Tactile and finger-force sensors are indispensable interfaces for both human and robot interaction with the surroundings and they constitute the core sensing technology used for finding and handling of objects. Human skin is the most advanced but complex type of tactile sensor, thereby development of synthetic sensors to perform as precise as biological skin is very challenging.
Tactile or pressure sensors, according to transducing mechanisms, are generally divided into the following categories: capacitive, piezoelectric, resistive, and optical. Irrespective of the type of the mechanism, the tactile sensor functions through the mechanical deformation of the sensing unit upon contact with an object. In other words, the applying force will change mechanically (but not permanently) the arrangement of the units of the sensor. This deformation induces a change of the properties of the structure (e.g. electrical, optical or thermal properties). The resulting changes in comparison to the undeformed state will be transformed to a signal which will be ultimately reported as applied pressure. Although the development of a bulky tactile sensor is nontrivial, miniaturization of the system in order to be used in advance devices such as the skin of robot is rather challenging.
Therefore, sensors which works based on electrical conductivity (or resistivity) changes are more suitable for small devices because the collection of the response from the sensors is rather easy and does not require large detectors. On the other hand, optical sensors are rather bulky and hardly can be implemented for robotic industry. Nevertheless, the high speed of optical devices is a great advantageous compare to other alternative sensors and hence it turn the attention of sensor community to find a solution to miniaturize optical tactile sensor.