Tactile sensors, also called touch sensors, help determine an object’s physical characteristics when they are close to the objects. They are also called transducers which are used in various applications worldwide.

Psychophysical studies have determined the importance of touch, which is used for interpretation, extraction, and manipulation of the object within our range that is registered in various types of receptors like thermoreceptors, nociceptors, mechanoreceptors, etc., which are connected to a system to scrape the necessary information.

Tactile sensor technologies

Tactile sensors are sensitive to static or dynamic forces, which can be used for proprioception or exteroception. These sensors measure the internal states of the robot, while the latter is used to measure physical characteristics like the sense of touch in the environment.

All tactile sensor technologies are classified based on the transduction method, which helps convert stimuli from the external environment into intelligent systems. The most widely used tactile sensors in robots are based on the following transduction methods:

• Capacitance
• Piezoresistivity
• Optics
• Magnetic
• Binary
• Piezoelectric

Capacitive sensors

These types of sensors measure the variations in capacitance from an applied load over a parallel plate capacitor in the sensor. Capacitive sensors are susceptible to the external field, which in robotics is called taxels mimic the operation of human fingers. Since these sensors can be fabricated in compact spaces, they are used at the fingertips of robots.

Piezoresistive sensors

These sensors detect the resistance of a contact when an external force is applied. These sensors are generally fabricated on conductive rubber made of piezoresistive ink. These sensors produce a wide dynamic range, good load tolerance, durability, and low-cost fabrication. These sensors are used in robots where high accuracy is not a prim criterion.

Optical sensors

These sensors transduce mechanical contact, movement, and pressure into changes in refractive index or light intensities, detected using state-of-the-art vision sensors. They provide robustness to electrical interference with the capability of resolving high-level problems. Due to this, they are used in dexterous object manipulation and various other robotic systems. It is also used as one of the primary sensors in a three-finger robot hand to grasp particular objects in close vicinity.

Piezoelectric sensors

They produce an electrical charge when a force, pressure, or deformation is applied to them. They are best used to measure vibrations and are widely used for their high-frequency response, sensitivity, and availability in various forms like ceramics, crystals, etc. They are also preferred for the fabrication of tactile sensors, given their workability, flexibility, and chemical stability. Recently, they were best used in integrating tactile sensors with robotic grippers in robotic hands. Along with binary sensors, they are the best choice for using them in five–finger robotic hands as well