This invention relates to multi-point sensing of a contact with an external surface of a device. The device may be a robot hand, a robot body part, or any other component or part of any system that requires sensing an external contact with another body or part in the proximity environment with certain preferred parameters. For example, in order to be as similar to human skin as possible, robot skin needs to be flexible to enable draping over curved surfaces such as robot fingers, and facilitate sensing of multi-point contact at close resolution. A tactile array sensor is a very useful means to take artificial human made devices closer to live beings in terms of sensing and therefore has broad application in several fields. Pressure profile sensing using a sensor array linked to Capacitance based transducers is the most established means reported to date.
There have been some disclosures to develop a multi-point array sensor. An early disclosure by Miller et. al. (U.S. Pat. No. 5,374,787 dated Dec. 20, 1994) describes a sensor matrix array made of sensor pads in which capacitance changes as an object approaches a sensor pad, thus computing the position of the approaching object in relation with the sensor array. Wellman et. al. (U.S. Pat. No. 5,983,727, Issued Nov. 16, 1999) used an array of cavities formed in a substantially incompressible mounting structure. The cavities are filled with fluid, and covered by flexible elastic membrane; and the fluid pressure caused by touch with external objects is measured by transducers connected with the respective cavities. The transducers are capacitors or alternatively radiation (optical or ultrasonic) emitter-receiver sets. Peine et. al. (U.S. Pat. No. 7,378,856, Issued May 27, 2008) disclose elimination of cavities and fluid; instead, rows and columns of conductive strips separated by a thin layer of flexible deformable material such as silicone gel are used to form a capacitance array. Son et. al. (U.S. Pat. No. 7,430,925, Issued Oct. 7, 2008) disclose a Hybrid Tactile Sensor that introduces an intermediate connecting harness made from a flexible film claimed to overcome the problems of shorting between connections, mechanical stress, bulkiness, and fabrication difficulties as regards layout of electrical connections faced. Son et. al. (US Patent Application #US 2009/0033341 pub. dated Feb. 5, 2009) have disclosed a rectangular array of so called tile sensors. A different conduction contact based sensing approach is disclosed by Swallow et. al. (PCT Int. Publication #WO 01/75778 dated 11 Oct. 2001) which describes a pressure sensitive textile woven with orthogonal strands of conductive and insulative yarn. Other textile based approaches have also been reported by Gibson (U.S. Pat. No. 4,659,873) and Sandbach (PCT Int. Pub. #WO 01/75924 dated 11 Oct. 2001). Sandbach has disclosed a multi-layer fabric with two conductive layers separated by insulative layer, the conductive strands coming in contact with each other due to pressure from touch with an object, thus sensing the touch. A multi-layer conductive fabric based approach comprising of two layers of orthogonally laid conductive fabric strands separated by an elastically compressible dielectric material layer; that uses the capacitance generation at the crossover pixel points of the conductive strands of the fabric is disclosed by Manaresi et. al. (U.S. Pat. No. 6,826,968 dated Dec. 7, 2004).
Recent research is leading towards micro and nano resolution new materials development, for example, Li et. al. (“Multifunctional Graphene Woven Fabrics”, Li, X., Sun, P., Fan, L., Zhu, M., Wang, K., Zhong, M., Wei, J., Wu, D., Cheng, Y., Zhu, H., Nature, Scientific Reports 2, Article #395, 4 May 2012) have reported development of graphene-based woven fabric (GWF) by interlacing two sets of orthogonal graphene micron-ribbons embedded with Polydimethylsiloxane (PDMS). Further, the research is progressing towards wearable conductive fiber integrated sensors, for example, Gibbs & Asada (“Wearable Conductive Fibre Sensors for Multi-Axis Human Joint Angle Measurements”, Gibbs, P. T., Asada, H. H., Journal of Neuro-Engineering and Rehabilitation 2005, 2:7.
However, in spite of the above disclosures and research work reported in the prior art, there is no sub-millimeter resolution multi-point array tactile sensor available in the market to date. Functional requirement for an artificial robot skin type product with resolution in the range of 0.1 mm-1 mm is a critical parameter (Srinivasan, M. A., & Dandekar, K., “An investigation of the mechanics of tactile sense using two-dimensional models of the primate fingertip”, Biomech. Eng, 118:1, pp. 48-55, February 1996; Srinivasan, M. A., & Gulati, R. J., “In vivo compressibility of the human fingerpad”, Advances in Bioengineering, 22, pp. 573-576, 1992; Srinivasan, M. A., & LaMotte, R. H., “Encoding of shape in the responses of cutaneous mechanoreceptors”, in O. Franzen, & J. Westman-Eds., Wenner-Gren Intl. Symposium Series, pp. 59-69, 1991, New York: Macmillan). One of the crucial factors determining the success of a widely applicable robotic skin product is its crossing the so called tactile sensing two-point limen threshold. This threshold is defined by the smallest separation at which two points applied simultaneously to the human finger skin can be distinguished from one another, and is close to 1 mm. Another drawback in the prior art is that the sensing electrodes need a means for connection. Although an intermediate connecting harness made from a flexible film to enable connection with an IC is reported as a solution for this problem in the prior art, it introduces an additional member and thus complexity in the sensing system. Furthermore, the prior art has not disclosed configurations to enable easy and low cost assembly of sensor arrays using off-the-shelf components, using methods more suitable for automation of manufacturing process, avoiding use of expensive infrastructure. The prior art also has constraints such as excessive connections, lack of flexibility in deploying different resolutions in different sensing areas particularly relevant to biomimetic robotics, and lack of modularity in sensor pixels which makes replacement of individual or at least a small group of sensor pixels difficult.
Embodiments and aspects of the present invention overcome some of the difficulties in prior art either separately, individually or in combination with each other. The advantages of the present invention will become apparent from the description and accompanying drawings.
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