High speed high resolution ultrasonic position and orientation tracker

An ultrasonic position tracking system for monitoring the position and orientation of a movable object in a closed frame of reference, said system comprising:

• means for transmitting n respective ultrasonic signals from respective predetermined transmitter locations fixed with respect to one of said movable object and said frame of reference;
• means for receiving said n respective ultrasonic signals at m predetermined receiver locations fixed with respect to the other of the movable object and said frame of reference to provide n.times.m received signals;
• m.times.n channels for processing respective ones of said in m.times.n received signals, wherein each of said channels comprises:
• (a) means connected to said transmitter and to said receiver for measuring a phase difference between a respective pair of said transmitted and received signals and for generating an incremental or decremental count whenever an increase or decrease, respectively in said phase difference results in a phase difference of zero;
• (b) means for maintaining an algebraic sum of said incremental and decremental counts indicative of an integral number of wavelengths of said transmitted signal by which a respective pair of said first and second locations are displaced from one another relative to an initial displacement, said algebraic sum being updated with each zero crossing of said phase difference;
• and
• (c) means for sensing a number of predetermined fractions of one wavelength of the respective transmitted signal by which said respective pair of transmitted and received signals are out of phase with respect to one another, said number of predetermined fractions being updated with each cycle of said respective transmitted ultrasonic signal, and the displacement of said respective pair of first and second locations being said integral number of wavelengths plus said number of predetermined fractions of said wavelength.

A method of tracking displacement between an ultrasonic transmitter acoustically radiating a continuous wave ultrasonic transmitted signal and an ultrasonic receiver detecting said transmitted signal as a received signal, said method comprising:

• creating an interference pattern characterized by interference fringes by mixing said transmitted and received signals;
• counting the number of said fringes by which said pattern changes with respect to a fixed point and storing said number as a number of wavelengths of said displacement;
• interpolating a location of said fixed point between successive ones of said interference fringes as a fraction of a wavelength and storing said fraction of a wavelength;
• and
• computing said displacement by combining said number of wavelengths and said fractional number of wavelengths.

A method of overcoming drift error in an inertial measurement unit which continuously generates information describing its position and orientation in a frame of reference, said method comprising:

• operating said inertial measurement unit (IMU) to generate continuous position and orientation information relative to said frame of reference for all times following a initialization time wherein its reference is established from another source;
• acoustically radiating respective continuous wave ultrasonic transmitted signals from a plurality of first respective locations;
• detecting each of said transmitted signals as a received signal at a plurality of second respective locations, wherein one of said pluralities of first and second locations are fixed to a frame of reference and the other comprises locations on said inertial measurement unit;
• processing said transmitted and received signals so as to generate a second source of position and orientation of said inertial measurement unit;
• and
• periodically initializing position and orientation information generated by said inertial measurement unit with said second source position and orientation information.

A method of overcoming drift error in an inertial measurement unit which continuously generates information describing its position and orientation, said method comprising:

• acoustically radiating respective continuous wave ultrasonic transmitted signals from a plurality of first respective locations;
• detecting each of said transmitted signals as a received signal at a plurality of second respective locations, wherein one of said pluralities of first and second locations are fixed to a frame of reference and the other comprises locations on said inertial measurement unit;
• processing said transmitted and received signals so as to generate a position and orientation of said inertial measurement unit;
• and
• periodically correcting position and orientation information generated by said inertial measurement unit with the position and orientation generated in said processing step, and further in which, for each respective pair of transmitted and received signals, the steps of:
• (a) creating an interference pattern characterized by interference fringes by mixing said transmitted and received signals;
• (b) counting the number of said fringes by which said pattern changes with respect to a fixed point and storing said number as a number of wavelengths of said displacement;
• (c) interpolating a location of said fixed point between successive ones of said interference fringes as a fraction of a wavelength and storing said fraction of a wavelength;
• and
• (d) computing said displacement by combining said number of wavelengths and said fractional number of wavelengths;
• and
• for all pairs of transmitted and received signals, deriving from the corresponding displacements computed in said computing step said orientation and position of said inertial measurement unit.

Apparatus for tracking displacement between an ultrasonic transmitter acoustically radiating a continuous wave ultrasonic transmitted signal and an ultrasonic receiver detecting said transmitted signal as a received signal comprising:

• means for transmitting an ultrasonic signal from a first location with a space,
• means for receiving ultrasonic signals at a record location within said space,
• means for combining said transmitted and received signals to produce an interference pattern therefor having maxima and minima,
• means for tracking the movement of said receiving means relative to said transmitting means by counting shifts in said interference pattern through maxima and minima that occur as a result of movement of said receiving means relative to said transmitting means.

A method for tracking displacement between an ultrasonic transmitter acoustically radiating a continuous wave ultrasonic transmitted signal and an ultrasonic receiver detecting said transmitted signal as a received signal comprising the steps of:

• transmitting an ultrasonic signal from a first location with a space,
• receiving ultrasonic signals at a record location within said space,
• combining said transmitted and received signals to produce an interference pattern therefor having maxima and minima, and
• tracking the movement of said receiving means relative to said transmitter by counting shifts in said interference pattern through maxima and minima that occur as a result of movement of said receiver relative to said transmitter.