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The perception of human skin is all qualitative but not quantitative. The tactile sensor can imitate human skin. More amazingly, it can also express the sensations of temperature, humidity, force, etc. in a quantitative way, and can even help disabled people to gain lost sensory abilities. A new hair-like electronic skin, for example, enables robots to quickly distinguish the slight air fluctuations caused by breathing or faint heartbeat vibration. This sensor is even more sensitive than human skin and can be widely used in prosthetics, heart rate monitors, and robots.
The detection function includes the detection of the state of the object, the contact state between the robot and the object, and the physical properties of the object.
Robot hand with tactile sensation
The recognition function extracts the shape, size, stiffness, and other characteristics of the operating object based on the detection to perform classification and target recognition.
In the '70s, robotics research has become a hot spot, but the study of tactile technology has only begun. At that time, the research on tactiles was limited to contact with the object and the size of the contact force. Although there were some good ideas, the developed sensors were few and rudimentary.
The 1980s was a period of rapid growth in the research and development of tactile sensing technology for robots, during which a lot of research was done on sensor design, principles, and methods, mainly resistive, capacitive, piezoelectric, thermoelectric, magnetoelectric, force, optical, ultrasonic, and resistive strain principles and methods. In general, the research in the 1980s can be divided into three parts: sensor development, tactile data processing, and active tactile sensing, with the prominent feature of sensor device research centered on industrial automation.
Research on tactile sensing technology continued to grow and develop in multiple directions in the 1990s. According to a broad taxonomy, the literature on tactile research can be divided into sensing technology and sensor design, tactile image processing, shape recognition, active tactile perception, and structure and integration.
Sensors on the robot hand
In 2002, American researchers installed tactile sensors on the top of catheters for endoscopic surgery, which can detect the stiffness of diseased tissues and apply the right amount of force according to the softness of the tissues to ensure the safety of surgical operations.
In 2008, Kazuto Takashima et al. in Japan designed a piezoelectric three-dimensional force tactile sensor, installed it on the end of the robot's dexterous finger, and established a liver simulation interface, which allows surgeons to feel the information of the liver lesion site through the control of the robot's dexterous hand and perform closed surgery.
In 2009, Markus Mewer of the Fraunhofer Institute for Manufacturing Technology and Applied Materials in Germany developed an octopus underwater robot with a new tactile system that can accurately sense the condition of obstacles and can automatically complete the survey of the undersea environment.
Tactile sensors are extremely important in the technological research of robot sensing capabilities. Tactile sensor research is divided into broad and narrow senses. The broad sense of tactiles includes touch, pressure, force, slip, heat and cold, etc. The narrow sense of tactiles includes the force sensation on the contact surface of the robot and the object. From a functional point of view, tactile sensors can be broadly classified into contact sensing sensors, force-moment sensing sensors, pressure sensing sensors, and slip sensing sensors.
Piezoresistive tactile sensors are made by using the resistivity of elastomeric materials to vary with the amount of pressure and change the pressure signal on the contact surface into an electrical signal.
In 1981, researchers sandwiched carbon fiber and carbon felt between metal electrodes to form a piezoresistive sensor; in 1999, the Chinese Academy of Sciences used force-sensitive resistors to create an array of tactile sensors capable of detecting three-dimensional contact force information; in 2007, National Taiwan University used a polymer piezoresistive composite film to design and develop a three-axis tactile sensor with adjustable sensing range and sensitivity. The three-axis tactile sensor consists of four sensing cantilever beams and polymer piezoresistive composite films pasted on the surface and side of each cantilever beam.
A tactile sensor designed by Nanjing University of Aeronautics and Astronautics based on the optical waveguide principle that can detect three-way forces. The tactile sensing system consists of force-sensitive silicone rubber cylindrical contacts, cone contacts, and the cylindrical contacts correspond to the cone contacts on the other side of the rubber pad. The new photoelectric sensitive device, PSD, can not only detect the three-way force but also determine the force location information. And the output of the tactile sensor is compatible with that of the vision sensor, which is suitable for robot real-time force control and active tactile system.
Tactile sensor based on optical waveguide principle
The principle of the capacitive tactile sensor is that the relative position between two pole plates changes under the action of external force, which leads to the change of capacitance, and the force information is obtained by detecting the amount of capacitance change.
capacitive tactile sensor
In 2008, the flexible capacitive tactile sensor developed by the State Key Laboratory of Sensing Technology of Shanghai Institute of Microsystems and Information Technology can measure the contact force on the surface of arbitrarily shaped objects.
The magnetic field of a magnetically conductive tactile sensor changes under the action of an external force and converts the change in the magnetic field into an electrical signal through a magnetic circuit system, thus sensing the pressure information on the contact surface.
Magnetic Conductive Tactile Sensor
Magnetically conductive tactile sensors have the advantages of high sensitivity and small size but are less practical than other types of robotic tactile sensors.
The piezoelectric conversion element is a typical force-sensitive element with the important characteristic of reversible spontaneous charge and has the advantages of small size, light mass, simple structure, reliable operation, high intrinsic frequency, high sensitivity, and signal-to-noise ratio, and stable performance.
Piezoelectric robotic tactile sensors
In 2004, Chongqing University designed a tactile sensor using piezoelectric sensitive material to detect three-way force. The sensing head part is mainly composed of a base, cover, the inner core of the sensor, and adjustment mechanism. The inner core part of the sensing head is mainly composed of five identical piezoelectric elements, a square carbide, a section of cylindrical carbide, and a section of the copper column.
Prosthetic limbs can miraculously restore some of the lost functions of amputees, but one thing they have not been able to accomplish so far is to restore accurate tactile sensation. Now, researchers report that in the near future, it is possible for these artificial arms and legs, and feet to acquire a sense of touch that approaches the real thing. Using a two-layer thin, flexible plastic, scientists have developed a new electronic sensor that can mimic the neural information from tactile sensors in human skin and transmit signals to mouse brain tissue.
Several research teams have long tried to restore the sense of touch to prosthetic wearers. For example, two years ago, researchers at Case Western Reserve University in Cleveland, Ohio, reported that a prosthetic hand user was able to gain a sense of touch by attaching pressure sensors to the peripheral nerves in the arm.
However, although these results have restored basic tactile sensation, the sensors and signals are still vastly different from those sent by mechanoreceptors, the natural tactile sensors in the skin.
When mechanoreceptors in the body sense pressure, they send a stream of neural impulses; the greater the pressure, the higher the pulse frequency. Previous tactile sensors, on the other hand, produced stronger electrical signals at greater pressures, rather than a stream of high-frequency pulses. The electrical signal must be sent to another processing chip, which converts the strength of the signal into a digital pulse stream before it is sent to the surrounding nerve or brain tissue.
An important player in today's hot Industrial Internet is the industrial robot. Famous car manufacturers such as Tesla, BMW, etc. hardly see a single person in the workshop, relying on industrial robots to achieve assembly, painting, inspection, and other work. This year, Foxconn introduced thousands of robots to replace workers in China, proving that the adoption of industrial robots in manufacturing is the trend of the future. Force sensors give robots a sense of touch in the wrist. The force sensor is installed between the robot and the machine it operates so that all forces between the two can be sensed and monitored by the robot and the machine.
In recent years, portable smart electronics have evolved rapidly and many multifunctional wearable devices have emerged. It has become a new fashion to "wear" electronics on the body like bracelets, glasses, and shoes. Among them, the wearable tactile sensor is one of the most cutting-edge areas of the current technology circle. It can imitate the tactile function of direct contact with the external environment, mainly including the detection of force signals, heat signals, and wet signals, is the nerve endings of the Internet of Things and assist humans to fully perceive nature and its core components.
It is crucial to develop new tactile sensor devices that are wearable. It can adapt to arbitrary deformation of the substrate, and at the same time have accurate responses to a variety of irregular tactile stimuli. With the emergence of new functional materials such as graphene, carbon nanotubes, zinc oxide, and liquid metals, and the innovation of preparation technologies related to flexible electronics, the research of wearable tactile sensors has been rapidly developed in recent years.
Wearable tactile sensors are usually built on a skin-like elastic substrate or stretchable fabric to obtain flexibility and stretchability. With the rapid development of materials science, flexible electronics, and nanotechnology, the basic properties of the devices such as sensitivity, range, scale size, and spatial resolution have improved rapidly, even beyond the human skin. At the same time, in order to adapt to the sensing requirements of force, heat, moisture, gas, biological, chemical, and other multi-stimulus resolution, the device design is more and more sophisticated, and the integration scheme is more and more mature.
Smart sensor devices with practical functions such as biocompatible, biodegradable, self-healing, self-energy supply, and visualization have also come into being. In addition, wearable electronic products towards the direction of integration, that is, for specific applications will be tactile sensors and related functional components (such as power, wireless transceiver module, signal processing, actuators, etc.) effectively integrated to create a wearable platform with good flexibility, spatial adaptability, and functionality.
Currently, wearable tactile sensors still face many challenges in practical applications, such as performance degradation of sensors during repeated deformation, crosstalk decoupling for simultaneous detection of multiple stimuli, and matching of force, thermal and electrical properties between devices inside the wearable platform. Addressing these challenges will bring new opportunities and point to future directions for related material preparation, device processing, and system integration. There is no doubt that wearable tactile sensors will develop in the direction of more flexibility, miniaturization, intelligence, multi-functionality, and humanization. The scope of application of tactile sensors will be greatly broadened, with great prospects for application in human-computer interaction systems, intelligent robots, mobile medical, and other fields.