Integrated magnetometer and method of detecting a magnetic field

An integrated magnetometer comprises at least one field sensor unit including a first transducer element for generating, in response to a detected magnetic field, a first electrical output signal. At least one gradient sensor unit of the magnetometer includes at least a pair of second transducer elements which are arranged to detect the magnetic field at two different locations and generate, in response to the detected magnetic field, a second electrical output signal. The first and second transducer elements are formed on a common substrate and are encompassed by a common protective layer and/or housing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 to European Patent Application No. 20163834.3, filed on Mar. 18, 2020.

FIELD OF THE INVENTION

The present invention relates to an integrated magnetometer, and more particularly, to a sensor for detecting ferro- or superparamagnetic structures on material surfaces.

BACKGROUND

Highly sensitive magnetometers are used to detect the very weak magnetic material contained in some magnetic security features of a banknote when preforming security checks of the note. These security features may comprise magnetic pigments in the printing color pigments, but also metallized safety threads, or other specifically designed magnetic features. Due to their high sensitivity and low noise, anisotropic magnetoresistance (AMR) sensors may be used for this purpose. However, the electrical signals generated are still very weak.

In the past, banknote line sensor arrays had a reading width of approximately 10 mm per channel. However, recent efforts are being made to increase the resolution in order to be able to resolve more magnetic details on the banknotes. Smaller magnetic security features are expected due to the increasing number of counterfeit banknotes and improved note quality among counterfeiters. With a higher resolution of the magnetic image of a banknote, the verification of the features must increase in accuracy. For example, a banknote validator system may require 112 channels with a total scan length of 196 mm. This corresponds to a resolution of 1.75 mm per channel.

Generally, magnetoresistive sensors (also referred to herein as magnetic sensors) measure the direction and intensity of magnetic fields. Such arrangements are used to identify magnetic materials such as magnetic markings in banknotes, as well as to detect angles and positions by means of magnetometers. The sensors make use of the magnetoresistive effect, which is the tendency of a material (e.g., ferromagnetic) to change the value of its electrical resistance in an externally applied magnetic field. In particular, in multicomponent or multilayer systems, giant magnetoresistance (GMR), tunnel magnetoresistance (TMR), colossal magnetoresistance (CMR), and extraordinary magnetoresistance (EMR) are observed, while the anisotropic magnetoresistive effect (AMR) needs only one layer to occur.

Magnetic sensors provide high accuracy and robustness against challenging environmental conditions and play an important role in various applications such as manufacturing and transportation applications. In more detail, sensors based on a magnetoresistance effect are important components because of their low intrinsic measurement error and high stability. Additionally, the favorable temperature characteristic as well as the robustness against harsh environmental conditions leads to the relevance of magnetoresistive sensors in many important applications.

The strength of the magnetic field is usually described by the magnetic flux density B. The gradient of the magnetic flux density results from a change of B per unit distance along the direction of the greatest change of B. As B is a vector, which usually is also a function of all three spatial dimensions, the gradient is a tensor comprising all partial derivatives of the three spatial B-field components with respect to the spatial coordinates. In practice, for measuring the gradient two magnetic field sensors are used which measure the magnetic flux density at two different locations, separated by a well-defined distance. Then, the difference of the two measured flux density values is calculated and divided by the distance. The distance is chosen in a way that the B-field varies linearly within the distance, in other words, by choosing a distance that is small compared to the distance between each of the sensors and the magnetic field source.

When reading magnetic safety structures on documents such as banknotes with conventional magnetoresistive sensor arrays, it is known to either detect the intensity of the magnetic field as an absolute value or to detect a local field gradient. For each technology there exist different sensor designs. Specifically, field sensors which detect the absolute intensity of the magnetic field have the advantage of a high sensitivity especially over higher distances. However, field sensors have the disadvantage that their output signal may be impaired by external disturbing magnetic fields. On the other hand, gradient sensors preform a differential measurement and are therefore insensitive to external disturbing magnetic fields. However, gradient sensors have the disadvantage of a much lower sensitivity compared to field sensors, in particular for larger distances from the magnetic objects to be detected.

Accordingly, there is a need for a magnetic sensor suitable for banknote sensing which avoids the above drawbacks of both field and gradient sensors.

SUMMARY

In one embodiment of the present disclosure, an integrated magnetometer includes at least one field sensor unit having a first transducer element for generating, in response to a detected magnetic field, a first electrical output signal. At least one gradient sensor unit of the magnetometer includes at least a pair of second transducer elements which are arranged to detect the magnetic field at two different locations. The second transducer elements are adapted to generate, in response to the detected magnetic field, a second electrical output signal. The first and second transducer elements are formed on a common substrate and are encompassed by a common protective layer and/or housing.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Technical solutions of the present disclosure will be described hereinafter in detail through embodiments and with reference to the attached drawings. In the specification, the same or the like reference numerals refer to the same or the like elements. The illustration of the embodiments of the present disclosure made with reference to the attached drawings is aimed to explain the general inventive concept of the present disclosure, not to be construed as a limitation of the present disclosure.

Referring toFIG.1, a schematic diagram of the detection of a moving (ferro-) magnetic marking on a banknote by means of a magnetic gradient sensor is shown. In particular, a banknote100with a magnetic safety feature102moves with respect to a magnetic gradient sensor104. The safety feature102is generally referred to as a magnetic field source with a north and south pole and magnetic flux lines106. The magnetic source102may be attached to any other movable object for different measurement applications, for example, determination of a position. However, the following description is made in the context of a banknote authentication system as an example of how the integrated magnetometer according to the present invention works and can be used.

The diagram ofFIG.1shows as a curve108the measurement signal δ as a function of the time t. In particular, δ(t) is the difference between the measurement values of two transducer elements112A,112B. Illustrations A, B, and C show respective positions of the magnetic source102with respect to the magnetic gradient sensor104at specific instants while the banknote100is moving in the direction of the arrow110. Without loss of generality, this direction is assumed to be along the y-direction, whereas the banknote100is distanced or spaced apart from the magnetic gradient sensor104in the z-direction. The directions of the corresponding Cartesian magnetic field components Hyand Hzare schematically indicated inFIG.1.

By way of example, the banknote100may be moving in the direction110with a velocity of 1 m/s to 10 m/s. The magnetic gradient sensor104has two transducer elements112A,112B. Each of the transducer elements112A,112B detects a magnetic flux in the y-direction. When the magnetic source102is in the position A shown inFIG.1, a maximal difference is measured in the negative direction because only transducer element112A, which is on the left side of the magnetic gradient sensor104, measures a magnetic flux as indicated by arrow114.

When the banknote100containing the magnetic source102moves further in the direction110, the measurable magnetic flux for the left-hand transducer element112A becomes smaller, whereas the measurable magnetic flux for the right-hand side transducer element112B increases. Because the measurement signal δ(t) represents the difference between the signals of the two transducer elements112A and112B, the configuration for positon B, where the magnetic source102is symmetric with respect to the transducer elements112A,112B, results in a zero signal.

A further (positive) maximum of the curve108can be seen for position C, where the right-hand side transducer element112B is in closest proximity to the magnetic source102.

The advantage of such a gradient measurement is the fact that inherent to the differential measurement disturbing magnetic fields do not impair the accuracy of the measurement. However, the signal amplitude is lower than for magnetic field sensors which detect the absolute value of the flux density.

FIG.2illustrates a banknote authentication system, corresponding to the system shown inFIG.1, having a magnetic field sensor116instead of the magnetic gradient sensor104. The magnetic field sensor116has only one transducer element118which outputs a signal H(t) which is indicative of the absolute value of the magnetic flux in the y-direction. Curve120shows the progression of the signal with the time t while the banknote100is moving along the direction110. As can be seen from the comparison withFIG.1, the measurable signal amplitude is higher. However, a problem with magnetic field sensors is that they are sensitive towards disturbing magnetic fields which directly influence the measurement signal.

Furthermore,FIG.3illustrates another aspect of the magnetic gradient sensor shown inFIG.1. InFIG.3the top view of the magnetic gradient sensor104is shown together with the side view according toFIG.1. The distance between the first and second transducer elements112A and112B is denoted by Δy. The distance Δy limits the resolution of the gradient sensor in the scan direction, i.e. the direction110in which the banknote100moves (seeFIG.1).

In the shown example, there is not only one pair of transducer elements112A and112B, but a second pair of transducer elements122A and122B. The distance Δx between the transducer elements112and122determines the minimal lateral resolution between two magnetic features which are arranged along the x-direction. The output signals of the two pairs of transducer elements112,122form channels, with each channel containing the information about a particular region in the x-direction.

The amount of channels is not limited to one or two, also three channels or more are possible.FIGS.4and5illustrate examples of three-channel magnetic sensors. In particular,FIG.4shows a three-channel magnetic gradient sensor400. The magnetic gradient sensor400comprises three pairs of transducer elements412A,412B. Each pair of transducer elements412A,412B is connected in series by means of electrically conductive leads424. At the connecting line between the first and second transducer elements412A,412B a central node426is connected to contact pads428. Via the contact pads428, the central node426may be connected to signal processing components (not shown). In particular, the differential signal is generated for each pair of transducer elements as explained above with reference toFIG.1.

FIG.5illustrates a three-channel magnetic field sensor500according to the principle explained with reference toFIG.2. It is noted that the transducer element512may be formed as a pair of magnetoresistive elements512A,512B. These magnetoresistive elements512A,512B are arranged directly adjacent to each other, so that they essentially experience the same magnetic flux at any given time. Unlike inFIG.4, wherein both resistors412A and412B react in the same way to an external field, resistors512A and512B are designed to react to a homogeneous field with opposite resistance changes, so that a homogeneous magnetic field across both resistors in this case causes a strong output signal from the voltage divider arrangement.

When using AMR magnetoresistive elements, such opposing reaction of the two resistors can be achieved by providing a different tilting direction of the barber poles arranged on the nickel iron strips. In case of GMR and TMR elements, different pinning directions of the pinned layers are used.

With respect to signal evaluation, the magnetoresistive elements512A,512B may also be treated as a half-bridge (or voltage divider). At the connecting line between the first and second transducer elements512A,512B a central node526is connected via electrically conductive leads534to contact pads528. Via the contact pads528, the central node526may be connected to signal processing components (not shown).

The arrangements shown inFIGS.4and5, however, have the drawbacks explained above and inherent to using separate chips for measuring either the magnetic gradient or the magnetic field. In contrast thereto, with reference toFIGS.6and7an improved magnetometer600according to the present invention will be explained.

FIG.6illustrates the layout of an integrated magnetometer600according to the present invention, combining the advantages of magnetic gradient sensing and magnetic field sensing on one integrated chip with a common substrate, and overcoming the problems of known magnetic sensors.FIG.7shows the corresponding equivalent circuit diagram of the integrated magnetometer600.

The integrated magnetometer600is designed for detecting a two channel magnetic feature with the two separate parts of the feature being distanced apart by a channel distance602. The channel distance602may for instance be 1.75 mm. The integrated magnetometer600comprises, for each of the two channels, a magnetic field sensor604,606, which comprises as transducer elements two magnetoresistive elements RH1, RH2and RH3, RH4, respectively. The two magnetoresistive elements RH1, RH2of the first magnetic field sensor604are arranged closely adjacent to each other, and the two magnetoresistive elements RH3, RH4of the second magnetic field sensor606are arranged closely adjacent to each other. Thus, the magnetoresistive elements of each magnetic field sensor experience essentially the same magnetic flux to be measured. The voltage measured at the terminal UoH1can be evaluated for generating a first output signal indicative of the absolute value of the magnetic field strength.

Furthermore, the integrated magnetometer600comprises, for each of the two channels, a magnetic gradient sensor608,610, which comprises two magnetoresistive elements RG1, RG2and RG3, RG4, respectively. The two magnetoresistive elements RG1, RG2of the first magnetic gradient sensor608are arranged distanced apart from each other by a distance Δy, and the two magnetoresistive elements RG3, RG4of the second magnetic field sensor610are also arranged distanced apart from each other by the distance Δy. Thus, the magnetoresistive elements of each magnetic gradient sensor work as a gradient sensor as explained above referring toFIG.1. The voltage measured at the terminal UoG1can be evaluated for generating a second output signal indicative of the gradient of the magnetic field strength. Advantageously, when forming a differential output signal, this output signal using the magnetoresistive elements RG1, RG2and RG3, RG4, respectively, is not impaired by the presence of disturbing external magnetic fields. The strong first output signals of the magnetic field sensors604,606may also be corrected taking into account the differential output signal of the gradient sensors608,610.

According to the present disclosure, all transducer elements, the interconnecting electrically conductive leads612and the contact pads614are integrated on one common substrate616an are covered by a common protective layer and/or housing618. In other words, the integrated magnetometer600combines a gradient sensor and a field sensor on one sensor chip.

In order to meet various demands regarding the channel width and number, by combining channels also channel widths being an integer multiple of the channel distance can be realized. In the simplest case, the output signals of neighboring channels can be mixed by directly connecting them. However, individual half bridge signals or paired differential signals analog to a complete Wheatstone bridge may also be generated.

In practice, larger channel widths and a smaller number of channels distributed over the length of the entire sensor array are often used to simplify evaluation. In such cases, the individual outputs of one or more integrated magnetometers can be connected directly to each other. Alternatively, two separate groups of channels, each using one or more connected channel outputs, with opposite signal polarity can be used to generate a differential signal. In order to obtain the same or opposite signal polarities required for this purpose, one can either select the direction of the auxiliary magnetic fields required for the operation of MR sensors accordingly or also operate adjacent integrated magnetometers with the same or opposite supply voltages.

This solution allows the measurement of a high-resolution field signal and a gradient signal whose integral provides a reconstructed field signal with lower resolution but without interference. The advantages of both operating modes can thus be used together without disadvantages.

Furthermore, as can be seen fromFIG.6, the pin assignment of the sensor housing has been selected so that when using only one operating mode, it is possible to choose between the operating modes by soldering the terminals to a PCB rotated 180°. In particular, the terminals belonging to the field sensors (UoH2, UoH1, GND, and VccH), which are encircled by solid lines, are arranged in a pattern corresponding to the terminals belonging to the gradient sensors (UoG2, UoG1, GND, and VccG), which are encircled with broken lines inFIG.6. The substrate616may comprise further electronic elements, such as protection components, for example, ESD protection diodes, pre-amplifiers, and/or digitizer circuitry. According to the present disclosure, at least three magnetoresistive transducer elements are arranged in a way that both a high-resolution field signal and a robust gradient field signal is obtained, whose integral provides a reconstructed field signal with lower resolution but without interference. Moreover, a comparison of both signals gives information on the quality of the measurement.

The above-described concept enables optimized signal quality and information retrieval. The integration of a field sensor with a gradient sensor does not increase the space requirement as compared to the gradient sensor taken alone. No different sensor types are required for field and gradient measurement. This simplifies and reduces the cost of sensor production, warehousing, and logistics. The pin assignment of this surface-mount technology (SMT) package allows changing the operating mode without PCB modification. This simplifies the adaptation of sensor modules to customer requirements and increases the flexibility in development as well as in production. The inventive concept is in particular suitable for magnetoresistive transducer elements, AMR or TMR transducers.

The operation of the exemplary integrated magnetometer600will be explained in more detail with respect toFIGS.8and9.

FIG.8shows the operational mode where only the magnetoresistive field sensors604,606(only one sensor is visible in the cross-sectional view ofFIG.8) are measuring the magnetic field of a moving magnetic source620. Schematically, the magnetic flux lines622are shown with the tangential components (broken arrows) and the component along the sensitive plane of the magnetoresistive transducer element (i.e. along the y-direction, solid arrows).

As with the arrangement shown inFIG.2, the maximum signal can be measured when the magnetic flux lines622are parallel to the sensitive plane of the magnetic field sensor604,606. The curve H(t)624has an H value of zero, when the magnetic flux lines are orthogonal to the sensitive plane of the magnetic field sensor604.

As shown inFIG.9, the magnetic gradient sensor608has two magnetoresistive transducer elements608A,608B, which are distanced or spaced apart by a distance Δy in scan direction. The gradient measurement yields an output signal which is the difference of the voltage measured across the magnetoresistive element608B and the voltage measured across the other magnetoresistive element608A. The magnetoresistive transducer elements608A,608B thus measure the local difference of the magnetic field distribution. The distance Δy determines the resolution in the scan direction and may for instance be 0.75 mm.

According to the above-described embodiments, the present disclosure provides a device for the detection of ferro- or superparamagnetic structures on material surfaces using magnetic field sensitive sensor elements. These magnetic field sensitive sensor elements are assembled on a common carrier substrate. A spatially small sensor element array is responsive to homogeneous magnetic fields which generates a sensor signal which is initially approximatively proportional to a component of the magnetic field generated by the field generating structures. A second sensor element arrangement reacts to field differences between two respectively spatially small but spaced apart sensor elements. The sensor element arrangement responding to homogeneous fields is arranged in the space between the two sensor elements which form the sensor arrangement sensitive to local field differences. Specifically, the sensor elements each may comprise two resistors connected to a voltage divider arrangement and a magnetoresistive effect is used for signal generation. Further, separate supply voltage terminals may be provided for the sensor element arrays responsive to homogeneous fields and local field differences. An exemplary terminal assignment of the sensor package may be provided, which allows a change between a measurement of local field differences and a measurement of homogeneous fields by means of a 180° rotated placement on a carrier plate.

It should be appreciated by those skilled in this art that the above embodiments are intended to be illustrative, and many modifications may be made to the above embodiments by those skilled in this art, and various structures described in various embodiments may be freely combined with each other without conflicting in configuration or principle.

Although the present disclosure have been described hereinbefore in detail with reference to the attached drawings, it should be appreciated that the disclosed embodiments in the attached drawings are intended to illustrate the preferred embodiments of the present disclosure by way of example, and should not be construed as limitation to the present disclosure.

Although several exemplary embodiments have been shown and described, it would be appreciated by those skilled in the art that various changes or modifications may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.

It should be noted that, the word “comprise” doesn't exclude other elements or steps, and the word “a” or “an” doesn't exclude more than one. In addition, any reference numerals in the claims should not be interpreted as the limitation to the scope of the present disclosure.