Electromagnetic position and orientation sensing system
Magnetic tracking systems and methods for determining the position and orientation of a remote object. A magnetic tracking system includes a stationary transmitter for establishing a reference coordinate system, and at least one receiver. The remote object is attached to, mounted on, or otherwise coupled to the receiver. The transmitter can include a set of three mutually perpendicular coils having a common center point, or a set of three coplanar coils with separate centers. The receiver can include a set of three orthogonal coils. The position and orientation of the receiver and the remote object coupled thereto is determined by measuring the nine mutual inductances between the three transmitter coils and the three receiver coils. The magnetic tracking system provides reduced power consumption, increased efficiency, digital compensation for component variation, automatic self-calibration, automatic synchronization with no connections between transmitter and receiver, and rapid low-cost implementation.
1. A method of sensing a position and an orientation of a remote object, for use in a system comprising a transmitter having a plurality of transmitter coils and at least one receiver having a plurality of receiver coils, the receiver being associated with the remote object, the plurality of transmitter coils for generating a plurality of electromagnetic fields in response to a plurality of excitation currents, respectively, thereby inducing a corresponding plurality of receiver voltages in each of the plurality of receiver coils, the plurality of excitation currents and the plurality of receiver voltages being substantially sinusoidal, the plurality of electromagnetic fields defining a reference coordinate system for the sensing of the position and the orientation of the remote object, the method comprising the steps of:
detecting, within a sampling window, the plurality of receiver voltages induced in each receiver coil by the plurality of excitation currents, the sampling window having a start and a duration,
calculating a time shift between a common zero crossing of the plurality of receiver voltages and the start of the sampling window;
converting each of the plurality of receiver voltages induced in each receiver coil into a set of digital samples, and, for each set of digital samples, calculating an inner product of the respective set of digital samples and a predetermined set of basis functions, thereby obtaining sine and cosine component amplitudes for each receiver voltage at each receiver coil; and
using the sine and cosine component amplitudes for each receiver voltage and taking into account the time shift, calculating a real phasor component for each receiver voltage induced in each receiver coil, each real phasor component having an amplitude and a sign at least partially indicative of the position and the orientation, respectively, of the remote object within the reference coordinate system.
2. The method of claim 1 further including:
deriving excitation frequencies for the respective excitation currents from a transmitter clock associated with the transmitter; and
deriving a sampling frequency for the receiver voltages induced in each receiver coil from a receiver clock associated with the receiver, the receiver clock being unsynchronized with the transmitter clock,
wherein the duration of the sampling window corresponds to an integer number of cycles of the sampling frequency,
wherein the plurality of excitation currents are periodic within the duration of the sampling window, and
wherein the duration of the sampling window represents a minimum time resolution for the sensing of the position and the orientation of the remote object.
3. The method of claim 1 further including:
deriving excitation frequencies for the respective excitation currents from a transmitter clock associated with the transmitter; and
deriving a sampling frequency for the receiver voltages induced in each receiver coil from a receiver clock associated with the receiver, the receiver clock being unsynchronized with the transmitter clock,
wherein the excitation frequencies correspond to different integer sub-multiples of a predetermined master clock frequency, the excitation frequencies having values that are in integer ratios to each other, and
wherein the sampling frequency corresponds to an integer sub-multiple of the predetermined master clock frequency.
4. The method of claim 1 wherein the calculating of the time shift includes calculating the time shift between the common zero crossing of the plurality of receiver voltages and the start of the sampling window at a time when the plurality of receiver voltages transition from negative to positive.