# Upper Limb Bionic Orthoses: General Overview and Forecasting Changes
## 5 Sensory System  
Sensors corresponding to human senses are a necessary equipment for a bionic orthosis as both play control and safety functions. They allow the patient and physiotherapist to determine forces, which are essential for proper functioning of the orthosis/exoskeleton system and providing protection against injury. Exoskeletons share the same transducer technologies as other robotics. We can designate several popular sensor technologies: touch sensors, encoders, force/torque sensors, relative position sensors, absolute position sensors and distance sensors. Moreover, different sets of cameras are used more often for distance measurement or shape recognition and interpretation. [PERSON] et al. [123] divided sensors into two basic groups: position-movement and force-pressure sensors. The first group includes: encoders, linear variable differential transformers, potentiometers, accelerators, inclinometers, magnetic sensors, electro-goniometers, and MEMS Inertial Sensor Devices. The second group includes: strain gauges, force/torque sensors, pressure sensors, piezoelectric sensors, piezoresistive polymers, and capacitive force sensors. A basic comparison of transducer technologies used in orthoses is presented in Table 4.  
First of all, limit switches should be installed in all moving parts of a bionic orthosis, which cut off power supply to the orthosis in an emergency situation (e.g., when the inclination limit is exceeded). These sensors are passive type--they do not affect the control (no feedback) and operate only in two positions (on or off), without intermediate states. The electrical limit switches have a very short response time, which makes them ideal for protection against undesirable inclination of a bionic orthosis. They also support first level safety mechanism--passive mechanical end stops [124]. The Hall effect sensor was used by [PERSON] et al. to measure inclination angles of individual parts of an orthosis [125]. Scientists used also a force sensor acting in three planes with the accuracy of 0.08 N to control strength of joints in an arm exoskeleton [126]. An important element of the bionic upper limb orthosis is the hand grip force measurement, consequently a special sensor was designed for this task [127]. Its purpose is to adjust the hand grip force to prevent damaging the grasped object. For this purpose, a [PERSON] hair (a monofilament fibre) was used. Thanks to that, the sensor senses pressure as low as 0.087 N/mm\({}^{2}\). \begin{table}
\begin{tabular}{l c c c c c} \hline \hline  & **Power Density** & **Average** & **Density (kg/m\({}^{3}\))** & **Product Life** & **References** \\  & **(W/kg)** & **Efficiency (\%)** & & **Cycle (Number of Cycles)** & \\ \hline SMA & 1000–50,000 & \(<\)5 & 6450 & 10\({}^{7}\) & [71; 109] \\ CNT & 10–270 & \(>\)22 & 1000 & 140,000 33\% & [71; 114; 120; 121] \\ Elastomer & 500–5000 & 25 & 1000 & 10\({}^{7}\) & [71; 110; 111] \\ MRF & 690 & NDA \({}^{*}\) & 3000 & NDA \({}^{*}\) & [116] \\ Ultrasonic motor & 36 & 18–80 & 1620 & NDA \({}^{*}\) & [117; 122] \\ \hline \hline \end{tabular}  
* No data available.  
\end{table} 
Table 3: Characteristics of unconventional bionic orthosis drives. In research associated with balance assistance of a hip exoskeleton robot the control strategy is based on sensors data as interaction force between exoskeleton and human thigh (force sensors), posture detection (inertial measurement units), ground contact detection (pressure insoles) [128]. Some of these sensors can be successfully implemented in upper-limb orthosis control system. Some robotic devices can have a camera that recognises shapes using advanced algorithms. Use of the camera is designed to adjust the strength when gripping and lifting objects. The most important tasks of shape recognition algorithms are: extracting the object from the environment, determining the distance and position of the object, and passing this information as soon as possible to the main processor.