US 7,204,123 B2
`(10) Patent No.
`a2) United States Patent
`McMahanetal.
`(45) Date of Patent:
`Apr. 17, 2007
`
`
`US007204123B2
`
`(54) ACCURACY ENHANCEMENT OF A SENSOR
`DURING AN ANOMALOUS EVENT
`
`(75)
`
`Inventors: Lisa E. McMahan, Tampa, FL (US);
`Joseph G. Protola, Clearwater, FL
`(US); Bruce Wayne Castleman,
`Pinellas Park, FL (US)
`
`(73) Assignee: Honeywell International Inc.,
`Morristown, NJ (US)
`
`(*) Notice:
`
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 94 days.
`
`(21) Appl. No.: 10/842,063
`
`(22)
`
`Filed:
`
`May 10, 2004
`
`(65)
`
`Prior Publication Data
`US 2005/0210952 Al
`Sep. 29, 2005
`
`(51)
`
`Related U.S. Application Data
`(60) Provisional application No. 60/557,109, filed on Mar.
`26, 2004.
`Int. CL
`(2006.01)
`GOIP 21/00
`(52) US. Che oe cceccceeeesesesenseeseesenenenscassenseees 73/1.37
`(58) Field of Classification Search........ 73/1.37-1 39;
`324/602; 702/141
`See application file for complete search history.
`References Cited
`U.S. PATENT DOCUMENTS
`
`(56)
`
`.......... 360/78.04
`4,488,189 A * 12/1984 Axmear et al.
`4,606,316 A *
`8/1986 Komurasaki
`........... 123/406.16
`4,712,427 A
`12/1987 Peters oo. eee 73/514.29
`5,175,438 A * 12/1992 Ikeda wececssseecscesteeseen 250/574
`5,371,718 A * 12/1994 Ikeda et al. veces 367/91
`
`5,479,161 A * 12/1995 Keyes et al. cesses: 340/870.04
`
`4/1997 Gaubatz, ecco 376/242
`5,621,776 A *
`
`FOREIGN PATENT DOCUMENTS
`
`JP
`
`63046961 A *
`
`2/1988
`
`OTHER PUBLICATIONS
`
`Honeywell, Accelerex RBA5O0 Accelerometer, Jan. 2001, Pub-
`lisher: Honeywell International Inc. (2 pgs.).
`
`*
`
`i
`cited b
`OY EXAMINET
`CHE
`Primary Examiner—Hezron Williams
`Assistant Examiner—Tamiko Bellamy
`(74) Attorney, Agent, or Firm—Fogg & Powers LLC
`(57)
`ABSTRACT
`
`A method for enhancing the accuracy of a sensor is pro-
`vided. The method includes determining a measure of the
`output of the sensor, determining whether the measure falls
`outside of an acceptable range for the output of the sensor,
`and, when the measure falls outside the acceptable range,
`modifying the measure of the output such that the measure
`falls within the acceptable range for the sensor.
`
`4,385,699 A *
`
`5/1983 Ashina .....ccceeeeeeee 209/538
`
`22 Claims, 6 Drawing Sheets
`
`202
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`SENSOR
`
`ELECTRONIC
`
`CIRCUIT
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`L
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`\
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`Page | of 13
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`SAMSUNG EXHIBIT 1005
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`SAMSUNG EXHIBIT 1005
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`Page 1 of 13
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`U.S. Patent
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`Apr. 17, 2007
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`Sheet 1 of6
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`US 7,204,123 B2
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`100
`
`
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`SENSOR
`
`ENHANCEMENT
`CIRCUIT
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`ELECTRONIC
`CIRCUIT
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`Fig. 1
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`CIRCUIT
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`ELECTRONIC
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`202
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`SENSOR
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`U.S.
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`Patent
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`Apr. 17, 2007
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`Sheet 2 of 6
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`US 7,204,123 B2
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`BEGIN
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`
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`302 PROCESS
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`300
`SENSOR OUTPUT
`MODIFY
` 308
`PASS VALUE
`
`Fig. 3
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`Page 3 of 13
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`U.S. Patent
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`Apr. 17, 2007
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`US 7,204,123 B2
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`U.S. Patent
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`Apr. 17, 2007
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`Sheet 4 of6
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`U.S. Patent
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`Apr. 17, 2007
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`Sheet 5 of6
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`US 7,204,123 B2
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`VELOCITY
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`VELOCITY
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`-1000
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`-2000
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`-3000
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`-4000 -5000
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`0
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`90
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`100
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`150
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`Fig. 7
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`TIME
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`Fig. 8
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`U.S. Patent
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`Apr. 17, 2007
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`Sheet 6 of6
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`US 7,204,123 B2
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`908
`~---/-----
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`.
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`FILTER
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`ELECTRONIC
`CIRCUIT
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`ENHANCEMENT CIRCUIT|
`\
`904
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`Fig. 9
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`906
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`1000
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`1010
`1008
`--\----------f-
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`ENHANCEMENTCIRCUIT
`Se eee 1006
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`ELECTRONIC
`CIRCUIT
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`902
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`SENSOR
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`1002
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`US 7,204,123 B2
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`1
`ACCURACY ENHANCEMENT OF A SENSOR
`DURING AN ANOMALOUS EVENT
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`
`This application is related to, and claimsthe benefit of the
`filing date of U.S. Provisional Application No. 60/557,109,
`filed on Mar. 26, 2004.
`
`GOVERNMENTLICENSE RIGHTS
`
`The U.S. Government may have certain rights in the
`present invention as provided for by the terms of Lockheed
`Subcontract
`No.
`LHOIN1801N/DASG60-00-C-0072
`
`awarded by the Dept. of Army.
`
`BACKGROUND
`
`Many modem guidance and navigation systems use
`vibrating beam sensors to measure parameters used in
`controlling the flight path of aircraft, missile, or other flight
`vehicle. Vibrating beam sensors typically depend upon crys-
`tal beam oscillators to provide a frequency output
`that
`changes frequency as strain in the beam changes. As an
`example, in a typical accelerometer application, the beam is
`connected to a proof mass supported by flexures connected
`to another structure. When the proof mass is acted upon by
`acceleration, the proof mass deflects about the flexures, and
`stretches or compresses the crystal beam. In some applica-
`tions, two crystal beamsare used in such a way that one is
`compressed and the other is stretched as the proof mass
`deflects. The frequency of the beam in tension increases and
`that of the beam in compression decreases. In these types of
`accelerometers both frequencies are used to provide better
`performance.
`An example of an accelerometer with two crystal beams
`is the Accelerex® RBA-500, made by Honeywell Inc.,
`Redmond, Wash.In this accelerometer, the crystal beams are
`driven at one of their natural resonant frequencies and the
`oscillations generate nearly sinusoidal waveforms in closed
`loop electronics. The sinusoidal waveforms are internally,
`electronically converted to square wave output signals from
`the accelerometer.
`
`The frequency output of a crystal beam accelerometeris
`dependent on the input acceleration. The frequency outputis
`limited by the mechanical structure of the accelerometer as
`well as its internal electronics. Further, deflection of the
`proof mass is limited by physical stops. The stops are
`designedto allow the desired acceleration dynamic range for
`the accelerometer. Further, the stops limit the travel of the
`proof mass to keep from damagingthe crystals and flexures
`from excessive strain. Since the proof mass deflection is
`limited,the strain in the crystal beams should be limited and
`the expected frequency change of the crystal beams should
`fall within an established frequency band.If the acceleration
`exceeds the magnitude at which the proof mass hits the
`stops, it is expected that the frequency output of the crystal
`beams would be limited to the values corresponding to the
`proof mass deflected at the stops. For example, the nominal
`output of an RBA-500 is two square waves with frequencies
`of 35 kHz. The frequencies vary with acceleration until the
`stops are contacted. When the stops are contacted,
`the
`frequency of one crystal is about 30 kHz and the frequency
`of the other crystal is about 40 kHz. These are only illus-
`trative values and will vary for each accelerometer.
`
`10
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`25
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`2
`Typically, guidance and navigation systems determine the
`meaning of the output signals of the accelerometer with
`digital electronics. In some systems, the digital electronics
`count the numberofrising or falling edges in a square wave
`signal output by the accelerometer. This provides a measure
`of the frequency of the output signal and, in turn, a measure
`of acceleration since the frequency of the output signal is
`related to the acceleration.
`Unfortunately, the crystals of an accelerometer are known
`to output higher frequencies or lower frequencies than
`normal under high dynamic environments. This may be due
`to other resonant frequencies of the crystal beamsor it may
`be dueto transient strains on the crystal beamsasa result of
`high velocity paddle impacts with the stops. The number of
`occurrences and the duration of the occurrences are unpre-
`dictable.
`
`The anomalousoutput of higher or lower frequencies can
`lead to a greater or lesser number of counts than should be
`possible, leading to the types of errors already described.
`The effect of these higher or lower than expected counts is
`to cause the acceleration and velocity to be incorrectly
`computed, leading to an apparent velocity shift and a sub-
`sequent error in guidance or navigation.
`Therefore, there is a need in the art for enhancing the
`accuracy of the output of a sensor.
`
`SUMMARY
`
`Embodiments of the present invention address problems
`with sensors which can be solved by enhancing the accuracy
`of the output of the sensor during anomalousevents. In one
`embodiment, errors in vibrating beam sensors are reduced
`by eliminating output frequencies that have impossible
`values under normal operation. The signals being processed
`may be analog or digital. In one embodiment, the method
`includes determining a measure of the output of the sensor.
`The method further includes determining whether the mea-
`sure falls outside of an acceptable range for the output of the
`sensor. When the measurefalls outside the acceptable range,
`the method modifies the measure of the output such that the
`measure falls within the acceptable range for the sensor.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram of one embodiment of system
`with a sensor with enhanced accuracy during an anomalous
`event.
`
`FIG. 2 is a block diagram of another embodiment of
`system with a sensor with enhanced accuracy during an
`anomalous event.
`FIG.3 is a flow chart of one embodimentof a process for
`enhancing the accuracy of a sensor during an anomalous
`event.
`
`FIG.4 is a graph thatillustrates an example of the output
`of a sensor with an anomalous event.
`
`FIG.5 is a graph of a corrected output of the example of
`FIG. 3 using an existing correction technique.
`FIG.6 is a graph of a corrected output of the example of
`FIG. 3 with correction of the sensor output according to an
`embodiment of the present invention.
`FIG.7 is a graphthatillustrates a signal generated based
`on the output of the sensor during an anomalous event using
`existing techniques correct the sensor output.
`FIG.8 is a graphthatillustrates a signal generated based
`on the output of the sensor during an anomalous event with
`the sensor output corrected according to an embodiment of
`the present invention.
`
`Page 8 of 13
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`US 7,204,123 B2
`
`3
`FIG. 9 is a block diagram of another embodiment of
`system with a sensor with enhanced accuracy during an
`anomalous event.
`
`FIG. 10 is a block diagram of another embodiment of
`system with a sensor with enhanced accuracy during an
`anomalous event.
`
`DETAILED DESCRIPTION
`
`In the following detailed description, reference is made to
`the accompanying drawings that form a part hereof, and in
`which is shown by way ofillustration specific illustrative
`embodiments in which the invention may be practiced.
`These embodiments are described in sufficient detail
`to
`
`enable those skilled in the art to practice the invention, and
`it is to be understoodthat other embodiments maybeutilized
`and that logical, mechanical and electrical changes may be
`made without departing from the spirit and scope of the
`present
`invention. The following detailed description is,
`therefore, not to be taken in a limiting sense.
`FIG. 1 is a block diagram of one embodimentof system,
`indicated generally at 100, that enhances the accuracy of a
`sensor 102 during an anomalousevent. For purposes ofthis
`specification, an anomalousevent is an event that causes the
`sensor 102 to provide an output that is outside a range of
`normally expected outputs for the sensor. This output is also
`referred to herein as an “anomalous” output.
`System 100 includes enhancement circuit 104 coupled
`between sensor 102 and electronic circuit 106.
`In one
`embodiment, sensor 102 comprises an accelerometer or
`other appropriate sensor for monitoring a selected stimulus.
`In one embodiment, sensor 102 comprises a vibrating beam
`accelerometer. Further,
`in one embodiment, sensor 102
`comprises a vibrating beam accelerometer with two comple-
`mentary, vibrating beams such as the Accelerex® RBA-500
`commercially available from Honeywell International, Red-
`mond, Wash. In other embodiments, sensor 102 comprises
`any other appropriate sensor that is subject to a definable
`range for output signals such that an anomalous event may
`be detected based on the output of the sensor 102. In other
`embodiments, sensor 102 comprises any appropriate device
`with a known range of physically possible values being
`output as either an analog or digital signal, which makes
`dynamic measurements in the form of a frequency shift of a
`modulated oscillation frequency (i.e. resonant frequency as
`a function of the stress/strain applied).
`In one embodiment, electronic circuit 106 comprises a
`guidance and navigation system used, for example, in an
`aircraft, missile or other flight vehicle. In further embodi-
`ments, the electronic circuit 106 comprises any appropriate
`circuit or system that uses the output of a sensor 102 in its
`operation.
`Enhancement circuit 104 receives the output of sensor
`102. Enhancementcircuit 104 determines when the output
`of sensor 102 is not within the normal operating range for
`sensor 102. Further, enhancement circuit 104 provides a
`signal to electronic circuit 106. When the output of sensor
`102 is within its normal operating range, enhancement
`circuit 104 provides the output of sensor 102 to electronic
`circuit 106. When the output of sensor 102 is not within its
`normal operating range, enhancementcircuit 104 enhances
`the output of sensor 102 by not passing the anomalous
`output
`to electronic circuit 106.
`In one embodiment,
`enhancementcircuit 104 passes a nominal value within the
`normal operating range of the sensor. In other embodiments,
`enhancementcircuit 104 passes a value extrapolated from
`other values output by sensor 102.
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`4
`In embodiments based on a sensor with two crystal
`oscillator beams, such as the Accelerex® RBA-500, an
`additional technique is available to enhance the accuracy of
`the sensor. It is the nature of this device that one crystal
`beam is in a state of tension when the otheris in a state of
`
`compression and the two frequencies are displaced from
`their unstrained value by approximately the same amount
`but with different signs. In other words, the nominal value of
`both crystals may be 35 kHz. When subjected to an accel-
`eration, one crystal may read 33 kHz andthe other 37 kHz.
`Thatis, both are displaced 2 kHz, but in opposite directions
`from the nominal. It is highly unlikely that both crystals will
`experience the anomalous behaviorat exactly the same time.
`It is highly probable that when onecrystal is experiencing a
`problem, the other one will be providing good data. There-
`fore, the accuracy of the sensor 102 can be enhanced during
`anomalous events by using the data from the good crystal,
`e.g., the crystal with the value within the normal operating
`range. From this value,
`the response expected from the
`anomalous crystal
`is calculated and passed to electronic
`circuit 106. In most cases, this will be a more accurate
`adjustment of the device’s output during anomalous events
`than simply using a nominalor extrapolated value.
`In operation, enhancementcircuit 104 receives the output
`of sensor 102, and, selectively modifies the output of sensor
`102 when an anomalousoutput is detected. When the output
`of sensor 102 is not within the expected range of its normal
`operation,
`it is presumed that the output is in error. This
`means that
`the output of the sensor is not an accurate
`reflection of the stimulus that the sensor is designed to
`monitor. If the error is allowed to propagate to the electronic
`circuit 106, the operation of electronic circuit 106 is likely
`to be compromisedsince the error may be magnified when
`relied on in further operations by electronic circuit 106. In
`one embodiment, enhancementcircuit 104 advantageously
`overcomes this problem during an anomalous event by
`providing a value to electronic circuit 106 that is within the
`normalrange of the output of sensor 102 as discussed above.
`By enhancing the output from sensor 102 during anomalous
`events in this manner, enhancementcircuit 104 improves the
`performance of electronic system 106 by reducing the
`impact of incorrect readings from sensor 102 on the opera-
`tion of electronic system 106.
`FIG. 2 is a block diagram of another embodiment of
`system, indicated generally at 200, with a sensor 202 with
`enhanced accuracy during an anomalous event.
`In this
`embodiment, sensor 202 is an accelerometer with a square
`wave output. The frequency of the square wave output of
`sensor 202 is dependent on the acceleration applied to the
`sensor 202.
`In one embodiment,
`the sensor 202 is an
`Accelerex® RBA-500 with a square wave output with a
`nominal frequency of 35 kilohertz (KHz) at zero accelera-
`tion or “zero g’s.” The operating range of the RBA-500is
`typically in the range from 35 to 42 KHz for one vibrating
`beam and 28 to 35 KHz for the other vibrating beam.
`Enhancementcircuit 204 determines when the output of
`sensor 202 is not within the expected or normal operating
`range. Enhancement circuit 204 receives the output of
`sensor 202 at counter 210. Counter 210 is programmed to
`count the numberof leading edges in the output of sensor
`202. In one embodiment, the output of sensor 202 is moni-
`tored over 0.5 millisecond (ms) intervals. With this time
`interval, the expected numberof leading edges for one beam
`is 17.5 to 21 given a frequency range from 35 to 42 KHz.
`This count values is provided to output circuit 214.
`Enhancementcircuit 204 determines when the output of
`sensor 202 falls outside the normal operating range using
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`US 7,204,123 B2
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`6
`lous event. As can be see at 500, the output of the sensorstill
`falls outside the normal range for a portion of the time
`between 20 and 40 milliseconds. In FIG. 6, anomalous
`readings from a sensor are replaced with a nominal output
`that falls within the acceptable range for the sensor. With this
`replacement, the sensor output falls within its normal oper-
`ating range during the full 200 msinterval. The effect of this
`replacement technique is shown by comparing the graphs of
`FIGS. 7 and 8.
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`comparator 212. In one embodiment, comparator 212 com-
`pares the output of one beam with the maximum 22.5. If the
`count exceeds this value, then comparator 212 provides a
`signal to output circuit 214 that indicates that sensor 202 has
`provided an anomalousreading. Otherwise, if the count falls
`below this value, then the comparator 212 provides a signal
`to output circuit 214 that indicates that the sensor 202 output
`is acceptable.
`Output circuit 214 provides an output to electronic circuit
`FIG. 7 is a graph that illustrates the velocity along the
`206 based on the value produced by counter 210 and the
`vertical axis and time along the horizontal axis. This graph
`output of comparator 212. When comparator 212 determines
`illustrates one example of the output of a system that uses a
`that the output of sensor 202 is within its normal operating
`sensor reading with an unmodified anomalous event. It can
`range, then output circuit 214 provides the value output by
`be seen that during the anomalous event, a velocity shift of
`counter 210 to electronic circuit 206. When comparator 212
`over 4000 cm/s was reported based on the sensor reading.
`determines that the output of sensor 202 is not within its
`This is an unacceptable velocity shift. FIG. 8 is a graph that
`normal operating range, then output circuit 214 provides a
`illustrates the velocity versus time curve when the sensor
`value other than the output of counter 210 to electronic
`output is modified to remove any values outside the nominal
`circuit 206. For example, in one embodiment, output circuit
`range expected of the sensor. As can be seen from the graph,
`214 provides a value of 17.5 to the electronic circuit 206. In
`by removingthe valuesthatfall outside the normal operating
`other embodiments, output circuit 214 provides a value
`range and replacing the values with a nominal 17.5 count,
`extrapolated from other counts produced by counter 210. In
`the 4000 cm/s velocity shift is removed. Thus, the accuracy
`yet further embodiments, output circuit 214 generates a
`of the output of the sensor is enhanced.
`value within the normal operating range based on a count
`FIG. 9 is a block diagram of another embodimentof a
`value for another vibrating beam in sensor 202. For
`system, indicated generally at 900, for providing enhanced
`example, when one beam produces a count that exceeds 21
`operation of a sensor 902. In the system 900, sensor 902 is
`and the other, complementary beam producesa value within
`coupled to electronic circuit 906 through enhancementcir-
`its acceptable range, e.g., 15, the output circuit determines a
`cuit 904. As in the other embodiments, enhancementcircuit
`value in the range from 17.5 to 21 that corresponds with the
`904 enhances the accuracy of the output of sensor 902 by
`value produced by the other beam, e.g., 20.
`suppressing outputs from sensor 902 that are not within the
`FIG.3 is a flow chart of one embodimentof a process for
`normal operating range of the sensor 902. In this embodi-
`enhancing the accuracy of a sensor during an anomalous
`ment, enhancement circuit 904 comprises filter 908. The
`event. The process begins at block 300. At block 302, the
`sensor 902 produces an output signal with a frequency that
`process determines a measure of the sensor output. In one
`is related to a measured stimulus. The sensor 902 produces
`embodiment, the process analyzes a square wave output. In
`an analog signal. The bandwidth offilter 908 is chosen such
`this embodiment, the process counts the number of leading
`that signals with frequencies within the normal operating
`edges in the output signal during a specified interval, e.g.,
`range of sensor 902 are passed and signals with other
`0.5 ms. At block 304, the process determines whether the
`frequencies are suppressed. A bandpassfilter, well known in
`measure of the sensor output falls in an expected range. In
`the state of the art, can be designed to pass only frequencies
`one embodiment,
`the expected range is determined by
`40
`
`physical and electrical characteristics of the sensor. For above a certain value and belowacertain, but higher value.
`example, with the RBA-500, the output range of the accel-
`For example, if the lowest and highest frequencies expected
`erometer is limited to a square wave with a frequency
`are 30 kHz and 40 kHz,a 6 db/octave bandpassfilter is used
`bounded between 35 KHz and 42 KHz. With this sensor, the
`to pass 30-40 kHz sinewaves unattenuated and frequencies
`range of expected counts is from 17.5 to 21 in a 0.5 ms
`outside this range would be highly attenuated. The subse-
`interval.
`quent processing and computations in electronic circuit 906,
`whether analog or digital, would only have realistic values
`to process, leading to greater accuracy.
`FIG. 10 is a block diagram of another embodiment of a
`system, indicated generally at 1000, for providing enhanced
`operation of a sensor 1002. In the system 1000, sensor 1002
`is coupled to electronic circuit 1006 through enhancement
`circuit 1004. As in the other embodiments, enhancement
`circuit 1004 enhances the accuracy of the output of sensor
`1002 by suppressing outputs from sensor 1002 that are not
`within the normal operating range of the sensor 1002. In this
`embodiment, enhancementcircuit 1004 comprises process-
`ing circuit 1008 and output circuit 1010. The sensor 1002
`produces an output signal with a frequency that is related to
`a measured stimulus. In one embodiment, the sensor 1002
`produces an analog signal and in other embodiments, sensor
`1002 produces a digital signal. Processing circuit 1008
`determines the frequency of the output of the sensor 1002.
`If the frequency is within the normal operating range of
`sensor 1002, then output circuit 1010 passes the signal to
`electronic circuit 1006. If, however, the processing circuit
`1008 determines that the frequency of the output signal is
`not within the normaloperating range, then the outputcircuit
`
`If the measure is within the range, the output is passed
`without correction at block 306. If, however, the measureis
`not within the range, the output is modified at block 308. In
`one embodiment, the output is replaced with a value that is
`within the expected range, e.g., a measure over 21 would be
`replaced with a measure of 17.5. In other embodiments, the
`measure is replaced with a value chosen by interpolation
`between values produced by the sensor that fall within the
`range. In yet further embodiments, the value is replaced with
`a value dependent on another output of the sensorthat falls
`within the range during the anomalousevent.
`FIG.4 is a graph thatillustrates an example of the output
`of a sensor with an anomalousevent. In the graph, the output
`of a sensor, e.g., the frequency of the output of an Accel-
`erex® RBA-500 accelerometer is plotted along the vertical
`axis. The horizontal axis represents the time at which the
`accelerometer reading was taken in milliseconds. As indi-
`cated at 400, an anomalous event occurs between 20 and 40
`milliseconds. This event is detectable because the frequency
`of the output of the sensor exceeds the nominal 35 to 42 KHz
`expected range.
`In FIG. 5,
`the output of the signal
`is
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`US 7,204,123 B2
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`7
`suppressesthe signal from sensor 1002. In one embodiment,
`a signal is substituted for the signal from sensor 1002. For
`example, a signal at a nominal operating point of the sensor
`1002 is forwardedto electronic circuit 1006 by output circuit
`1010. In other embodiments, an interpolated value is pro-
`vided by output circuit 1010 to electronic circuit 1006. In
`further embodiments, a value is passed by output circuit
`1010 to electronic circuit 1006 based on other data from
`sensor 1002.
`
`The methods and techniques described here may be
`implemented in digital electronic circuitry, or with a pro-
`grammable processor (for example, a special-purpose pro-
`cessor or a general-purpose processor such as a computer)
`firmware, software, or in combinations of them. Apparatus
`embodying these techniques may include appropriate input
`and output devices, a programmable processor, and a storage
`medium tangibly embodying program instructions
`for
`execution by the programmable processor. A process
`embodying these techniques may be performed by a pro-
`grammable processor executing a program of instructions to
`perform desired functions by operating on input data and
`generating appropriate output. The techniques may advan-
`tageously be implemented in one or more programsthat are
`executable on a programmable system including at least one
`programmable processor coupled to receive data and
`instructions from, and to transmit data and instructionsto, a
`data storage system, at least one input device, and at least
`one output device. Generally, a processor will receive
`instructions and data from a read-only memory and/or a
`random access memory. Storage devices suitable for tangi-
`bly embodying computer program instructions and data
`includeall forms of non-volatile memory, including by way
`of example semiconductor memory devices,
`such as
`EPROM, EEPROM,and flash memory devices; magnetic
`disks such as internal hard disks and removable disks;
`magneto-optical disks; and DVD disks. Any of the foregoing
`may be supplemented by, or incorporated in, specially-
`designed application-specific integrated circuits (ASICs).
`It is apparent that these techniques will work for any
`source of transients or noise on the signals, whether from
`shocks, vibration, electronic noise or other sources. There-
`fore, embodiments of this invention are useful for any device
`that has limited bandwidth that can define boundaries
`
`beyondwhich the data (frequencies, counts, etc.) are detect-
`able as invalid. It is particularly effective where there are
`multiple outputs which have a relationship with each other,
`such as the two square wave output signals in the RBA-500.
`A number of embodiments of the invention defined by the
`following claims have been described. Nevertheless, it will
`be understood that various modifications to the described
`
`embodiments may be made without departing from the
`scope of the claimed invention.
`In FIGS. 1-10, the exemplary embodiments have been
`described in terms of improving the accuracy of an accel-
`erometer. It is understoodthat this application is not limited
`to improving the accuracy of an accelerometer. The sensors
`102, 202, 902 and 1002 are implemented in other embodi-
`ments as other types of sensors. For example,
`in some
`embodiments, the sensors are implementedas strain sensors.
`In other embodiments,
`the sensors are implemented as
`micro-electro-mechanical systems (MEMS)sensors. Many
`MEMSaccelerometers and gyros have a very similar output
`to the Accelerex® RBA-500. In other embodiments,
`the
`sensors
`include piezoelectric
`sensors. Crystals, which
`acquire a charge when compressed, twisted or distorted are
`said to be piezoelectric. This provides a convenient trans-
`ducer effect between electrical and mechanical oscillations.
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`Quartz demonstrates this property and is extremely stable.
`Quartz crystals are used for watch crystals, precise fre-
`quency reference crystals for radio transmitters, precision
`accelerometers, etc. An oscillating electric field makes the
`quartz crystal resonate at its natural frequency. The vibra-
`tions of this frequency are counted and are used to keep the
`clock or watch on time. Barium titanate, lead zirconate, and
`lead titanate are ceramic materials which exhibit piezoelec-
`tricity and are used in ultrasonic transducers as well as
`microphones. If and electrical oscillation is applied to such
`ceramic wafers, they will respond with mechanical vibra-
`tions which provide the ultrasonic sound source. The stan-
`dard piezoelectric material for medical imaging processes
`has been lead zirconate titanate (PZT). Piezoelectric ceramic
`materials have found use in producing motions on the order
`of nanometers in the control of scanning tunneling micro-
`scopes. In other embodiments, the sensors are microphones.
`Microphones are used to convert acoustical energy into
`electrical energy. The microphone serves as an example of
`the idea that a specific purpose can be accomplished using
`many different physical principles.
`In other embodiments, the sensors are biopotential sen-
`sors. The surface recording electrode can be used to measure
`many different biopotentials. For, example, it can be used to
`measure electrical signals generated from the flexion and
`extension of the muscles. This signal is referred to as the
`electromyogram or EMG. This signal varies in frequency
`from approximately 50 Hz to 1000 Hz. Its amplitude varies
`from approximately 10 uV to 1 mV depending on properties
`such as the size of the muscle and the amountof exertion.
`Another common signal measured by electrodes is the
`electroencephalogram or EEG. This is the signal caused by
`neural activity in the brain. It contains frequencies from less
`than 1 Hz up to 50 Hz and amplitudes which are usually less
`than 10 uV.
`
`The exemplary embodiments described above also have
`focused on the electrical circuits 106, 206, 906 and 1006 as
`being guidance and navigation circuitry.It is understood that
`this description is provided by way of example and not by
`wayoflimitation. For example, other electrical systems may
`benefit from the improved sensor as described above. For
`example, systemsrelating to needle control in textile weav-
`ing,
`small-volume pumping devices, micro-positioning:
`machinery, cutting tools, mirrors, etc., stabilizing mechani-
`cal arrangements, fiber optics, vibration control, ultrasonic
`cleaners and welders, deep water hydrophones, medical
`probes, piezoelectric
`actuators,
`and toys/games. For
`example, a manufacturer has embedded piezoelectric mate-
`rials in skis in order to damp outthe vibrations of the skis
`and help keep the ski edges in contact with the snow. The
`piezoelectric material converts each mechanical vibration
`into an electric voltage, which is processed by a semicon-
`ductor electronic circuit. The circuit then sends a counter
`voltage to the piezoelectric material, which produces an
`opposing mechanical force to damp out the vibrations.
`Whatis claimedis:
`
`1. A system comprising:
`an accelerometer having an output;
`an enhancement circuit, coupled to the output of the
`accelerometer, the enhancementcircuit comprising:
`a counter that counts a number of leading edges of
`pulses in the output of the accelerometer over a
`selected period of time;
`a comparator, responsive to the counter, that determines
`when the count in the selected period of time falls
`outside a selected range of values; and
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