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‘Clothes’ for robots: A Flexible Large Scale Capacitive Tactile System

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Posted November 26, 2014

Modern technology of sensor manufacturing already achieves miniature dimensions and high sensitivity. This is also the case for capacitive sensor technology, which is considered to be a basis of future generations of robots with highly advanced tactile sensing functionality.

The patch and the dielectric layer that have been integrated on iCub forearm. (a) front and (b) back side of the patch (16 triangular modules). (c)  the front side of the dielectric cover; it is possible to notice that the cover has been formed to adapt to the shape of the forearm. (d) the back side of the dielectric cover. Here it is possible to notice the grid that characterises the 3D air mesh fabric. Image courtesy of the researchers.

The patch and the dielectric layer that have been integrated on iCub forearm. (a) front and (b) back side of the patch (16 triangular modules). (c) the front side of the dielectric cover; it is possible to notice that the cover has been formed to adapt to the shape of the forearm. (d) the back side of the dielectric cover. Here it is possible to notice the grid that characterises the 3D air mesh fabric. Image courtesy of the researchers.

The importance of robots having ability to sense various physical parameters in different locations of their body surface becomes increasingly obvious. Machines have to be able to operate in so-called unstructured environments and in close proximity to humans while maintaining high degree of safety and reliability. This functional need drives a multitude of research in the field of transduction methods that could be used to grant robots the sense of touch. However, some technical limitations are still obstructing successful practical implementation of skin-like tactile systems for robots. Requirement to embed a deformable dielectric layer covered by a conductive layer is currently the main limitation of basically all approaches based on capacitive technology.

An effective integration of tactile sensors on the body surface of real robots poses one unavoidable requirement: sensors have to be flexible to be suitable for attaching on curved surfaces. This requirement also has to be fulfilled while dealing with other system-level factors like wiring, networking, efficient power consumption, not to mention issues like cost of manufacturing, integration and maintenance. Now, a research conducted by scientists from Genova, Italy, promises to advance the robotic tactile technology much further than before.

In the scientific paper published on arXiv.org on 25th of November, the team presents the revised version of the robotic skin system designed to overcome main limitations of capacitive tactile technology. The previous version successfully demonstrated features of modular design and achieved food performance in terms of sensitivity and resolutions. However, as the authors note, some of the problems still remained: hysteresis caused by silicone foam used to make the dielectric layer, reduced sensitivity due to ageing of the elastomer material, combined with a poor mechanical strength to wear and tear of the external conductive layer, and sensor output drift under thermal variations.

4layer flexible fpcb

The vertical section of the structure of the new tactile module. Image courtesy of the researchers.

The sandwich of fabric that constitutes the new dielectric for the sensor. The deformable bottom layer is made with 3D air mesh fabric, the medium layer is made with conductive Lycra that acts as common ground plane for all the taxels. The third layer protects the Lycra and improves the mechanical property of the sensor.

The sandwich of fabric that constitutes the new dielectric for the sensor. The deformable bottom layer is made with 3D air mesh fabric, the medium layer is made with conductive Lycra that acts as common ground plane for all the taxels. The third layer protects the Lycra and improves the mechanical property of the sensor. Image courtesy of the researchers.

To eliminate these negative factors, the scientists used a deformable 3D fabric to produce the dielectric layer. On top of this layer conductive and protective layers of artificial robotic skin are glued. A scalability is ensured by forming self-contained sensory modules that can be interconnected. In the current work, each module was made of flexible printed circuit boards (FPCBs) combined with soft dielectric, and shaped as a triangle hosting 12 sensors or ‘taxels’ (see the figures above). Under pressure, the capacitor deforms; the capacitance signal is picked up by digital converter, converted to code and transmitted in numerical form via serial line.

“Several such triangles can be interconnected to form a mesh of sensors to cover the desired area”, the authors of the paper write. Certainly, the triangles and the interconnecting lines between them are flexible, thus providing possibility to shape the entire tactile layer in any (or almost any) desired form.

The integration of the sensor on the iCub forearm: (a) the sensor patch is first glued on top of the iCub forearm cover then, (b) the dielectric layer is fixed on top of the sensor with screws. Image courtesy of the researchers.

The integration of the sensor on the iCub forearm: (a) the sensor patch is first glued on top of the iCub forearm cover then, (b) the dielectric layer is fixed on top of the sensor with screws. Image courtesy of the researchers.

The team notes, that the thermal compensation technology was used in the revised version of triangular FPCB modules to eliminate output signal drift emerging due to temperature variations. Certain sensory module shape optimizations were also implemented to increase the signal to noise ratio and simplify the integration process (e.g. triangular modules with smooth edges are easier to glue).

The replacement of conductive Lycra and silicone foam with a sandwich made of 3D fabric used for clothing, Lycra and protective fabric, became the second major improvement of the modular tactile system. In result, the module flexibility was increased and this allowed to achieve larger sensitivity and facilitated the production because available industrial manufacturing processes can be applied.

The integration of the sensor on different robots: (a)iCub, (b)WAM arm from Barret Technology. Image courtesy of the researchers.

The integration of the sensor on different robots: (a)iCub, (b)WAM arm from Barret Technology. Image courtesy of the researchers.

During experimental investigations, the capacitive skin was integrated and tested on the iCub robot forearm and WAM arm from Barrett Technology. The scientists confirmed that the sensory tactile system can be mounted on the robotic surfaces easily without special glues and can be also replaced if damaged.

Currently the team of developers has not provided any comments about how soon their technology can appear in mainstream robotics.

Written by Alius Noreika

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