Explore how electromagnetic tracking sensor coils use induction to generate tracking signals and how winding turns, coil size, and core materials affect performance – by Peter Matthews.
In an electromagnetic tracking system (EMTS or EM tracking system), a winding of fine wire produces the signal used to determine where a medical instrument is and which way it points. Understanding how a signal becomes position data, and how coil design influences system effectiveness, starts with the physics of electromagnetic induction.
How Induced Voltage Becomes Position Data
EM tracking systems use a field generator to produce controlled, low-frequency alternating magnetic fields. As the magnetic flux through the sensor coil changes, a voltage is induced according to Faraday’s law of electromagnetic induction.
For an alternating field, the relationship can be expressed as:
V ∝ N × A × B × ω × cos(θ)
Here, N is the number of turns, A is the coil’s cross-sectional area, B is the magnetic flux density, ω is the angular frequency, and θ is the angle between the field direction and the coil’s axis. The signal grows with more turns, larger coil area, and a stronger or faster-changing field. The signal also scales with cos(θ), the orientation term. This factor enables tracking by making it possible to understand how the coil is tilted relative to the field.
A typical field generator produces distinguishable magnetic fields along three known, perpendicular axes, and the EM tracking system measures the sensor coil’s response to each field separately. Since field strength varies with distance, and induced voltage depends on orientation, the system can use these measurements to calculate where the sensor sits and which way it points.
A single, axially wound coil provides five of the six degrees of freedom (5DOF vs 6DOF), including three-dimensional position, pitch, and yaw. It cannot detect roll, or rotation around its own axis, because that movement is electromagnetically invisible in a rotationally symmetric coil. This is relevant for straight, symmetric instruments like biopsy needles. Full 6DOF tracking requires breaking that symmetry and building the coil differently.
The Design Trade-Offs Behind EM Tracking Sensor Coils
The relationship that makes tracking possible, V ∝ N × A × B × ω × cos(θ), demonstrates key design choices available to designers. Adding turns (N) directly raises the induced voltage for a stronger, clearer signal, but it raises inductance and resistance too. Higher inductance lowers the coil’s self-resonant frequency (SRF), narrowing its usable bandwidth.
Increasing cross-sectional area (A) boosts signal without adding turns, but diameter is constrained by the size of the medical instrument itself.
A high-permeability core can concentrate magnetic flux and strengthen the signal response within a small volume, but it introduces tradeoffs. An air core responds linearly with no hysteresis, while a high-permeability core buys sensitivity at the cost of some non-linearity.
The wire itself adds another layer of manufacturing complexity. Fitting thousands of turns into a sub-millimeter part requires extremely fine wire on the order of 10 microns across.
Design decisions become more consequential as sensor diameters shrink toward 200-400 microns. Reducing diameter decreases coil area and signal strength. Even if you compensate with additional turns or different core materials, those design changes affect other key performance characteristics.
The challenge is finding a combination of winding geometry, materials, and electrical characteristics that delivers on performance and can be manufactured consistently. Because these factors are interconnected, decisions early in development can determine whether a device meets its targets.
At Knowles, we manufacture micro coils with more than 10,000 turns using wire finer than 58 American Wire Gauge (AWG) in a Class 10,000 cleanroom. Design, prototyping, and production sit under one roof, so we can help you evaluate trade-offs early.
If you’re developing the next generation of EM-navigated devices, talk to our engineers.
This blog is the second part of a four-part series exploring the design, performance, and manufacturing considerations behind micro coiling for medical device applications.
Get in touch for orders or any queries: sales@rfdesign.co.za / +27 21 555 8400
Courtesy of Knowles
