Note: Descriptions are shown in the official language in which they were submitted.
CA 02283209 1999-09-10
Process for detecting the position of a magnet for generating
a magnetic field and device for this purpose
Technical area:
The invention relates to a process for detecting the position of a moveably
arranged control element of an actuating drive behind a housing wall made of
a ferromagnetic material, whereby the control element has a magnet for
generating a magnetic field through the housing wall, and a magnetic field
sensor is positioned in front of the housing wall, according to the generic
part
of Claim 1 as well as a device for this purpose according to the generic part
of
Claim 2.
State of the art:
Magnetic sensors serve for contact-free detection or measurement of physical
quantities such as position, path, distance, speed or angle of rotation. In
many
applications, the sensor is controlled by a permanent magnet and then
converts the position of this magnet relative to the sensor element into an
electrical signal. Examples of such sensors are magnetic field sensors, for
example, saturation core probes, GMR (Giant Magneto Resistive sensors),
magneto-resistive sensors or Hall elements that are suited for control with
permanent magnets as well as for detecting iron parts. Only the magnetic
field component that is parallel to the sensor axis is effective in driving
the
position sensor.
EP 0,45 7 ,762 A describes an actuating drive with a control element that can
be moved behind a housing wall, whereby a device for generating a magnetic
field is attached to said control element and there is a magnetic field sensor
in
front of the housing wall of the actuating drive. The housing wall is made of
a
magnetically conductive material, whereby the field lines of the magnetic
field
in the housing wall form a main flux that is shielded relative to the front of
the housing wall. In order to generate a magnetic secondary flux on the front
of the housing wall, there is a magnetic conductor having two ends, whose
first end is adjacent to the housing wall and whose second end delimits an air
gap in which the magnetic field sensor is installed. Likewise, there can also
be
3~ two magnetic conductors whose second ends are arranged opposite from each
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other, thus forming an air gap. The actuating drive is
configured as a hydraulic high-pressure cylinder with a
piston connected to a piston rod as the control element,
whereby the cylinder wall forms the housing wall and the
device for generating the magnetic field is configured as a
permanent magnet. Likewise, in this publication, it was
already suggested that it would be conceivable to arrange a
Hall probe directly on the outside of the cylinder wall and
then to only provide a magnetic conductor whose second end
covers the back of the Hall probe, whereby an air gap
relative to the cylinder wall is also provided.
Technical task:
The invention is based on the objective of
providing a process with which an actuating drive can be
improved in such a way that said actuating drive is capable
of reliably detecting the position of the control element in
a simple manner with simple means, even in the case of a
magnetically shielding housing wall, and an improved
actuating drive is also to be provided.
Disclosure of the invention and its advantages:
In one aspect of the present invention, there is
provided a device for detecting the position of a moveably
arranged control element of an actuating drive behind a
housing wall made of a ferromagnetic material comprising a
housing wall made of a ferromagnetic material; a relatively
moveable control element having a magnet disposed behind the
housing wall, wherein the control element having the magnet
is moved relative to the housing wall; an actuating drive
supporting the moveable control element and the magnet for
generating a magnetic field through the housing wall,
wherein field lines of the magnetic field build up a main
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flux which progresses within said housing wall, whereby said
main flux forms a magnetic remanence which is retained after
the magnet has passed by and wherein the field of the
magnetic remanence is directed along the movement axis of
the magnet in accordance with the polarity, and builds up a
leakage flux towards the front of the housing wall to the
outside of the housing wall; a magnetic field sensor
comprising a magnetic field sensor element located outside
of said housing wall, wherein a leakage flux is directed in
an opposite direction relative to the field direction of the
main flux, and wherein the magnetic field sensor element
detects the field direction of the leakage flux and, wherein
the leakage field is superposed by the magnetic field of the
magnet when the control element with the magnet passes by,
and wherein the magnetic field sensor element is capable of
registering the change in the polarity of the magnetic
field; an evaluation electronic unit post-connected to the
magnetic field sensor element for deriving a switching
signal from the signal of the magnetic field sensor element
in the electronic evaluation unit.
In a second aspect, there is provided a process of
detecting the position of a moveably arranged control
element comprising moving a control element of an actuating
drive behind a housing wall made of a ferromagnetic
material, together with a magnet relative to the housing
wall; generating magnetic field through the housing wall
with a magnet forming part of the control element; thereby
building up a main flux for magnetic field lines of a
magnetic field, wherein the main flux progresses within said
housing wall, with a magnetic remanence; building up a
leakage flux towards a front of the housing wall with the
magnetic remanence; thereby directing the leakage flux in an
opposite direction from a direction of the main flux at a
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location outside of said housing wall; positioning a
magnetic field sensor outside of the housing wall; detecting
the field direction of the leakage flux with the magnetic
field sensor; passing the control element with the magnet by
the main flux; superposing the leakage field by the magnetic
field of the magnet; registering the change in the polarity
of the magnetic field with the magnetic field sensor
element; deriving a switching signal from the signal of the
magnetic field sensor in a post-connected electronic
evaluation unit.
In a third aspect, there is provided a device or
the execution of the process according to the second aspect,
characterized by the following features: behind a housing
wall made of a ferromagnetic material, there is a relatively
moveable control element of an actuating drive that has a
magnet for generating a magnetic field through the housing
wall, whereby a magnetic field sensor, which is a magnetic
field sensor element, is located outside of the housing
wall, an evaluation electronic unit is post-connected to the
magnetic field sensor element, when the device is used and
the control element with the magnet is moved relative to the
housing wall, the field lines of the magnetic field build up
a main flux which progresses within said wall, whereby said
main flux forms a magnetic remanence which is retained after
the magnet has passed by and it is directed along the
movement axis of the magnet in accordance with the polarity,
and builds up a leakage flux towards the front of the
housing wall to the outside thereof, where the magnetic
field sensor element is positioned, whereby, outside of said
housing wall, the leakage flux is directed in the opposite
direction from the main flux, and the magnetic field sensor
element detects the field direction of the leakage flux and,
when the control element with the magnet passes by, the
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leakage field is superposed by the magnetic field of the
magnet, and the magnetic field sensor element is capable of
registering the change in the polarity of the magnetic
field, and a switching signal can be derived from the signal
of the magnetic field sensor element in the electronic
evaluation unit.
In a fourth aspect, there is provided a process
for detecting the position of a moveably arranged control
element of an actuating drive behind a housing wall made of
a ferromagnetic material, whereby the control element has a
magnet for generating magnetic field through the housing
wall and a magnetic field sensor is positioned outside of
the housing wall, characterized in that the control element,
together with the magnet, is moved relative to the housing
wall, as a result of which the field lines of the magnetic
field build up a main flux, which progresses within said
housing wall, with magnetic remanence, which builds up a
leakage flux towards the front of the housing wall, whereby,
outside of said housing wall, the leakage flux is directed
in the opposite direction from the main flux, and the
magnetic field sensor detects the field direction of the
leakage flux and, when the control element with the magnet
passes by, the leakage field is superposed by the magnetic
field of the magnet, and the magnetic field sensor element
registers the change in the polarity of the magnetic field,
and a switching signal is derived from the signal of the
magnetic field sensor in a post-connected electronic
evaluation unit.
According to the invention, the objective is
achieved in that the control element, together with the
magnet, is moved relative to the housing wall, as a result
of which the field lines of the magnetic field build up a
main flux, which progresses within said housing wall, with
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magnetic remanence, which builds up a leakage flux towards
the front of the housing wall, whereby, outside of said
housing wall, the leakage flux is directed in the opposite
direction from the main flux, and the magnetic field sensor
detects the field direction of the leakage flux and, when
the control element with the magnet passes by, the leakage
field is superposed by the magnetic field of the magnet and
the magnetic field sensor element registers the change in
the polarity of the magnetic field, and a switching signal
is derived from the signal of the magnetic field sensor in a
post-connected electronic evaluation unit.
A device for the execution of the process is
characterized by the following features: behind a housing
wall made of a ferromagnetic material, there is a relatively
moveable control element of an actuating drive that has a
magnet for generating a magnetic field through the housing
wall, whereby a magnetic
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field sensor, which is a magnetic field sensor element, is located at a slight
distance in front of the housing wall, an evaluation electronic unit is post-
connected to the magnetic field sensor element, when the device is used and
the control element with the magnet is moved relative to the housing wall, the
field lines of the magnetic field build up a main flux which progresses within
said wall, whereby said main flux forms a magnetic remanence which is
retained after the magnet has passed by and it is directed along the
movement axis of the magnet in accordance with the polarity, and builds up a
leakage flux towards the front of the housing wall to the outside thereof,
where the magnetic field sensor element is positioned, whereby, outside of
said housing wall, the leakage flux is directed in the opposite direction from
the main flux, and the magnetic field sensor element detects the field
direction of the leakage flux and, when the control element with the magnet
passes by, the leakage field is superposed by the magnetic field of the
magnet,
and the magnetic field sensor element is capable of registering the change in
the polarity of the magnetic field, and a switching signal can be derived from
the signal of the magnetic field sensor element in the post-connected elec-
tronic evaluation unit.
In an advantageous embodiment of the invention, the magnetic field sensor
element is a magneto-resistive sensor and/or a saturation core probe and/or a
Hall element and/or a GMR sensor (Giant Magneto Resistive sensor), whereby
the magnetic field sensor element is attached directly onto the wall.
Likewise,
the magnetic field sensor element can be arranged inside a housing which, in
turn, is attached, preferably directly, onto the housing wall; the material of
the housing is such that it practically does not influence the magnetic field
lines that pass through the housing.
The magnetic field sensor element is capable of adequately detecting the weak
leakage field emerging from the housing wall. Of course, the magnetic field of
a permanent magnet can also be detected with such an arrangement. A
saturation core probe consists of a long coil with a core made of highly
permeable material such as, for example, amorphous metal; once the core is
magnetically saturated, the impedance of the coil diminishes. A magneto-
resistive element is a component made of a magnetically conductive material
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(permalloy strips), whose resistance changes under the influence of an
external magnetic field. GMR sensor elements are a further development of
the magneto-resistive sensor element.
Preferably, the housing wall is the wall of a cylinder and the control element
is a piston connected to a piston rod, whereby the device for generating the
magnetic field is arranged on the piston or else on the piston rod, and said
device is a permanent magnet that is connected to one or more pole rings
which have faces across from the cylinder wall for feeding the magnetic field
into the cylinder wall.
The pole ring or pole rings consist of ferromagnetic material, whereby the
permanent magnet can be a magnet ring or can be made up of a plurality of
magnets, which is or are arranged in a receiving ring made of a non-
magnetizable material, whereby the receiving ring is attached to the piston.
An appropriate electronic evaluation unit for processing the sensor signal and
for connecting the magnetic field sensor to an SPS or another peripheral
device is post-connected to each sensor element.
In order to better evaluate the leakage flux, the magnetic field sensor can be
arranged in a recess of the housing wall of the actuating drive.
IVToreover, the pole ring consists of soft-magnetic steel and has several
permanent magnets, for example, in a cylindrical arrangement, in a receiving
ring made of non-magnetizable material. The piston and/or the piston rod of
the actuating drive can be made of magnetizable or non-magnetizable
material such as, for example, brass. The magnetic field sensor can consist of
several spatially differently arranged magnetic field sensor elements for
purposes of differential evaluation of the magnetic flux density change and
for
generating a differential signal. On the piston or on the piston rod, there
are
pole shoes made of soft-magnetic or ferromagnetic material, between which at
least one magnet is held. Likewise, the pole shoes are arranged along a circle
and a plurality of magnets are arranged between these pole rings in a cross
section plane of the cylinder. The magnets can be arranged in a receiving ring
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made of non-magnetizable material and preferably be equidistant from each
other, whereby the receiving ring is arranged between the ferromagnetic pole
rings in such a way that the north or south poles of the magnets are directly
across from the pole rings or else touch them.
An essential advantage of the invention lies in the fact that, in contrast to
the
principle established so far, as described in EP 0,457,762 A (Hall element
with flux baffles), due to the specific arrangement of the sensor element and
the improvement of the electronics, it is possible to totally dispense with
the
flux baffles, since the magnetic flux directly enters the sensor element,
whose
output signal is converted into a corresponding switching signal.
The invention is based on the physical principle that the housing wall of the
actuating drive or the cylinder wall has to be made of a ferromagnetic
material, whereby a magnetic system has to be attached on the actuating
drive, whereby said magnetic system has to generate an adequate magnetic
field. When the magnet of the magnetic system of the control element moves
past the magnetic field sensor, the magnetically excitable elementary
magnets are directed and this is retained as remanence within the housing
wall, depending on the material of which it is made. In this process, the
elementary magnets, which had been non-ordered until then, are rendered
into an ordered state, which forms the remanence field. This generates a more
or less weak magnetic leakage field, which emerges from the surface of the
housing wall, whose field lines run opposite to the direction of the remanence
field within the housing wall. The remaining residual magnetism within the
housing wall is directed in accordance with the polarity of the magnetic
system of the control element.
The magnetic field sensor mounted on the outside of the housing wall scans
the magnetic leakage flux along the housing wall. In doing so, the magnetic
field sensor either detects only the field of the remanence or else the
polarity,
that is to say, the field direction of the remanence.
If the control element approaches the magnetic field sensor, then the strong
field of the magnet system generates a leakage field on the outer wall of the
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housing wall that is superposed over the field of the remanence field; the
magnetic field sensor registers this change in the magnetic field.
A brief description of the drawing in which the following is shown:
Figure 1 a section from a cylinder wall with a piston rod and piston that has
a permanent magnet for generating the magnetic field,
Figure 2 a schematic representation of the remanence field within the
cylinder wall together with the resultant leakage field as well as a
magnetic field sensor that is mounted directly on the cylinder wall,
Figure 3 the magnetic field that forms inside the cylinder wall and changes
when the permanent magnet of the control element passes over it
once again,
Figure 4 a schematic representation of the course of the magnetic flux
density on the cylinder wall,
Figure 5 a plurality of magnetic field sensors which are arranged separately
from each other for purposes of differential measurement,
Figure 6 a top view of a receiving ring with a plurality of magnets arranged
at equidistant intervals and
Figure 7 a section of a cylinder wall with a piston made of non-magnetizable
material as well as a push rod, whereby there are pole rings
arranged on the piston, and there is at least one magnet held
between these pole rings.
Amended Pale
