DE102008032329B4 - Force measuring system and method for measuring a force - Google Patents

Abstract

Force measuring system comprising:
- A plate-shaped force transducer (2) made of a magnetoelastic material;
- A measuring unit (13) having at least one permanent magnet (8) and at least one magnetic field sensor (5);
wherein the measuring unit (13) can be arranged to the force transducer (2) in such a way that the permanent magnet (8) forms a magnetic circuit with the force transducer (2),
wherein the permanent magnet (8) projects at least partially into an opening (7) in the magnetoelastic material of the force transducer (2).

Description

The invention relates to a force measuring system with a force transducer made of a magnetoelastic material. It further relates to a method for measuring a force.

From the DE 30 04 592 A1 An electrohydraulic control device is known, in which a force measuring pin utilizes the effect of a force on a magnetoelastic material for force measurement by means of a differential measuring method. The force pin is constructed relatively complicated.

The DE 10 2006 007 385 A1 discloses a force sensor that allows force measurement by means of a strain gauge transducer. The use of strain gauges for force measurement is well known, but often associated with a high level of technical and therefore economic effort.

The US 5,195,377 A shows a magnetoelastic force transmitter, in which a magnet is arranged adjacent to the power transformer. The DE 37 19 322 A1 shows a measuring system with a bolt-like, tubular component.

The object of the invention is therefore to provide a measuring system for force measurement, which is particularly simple and inexpensive to produce, but at the same time robust and suitable for measuring larger forces in the kN range.

Moreover, it is a further object of the present invention to provide a simple and inexpensive method for measuring a force.

According to the invention, this object is achieved with the subject matter of the independent patent claims. Advantageous developments of the invention are the subject of the dependent claims.

A force measuring system according to the invention comprises a force transducer made of a magnetoelastic material. It further comprises a measuring unit which has at least one permanent magnet and at least one magnetic field sensor. In this case, the measuring unit can be arranged in such a way to the force transducer, that the permanent magnet forms a magnetic circuit with the force transducer.

The term "can be arranged in this way" also constellations of measuring unit and force transducer are taken here, in which they are already arranged in the required manner to each other and form a magnetic circuit. This is the case, for example, when the measuring unit and the magnet are firmly connected to each other, so that their arrangement with each other is already determined in the appropriate manner.

According to a concept underlying the invention, the action of a force on a magnetoelastic material distorts the magnetic fields of a magnetic circuit comprising this magnetoelastic material. This distortion can be measured by a magnetic field sensor, for example a Hall sensor. If the magnetic field sensor is arranged accordingly, it is possible to obtain a directly proportional output variable of the sensor as a measured variable. The measurement can be carried out contactless and thus wear-free.

The measuring principle is preferably based on a rotation of the stray field above the magnet. For this purpose, an air gap - for example, a non-magnetic material of a magnet holder - is arranged between the magnet and the magnetoelastic material.

The design of the magnetic field sensor as a Hall sensor has, inter alia, the advantage that Hall sensors in standard semiconductor processes are very easy to produce.

In this way, a particularly simple and robust measuring system is provided, which is thus suitable, for example, for use in agriculture. The use of a permanent magnet is advantageous because it allows different components to be saved compared to prior art measuring systems and eliminates power supply to the magnet.

In one embodiment, the magnetic field sensor is aligned such that it detects a zero signal in an unloaded state of the force transducer and a load signal deviating from the zero signal in a loaded state of the force transducer. When aligning the magnetic field sensor, both material properties of the force transducer and the magnetization of the magnet are included. The alignment and calibration of the magnetic field sensor can for example be such that no signal is detected as a zero signal and as a load signal, a signal which also increases with increasing load of the force transducer.

The permanent magnet may be formed, for example, as a surface-magnetized body or as a diametrically magnetized body. It can also be provided a plurality of permanent magnets, for example, if a certain geometry of the magnetic field is desired.

In one embodiment of the invention, the force transducer has an opening into which the permanent magnet is at least partially insertable. In this case, the term "einbringbar" also arrangements are taken in which the permanent magnet is already fixed or releasably disposed in the opening.

With this arrangement is achieved with a corresponding orientation of the magnet, that the magnetic flux density in the magnetoelastic material is as large as possible, because a large part of the field lines extends within the magnetoelastic material. It is thus achieved optimal use of the magnetic field of the permanent magnet.

With the arrangement according to the invention, it is possible to measure even very small forces on the basis of the rotation of the stray field caused by them.

In one embodiment of the invention, the measuring unit further comprises a holder for the permanent magnet and an electronic unit. The holder has the task to bring the permanent magnet into place within the opening of the force transducer and to hold there. It also forms the air gap between the magnet and the carrier. The electronics unit may comprise at least parts of the measuring and evaluation electronics. For example, it can have a circuit board on which the magnetic field sensor is also arranged.

In one embodiment, the measuring unit is locatable to the force transducer such that a permeability change in the magnetoelastic material induced by the application of a force to the force transducer causes the magnetic field lines of the magnetic field measured by the magnetic field sensor to rotate through an angle α.

The term "can be arranged in this way" constellations of measuring unit and force transducer are taken here, where they are already arranged in the required manner to each other, for example, because the measuring unit is firmly connected to the force transducer.

In one embodiment of the invention, at least two magnetic field sensors are provided, which are aligned differently with respect to the magnetic field of the permanent magnet.

According to the present invention, in a method for measuring a force, the force to be determined acts on a force transducer made of a magnetoelastic material. In this case, the force transducer is part of a magnetic circuit comprising a permanent magnet. The type of force introduction causes a shear stress distribution in the region of a measuring zone, which in turn alters the magnetic permeability in the magnetoelastic material in a measuring zone. The magnetic stray field measured by a sensor is thereby distorted and in particular rotated. It is measured in this way by the action of the force to be determined on the magnetoelastic material caused change in the magnetic field of the permanent magnet.

In one embodiment of the invention, the vector of the force to be measured with a major component of the magnetic field of the permanent magnet biases a plane and the force in the magnetoelastic material of the force transducer causes a permeability change, the rotation of the main component of the magnetic field by an angle a in the Vector of the force to be measured and the main component of the magnetic field spanned plane causes.

In this case, the change of the magnetic field of the permanent magnet to be measured substantially consists in the change of the angle of the magnetic field relative to the magnetic field sensor. The angle measurement has the advantage that it is very robust against temperature fluctuations.

In one embodiment of the invention, the force transducer is first introduced into the effective range of the force to be measured, and then a measuring unit comprising the permanent magnet and the magnetic field sensor is attached to the force transducer such that the permanent magnet at least partially dips into an opening in the force transducer. Measuring unit and force transducer can thus be applied separately if necessary.

• 1 zeigt schematisch eine perspektivische Ansicht eines Messsystems gemäß einer Ausführungsform der Erfindung;
• 2 zeigt ebenfalls schematisch eine perspektivische Ansicht eines Messsystems gemäß einer Ausführungsform der Erfindung;
• 3 zeigt schematisch einen Querschnitt durch das Messsystem gemäß einer Ausführungsform der Erfindung;
• 4 zeigt schematisch das Messsystem in einem unbelasteten Zustand;
• 5 zeigt schematisch das Messsystem unter der Einwirkung einer zu messenden Kraft und
• 6 zeigt schematisch einen Querschnitt durch eine Messeinheit des Messsystem gemäß einer Ausführungsform der Erfindung.

Embodiments of the invention are explained below with reference to the accompanying figures.

• 1 shows schematically a perspective view of a measuring system according to an embodiment of the invention;
• 2 also schematically shows a perspective view of a measuring system according to an embodiment of the invention;
• 3 schematically shows a cross section through the measuring system according to an embodiment of the invention;
• 4 schematically shows the measuring system in an unloaded state;
• 5 schematically shows the measuring system under the action of a force to be measured and
• 6 schematically shows a cross section through a measuring unit of the measuring system according to an embodiment of the invention.

The same parts are provided in all figures with the same reference numerals.

The measuring system 1 according to 1 includes a force transducer 2 which has a magnetoelastic material and which is plate-shaped in the embodiment shown. The force converter 2 has an opening in the form of a bore, which in the 1 is not visible. On his top 6 is a holder 3 arranged with a permanent magnet also not visible in this view, wherein the permanent magnet is arranged in the opening.

Above the opening and this in the illustration in 1 concealing is an electronics unit 4 with a Hall sensor 5 arranged. The electronics unit 4 includes, for example, a circuit board with electronic components arranged thereon for measurement and evaluation.

2 shows the measuring system 1 according to 1 , wherein in this illustration the electronics unit 4 with the Hall sensor 5 was omitted.

Thus, the opening 7 in the force transducer 2 recognizable, the plate-shaped force transducer 2 completely penetrated as a bore in this embodiment. A holder 3 carries a permanent magnet 8th and let him in the opening 7 protrude.

This is in 3 particularly clearly visible in cross section. The holder 3 has a holding device 9 , For example, a clamping ring or a plateau on which the permanent magnet 8th is glued on. The holder 3 consists in the embodiment shown of plastic and thus of a non-ferromagnetic material. The holder 3 is between the permanent magnet 8th and the force transducer and thereby forms an air gap. He is therefore not part of the magnetic circuit.

The holder 3 leaves the permanent magnet held on it 8th in the opening 7 the force transducer 2 with the depth t protrude or dip, leaving the permanent magnet 8th laterally completely from the magnetoelastic material of the force transducer 2 is surrounded. In this case, the orientation of the permanent magnet 8th and the force transducer 2 advantageously chosen so that the magnetic flux of the permanent magnet 8th as much material as possible from the force transducer 2 penetrates.

On the holder 3 is the electronics unit 4 with the sensor 5 arranged.

The measuring system 1 can be used for example in a measuring pin, as in the DE 10 2006 007 385 is described. Such an arrangement is favorable, because the measuring bolt itself need not be made of a special, magnetoelastic material, but may consist of almost any material. In an alternative embodiment, however, the holder is inserted with the magnet directly into the bore of the measuring bolt, so that this takes over the role of the carrier.

4 shows a section of the force transducer 2 in the unloaded condition, whereby here also the electronic unit 4 with the sensor 5 not shown.

The arrows 10 indicate the field lines of the magnetic field of the permanent magnet 8th , The field lines in this illustration are idealized parallel and at the same distance from each other. In reality, the permanent magnet 8th no such homogeneous magnetic field.

5 shows the excerpt from the force transducer 2 in the loaded condition, with a force in the direction of the arrows 12 on the force transducer 2 acts and induces a shear stress in its material. In the case shown, the local change in the permeability of the magnetoelastic material causes a rotation of the arrows 11 marked field lines by the angle a.

Here is the Hall sensor 5 only symbolically represented in the figures. In an advantageous embodiment, the system comprises an arrangement of, for example, two mutually perpendicularly arranged individual Hall sensors, which are the two components of the magnetic field in the plane of the force transducer 2 measure up. From the change of both components, the angle a can then be determined by which the magnetic field under the load of the force transducer 2 has turned.

6 schematically shows a cross section through the measuring unit 13 of the measuring system 1 holding the holder 3 , the electronics unit 5 with the sensor 5 and the permanent magnet 9 includes.

The measuring unit 13 is separate from the force transducer in this embodiment 2 trained and is connected to the force measurement with this. This is the holder 3 so on the top of the force transducer 2 put on that a bottom 14 of the owner 3 at the top 6 abuts and the permanent magnet 8th in the opening 7 the force transducer 2 protrudes. The holding device 9 points together with the permanent magnet 8th a thickness d which is at most as big as the depth t the opening 7 such that the largest possible area of the magnetic field is the magnetoelastic material of the force transducer 2 penetrates.

LIST OF REFERENCE NUMBERS

1

measuring system

2

power converter

3

holder

4

electronics unit

5

Hall sensor

6

Top of the force transducer

7

opening

8th

magnet

9

holder

10

arrow

11

arrow

12

arrow

13

measuring unit

14

Bottom of the holder

d

thickness

t

depth

Claims (14)

Force measuring system comprising: - A plate-shaped force transducer (2) made of a magnetoelastic material; - A measuring unit (13) having at least one permanent magnet (8) and at least one magnetic field sensor (5); wherein the measuring unit (13) can be arranged to the force transducer (2) in such a way that the permanent magnet (8) forms a magnetic circuit with the force transducer (2), wherein the permanent magnet (8) projects at least partially into an opening (7) in the magnetoelastic material of the force transducer (2). Force measuring system after Claim 1 , wherein the magnetic field sensor is aligned such that it detects a zero signal in an unloaded state of the force transducer (2) and in a loaded state of the force transducer (2) a deviating from the zero signal load signal. Force measuring system after Claim 1 or 2 in which the permanent magnet (8) is introduced into the opening (7) such that the magnetic flux of the permanent magnet (8) in an unloaded state of the force transducer (2) in the direction of the force to be determined by the force measuring system to the force transducer (2) is aligned ,. Force measuring system according to one of Claims 1 to 3 , wherein the permanent magnet (8) is formed as a surface-magnetized body. Force measuring system according to one of Claims 1 to 3 , wherein the permanent magnet (8) is formed as a diametrically magnetized body. Force measuring system according to one of Claims 1 to 5 wherein the opening (7) is configured such that the permanent magnet (8) can be introduced into the opening (7) such that the permanent magnet (8) is laterally completely surrounded by the magnetoelastic material of the force transducer (2). Force measuring system according to one of Claims 1 to 6 wherein the measuring unit (13) further comprises a holder (3) for the permanent magnet (8) and an electronic unit (4), wherein the holder (3) between the permanent magnet (8) and the force transducer (2) is arranged and thereby a Air gap between the permanent magnet (8) and the force transducer (2) is formed. Force measuring system according to one of Claims 1 to 7 in that the measuring unit (13) can be arranged to the force transducer (2) in such a way that a permeability change in the magnetoelastic material induced by the action of a force on the force transducer (2) causes a rotation of the magnetic field lines of the magnetic field measured by the magnetic field sensor (5) by an angle α causes. Force measuring system according to one of Claims 1 to 8th , wherein the measuring unit (13) with the force transducer (2) is firmly connected. Force measuring system according to one of Claims 1 to 9 wherein at least two magnetic field sensors (5) are provided, which are aligned differently with respect to the magnetic field of the permanent magnet (8). Method for measuring a force, in which the force to be determined acts on a plate-shaped force converter (2) made of a magnetoelastic material, wherein the force transducer (2) is part of a magnetic circuit comprising a permanent magnet (8) which at least partially in one Opening (7) in the magnetoelastic material of the force transducer (2) protrudes. and wherein a change in the magnetic field of the permanent magnet (8) caused by the action of the force to be determined on the magnetoelastic material is measured. Method according to Claim 11 , wherein the change of the magnetic field of the permanent magnet (8) by a Hall sensor (5) is measured. Method according to Claim 11 or 12 , wherein the vector of the force to be determined with a major component of the magnetic field of the permanent magnet (8) spans a plane and the force in the magnetoelastic material of the Force transducer (2) causes a permeability change, which causes a rotation of the main component of the magnetic field by an angle α in the plane defined by the vector of the force to be determined and the main component of the magnetic field. Method according to one of Claims 11 to 13 in which first the force transducer (2) is introduced into the effective range of the force to be determined and then a measuring unit (13) comprising the permanent magnet (8) and the magnetic field sensor (5) is attached to the force transducer (2) such that the permanent magnet ( 8) at least partially into the opening (7) of the force transducer (2) is immersed.

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