CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of PCT/AT2017/060082 filed on Mar. 31, 2017, the disclosure of which is incorporated by reference. The international application under PCT article 21(2) was not published in English.

FIELD OF THE INVENTION

The present invention relates to a force sensor comprising a frame as well as an oscillation structure, which has multiple arms and can oscillate freely in the frame.

STATE OF THE ART

In practice, strain gauges are generally used for measuring forces, which gauges are attached to components that are exposed to forces and deform when forces occur. In this way, it is possible to calculate the force indirectly by way of the strain that is determined. It is true that strain gauges are relatively precise and can also be used for measuring force vectors in at least two dimensions if multiple strain gauges, disposed transverse to one another, are used. However, fundamental problems occur in the determination of pressure forces as well as due to imperfections in the installation of strain gauges, which usually takes place by means of gluing.

Furthermore, in certain applications, in particular in applications of biology and/or medicine, very small forces in the range of less than 1 μN or even smaller, equal to 1 nN (in other words in the pico-Newton range) occur. Such small forces usually cannot be measured using strain gauges.

TASK OF THE INVENTION

It is therefore the task of the present invention to create an improved possibility for measurement of forces, which avoids the disadvantages described above. In particular, it is supposed to be possible to measure very small forces, preferably down to the pN range.

PRESENTATION OF THE INVENTION

The core of the invention for accomplishing the stated task is to make available a force sensor having an oscillation structure, which oscillation structure can be excited to oscillate in a magnetic field, by means of having alternating current applied to it, and the resonance frequency of which structure changes if a force that acts on the force sensor leads to a distortion of the force sensor. The resulting detuning of the resonance frequency is a measure of the magnitude of the distortion and thereby of the magnitude of the acting force. It is therefore possible to measure very small forces, which lie in the nN range or even the pN range, by means of the dimensioning and design of the structure of the force sensor, in particular as a MEMS (micro-electro-mechanical system), and as a function of the resolution capacity of the frequency measurement.

For this reason, according to the invention, a force sensor is provided, comprising a frame as well as an oscillation structure, which has multiple arms and can oscillate freely in the frame, wherein the arms are fixed in place on suspension regions of the frame and run transverse to one another, at least in certain sections, wherein at least one conducting means is provided, which extends along at least two arms, so as to allow a flow of current in the form of moving charge carriers at least between two suspension regions, wherein the at least one conducting means can have an alternating voltage applied to it, so as to excite at least one oscillation mode of the oscillation structure with a resonance frequency, particularly in an unstressed state of the force sensor, making use of the Lorentz force, when the oscillation structure is situated in a magnetic field, wherein the force sensor is designed in such a manner that the suspension regions are displaced spatially relative to one another, at least in part, when the frame has a force applied to it, that the magnitude of the spatial displacement of the suspension regions depends on the magnitude of the force, that the spatial displacement of the suspension regions brings about detuning of the resonance frequency, and that the magnitude of the detuning depends on the magnitude of the spatial displacement of the suspension regions.

In other words, each arm has an end or an end region with which it is fixed in place on one of the suspension regions.

Furthermore, each arm has a section that runs transverse at least to a section of another arm. Preferably, in this regard, each arm runs in a straight line, at least in certain sections, wherein, however, a curved progression, at least in certain sections, is also conceivable.

The expanse of the conducting means along the at least two arms is preferably understood to be continuous, so as to allow the flow of current along these arms. Based on the coupling of the at least one conducting means with the arms or with the oscillation structure, the Lorentz force can also act on the oscillation structure when the oscillation structure or the force sensor is situated in the magnetic field. If the at least one conducting means now has an alternating voltage applied to it, and if the oscillation structure or its arms is/are situated in a magnetic field, then on the basis of the resulting oscillating Lorentz force, oscillation of the oscillation structure is excited. In this regard, in particular in the case of a magnetic field that is static over time or changes only comparatively slowly, particularly effective excitation can take place in that the frequency of the alternating current corresponds to a resonance frequency of the oscillation structure or to an oscillation mode of the oscillation structure.

An embodiment of the force sensor that is advantageous in terms of production technology occurs in that a conductor track, for example composed of gold or platinum, is provided as the at least one conducting means. Accordingly, the conductor track runs, at least in certain sections, along the longitudinal direction of one arm, and, at least in certain sections, along the longitudinal direction of another arm.

In order to ensure protection of the at least one conducting means against external influences, it can also be provided that the at least one is integrated into the oscillation structure or into its arms. In this case, the at least one conducting means can be structured of metal, in particular of aluminum, for example as a micromechanical structure.

It would also be conceivable to structure the oscillation structure or its arms, as well as the at least one conducting means, in one piece, at least in certain sections, so as to exclude that the at least one conducting means is mechanically damaged or actually destroyed by the oscillations of the oscillation structure. In other words, in this case the oscillation structure or its arms is/are itself/themselves electrically conductive, at least in certain sections.

With regard to the spatial displacement of the suspension regions, it should be noted that it is conceivable, for example, that the acting force or the acting forces is/are oriented in such a manner that two suspension regions maintain the same distance from one another but not from other suspension regions.

Detuning of the resonance frequency is achieved, in particular, as compared with an unstressed state, in other words one in which no force of any kind is acting on the force sensor. Furthermore, of course detuning of the resonance frequency also occurs as compared with a state that was previously present, where an (at least slightly different) force effect already prevailed, and a resonance frequency that was different as compared with the unstressed state was present. In other words, a change in a force effect also results in a change in or detuning of the resonance frequency.

It should be noted that in principle different oscillation modes can be excited. In particular, oscillation modes having a resonance frequency that is particularly sensitive to distortions or displacements only in a specific direction can also be excited, in targeted manner, and thereby a direction-resolved force measurement can take place. In the case of excitability of such oscillation modes for two linearly independent directions, two-dimensional resolution of the force can take place; in the case of excitability of such oscillation modes for three linearly independent directions, three-dimensional resolution can take place. In other words, a multi-axial force sensor can be implemented.

Furthermore, it should be noted that the measured force can also result from a torque that acts on the frame, which can lead to torsion of the frame and corresponding displacements of the suspension regions. Accordingly, the force sensor according to the invention can therefore fundamentally be used also for measurement of torques or torsions.

Finally, it should be noted that in the case of multiple conducting means in the form of conductor tracks, not all the conductor tracks necessarily have to be used simultaneously for excitation of oscillations or to have alternating voltage applied to them simultaneously. For example, one of the conductor tracks can be used for temperature measurement, in that this conductor track does not have alternating voltage applied to it, but rather the electrical resistance of this conductor track is measured. In addition or alternatively, one or more additional conductor tracks, particularly composed of platinum, can be provided for the temperature measurement, the electrical resistance of which tracks is used to determine the temperature. In this regard, these additional conductor tracks can run, at least in certain sections, over the oscillation structure, in particular, at least in certain sections, over at least one of the arms of the oscillation structure and/or in at least one of the suspension regions.

With regard to installation of the force sensor according to the invention on an object that is exposed to force effects to be determined, it should be stated that the most varied attachment possibilities exist, which cannot be conclusively listed at this point. Typically, in this regard, the force sensor according to the invention is connected with the object in fixed manner, and is disposed in level manner when no force effect is present. For example, the force sensor according to the invention can be introduced, in the case of monitoring of a specific component, into the surface of the component, i.e. into a recess of the component provided for this purpose, and then be embedded (naturally without gluing up the oscillation structure). A further, more specific application example would be use of the force sensor according to the invention as a replacement for and/or expansion of a load cell, where the force is applied from the outside. In this regard, it would also be conceivable to mount the force sensor only on one side (upright).

In order to allow particularly stable oscillation of the oscillation structure with regard to external influences, it is provided, in the case of a preferred embodiment of the force sensor according to the invention, that the suspension regions are disposed in the region of corners of the frame.

In order to allow a solution that is simple in terms of production technology, it is provided, in the case of a preferred embodiment of the force sensor according to the invention, that four corners of the frame are provided, which are disposed in a plane of the frame and preferably form a rectangle, particularly preferably a square in the plane.

Typically, the expanse of the frame is clearly less in a normal direction that stands normal to the plane of the frame than in the plane. For example, the frame can have a size of 1 mm*1 mm to 15 mm 15 mm in the plane, and have a size of 100 μm to 500 μm, for example, preferably 250 μm, in the normal direction. In the normal direction, the oscillation structure can have a size that is reduced once again, for example from 10 μm to 50 μm, preferably 20 μm.

In order to expand the sensitivity of the force sensor in the direction of particularly small forces, it is provided, in the case of a preferred embodiment of the force sensor according to the invention, that the frame has at least one meander-shaped section, which is preferably disposed between at least two suspension regions. In this regard, the meander structure brings about the result—similar to a helical spring—of elastic deformability of the corresponding section of the frame along at least one direction in the case of a reduced force effect, in comparison with the force effect required for the same deformation without a meander structure.

In other words, deformability of the frame is already made possible at very small forces. Preferably, in this regard, the at least two suspension regions are connected with one another by means of a section of the frame, which section contains the meander-shaped section.

Preferably, each side of the frame has such a meander-shaped section and/or a meander-shaped section is provided along the circumference of the frame, between two consecutive suspension sections, in each instance, so as to achieve an increased force sensitivity in at least two dimensions, i.e. for forces in all the directions within the plane of the frame.

Vice versa, it is also possible to intentionally provide for no meander-shaped section in the frame, in order to shift the measurement range in the direction of higher forces, in targeted manner, analogous to what was stated above.

In order to guarantee that the oscillation structure has oscillation modes in the most varied directions, which modes can subsequently be excited, it is provided, in the case of a preferred embodiment of the force sensor according to the invention, that the number of arms is a whole-number multiple of four, preferably precisely four. The latter number in a way represents the optimum between the possible oscillation modes to be excited, precise splitting of the stress to be measured in the x and y direction (or in two directions at a right angle to one another, in particular in the plane of the frame), as well as the moments that occur, relative to one another. Furthermore, in the case of more than four arms, the system becomes more complicated again with regard to the possible oscillations as well as the temperature dependence.

In the case of a preferred embodiment of the force sensor according to the invention, it is provided that the oscillation structure has a coupling element by means of which the arms are mechanically coupled with one another, wherein each arm is preferably fixed in place on the coupling element with one end. By means of a suitable selection or structural configuration of the coupling element, the oscillation modes of the oscillation structure can be structured in targeted manner, so as to achieve particularly stable oscillation behavior, for example, in particular in preferred directions.

In the simplest case, the coupling element could also simply be essentially point-shaped, in other words if the arms meet at this point and, in particular, project away from this point in star shape. In the case of four straight-line arms, this then results in a simple cross or X shape.

In the case of a preferred embodiment of the force sensor according to the invention, it is provided that the coupling element forms an edge around a surface that preferably lies in a plane of the frame. It has been shown that very stable oscillation modes can be achieved by means of this configuration.

In this regard, the coupling element does not have to be angular or polygonal, but rather can also be round or at least round in certain sections.

In the case of a particularly simple embodiment of the force sensor according to the invention, in terms of production technology, it is provided that the coupling element is polygonal, preferably four-cornered, preferably rectangular, particularly preferably square.

In a particularly preferred embodiment of the force sensor according to the invention, it is provided that each arm is fixed in place on the coupling element in the region of a corner point of the element.

Preferably, in this regard, each arm is fixed in place on the coupling element with an end.

It has been shown that this arrangement guarantees particularly stable oscillation modes, which can, in particular, form normal modes.

In order to prevent arching of the arms—and thereby detuning of the resonance frequency—on the basis of thermally caused expansion or contraction, it is provided, in the case of a preferred embodiment of the force sensor according to the invention, that the arms are fixed in place on the respective suspension region, in each instance, by means of a temperature-compensation structure that is meander-shaped at least in certain sections. The respective temperature-compensation structure can be formed, in this regard, by a cantilever arm. The temperature-compensation structures structured in this way and fixed in place on the frame are themselves subject to thermal expansion or contraction, and are assigned to the arms in such a manner that the thermal expansion of the arms is compensated and thereby arching is prevented.

Preferably, the temperature-compensation structures are each structured in one piece with the related arm.

In order to be able to produce force sensors having a well-defined structure and well-defined mechanical properties of the frame as well as of the oscillation structure, it is provided, in the case of a preferred embodiment of the force sensor according to the invention, that the frame and the oscillation structure are produced in one piece from silicon, preferably mono-crystalline silicon. This allows production on the basis of a silicon wafer, for example, in what is called a silicon-on-insulator (SOI) method, which is known.

In order to determine the (resonance) oscillations of the oscillation structure, in particular the frequencies of the (resonance) oscillations, known read-out means can be used. Accordingly, a system comprising a force sensor according to the invention as well as read-out means for determination of the resonance frequency is provided, according to the invention, wherein the read-out means preferably comprise at least one optical sensor and/or at least one capacitive sensor. As has been said, such sensors are known.

Capacitive sensors are particular suitable for measurement of oscillation modes, the oscillation plane or amplitude of which stands normal to the plane of the frame, wherein the capacitive sensors can then be disposed above and/or below the plane of the frame. Using optical sensors, it is particularly possible to measure oscillation modes, the oscillation plane or amplitude of which lies in the plane of the frame. For example, an optical measurement can take place in the transmission geometry (normal to the plane of the frame), and in this way, the periodic change in the placement or geometry of the oscillation structure in the plane of the frame can be detected.

In a preferred embodiment of the system according to the invention, it is preferred that a control unit is provided, using which an alternating voltage can be applied to the at least one conducting means, so as to excite the oscillation structure to cause it to oscillate, and wherein the control unit is connected with the read-out means for determination of the resonance frequency, wherein preferably at least two conducting means are provided and the control unit is designed for applying counter-phase alternating voltages to the at least two conducting means. Therefore the control unit can use alternating voltages of different frequencies—in particular one after the other—for application, and can determine at least one resonance frequency on the basis of detection of the resulting oscillation of the oscillation structure, in each instance, wherein the selection of the next frequency can be made dependent on the current detection result.

Application of counter-phase alternating voltage can have advantages in terms of measurement technology, in particular if not only the frequency but also the amplitude is supposed to be determined, since in this way, a difference signal of the read-out means, using which the amplitudes of the oscillation modes are measured, can be generated very easily and without additional computation effort. For example, implementation of a differential capacitor would be possible in the case of a capacitive read-out of the amplitudes.

In order to be able to determine at least one resonance frequency particularly rapidly, it is provided, in the case of a preferred embodiment of the system according to the invention, that the control unit is designed for applying at least one pulse of alternating voltages to the at least one conducting means, so as to excite the oscillation structure to cause oscillations, wherein the at least one pulse has a bandwidth of frequencies, which bandwidth comprises the resonance frequency of at least one oscillation mode of the oscillation structure, preferably in an unstressed state of the force sensor. The sequential application of alternating voltages at different frequencies as described above is clearly more time-consuming in comparison. In particular proceeding from knowledge of the at least one resonance frequency in the unstressed state (i.e. without force effect on the sensor), a suitable or sufficiently great frequency band can be selected, so as to determine the resonance frequency that has been detuned as the result of the force effect.

In practice, use of a scanner, in particular a hand scanner, for example, would be possible, which scanner is brought into the vicinity of the force sensor and brings about application of the at least one pulse to the at least one conducting means (in particular inductively). Furthermore, the read-out means as well as the control unit can be integrated into the hand scanner. Accordingly, the force sensor can remain mounted on an object, in the case of which object one wishes to determine possible force effects on this object only from time to time (and not continuously). Such an object can be, for example, a building or a prosthesis worn by a human being.

Fundamentally, it is possible, in this regard, to also excite multiple resonance frequencies for different modes with one pulse.

In order to excite different resonance frequencies in targeted manner, for example in order to conveniently make use of direction dependencies of the corresponding oscillation modes for a direction-resolved force measurement, in terms of measurement technology, it is provided, in the case of a particularly preferred embodiment of the system according to the invention, that the control unit is designed for application of multiple pulses of alternating voltages to the at least one conducting means, one after the other, wherein the different pulses comprise resonance frequencies of different oscillation modes of the oscillation structure, preferably in the unstressed state of the force sensor. In other words, a first pulse comprises at least one first resonant oscillation mode of the oscillation structure, preferably in the unstressed state; a further pulse comprises at least one further resonant oscillation mode of the oscillation structure, preferably in the unstressed state, wherein the first oscillation mode and the further oscillation mode are different.

In the case of a preferred embodiment of the system according to the invention, it is provided that the system comprises means for generation of the magnetic field, preferably at least one permanent magnet and/or at least one Helmholtz coil. In this way, it can be ensured that an at least somewhat defined magnetic field is always present for the force measurements, and thereby desired oscillation modes of the oscillation structure can be set or detected very precisely, so as to allow very precise force measurements.

In this regard, the magnetic field does not need to be defined particularly precisely, since it is sufficient for excitation of a desired oscillation mode of the oscillation structure to arrive at the corresponding resonance frequency by means of a suitable selection of the frequency of the alternating voltage. The means for generation of the magnetic field therefore also do not need to be attached or disposed on the force sensor according to the invention in fixed or permanent manner.

Instead, the means for generation of the magnetic field, in particular a permanent magnet, can be disposed in the scanner, in particular the hand scanner already mentioned above. The scanner then serves not only for application of alternating voltage, but at the same time also makes the magnetic field required for oscillation excitement by means of the Lorentz force available.

In the manner described, excitation of what are called in-plane oscillations, in other words of oscillation modes, the oscillation plane or amplitude of which lies in the plane of the frame, can also be easily guaranteed. In this way, torsions of the frame or torques applied to it can be measured particularly well. Accordingly, it is provided, in the case of a particularly preferred embodiment of the system according to the invention, that the control unit is designed for excitation of oscillations of the oscillation structure in a plane of the frame.

As has already been stated, the force sensor according to the invention is particularly suitable for applications in biology or medicine, for example on medical instruments, as well as in prosthetics. The comfort of a prosthesis and the prevention of injury to the body, in particular, are essentially dependent on an optimal distribution of force. It is conceivable to equip prostheses and/or implants with force sensors according to the invention, so as to obtain feedback regarding the respective stress state when using the prostheses or implants.

Accordingly, a prosthesis comprising a force sensor according to the invention and/or a system according to the invention is provided, according to the invention.

