# Artificial-Hand Technology--Current State of Knowledge in Designing and Forecasting Changes
## 4 Actuating System  
### Actuator Types  
A type of actuating device has a major impact on artificial-hand dynamics. Varying-drive responses affect joint velocity, while power (with transmission efficiency) has influence on the force produced by the fingertips and the whole mechanism. It should be noted that these are not the only aspects that have to be considered during the design phase, since size, weight, and noise are generally restricted. There are a number of parameters that specify a chosen unit, so general categorization should be employed. [PERSON] et al. [132] and [PERSON] [134] described actuators typically divided into DC motors, pneumatic/hydraulic actuators, and a few new types, such as SMAs and EAPs. Instead of that, [PERSON] et al. [135] briefly presented the existence of motors and active materials that leads to linear or rotational movement, where all could be categorized by experience into conventional and nonconventional. This article combines all of the above, so no homogeneous classification is adopted. The first group consists of actuators based on rotary motion; devices like motors, electric actuators, servos are specified. Another fuses both pneumatics and hydraulics; broadly speaking, these use pressurised gas or a liquid medium. The last gathers active materials also called artificial muscles, a relatively new group of actuating units. There are two main types of DC motors that are related to inner design. Brushed motors have rotating windings so a commutator and brushes are used to change polarization. Mechanical contact between them wears down both parts in time and shortens the device lifetime. It also generates noise that is not desirable in prosthetics. A better solution is a brushless motor that has stationary windings, making it contactless. Hence, it is not only noiseless, but also has simpler construction, higher efficiency, power density, and generated torques (see Table 2). Regarding these parameters, both motor types require a reduction system to decrease speed and increase momentum for workable applications. Miniature gearmotors and standalone gearbox modules are used. The authors of [116] used a miniature Pololu with a 35:1 reduction that allowed to rotate joints with a speed of 20 deg/s. RCH1 Hand [45] with Maxon RE-maxl7 motors and separate gears (64:1) achieved 2750 deg/s in the DIP joint with a total grasping force of 30 N. These and other examples [40, 75, 77, 79, 94, 136, 137] showed that a free-standing motor, regardless if it is brushed or brushless, is not able to drive such mechanisms like a robotic hand. At the expense of size, servos provide an inbuilt gear reduction mechanism with a potentiometer so that precise positioning is achievable. Furthermore, due to increased output torque reaching 7 Nm in [73], angular velocity was reduced, interestingly to the same average value of 270 deg/s [24, 36, 108, 138, 138]. This is similar to linear actuators, where instead of rotational, linear displacement is produced [39, 81, 106]. Solutions like [88, 137] rotated the fingertip with 1396 and 450-800 deg/s, simultaneously delivering 30 (grasp) and 15 N (fingertip) force, respectively. A stepper motor is another type of actuator that provides position control, but as other motors,  
Figure 4: Timeline of configurations of degrees of motion in robotic hands. it requires a gearshift. [PERSON] et al. [139] does not mention a gear ratio, but 5 V Escap motors used in their hand allowed to generate 10 N fingertip force, which does not differentiate it from the above solutions [24; 39; 86; 99]. Alternatively to the electromagnetism phenomenon, a reverse piezoelectric effect may be used to produce force and movement. Strictly in piezoelectric motors, an applied electric field causes the shape change of material that, for example, repeatedly pushes the rotor and makes it rotate. Such articulation provides high torque, but the construction is complicated and expensive; hence, robotic-hand applications have not been reported yet [129; 130; 132; 135].