Patent ID: 12189882

DETAILED DESCRIPTION

FIG.1illustrates an example of a force sensor1according to the invention. The sensor1comprises a cavity2assuming a block shape, defining a rectangular bottom surface2aand four rectangular surfaces2boriented perpendicular to the surface2a, forming the side faces of the cavity. The cavity2is integrated in a hollow support7, made, for example, from an electrically insulating and preferably rigid material, for example, a rigid plastic, a ceramic or glass. As an alternative embodiment, the support7can be made from a metal material, being electrically insulated from the detection structures in contact therewith, if applicable.

A detection structure3is located on each of the surfaces2aand2b.

The cavity2is filled with a deformable medium4, defining a detection surface S, on which an external force F is applied.

The forces generated by applying the force F are transmitted by the deformable medium4to the detection structures3, which can detect a pressure applied onto their respective surface, with an orientation normal to this surface, and corresponding to the transmitted forces.

The deformable medium4can fill the cavity2in various ways, and the detection surface can assume different shapes. For example, as illustrated inFIG.2a, the deformable medium4fills the cavity2so as to be flush with the opening thereof. In an alternative embodiment, the deformable medium4protrudes out of the cavity2, as illustrated inFIG.2b. In the examples ofFIGS.2aand2b, the detection surface S is a flat surface, parallel to the bottom surface2aof the cavity. In another alternative embodiment, illustrated inFIG.2c, the deformable medium4covers the opening of the cavity2with a variable thickness, with the covering being thicker at the center of the cavity2, the detection surface S forming a dome above the cavity2.

The detection structure3can be produced in various ways, some of which are shown inFIGS.3a,3b,3c,3c,3eand3f.

As illustrated inFIG.3a, a first electrode30can be printed on a layer31of material with low conductivity, for example, a polymer filled with carbon particles, and a second electrode32can be printed on another layer33of material with low conductivity. The layers31and33are then overlaid so that the electrodes30and32are disposed on the outside of the assembly. The two layers31and33are separated, for example, by a very thin air gap34, and only touch each other with satisfactory electrical contact when pressure is exerted on the assembly. In the considered example, the electrodes30and32are each linear shaped and are arranged perpendicular to each other. The electrode32is in contact with the deformable medium4, whereas the electrode30is in contact with an underlying support (not shown herein). The forces transmitted by the deformable medium4when a force F is applied onto the detection surface S vary the contact surface between the layers31and33. When a voltage is applied between the electrodes30and32, the variation in the contact pressure between the layers31and32causes a change in the electrical resistance measured in the zones where the electrodes intersect (which correspond to the measurement points).

In an alternative embodiment illustrated inFIG.3b, the electrodes30and32are printed on either side of the same layer35of intrinsically piezo-resistive material, which deforms under the action of the forces transmitted by the deformable medium4, causing, similarly to the previous example, a variation in the electrical resistivity in the intersection zones of the electrodes.

In other alternative embodiments, the electrodes are supported by a flexible substrate, for example PET. The electrodes30and32are, for example, each printed or deposited on a flexible electrically insulating layer36and face each other, as illustrated inFIG.3c. A layer37with low conductivity, for example, a polymer, optionally cellular, or even a piezoelectric material, is sandwiched between the electrodes30and32and separated from each electrode30or32by a thin air gap34.

In the alternative embodiment ofFIG.3d, the electrodes30and32are supported by a single electrically insulating substrate36, facing a layer37with low conductivity, for example, a foam or a non-cellular material, in contact with the deformable medium4. The electrodes are separated from the layer37by a thin air gap34. When a force is transmitted by the deformable medium4, the layer37can come into contact with part of the electrodes.

As previously explained, measuring the variation in the electrical resistance between two electrodes of such detection structures3allows the forces transmitted by the deformable medium to be estimated.

The pressure generated by the forces transmitted to the detection structures3can be measured with discrete sensors, if applicable.

In the example illustrated inFIG.3e, the detection structure3comprises strain gauges38, for example, of the metal type, disposed on the surface of the cavity and extending inside the deformable medium4.

In the alternative embodiment illustrated inFIG.3f, the strain gauges38are at least partially integrated into the support7and partially come into contact with the deformable medium4. Other types of strain gauges still can be used, for example, Micro Electro-Mechanical System (MEMS) pressure sensors.

The detection structures3can be multi-point or single-point measurement structures.FIG.4illustrates the measurement zones5obtained on the surfaces of the cavity of a sensor comprising multi-point detection structures. In the considered example, the detection structure3disposed on the bottom surface2aof the cavity2defines a grid of 12×12 measurement zones5. On the side surfaces2b, and as illustrated inFIG.5, the detection structure3defines a row of measurement zones5. The number and distribution of the measurement zones5on the detection structures3is, of course, not limited to the illustrated example, any arrangement can be contemplated, depending on the nature and the arrangement of the detection means equipping the detection structure.

In order to obtain the grid of measurement zones as described above, detection structures3, such as those shown inFIG.6, can be integrated on the surfaces2aand2bof the cavity2. In this example, each detection structure3comprises, as described above, two layers31and33of material with low electrical conductivity, on which, for example, a plurality of linear electrodes, parallel to each other and evenly spaced apart, have been printed or screen printed. InFIG.6, only the electrodes32of the layers33are visible. Electrodes30are disposed on the non-visible face of the layers31and are arranged, for example, perpendicular to those of the layer33. The layers31and33disposed thus are integrated into the cavity2of the support7. The deformable medium4is then added and comes into contact with the layers33. The electrodes printed on the layer33of the side surfaces2bof the cavity are vertical in the illustrated embodiment; however, any other orientation can be contemplated, for example, horizontal or oblique, the electrodes can assume a U-shape or V-shape, for example.

FIG.7illustrates another way of producing multi-point detection structures inside a cavity in the shape of a block. In this example, linearly shaped electrodes30and32are disposed on layers31and33, respectively, of material with low conductivity, which will then be assembled together so that the electrodes30and32are located on the outside of the assembly.

The four-branch cross shape50of the layers31and33represents the flattened cubic cavity. All the electrodes are printed in the same plane and the assembly is then folded at a right angle to the central square51of the cross in order to be placed in the hollow support7. Thus, the detection structures3of all the surfaces of the cavity2are integrated into the support as one piece, with the central square51of the cross forming the detection structure of the bottom surface2aand the branches52of the cross each forming the detection structure3of a side surface2bof the cavity. The arrangement of the electrodes is such that, on each detection structure, the electrodes30perpendicularly intersect the electrodes32, with each intersection corresponding to a measurement zone5. All the electrodes30and32emerge on the upper part of the cavity. On two opposite branches52of each layer, the electrodes are U-shaped, with branches300or320that extend up to the free edge53of the branch in order to facilitate electrical connection to a reading circuit.

In another embodiment, the sensor1comprises a plurality of single detection cells6only comprising single-point measurement detection structures3. A single detection cell6comprises, for example, a single cubic cavity2, as illustrated inFIG.8, and has five separate measurement zones5, that is one per detection structure3. A matrix9of single detection cells6then can be formed, as illustrated inFIG.9, by integrating the cavities on the same support7, for example, on the same plane. The support is preferably rigid, and can be formed from a monolithic block, as in the considered example, or as an alternative embodiment it can be obtained by assembling separate elements. The deformable medium4of the sensor1thus obtained preferably forms a monolithic block, as shown inFIG.10, defining a detection surface S common to all the assembled single detection cells6. The advantage of such a sensor is that it is able to measure a continuous distribution of the forces exerted on the detection surface S. In particular, and unlike a single-cavity sensor, with a matrix9of single detection cells it is possible to distinguish the application of two distinct simultaneous forces.

A single detection cell6can be produced by integrating two layers31and33of material with low conductivity in the shape of a cross50into the cavity2, similar to the previous description. In the considered example, in order to obtain single-point detection structures3, for example, three electrodes30are printed on the layer31, and three electrodes32are printed on the layer33. The arrangement of the electrodes is such that, once the two layers as a whole are assembled in the cavity, an electrode30perpendicularly intersects an electrode32on each single detection structure3, corresponding to the single measurement zone5. As previously described, all the electrodes30and32emerge on the upper part of the cavity, and the electrodes located on two opposite branches52of the cross are U-shaped, with branches300or320that extend up to the free edge53of the branch.

As an alternative embodiment, the detection structures of a line of single detection cells6of a matrix9can be formed as a “strip” on the same substrate, in particular on a flat substrate. The strip is then folded in order to be integrated into the support7in a serrated shape that conforms to the series of cavities.

The force sensor according to the invention preferably comprises, as illustrated inFIG.12, a processing circuit8in order to process the signals delivered by the detection structures3in order to measure at least one, and preferably all, the components of a force exerted on the detection surface S. It comprises a central unit80and a memory81, for example.

The components of the force exerted on the detection surface S can be determined on the basis of the processed signals by following, for example, the steps of the method that are shown inFIG.13and are described hereafter. By way of an example, a sensor is considered that comprises a cavity in the form of a block, as illustrated inFIG.6, and that comprises a multi-point detection structure3on each of the surfaces of its cavity that defines a grid of measurement zones similar to that shown inFIG.4. In the considered example, the measurement zones5are square shaped and all have an identical surface, as illustrated inFIG.12.

In the first step10, the voltage on each measurement zone5of each detection structure3is measured sequentially. For a detection structure, the voltage measured in the measurement zone (i, j) located at the intersection of row i with column j of the grid is denoted vij, with i and j being integers varying from 1 to the number of rows and columns of the grid, respectively. In step11, the single pressure pijexerted on the measurement zone (i, j) is estimated as a function of the measured voltage, using the following relation, for example:
pij=κvij

with κ being a conversion factor expressed as Newtons/(mm2·volts), which depends on the materials that are used. A distribution of the pressures on the relevant detection structure is then advantageously obtained, the resolution of which can be below one millimeter, for example.

The resultant of the normal forces FNexerted on the detection structure3can be obtained by adding the corresponding pressures:
FN=ΣΣpijds=ΣΣavijds

with ds denoting the surface of the measurement zone5, as shown inFIG.14, in a standardized coordinate system, with ds being identical for all the measurement zones.

In step12, for example, the three components of the force F exerted on the detection surface S of the sensor1are computed. In view of the geometry of the sensor in the considered example, the following description is based on a Cartesian coordinate system with directions (X, Y, Z), with Z denoting a direction normal to the detection surface S and to the surface of the bottom of the cavity, and X and Y denoting the tangential directions, as illustrated inFIG.15. The normal component Fzof the force F, i.e. the force in the direction Z, is simply provided by the resultant of the normal forces FNexerted on the detection structure of the bottom of the cavity2a.

Depending on the nature of the deformable medium4, a force exerted on the sensor in a normal direction, i.e. in the considered example in the direction Z, can lead to forces40transmitted in all directions, and in particular can cause non-zero pressure measurements on the lateral detection structures. This phenomenon, illustrated inFIG.15, must be taken into account when computing the tangential components Fxand Fyof the force F. These tangential pressure measurements can be estimated and then compensated by modelling the response of the deformable medium4to a normal force, for example, in accordance with a Gaussian type model. Examples of such modelling are shown inFIGS.16aand16b. The example is provided in the direction X, but the reasoning is similar for the direction Y in view of the symmetry of the problem. In the considered example, the deformable medium4is subjected to two normal forces exerted at separate positions A1and A2, corresponding to the maximum of the curves shown inFIGS.16aand16b. The tangential force Fttransmitted by the normal force at the point x depends on the normal force FNexerted in accordance with the following relation, for example:

Ft⁡(x,FN,Bx)=α⁢⁢FN⁢e-(x-Bx)2⁢σ2

with Bxdenoting the component x of the barycenter of the normal forces on the surface α and σ and a being two parameters that depend on the properties of the deformable medium4and on the dimensions of the sensor. A relatively low value of σ, as illustrated inFIG.16b, implies that the transmission of the normal force in the tangential directions is limited. Conversely, a relatively high value of σ, as inFIG.16a, implies that the transmission of the normal force in the tangential directions is high and that the corresponding measurements must be compensated.

In order to obtain the tangential component Fxof any force F exerted on the detection surface S, as shown inFIG.17, the previously described compensations are taken into account using the following relation, for example:
Fx=β[(FR−Ft(1,FN,Bx))−(FL−Ft(−1,FN,Bx))]

with FRand FLbeing the resultant of the measured forces40on the detection structures of the lateral surfaces located at x=1 and x=−1, respectively. β is a parameter that depends on the nature of the deformable medium and on the dimensions of the sensor. If the force exerted on the detection surface S is normal to the bottom surface of the cavity and is centered, as illustrated inFIG.15, the forces40measured on the 2 opposite lateral surfaces of the sensor are similar. Otherwise, as illustrated inFIG.17, the forces40are greater on one side surface than on the other, allowing the direction of the force F to be determined in the corresponding direction.

The tangential component Fycan be computed using a similar method.

If desired, the moment C of the force F exerted on the sensor1can be determined in step13on the basis of distributions of pressure detected by detection structures on opposite surfaces of the cavity. By way of an example,FIG.18ashows the distribution of the forces40exerted on the side surfaces of the cavity when a positive moment normal force is applied at the center of the sensor, and, similarly,FIG.18bshows the distribution of the forces40when a positive moment force is applied at the same point. The moment QNis obtained by virtue of the following relation:
QN=γ(∇l+∇r+∇+∇d)

with ∇l,∇r, ∇u,∇d being the gradients of the forces40measured on each of the side surfaces of the cavity of the sensor. γ is a factor depending on the nature of the deformable medium4and on the dimensions of the sensor.

The set of parameters (α,β,σ,γ) can be determined during a calibration phase, during which the sensor is subjected to known stresses, which are compared with the measured forces. The calibration can be carried out in several ways. For example, a model, such as a linear or non-linear regression model, can be used in order to find the parameters that minimize an error criterion between the measurement and the model, with the error criterion being selected, for example, in accordance with the known Mean Square Error (MSE), Root Mean Square Error (RMSE), Mean Absolute Error (MAE), or other methods.

The method for determining the components of a force exerted on the sensor is not limited to the model described above, in particular if the detection surface of the sensor has a relatively complex shape. For example, a pre-trained neural network can be used to obtain the force applied onto the detection surface and determined as a function of the signals detected by the detection structures. If applicable, the neural network is trained beforehand during a calibration phase, as shown in step19ofFIG.19, in which, similar to the previously described model, the forces exerted on the detection structures are measured when the sensor is subjected to known stresses, in order to provide the neural network with examples for the training thereof. Once the neural network has been trained for a given sensor, it can be used in step20to determine all kinds of forces exerted on its detection surface. In addition to the neural network, other machine learning algorithms can be used, such as, for example, Support Vector Machine (SVR), GradientBoosting, or even Random forest methods.

In the case of a sensor comprising a matrix9of single detection cells6, a single processing circuit8can process, for example, all the signals of the matrix9in order to compute, for example, the distribution of the one or more force(s) exerted on the detection surface S of the device, and/or the moment corresponding to said forces. This method is illustrated inFIG.20. In step15, and similar to the previous description, the voltage on the single measurement zone5of each detection structure3is measured for each detection cell6. In step16, the pressure exerted on the detection structure5is estimated as a function of the measured voltage and the components of the force exerted on the single detection cells6are computed in accordance with the same method as previously described, with or without compensation of the tangential pressures due to a normal force. In step17, a distribution of forces over the entire matrix9is advantageously obtained on the basis of the computed forces, which can provide new information relating to the nature of the forces exerted on the sensor. In particular, it is possible to detect and quantify a “multi-layer” contact, i.e. the simultaneous application of at least two separate forces, as illustrated inFIG.22. In step18, and as illustrated inFIG.23, the moment {right arrow over (Q)}tof a force exerted on the sensor comprising the matrix9of single detection cells6can be computed on the basis of the estimated forces {right arrow over (F)}ifor each single detection cell6and on the following relation:
{right arrow over (Q)}t=Σ{right arrow over (r)}i×{right arrow over (F)}i

where the number of single sensors is added and the vector {right arrow over (r)}idenotes the position vector between the center of the single sensor cell i and the point relative to which the moment is computed, i.e. the point of application of the total force.

When the sensor comprises multi-point detection structures3with a matrix arrangement of electrodes resulting in a grid of measurement zones, as illustrated inFIG.4, for example, a method for reading measurement zones5by scanning, such as that shown inFIG.24, can be implemented. This method, which is described hereafter, uses a method similar to that proposed in the article entitled “The UnMousePad—An Interpolating Multi-Touch Force-Sensing Input Pad” by Rosenberg et. Al (ACM SIGGRAPH2009 papers. 2009. 1-9).

In the considered example, N denotes the number of electrodes disposed on a face of the detection structure3, that is the number of columns in the grid, M denotes the number of electrodes disposed on the opposite face of the detection structure3, that is the number of rows in the grid. As previously described, the intersection of the electrodes defines the measurement zones5. In the first step71of the reading method, all the electrodes are grounded. The following steps are then repeated for i, which is an integer ranging between 1 and N:in step72, powering the electrode i; andrepeating, with j being an integer between 1 and M:in step73, the electrode j is connected to the input of a reading circuit;in step74, the signal is read on the measurement zone corresponding to the intersection of the electrodes i and j; andin step75, the electrode j is grounded; andthe electrode i is grounded.

This sequential reading method, by supplying each of the measurement zones5in turn, as illustrated inFIG.25, while connecting the other electrodes to ground (not shown inFIG.25), advantageously allows measurement artefacts, such as “ghost” signals, to be limited that are associated with the leakage paths of the current inside the detection structure. The electronic components, by virtue of the multiplexing of the channels, also can be pooled, which can reduce the manufacturing cost of the sensor. As an alternative embodiment, a plurality of analogue-digital converters can be used with or without channel multiplexing, in particular one converter per electrode j connected to the reading circuit.

A sensor according to the invention can be manufactured by following, for example, the steps of the method illustrated inFIG.26. In step21, at least one first electrode30assuming a linear shape, for example, is printed on a first layer31of material with low conductivity. In step22, at least one second electrode32, in particular similar to the first electrode, is printed on a second layer33of material with low conductivity. In step23, the layers31and33are assembled so that the electrodes30and32are on the outer faces of the assembly and so that the electrode32intersects, in a front view, the electrode30. In step23, the assembly that is obtained is then cut into a shape corresponding to the pattern of the cavity2, before being integrated into the support7in step24, with a detection structure3thus being located on each surface of the cavity. In step25, the cavity is at least partially filled with a resiliently deformable material, in particular an elastomer, and each electrode of the sensor thus obtained is electrically connected in step26, which step can occur before step25, if applicable.

Of course, the invention is not limited to the examples described above.

For example, the detection of the pressures exerted on the detection structures3is not necessarily obtained by resistive measurement. It can be achieved, for example, by capacitive measurement, for example, by replacing the conductive material with a material with a high dielectric constant, and by subjecting the measurement electrodes to alternating voltages, for example, or by any other method suitable for measuring a capacitance.

The cavity can assume any shape, and any depth as required. For example, it can be very shallow. The dimensions of the corresponding sensor can vary from a few millimeters to a few centimeters by width and by length, for example, and the thickness is a few millimeters, for example.

The sensor also can be equipped to measure other quantities in addition to the force exerted on its detection surface. For example, it can comprise measurement means for estimating the temperature of an object exerting a force on its detection surface, in particular by virtue of a temperature sensor below the bottom surface of the cavity. A piezoelectric layer can be added in order to measure any vibrations or accelerations.

The sensor according to the invention can be used, among other things, for robotics applications, in particular within the context of the dexterous robotic handling of objects. For example, it allows the qualities for handling objects by robots to be improved, in particular for detecting and identifying slippage of these objects.

In other application examples, the sensor is integrated into a human-machine touch interface.