# Artificial-Hand Technology--Current State of Knowledge in Designing and Forecasting Changes
## 5 Sensors  
Humans without their senses could be compared to the first automatic machine that could execute some work but did not respond to any information from the environment. This led to evident problems, where the most important and dangerous were self-damage or injury to someone else. Since the purpose of artificial hands is to imitate humans' most advanced abilities, it must be considered with the appropriate senses-sensors. A basic type of feedback is the angle position that may relate to both joint and actuator position. Since servomotor electronic systems provide angular information, and DC motors can easily be supplemented with encoders as well as pneumatic actuators with pressure gauges/sensors [89; 110; 136], more challenging is to apply an appropriate aperture in the finger joints. Instead of potentiometers [82; 115], which introduce unwanted mechanical correlation and friction, contactless Hall-effect sensors with high resolution and good repeatability are a better solution [121; 138]. The working principle is based on a diametrically magnetized magnet placed concentrically in a joint that is detected by a sensor. Contrary to commonly used circular magnets, ellipsoidal magnets are used in Robonaut Hand 2 for that characteristic to be more linear [168]. Strain gauges and bend sensors are rarely used due to problematic installation, nonlinear characteristics, and low resolution [80; 108]. A robotic hand with a closed position loop is able to perform all of the possible gestures, limited only by joint range. This situation changes diametrically when external force is applied either from a manipulated object or finger collision. To prevent potential damage, force, torque, and tension data-collection methods are applied. The conventional technique assumes estimating torque by measuring the actuator circuit current, used in [73] and [90], which is well-known and developed, but a number of factors could disturb the mathematical model. One of them was included in the RoboRay Hand, so the control algorithm compensated for friction occurring in the entire construction [137]. In opposition to angular positioning, a force tasks implementation of strain gauges in various forms. For example, in [38], the tendon slid over a pulley that was placed on the strain gauge so that tension was checked with joint torques of up to 130 N; in Gifu Hand II, a six-axis force sensor was included in the fingertips, which was based on gauges [36] and many others [23; 139]. Once more, Robonaut Hand 2 presented an original solution where it was observed that conduit (covering tendon) compressive force was equal to tendon tension within an error of 5%-10% [168]. Going forward, angular-position disparity with spring displacement [82; 111] and differences in actuator-chamber pressure [93] were presented, but the most innovative method involved photoactive elements [115]. The authors in [178] constructed a miniature tension sensor consisting of a fork shape that had an LED diode and phototransistor on the gap sides. An elastic frame covered the whole part, while a screen attached to the inner side of frame interrupted the light beam. When the tendon pulled the elastic surface, the amount of detected light increased, hence the voltage changed. Extensive tests confirmed that sensors placed in fingertips can withstand forces up to 200 N with very linear characteristic and generally no histeresis. The missing factor separating from inland manipulation as well as accurate grasp parameters is tactile prehension of the human hand. The major challenge is to mimic human-skin mechanoreceptors responsible for interaction with the environment and providing sophisticated prehensile information. While natural mechanisms allow to detect such phenomena as vibration, touch, and pressure, the state of the art in robotics was found to be at a phase of introducing thin elastic sensors.