# Artificial-Hand Technology--Current State of Knowledge in Designing and Forecasting Changes
## 4 Actuating System
### Actuator Types
However, the same work principle is used in ultrasonic motors, where the transmission method is different. Its approach is to generate a propagation wave on a stationary elastic ring that, due to mechanical contact with the rotor, causes its rotation [135]. This type of motor is magnetically resistant, has high torque, and a fast response, but physical contact drastically shortens its lifetime. In [111], the disadvantage of a nonlinear torque characteristic was considered and, in order to prevent it from potential damage, motors were coupled with fingers by elastic wire so that uncontrolled impact could be softened. The authors declared that the velocity of MCP and the others joints was 486 and 900 deg/s, respectively, while a value of only 2 N of fingertip force during a force-control test was noted. Noteworthy is the solution of [PERSON] and [PERSON] [140]. They proposed an application of a DC motor, but instead of conventional reduction system, a \"Twist Drive\" was used. Two tendons were linked to a motor shaft at one end and to a digit at the other with an offset between them. When the motor shaft rotated, the wires would twist, adequately shortening their length to that connection so that pulling occurred. In spite of nonlinear transmission characteristics, the authors proved that a simple P/Pi controller allowed stable hand movement as well as 10 N force per each finger. Another group of actuators consist of pressurised-medium-driven devices such as actuators, muscles, and custom-made ones. Generally, they are specified as compliant, impact-proof, and of low precision, since the air (typically exploited gas) has a large compressibility coefficient. Furthermore, dynamics is strongly dependent on pressure used in the system. First, a linear actuator is a rigid unit that could be both one- or two-way. It is not possible to miniaturize it because of the piston fitted inside, as well as square reliance between a cylinder diameter and generated force [135]. Moreover, additional space for equipment, i.e., valves, pressure regulators, and a gas reservoir, must be prepared. Owing to these limitations, actuators were used for test \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Actuator** & **Model** & **Power Supply (V/A)** & **Torque/Force (Stall)** & **Power Density [W/kg]** & **Speed** & **Weight [g]** & **Cost [s]** \\ \hline Brushed & \begin{tabular}{c} Maxon \\ motor \\ DC motor \\ \end{tabular} & 12/0.01–0.16 & 0.905 (1.37) mNm & 159 & 4530 rpm & 6.3 & 104 \\ Brushless & \begin{tabular}{c} Maxon \\ motor EC20 \\ \end{tabular} & 24/0.03–0.84 & 7.74 (19.9) mNm & 227 & 5220 rpm & 22 & 93 \\ Ultrasonic & \begin{tabular}{c} Shinei \\ USR30-B4 \\ \end{tabular} & 110/ & 50 (100) mNm & 132 & 250 rpm & 19 & – \\ Servomotor & \begin{tabular}{c} Hite \\ HS-475B \\ \end{tabular} & 6/0.18–1.1 & (539) mNm & – & 55 rpm & 40 & 28 \\ Stepper & \begin{tabular}{c} Escap \\ motor \\ \end{tabular} & 24/0.65–0.9 & 3–5.5 (7) mNm & 157 & \textless{}10,000 rpm & 23 & 143 \\ Linear & \
\begin{tabular}{c} Actuonix \\ \end{tabular} & 12/0.2 & 8 (15) N & – & 15 mm/s & 15 & 65 \\ actuator &
\begin{tabular}{c} PQ12-P \\ \end{tabular} & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of motor-based actuators. purposes where soft, humanlike moves were required [84; 93; 100], or in automatics like the Utah/M.I.T hand that allowed not only 31 N fingertip force, but also a 10 Hz open-close cycle. In order to tackle the precision problem, [PERSON] et al. [115] proposed a two-mode actuator. The first cylinder is for big and fast displacements (468 deg/s) and, when an object is reached, the second is activated (250 ms delay) to execute small movements with a sufficient amount of power (29.1 N in fingertips). As the hand is supplied by hydrogen peroxide decomposition, a complex transmission system allows to lock joints on the desired positions in order to minimize air consumption.