A tactile sensor is a device used in robots to replicate the sense of touch.
Tactile sensation is a crucial sensory function for individuals when they come into direct contact with the environment. Developing a tactile sensor that meets the necessary requirements is a critical component in the advancement of robotics technology.
With advancements in microelectronics and the advent of various organic materials, various solutions for developing tactile sensors have been proposed. However, many of these solutions are still in the experimental stage and there are relatively few products that have been commercialized.
Tactile sensors can be broadly classified into four categories: contact sensors, force-torque sensors, pressure sensors, and sliding sensors, based on their functions.
A touch sensor is used to determine whether a robot, typically its limbs, is in contact with an external object or to measure the properties of the object being touched.
There are several types of contact sensors including micro switches, conductive rubber, carbon sponge, carbon fiber, and pneumatic reset devices.
① Micro switch
The touch sensor consists of a spring and a contact. When the contact touches an external object and moves away from the substrate, the signal path is interrupted, indicating contact with the external object.
The advantage of this type of normally closed (always on when not in contact) microswitch is its ease of use and simple design. However, it is prone to mechanical vibrations and the contacts are vulnerable to oxidation.
② Conductive rubber type
This type of touch sensor utilizes conductive rubber as its sensitive component. When pressure is applied against an external object, the conductive rubber compresses, altering its resistance, causing a change in the current flowing through the conductive rubber.
The disadvantage of this type of sensor is the inconsistent drift and hysteresis characteristics due to variations in the formulation of the conductive rubber. The advantage is its flexibility.
③ Carbon sponge type
This touch sensor features an elastomer made of sponge material on the substrate, with an array of carbon sponge arranged within. When an object is pressed against the sensor, the resistance of the carbon sponge decreases, and the magnitude of current flowing through the carbon sponge is measured to determine the extent of compression.
This type of sensor can also be used as a pressure sensor. It has the benefits of a simple structure, good elasticity, and ease of use. However, the uniformity of carbon distribution directly impacts the measurement results and the ability to recover after compression.
④ Carbon fiber type
In this touch sensor, carbon fiber is used as the upper layer, the lower layer is the substrate, and the middle layer is filled with urethane and metal electrodes. When an external object is in contact, the carbon fiber is compressed and makes electrical contact with the electrode.
The advantage of this touch sensor is its flexibility, allowing it to be mounted on the curved surface of a robot arm. However, it has a significant amount of hysteresis.
⑤ Pneumatic reset type
This touch sensor has a flexible insulating surface that deforms when pressed and uses compressed air as a resetting force when disengaging from an object. When the sensor comes into contact with an external object, the internal elastic bubble (made of yttrium copper foil) makes contact with the lower electrode and conducts electricity.
The advantage of this touch sensor is its flexibility and reliability, but it requires a source of compressed air.
The slip sensor is a displacement sensor used to measure the amount of slip experienced by an object when it is grasped or transported by a robot. It can be classified into three categories based on the presence or absence of a sliding direction detection function: non-directional, unidirectional, and omnidirectional.
① The non-directional sensor has a probe earphone type design and consists of components such as a sapphire probe, a metal bumper, a piezoelectric Rochelle salt crystal, and a rubber bumper. The sapphire probe vibrates as it slides and the movement is converted into an electrical signal by the Rochelle salt. The buffer serves to reduce noise.
② The unidirectional sensor has a roller photoelectric type design. The slip of the grasped object causes the drum to rotate, which results in the photodiode receiving an optical signal transmitted through the code wheel (mounted on the drum’s circular surface). The slip of the object is detected by the drum’s corner signal.
③ The omnidirectional sensor uses a metal ball that is coated with an insulating material, creating a pattern of conductive and non-conductive regions on its surface. When the sensor comes into contact with an object and sliding occurs, the ball rotates, causing the conductive and non-conductive regions on its spherical surface to alternately touch the electrodes.
This generates an on-off signal, and the magnitude and direction of the slip can be determined by counting and interpreting the on-off signals.
This sensor requires a sophisticated manufacturing process.