U.S. patent application number 13/696262 was filed with the patent office on 2013-08-15 for detection of a metal or a magnetic object.
This patent application is currently assigned to Robert Bosch GmbH. The applicant listed for this patent is Andrej Albrecht, Tobias Zibold. Invention is credited to Andrej Albrecht, Tobias Zibold.
Application Number | 20130207648 13/696262 |
Document ID | / |
Family ID | 44626148 |
Filed Date | 2013-08-15 |
United States Patent
Application |
20130207648 |
Kind Code |
A1 |
Zibold; Tobias ; et
al. |
August 15, 2013 |
Detection of a Metal or a Magnetic Object
Abstract
A measuring apparatus for detecting a metal object includes two
emission coils, a magnetoresistive measuring device, and a control
device. The emission coils are configured to produce superimposed
magnetic fields. The magnetoresistive measuring device is in the
region of both magnetic fields, and is configured to emit an output
signal which is dependent on the magnetic field. The control device
is configured to supply the emission coils with alternating
voltages such that the value of the alternating voltage component
of the output signal, which is time synchronized with the
alternating voltages, is minimized. The control device is further
configured to detect the object when the ratio of the alternating
voltages does not correspond to the distances between the
magnetoresistive measuring device and the emission coils.
Inventors: |
Zibold; Tobias; (Stuttgart,
DE) ; Albrecht; Andrej; (Stuttgart, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Zibold; Tobias
Albrecht; Andrej |
Stuttgart
Stuttgart |
|
DE
DE |
|
|
Assignee: |
Robert Bosch GmbH
Stuttgart
DE
|
Family ID: |
44626148 |
Appl. No.: |
13/696262 |
Filed: |
April 15, 2011 |
PCT Filed: |
April 15, 2011 |
PCT NO: |
PCT/EP2011/056023 |
371 Date: |
February 18, 2013 |
Current U.S.
Class: |
324/232 |
Current CPC
Class: |
G01R 33/091 20130101;
G01V 3/104 20130101; G01R 33/0029 20130101; G01V 3/107
20130101 |
Class at
Publication: |
324/232 |
International
Class: |
G01R 33/09 20060101
G01R033/09 |
Foreign Application Data
Date |
Code |
Application Number |
May 7, 2010 |
DE |
102010028721.0 |
Jul 9, 2010 |
DE |
102010031142.1 |
Claims
1. A measuring apparatus for detecting a metallic object,
comprising: two transmitting coils configured to generate
superimposed magnetic fields; a magnetoresistive measuring device
(i) in the region of the two magnetic fields, and (ii) configured
to provide an output signal dependent on the magnetic field; and a
control device configured to supply the transmitting coils with
alternating voltages in such a way that a magnitude of an AC
voltage component, which is synchronous with the alternating
voltages, of the output signal of the magnetoresistive measuring
device is minimized, wherein the control device is configured to
detect the metallic object if a ratio of the alternating voltages
does not correspond to a ratio of distances of the magnetoresistive
measuring device from the transmitting coils.
2. The measuring apparatus as claimed in claim 1, wherein the
alternating voltages are AC voltages that are phase-shifted to one
another, in order to change the magnitude and phase of the magnetic
fields of the transmitting coils periodically.
3. The measuring apparatus as claimed in claim 1, further
comprising: a plurality of sensors spaced apart from one another
for magnetic field determination, wherein the plurality of sensors
are aligned with one another and connected to one another in such a
way that output signals from the sensors add up to zero when the
magnetic fields at the sensors are equal, and wherein main field
directions of the transmitting coils and preferred directions of
the sensors are essentially parallel to one another.
4. The measuring apparatus as claimed in claim 3, wherein the
transmitting coils lie on top of each other in layers parallel to
one another.
5. The measuring apparatus as claimed in claim 3, wherein one of
the sensors of the plurality of sensors is surrounded by one of the
transmitting coils and another of the sensors of the plurality of
sensors lies outside the transmitting coils.
6. The measuring apparatus as claimed in claim 1, further
comprising: a sensor for determining a magnetic field gradient,
wherein main field directions of the transmitting coils run
parallel to each other, and wherein a preferred direction of the
sensor runs essentially perpendicular or parallel to the main field
directions.
7. The measuring apparatus as claimed in claim 6, wherein: the
transmitting coils are arranged essentially next to one another in
a layer, and the preferred direction of the sensor runs essentially
parallel to this layer.
8. The measuring apparatus as claimed in claim 6, wherein: the
transmitting coils are essentially D-shaped, with the backs of the
D-shapes facing one another, and the sensor is arranged between the
backs of the D-shapes.
9. The measuring apparatus as claimed in claim 7, wherein: the
sensor is arranged essentially in the layer of the transmitting
coils, another sensor is provided in a layer parallel to this
layer, and preferred directions of the sensor and the other sensor
are perpendicular or parallel to one other.
10. A method for detecting a metallic object, comprising: supplying
two transmitting coils with alternating voltages in order to
generate superimposed magnetic fields; determining an output
signal, which is dependent on magnetic field, of a magnetoresistive
measuring apparatus in the region of the superimposed magnetic
fields; wherein the supplying the transmitting coils with
alternating voltages takes place in such a way that the magnitude
of an AC voltage component, which is synchronous with the
alternating voltages, of the output signal of the magnetoresistive
measuring device is minimized; and detecting the metallic object if
the ratio of the alternating voltages does not correspond to a
ratio of the distances of the magnetoresistive measuring device
from the transmitting coils.
11. The method of claim 10, wherein a computer program product
includes a program code means configured to perform the method if
the computer program product runs on a processing device or is
stored on a computer-readable data carrier.
Description
[0001] When performing certain types of work on workpieces, there
is a risk that an object concealed in the workpiece may be damaged
by the work. For example, when drilling into a wall, a water pipe,
electrical cable, or gas line running inside the wall may be
damaged. On the other hand, it may be desirable to perform the work
in such a precise manner that work is also performed on an object
concealed in the workpiece, for example, if the hole from the above
example is to run through an iron reinforcement or a supporting
structure inside the wall.
BACKGROUND OF THE INVENTION
[0002] Coil-based metal detectors for detecting such a concealed
object are known in the art. Such detectors generate a magnetic
field in a measurement region. If a metallic object is in the
measurement region, the object is detected because of its influence
on the generated magnetic field. To determine the generated
magnetic field, at least two receiving coils are often used, which
are oriented and connected to one another in such a way that the
measurement signal provided jointly by both receiving coils
approaches zero in the absence of a metallic object in the
measurement region (differential measurement). In one variant, a
plurality of transmitting coils is used to generate the magnetic
field, the coils being activated in such a way that the measured
signal in the two receiving coils approaches zero, independently of
the presence of a metallic object in the measurement region
(field-compensated measurement).
[0003] DE 10 2007 053 881 A1 discloses a measurement method for
determining the position or the angle of a coil with respect to two
other coils. In order to do this, an alternating magnetic field is
generated by means of two transmitting coils arranged at an angle
to one another. A receiving coil is brought into the alternating
magnetic field and the activation of the transmitting coils is
changed such that the same voltage is induced in the receiving coil
by each of the transmitting coils. A ratio of current values
supplied to the transmitting coils provides a measure of a
determination of a position and/or angle of the receiving coil with
respect to the transmitting coils.
[0004] DE 10 2004 047 189 A1 discloses a metal detector having
printed coils.
[0005] The object of the invention is to provide a simple and
accurate detector for a metallic object. An additional object of
the invention is to specify a method for determining the metallic
object.
DISCLOSURE OF THE INVENTION
[0006] The invention achieves these objects by means of a measuring
apparatus having the features of claim 1 and a method having the
features of claim 10. Dependent claims provide preferred
embodiments.
[0007] A measuring apparatus for detecting a metallic object
comprises two transmitting coils for generating superimposed
magnetic fields, a magnetoresistive measuring device, in
particular, a measuring device having Hall sensors, in the region
of the two magnetic fields for providing an output signal dependent
on the magnetic field, and a control device for supplying the
transmitting coils with alternating voltages in such a way that the
magnitude of an AC voltage component, which is synchronous with the
alternating voltages, of the output signal of the measuring device
is minimized. The control device is adapted to detect the object if
the ratio of the alternating voltages does not correspond to the
ratio of the distances of the measuring device from the
transmitting coils.
[0008] The measuring apparatus can perform a field-compensated and
differential measurement and thereby provide an exact measurement
result that is resistant to interference. In addition,
magnetoresistive measuring devices can be used, which are
substantially smaller than conventional coils for determining
magnetic fields. This makes possible a highly compact construction
of the measuring apparatus and highly integrated measuring
arrangements in close physical proximity. Unlike coils,
magnetoresistive sensors measure the magnetic field and not the
time-based change in the magnetic flux. When generating alternating
fields by means of square-wave signals, this has the advantage of
allowing the influence of the object to be measured over the entire
duration of the half-cycle of the square-wave excitation, instead
of only over the short period of field change in the slope region.
In this way, it is possible to increase measurement accuracy.
[0009] The alternating voltages are preferably AC voltages that are
phase-shifted to one another, preferably phase-shifted by
180.degree., in order to change the magnitude and phase of the
magnetic fields of the transmitting coils periodically. The AC
voltages enable synchronous demodulation, which makes it possible
to suppress interfering signals having frequencies unequal to the
modulation frequency in a highly effective manner. In addition, it
is possible to generate alternating magnetic fields via the AC
voltages in order to induce eddy currents in non-magnetic materials
such as copper, with which they can then be detected.
[0010] In a first variant, the measuring device can comprise a
plurality of sensors spaced apart from one another for magnetic
field determination, with the sensors being aligned with one
another and connected to one another in such a way that output
signals from the sensors add up to zero when the magnetic fields at
the sensors are equal, and main field directions of the
transmitting coils and preferred directions of the sensors are
parallel to one another. For example, Hall sensors oriented
antiparallel can be used in a series connection in order to
determine a resulting magnetic field in the region of the two
transmitting coils economically and precisely.
[0011] The sensors can have preferred directions that run parallel
to one another, and the signals from the sensors can be subtracted
from one another. Alternatively to this, the preferred directions
of the sensors can be aligned antiparallel, and the signals from
the sensors can be added to one another. A differential amplifier
can be used for addition or subtraction, or the sensors can be
correspondingly connected to one another.
[0012] In principle, any kind of sensor that determines a magnetic
field is suitable. Such sensors can have small dimensions so that
the measuring device can be miniaturized. A spatial resolution can
thus be increased in an embodiment up to a graphically
representable range.
[0013] The transmitting coils lie advantageously on top of each
other in layers parallel to one another, thus facilitating a
matrix-like arrangement of a plurality of transmitting coils for
one or a plurality of measuring devices. The transmitting coils can
be air coils, in particular printed circuits ("printed coils")
formed on a printed circuit board, so that manufacturing can be of
low complexity and therefore inexpensive.
[0014] One of the sensors can be surrounded by one of the
transmitting coils and another of the sensors can lie outside the
transmitting coil. By choosing the specific positions of the
sensors, two or more sensors can be used in order to provide a
signal which is in total proportional to the resulting magnetic
field, and which relates to a plurality of points in the region of
the transmitting coils.
[0015] In a second variant, the measuring device can comprise a
sensor for determining a magnetic field gradient, with main field
directions of the transmitting coils running parallel to each other
and a preferred direction of the sensor running perpendicular or
parallel to the main field directions. High-precision sensors for
magnetic field gradients are available, for example, as AMR
(anisotropic magnetoresistive effect), GMR (giant magnetoresistive
effect), CMR (colossal magnetoresistive effect), TMR (tunnel
magnetoresistance), or planar Hall sensors. Such sensors can also
be obtained inexpensively as standard components. In another
embodiment, a sensor not based on the magnetoresistive effect, for
example a SQUID sensor, can also be used.
[0016] The transmitting coils can be arranged essentially next to
one another in a layer, with the preferred direction of the sensor
running parallel or perpendicular to this layer. As a result, a
measuring arrangement can be accommodated on a printed circuit
board that is populated only on one side, which can lower a unit
price of the measuring apparatus.
[0017] The transmitting coils can essentially be D-shaped, with the
backs of the D-shapes facing one another and the sensor being
arranged between the backs of the D-shapes. In this way, it is
possible to achieve a very compact construction in connection with
a sensor for magnetic field gradients.
[0018] The sensor is preferably arranged essentially in the layer
of the transmitting coils, and another sensor is provided in a
layer parallel to this layer, with preferred directions of the
sensor and the other sensor being perpendicular to one another. As
a result, the accuracy and universality of the measuring apparatus
can be increased.
[0019] Furthermore, the invention comprises a measuring method for
detecting a metallic object comprising steps of supplying two
transmitting coils with alternating voltages in order to generate
superimposed magnetic fields, of determining an output signal,
which is dependent on the magnetic field, of a magnetoresistive
measuring device in the region of the two magnetic fields, with the
supply of the transmitting coils with alternating voltages taking
place in such a way that the magnitude of an AC voltage component,
which is synchronous with the alternating voltages, of the output
signal of the measuring device is minimized, and of detecting the
object if the ratio of the alternating voltages does not correspond
to a ratio of the distances of the measuring device from the
transmitting coils.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The invention is described in greater detail below with
respect to the included drawings, where:
[0021] FIG. 1 shows a block diagram of a measuring apparatus;
[0022] FIG. 2 shows an arrangement of magnetoresistive measuring
devices for the measuring apparatus of FIG. 1;
[0023] FIG. 3 shows arrangements of a plurality of transmitting
coils on the measuring apparatus of FIG. 1;
[0024] FIG. 4 shows arrangements of magnetic field sensors and
transmitting coils for the measuring apparatus of FIG. 1;
[0025] FIG. 5 shows arrangements of magnetic field gradient sensors
and transmitting coils for the measuring device of FIG. 1; and
[0026] FIG. 6 shows a flow diagram for a method for detecting a
metallic object using the measuring apparatus of FIG. 1.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0027] FIG. 1 shows a block diagram of a measuring apparatus 100.
The measuring apparatus 100 is part of a metal detector 105 for
detecting metallic objects made, for example, of material
containing iron.
[0028] A clock generator 110 has two outputs at which it provides
phase-shifted periodic alternating signals, preferably
phase-shifted by 180.degree.. The alternating signals can in
particular comprise square-wave, triangular, or sinusoidal signals.
The outputs of the clock generator are connected to a first
controllable amplifier 115 and a second controllable amplifier 120.
Each of the controllable amplifiers 115, 120 has a control input
via which it receives a signal, which controls a gain of the
controllable amplifier 115, 120. An output of the first
controllable amplifier 115 is connected to a first transmitting
coil 125 and an output of the second controllable amplifier 120 is
connected to a second transmitting coil 130. Remaining ends of the
transmitting coils 125 and 130 are respectively electrically
connected to a defined potential.
[0029] As indicated by the dots at the transmitting coils 125 and
130, the transmitting coils 125 and 130 are oriented in opposite
directions. When supplied with opposite voltages with respect to
the defined potential, the transmitting coils 125, 130 establish
magnetic fields having the same orientations.
[0030] The same effect can also be achieved by supplying rectified
voltage having alternately varying amplitude with respect to the
defined potential; in this case, currents having a superimposed DC
component flow. In this case, the orientation of the transmitting
coil in the same direction as well as in the opposite direction
makes sense. In all cases, the magnetic field sensors are to be
aligned with one another and connected in such a way that there is
a constant signal at the output of the magnetoresistive measuring
apparatus 135 in the object-free case. This signal component
corresponds to the superimposed DC component of the currents. In
order not to control the input amplifier 140 unnecessarily, the
input amplifier can be DC-decoupled using a capacitor.
[0031] A magnetoresistive measuring device 135 is connected to an
input amplifier 140. The input amplifier 140 is shown with a
constant gain; however, in other embodiments, a gain of the input
amplifier 140 can also be controllable. As a result, for example, a
spatial resolution and/or sensitivity of the measuring apparatus
100 can be capable of being influenced and controlled, for example,
based on a measured value.
[0032] The output of the input amplifier 140 is connected to a
synchronous demodulator 145. The synchronous demodulator 145 is
furthermore connected to the clock generator 110, from which it
receives a clock signal that indicates the phase angle of the
signals provided at the outputs of the clock generator 110. In a
simple embodiment in which the signals provided by the clock
generator 110 are symmetrical square-wave signals, one of the
output signals can be used as a clock signal. The synchronous
demodulator 145 essentially interconnects the measurement signal
received from the input amplifier 140 alternately at its upper and
lower output based on the clock signal provided by the clock
generator 110.
[0033] The two outputs of the synchronous demodulator 145 are
connected to an integrator (integrating comparator) 150, which is
shown here as an operational amplifier connected to two resistors
and two capacitors. Other embodiments are also possible, for
example as an active low pass filter. A digital embodiment
following the synchronous demodulator is also conceivable, in which
the signal at the outputs of the synchronous demodulator is
converted from analog to digital at one or a plurality of instants
within a half-cycle and then compared to the corresponding value
from the next half-cycle. The difference is integrated and, for
example, reconverted to an analog signal and used to control the
amplifiers. Whereas the synchronous demodulator 145 provides the
measurement signal received from the input amplifier 140 at its
lower output, the integrator 150 integrates this signal over time
and provides the result at its output. Whereas the synchronous
demodulator 145 provides the measurement signal received from the
input amplifier 140 at its upper output, this signal is inverted
and integrated over time by the integrator 150, and the result is
provided at the output of the integrator 150. The voltage at the
output of the integrator 150 is the integral of the difference of
the low pass-filtered outputs of the synchronous demodulator
145.
[0034] If the superimposed magnetic field of the transmitting coils
125 and 130 is equal in magnitude and direction at the
magnetoresistive measuring device 135, then the signals provided at
the outputs of the synchronous demodulator 145 are on average equal
over time, and a signal that approaches zero (ground) is provided
at the output of the integrator 150. However, if the influence of
the magnetic field of one of the transmitting coils 125, 130
predominates, then the signals provided at the outputs of the
synchronous demodulator 145 are on average no longer equal, and a
positive or negative signal is provided at the output of the
integrator 150.
[0035] The signal provided by the integrator 150 is provided for
further processing via a connector 155. In addition, a
microcomputer 175 is connected to the control inputs of the
controllable amplifiers 115, 120. The microcomputer 175 compares
the provided signal to a threshold value and outputs a signal at an
output 180, which indicates the metallic object. The signal can be
presented to a user of the metal detector 105 optically and/or
acoustically.
[0036] Furthermore, the microcomputer 175 can perform additional
processing of the signals tapped from the control inputs of the
controllable amplifiers 115, 120, and can control parameters of the
measuring apparatus 100 based on the signals. For example, a
frequency or signal shape of the alternating voltages at the
outputs of the clock generator 110 can be varied, or a sensitivity
of the receiving amplifier 140 can be changed. In another
embodiment, more of the displayed elements of the measuring
apparatus 100 are implemented by the microcomputer 175, for example
the clock generator 110, the synchronous demodulator 145, or the
integrator 150.
[0037] The same signal from the integrator 150 is also used to
control the gains of the controllable amplifiers 115 and 120, with
the second controllable amplifier 120 being directly connected to
the output of the integrator 150, and the first controllable
amplifier 115 being connected to the output of the integrator 150
by means of an inverter 160. The inverter 160 causes an inversion
of the signal provided to it in such a way that the gain of the
first controllable amplifier 115 increases depending on the output
signal of the integrator 150 to the degree that the gain of the
second controllable amplifier 120 decreases, and vice versa. It is
also conceivable that only the gain of one of the controllable
amplifiers is controlled, while the gain of the second controllable
amplifier is kept at a fixed value.
[0038] A metallic object 170 is depicted in the region of the
transmitting coils 125, 130. The metallic object 170 is at
different distances from the transmitting coils 125 and 130 and
therefore has a different degree of influence on magnetic fields of
the transmitting coils 125, 130 having equal strength. The
measuring apparatus 100 is adapted to balance out this influence
via an opposite change in the gains of the controllable amplifiers
115, 120. If only one of the two controllable amplifiers 115, 120
is controlled and the other is fixed, then balancing out is not
achieved by means of an opposite change, but rather only with a
directed change.
[0039] FIG. 2 shows an arrangement 200 of two magnetoresistive
magnetic field sensors for use with the measuring device of FIG. 1.
A first Hall sensor 210 and a second Hall sensor 220 have opposite
preferred directions and are connected in series in such a way that
they provide a signal that totals zero in a homogeneous magnetic
field. The Hall sensors 210, 220 are aligned with their preferred
directions parallel (210) and antiparallel (220) to the main field
direction of the magnetic field generated in the object-free case
by the transmitting coils. The signal from the Hall sensors 210,
220 is routed to the input amplifier 140 in FIG. 1.
[0040] The Hall sensors 210 and 220 are necessarily at a certain
distance from one another so that they measure magnetic fields at
different locations. An output signal from the sensors 210, 220
thus results if the sensors 210, 220 are exposed in their preferred
directions to magnetic fields of varying strengths. For example,
this is the case if the magnetic field of one of the transmitting
coils 125, 130 of FIG. 1 is more strongly influenced by a metallic
object than the magnetic field of the other transmitting coil 125,
130. Based on the influence on the magnetic fields by a metallic
object, a non-zero synchronous AC voltage component in particular
results in the output signal of the magnetoresistive measuring
device. In this case, the measuring apparatus 100 changes the
voltages with which the transmitting coils 125, 130 are supplied in
the opposite direction until the magnetic fields at the Hall
sensors 210, 220 again have the same strength in the respective
preferred directions. The voltage present at the connector 155 can
be evaluated as a measure of the inequality of the alternating
voltages of the transmitting coils 125, 130.
[0041] In a second variant, instead of the Hall sensors 210, 220,
which determine magnetic fields, sensors are used to determine
magnetic field gradients, which are then oriented perpendicular to
a magnetic field aligned in the object-free case. In another
embodiment, only one such magnetic field gradient sensor is
provided.
[0042] FIG. 3 shows arrangements of transmitting coils on the
measuring apparatus 100 of FIG. 1. For the sake of clarity, pairs
of transmitting coils 125/130 lying on top of one other are
depicted as circles; magnetic field sensors or magnetic field
gradient sensors are not shown. Each of the circles corresponds to
one of the arrangements shown in FIG. 4 or FIG. 5. One or a
plurality of measuring apparatuses 100 is provided for activating
the arrangements 310, with each measuring apparatus 100 being
connected to only one of the arrangements 310 during a measurement.
A switchover of a plurality of arrangements 310 to one of the
measuring apparatuses 100 can occur. Information about the
direction, depth, or size of the metallic object to be detected can
be determined through appropriate geometric distribution of a
plurality of arrangements 310.
[0043] It is possible to determine depth with the arrangements in
FIG. 3a or 3b. It is possible to determine direction or orientation
with the arrangements in FIG. 3c or 3d. The arrangement in FIG. 3e
can be used for imaging.
[0044] FIG. 4 shows arrangements of magnetic field sensors and
transmitting coils for the measuring apparatus of FIG. 1. All
magnetic field sensors shown in FIG. 4 are oriented vertically in
terms of the preferred direction in which they must be exposed to a
magnetic field in order to generate a maximum output signal, that
is, parallel to a main magnetic field that exists inside the
transmitting coils 125, 130. Deviations of the preferred directions
of the magnetic field sensors from the specified orientations are
innocuous, as long as the deviations are sufficiently small to
allow the magnetic field sensors to determine the magnetic field in
the orientation used. The transmitting coils 125, 130 are formed as
coreless printed coils on opposite sides of a printed circuit
board.
[0045] In FIG. 4a, the Hall sensors 210 and 220 of FIG. 2 are
respectively arranged centered in one of the transmitting coils 125
or 130. The alignment of the Hall sensors 210 and 220 is
antiparallel and their connection is parallel, as shown in FIG.
2.
[0046] In FIG. 4b, the Hall sensors 210 and 220 are oriented in
parallel and connected in parallel. The magnetic field generated by
the transmitting coils 125, 130 has different orientation inside
and outside the transmitting coils 125, 130. The Hall sensors 210,
220 are at a distance from the turns of the transmitting coils 125,
130 such that the magnitudes of the magnetic fields in the
object-free case correspond to one another.
[0047] In FIG. 4c, in contrast to the illustration of FIG. 4b,
another, second Hall sensor 220 is provided, with the two second
Hall sensors 220 lying opposite one another with respect to the
first Hall sensor 210. The alignment of the Hall sensors is
parallel, with the distances of the second Hall sensors 220 from
the turns of the transmitting coils being chosen such that the
total of the magnitudes of the magnetic fields existing at the
second Hall sensors 220 in the object-free case corresponds to the
magnitude of the magnetic field existing at the first Hall sensor.
The Hall sensors 210, 220 are connected in such a way that their
output signals are added.
[0048] In FIG. 4d, the principle of the arrangement of FIG. 4c is
extended by providing an additional first Hall sensor 210 in the
interior of the first transmitting coil 125. The Hall sensors are
connected in such a way that their output signals are added. All
Hall sensors are aligned parallel to one another. The distances of
the Hall sensors from the turns of the transmitting coil 125 are
chosen such that the sum of the magnitudes of the magnetic fields
existing at the first Hall sensors 210 corresponds to the sum of
the magnitudes of the magnetic fields existing at the second Hall
sensors 220.
[0049] FIG. 5 shows arrangements of magnetic field gradient sensors
and transmitting coils for the measuring device of FIG. 1. In
contrast to the illustrations of FIG. 4, the sensors used are
magnetic field gradient sensors 510 whose preferred directions run
parallel to the surface of the illustrated printed circuit board.
In another embodiment, the preferred directions of the magnetic
field gradient sensors run parallel to the main field direction.
Here as well, deviations of the preferred directions of the
magnetic field gradient sensors from the specified orientations are
innocuous, as long as the deviations are sufficiently small to
allow the magnetic field gradient sensors to determine the magnetic
field in the orientation used.
[0050] A plurality of magnetic field gradient sensors 510 is
respectively connected to one another in such a way that their
output signals are added. It is also conceivable that the magnetic
field gradient sensors 510 are evaluated separately, for example,
in succession. To do this, the outputs of the magnetic field
sensors 510 must be connected to the input of the input amplifier
140 by a switch in an alternating manner.
[0051] In FIG. 5a, the transmitting coils 125, 130 are formed on
opposite sides of the printed circuit board. The transmitting coils
125, 130 are located next to one another and essentially have a
D-shape, with the backs of the D-shapes being parallel and facing
one another. The preferred direction of the magnetic field gradient
sensor 510 is at an angle of 90.degree. to the direction of the
backs of the D-shapes.
[0052] In FIG. 5b, in contrast to the illustration in FIG. 5a, the
magnetic field gradient sensor 510 is arranged on the same side of
the printed circuit board as the transmitting coils 125, 130. The
backs of the D-shapes surround the place at which the magnetic
field gradient sensor 510 is located.
[0053] In FIG. 5c, another magnetic field gradient sensor 510 on
the lower side of the printed circuit board is provided in addition
to the illustration in FIG. 5b. The preferred direction of the
upper magnetic field gradient sensor 510 is perpendicular to the
direction of the backs of the D-shapes, and the preferred direction
of the lower magnetic field gradient sensor 510 is perpendicular to
the preferred direction of the upper magnetic field gradient sensor
510. In another embodiment, both preferred directions can also be
perpendicular to the direction of the backs of the D-shapes.
[0054] In FIG. 5d, a respective arrangement from FIG. 5b is
arranged on the upper side and the lower side of the printed
circuit board, with the arrangements at the printed circuit board
level being rotated by 90.degree. to one another. The preferred
directions of the magnetic field gradient sensors 510 lie
perpendicular to one another.
[0055] FIG. 6 shows a flow diagram 600 of a method for detecting a
metallic object using the measuring apparatus of FIG. 1. In a first
step 610, the transmitting coils 125, 130 are supplied with
alternating voltages in order to generate oppositely oriented
magnetic fields. Next, in a step 620, an output signal, which is
dependent on the magnetic field, of a magnetoresistive measuring
device in the region of the two magnetic fields is determined. In a
step 630, depending on the synchronous AC voltage component of the
determined signal, the supply of the transmitting coils with
alternating voltages is performed in such a way that the magnitude
of the synchronous AC voltage component of the output signal of the
measuring device is minimized. Finally, in a step 640, the metallic
object is detected if the ratio of the alternating voltages does
not correspond to a ratio of the distances of the measuring device
from the transmitting coils.
* * * * *