Patent Application
20090320611
The invention relates to an elastic covering for tactile sensors and a tactile sensor array fitted with elastic covering.
In the industry, tactile sensors are primarily used in robotics, where the elastic cover of tactile sensors plays an extremely important role in the functioning of tactile sensors. The elastic cover defines the basic sensory characteristics as well as the operating mechanism of the sensor unit. In prior art, generally flat covers have been applied for covering tactile sensors.
The paper “Effects of the elastic covering on tactile sensor arrays” written by Gábor Vásárhelyi, Mária Ádám, Éva Vázsonyi, István Bársony and Csaba Dücsö (Proceedings of Eurosensors 2005, Barcelona) describes a separate hemispheric cover arranged on each tactile sensor. The disadvantage of this approach is that the accurate positioning of each cover at the time of installation is a time consuming task, and furthermore it is not described in the paper how shear forces parallel with the surface can be measured reliably.
The design of the elastic cover is to be carried out carefully, because as soon as it is fitted on the sensor, the sensor will no longer measure the surface impacts, but only their distribution coded in a complicated way and passed on by the cover in the form of deformation or mechanical stress distribution. When evaluating the signals of sensors, in order to measure the original impacts, the measured signals must be decoded on the basis of material characteristics, which necessitates the solving of an extremely complicated inverse problem in the case of the prior art covers.
It is an object of the invention to provide an elastic cover and a tactile sensor array fitted therewith, which is exempt from the above disadvantages of prior art solutions, can be produced and fitted simply and which—due to its geometry and material parameters—simplifies the decoding of mechanical stress. It is a further object to provide such an elastic cover and a tactile sensor array fitted therewith, which also enables the sensing of shear forces in parallel with the surface defined by the tactile sensors. Yet another object is the implementation of the latter shear force sensing function by sensor units which originally measure forces only in a single dimension.
According to a first aspect, the invention is an elastic cover for tactile sensors, said cover comprising an inner surface suitable for being fixed to a surface defined by the tactile sensors, and an outer surface comprising a bump. The cover is characterised by comprising a base layer, one surface of which is the inner surface, and on its outer surface bumps integral with the base layer are formed.
The elastic cover according to the invention can be produced, positioned and fixed simply, and furthermore due to its geometry and material parameters it simplifies the decoding of mechanical stress.
According to a preferred embodiment, the cross section—being parallel with the base layer—of the bumps decreases starting from the base layer. This is advantageous from the aspect of both manufacturing and sophisticated sensing. The bumps are preferably axially symmetric, and the symmetry axes of the bumps are perpendicular to the base layer. The bumps can have e.g. a hemispheric, semi-ellipsoidal, tetrahedron, conic or pyramidal shape. For a direction-selective sensing, in the given case bumps designed as straight ridges may be applied.
The distance between the bumps is preferably between 0.5 mm and 5 mm, and the thickness of the base layer is between 0.2 mm and 2 mm.
According to a second aspect, the invention is a tactile sensor array with an elastic cover, said array comprising tactile sensors arranged along a surface, and being characterised in that on the surface defined by the tactile sensors an inventive elastic cover is arranged.
A specially preferred embodiment comprises tactile sensors suitable for three-dimensional force sensing, and bumps having their topmost points in a region above the tactile sensor. The bumps preferably have a hemispheric shape, and the tactile sensors are beneath the topmost points of the bumps, practically at a distance corresponding to the diameter of the hemispheric shape.
Another preferred embodiment comprises tactile sensors suitable for a single dimensional force sensing, and said bumps have their topmost points between two tactile sensors. For the sake of simple decoding, each of the topmost points of the bumps is preferably arranged in a region above a middle point of the distance between two tactile sensors.
The invention will hereinafter be described on the basis of preferred embodiments depicted by drawings, where
According to our invention, we do not apply a flat cover or a cover with separate hemispheres, but a cover of a geometry detailed below for covering the tactile sensors. The cover according to the invention can be used preferably in tactile sensors of three degrees of freedom, i.e. in tactile sensors which measure not only a force component perpendicular to the surface, but also the two shear direction components parallel with the surface. However, according to the discussion below, it may also be used in sensors of one degree of freedom. By way of example, a tactile sensor of three degrees of freedom is described in the paper referred to in the introduction.
In association with creating the elastic cover and the array, the following considerations apply.
The elastic cover according to the invention and shown in
The figure shows that the cross section—in parallel with the base layer 10—of the bumps 13 gradually decreases as the distance grows from the base layer 10. Preferably, the bumps 13 are designed in axial symmetry where the axis of symmetry of each bump 13 is perpendicular to the base layer 10. The bumps 13 preferably have a hemispheric, semi-ellipsoidal, tetrahedron, conic or pyramidal shape.
The cover shown in
As long as only pressure gauge (one degree of freedom) sensors existed, the elastic cover did not play any special role, because it only functioned as a type of spatial low-pass filter, with the effect mainly exerted in an attenuating manner. Consequently, in the case of those sensors, a cover with a flat surface was sufficient.
Contrary to a flat surface—where the sensor below the cover ‘senses’ the sum total of forces exerted on the surface of the cover above—the bumps, preferably hemispheres according to the invention, localize the attacking force at their topmost points with high efficiency. Because they protrude from the surface, in the case of a tactile event they represent the first contact points, around which generally the highest mechanical stresses are concentrated. In this way the sensing efficiency can be high. At the same time, in the case of holding functions, the hemispheres provide better adherence, and thereby they may support an improved surface contact.
When selecting the material for the elastic cover, the following factors must be considered. It is advisable to apply a material which is
By way of example, a suitable raw material is the two component RT-601 silicon rubber produced by Wacker Chemie AG. This material has a high homogeneity and behaves approximately in a linear way under small load, consequently it can be described with a linear continuum mechanical model. In the case of a linear system, in a way well-known in modelling, only two elasticity parameters of the material must be known. The Poisson ratio defining incompressibility is 0.5 with a good approximation in the case of rubber, which practically means that rubber cannot be compressed in volume. Young's modulus defining hardness is approx. 2.4 MPa in the case of the material used as an example.
To make an appropriate elastic cover, first a mould is to be made. Once the actual dimensions are known, various processes can be applied. In the case of bumps of a diameter of approx. 400 μm, we have etched a standard silicon plate in an isotropic way with the appropriate mask, and in the case of bumps having a diameter of 2 mm, steel roller balls with a given size were pressed in given positions into a smoothly polished aluminium plate. As an alternative, even a plastic mould can be applied in an arbitrary size.
The liquid, bubble-free stirred silicon rubber is to be poured into the mould. The smoothness of the surface opposite the mould can be ensured by a cleaned glass sheet placed on spacers of appropriate thickness. At the end of the setting time, the rubber can be separated from the mould and the glass, and it can be cut to size to achieve the proper lateral dimension. Using a thin layer of adhesive, the rubber layer can then be glued to the tactile sensor array. This adhesive can be the base layer in a liquid state or as an alternative a rubber glue having similar mechanical characteristics.
In designing the elastic cover, actually there is no need to insist on a hemispheric geometry. Any bumps meeting the above listed requirements can be applied.
In the embodiment shown in
The mechanical stress transferring characteristics of the elastic cover having the bumps 13 of hemispheric geometry and shown as a schematic cross sectional view in
It is also possible by means of the elastic cover according to the invention to obtain signals characterising the shear forces from (single dimensional) tactile sensor arrays measuring only pressure-like components. The essence of the approach according to the invention is that according to
In the top view according to
The 1D→3D conversion is possible by meeting the following conditions:
If the above three conditions are met, the components of our new 3D sensor element can be outlined by simple mathematics. The component normal to the surface will be proportional to the average of the signals from the four elements, while the two lateral components can be derived from the difference of signals provided by opposite elements, taking into consideration the appropriate proportionality factor. The pressure to the four tactile sensors is passed on by the material of the base layer according to the invention, enabling thereby the application of a simple sensor model.
If the topmost point of the bump 13 is not exactly above the middle, the simple decoding option is retained, because the calculation is to be expanded only by the compensation of the effect stemming from the difference in distance. It is of course also possible to apply the arrangement shown in
A design is also possible where the bumps are constructed as straight ridges. In the longitudinal direction of bumps, sensing is the same as with the flat covering, while in a crosswise direction a sensing function provided by the hemispheric bumps can be obtained. In this way a direction selective tactile sensing is achieved. Of course, it is also possible that the bumps are not located in a straight line, but they are situated along a fractional line or a curved line enabling the required direction selectivity.
The distance of bumps 13 from each other is preferably between 0.5 mm and 5 mm. The sensory function of a human finger enabling a resolution of some 1 mm falls into this region. In the case of hemispheric bumps, their radius is preferably not less than 0.15 mm.
The thickness of the base layer 10 is preferably between 0.2 mm and 2 mm. If the base layer is thinner, the cover becomes difficult to handle, tears easily and a sensing model provided by the passing of pressure by the base layer is not available.
Of course, the invention is not limited to the preferred embodiments shown by way of example in the figures, but further modifications are possible within the scope of the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| P0600892 | Nov 2006 | HU | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/HU07/00115 | 11/27/2007 | WO | 00 | 5/28/2009 |
