Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
2002-11-01
2004-06-22
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
Reexamination Certificate
active
06754596
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of measuring position and orientation with improved signal to noise ratio. The present invention contemplates transmitting a three axis pulsed DC transmit waveform that is received by a three axis receiver. This general concept is well known. However, in systems employing such structure, typically, only the leading edge of the waveform is used in conducting measurements. Typically, the signal to noise ratio is relatively low since only a portion of the signal energy is used.
The concept of using transmitting and receiving components incorporating electromagnetic coupling is well known in the field of bio-mechanics and in minimally invasive surgery. As one example, sensors transmit position information regarding the locations of surgical instruments within the body. This information is employed by a computer and display to precisely show relative motions of instruments so monitored, giving the surgeon valuable information regarding required actions.
Magnetic position and orientation measuring systems are also used in the field of motion capture and digitization in which an actor moves in such a manner as to animate a virtual character. When conductive materials are present adjacent the location where the actor is moving, they generate eddy current fields which distort the received magnetic field signals, thereby causing undesirable errors in the computed sensor position. Systems employing pulsed DC transmit waveforms and various magnetic sensor and signal processing techniques have been developed in an attempt to reduce these negative effects. Sensors employed in these applications measure both the H field and the derivative dH/dT field. The H field is generally measured by a flux gate magnetometer, Hall effect sensor, magneto-optical sensor or magneto-resistive sensor. Calculation of the dH/dT field is typically performed through the use of a coil in series with an integrator.
The following prior art is known to Applicant:
Volume 41
, Geophysics
, April, 1976, pages 287-299, describe the advantages and disadvantages of DC field (H) measurement systems using a flux gate magnetometer sensor means and measurement of dH/dT using a coil-integrator sensing means when employed in a pulse excited geomagnetic prospecting system. This publication fails to teach or suggest determination of the position of a sensor relative to a transmitter in three dimensions.
U.S. Pat. Nos. 4,849,692 and 4,945,305, both to Blood, disclose a position measuring system in which a pulsed DC waveform is transmitted and the transmitted signal plus eddy field distortion are sensed using a DC responsive sensor. The transmitted waveform is held in a steady state until the eddy current fields decay to a nominal value, at which time the remaining sensed field value is digitized and processed. The systems disclosed in the Blood patents require compensation for the presence of the Earth's magnetic field, which field is generally an order of magnitude larger than the sensed signal from the transmitter. This requirement adds cost and size to the signal processing system and the requirement to wait until the eddy current field distortion has decayed adds unacceptable additional processing time.
U.S. Pat. No. 4,868,498 to Lusinchi discloses an angular measurement device including a magnetic transmitter element affixed to a rotating body. A transmitted signal is sensed by a coil, the output of which is then integrated to provide a flux reading from the transmitter. The system disclosed by Lusinchi may measure the angular position of a rotating body but is not capable of determining position in three dimensions.
U.S. Pat. No. 5,272,658 to Eulenberg discloses a long-term integrator for integrating a voltage signal from a coil measuring magnetic induction. Eulenberg also discloses the use of a flux measuring coil followed by an offset reducing amplifier followed by a digital integrator consisting of an analog-to-digital converter and a DSP, the sum of which comprises a long-term flux meter. The system disclosed by Eulenberg does not include description of a method or apparatus for determining position from the coil-integrator magnetic field measurement system and only claims the long-term integrator portion of the disclosure.
U.S. Pat. No. 5,453,686 to Anderson discloses a position measuring system using the same type of transmit waveform and position algorithm as disclosed in U.S. Pat. No. 4,849,692 to Blood but with the addition of a coil-integrator sensor means similar to that which is disclosed in the Eulenberg patent. The coil-integrator sensing means which is comparable to a well known flux gate magnetometer is well known in the art to produce results equivalent to a flux gate magnetometer when measuring transient magnetic events. The transmitting waveform disclosed in U.S. Pat. No. 4,849,692 to Blood is precisely the same as that which is used in the Anderson patent. The transmitting waveform disclosed in Blood '692 and Anderson utilizes only half of the available transmit energy when the coil integrator is employed as the sensing element. While Anderson does not require compensation for the static portion of the Earth's magnetic field, when the sensor coils are rotated in the Earth's magnetic field, an undesired electromagnetic field is generated at the coil terminals and integrated. As a result, sensor offset errors occur due to dynamic sensor motion in a static magnetic field. The present invention contemplates reducing this type of error.
U.S. Pat. No. 5,767,669 to Hansen et al. discloses a system in which a triangular non-steady state transmit waveform is employed to overcome eddy current distortions that would otherwise be created by adjacent conductive metals. In one embodiment of the Hansen et al. patent, a transmit waveform is produced in such a manner that eddy current conditions in the conductive metal environment reach a steady state condition during both the rising and falling edges of the transmit waveform. The Hansen et al. patent also discloses numerous techniques for reducing the duration of either the rising and/or falling edges of the transmit waveform to increase the measurement rate. In all disclosed versions, the system requires that the integration reset and output digitization occur during transient conditions of the transmitted waveform, thereby requiring a high bandwidth signal chain. This mode of operation also requires extremely precise time synchronization between the transmitter and sensor signal processing. In motion capture applications, it is highly desirable to facilitate operation devoid of physical connections between the transmitter and signal processor so that the person who is performing the choreographed movements is unencumbered by attached cabling. Often, precision in measurements can be lost when attempting to synchronize when using a wireless configuration. The concept of time jitter is often encountered. Such time jitter results in reduction in synchronization precision which produces noise, offsets and other undesirable effects on system output. The present invention contemplates reducing these undesirable effects.
In the prior art, pulsed DC transmit waveforms are produced such that the falling edges of the X and Y axes overlap with the rising edges of the Y and Z axes, respectively, such that resetting the integration before the transient condition and integrating the coil output during the transient period produce a value that does not represent a useful quantity. Additionally, prior art systems require that the integrator be reset at the beginning of an X, Y, Z, OFF cycle and then read at the end of that cycle. The difference between the two values consists of the inherent drift of the integrator plus any output contribution due to motion in a static magnetic field. This difference is then divided proportionally according to time and subtracted from the measured values of X, Y and Z sensed fields. By contrast, in the preferred embodiment of the present invention, a pulsed DC tra
Ascension Technology Corporation
Barlow John
Lau Tung S
Spiegel H. Jay
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