346
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`Sensors for Mobile Robots
`
`KHz) and current swing of the oscillator circuit (Kim & Hawks, 1989). The
`square-wave oscillator output of each of three identical channels is fed directly to
`the onboard microprocessor without the need for complicated interface circuitry.
`
`
`
`Figure 12-18. The TCM compass employs a three-axis strap-down magnetometer in conjunction
`with a two-axis tilt sensor to compensate for variations in vehicle attitude (courtesy Precision
`Navigation,Inc.).
`
`Automatic distortion-detection algorithms are incorporated that raise a warning
`flag when magnetic disturbances (i.e., close-proximity metallic objects or
`electrical cabling) are compromising compass accuracy. Pitch-and-roll outputs
`are available for external use with 0.1-degree resolution at an accuracy of +0.2
`degrees. Ambient temperature information is also provided over a range of -20 to
`+70°C, with an accuracy of +0.5 degrees. Both digital outputs (RS-232 or
`NMEA-0183) and linear quadrature analog outputs (0-5 volts) are standard.
`Yee
`Veo
`Sensor
`AN
`|
`Ry
`
`Coil sil=
`
`
`x
`
`
`
`;
`>—> Oulput
`
`Vee
`I
`
`R 1
`
`ole
`
`
`
`Rs
`
`Figure 12-19. Block diagram ofa single-axis sense element as implemented on the TCM compass
`(adapted from Kim & Hawks, 1989).
`
`Power requirements for the TCM compass are 5 to 25 volts DC at 6 to 12
`milliamps, depending on user configuration. The OEM circuit board measures 2.5
`
`
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`
`

`

`Chapter 12 Magnetic Compasses
`
`347
`
`by 2 inches wide by 1.1 inches high, weighs 1.6 ounces, and withatilt range of
`+25 degrees costs only $700.
`(Additionaltilt ranges of 60 and 90 degrees are also
`available at slightly higher cost.) The moderate price, extremely low power
`consumption, elimination of gimbal-induced measurement errors, small size and
`weight, plus availability of pitch, roll, and ambient temperature outputs make the
`TCM-Series
`a
`strong contender
`for mobile
`robotic
`applications.
`Field
`performance evaluations are currently underway for early prototypes installed on
`both ROBARTIII and the MDARSInterior robot (Chapter 1).
`tilt
`An extremely low-cost
`($50)
`two-axis electronic compass without
`compensation, the Vector-2X, is available as well for less demanding applications,
`with an overall accuracy of +2 degrees.
`
`12.4 Hall-Effect Compasses
`
`Recall from Section 3.2.1 that Hall-effect sensors in the presence of an external
`magnetic field develop a DC voltage across a semiconductor region that
`is
`proportional to the magnetic field component at right angles to the direction of
`current flow (Wood, 1986). One advantage of this technology (i.e., relative to the
`fluxgate) is the inherentability to directly sense a static flux, resulting in much
`simpler readout electronics.
`Early Hall magnetometers could not match the
`sensitivity and stability of the fluxgate (Primdahl, 1979), but the sensitivity of
`Hall devices has improved significantly. The more recent indium-antimonidide
`devices have a lowersensitivity limit of 10° Gauss (Lenz, 1990).
`The Navy in the early 1960s showed considerable interest in a small solid-state
`Hall-effect compass for low-power extended operations in sonobuoys (Wiley,
`1964). A number of such prototypes were built and delivered by Motorola for
`evaluation. The Motorola compass employed two orthogonal Hall-effect devices
`for temperature-nulled non-ambiguousresolution of the geomagnetic field vector.
`Each sensor element was fabricated from a 2- by 2- by 0.1-millimeter indium-
`arsenide-ferrite sandwich and inserted between two wing-like mumetal
`flux
`concentrators as shown in Figure 12-20.
`It
`is estimated the 2-inch magnetic
`concentrators increased the flux density through the sensing elements by two
`orders of magnitude (Wiley, 1964). The output of the Motorola unit was a
`variable-width pulse train, the width of the pulse being proportional to the sensed
`Magnetic heading.
`Excellent response linearity was reported down to flux
`densities of 0.01 Gauss (Wiley, 1962).
`Maenaka, et al. (1990) report on the development of a monolithic silicon
`magnetic compass at the Toyohashi University of Technology in Japan, based on
`two orthogonal Hall-effect sensors. Their use of the terminology “magnetic
`compass” is perhaps an unfortunate misnomer, in that the prototype device was
`tested with an external field of 1,000 Gauss. Contrast this field strength with that
`of the earth’s magnetic field, which varies from only about 0.1 Gauss at
`the
`equator to about 0.9 Gauss at the poles. Silicon-based Hall-effect sensors have a
`
`
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`
`

`

`348
`
`Sensors for Mobile Robots
`
`lowersensitivity limit of around 10 Gauss (Lenz, 1990). It is likely the Toyohashi
`University device was intended for other than geomagnetic applications, such as
`remote position sensing of rotating mechanical assemblies.
`
`
`
`re
`\
`
`‘
`
`Fe
`
` |
`
`Indiurn: Sa
`Arsenide
`
`|«—t— Indium
`Arsenide
`
`
`
`
`
`Fe
`=
`Fe
`
`Figure 12-20. A pair of indium-arsenide-ferrite Hall-effect sensors (one shown) are positioned
`between flux concentrating wings of mumeral in this early Motorola prototype (adapted from
`Wiley, 1964).
`
`This prototype Hall-effect magnetometeris still of interest in that it represents
`one ofthe first fully self-contained implementations of a two-axis magnetometer
`in integrated circuit form. Two vertical Hall cells (Maenaka, et al, 1987) are
`arranged at right angles as shown in Figure 12-21 on a 4.7-millimeter square chip,
`with their respective outputs coupled to a companion signal processing IC of
`identical size.
`(Two separate chips were fabricated for the prototype instead of a
`single integrated unit
`to enhance production yield.) The sensor and signal
`processing ICs are interconnected (along with some external variable resistors for
`calibration purposes) on a glass-epoxyprinted circuit board.
`
`J|
`
`
`
`
`
`
`
`
`
`Figure 12-21. Twovertical Hall cells are arranged at right angles on a 4.7-millimeter-square chip
`in this two-axis magnetometer developed at the Toyohashi University of Technology in Japan
`(adapted from Maenaka,etal., 1990).
`
`The dedicated signal-processing circuitry converts the B-field components B,
`and B, measured by the Hall sensors into an angle 8 by means of the analog
`operation (Maenaka,et al, 1990):
`
`
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`
`

`

`Chapter 12 Magnetic Compasses
`
`349
`
`x
`
`6 =arctan
`
`where:
`
`@ = angle between B-field axis and sensor
`B, = x-component ofB field
`By = y-componentofB field.
`
`The analog output of the signal-processing IC is a DC voltage that varies
`linearly with vector orientation of the ambient magnetic field in a plane parallel to
`the chip surface. Reported test results show a fairly straight-line response (i.e., +2
`percentfull scale) for external field strengths ranging from 8,000 Gauss down to
`500 Gauss; below this leyel performance begins to degrade rapidly (Maenaka,et
`al., 1990). A second analog output on the IC provides an indication of the
`absolute value offield intensity.
`While the Toyohashi “magnetic compass” prototype based on silicon Hall-
`effect
`technology is incapable of detecting the earth’s magnetic field,
`it
`is
`noteworthy nonetheless,
`A two-axis monolithic device of similar nature
`employing the more sensitive indium-antimonide Hall devices could potentially
`have broad appeal for low-cost applications on mobile robotic platforms. For
`increased sensitivity, an alternative possibility would be to use magnetoresistive
`sensor elements, to be discussed in the next section.
`
`12.5 Magnetoresistive Compasses
`
`The general theory of operation for anisotropic magnetoresistive (AMR) and giant
`magnetoresistive (GMR) sensors as used in short-range proximity detection was
`previously presented in Chapter 3. Recall three properties of the magnetoresistive
`magnetometer make it well suited for application as a geomagnetic sensor:
`1)
`high sensitivity,
`2) directionality, and
`3)
`in the case of AMR sensors, the
`characteristic
`“flipping”
`action associated with the direction of
`internal
`magnetization.
`AMR sensors have an open-loop sensitivity range of 10° to 50 Gauss (which
`easily covers the Q.1- to 1.0-Gauss range of the earth’s horizontal magnetic field
`component), and limited-bandwidth closed-loop sensitivities approaching 10°
`Gauss (Lenz, 1990), Excellent sensitivity, low power consumption, small package
`size, and decreasing cost make both AMR and GMR sensors increasingly popular
`alternatives to the more conventional fluxgate designs used in robotic vehicle
`applications.
`
`
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`

`350
`
`Sensors for Mobile Robots
`
`12.5.1 Philips AMR Compass
`
`Oneof the earliest magnetoresistive sensors to be applied to a magnetic compass
`application is
`the KMZIOB offered by Philips Semiconductors BV, The
`Netherlands (Dibburn & Petersen, 1983; Kwiatkowski & Tumanski, 1986;
`Petersen, 1989).
`The limited sensitivity of this device (approximately 0.1
`mV/A/m with a supply voltage of SV DC) in comparison to the earth’s maximum
`horizontal magnetic field (15 A/m) means that considerable attention must be
`given to the error-inducing effects of temperature and offset drift (Petersen, 1989).
`One way around these problems is to exploit
`the “flipping” phenomenon
`(Chapter 3) by driving the device back and forth between its two possible
`magnetization states with square-wave excitation pulses applied to an external
`coil (Figure 12-22). This switching action toggles the sensor’s axial magnetic
`field as
`shown in Figure 12-22A,
`resulting in the alternating response
`characteristics depicted in Figure 12-22B.
`Since the sensor offset
`remains
`unchanged while the signal output due to the external magnetic field H,
`is
`inverted (Figure 12-22A),
`the undesirable DC offset voltages can be easily
`isolated from the weak ACsignal.
`
` Magnetizing Current Magnetization
`
`
`
`
`Figure 12-22. External current pulses set and reset the direction of magnetization, resulting in the
`"flipped" response characteristics shown by the dashed line. Note the DC offset of the device
`remains constant, while the signal output is inverted (adapted from Petersen, 1989).
`
`A typical implementation of this strategy is shown in Figure 12-23. A 100-Hz
`square-wave generator is capacitively coupled to the external excitation coil L
`which surrounds two orthogonally mounted magnetoresistive sensors.
`The
`sensors’ output signals are amplified and AC-coupled to a synchronous detector
`driven by the same square-wave source. The rectified DC voltages Vy; and Vy2
`are thus proportional to the measured magnetic field components H; and Hp.
`Determination of applied field direction is dependent on the ratio as opposed to
`absolute values of these output signals, and so as long as the two channels are
`calibrated to the same sensitivity, no temperature correction is required (Fraden,
`1993).
`
`
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`
`

`

`Chapter 12 Magnetic Compasses
`
`351
`
`I
`
`juore—waove
`nerolor
`
`
`
`
`
`
`
`
`
`p> Hm}.
`
`
`
`
`
`——l
`ss
`Amplifier
`
`Synchronous
`Bete
`
`
`
`DH}.
`
`Figure 12-23. Block diagram of a two-axis magnetic compass system based on a commercially
`available anisotropic magnetoresistive sensor such as the Philips KMZ/0B (Petersen, 1989).
`
`12.5.2 Space Electronics AMR Compass
`
`The Space Electronics Micro-Mag sensor introduced in Chapter 3 (SEI, 1994;
`Lao, 1994) can be configured as shown in Figure 12-24 to function as an
`anisotropic magnetoresistive (AMR) compass. The integral 350-ohm temperature
`compensation resistor (RTD) is connected in the lower arm of a Wheatstone
`bridge in series with a 100-ohm 10-turn trimming resistor.
`Two identical
`channels are required, with their associated AMR sensors mounted in an
`orthogonal fashion to yield output voltages proportional to the sine and cosine of
`magnetic field azimuth.
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`“WNSIO1
`Sensor
`{tow
`Pass
`
`|FilterJ
`Figure 12-24. Typical application circuit for the SEI MMSJ0/ MicroMag that provides an output
`voltage proportional to the cosine of magnetic azimuth for a gimbaled sensor in the horizontal
`plane (courtesy Space Electronics, Inc.).
`
`
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`
`

`

`352
`
`Sensors for Mobile Robots
`
`12.5.3 Honeywell HMR Series Smart Digital Magnetometer
`
`The Honeywell Magnetoresistive (HMR) Series of magnetometers incorporates
`three orthogonal sensor axes, each consisting of a permalloy thin-film Wheatstone
`bridge configuration deposited on a silicon substrate as discussed in Chapter 3
`(Honeywell, 1994b). Changes in bridge resistance are converted to a digital
`output signal (prespecified RS-232 or RS-485) by internal A/D converters and a
`dedicated microprocessor, with 12-bit output resolution (11 bits plus sign), A
`switching technique is employed to “flip” the sensor characteristics back and forth
`between the two possible magnetic states (see again Chapter 3), thus canceling the
`DC offset and past magnetic history of the permalloy bridges, in addition to any
`offset
`introduced by the sensor electronics (Honeywell, 1994a). The unit
`is
`packaged in a compact rectangular enclosure measuring 1.12 by 1.75 by 3 inches
`as shownin Figure 12-25.
`
`
`
`is a three-axis
`The Honeywell HMR-Series Smart Digital Magnetometer
`Figure 12-25.
`magnetoresistive magnetometer with a sensitivity of | milliGauss over a measurement range of +1
`Gauss (courtesy HoneywellSolid State Electronics Center).
`
`Output values for the three axes (X, Y, and Z) are transmitted in two-byte
`hexadecimal format upon request from the external host processor, where they can
`be combined with externally supplied information regarding vehicle attitude to
`calculate a tilt-compensated magnetic heading solution. At 38.4 kilobaud, the
`maximum update rate is 54 Hz. The current bridge temperature reading is also
`made available with 8-bit resolution. The magnetometer has a measurement range
`of +1 Gauss (each axis) with a sensitivity level of 1 milliGauss and provides a
`digital resolution of 0.5 milliGauss per least-significant bit. Overall accuracy is
`+1 percent of full scale. Power requirements are 12 to 15 volts DC (single supply)
`at 40 milliamps. An HMR Series Development Kit is now available from the
`Honeywell Solid State Electronics Center, Plymouth, MN,
`that
`includes the
`
`
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`

`

`Chapter 12 Magnetic Compasses
`
`353
`
`magnetometer, power supply, cabling, operating manual, and IBM-compatible PC
`software.
`
`12.6 Magnetoelastic Compasses
`
`A number of researchers have recently investigated the use of magnetoelastic
`(also known as magnetostrictive) materials as
`sensing elements for high-
`resolution magnetometers. The principle of operation is based on the changes in
`Young’s modulus experienced by magnetic alloys when exposed to an external
`magnetic field. The modulus of elasticity E of a given material is basically a
`measure ofits stiffness, and directly relates stress to strain as follows:
`o
`E=—
`E
`
`where:
`
`E = Young’s modulus ofelasticity
`© = applied stress
`€ = resulting strain.
`
`Any ferromagnetic material will experience some finite amount of strain
`(expansion or shrinkage)
`in the direction of magnetization due
`to this
`magnetostriction phenomenon,
`It stands to reason that if the applied stress o
`remains the same, strain € will vary inversely with any change in Young’s
`modulus £. In certain amorphous metallic alloys, this effect is very pronounced.
`Barrett, et al. (1973) propose a qualitative explanation, wherein individual
`atoms in the crystal lattice are treated as tiny magnetic dipoles. The forces exerted
`by these dipoles on one another depend upon their mutual orientation within the
`lattice; if the dipoles are aligned end to end, the opposite poles attract, and the
`material shrinks ever so slightly. The crystal
`is said to exhibit a negative
`magnetostriction constant in this direction. Conversely, if the dipoles are rotated
`into side-by-side alignment through the influence of some external field,
`like
`poles will repel, and the result is a small expansion.
`It follows the strength of an unknown magnetic field can be accurately
`measured if suitable means is employed to quantify the resulting change in length
`of some appropriate material displaying a high magnetostriction constant. There
`are currently at least two measurement technologies with the required resolution
`allowing the magnetoelastic magnetometer to be a realistic contender for high-
`sensitivity low-cost performance: 1) interferometric displacement sensing and 2)
`tunneling-tip displacement sensing.
`Lenz (1990) describes a magnetoelastic magnetometer which employs a Mach-
`Zender
`fiber-optic interferometer
`to measure the change in length of
`a
`magnetostrictive material when exposed to an external magnetic field. A laser
`
`
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`

`354
`
`Sensors for Mobile Robots
`
`source directs a beam oflight along two optical fiber paths by way of a beam
`splitter as shown in Figure 12-26. One of the fibers is coated with a material
`(nickel iron was used) exhibiting a high magnetostrictive constant. The length of
`this
`fiber
`therefore is
`stretched or compressed in conjunction with any
`magnetoelastic expansion or contraction of its coating. The output beam from
`this fiber-optic cable is combined in a light coupler with the output beam from the
`uncoated reference fiber and fed to a pair of photodetectors.
`
`Optica
`= Fiber ~-~§——
`Laser
`Diode
`
`
`
`Sensing Leg
`
`
`
`WW
` _ ight Coupler = Phatodetectors
`I
`
`ae
`
`___
`
`Reference Leg
`
`fi
`
`Figure 12-26. Fiber-optic magnetometers, basically a Mach-Zenderinterferometer with one fiber
`coated or attached to a magnetoelastic material, have a sensitivity range of 107 to 10 Gauss
`(adapted from Lenz, 1990, ® IEEE).
`
`Constructive and destructive interferences caused by differences in path lengths
`associated with the two fibers will cause the final output intensity as measured by
`the photodetectors to vary accordingly. This variation is directly related to the
`change in path length of the coated fiber, which in turn is a function of the
`magnetic field strength along the fiber axis. The prototype constructed by Lenz
`(1990) at Honeywell Corporation measured 4 inches long by | inch wide and was
`able to detectfields ranging from 10” Gauss up to 10 Gauss.
`Cantilever
`
`
`
` Surface
`Figure 12-27, Scanning tunneling microscopy, invented at IBM Zurich in 1982, uses quantum
`mechanical
`tunneling of electrons across a barrier to measure separation distance at the gap
`(courtesy T.W. Kenny, NASA JPL).
`
`the Naval Research Laboratory (NRL) have developed a
`Researchers at
`prototype magnetoelastic magnetometer capable of detecting a field as small as 6
`x 10° Gauss using the tunneling-tip-transducer approach (Brizzolara, et al.,
`1989). This new displacement sensing technology, invented in 1982 at IBM
`Zurich, is based on the measurement of current generated by quantum mechanical
`tunneling of electrons across a narrow gap (Figure 12-27). An analog feedback
`circuit compares the measured tunnel current with a desired setpoint and outputs a
`drive signal to suitably adjust the distance between the tunneling electrodes with
`an electromechanical actuator (Kenny, et al., 1991). The instantaneous tunneling
`
`
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`
`

`

`
`
`
`
`
`
`=]
`
`Feedback
`Flectranics
`
`Leeroy
`Seape
`
`Soleneid Coils
`
`
`
`
`Quartz Tube
`
`Chapter 12 Magnetic Compasses
`
`355
`
`currentis directly proportional to the exponential of electrode displacement. The
`most common actuators employed in this role are piezoelectric and electrostatic,
`the latter lending itself more readily to silicon micromachining techniques.
`The active sense element
`in the NRL magnetometer is a 10-centimeter
`metallic-glass ribbon made from METGLAS 260582, annealed in a transverse
`magnetic field to yield a high magnetomechanical coupling (Brizzolara,et al.,
`1989). The magnetoelastic ribbon elongates when exposed to an axial magnetic
`field, and the magnitude of this displacement
`is measured by a tunneling
`transducerasillustrated in Figure 12-28.
`
`
`Approach
`Mehran
`
`Tunneling
`Tp
`
`Mognetostrictive
`iRibbon
`
`
`
`The NRL tunneling-transducer magnetometer employed a 10-centimeter
`Figure 12-28.
`magnetoelastic ribbon vertically supported in a quartz tube (Brizzolara, et al., 1989).
`
`An electrochemically etched gold tip is mounted on a tubular piezoelectric
`actuator and positioned within about
`| nanometer of the free end of the
`METGLASribbon. The ribbon andtip are electrically biased with respect to each
`other, establishing a tunneling current that is fed back to the piezo actuator to
`maintain a constant gap separation.
`The degree of magnetically induced
`elongation of the ribbon can thus be inferred from the driving voltage applied to
`the piezoelectric actuator. The solenoidal coil shown in the diagram supplies a
`bias field of 0.85 oersted to shift
`the sensor into its region of maximum
`sensitivity.
`The NRL group in collaboration with the Jet Propulsion Laboratory, Pasadena,
`CA, has more recently developed an alternative magnetic sensor that uses a
`tunneling transducer to measure the induced torque on a suspended magnet due to
`low-frequency field changes (DiLella, et al., 1995). The sensor consists of two
`
`micromachined a_structure measuringsilicon wafers assembled into
`
`
`
`approximately |
`inch by 1
`inch by 0.1 inch (Figure 12-29), The upper wafer
`includes a permanent magnet attached to a rectangular support suspended from a
`pair of torsion beams. The underside of the magnet faces the tunneling tip and
`serves as both the tunneling counter electrode and one of two rotation control
`electrodes. The lower componentconsists of the other rotation control electrode
`and the tunneling tip as illustrated below.
`
`
`SilverStar Exhibit 1016 - 370
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`
`

`

`356
`
`Sensors for Mobile Robots
`
`Torsion Beam
`
`Deflection
`
`Electrode
`
`Figure 12-29. Cross-sectional diagram of the NRL/JPL micromachined magnetic-field sensor
`based on an electron-tunneling displacement transducer (courtesy Naval Research Lab).
`
`Because of the offset placement of the lower rotation control electrode with
`respect to the longitudinal axis of the torsion beams, an electrostatic torque is
`generated by the voltage difference between the electrodes, rotating the magnet
`assembly into tunneling range of the tip. This electrostatic torque about the
`torsion-beam axis is balanced by the resulting torsional stress in the beams and a
`magnetically induced torque generated by the ambient magnetic field acting upon
`the permanent-magnet dipole. Once the desired tunneling current is established
`and maintained by a simple feedback control circuit, any subsequent change in
`electrode voltage can be attributed to variations in the ambient magnetic field,
`The calculated sensitivity limit of this sensor configuration based on fundamental
`noise sources is 0.002 nT/VHz at
`1 Hz, while the actual measured sensitivity of
`the prototype is 0.3 nT/VHz at 1 Hz (DiLella, et al., 1995),
`Fenn,et al. (1992) propose yet another tunneling magnetoelastic configuration
`with a predicted sensitivity of 2 x 10°'' Gauss, along the same order of magnitude
`as the cryogenically cooled SQUID. A small cantilevered beam of METGLAS
`260552, excited at its resonant frequency by a gold-film electrostatic actuator, is
`centered between two high-permeability magnetic flux concentrators as illustrated
`in Figure 12-30. Any changes in the modulus of elasticity of the beam will
`directly affect its natural frequency;
`these changes in natura! frequency can then
`be measured and directly related to the strength of the ambient magnetic field.
`The effective shift in natural frequency is rather small, however (Fenn reported
`only a 6-Hz shift at saturation), again necessitating a very precise method of
`measurement,
`
`=
`:
`METGLAS
`Q.7mm
`P
`Cantilever
`
`
`|
`|
`
`
`
`
`Ins | == Guide
`SS hs libs
`|
`Guide
`
`1
`OP
`|
`
`
`
`
`1 or Sem
`7
`Figure 12-30. Top view of the single cantilevered design (adapted from Fenn, et al., 1992)
`
`ra
`
`Flux
`
`Flux
`
`Substrate
`
`
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`

`Chapter 12. Magnetic Compasses
`
`357
`
`is employed to track the
`A second (non-magnetic) cantilever element
`displacement of the METGLASreed with subangstrom resolution using tunneling-
`transducer displacement sensing as illustrated in Figure 12-31.
`A pair of
`electrostatic actuator plates dynamically positions the reed follower to maintain
`constant
`tunneling current
`in the probe gap,
`thus ensuring a constant
`lateral
`separation between the probe tip and the vibrating reed. The frequency of the
`excitation signal applied to the reed-follower actuator
`is
`therefore directly
`influenced by any resonant frequency changes occurring in the METGLAS reed.
`The magnetometer provides an analog voltage output which is proportional to this
`excitation frequency, and therefore indicative of external magnetic field
`amplitude.
`
`
` Excitation
`
`
`BERNRITRI
`
`Actuator
`
`
`
`
`
`METGLAS Reed
`
`
`
`
`
`
`
`
`oy
`i>ROYCOG.RA sis pow
`LeP
`Lo KARes vs CUGE
`Figure 12-31. Side view of the double cantilevered design (adapted from Fenn,et al., 1992).
`
`~ Tunneling—Tip
`Cantelever
`
`
`
`Oneanticipated problem associated with such magnetoelastic devices is that
`changes in Young’s modulus also occur due to temperature shifts. Fenn, et al.
`(1992) report a 1-Hz bandwidth sensor would require a temperature stability of
`10’°K during the measurement period and suggest thermal
`isolation using a
`vacuum jacket and multilayer insulation.
`
`12.7 References
`
`Acuna, M.H., Pellerin, C.J., “A Miniature Two-Axis Fluxgate Magnetometer,”
`IEEE Transactions on Geoscience Electronics, Vol. GE-7, pp. 252-260, 1969
`Barrett, C.R., Nix, W.D., Tetelman, A.S., The Principles of Engineering
`Materials, Prentice Hall, Englewood Cliffs, NJ, 1973.
`Bolz, R.E., Tuve, G.L., eds., CRC Handbook of Tables for Applied Engineering
`Science, CRC Press, Boca Raton, FL, 1979.
`Bozorth, R.M., Chapin, D.M., “Demagnetizing Factors of Rods,” Journal of
`Applied Physics, Vol. 13, pp. 320-326, May, 1942.
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`Magnetometer,” Sensors and Actuators, Vol. 20, pp. 199-205, 1989.
`
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`SilverStar Exhibit 1016 - 372
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`358
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`Sensors for Mobile Robots
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`Carlson, A.B., Gisser, D.G., Electrical Engineering: Concepts and Applications,
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`SilverStar Exhibit 1016 - 373
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`Chapter 12 Magnetic Compasses
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`SilverStar Exhibit 1016 - 374
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`360
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`
`SilverStar Exhibit 1016 - 375
`SilverStar Exhibit 1016 - 375
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`

`

`13
`Gyroscopes
`
`insensitive to the electromagnetic and
`the most part
`Gyroscopes are for
`ferromagnetic anomalies that affect the accuracy of compasses and are particularly
`useful in applications where there is no geomagnetic field present at all (i.c., deep
`space), or in situations where the local field is disturbed. Two broad categories of
`gyroscopes will be discussed:
`1) mechanical gyroscopes and
`2) optical
`gyroscopes.
`Mechanical gyroscopes operate by sensing the change in direction of some
`actively sustained angular or linear momentum, which in either case can be
`continuousor oscillatory in nature (Cochin, 1963). Probably the most well-known
`mechanical configuration is the flywheel gyroscope, a reliable orientation sensor
`based on the inertial properties of a rapidly spinning rotor, first demonstrated in
`1810 by G.C. Bohnenberger of Germany.
`In 1852, the French physicist Leon
`Foucault showed that such a gyroscope could detect
`the rotation of the earth
`(Carter, 1966). More recently there has been considerable interest shown in a
`number of new products classified as vibrating structure gyroscopes earmarked
`for applications in stabilized camera optics,
`robotics, an