# Upper Limb Bionic Orthoses: General Overview and Forecasting Changes
## 4 Actuators Overview  
Most frequently encountered upper limb hardware systems are based on electric, hydraulic and pneumatic actuation [85; 86; 87; 88; 89; 90]. In the past, primitive hydraulic drives were used in exoskeletons, as it was the only type of drive that could set in motion a heavy steel frame. The aforementioned Hardiman project was composed of a complex hydraulic and electronic network. With the technology development and new materials, engineers started to adapt, first of all, new lightweight motor based drives. Moreover, the number of smart materials used as an actuator in bionic orthoses is rising increasingly. In this type of materials we can observe change of physical properties or shape under the influence of, e.g., an electric, magnetic and temperature field [71; 91]. An exemplary classification of described and others actuators is shown below (Figure 3). Easily accessible, cheap and with wide range of power and size DC motors are the most common types of drives. Compared to others, they are distinguished by extensive experience and knowledge in control methods and that is why they are used in limb exoskeletons [92; 93; 94] and in stationary rehabilitation robots [95; 96; 97; 98].These electric actuators can be divided into two basic groups: brushed and brushless. Brushless motors (BLDC) have much higher torque and are more compact than brushed (Table 2), what is frequently used in elbow direct actuation [15; 99], but that is not a rule and brushed motors also appear [87]. On the other hand, they require a dedicated control board to correctly change polarisation and so regulate speed/torque. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & **Tensile Strength (MPa)** & **Yield Strength (MPa)** & **Density (kg/m\({}^{3}\))** & **Processing Difficulty** & **Price (USD/kg) \({}^{*}\)** & **References** \\ \hline Stainless steel & 500–700 & 200 & 8000 & Medium & 25 & [50] \\ Aluminium alloy & 510–540 & 430–480 & 2810 & Difficult & 15 & [68; 69] \\ PLA & 800 & 70–100 & 900–1500 & Easy & 2 & [72; 73] \\ SMA & 1000 & 200 & 6500 & Difficult & 100 & [70; 71; 76] \\ Carbon fibre & 2800–5000 & 840 & 1600–2000 & Very difficult & 25 & [75] \\ \hline \hline \end{tabular} \({}^{*}\) Estimated values based on wholesale prices of 2016-12-05 (not applicable to SMA [76]).  
\end{table} 
Table 1: Properties of materials used in the production of bionic orthoses. Brushed motors could be simply controlled by changing the voltage level, which makes them easy applicable; however, brushes wear over time, which shortens their lifetime. They are a base for other types of actuators such as servomotors or linear actuators--devices which consist of an integrated gearbox and feedback circuit and allows for precise displacement control, speed, and acceleration of the motor shaft [66,100]. It should be noted that both brushed and BLDC motors require an additional gearbox to generate appropriate power and speed [59,65]  
Currently, the stepper motor is one of the simplest types of drive to control. Unfortunately, these motors have many serious disadvantages, such as discontinuous work (jumping), large dimensions (at high torque values), high energy demand, and high heat output during operation. In reference to the next group, pneumatic and hydraulic actuators, there are no applications using this type of drive due to oversized end-effectors and additional equipment (valves, regulators, gas tank and a compressor) which results in the relatively large and bulky system. A particular type of such actuators is the McKibben muscle modelled after the human muscle. A pneumatic muscle, used in several research exoskeleton projects [101,102], is a first applied and developed drive considered as a artificial muscle, because of a human muscle properties such as light weight and elasticity [103,104]. Nevertheless, it still possesses imperfections of pressurized medium actuators, more precisely, equipment requirements. Recently, a growing interest to unconventional solutions is observed (Figure 4).