Method for measuring a position

A method for measuring a position using a magnet and a sensor for detecting the magnetic field strength of the magnet. The magnet and/or the sensor interact with a movable element so that a relative movement between the sensor and the magnet can be effected. The position of the movable element in accordance with the coordinates in a system of coordinates can be ascertained on the basis of the magnetic field having a predetermined shape generated by the magnet and detected by the sensor. The sensor ascertains three linearly independent spatial direction components of the magnetic field strength of the magnetic field acting at the location of the sensor. Each coordinate along a coordinate axis of the system coordinates of the magnetic field is determined individually and unambiguously by the combination of the three linearly independent spatial direction components of the magnetic field strength detected by the sensor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for measuring a position and a device for measuring a position.

2. Description of Related Art

Such a method can be applied as evaluating method for the three-dimensional determination of a position in a magnetically operating position sensor.

From DE 10 2008 024 103 A1, a device for measuring a position comprising a magnet and a sensor detecting the magnetic field strength of the magnet is known, wherein the magnet and/or the sensor interacts with a movable element. By means of the movable element, a relative movement between the sensor and the magnet can be effected in such a manner that the position of the movable element in accordance with the coordinates (x, y, z) in a system of coordinates can be ascertained on the basis of the magnetic field generated by the magnet and detected by the sensor. For this purpose, the sensor ascertains the components of the magnetic field strength (Bx, By, Bz) of the magnetic field, acting in the sensor at a location, or, respectively, at the same location in three linearly independent spatial directions. The magnet is selected in such a manner that it generates an analytically describable magnetic field.

SUMMARY OF THE INVENTION

The invention is based on the object of creating a method for measuring a position for such a device which operates in a simple manner.

In the method according to the invention, each coordinate x, y, z along a coordinate axis of the system of coordinates in a half-space of the magnetic field is determined individually and unambiguously by the combination of the components of the magnetic field strength (Bx, By, Bz), detected by the sensor, in all spatial directions of the system of coordinates. For determining the respective position, it is thus sufficient to measure the components of the magnetic field strengths Bx, By, Bzat the one location in the sensor, that is to say to measure only a triple of values from which the coordinates x, y, z for the spatial position can then be calculated in a simple manner. It can thus be found that the method according to the present invention requires only little equipment expenditure and operates very rapidly.

The method for determining a position can be simplified further in that an essentially ideal dipole field is used as analytically describable magnetic field. For this purpose, a field of a cylindrical magnet having a ratio of diameter to cylinder height of about 1:1 is preferably provided since such a cylindrical magnet can be produced simply and cost-effectively. However, the field of a spherical magnet can also be used as dipole field.

The method can be developed further by describing the coordinates x, y, z from a quotient relation between the respective magnetic field strength (Bx, By, Bz), corresponding to the coordinate axis, a magnetic field value B0depending on the location, and a constant system value. In this context, the system value comprises the magnetic permeability constant and the dipole moment of the magnet. The magnetic field value B0in turn comprises a component (Bx, By, Bz) in a spatial direction and the amount of the magnetic field strength. Since these are only simple calculating steps, a low-power cost-effective microprocessor is adequate for calculating them.

In particular, the coordinates are here determined as follows:

x=M0B032M0B03⁢13⁢(BzB0+1)⁢13⁢BxB0y=M0B032M0B03⁢13⁢(BzB0+1)⁢13⁢ByB0z=M0B03⁢13⁢(BzB0+1)whereB0=-Bz+Bz2+8⁢B24B2=Bx2+By2+Bz2M0=μ0⁢pz4⁢π
and where pzis the dipole moment of the magnet and μ0is the permeability constant.

In some instruments in which the magnetically operating device is used for measuring a position, disturbances of the magnetic field can be produced due to the installed situation in the instrument. In such applications, the magnet thus generates a magnetic field which is distorted at least at one surface not located between the sensor and the magnet. In particular, such a surface, for example a metallic surface in the instrument, can produce shielding of the magnetic field. To guarantee a correct determination of a position even in such cases, the method for measuring a position can be designed as follows: In a first step, each coordinate x, y, z along a coordinate axis of the system of coordinates is ascertained in a half-space of an undistorted magnetic field as an initial value. In a second step, a magnetic field model of an arrangement which is plane-symmetrical, in particular mirror-like, with respect to the surface effecting the distortion, of at least two undistorted magnetic fields is described. In a third step, finally, a position determination in the magnetic field model is undertaken on the basis of the initial value by iterative combination of the components, detected by the sensor, of the magnetic field strength (Bx, By, Bz) in all spatial directions of the system of coordinates and of the magnetic field model. It can be found here, too, that this method provides for determining a position in a simple and rapid manner.

The ascertainment of each coordinate x, y, z along a coordinate axis in a half-space of an undistorted magnetic field is expediently determined as an initial value in accordance with the method described above for an undisturbed magnetic field. In a simple case, however, it may also be sufficient to select the ascertainment of each coordinate x, y, z along a coordinate axis freely in a half-space of an undistorted magnetic field as an initial value.

In a further embodiment which is characterized by a simple evaluation with regard to determining a position, the magnetic field model can be selected as a mirror-symmetrical magnetic field of two essentially ideal dipole fields mirrored at the at least one surface effecting the distortion and unilaterally pole-inverted. It is also possible for the magnetic field model to comprise an area constant for taking into consideration boundary area effects of the at least one surface effecting the distortion. For example, the area constant takes into consideration a nonideal, and thus incomplete, shielding, which is frequently present in reality, of the magnetic field by a metallic surface.

In particular, the magnetic field model can be determined as follows:

B→=(BxByBz)=μ04⁢π⁢3⁢(p→·r→)⁢r→-r2⁢p→r5+η⁢⁢μ04⁢π⁢3⁢(p→·r→S)⁢r→S-rs2⁢p→rS5
where r=(x y z) is the position vector for the magnet, rs=(xsyszs) is the position vector for the mirror magnet and η is the effectively active permeability of the surface effecting the distortion and of the half-space lying behind it.

A position measuring device operating in accordance with the method according to the invention can be advantageously used in a device having a metal housing. This can be, for example, a laundering machine. The machine has a housing which, in particular, consists at least partially of metal, and a rotatable drum suspended in the housing. The relative position of the drum in the housing is then determined by one of the methods described above. For example, the position measurement can be used in a washing machine for detecting the loading of the washing drum and/or for detecting the vibrational behavior of the washing drum. It is then advantageously possible to implement larger drums having a capacity of 7 or 8 kg laundry in a washing machine housing having a standard width of 60 cm. This is made possible due to the fact that the position of the drum with respect to the housing can be measured. On the basis of this signal, the washing machine control can recognize threatening impacts of the drum on the housing and respond accordingly.

The advantages achieved by means of the invention consist, in particular, in that the measuring of a position can be implemented with little expenditure and inexpensively due to the simple evaluating method. The position measurement can thus be used in cost-sensitive mass produced articles such as domestic appliances, electric tools or the like. Nevertheless, the position measurement operates very accurately and more precisely than previously and also sensitively so that it can be used in safety-critical applications, for example in motor vehicles. Finally, this is also a faster evaluating method than previously.

DETAILED DESCRIPTION OF THE INVENTION

InFIG. 1, used as schematic diagram, a position measuring device1is shown which comprises a magnet2and a sensor3detecting the magnetic field strength of the magnet2. The magnet2and/or the sensor3interacts with a movable element4drawn only diagrammatically, so that a relative movement between the sensor3and the magnet2can thus be effected by means of the movable element4. At present, the magnet2is arranged at the movable element4, whereas the sensor3is arranged to be stationary. Naturally, the sensor3can also be arranged conversely at the movable element4and the magnet2can be arranged to be stationary which, however, is not considered in further detail. The magnet2is designed in such a manner that it generates an analytically describable magnetic field5. As can be seen from the field lines of the magnetic field5, drawn inFIG. 1, this is an essentially ideal dipole field, the magnetic dipole field5being generated by a cylindrical magnet2having a ratio of diameter to height of cylinder of about 1:1. The magnetic dipole field5can also be generated by means of a spherical magnet which, however, is not shown in further detail.

The position of the movable element4in accordance with coordinates x, y, z in a system of coordinates which is in this case a Cartesian system of coordinates can be ascertained on the basis of the magnetic field5generated by the magnet2and detected by sensor3. For this purpose, the position measuring device1operates in accordance with the following method for measuring a position.

Firstly, the components, acting at a single, sensitive point7and, in consequence, at the one same location7, of the magnetic field strength of the magnetic field5are ascertained in sensor3in three linearly independent spatial directions6, the so-called B-field components Bx, By, Bzas is indicated diagrammatically inFIG. 1. Each coordinate x, y, z along a coordinate axis of the system of coordinates is determined in a half-space of the magnetic field5, this being presently the half-space located to the right of the magnet2inFIG. 1, individually and unambiguously by the combination of the components of the magnetic field strength Bx, By, Bz, detected by the sensor3, in all spatial directions6of the system of coordinates. To determine the coordinates x, y, z, a quotient relation between the respective magnetic field strength Bx, By, Bzcorresponding to the coordinate axis, a magnetic field value B0depending on the location and a constant system value is used. The system value comprises the magnetic permeability constant or the magnetic field constant μ0, respectively, and the dipole moment pzof the magnet2. The magnetic field value B0comprises a component Bx, By, Bzin one spatial direction6and the amount of the magnetic field strength β. In particular, coordinates x, y, z can be determined with the aid of the quotient relation as follows:

The formulae for determining the coordinates can be simplified still further by corresponding reformulation as follows:

FIG. 2shows the use of the position measuring device1in a domestic appliance, namely in a laundering machine drawn only diagrammatically, for example a washing machine8having a housing10. The washing machine8has a washing drum9which is supported rotatably at a bearing arrangement4. The bearing arrangement4in turn is attached via suspensions11in the washing machine8in such a manner that vibrations, movements or the like of the drum9are transferred to the bearing arrangement4. The bearing arrangement4thus represents the movable element with which the magnet2interacts. For this purpose, the magnet2is arranged in a holder12attached to the movable element4. The sensor3has a sensor housing13which is fixed in the vicinity of the holder12and allocated to the magnet2at an element stationary with respect to the movable element4in the washing machine9, namely at the rear housing wall14of the housing10. The coordinates x, y, z, determined by the position measuring device1, of the drum9are transmitted to a microcomputer in the washing machine8where vibrations, resonances or the like are then compensated for by means of corresponding control of the drive for the washing drum9.

The housing10of the washing machine8consists of a metal housing. However, metal causes a change in the magnetic field5in the manner of a shielding. Since the sensor3is located in the vicinity of the metallic rear housing wall14, the magnetic field5generated by the magnet2is distorted at this at least one surface14not located between the sensor3and the magnet2in that the field lines of the magnet2are deflected at the surface14in such a manner that they extend largely in the surface14as can be seen diagrammatically inFIG. 3. In order to take into consideration this effect, the method for measuring a position comprising the magnet2and the sensor3detecting the magnetic field strength, that is to say the determination of the relative position of drum9in the housing10, is extended as follows.

In a first step, each coordinate x, y, z along a coordinate axis6of the system of coordinates in a half-space of an undistorted magnetic field5is assigned an initial value x0, y0, z0. The ascertainment of each coordinate x, y, z is preferably effected along a coordinate axis6in the half-space of an undistorted magnetic field5as an initial value x0, y0, z0in accordance with the above formulae (1*), (2*) and (3*) in accordance with the method for determining a position for an undisturbed magnetic field5. On the other hand, it is also possible to select the ascertainment of each coordinate x, y, z along a coordinate axis6in the half-space of an undistorted magnetic field5freely in a suitable manner as an initial value x0, y0, z0.

In a second step, a magnetic field model of a plane-symmetrical, in particular mirror-like arrangement with respect to the surface14causing the distortion, of at least two undistorted magnetic fields5,5′ is described as is shown inFIG. 4. This magnetic field model expediently contains a mirror-symmetrical magnetic field5,5′, the magnetic field5being generated by the magnet2and the magnetic field5′ being generated by a similar mirror magnet2′ with respect to the surface14. The magnetic fields5,5′ as shown inFIG. 4are thus two essentially ideal dipole fields which are mirrored at the at least one surface14causing the distortion and are unilaterally pole-inverted. The magnetic field model can also comprise an area constant η which is used for taking into consideration boundary area effects of the at least one surface14causing the distortion. For example, the area constant takes into account the fact that the real metal of the rear housing wall14is not an ideal “magnetic conductor”. In other words, it is taken into consideration that, in the real case of the rear housing wall14, a certain proportion of the magnetic field lines also extends outside the rear housing wall14, as a result of which η represents the effectively active permeability of the rear housing wall.

In a third step, finally, the actual determination of a position occurs in this magnetic field model. This is determined, starting from the initial value x0, y0, z0, by iterative combination of the components, detected by the sensor3, of the magnetic field strength Bx, By, Bzin all spatial directions6of the system of coordinates and of the magnetic field model. In other words, the initial value x0, y0, z0is firstly inserted into the magnetic field model and from this a first approximate value x1, y1, z1is determined. This first approximate value is subsequently in turn inserted into the magnetic field model and from this a second approximate value x2, y2, z2is determined. The iteration is then continued until two successive approximation values xn-1, yn-1, zn-1and xn, yn, znare approximately equal for the position, that is to say, for example, until
|xn-1−xn|≤ϵ
|yn-1−yn|≤ϵ
|zn-1−zn|≤ϵ
holds true, where ϵ is a predetermined barrier determining the required accuracy of the evaluating method.

In particular, the magnetic field model consisting of the arrangement of magnet2and mirror magnet2′ according toFIG. 4can be determined as follows:

Here, r=(x y z) is the position vector for magnet2, rs=(xxyszs) is the position vector for the mirror magnet2′ and η is the effectively active permeability of the surface14causing the distortion and of the half-space lying behind it. As has been found, the value for η is approximately 0.7 in the usual materials for the rear housing wall14in a washing machine8.

The invention is not restricted to the exemplary embodiments described and represented. Instead, it comprises also all technical developments within the context of the invention defined by the patent claims. Thus, the method for measuring a position can also be used, apart from in other domestic appliances, in motor vehicles, for example in motor vehicle locks, for chassis identification, for multimedia operating elements or the like. In addition, this measuring method offers many other possible applications for the general recognition of position in production and automation technology.

LIST OF DESIGNATIONS