High Speed High Resolution Ultrasonic Position and Orientation Tracker

1. Technical Field

The invention is related to position tracking systems generally and particularly to tracking pilot helmet position in the cockpit of a flight vehicle or simulator using ultrasonic transmitters mounted in the cockpit and ultrasonic transducers mounted on the helmet, or vice versa.

2. Background Art

Position tracking systems for monitoring the position of an article such as a pilot's helmet in a cockpit are useful for such purposes as maintaining a computer-generated projected image display in the pilot's field of view, for example. Other purposes include tracking any part of a person's body such as the head, hand or finger, so that the person may generate inputs to a computer by the movement of his head, hand or finger, for the control of cursor movement in a computer display, for example. Other applications include robotics and weapon systems.

The advantages of employing ultrasonic waves to perform such position tracking over techniques employing electromagnetic signals (such as those disclosed in U.S. Pat. No. 4,742,356 to Kuipers and U.S. Pat. No. 4,303,394 to Berke et al.) are well-known. Essentially, electromagnetic systems are far more vulnerable to interference from many noise sources. Ultrasonic position tracking techniques are well-known. For example, U.S. Pat. No. 4,807,202 to Cherri et al. discloses an ultrasonic tracking system in which the orientation and position of a movable object (such as a helmet) in a closed frame of reference (such as a cockpit) is continually tracked by following the six degrees of freedom of movement of the moveable object. This is accomplished, as illustrated in FIG. 1 hereof, by mounting three ultrasonic transmitters 100, 102, 104 in three different locations in the closed frame of reference 106 and mounting three ultrasonic transducers 108, 110, 112 in three different locations on the moveable object 114. Each one of the three transmitters 100, 102, 104 transmits an ultrasonic acoustic wave signal at a different ultrasonic frequency, all of which are received at each one of the three transducers 108, 110, 112. The three ultrasonic frequencies received at each transducer are separated into three received signals, so that a total of nine signals are received and processed. A tracking processor 116 processes each of the nine signals to provide the distance between the transducer and the transmitter corresponding to the frequency of the received signal, thus providing nine distances. Using well-understood principles, the tracking processor 116 computes the instantaneous position and orientation of the moveable object 114 with respect to the closed frame of reference from the nine distances. This computation uses the locations of the transmitters in the closed frame of reference and the locations of the transducers with respect to the frame of reference of the moveable object.

Various methods of transmitting and processing the ultrasonic signals are employed, all with varying degrees of limited performance. The basic limitation of such methods is that they are slow, limiting the rate at which the position and orientation of the movable object can be repeatedly computed. Most of the methods employ a pulsed ranging ultrasonic technique, such as the techniques disclosed in U.S. Pat. No. 4,853,863 to Cohen et al., U.S. Pat. No. 4,807,202 to Cherri et al., U.S. Pat. No. 3,836,953 to Rotier and U.S. Pat. No. 3,777,305 to Stoutmeyer.

The above-referenced patent to Cohen et al. discloses an ultrasonic position tracking technique in which the frequency shift of the ultrasonic signal due to helmet movement is measured and integrated to provide a displacement value from which the transmitter-to-receiver range is computed.

The problem with the pulse or sequential techniques is that the time required for the receiver to acquire a sufficient ultrasonic signal from which the transmitter-to-receiver range can be inferred limits the rate at which the helmet position can be tracked. For example, the ultrasonic pulsed ranging techniques are limited by the time of flight between the transmitter and the receiver. Generally, it is believed that such sequential techniques produce an updated position measurement for a given sensor at a rate not exceeding on the order of 10 Hz. Most users would be pleased by a 100 Hz update rate. Pulse or sequential techniques are limited to a total time of 9 time-of-flight intervals, or about 30 ms. In a room with echoes, this figure may be 3 or 4 times as large. Consequently, the maximum update rate for sequential techniques is 30 Hz, although a more practical update rate is 10 Hz. Another problem with sequential techniques is that the data is incoherent. The 9 ranges are not simultaneous. As will be shown below, the present invention provides the 9 ranges simultaneously as coherent data, a significant advantage over the prior art.

The problem with the technique of integrating the frequency shift is that its maximum update rate is limited by the time required to measure frequency, compute the doppler shift and then integrate it so as to produce a displacement value from which to compute a range.

What is needed is an ultrasonic position tracker which will track the moveable object position at far higher rates and provide the nine measurements coherently or simultaneously. However, given the present state of the art, such tracking rates do not seem possible.

The invention is an ultrasonic position tracking system which tracks the position of a moveable object with respect to a fixed frame of reference to within a fraction of the ultrasonic wavelength at a maximum rate of tens of thousands of times per second, or about the frequency of the ultrasonic carrier employed. The ultrasonic position tracking of the invention is a continuous wave ultrasonic interferometric displacement detector in which displacement of a movable object is detected in the wave interference or relative phase between the transmitted and received ultrasonic waves. The displacement is measured to within one ultrasonic carrier wavelength by counting interference fringes or zero phase crossings. The displacement measurement is then refined to within a fraction of the ultrasonic carrier wavelength by interpolating between adjacent interference fringes. In order to provide a rough position within the closed frame of reference from which the fringe count can be initialized, the ultrasonic carrier is modulated by a low frequency acoustic signal whose wavelength is at least as great as the diameter of the closed frame of reference. The low frequency signal is detected and its phase determined to provide a rough but unambiguous estimate of the range between the transmitter and receiver. Either one of the transmitter and receiver is fixed on the movable object while the other is stationary with respect to the closed frame of reference.

The key to the high speed of the invention is that a new wavelength count or displacement measurement is always available. In an ideal implementation of the invention, changes in the fringe count register immediately upon the occurence of a phase coincidence. The only delay is one time-of-flight interval. The invention avoids sequencing of measurements. A new fractional wavelength count is always available with each cycle of the received signal, so that it is available at the frequency of the ultrasonic carrier.

In another aspect of the invention, the moveable object is an inertial measurement unit, and the ultrasonic position tracker updates the inertial measurement unit's internal position information to correct for drift to which all inertial measurement units are prone. The ultrasonic position tracker of the invention is sufficiently fast to update the inertial measurement unit in a meaningful way, while the inertial measurement unit provides a continuous measurement of position and orientation and therefore maintains accurate position measurements between updates provided by the ultrasonic position tracker.

FIG. 1 is a simplified diagram of an ultrasonic position tracking system of the prior art employing three transmitters operating at three different frequencies and three transducers.

FIG. 2 is a schematic block diagram illustrating one channel of a circuit embodying the invention.

FIGS. 3A through 3C are contemporaneous diagrams of time domain waveforms generated in the operation of the circuit of FIG. 2.

FIGS. 3D through 3G are diagrams of correlative phase domain waveforms generated in the operation of the circuit of FIG. 2.

FIG. 4 is a schematic block diagram illustrating a digital implementation of three channels of the type illustrated in FIG. 2.

FIG. 5 is a simplified block diagram illustrating the application of the circuit of FIG. 2 with an inertial measurement unit in accordance with an alternative embodiment of the invention.

FIG. 2 illustrates a typical circuit of the invention used in each one of the nine channels in the tracking processor 116 of FIG. 1. In the example illustrated in FIG. 2, the circuit processes the one signal received by the receiving transducer 110 from the transmitter 100, it being understood that there are a total of nine such processing tasks to be performed, corresponding to the nine received signals discussed above with reference to FIG. 1.

The main advantage of the invention is that it exploits the C.W. form of the received ultrasonic signal by an interferometric comparison of the transmitted and received versions of a signal in which any change by a single fringe of the resulting interference pattern produces a change in the computed tracking position. The fastest rate at which this change can occur is the frequency of the ultrasonic signal, or about 40 kHz in one implementation of the invention. This is an increase in tracking rate of about two orders of magnitude over the prior art.

The interferometric displacement detector circuit of FIG. 2 includes a fringe processor 200 having inputs connected to receive both the transmitted and received signals, a fringe count latch 202 connected to one output of the fringe processor 200 and a fringe fraction latch 204 connected to another output of the fringe processor 200. A master timing generator 206 synchronizes the operation of the circuit of FIG. 2 and the generation of the ultrasonic signal (on the order of 40 kHz, for example) produced by the transmitter 100. The pair of latches 202 and 204 are periodically sampled by a computer 208. As will be described below herein, the computer 208 samples a total of nine such pairs of latches corresponding to the nine received ultrasonic signals discussed hereinabove. The computer 208 is programmed in accordance with the prior art techniques referred to previously herein to compute the position and orientation of the helmet or tracked object 114.

Accompanying the interferometric detector circuit is a rough range processor 210 which responds to a low frequency (480 Hz, for example) signal modulating the ultrasonic carrier transmitted by the transmitter 100 to provide a rough but unambiguous position of the helmet 114 within the cockpit or frame of reference 106. The output of the rough range processor 210 is held in a rough range latch 212 which is sampled by the computer 208 as desired. The wavelength of the rough range modulation signal must be at least on the order of the diameter or greatest length of the cockpit or closed frame of reference 106, and so its frequency is typically on the order of 480 Hz.

The fringe processor 200 detects the phase differences between the received signal of the transducer 110 and the transmitted signal in quadrature pair and deduces therefrom each zero-crossing of the phase difference. The fringe processor 200 includes a phase detector 214 which detects the phase difference between two signals: (a) the signal transmitted by the transmitter 100 and signal phase-shifted by 90 degrees by a phase shifter 216 and (b) the signal received by the receiver 110 and bandpass filtered and converted to square wave by a phase-locked loop 218. (For this purpose, the phase-locked loop 218 is of the type well-known in the prior art having a square wave oscillator.) Another phase detector 220 detects the phase difference between the transmitted signal and the output of the phase-locked loop 218. The output signal amplitudes of the detectors 214, 220 are proportional to the phase differences of the signal pairs received by the phase detectors. The output signals of the pair of phase detectors 214, 220 are low pass filtered by a pair of filters 222, 224 and are compared with a threshold by a pair of Schmitt triggers 226, 228, respectively.

FIG. 3A illustrates the time domain pulsed waveform (a square wave) of the signal furnished by the master timing generator 206 to the transmitter 100. The reactive elements of the transmitter 100 (or, alternatively, an active filter not shown in the drawings) convert the square waveform to a corresponding sine wave shown in FIG. 3A, so that a sine wave C.W. signal is received by the transducer 110. FIG. 3B illustrates the waveform at the output of the phase shifter 216. FIG. 3C illustrates the signal received by the transducer 110 as converted to a square wave at the output of the phase-locked loop 218. FIGS. 3D and 3E illustrate in the phase domain the magnitude of the signal at the output of the low pass filters 222 and 224, respectively, as a function of increasing phase difference detected by the phase detectors 214, 220, respectively.

FIGS. 3F and 3G illustrate the outputs of the Schmitt triggers 226, 228, respectively. Essentially, each time the input to the Schmitt trigger 226 or 228 falls below its trigger threshold (dashed line of FIGS. 3D or 3E), the Schmitt trigger output goes high, and each time the input rises above the trigger threshold the output goes low. Note that the leading edges of the output of the Schmitt trigger 228 (FIG. 3F) coincide with the zero phase crossings of the phase detector 220 (FIG. 3D), which is the only one of the two phase detectors whose input is not phase shifted. These leading edges of the Schmitt trigger 228 are detected by employing the output of the other Schmitt trigger 226 as a gate: whenever the output of the other Schmitt trigger 226 is low, sampling of the output of the Schmitt trigger 228 is enabled. For this purpose, the fringe counter 230 has its clock input connected to the Schmitt trigger 228 and its enable input connected to the other Schmitt trigger 226. The fringe counter 230 is of the well-known type in which up-going transitions at its clock input cause the fringe counter 230 to count up while down-going transitions at its clock input cause the fringe counter 230 to count down. The contents of the fringe counter 230 indicate the number of fringes or zero crossings caused by displacement of the helmet 114.

The master timing generator 206 produces fringe interpolation clock pulses at a rate which is an integral multiple (e.g., 4, 8, or 16 times) the frequency of the transmitted ultrasonic signal. A fringe interpolation counter 240 counts the fringe interpolation clock pulses. The rate of the fringe interpolation clock pulses (e.g., n times the transmitted ultrasonic signal frequency) and the modulus of the counter 240 (e.g., n) are selected so that the counter 240 automatically resets at the end of each sinusoidal period of the transmitted ultrasonic signal. The fringe fraction latch 204 latches the output of the fringe interpolation counter 240 in synchronism with the received ultrasonic signal at the output of the phase-locked loop 218. Thus, the change in phase (range) between fringes is measured to within a fraction (1/n) of the wavelength of the ultrasonic signal.

In the example of FIG. 2, the master timing generator 206 derives the ultrasonic carrier for the transmitter 100 by dividing down the fringe interpolation counter by some integer (e.g., 16). However, the ultrasonic carrier may be derived by simply connecting the transmitter 100 to the most significant bit of the fringe interpolation counter 240.

The rough range processor 210 operates simply by measuring the phase of the low frequency (e.g., 480 Hz) modulation signal imposed on the ultrasonic carrier. Since the wavelength of the low frequency modulation is at least as great as the longest dimension of the closed frame of reference or cockpit 106, the phase difference between the transmitted and received signals is a unique measure of the range from transmitter to receiver in the cockpit 106.

As illustrated in FIG. 2, a low frequency (e.g., 480 Hz) oscillator 250 modulates the ultrasonic signal transmitted by the transmitter 100. A conventional AM demodulator 252 in the rough range processor 210 demodulates the low frequency (e.g., 480 Hz) signal received at the receiver 110. In the meantime, a rough range counter counts 256 up in synchronism with the low frequency oscillator 250 and is latched in the latch 212 by the output of the demodulator 252. Preferably, the modulus of the rough range counter 256 and rate at which it counts are selected so that the counter 256 resets at the end of each period of the low frequency modulation signal. Thus, the contents of the latch 212 indicates the phase of the modulation, which is a direct measure of the range between the transmitter and receiver.

FIG. 4 is a diagram corresponding to FIG. 2 but showing at a higher level how the three ultrasonic frequencies detected by the receiver 110 are processed. In one implementation of the invention, the three ultrasonic frequencies are 37 kHz, 40 kHz and 43 kHz. Thus, in FIG. 4 there are three fringe processors 200a, 200b and 200c corresponding to the fringe processor 200 of FIG. 2, three ultrasonic received carrier signals RC1, RC2 and RC3 and three rough range received signals RR1, RR2 and RR3. In addition, an address decoder 260 permits the computer 208 to selectively address any one or portions of the various latches 204, 204 and 212, in accordance with well-known techniques, as required by the computer's program. The apparatus of FIG. 4 is associated with one of the transducers, there being three transducers in all, so that FIG. 4 illustrates about one third of the system of the invention comprising a complete tracking processor of the type illustrated in FIG. 1.

The contents of the latches 202 and 204 provide the range between the transmitter 100 and the receiving transducer 110 in the form of a wavelength count (the latch 202) and a fraction of a wavelength (the latch 204). The computer 208 is programmed in accordance with the techniques of the prior art discussed hereinabove to periodically sample the contents of the latches 202 and 204 for each of the nine received ultrasonic signals and compute therefrom the helmet position and orientation. The computer 208 also periodically samples the contents of the rough range latch 212, to verify the value of the fringe count and correct it if necessary.

Referring to FIG. 5, the system 500 of FIG. 1 implemented with three circuits of FIG. 4 for each of the three transducers 108, 110, 112 is used to periodically update an inertial measurement unit 502. The inertial measurement unit constantly provides position and orientation data, while the ultrasonic position tracking system of the invention 500 provides measurements only at periodic intervals corresponding to the position tracking rate discussed above. However, while the inertial measurement unit is therefore superior in its ability to continuously provide such measurements, it suffers from a tendency to drift. The advantage of the embodiment of FIG. 5 is that drift errors are minimized by periodically updating the inertial measurement unit 502 with its correct position and orientation as measured by the ultrasonic tracking system 500 at a very high rate. For this purpose, the three transducers 108, 110, 112 are placed on or mechanically coupled to the inertial measurement unit 502, while the output of the computer 208 embodying the tracking processor 116 of FIG. 1 is transmitted to the inertial measurement unit 502. The inertial measurement unit updates its position and orientation data with the measurements received from the ultrasonic tracking system 500. The position tracking system 500 may be the ultrasonic position tracking system described above or may be a magnetic, infared or electromagnetic position tracking system of the type well known in the art.

While the invention has been described in detail by specific reference to preferred embodiments thereof, it is understood that variations and modifications may be made without departing from the true spirit and scope of the invention.