# Artificial-Hand Technology--Current State of Knowledge in Designing and Forecasting Changes
## 4 Actuating System
### Number of Actuators  
Robotic hands mainly have a limited number of actuators due to different reasons. For example, prosthetic hands (at least for now, when EMG is used) do not have enough control signals from amputees so actuators could be reduced. On the other hand, some applications require a specific type of drive unit of which the size may also decrease the instaleld parts.  
Nevertheless, the required number for proper control depends only on the kinematic model, since it was proven by [PERSON] that any kinematic chain with \(n\) degrees of freedom can be fully controlled by a minimal number of \(n+1\) independent tendons. Hence, underactuated hands with an actuator number that is lower than the number of DOF are perceived as unstable. Moreover, [PERSON] showed that the same chain with more than \(2n\) tendons is redundant; [67] and [PERSON] et al. [177] defined an \(n\) number as the minimum for bidirectional actuators coupled with a joint by an endless tendon loop. As \(n+1\) construction tends to exhibit buckling, as well as other instability problems [19], most devices use more than the \(n+1\) strategy. Such solutions have reduced the number of DOF and actuators, so they are not more complicated and rather practical. However, decreasing degrees of freedom in robotic hands takes us farther from the purpose of devices with human dexterity and versatility.  
Although the literature proves that the largest possible number of DOF/actuators is desired [131], Figure 6 shows that the number of actuating devices installed in hands (four- and five-fingered) has not been increasing over the years. Furthermore, stabilization of solutions was observed, which is related to the conclusions from the previous sections. More specifically, the two biggest groups can be distinguished that have a mean value of 16 and 20 degrees of motion, and are marked in green and orange, respectively. Most of the first group, as described earlier, consists of prosthetic hands that have restricted space; hence, the average number of actuators is five. The same can be deducted from [PERSON] et al. 2013 research, where commercial and research constructions are compared on the graph of number of joints-actuators [130]. Another group, concentrated on the 20 DOM point, constitutes more sophisticated devices with a number of joints closer to that of humans. Nonetheless, the number of actuators is slightly higher also due to its size, which leads to the problem of an appropriate actuating unit.  
Few advanced robotic hands can not be ommited. Solutions like SBC Hand driven by 32 SMA wires [37], multiply mentioned Shadow Hand with 23 DOM and 32 McKibben muscles [131] and DLR  
Figure 6: Number of degrees of motion to number of actuator ratio with two main groups of artificial hands highlighted: common, green; more advanced, orange. Statistics presented for three periods of time: (**a**) 1980–1999, (**b**) 2000–2009, (**c**) 2010–03.2019  
Hand with high biomimetic construction and 38 DC motors [95] prove how advanced human like performances are available and they will indisputably be icons in future, as Utah/MIT Hand [38] with 32 pneumatic actuators from 80' is now.