# Artificial-Hand Technology--Current State of Knowledge in Designing and Forecasting Changes
## 4 Actuating System
### Actuator Types
Although it was reduced to 23.5% compared to conventional actuators, the occupied space limited the number of actuators in a five-fingered 11 DOM hand to four. A well-known one-way type of actuator is the McKibben muscle, a so-called pneumatic muscle. It is so named due to its build and working principle. It is made of an elastic tube covered with a braided mesh sleeve, where one end is plugged and another is the air inlet. When pressure increases, the tube expands, and the mesh architecture imposes longitudinal contraction [141]. It features elasticity, low weight, up to 150 Hz work frequency and large available forces (see Table 3), making it a prospective solution for robotics [110; 142]. Despite the high characteristic nonlinearity and low precision of pneumatic actuators, the creators of the Shadow hand [131] limited its actuating speed to 0.5 Hz so that 0.6\({}^{\circ}\) joint-positioning accuracy was feasible. A relatively new group consists of soft pneumatic actuators. They also require a pressurised medium, but control is more difficult. Some applications use silicone molds that have an inner chamber system because of which all parts bend in a defined direction. Such a solution was applied in [104], where the hand was single-mold with four air channels. Similarly, in [105] three additional fluidic elastomer actuators (FEAs) were applied to MCP joints. As FEAs are silicone tubes with special windings, connecting three of them in a joint allows the precise control of movement by the appropriate activation order. Dexterity tests demonstrated that 1 bar was required to bend the MCP joint by 90\({}^{\circ}\). Other solutions assumed pulling the tendon with expanding balloons [120], but even in [87], where 35 N wire tension was reached, authors did not mention its lifetime. Direct applications in joints are also used, like in [114] in which a tube covered by two materials with different elasticity moved the fingers, or the device presented in [71; 127], where the advantage of fluid medium, that is, the highest achievable forces, occurred with 25 Nm joint torque. The last actuator group is active materials, where changes in a crystal lattice are the movement source. The most popular are shape memory alloys (SMA) [102; 143]. They are based on a phase transformation between two states, martensite and austenite. Generally, when an alloy is under load and its temperature is increased, the material recovers to its original shape [33; 144; 145]. The most important advantage of this material is its small weight and noiseless work, appreciated in prosthetics [125]. Unfortunately, the phase-temperature characteristic has big hysteresis that causes control problems and was actually reported by many researchers [37; 98; 146]. To solve this problem, many schemes were applied, including a conventional PID [80], C-segmentation with Peltier modules [37; 98] and fuzzy algorithms [147]. Moreover, to manage nonlinear systems, a neural network \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Actuator** &
\begin{tabular}{c} **Nominal** \\ **Length** \\ **[mm]** \\ \end{tabular} & **Displacement** & **Force** [N]** & **Speed** & **Life Time** & **Weight** & **Cost** \\  & **[mm]** & & **[N]** & **[Hz]** & **[Cycles]** & **[g]** & **[s]** \\ \hline McKibben muscle & & & & & & & \\ (Festo \(\varnothing\)5) & 30–1000 & 20\% & 140 & \textless{}150 & 10\({}^{7}\) & 77 & – \\ Linear actuator & 68 + & 12.5–300 mm & 47 & – & \textgreater{}10\({}^{7}\) & 36–144 & 105 \\ (Airpel cylinder M9) & stroke & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of conventional pneumatic actuators. was speculated [80; 148]. Despite the employed complex control methods, the main problem of heat exchange occurs. Because of this, time response is poor, 1 Hz in [80; 149] and less than 0.1 Hz in [147]. The typical strain varies between 2% and 3% (see Table 4), so long wires must be used [149]. To shorten actuator length, springs are a better solution [126; 150].