# US 2021/0240273 A1 · Object Tracking Using Magnetic Field Sensing and Backscatter Transmissions > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2021/0240273 A1 - Document ID: US20210240273A1 - Status: Published application - Kind: A1 - Application number: 16/768,761 - Filed: 2019-10-24 - Published: 2021-08-05 - Earliest claimed: 2018-10-24 - Assignees: Google Inc. - Applicants: Google LLC - Inventors: Alejandro Kauffmann - Web page: https://patentgrep.com/patent/US20210240273A1 ## CPC - G06F 3/017: {Gesture based interaction, e.g. based on a set of recognized hand gestures (interaction based on gestures traced on a digitiser G06F3/04883)} - G01R 33/0094: {Sensor arrays} - G06F 3/03545: {Pens or stylus} - G06F 3/038: Control and interface arrangements therefor, e.g. drivers or device-embedded control circuitry - G06F: ELECTRIC DIGITAL DATA PROCESSING (computer systems based on specific computational models G06N) - G01R: MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES (indicating correct tuning of resonant circuits H03J3/12) ## Abstract A low-power object tracking system is disclosed that includes an object tracking device that senses one or more magnetic field(s) to determine a position of the object tracking device. The object tracking device includes a magnetic field sensor including one or more receiving coils and position tracking circuitry in communication with the magnetic field sensor. The position tracking circuitry is configured to determine at least one field strength associated with at least one stationary magnetic field sensed at the one or more receiving coils, and to determine position information associated with the housing based at least in part on the at least one field strength. The object tracking device includes a communication interface configured to transmit the position information to at least one remote computing device. ## Claims ### Claim 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; and 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; a communication interface configured to transmit the position information; and a computing device configured to receive the position information from the communication interface and to initiate at least one action based on the handwriting input. ### Claim 2 (depends on 1) The object tracking system of claim 1, wherein: the magnetic field transmitter operates at one or more frequencies less than 1 kHz; and the communication interface operates at one or more frequencies greater than 1 MHz. ### Claim 3 (canceled) ### Claim 4 (depends on 1) The object tracking system of claim 1, wherein: the communication interface comprises backscatter circuitry configured to generate at least one output signal using radio-frequency signals, the backscatter circuitry configured to encode the position information into the at least one output signal. ### Claim 5 (depends on 4) The object tracking system of claim 4, wherein: the object tracking device does not include an on-board power source. ### Claim 6 (depends on 4) The object tracking system of claim 4, further comprising: a radio-frequency (RF) transmitter remote from the object tracking device, the RF transmitter configured to generate RF signals. ### Claim 7 (depends on 6) The object tracking system of claim 6, wherein: the backscatter circuitry is configured to generate at least one clock signal based on at least one RF signal; and the backscatter circuitry is configured to modulate the at least one clock signal to encode the position information in the at least one output signal. ### Claim 8 (depends on 7) The object tracking system of claim 7, wherein the at least one clock signal includes a baseband clock signal and a reference clock signal, wherein the backscatter circuitry comprises: a frequency synthesizer configured to generate the baseband clock signal and the reference clock signal from the 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 RF signal based on the phase modulated data. ### Claim 9 The object tracking system of 7, 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. ### Claim 10 (depends on 1) 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. ### Claim 11 (canceled) ### Claim 12 (canceled) ### Claim 13 (depends on 6) The object tracking system of claim 6, wherein: the RF transmitter is at least partially disposed within the housing; and the computing device is at least partially disposed within the housing. ### Claim 14 (depends on 1) 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; the housing comprises one or more second surfaces remote from the first surface. ### Claim 15 (canceled) ### Claim 16 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 a communication interface configured to transmit the position information to at least one remote computing device. ### Claim 17 (depends on 16) The object tracking device of claim 16, wherein: the communication interface comprises backscatter circuitry configured to generate an output signal using a radio frequency signal from a radio frequency transmitter remote from the object tracking device, the backscatter circuitry configured to encode the position information into the output signal; and the backscatter circuitry is configured to generate at least one clock signal and the output signal using power derived from the radio frequency signal. ### Claim at position 18 18-22. (canceled) ### Claim 23 (depends on 16) The object tracking device of claim 16, 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. ### Claim 24 (depends on 16) The object tracking device of claim 16, 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. ### Claim at position 21 25-37. (canceled) ### Claim 38 (depends on 16) The object tracking device of claim 16, further comprising: backscatter circuitry 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. ### Claim 39 (canceled) ### Claim 40 (canceled) ### Claim 41 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 a communication interface configured to transmit 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. ### Claim 42 (depends on 41) The object tracking system of claim 41, 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. ### Claim 43 (depends on 41) The object tracking system of claim 41, 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. ### Claim 44 (depends on 41) The object tracking system of claim 41, further comprising: a radio-frequency (RF) transmitter remote from the plurality of object tracking devices, the RF transmitter configured to generate an RF signal; wherein each of the plurality of object tracking devices comprises backscatter circuitry configured to generate an output signal using the RF signal, the backscatter circuitry configured to encode the position information into the output signal.