A team of researchers at the Indian Institute of Science, Bengaluru, has developed a ‘diaphragm and micro-stylus’ based Fiber Bragg Grating tactile sensor. This is a big improvement in tactile sensing—or ‘touch’ sensing—used in (among other areas) robotics.
A robot has to grip an object correctly. To do this its arms are fitted with tactile sensors that sense the the surface of the object by touch. The touch provides it with several information about the texture, shape, composition and temperature of the object. Tactile sensors are also used to read Braille scripts, locate tumors in open surgeries, monitor pulse rate and heartbeat in wearable devices and also in a host of industries.
However, tactile sensors are expensive, difficult to make and not quite efficient. To address this, researchers from the Indian Institute of Science (IISc) have developed a novel diaphragm and micro-stylus based Fiber Bragg Grating (FBG) tactile sensor.
To learn further about this, it is first necessary to understand what Fiber Bragg Grating is.
Normal optical fibers are uniform along their lengths. In a simple fiber Bragg grating, the refractive index of the fiber core varies periodically along the length of the fiber. This is done by transversely illuminating the fiber with a UV laser beam and inscribing a pattern.
When a light with a broad spectrum is launched into one end of fiber containing a fiber Bragg grating, the part of the light with wavelength matching the Bragg grating wavelength will be reflected back to the input end, with the rest of the light passing through to the other end. The fundamental principle behind the operation of fiber Bragg grating (FBG) is Fresnel reflection—light traveling between media of different refractive indices may both reflect and refract at the interface.
If the cable and subsequently the FBG are subjected to a force or a change in temperature by some random object, the reflected wavelength shifts by some amount. Monitoring this shift gives some information about the object. Fiber Bragg Gratings can then be used as direct sensing elements for strain and temperature.
In the newly-developed tactile sensor, the IISc team used a combination of a diaphragm and a freely moving micro-stylus (MS) ‒ which looks like the tip of a ballpoint pen ‒ as the “random object”, which “disturbs” the FBG depending on the surface texture. Their sensor exhibits nearly 80 times higher sensitivity compared to existing devices, the researchers say.
The team used their sensor to “read” Standard English Grade-1 Braille cells, a functionality that might help people who have lost their eyesight during the course of their life avoid having to learn Braille at such a late stage. While scanning the surface of the Braille-embossed paper, the tip of the MS touches the curved surface of the hemispherical Braille dot. This pushes the MS upward, which deflects the diaphragm, which in turn compresses the attached FBG sensor. The data recorded by the sensor is then converted to English alphabets using an algorithm. These alphabets could potentially be communicated to the user as an audio signal.
Compared to existing tactile sensors, FBG sensors have many advantages: small size, high sensitivity, fast response, ease of remote deployment, reduced connectivity requirements, immunity to electromagnetic interference, and multiplexing capability. This FBG tactile sensor is a promising candidate for various applications related to surface measurement.
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