DC electromagnetic tracking

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A 3D Guidance trakSTAR system, one of Ascension's DC tracking systems. Shown is the 4-inch wide cube transmitter on the left

Direct current (DC) electromagnetic tracking is a spatial tracking technology that doesn't require a line of sight and is not affected by aluminum and copper. It is highly accurate.

DC is one type of electromagnetic 3D tracking, along with AC. DC uses pulsed signals, and subtracts the earth's magnetic field from each, to get measurements.

DC tracking was developed by Ascension to overcome some of the limitations of AC electromagnetic tracking, specifically the metallic distortion problems.[1] As of 2023, all patents on the initial invention are no longer in effect.

Ascension claims that DC tracking technology is 3 to 10 times less affected by conductive metal than AC technology.[1]

DC trackers are suitable for use near aluminum and copper.[2] Ascension claims that DC trackers typically report one-fifth the errors of a AC system in real-world settings containing steel rebar in floors and ceilings, but admits that ferrous objects such as iron or steel objects within a few feet of a tracking area can be a problem.[1]

For a pulsed-DC tracking system operating at 400Hz, the skin depth of aluminum is 0.165 inch. Thus, aluminum thicker than 0.165 inch (about 4 millimeters) distorts the magnetic fields and makes tracking inaccurate.[3]

DC trackers have problems near ferrous metals, such as iron.[2]

How it works[edit]

Each transmitting coil component (X, Y, or Z) is driven one at a time by a pulsed direct current signal. The receiving antennas measure the values of transmitted direct current magnetic fields one dimension at a time and those of the Earth's magnetic field as well, one dimension at a time.[4]

In U.S. Pat. No. 4,945,305 issued to Ernest B. Blood for a DEVICE FOR QUANTITATIVELY MEASURING THE RELATIVE POSITION AND ORIENTATION OF TWO BODIES IN THE PRESENCE OF METALS UTILIZING DIRECT CURRENT MAGNETIC FIELDS, a remote object position and orientation determining system is disclosed in which the transmitting antennas are driven sequentially by a pulsed, direct current signal. The generated electromagnetic fields are sensed by a DC-field-sensitive sensor in each of the three orthogonal components of the sensor reference coordinate frame and are resolved into remote object position and orientation relative to the source reference coordinate frame. The purpose of the system disclosed in Blood is to reduce field distortions resulting from the decay of eddy currents induced in electrically conductive materials by magnetic fields. Blood teaches that these disadvantages are overcome by applying a pulsed-DC signal to a source axis, which will induce an eddy current in any surrounding conducting metal only at the beginning of the pulse. The Blood system waits enough time for the eddy current to decay before measuring the transmitted flux. Alternatively, the received signal is measured several times as the eddy current is dying out and curve-fitted to an exponential decay math function in order to remove the effect of the eddy current field distortion.[5]

Because the sensor employed in Blood is a DC field sensor, the sensor measures a composite static magnetic field made up of the pulsed-DC field from the source and a constant DC field surrounding the earth, i.e., the earth's magnetic field. Prior to resolving the sensor measurements into position and orientation, the Blood system must subtract out the effect of the earth's magnetic field. Because, at nominal source/sensor spacing, the earth's field is an order of magnitude greater than the DC field generated by the source, it must resort to hardware to subtract out the earth's field. This is necessary in order to avoid limitations in the dynamic range of the analog-to-digital converter. Furthermore, a quiescent period during which no source coil is being excited is necessary in order to allow the magnitude of the earth's magnetic field to be measured.[5]

Hardware and algorithm[edit]

DC magnetic tracking systems typically require an electronics unit that connects to a host computer. The electronics box has analog-to-digital converters for receiving the sensor signal.[6]

The systems require digital signal processing.

As of 2025, NDI claims the 3D Guidance systems are protected by US patents 6754596, 6528991, and 6856823, but those are all expired as of 2023, meaning the technology is free to be copied.

References[edit]

  1. ↑ 1.0 1.1 1.2 "The Advantage of DC Magnetic Tracking". https://web.archive.org/web/19970714033826if_/http://www.ascension-tech.com/dcadvantage.htm.
  2. ↑ 2.0 2.1 Menache, AuthorsAlberto (2011-01-01). "Understanding Motion Capture for Computer Animation". https://www.sciencedirect.com/book/monograph/9780123814968/understanding-motion-capture-for-computer-animation.
  3. ↑ "US Patent US7761100". https://patentimages.storage.googleapis.com/35/d5/a6/9085c9e9991319/US7761100.pdf.
  4. ↑ "The first DC electromagnetic 3D tracking system patent". https://patentimages.storage.googleapis.com/18/64/9d/900cb662e909bb/US4849692.pdf.
  5. ↑ 5.0 5.1 "Pulsed-DC position and orientation measurement system". 1993-04-08. https://patents.google.com/patent/US5453686A/en.
  6. ↑ "Method of measuring position and orientation with improved signal to noise ratio". 2002-11-01. https://patents.google.com/patent/US6754596B2.
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