Eugene Paperno

A new method for magnetic position and orientation tracking

The method is based on two-axis generation of a quasi-static rotating magnetic field and three-axis sensing. Two mutually orthogonal coils fed with phase-quadrature currents comprise the excitation source, which is equal to a mechanically rotating magnetic dipole. The resulting excitation field rotates elliptically at any position in the near-field region. The AC part of the squared field magnitude is a sinusoidal wave at twice the excitation frequency. The following set of parameters uniquely characterize the excitation at the sensors position: the phase of the squared field waveform, relative to the excitation currents, the minimum field value, the ratio of the field extremes, and the orientation of the excitation field plane. Simple and explicit analytical expressions are given which relate the first three parameters to the azimuth, elevation, and distance from the source to the sensor, respectively. The orientation of the sensor axes, relative to the plane of the excitation, can easily be determined by comparing the phase and amplitude of the measured signals against the phase and amplitude of the excitation field at the sensors position. Apart from simplicity, the proposed method increases the speed of tracking; a single period of excitation is in principle sufficient to obtain all of the information needed to determine both the sensors position and orientation. A continuous sinusoidal excitation mode allows an efficient phase-locking and accurate detection of the sensor output. It also improves the electromagnetic compatibility of the method.

3-D magnetic tracking of a single subminiature coil with a large 2-D array of uniaxial transmitters

In this paper, we describe a novel system and method for magnetic tracking of a single subminiature coil, which can be used, for example for intrabody navigation of flexible medical instruments.

Magnetic Eye Tracking: A New Approach Employing a Planar Transmitter

A new scleral search coil (SSC) tracking approach employing a planar transmitter has been developed theoretically and tested experimentally. A thin and flat transmitter is much more convenient in installation, operation, and maintenance than the conventional large cubic one. A planar transmitter also increases the mobility of SSC systems, simplifies their accommodation in a limited clinical space, enables bedside testing, and causes no visual distractions and no discomfort to the users. Moreover, it allows tracking not only the SSC orientation, but also its location, which is very important for many medical and scientific applications. The suggested approach provides the speed and precision that are required in SSC applications. The experimental results show that it can be used for the diagnosis of vestibular disorders. The tracking precision is in good agreement with its theoretical estimation.

A Three-Axial Search Coil Magnetometer Optimized for Small Size, Low Power, and Low Frequencies

A compact and sensitive three-axial search coil magnetometer has been designed, built, and tested. The magnetometer sensitivity threshold equals 12 pT/Hz0.5 at 1 Hz, and the magnetometer dimensions are 72 mm × 69 mm × 69 mm. All the magnetometer coils, the electronics, and batteries are accommodated within a single electrostatic shield and a single housing. A close to 2 aspect ratio (30 mm diameter, 58 mm total length) of the search coils provides a very high (~70%) volume utilization factor. Such a small aspect ratio is obtained due to employing 30-mm diameter, 4-mm thick flux concentrators. The magnetometer is optimized for 20 mHz to 7 Hz frequencies and for ultra-low (252 μW) power consumption. The ultra-low-power consumption enables a seven-year continuous operation from the four 1/2AA lithium batteries. The effect of the integration of three orthogonal search coils on the magnetometer sensitivity and accuracy has been investigated.

Analytical Optimization of Low-Frequency Search Coil Magnetometers

An analytical optimization is proposed for the sensitivity threshold of low-frequency search coil magnetometers employing disk-shape flux concentrators. The optimal diameters of the core and the wire are found for a given set of the optimization parameters: frequency, search coil volume and aspect ratio, relative permeability of the core and the flux concentrators, and the noise of the preamplifier. The proposed analytical optimization allows an immediate analysis of the theoretical limits of the magnetometer sensitivity threshold. An approximation is obtained to simplify and clarify the relationship between the minimum possible sensitivity threshold and the optimization parameters. Measurements performed with an experimental magnetometer model confirm the optimization.

Magnetic Tracking of Eye Motion in Small, Fast-Moving Animals

Here, we present a new approach for the magnetic tracking of eye motion in small, fast-moving animals. We employ a thin, flat magnetic tracking transmitter instead of the conventional bulky, cubic-shape transmitting frame. The new transmitter enables convenient access to the tracked animal, causes no visual distractions, and occupies much less space. We also employ a tiny solenoidal search coil instead of the conventional scleral search coils. Such a small solenoidal search coil attached laterally to the eye does not limit the peripheral field of view and allows the animal to perform its standard behavioral tasks. The flat transmitter comprises eight transmitting coils that allow us to monitor not only the orientation of a search coil but also its location. To test the efficiency of the new approach, we have measured the location and orientation of the solenoidal search coils attached to the eyes and head of an archer fish during swimming, targeting, and shooting. The size of the coils attached to the fish eyes was 2 mm in diameter and 2 mm in length, and the size of the coil attached to the fish head was 4 mm in diameter and 4 mm in length. The transmitter size was 60 cm times 60 cm times 2 cm. At a 25 cm from the transmitter, we have obtained the tracking resolution of 3 millidegree and 8.3 mu m rms for a 200-Hz bandwidth. Such a performance is good enough to precisely monitor the fastest component in the fish eyes movements. The fish with the search coils on the eyes and head correctly hits the target up to 20 times during an experimental session, which is similar to the shooting rate of the fish without the search coils. This implies that our new design does not introduce much discomfort for the fish.

A Tube-Core Orthogonal Fluxgate Operated in Fundamental Mode

In this paper, we suggest applying fundamental-mode operation to orthogonal fluxgates with tube cores. Excitation current in these fluxgates flows through a toroidal coil wound around the tube core, whereas in the orthogonal fluxgates with amorphous wires, it flows directly through the core. Having no excitation current inside the core reduces its heating and, hence, decreases the fluxgate thermal drift. Employing the toroidal coil also allows decreasing the excitation current by simply increasing the number of coil turns, while keeping the same intensity of the excitation field. Our experiments have shown a much higher efficiency of the new operating mode as compared with the second-harmonic mode. Adding a great enough dc bias to the ac excitation has caused a dramatic noise reduction. This effect is especially pronounced at relatively low frequencies, below 10 kHz. The fluxgate resolution in the fundamental mode, 10 pT/radicHz at 1 Hz , is by a factor of 30 better than in the second-harmonic mode. The sensitivity in the fundamental mode exceeds by a factor of 12.5 the sensitivity in the second-harmonic mode. We have also observed an about two times lower thermal drift of the fluxgate output. We have also found in this work that the phase noise of the excitation current is the main contributor to the fluxgate noise at low frequencies. It contributes about 67% to the fluxgate noise power density near the fundamental.

A miniature and ultralow power search coil optimized for a 20 mHz to 2 kHz frequency range

We describe the design of a miniature search-coil magnetometer that is optimized in terms of resolution and power consumption for frequencies down to 20 mHz. Our aim is to come close to the size and resolution of fluxgate magnetometers, while reducing the power consumption by at least an order of magnitude. To reach this goal, we attach flux concentrators in the shape of thin disks to a ferrite core, employ an ultralow power, zero 1/f noise preamplifier, and finally optimize the diameters of the coil core and wire. The optimized search coil is of 54 mm length, 30 mm outer diameter, and includes 160 000 turns of a 50 μm copper wire. The coil resistance is 86 kΩ, the self-resonance frequency is 250 Hz, and the total weight is 210 g. Our experimental results are in close agreement with the theoretical calculations. For a power consumption of 5 mW, the coil resolution is 14 pT/√Hz at 1 Hz and reaches 350 fT/√Hz in the frequency range from 100 Hz to 2 kHz. For a power consumption of 0.17 mW, the coil resolutio...

A New Calibration Procedure for Magnetic Tracking Systems

In this study, we suggest a new approach for the calibration of magnetic tracking systems that allows us to calibrate the entire system in a single setting. The suggested approach is based on solving a system of equations involving all the system parameters. These parameters include: 1) the magnetic positions of the transmitting coils; 2) their magnetic moments; 3) the magnetic position of the sensor; 4) its sensitivity; and 5) the gain of the sensor output amplifier. We choose a set of parameters that define the origin, orientation, and scale of the reference coordinate system and consider them as constants in the above system of equations. Another set of constants is the sensor output measured at a number of arbitrary positions. The unknowns in the above equations are all the other system parameters. To define the origin and orientation of the reference coordinate system, we first relate it to a physical object, e.g., to the transmitter housing. We then use special supports to align the sensor with the edges of the transmitter housing and measure the sensor output at a number of aligned positions. To define the scale of the reference coordinate system, we measure the distance between two arbitrary sensor locations with a precise instrument (a caliper). This is the only parameter that should be calibrated with the help of an external measurement tool. To illustrate the efficiency of the new approach, we applied the calibration procedure to a magnetic tracking system employing 64 transmitting coils. We have measured the systematic tracking errors before and after applying the calibration. The systematic tracking errors were reduced by an order of magnitude due to applying the new calibration procedure.

Compensation of the thermal drift in the sensitivity of fundamental-mode orthogonal fluxgates

A method is suggested that reduces the temperature coefficient of the sensitivity of fundamental-mode orthogonal fluxgates by an order of magnitude. For the background magnetic fields greater than 20μT, the method provides even better reduction, down to 100ppm∕°C, which is comparable with the temperature coefficient of conventional parallel-type fluxgates operated in closed-loop configuration. The fluxgate prototype has demonstrated a 20-fold reduction of the thermal drift in its sensitivity. The suggested method is solely based on the processing of signals generated by fundamental-mode orthogonal fluxgates in open-loop configuration and does not require any additional hardware. This makes the open-loop fundamental-mode orthogonal fluxgates competitive with closed-loop parallel-type ones in terms of both resolution and accuracy, while still keeping them simpler than the parallel fluxgates.