An energized electric power cable generates low-frequency electric and magnetic fields that are related to the voltages and currents. Especially at wavelengths λ>>d, where d is the distance away from an energized conductor and λ is the signal wavelength, we can extract electrical information with quasi-static near-field electric and magnetic field theory and the Principle of Superposition.
Coulomb’s and Biot-Savart Laws, respectively, relate the electric and magnetic fields to the voltage V and current I on a single straight energized wire (See Figure 1) where μ0 and ε0 are magnetic permeability and electric permittivity constants, ρ is the surface charge density on the wire, and aφ and ar are vectors pointing in the direction of the field in cylindrical coordinates. Superposition principles are used with Coulomb and Biot-Savart Laws in multi-wire configurations (i.e., 3-phase power cables). The boundary element method or electromagnetic models are typically used to solve for ρ(V), which changes minimally along the length of the wire at quasi-static frequencies.
There are many commercial-off-the-shelf (COTS) and Government-off-the-shelf (GOTS) sensors for making noncontact electric- and magnetic-field measurements. The electric-field (Efield) sensing team at ARL has previously characterized several E-field sensors, including SAIC Steered-Electron Electric-Field (SEEF) sensor, Srico optical sensor, Plessey EPIC sensors, QUASAR non-contact capacitive “dime” sensors, QFS 3-axis low-frequency (LF) E-field sensors, and the University North Carolina Charlotte (UNCC) custom E-field sensor. These sensors have produced good results, but do not include real-time programming abilities for signal conditioning. Furthermore there are no COTS or GOTS 1-Wire compatible electric- and magnetic-field sensors. Therefore, there is a need for integrated multimodal (electric- and magnetic-field) programmable sensors that are specifically designed for sensing the fields near energized cables. ARL’s multimodal D-dot (MD-dot) sensor addresses these needs, and additionally has “smart” features that adjust integrated circuits (ICs) on the sensor during start-up based upon the electrical characteristics of the attached cable.
The circuit’s primary design consists of a microcontroller, 8-channel digital-to-analog converter (DAC), variable gain chips, differential operational amplifiers, and E-field and B-field sensors. These components work together to form an interface that allows both onboard and external programming of the sensor, all while measuring electric and magnetic fields. Their connections are summarized in the block diagram.
The circuit works in the following manner to set its own gain voltage and offset voltage, resulting in optimized output signal characteristics:
- Microcontroller reads ferroelectric random-access memory (FRAM) for current DAC output on 1 of its 8 channels.
- Microcontroller changes digital value for 1 of 8 DAC channels by serial peripheral interface (SPI) protocol (green).
- DAC sets corresponding analog output voltage to desired variable circuit input (red).
- Variable gain and/or variable offset stage outputs reflect changes (thick blue).
- Output signal is measured by microcontroller’s analog-to-digital converters (ADCs) (thin blue).
- Microcontroller increases or decreases (repeat 1–5) DAC setting, or keeps DAC setting based on ADC measurement and stores in FRAM.
The magnetic field is measured by MLX91205 Hall effect sensors. Three 1-dimensional Hall effect sensors were arranged in a line across an energized power cable to measure the magnetic field at 3 spatially diverse locations. These sensors can give an accurate measurement of a +/–10 mT magnetic field in the azimuthal direction. The MLX91205 output voltage is linear with magnetic-field strength up to a maximum chip output of 50-mV amplitude; therefore, the magnetic sensitivity Ω for the sensor can be computed.
This work was done by Sean M Heintzelman for the Army Research Laboratory. ARL-0195
This Brief includes a Technical Support Package (TSP).
Non-Contact Circuit for Real-Time Electric and Magnetic Field Measurements
(reference ARL-0195) is currently available for download from the TSP library.
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