to shift the
  resonant frequency where max power is received

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Antenna Factor (AF) Theory

• Used for loop coil and periodic dipole
  calibrations
• AF Einc / Vreceived
• Einc ?oHinc where ?o 377?
• ?o is the intrinsic impedance of free space

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Calibration of Loop Coil
Reading from Calibrated Antenna
Electric/Magnetic Field
AF for Loop Coil
Source Signal
Reading from Loop Coil
Experimental Setup
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Loop Antenna Calibration
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Final Calibrated Loops
1-30 MHz
40 200 MHz
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High Frequency Electric/Magnetic Field
consideration

• Half wave dipole
• Log Periodic Dipole Array
• PCB patch antenna
• Loop Coil

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Log Periodic Dipole

• Measure magnetic and electric fields from 600MHz
  to 2.4GHz
• Field magnitude is determined from a calibrated
  Antenna Factor

• Commonly used for broadband UHF and VHF
  applications

• Low VSWR over operating range minimizes
  reflections

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Dipole Calibration
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Final Dipole
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Budget
Discrete (R,L,C, Diodes etec) 65.96
IC's PIC's, 555 timers, etc 29.74
Hardware(Motors, PCB's, LCD's) 136.50
Power Supplies 55.89
Enclosure(BNC, Headers, Cables, Wires) 155.61
   
Total 443.70
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Future Work

• Field Mill
• Miniaturize
• Distributed Sensor Network
• Adjustable Gain
• Relate Charge to Electric Field
• Remove Distance Dependency
• Flux-Gate
• Wider Calibration Bandwidth
• Adjustable Gain
• Calibration Below Earths Field
• Add 2 more dimensions
• Low Power Design

• Loop Coil
• Downshift and Sample
• Use FFT Calculation
• Better Variable Caps
• Increase Sensitivity
• Try to reduce capacitance
• Log Period Dipole
• More rugged antenna
• Antenna stand
• Obtain more accurate simulation
• Study which end to put the load
• Experiment with wire sizes

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Problems Encountered

• Best sensors to measure each range
• Interfacing the sensors
• Impedance matching issues
• Power sources

Lessons Learned

• Start early
• Finalize the requirements early
• Be prepared to make changes, nothing is
  concrete
• Know the software is working or being handled
  correctly
• Target the harder problems sooner rather than
  later
• Test all circuits before building any boards

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Summary
Coverage Range of Devices Coverage Range of Devices Coverage Range of Devices Coverage Range of Devices Coverage Range of Devices Coverage Range of Devices Coverage Range of Devices
Device Static Charge DC Magnetic Field 1-200 MHz Magnetic Field 1-200 MHz Electric Field 600-1800 MHz Electric 600-1800 MHz Magnetic
Field Mill            
Fluxgate            
Loop Coil            
Log Periodic            
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• Questions?

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Antenna Equations
Vreceived(dBuV) Vreceived(dBm) 107