Object tracking using magnetic field sensing and backscatter transmissions

1. An object tracking system, comprising:

a magnetic field transmitter disposed within a housing and comprising one or more transmitting coils configured to generate at least one stationary magnetic field;

an object tracking device configured to be engaged by a user for movement in response to a handwriting input provided by the user, the object tracking device comprising:

a magnetic field sensor in communication with the magnetic field transmitter and comprising one or more receiving coils configured to sense the at least one stationary magnetic field in response to movement of the object tracking device relative to a surface remote from the housing of the magnetic field transmitter;

position tracking circuitry configured to determine at least one field strength associated with the at least one stationary magnetic field and to determine position information associated with the handwriting input provided by the user based on the at least one field strength; and

backscatter circuitry including a frequency synthesizer configured to generate a baseband clock signal and a reference clock signal from at least one RF signal, a baseband processor configured to generate from the position information and the baseband clock signal one or more baseband packets, and a backscatter modulator configured to mix the one or more baseband packets with the reference clock signal to generate phase modulated data encoding the position information, the backscatter modulator configured to backscatter the at least one RF signal based on the phase modulated data; and

a computing device configured to receive the position information from the backscatter circuitry and to initiate at least one action based on the handwriting input.

2. The object tracking system of claim 1 , wherein:

the magnetic field transmitter operates at one or more frequencies less than 100 kHz; and

the communication interface operates at one or more frequencies greater than 1 MHz.

3. The object tracking system of claim 1 , wherein:

the object tracking device does not include an on-board power source.

4. The object tracking system of claim 1 , further comprising:

a radio-frequency (RF) transmitter remote from the object tracking device, the RF transmitter configured to generate RF signals.

5. The object tracking system of claim 4 , wherein:

the RF transmitter is at least partially disposed within the housing; and

the computing device is at least partially disposed within the housing.

6. The object tracking system of claim 1 , wherein:

the backscatter circuitry is configured to generate the at least one clock signal and the at least one output signal using power derived from the at least one RF signal.

7. The object tracking system of claim 1 , wherein:

the magnetic field transmitter is configured to generate a plurality of magnetic fields in a sequence, each of the plurality of magnetic fields including a different frequency;

the object tracking device is a first object tracking device; and

the object tracking system further comprises a plurality of object tracking devices including the first object tracking device;

wherein each of the plurality of object tracking devices is associated with a different one of the plurality of magnetic fields.

8. The object tracking system of claim 1 , wherein:

the surface is a first surface;

the object tracking device is a stylus configured to write on the first surface; and

the housing comprises one or more second surfaces remote from the first surface.

9. An object tracking device, comprising:

a magnetic field sensor comprising one or more receiving coils;

position tracking circuitry in communication with the magnetic field sensor, the position tracking circuitry configured to determine at least one field strength associated with at least one stationary magnetic field sensed at the one or more receiving coils in response to movement of the object tracking device relative to a surface remote from a source of the at least one stationary magnetic field, the position tracking circuitry configured to determine position information associated with a handwriting input to the object tracking device based at least in part on the at least one field strength; and

backscatter circuitry including a frequency synthesizer configured to generate a baseband clock signal and a reference clock signal from at least one RF signal, a baseband processor configured to generate from the position information and the baseband clock signal one or more baseband packets, and a backscatter modulator configured to mix the one or more baseband packets with the reference clock signal to generate phase modulated data encoding the position information, the backscatter modulator configured to backscatter the at least one RF signal based on the phase modulated data to transmit the position information to at least one remote computing device.

10. The object tracking device of claim 9 , further comprising:

an inertial measurement unit in communication with the position tracking circuitry;

wherein the position tracking circuitry is configured to determine the position information based at least in part on an output of the inertial measurement unit.

11. The object tracking device of claim 9 , wherein:

the magnetic field sensor includes three magnetic coils, each magnetic coil configured to sense a field strength associated with an individual one of three orthogonal magnetic fields; and

each of the three magnetic coils is arranged orthogonal to other ones of the magnetic coils.

12. The object tracking device of claim 9 , wherein:

the backscatter circuitry is configured to generate power from one or more ambient radio-frequency signals;

wherein the magnetic field sensor is powered by an output of the backscatter circuitry.

13. An object tracking system, comprising:

a magnetic field transmitter comprising one or more transmitting coils configured to generate at least one stationary magnetic field;

a plurality of object tracking devices, each object tracking device comprising a magnetic field sensor configured to sense the at least one stationary magnetic field in response to movement of the object tracking device relative to a surface remote from the magnetic field transmitter, each object tracking device comprising backscatter circuitry including a frequency synthesizer configured to generate a baseband clock signal and a reference clock signal from at least one RF signal, a baseband processor configured to generate from position information and the baseband clock signal one or more baseband packets, and a backscatter modulator configured to mix the one or more baseband packets with the reference clock signal to generate phase modulated data encoding the position information, the backscatter modulator configured to backscatter the at least one RF signal based on the phase modulated data to transmit the position information based on a sensed strength of the at least one stationary magnetic field; and

one or more computing devices configured to receive the position information from the communication interface and to initiate at least one action based on the position information.

14. The object tracking system of claim 13 , wherein:

each of the plurality of object tracking devices is configured to transmit corresponding position information to a different one of the one or more computing devices.

15. The object tracking system of claim 13 , wherein:

each of the plurality of object tracking devices is configured to transmit corresponding position information to a first computing device of the one or more computing devices.

16. The object tracking system of claim 13 , further comprising:

a radio-frequency (RF) transmitter remote from the plurality of object tracking devices, the RF transmitter configured to generate an RF signal.