# Artificial-Hand Technology--Current State of Knowledge in Designing and Forecasting Changes
## 4 Actuating System
### Transmission Approach  
There is no actuator that could at once be directly placed in a joint and deliver enough torque. Some robotic hands try to face that problem, but either motors installed in the digits, farthest from the proximal part, require bevel gears or custom-made pneumatic/fluidic occupied space above the joints [71; 99; 127; 140]. The need for a sufficiently dynamic and humanlike appearance led to palm or external (forearm) location; thus, a transmission mechanism must be provided [167]. Furthermore, the number of actuators in the bulk of robotic hands is lower than the degrees of motion, so some of the joints are coupled in different ways, most frequently, DIP and PIP [78]. Stiff connections are preferred for drive units installed in the immediate vicinity of the finger base. Such solutions as four-bar linkage were applied in [88; 122; 136; 139], which allowed precise and stable positioning with the use of servomotors and DC motors. In the Robonaut hand, an original solution used a flexshaft to transmit rotation from BLDC motors to the leadscrew in the palm, where it was converted and applied to four-bar linkage [20; 118]. Another basic mechanism was included in [84] and in [39]. Both of them were equipped with linear actuators. For that reason, a slider rocker mechanism secured the motor shaft from vertical forces, especially in Rehand, which is a prosthesis, so it was exposed to daily leaving activities. The next example of rigid parts in transmission systems are various types of gears. In [86], [PERSON] et al. implemented a complex chain of shafts and bevel  
Figure 5: Application timeline of different types of actuators. gears. A 2 DOF MCP joint moved thanks to more than eight gears. A DC motor placed in the proximal part responded with six gears and three shafts for simultaneous PIP and DIP joint rotation. Such an application requires precise and decent assembly and components, so that slipping or locking does not occur. A gear rack is the other kind, where not only is conversion from rotating to linear movement (and vice versa) possible [97; 115], but gearbox features as well. Although all above constructions were able to execute two-way movement, a significant amount of space must be considered during the design phase. More importantly, none of them was backdrivable, so any impact to the active axis could damage both transmission system and actuator. The biomimetic method of transmission refers to fibrous connective tissue, the tendon. It is used more or less in all types of actuators. Since McKibben and artificial muscles are one-way actuators in cooperation with elastic filament, actuation must be considered so fingers can be controlled by pulling links. It works as described in SMA-actuated hands [98; 146; 149] and in the SBC hand [37], in which abduction-adduction was possible. In the Shadow hand [131] an impressive number of 40 tendons were grouped and redirected from the forearm, through the wrist, to a 24 DOM hand. It should be noted that pneumatic muscles used in this hand require prestretching; thus, all construction was metallic in order to counteract occurring forces and stresses. Moreover, the authors pointed out that the hand is controlled both in a joint-angle and joint-stiffness loop, which partially solves the impact problem. Tendons are also applied in devices with conventional two-way actuators. In such applications, they are directly routed and attached to phalanges, and pulleys are also used. In Robonaut Hand 2 [168], another original actuation method was presented based on linear movement like previously [20; 118], with tendons being immediately connected to joints. Most important in that research was the fact that the authors carried out tests of a tendon work environment. In this project, such factors as sliding material and lubricant were considered to elongate tendons' lifetime, which has not been included anywhere else yet. Directly attached ligaments were also used in [72; 78; 108], where extension was handled by compression or torsion springs.