MAGNETORESISTIVE PROXIMITY SENSOR

Technical field

The present invention regards a magnetoresistive proximity sensor, in particular adapted to detect the position of a ferromagnetic body in a plane.

Prior art

The use of proximity sensors, for example of the optical, inductive or capacitive type for measuring the distance from the outer surface of determined bodies or products, is known.

Such sensors detect the distance of a body observed without requiring contact, though detecting - for example - a light beam, an induced current or a variation of the capacity of the involved sensitive element.

In some industries, such as for example those operating in the field of processing metal materials of various shapes, there arises the need to detect the presence and/or position of the products in the various steps of the manufacturing process.

Such need mainly arises from the industrial necessity of monitoring the production process, and thus the feedback correction of the parameters of the process in question as a function of the detections carried out, by means of sensors.

In the absence of efficient sensors, the correction activity on the production process is carried out by sight and by the intervention of the operator, preferably an expert in the production process to be monitored, thus requiring the interruption of the production process and the production of a series of test pieces, subsequently discarded as production by-products.

The proximity sensors of the known type currently do not allow to satisfactorily meet the requirements of some technical sectors.

For example, there may arise problems regarding the use of proximity sensors of the known type, in case of production processes taking place in the presence of dusts, cooling liquids and/or lubricants, which thus "dirty" the measurement environment, jeopardising the correct operation of the sensors and increasing measurement uncertainty to unacceptable levels.

In addition, some proximity sensors of the known type must be positioned so as to be able to "see" the body to be measured, thus unacceptably occupying the working space of the production process and thus being easily exposed to dirt, hence relatively unreliable.

It should be observed that such proximity sensors may directly detect the distance from the outer surface of an observed body while the position of the barycentre can be estimated only by means of complex calculation processes based on the geometry of the body, thus indirectly. [1 1 ] In addition, further proximity sensors comprising magnetoresistive sensors have been proposed.

[12] For example, patent application US 2005/028041 1 A1 comprises a first magnetoresistive sensor and a second magnetoresistive sensor connected in a bridge configuration. A magnetic source is positioned behind a proximity sensor, while a ferrous object passes in front of the proximity sensor, thus altering the direction of the field generated by the magnetic source.

[13] Patent US 2,596,272 instead illustrates a magnetic sensor associated to a permanent magnet provided with a surface rounded at one end. The permanent magnet is associated to a sensitive component which comprises a first and a second magnetoresistive element, each in turn comprising a pair of magnetic resistors. The four magnetic resistors are connected to each other in electric switching, so as to form a Wheatstone bridge, to provide an output signal representing the magnetic field in the detection plane of the sensitive component.

[14] In addition, the international patent application WO 95/18982 illustrates a sensor comprising a pair of magnetoresistive elements arranged in a shared plane and spaced from a lateral surface of a permanent magnet. A detection area is defined with respect to a preselected face of a pole of the permanent magnet. The magnetoresistive elements provide a first and a second signal which can be compared to define a third signal representing the presence or absence of a magnetically permeable object in the aforementioned detecting area.

[15] Lastly, patent application US 2013/0320970 illustrates a particular configuration of a sensor comprising a circuit that generates a signal of a magnetic field, comprising at least a sensitive element for generating a magnetic signal indicating for example the features of a magnetic body passing in front of the sensitive element. Then, the sensor comprises a threshold detector, which reacts to the magnetic field signal, adapted to generate an output threshold signal, and a peak detector, which reacts to the magnetic field signal, to generate a respective output signal. The output signals have transitions associated to the magnetic field signal cycles. In addition, there is also included a circuit coupled to the threshold detector and peak detector, adapted to detect the aforementioned transitions, and use said detected transitions to detect an error if a sequence in which the transitions occur diverts from the expected sequence.

[16] Also such proximity sensors, which use magnetoresistive sensitive elements do not allow fully meeting the needs of the various sectors of use, in particular for an efficient monitoring of the parameters involved in the production processes.

Presentation of the invention

[17] The task of the present invention is to overcome the aforementioned problems, by providing a magnetoresistive proximity sensor capable of allowing reliably detecting the position, at least two-dimensional, of a ferromagnetic body, in any operating or application condition.

Within this task, a further object of the present invention is to provide a magnetoresistive proximity sensor, capable of allowing easily detecting data useful for directly identifying the instantaneous position of the barycentre of a section of a body observed on a detection plane.

A further object of the present invention is to provide a magnetoresistive proximity sensor that is easy to obtain and functional, with safe and reliable use as well as relatively inexpensive.

A further object of the present invention is to provide a magnetoresistive proximity sensor that is flexible in use, in particular that is applicable to variously shaped materials, for use in apparatus for machining ferromagnetic materials.

The aforementioned objects are attained according to the present invention, by the magnetoresistive proximity sensor according to claim 1 , as well as the method according to claim 12.

The magnetoresistive proximity sensor comprises a source body provided on a detection plane and adapted to generate a magnetic field. The sensor also comprises a first magnetoresistive sensitive element and at least one second magnetoresistive sensitive element, arranged in the aforementioned generated magnetic field, the first sensitive element being adapted to emit in output a first output signal and the second sensitive element being adapted to emit a second output signal. Such signals, the first and the second, indicate a local variation of the direction of the generated magnetic field, when the magnetic field is crossed by a body to be observed, having a ferromagnetic behaviour. In addition, the sensor comprises acquisition means and, preferably, means for processing the first signal in output from the first sensitive element and the second signal in output from the second sensitive element, to obtain from the combination of the acquired first signal and second signal a measurement, at least in two dimensions, of the instantaneous position of the aforementioned body to be observed, crossing according to any direction in the aforementioned detection plane.

A characteristic of the sensor lies in the fact that due to the provision of at least one pair of sensitive elements, the aforementioned first sensitive element and the aforementioned second sensitive element, the sensor provides a "two-dimensional" measurement indicating the instantaneous position on the detection plane.

More precisely, the sensor according to the invention is capable of detecting the instantaneous position of the barycentre of the section of the body to be observed, intercepted on the aforementioned detection plane, in that it directly "senses", through the aforementioned variations of the generated magnetic field, not only the position of the surface thereof but also the mass of the body observed on the measurement plane and thus provides a direct indication of such quantity. Such capacity is quite advantageous with respect to the known detection systems, of the optical type, which do not allow performing a direct measurement of the instantaneous position of the barycentre of the section of an observed body, but require complex computing starting from the detection of the outer surface of the body, also considering the fact that the section of the observed body is not usually constant. Thus, the complex calculations required by the prior art sensors to obtain the aforementioned measurement slow the detection process.

Thus, it is important to emphasize that, as concerns the above, the process for detecting the instantaneous position of the barycentre of the section of an observed body is, according to the invention, quite rapid with respect to optical sensors of the prior art.

In addition, with respect to the prior art sensors, which use magnetoresistive sensitive elements, it should be observed that the sensor according to the invention allows performing a two-dimensional measurement. As a matter of fact, the first output signal and the second output signal, respectively emitted by the aforementioned first sensitive element and the second sensitive element, are processed to provide a first value in a first dimension and a second value in a second dimension, in the detection plane, whose combination unambiguously provides the instantaneous position in the detection plane of the section of the detected body, which crosses and affects the magnetic field generated by the aforementioned magnetic source.

Contrary to the prior art sensors, which provide a single output signal in any case, possibly processed by a plurality of input signals. Thus, the prior art devices allow obtaining an only one-dimensional measurement, or a measurement of the presence or absence of a ferromagnetic body crossing the detection field.

On the contrary, according to the invention the signals in output from each sensitive element are however used for separating a first coordinate and a second coordinate which, in combination, provide the exact instantaneous position of the section of the ferromagnetic body observed in the detection plane.

In addition, it should be observed that the first sensitive element and the second sensitive element are electrically disjointed so as to provide a first signal and a second signal, whose processing allows providing the two coordinates unambiguously indicating the position of the barycentre of the section of the observed ferromagnetic body in the detection plane. The distinction of a first coordinate and a second coordinate may occur, for example, through an experimental multidimensional calibrating method.

In addition, given that the sensor according to the invention is substantially capable of detecting the instantaneous position of the mean mass, thus of the barycentre, of the section of the observed body on the detection plane, instead of one or more points of the relative contour, the measurement carried out by the sensor is not affected by defects of the shape or surface of the observed body which are usually present.

It should be observed that the sensor according to the invention is not subject of the "dirtying" problems that affect prior art sensors. As a matter of fact, the sensor may also be installed beneath or behind a work surface of any apparatus, for detecting the passage of products being machined on the work surface. Basically, the sensor may detect the variations of the magnetic field determined by such passage even without "seeing", in the optical sense, the product to be detected.

The sensor according to the invention also allows, by processing the relative data, measuring further quantities, such as the displacement speed, acceleration, dimensions and number of bodies that cross or are present in the measurement field at the detection plane.

As a matter of fact, for example it is possible to obtain results regarding the speed of an observed body, measuring at least one first value of the instantaneous position of the observed body, more precisely a section of the body, and a second value of the instantaneous position, observed in a determined time range.

In addition, the dimensions of the observed body may be obtained as a function of the distortion of the magnetic field generated by the source body, caused by the observed body and detectable by the aforementioned sensitive elements in form of the respective signals. The degree of such distortion actually indicates the mass of the body observed on the measurement plane, and thus the dimensions of the body where the material and geometry are known.

According to an aspect of the invention, the first sensitive element and the second sensitive element of the sensor comprise respective resistive elements, for example obtained in form of films made of magnetoresistive material provided with anisotropic and/or giant magnetoresistance.

Preferably, each sensitive element comprises four resistive elements, associated to a Wheatstone bridge, for obtaining a respective voltage sensor in output, starting from the resistance variations of the aforementioned resistive elements upon the passage of said body observed on the detection plane.

According to a further aspect of the invention, at least one of the aforementioned resistive elements comprises a film made of permalloy.

The aforementioned film may be covered by a plurality of aluminium strips oriented at 45° with respect to the direction of a magnetic field determined by the magnetisation of the film, according to an arrangement called "barber pole", to optimise the magnetoresistive effect. Preferably, the aforementioned sensitive elements or at least one from among the first sensitive element and the second sensitive element is of the multilayer type, comprising a superimposition of ferromagnetic layers alternated with non-ferromagnetic layers.

The source body is preferably a permanent magnet, for example a neodymium permanent magnet.

According to an aspect of the invention, the source body is cube-shaped, to improve the sensitiveness to the angular distortions of the magnetic field.

According to a preferred configuration, the first sensitive element and the second sensitive element are arranged equally spaced from the source body.

According to an optimal configuration, the sensor is symmetric with respect to an axis, with the axis of the generated magnetic body, parallel to such symmetric axis.

Such arrangement allows obtaining output signals with an acceptable dynamic and with monotone trend with respect to the displacement of the observed body.

Lastly, the sensor preferably comprises means for amplifying the signals in output from the sensitive elements, for improving the signal/noise ratio for the at least two output signals.

Description of the drawings

The details of the invention shall be more apparent from the description of the detailed description of a preferred embodiment of the magnetoresistive proximity sensor according to the invention, illustrated by way of example in the attached drawings, wherein:

figure 1 shows a schematic plan view of the sensor according to the invention;

figures 2, 2a and 2b respectively show a scheme representing the operation of the sensor according to the invention, as well as charts representing the trend of the signals in output from each sensitive element of the sensor.

Embodiments of the invention

With particular reference to the figures, the magnetoresistive proximity sensor according to the invention is indicated with 1 (see figure 1 ).

The magnetoresistive proximity sensor 1 comprises a source body 2 provided on a detection plane A and adapted to generate a magnetic field B, a first magnetoresistive sensitive element 3 and at least one second magnetoresistive sensitive element 4, arranged at a determined distance a in the aforementioned generated magnetic field B. More precisely, the first sensitive element 3 and the second sensitive element 4 are substantially transducer comprising at least one resistive portion, sensitive, in that it is subjected to a resistance variation due to the variation of the direction of the magnetic field B. Preferably, each sensitive element 3, 4 comprises four resistive portions associated to the four sides of a Wheatstone bridge, so as to emit a first signal and a second signal in output, in response to a local variation of the direction of the magnetic field B.

[51 ] Such resistive portion is preferably constituted by a film made of permalloy, a material with magnetic properties for example obtainable through a photolithographic deposition.

[52] In order to obtain a linear characteristic of the sensor 1 , as regards a reduced measurement range, such permalloy film is preferably covered by a plurality of aluminium strips oriented at 45° with respect to the direction of a magnetic field determined by the magnetisation of the aforementioned film, according to an arrangement called "barber pole".

[53] Or the first sensitive element 3 and the second sensitive element 4 are preferably of the multilayer type, comprising a superimposition of ferromagnetic layers alternated with non- ferromagnetic layers, so as to have the so-called "giant magnetoresistance".

[54] Basically, the first sensitive element 3 and the second sensitive element 4 behave like electronic "compasses", thus indicating the local orientation variation of the magnetic field. Such variation for example occurs in the presence of a body with a ferromagnetic behaviour in the magnetic field B generated by the source body 2.

[55] The magnetic field B, in particular, is modified by the appearance and displacement in the magnetic field B of a ferromagnetic body 5 to be observed (see figure 2).

[56] In order to measure the instantaneous position and possibly the displacement of the crossing ferromagnetic body, the first sensitive element 3 and the second sensitive element 4 are suitably fixed on a non-ferromagnetic support 6, thus incapable of interfering with the magnetic field B generated by the source body 2.

[57] The source body 2 is preferably a permanent magnet, preferably prism-shaped, in particular cube-shaped. Preferably, the magnet is a neodymium magnet.

[58] According to an optimal configuration, the sensor 1 is symmetric with respect to an axis along which the source body 2 is arranged. The first sensitive element 3 and the second sensitive element 4 are positioned on opposite sides of the aforementioned symmetric axis, equally spaced therefrom.

[59] The magnetic field B generated by the source body 2 is preferably oriented with the north- south axis thereof parallel to the aforementioned symmetry axis. In this case, the angle a between the axis of the field and the symmetric axis of the sensor 1 is zero (see figure 1 ). Such configuration allows obtaining monotone, repeatable and readable detections.

[60] The sensor 1 also comprises means for acquiring the signals emitted by the sensitive elements 3, 4, for example of the oscilloscope type.

[61 ] Preferably, the sensor 1 , comprises means for processing the acquired signals, capable of processing the acquired signals to obtain measurements of the instantaneous position of the barycentre of the section of the ferromagnetic body 5 present in the magnetic field B.

[62] Such processing may also provide the displacement speed of the aforementioned body 5, acquiring and suitably correlating the signals indicating the position of the ferromagnetic body in a determined time range.

The operation of the magnetoresistive proximity sensor according to the invention is clear from the description above.

When the ferromagnetic body 5 is not in front of the sensor 1 , the sensitive elements 3, 4 detect the direction of the magnetic field B produced by the source body 2. On the contrary, when the ferromagnetic body 5 is arranged before the sensor 1 , it acquires its own magnetisation and produces its own magnetic field in response to the magnetising one of the magnet or source body 2.

In the latter case, the sensitive elements 3, 4 detect the direction of the magnetic field resulting from the superimposition of the field B generated by the source body 2 and of the responding one of the ferromagnetic body 5.

In practice, the ferromagnetic body 5 to be detected produces a distortion of the magnetic field lines which impact the sensitive elements 3, 4, as a function of the relative position thereof with respect to the sensor 1 (see figure 2).

Each sensitive element 3, 4 transmits a voltage signal which is acquired by the acquisition means.

Lastly, the processing of the acquired signals, thus the combination thereof, allows obtaining the position and/or displacement speed of the barycentre of the section of the ferromagnetic body 5. More precisely, the processing means are configured to obtain a two-dimensional measurement from the at least two acquired signals. As a matter of fact, such means separate a first coordinate in a first dimension and a second coordinate in a second dimension, unambiguously defining the position of the section of the ferromagnetic body in the detection plane.

In the practical implementation of the invention, the used materials as well as the sizes may vary according to the needs.

Should the technical characteristics mentioned in the claims be followed by reference signs, such reference signs were included with the sole purpose of increasing the understanding of the claims and thus they shall not be deemed limiting the scope of the element identified by such reference signs by way of example.