CONTACTLESS COMPRESSION OR TRACTION STEP GENERATION DEVICE

Abstract

The invention relates to a device for characterizing a sensor capable of performing dynamic force or mass measurements, for example, a force sensor (12), comprising: - magnetic means (8) for generating a magnetic field along at least one axis (XX'); - means (2, 4, 6) for adjusting and then maintaining a relative position, along the axis (XX'), between a force sensor (12) to be tested and the magnetic means (8); - means (22-24) for measuring a signal representative of a force, in tension or compression, applied to a non-contact force sensor, directly using said magnetic means. Abstract figure: Figure 1A

Description

Title of the invention: DEVICE FOR GENERATING COMPRESSION OR TRACTION STEPS WITHOUT CONTACT TECHNICAL FIELD AND PRIOR ART

[0001] The invention relates to the field of metrology for sensors, in particular force sensors, with a view to their testing and / or characterization and / or dynamic calibration.

[0002] It makes it possible, for example, to generate compression or traction steps without contact, for example for in-situ calibration of force sensors in static mode and / or for dynamic characterization.

[0003] In particular, it allows us to go back to the transfer function of the measurement chain.

[0004] Currently, 3 methodologies exist in the literature to qualify force sensors in dynamics.

[0005] First, there is the impact shock method generated by a shock hammer, which generates "pulse" type signals. This method is suitable for force sensors with a large measuring range and its implementation, which requires the use of interferometers for acceleration measurement, makes the in-situ aspect difficult, even if efforts are made to work on an embedded system, as described in the article by N. Vlajic and A. Chijioke, "Traceable calibration and demonstration of a portable dynamic force transfer standard," published in Metrologia, vol. 54, no. 4, pp. S83-S98, 2017 / 07 / 18 2017.

[0006] There is also the sinusoidal excitation method, which subjects the sensor to periodic oscillations, by connecting the end of the latter to a vibrating pot; on this subject, one can refer to the article by C. Schlegel et al., "Traceable periodic force calibration," Metrologia, vol. 49, no. 3, pp. 224-235, 2012 / 03 / 12 2012.

[0007] Finally, there is also a method for generating a force step which can be done by the so-called "knife" or "cut-wire" method, in which the sensor is subjected to a compressive or tensile force using a wire connected to a known mass allowing a known force to be applied. When the wire is cut, the system returns to its equilibrium state after having been subjected to a step considered perfect. The frequency response of the measurement chain is obtained.

[0008] A generic method proposes to encompass the 3 types of calibration described previously. It is based on the principle of a levitated mass (Levitation Mass Method LMM) above a force sensor via the use of the pressure of an air column or a magnetic field, as described by Y. Fujii and A. Takita, in the article "Correction method for force transducers on dynamic condition with or without the Levitation Mass Method (LMM)," published in Procedia Engineering, vol. 32, pp. 13-17, 2012 / 01 / 01 / 2012. This method uses the inertial force generated by the mass impacting the sensor, obtained by the product of the mass and its acceleration. Depending on the experimental setup, the three types of stresses above can be obtained.

[0009] In the literature (see for example the article by DB Newell et al. "The NIST Microforce Realization and Measurement Project," Instrumentation and Measurement, IEEE Transactions on Instrumentation and Measurement, vol. 52, pp. 508-511, 05 / 01 2003) we find stresses based not on a mechanical force, but on an electrostatic force: this involves the application of force steps in ranges of the order of nN to pN, and in a static calibration context.

[0010] Generally speaking, there is no method that is free from implementation by direct contact, which generates a disturbance of the measurement. In particular, in the case of the technique by cutting a wire by a blade, a certain quantity of wire remains attached to the sensor and is set in motion, which distorts the equilibrium position. In addition, this technique requires the implementation of a force perpendicular to the wire (to actuate the cutting blade) which again generates a bias force which is disruptive.

[0011] 3 problems related to this technique are discussed later in this application.

[0012] Concerning the other techniques mentioned above, they are complex to implement: they require the presence of an operator and a modification of the experimental system when one wants to modify the forces or constraints implemented and in particular cannot be carried out in situ, on a measurement site and outside of a laboratory.

[0013] Furthermore, a technique is sought which can be implemented over a wide range of work, for example from the order of 1 / 100th of a Newton to beyond the Newton (N). Statement of the invention

[0014] The invention aims to solve all or part of the problems set out above or in the remainder of the present application.

[0015] The invention firstly relates to a device for testing and / or for calibrating / or characterizing the transfer function of a sensor (for example a force sensor), comprising:

[0016] - magnetic means for generating a magnetic field along at least one axis (XX'), for example an electromagnet;

[0017] - means for adjusting then maintaining and / or measuring a relative position, according to the axis (XX'), between a sensor to be tested and the magnetic means;

[0018] - means for measuring a signal representative of a force, in traction or compression, applied to a sensor directly using said magnetic means;

[0019] - possibly means for measuring a signal representative of the setpoint applied to magnetic means, for example, the supply current and / or voltage of the magnetic means.

[0020] According to one embodiment, the means for generating a magnetic field comprise an electromagnet.

[0021] Preferably, a device according to the invention further comprises means for reversing the direction of application of the magnetic field along the direction (XX'), for example means for:

[0022] - reverse the direction of current flow in an electromagnet, allowing the generation of a tensile or compressive force; these means then play the role of an actuator;

[0023] - and / or to cut the application of the magnetic field along the axis (XX'); these means then play the role of a switch.

[0024] According to an exemplary embodiment, a device according to the invention further comprises means, for example of the mechanical type, for limiting the distance between the magnetic means and a sensor to a minimum distance. The measuring chain can thus be secured.

[0025] The device according to the invention may also comprise means for adjusting and maintaining and / or measuring a relative position, along the axis (XX'), between a sensor to be tested and the magnetic means, may comprise at least one motor, for example a stepper motor.

[0026] The device according to the invention may also comprise motors, for example stepper motors, along at least 2 axes, for example the axes (YY') and (ZZ'), to center the sensor to be characterized and the magnetic means.

[0027] According to one embodiment, the mobile support is controlled by a 3-axis system controlled by 3 motors.

[0028] A device according to the invention may further comprise:

[0029] - means for centering and / or maintaining and / or measuring a relative position according to the axes (YY') and (ZZ') between the sensor to be characterized and the magnetic means;

[0030] - and / or means for measuring and / or digitizing and / or processing a signal taken from the terminals of a sensor tested or characterized using the device according to the invention, for example means for carrying out a Fourier transform of said signal;

[0031] - and / or means for measuring and / or digitizing and / or processing a signal taken from the terminals of the voltage or current setpoint of the magnetic means generating the stress using the device according to the invention, for example from the power supply system of the magnetic means;

[0032] - and / or means for storing response and / or calibration data and / or hysteresis of a sensor.

[0033] The invention also relates to a method for testing and / or calibrating and / or characterizing a sensor, for example a force sensor, for example implementing a device as described above and in the remainder of the present application.

[0034] Such a method of testing and / or calibrating and / or characterizing a sensor may comprise the following steps:

[0035] - maintaining said sensor fixedly relative to generation means of a magnetic field;

[0036] - possibly the positioning (or centering) of said sensor with the means magnetic;

[0037] - the generation of a magnetic field (B) allowing the application of a constraint, for example, traction or compression on the sensor, by direct interaction between this field and the sensor;

[0038] - measuring and / or recording and / or digitizing the response of the sensor to this constraint, for example in traction or compression;

[0039] - possibly the measurement and / or recording and / or digitization of the piloting magnetic means.

[0040] A method according to the invention may further comprise a step of mounting an accessory, for example a ring, on the sensor, for example a ring.

[0041] Such a method may include a prior step of mounting a magnetic part on the accessory.

[0042] In a method according to the invention, it is possible to apply alternately a tensile force or stress then a compressive force (or vice versa) to the sensor by reversing the magnetic field, for example by reversing the direction of circulation of a current in an electromagnet which generates said magnetic field.

[0043] If the body of the sensor is non-magnetic, the method may include a prior step of mounting a magnetic part on or in the sensor, for example, a screw. Note that the added magnetic part is of a chosen reference compatible with the measuring range of the sensor.

[0044] A method according to the invention may further comprise modifying or adjusting the distance between means for generating a magnetic field and said sensor. It is thus possible to carry out measurements for different distances between the sensor and the means for generating a specific force via the magnetic field.

[0045] A method according to the invention may comprise the application to the sensor:

[0046] - of a tensile stress then of a compressive stress;

[0047] - or a tensile or compressive stress, then an absence of stress (the sensor then being at rest).

[0048] The tensile or compressive stress can be applied along a first sensitivity axis of the sensor, preferably by limiting the transverse forces seen by the sensor. If the sensor comprises at least a second sensitivity axis, the method can further comprise a step of modifying the orientation of the sensor, and the application of a tension or a compression along at least this second sensitivity axis of the sensor.

[0049] A method according to the invention can be applied for example to a sensor capable of performing a dynamic weight or mass measurement, for example a force sensor of the piezoresistive, or piezoelectric or capacitive, monoaxial or triaxial type or a sensor comprising a scale.

[0050] This method according to the invention can be extended for example to a sensor capable of performing a dynamic mass measurement, for example a scale.

[0051] For example, a sensor to which the invention can be applied can comprise one or more faces, the magnetic field being applied in a direction perpendicular to this or these faces.

[0052] A method according to the invention may further comprise a step of storing data on the temporal variation of the response of the sensor from an initial state, when subjected to said stress, for example said traction or compression, as a function of a position of the sensor relative to means which generate said magnetic field (B), to its transition to a final state, which may be the sensor at rest or the sensor according to a stress different from its initial state, for example traction or compression, as a function of a new position of the sensor relative to means which generate said magnetic field (B). Such data are for example response and / or calibration and / or hysteresis qualification data of the sensor.

[0053] A device according to the invention or a method according to the invention makes it possible to test and / or characterize and / or calibrate a force sensor in dynamic mode (the response of the sensor is measured as a function of time) by applying one or more successive constraints, for example by generating one or more force steps (for example with return to its initial state at rest or under constraint), the amplitude of which is controlled by applying a magnetic field directly to the sensor and without contact with it.

[0054] This contactless request makes it possible in particular to trace the transfer function of the sensor tested, and / or of the measurement chain as a whole, for example. with signal acquisition and / or processing means, in the working environment of the sensor, taking into account any mechanical couplings, for example, when an accessory support must be mounted on the sensor for the purposes of the experiment.

[0055] The static phase of a bearing generated by application of a magnetic field constant over time also makes it possible to calibrate the force sensor in static mode: for the application of a given field, for example as a function of the distance between the magnetic means and the sensor, the sensitivity of the sensor can be measured.

[0056] A device or method according to the invention makes it possible to generate a step or a constraint with a magnetic field, but also to be able to test a sensor in tension and / or in compression (for example first in tension, then in compression, or vice versa) by adapting the orientation of the magnetic field, for example the polarity of the electromagnet: no modification or interference is necessary with the experimental setup. In addition, this calibration can be carried out at an experimental location, and not only in a laboratory.

[0057] A device or method according to the invention makes it possible to stress, in one direction, a sensor with one component (1 axis of sensitivity) or several, for example 3, components (for example 3 axes of sensitivity) by modifying the orientation of the sensor for the characterization and / or calibration of each axis.

[0058] In a method or device according to the invention, the sensor, in tension or in compression, may be:

[0059] - subjected to stress or force over a wide range, for example between 1 / 10 of Newton and 1 Newton or more than 1 N, for example up to 5 N (depending on the means for generating a magnetic field, the method or device according to the invention can be used in higher ranges; if the electromagnet is changed, it can be checked that it generates a sufficiently strong field, and possibly that, when its power supply is cut, it releases the sensor in a time preferably shorter than the response time of the sensor, in order to excite as best as possible in the bandwidth of the sensor, preferably in this entire bandwidth;

[0060] - and / or stressed on the measuring range of the sensor, but also in loading - unloading, without operator intervention from a mechanical point of view.

[0061] The invention makes it possible, for example, to characterize, or even calibrate, for example a force sensor such as a dynamic mass measurement sensor, for example to evaluate the mass loss of a material subjected to a heat flow.

[0062] The invention is compact and can be taken by users to experimental sites. It makes it possible to limit heat exchanges, for example thanks to a full cover. It also makes it possible to isolate the measurement chain possible interference linked to vibrations, for example thanks to its support, comprising for example anti-vibration pads. BRIEF DESCRIPTION OF THE FIGURES

[0063] Figures 1A and 1B represent 2 examples of embodiment of a device according to 2 embodiments of the invention;

[0064] [Fig.2A] represents a device according to one embodiment of the invention, equipped with its measuring chain;

[0065] [Fig.2B] represents another aspect of the measurement chain for a device according to an embodiment of the invention;

[0066] [Fig.3A] represents on the one hand the instruction applied to the electromagnet and the oscillatory response of the system to be characterized;

[0067] [Fig.3B] represents a transfer function of a measurement chain;

[0068] [Fig.4A], [Fig.4B], [Fig.4C] and ([Fig.4D]] represent the comparison of signals temporal (figures 4 A and 4C) and their frequency response (figures 4B and 4D) for compression and traction type stress.

[0069] Figures 5A - 5B represent time signals obtained for compressive ([Fig.5A]) or tensile ([Fig.5B]) loading.

[0070] [Fig.6] represents the evolution of the mass variation seen by a sensor (as a function of the movement of the motors) over a part of the full scale of the sensor (2 g > 7 g).

[0071] [Fig.7] illustrates the repeatability of a measurement.

[0072] [Fig.8A] represents an example of a sensor to which the invention can be applied.

[0073] Figures 8B-8D represent various steps of preparation of a sensor according to the invention.

[0074] [Fig.8E] represents a set of different elements of a device for implementing the invention.

[0075] Figures 9A - 9B represent steps of a method according to the invention, implemented by a user ([Fig.9A]) or by software ([Fig.9B]). DETAILED DESCRIPTION OF THE INVENTION

[0076] [Fig.1A] represents a first example of embodiment of a device 1 according to the invention.

[0077] It comprises at least one motor 2 or a set of motors 2 (for example: stepper motors) and a system or plate, preferably tri-axis, 4 which carries a bracket 6 on which are mounted means 8 for generating a magnetic field, for example an electromagnet.

[0078] Opposite these means 8 is a measuring plate or bench or support 14, which makes it possible to position and maintain a force sensor 12 (shown positioned on the support, but this sensor can be removed from the device according to the invention). Optionally, if this sensor is non-magnetic, it is provided with a magnetic (or ferroelectric) part, for example a magnetic screw 10. An example of such a sensor is a piezoresistive sensor, comprising in particular a Wheatstone bridge at the terminals of which a signal can be taken, which is representative of the constraint or force applied to the sensor, which is assimilated to a mass or a force, using the magnetic field generated by the means 8.

[0079] It is thus possible to generate a stress applied directly to the sensor 12, without contact, by using a magnetic field generated by the means 8 along an axis XX'; preferably, a sensitivity axis of the sensor 12 is aligned along this axis XX', so that a force is applied to it and along this sensitivity axis. This force results from the direct interaction of the field directed along this axis XX' with the sensor or its magnetic or ferromagnetic part. For example, a force step allowing the calibration of the sensor is generated from the change of state of the magnetic means. It is possible to measure, at the terminals of the sensor, the response of the latter as a function of the stress applied to it. It is therefore possible to calibrate the sensor.

[0080] If the sensor has several sensitivity axes, several measurements, possibly several calibrations, can be made along these different axes, by modifying the position or orientation of the sensor to move from the alignment, along the XX' axis, of one sensitivity axis of the sensor to another.

[0081] In general, the axis XX' will be perpendicular to a surface of the support 14 and / or to a surface of the sensor (see explanations below in connection with [Fig.8A], [Fig.8B], [Fig.8C], [Fig.8D] and [Fig.8E]) which is used to apply a stress to the sensor, for example in tension or compression.

[0082] As a variant, illustrated in [Fig.lB], the sensor 12 of the device 1 is mounted on the bracket 6 and the means 8 for generating a magnetic field are mounted on a plate or bench or measuring support 14 or form part of it. The rest of the device, and its operation, are the same as those indicated above.

[0083] In both cases, the means 2, 4, 6 make it possible to achieve precise positioning of the means 8 for generating a magnetic field relative to the sensor 12. The amplitude of the force or the force step applied to the sensor can be adjusted as a function of the distance between the means 8 for generating a magnetic field and the sensor. This distance can be known with great precision, it can be measured and stored in storage means in order to be associated with a response signal from the sensor for this same distance; by varying this distance, the evolution of the response of the sensor to a stress which thus varies in intensity is known.

[0084] The stress applied to the sensor using a device according to the invention is therefore directed precisely along the XX' axis.

[0085] On the contrary, in the cutting blade technique:

[0086] - the application of a force perpendicular to the wire, by the action of the blade, can generate a bias force that also changes the application of force at the sensor under stress;

[0087] - when cutting the wire with a blade, a certain quantity of wire remains attached to the sensor and is set in motion, which distorts the final state (the equilibrium position). On the other hand, in the context of the present invention, a sensor which interacts directly with a non-moving magnetic element 8 makes it possible to better characterize the final state of the sensor at complete rest. This is an even more important criterion as the measuring range of the sensor can be small (on a sensor with a measuring range of 10 g for example, a length of wire with a mass of 1 g can remain attached to the sensor and temporarily behave like a spring, which is not negligible). Furthermore, in the cutting blade technique, the quantity of wire remaining attached can have a specific movement which disrupts the oscillatory response of the complete system.

[0088] Means forming a mechanical stop may be implemented, in order to avoid possible damage to the calibrated sensor, which could come into contact with the means 8 for generating a magnetic field: it is possible, for example, to position a bar between the surface of the support 14 and the means 8, so that the latter is kept at a minimum distance from the sensor and does not damage the latter.

[0089] In addition, or as a variant, the motor(s) 2 can be controlled in such a way that the means 8 cannot approach below a certain distance from the sensor 12.

[0090] In particular, one or more stepper motors 2 can be used, which allow(s) precise centering of the means 8 relative to the sensor 12; this precision is much greater than that obtained in the case of a technique such as that of the knife.

[0091] With such a system, dynamic in-situ calibration is possible, for example on an experimental test site. Such an example is that of an experimental means making it possible to apply a brief and high intensity thermal stress: after in-situ calibration (therefore near the experimental device) by one of the techniques described in the present application, an object, for example a disk carried by a ring, can be positioned on the sensor to be subjected to a significant heat input by the experimental device; the material constituting the object (disk) is degraded and the sensor 12 follows the evolution of this process.

[0092] If the sensor in question has a non-magnetic structure, a magnetic part 10 can be mounted prior to the test and / or the calibration / or the characterization, preferably as little invasive as possible (in mass in particular), for example a magnetic screw.

[0093] The invention also makes it possible to stress a sensor alternately in tension then in compression (or vice versa) without modification of the assembly, without being invasive or dependent on an operator, simply by reversing the orientation of the magnetic field, for example by reversing the polarity of the means 8 for generating a magnetic field; on the contrary, the techniques of the prior art require action by the operator on the system, and in particular on the sensor, between the dynamic calibrations in tension and in compression.

[0094] Thus, compared to known techniques, and in particular that of "cut-wire", it is possible to carry out one or more complete cycles of stress tests (tension or compression) without any intervention from an operator. It is therefore possible to carry out:

[0095] - a transition from a 1st state of constraint of the sensor to a 2nd state of constraint (in reversing the direction of the magnetic field generated for example by an electromagnet) or to a state of equilibrium at rest (by cutting off the magnetic field generated by magnetic means, for example the electromagnet);

[0096] - then possibly a passage from said 2nd state of stress or from the state at rest to 1st state of constraint, by generating a force step by activating magnetic means, for example the electromagnet.

[0097] These 2 steps can be carried out in reverse order.

[0098] The invention is more convenient to implement than the "cut-wire" method: the operator does not waste time hooking the often thin wire, a step which, moreover, poses problems of repeatability since the knots are never made exactly identically. In addition, the principle of this "cut-wire" method is to tension the sensor using a taut wire at the end of which a mass is positioned; the sensor is then subjected to a static force. Cutting the wire very quickly releases the mass and thus generates a force step at the sensor which returns to its rest position. It is sometimes difficult to cut the taut wire very cleanly and very briefly (the blade of the cutting element may be more or less blunt), which makes the step increase less abrupt (one is never certain that the cut is instantaneous) than a power cut at an electromagnet 8.Thus, the balancing time of the sensor is faster and more accurate by a device or method according to the invention.

[0099] The invention can operate at least in the range of 0.1N to 1N (10g to 100g) but can be adapted, by choosing a magnetic means, for example, a suitable electromagnet, to the measuring range of the sensor in question.

[0100] An advantage of the invention is to allow better control of the amplitude of the force step applied to the sensor. In fact, there is no problem of flatness between the surface of the sensor and that of the electromagnet. A device or method according to the invention does not involve any friction, no transmission of stress waves, while eliminating the additional physical stresses which are generated when the wire is cut ("overshoots" in English terminology) and the oscillations of this same wire before reaching the equilibrium position of the sensor.

[0101] Furthermore, the invention is easier to implement than the “cut-wire” method on a series of measurements: in fact, unlike the “cut-wire” method, which requires an operation of careful installation and attachment of a wire to a mass which will be different depending on the force levels required to cover the entire measurement range of the sensor, it is possible to carry out repeatability and reproducibility measurements without intervening on the system between measurements, and this, for the entire measurement range of the sensor. For example, it is sufficient to gradually modify the distance between the electromagnet and the sensor and to renew the measurement(s) for each of the selected distances. This can be done using the motor(s) 2 and the means 4, 6, which can be controlled by various means as explained below in connection with FIGS. 2A and 2B.

[0102] [Fig.2A] represents a device according to one embodiment of the invention, equipped with its measuring chain. The device 1 is connected to power supply and amplification means 22 dedicated to the sensor tested, to means 28 for controlling the motors 2 and to means 29 for controlling the electromagnet 8. The signal generated by the sensor can be digitized and processed by processing means 24, for example in software form in a processor or a computer or a microcomputer programmed for this purpose. For example, the measured time signal can undergo processing by Fourier transform to identify the characteristic frequency(ies) of the sensor.

[0103] The entire device, comprising the support 12 and the electromagnet 8, can be mounted on feet 27 comprising damping means, for example made of rubber, to neutralize any external vibration.

[0104] [Fig.2B] allows the visualization of the different relationships and / or wiring in an assembly such as that of [Fig.2A].

[0105] The means 28 for controlling the motor(s) are controlled by the computer 24. These means may comprise a controller 282 (connected to the computer 24 by a Ethernet connection) and specific means 281 of controlling the motor along each of the X, Y, Z axes.

[0106] The means 29 for controlling the electromagnet 8 comprise a stabilized power supply 291 and a control box 292 for the electromagnet. The latter produces the magnetic field which acts on the magnetic material (or on the magnetic screw 10).

[0107] The means 24 comprise the computer 24i itself and a data acquisition chassis 242. The data relating to the electromagnet and the data coming from the sensor 12 are sent to the data acquisition chassis 242, which produces corresponding digital data 25.

[0108] [Fig.3A] represents on the one hand the setpoint applied to the electromagnet 8 (with its change of state) and the oscillatory response of the system to be characterized. The setpoint or state of the electromagnet 8 is characterized by the levels I and II represented in this figure: by passing from level I to level II, a stress step is applied to the sensor 12 that we want to calibrate or qualify. The temporal oscillatory response of the latter is represented in this same figure. [Fig.3B] represents this same response in frequency terms (curve IIr, curve Ie representing the excitation frequency), it is therefore the transfer function of the system.

[0109] Figures 4A - 4D represent the comparison of time signals (Figures 4 A and 4C) and frequency signals (Figures 4B and 4D, on which curve IIr represents the response of the sensor, curve L representing the excitation frequency) for a compression type stress:

[0110] a) figures 4A - 4B: we first apply a load of 4 g then an unloading to -2.5 g (which corresponds, in this example, to the final state "without stress", due to the mass of the magnetized screw; same comment for the other figures 4B-4D); we recognize, in [Fig.4A], the 2 states I and II of the step applied by the electromagnet;

[0111] b) figures 4C - 4D: the initial state "without stress" is at -2.5 g then a loading at 4 g is carried out (final state). In this case, the force is reversed compared to the previous figures, since we move from state II to state I.

[0112] The supply setpoint of the electromagnet is here 24 V: the “off-load” state, i.e. not generating any stress on the sensor (state II) being for a supply of 0 V, the stress state (state I) for a supply of 24 V.

[0113] These figures 4A - 4D show that the response of the sensor can be tested under loading or unloading, for example by going from an initial state to a final state or from a final state to an initial state just by changing the polarity of the electromagnet. There is no other modification of the device to be made, which guarantees identical and constant measurement conditions from one measurement to another.

[0114] Figures 5A - 5B represent time signals obtained for a request:

[0115] - in compression ([Fig.5A]) at 10 g, followed by unloading at -2.5 g (“empty” state ") ;

[0116] - or in traction ([Fig.5B]) at -10 g, followed by unloading at -2.5 g (empty state).

[0117] Here again, we recognize, in [Fig.5A], the 2 states I and II of the step applied by the electromagnet 8.

[0118] [Fig.6] represents the evolution of the mass variation (vertical axis, in g) seen by the sensor 12, as a function of the displacement of the electromagnet (along 2 axes X and Y perpendicular to each other), over a part of the full scale of the sensor (in this example: from 2 g to 7 g). This figure clearly shows the presence of a hysteresis of the sensor. The invention therefore makes it possible, by measuring the response of the sensor in compression, then in tension, or vice versa, to identify such hysteresis. It therefore makes it possible to identify data from the hysteresis curve of the sensor.

[0119] [Fig.7] illustrates the repeatability of a measurement according to the invention. 50 tests were carried out on a sensor with a measuring range of 10 g. The duration of each test is 5 seconds, and the sampling is 1 kHz (number of points: 5000).

[0120] The maximum (respectively: minimum) value of the standard deviation is 0.14 (respectively: 0.0085), and its average is 0.0233.

[0121] The repeatability observed for the same measurement according to the invention is therefore very good.

[0122] [Fig.8A] represents an example of a sensor 12 to which a device according to the invention or a method according to the invention can be applied. It comprises a housing, for example of substantially parallelepipedal shape, which contains a piezoresistive element associated with a Wheatstone bridge. The invention can be used on sensors allowing mass or weight measurements. It can be used on sensors of the capacitive, or piezoresistive, or piezoelectric type or such as a scale. The upper face 12i and / or the lower face 122 can be used to subject the sensor to a tensile or compressive stress, a sensitivity axis of the sensor being oriented along the X axis substantially perpendicular to each of these faces 12b 122.The sensor may include one or two other sensitivity axes, for example along the Y and / or Z axis, perpendicular to each other and to the X axis, in which case one or other of the faces perpendicular to at least one of these Y, Z axes may be used to carry out measurements, in tension or in compression, according to the chosen axis. One or more of the faces used may be provided with a hole 13, 15, to be able to apply the stress to be measured. These are for example tapped holes. The hole 13 may be used in particular to fix the sensor 12 on the plate 14 ([Fig.lA]) or on the bracket 6 ([Fig.lB]). The hole 15 may be used to introduce an element, . for example a screw 10, made of magnetic or ferromagnetic material, if the entire sensor is non-magnetic and with a view to implementing a method according to the invention.

[0123] One or more wire(s) 23 (see figures 8B-8E) make it possible to sample signals representative of the constraints applied to the sensor 12. A cover (not shown) makes it possible to protect the bench which is the subject of the invention from environmental disturbances (draft, temperature variations) during the measurements carried out.

[0124] Figures 8B - 8D represent:

[0125] - the support 14 and the sensor 12 ([Fig.8B]);

[0126] - an example of fixing a magnetic screw 10 (figures 8B and 8C), with, in the case of [Fig.8C], the addition of an accessory (here a ring) 31.

[0127] The sensor 12 is first screwed onto the metal support 14 ([Fig.8B]), then the magnetic screw 10 ([Fig.8C]) or the ring 31 and magnetic screw 10 assembly ([Fig.8D]) is added.

[0128] [Fig.8E] shows a view of the alignment between the different elements of the device: the support 14, the sensor 12 here with the magnetic screw (element 10), and the electromagnet 8.

[0129] In an example of implementation of a method according to the invention, which method can implement a device as described in the present application, the magnetic field B is applied along the X axis, in one direction or the other, and interacts directly either with the entire sensor 12, or with the element 10, in order to subject this sensor to the desired stress, in tension or in compression. The response of the sensor can for example be measured as a function of the distance between the means 8 for generating the magnetic field and the sensor.

[0130] [Fig.9A] represents steps of implementing an example of a method according to the invention, for example characterization of the transfer function.

[0131] In a first step (SI) the force sensor 12 is mounted, for example in an assembly according to one of figures 1A or 1B; the setpoint measurement for the operation of the electromagnet can be adjusted (here, the current applied to the electromagnet 8).

[0132] In a 2nd step (S2), it is possible to mount an adaptation part 10 made of ferrous material (for example in the case where the sensor 12 is non-magnetic), or an accessory part 29 on the sensor 12. If this accessory part 29 is non-magnetic, it will be necessary to mount an adaptation part 10 made of ferrous material.

[0133] The distance between the magnetic system and the sensor 12 can be precisely adjusted and / or measured, for example using a stepper motor (step S3). The centering between the magnetic system 8 and the sensor can be ensured in a plane (YZ) by a plate controlled by two stepper motors.

[0134] Initially, the system, essentially comprising the sensor 12, is in a certain state, linked to the application, or not, of a magnetic field along the axis XX'. When it is decided to change the state of the system (step S4), it passes, for example, from a constrained state (in tension or in compression) to its rest state (step S5): such a passage, from a 1st state to a 2nd state, is illustrated in figures 3A, 4A, 4C or 5A already commented on above. A measurement of the response of the system, for example according to one of these figures 3A, 4A, 4C or 5A, can then be carried out. The signal thus measured can be subjected to one or more processing operations, in particular a Fourier transform in order to identify the spectrum of its response and, for example, to identify one or more resonance frequencies.

[0135] It is then possible to modify the distance between the electromagnet and the sensor 12 (step S6) and then return for example to the setting of step S3.

[0136] Regardless of the embodiment implemented, the sensor response data may be stored, for example as a function of the distance between the electromagnet and the sensor. Optionally, it is possible to collect sensor response data as a function of temperature, which may be of interest for using the device in a certain temperature range.

[0137] It is thus possible to store, for one or more sensors, response data and / or hysteresis curve(s) data.

[0138] Steps of software for implementing a method according to the invention are illustrated in [Fig.9B].

[0139] In a 1st step (S10) the trigger level of the means for measuring a signal coming from the sensor 12 can be programmed.

[0140] The sensor data is then read (step S12)

[0141] The applied force can then be adjusted (step S13), for example as a function of the current level flowing in the electromagnet, and, if this adjustment is satisfactory, this current can be triggered (step S14) as well as the data acquisition (step S15).

[0142] These data are then recorded (step S16). This is the end of the acquisition (step S17).

Claims

Claims

1. Device for characterizing a force sensor (12) comprising a magnetic or ferromagnetic part, this device comprising: - magnetic means (8) for generating a magnetic field along at least one axis (XX'); - means (2, 4, 6) for adjusting and then maintaining a relative position, along the axis (XX'), between a sensor (12) to be tested and the magnetic means (8); - means (22-24) for measuring a signal representative of a force, in traction or compression, applied to the sensor directly using said magnetic means, the device further comprising means (29) for at least reversing the direction of application of the magnetic field along the direction (XX').

2. Device according to claim 1, comprising a fixed support (14) for holding the sensor (12) to be characterized and a mobile support (6) for holding the magnetic means (8).

3. Device according to claim 2, the mobile support (6) being controlled by a 3-axis system (4) controlled by 3 motors (2).

4. Device according to one of claims 1 or 2, the means (2, 4, 6) for adjusting and then maintaining a relative position, along the axis (XX'), between a force sensor (12) to be characterized and the magnetic means (8), comprising at least one motor (2), for example a stepper motor.

5. Device according to one of claims 1 to 4, the means (8) for generating a magnetic field comprising an electromagnet.

6. Device according to one of claims 1 to 5, further comprising means (29) for cutting off the application of the magnetic field along the axis (XX').

7. Device according to one of claims 1 to 6, further comprising means for limiting the distance between the magnetic means and a sensor to a minimum distance.

8. Device according to one of claims 1 to 7, further comprising means (2, 4, 6) for centering and / or maintaining and / or measuring a relative position along the axes (YY') and (ZZ'), perpendicular between

9.

10.

11.

12.

13.

14.

15.

16. them and to the axis (XX'), between the sensor to be characterized (12) and the magnetic means (8). Device according to one of claims 1 to 8, further comprising means (24) for digitizing, and / or processing, for example by Fourier transform, a signal taken at the terminals of a sensor (12) to be characterized. Device according to one of claims 1 to 9, further comprising means for digitizing and / or storing response and / or calibration and / or hysteresis data of a sensor (12) to be characterized. Method for characterizing a sensor (12) comprising a magnetic or ferromagnetic part, this method comprising: - holding said sensor (12) fixedly relative to means (8) for generating a magnetic field; - the generation of a magnetic field (B) in a direction (XX') allowing the application of traction or compression to the sensor, by direct interaction between this field and the sensor; - measuring the response of the sensor (12) to this traction or compression, this method comprising the application to the sensor (12) alternately of a tensile then compressive constraint force, or of a tensile then compressive constraint force, by reversing the direction of the magnetic field. Method according to claim 11, in which the sensor (12) is non-magnetic, the method comprising a prior step of mounting a magnetic part (10) on the sensor. Method according to one of claims 11 or 12, further comprising a step of mounting an accessory (29) forming a support, for example a ring, on the sensor (12). Method according to claim 13, comprising a prior step of mounting a magnetic part (10) on the accessory. Method according to one of claims 11 to 14, further comprising modifying or adjusting the gap between the means (8) for generating a magnetic field and said sensor (12). Method according to one of claims 11 to 15, comprising the application to the sensor (12) of a tensile or compressive stress, then of an absence of stress.

17. Method according to one of claims 11 to 16, the traction or compression being applied along a 1st axis of sensitivity of the sensor (12).

18. Method according to claim 17, the sensor (12) comprising at least a 2nd axis of sensitivity, the method further comprising: - a step of modifying the orientation of the sensor, - and the application of traction or compression along this 2nd axis of sensitivity of the sensor (12); - measuring the response of the sensor (12) to this traction or compression along this 2nd axis of sensitivity.

19. Method according to one of claims 11 to 18, the sensor (12) being of the piezoresistive or capacitive type, or piezoelectric or comprising a scale.

20. Method according to one of claims 11 to 19, the sensor (12) comprising one or more faces (12i, 122), the direction (XX') of the magnetic field being perpendicular to this or these faces.

21. Method according to one of claims 11 to 20, the stress implemented being between 1 / 10 of Newton and 1 Newton or more than 1 Newton.

22. Method according to one of claims 11 to 21, further comprising a step of storing response and / or calibration and / or hysteresis data of the sensor to said traction or compression, for example as a function of a position of the sensor (12) relative to means (8) which generate said magnetic field (B).

Images