Inside this Edition:

• Next Generation Tracking
• Smaller Aurora Sensor Coils
• Improved Neuroimaging
• AC vs. DC Technology
• Research Spotlight
• NDI Organizes for Research
• Events

Back to NDI InFocus July 2007

NDI Web Links:

• Aurora EM Tracking System
• Aurora Features & Benefits
• Aurora Tech Specs
• Electromagnetic Technology
• Tracking Technologies

AC Versus DC Electromagnetic Tracking

There are various magnetic spatial measurement systems on the market, and though they are in principal similar in operation, they differ in the manner in which they generate magnetic fields.

The Field Generator coils are responsible for creating the magnetic fields and can be driven either by direct current (DC) or alternating current (AC). This means that the resultant magnetic fields are either constant (DC) or changing (AC).

The advantage of a truly constant (DC) magnetic field is that eddy currents are not generated in nearby conducting objects. Eddy currents can interfere with the generated field, resulting in inaccurate sensor readings. The disadvantage of a DC-based system is that the magnetic fields can be affected by the earth's magnetic field. Compensation is necessary, dependent on sensor position and orientation.

With an AC-based system, eddy currents can be generated though they are frequency dependent. This dependency can be useful to tune the system to reduce the distortion effects on accuracy.

True DC-based systems are not feasible; instead, some manufacturers have a "pulsed DC" based system which have time-dependent magnetic fields, where the magnetic fields are turned off and on at a certain frequency. When magnetic fields are turned off and on, there is a potential for eddy currents to be generated. The degree of how pulsed DC systems are affected by eddy currents is related to the time dependence of the generated magnetic field (the same as AC systems). In this respect, there are no differences in the performance of "pulsed DC" based and AC-based systems with regards to tracking performance in an environment containing conductive materials.

All magnetic spatial measurement systems are affected by the close presence of ferromagnetic materials. Ferromagnetic materials distort the generated magnetic fields, leading to incorrect sensor readings. Proper placement of the Field Generator with respect to both the sensor coils and ferromagnetic materials will maximize accuracy. For further information on Electromagnetic Tracking and NDI Aurora technology, please visit our website.

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