# Design of a Two-DOFs Driving Mechanism for a Motion-Assisted Finger Exoskeleton
## 3 Kinematic Design  
The index finger of a healthy subject has been used as a reference to develop and validate the proposed novel exoskeleton. First, grasping motions have been analyzed by using the Computer Vision Toolbox of MATLAB (Mathworks, Natick, MA, USA). Markers have been attached to the joints of the index finger and its fingertip (FT). Distances between the markers have been also measured with a digital caliper for calibration purposes. The camera has been aligned perpendicular to the side of the finger under study, and a fixing frame has been used both for the hand and camera in order to achieve a planar motion of the finger and a fully orthogonal video recording. The acquired videos have been post-processed to collect positions of the joints versus time. Figure 3 shows the experimental setup for the finger motion tracking as well as the definition of the angles and reference frame with the x-axis horizontal oriented from left to right, and the y-axis vertical oriented from top to bottom. The z-axis is defined according to the right-hand rule. The origin of the reference frame is attached to the center of the MCP joint. The angle of the MCP joint is called \(\varphi\), the angle of the PIP joint is called \(\varepsilon\), and the angle of the DIP joint is called \(\tau\). The angles have a clockwise positive direction. The lengths of the phalanxes of a test subject were identified from set-up measurements as 25 mm for the distal phalanx, 28 mm for the medial phalanx, and 43 mm for the proximal phalanx. Twenty-two frames have been evaluated from the motion tests for a suitable finger motion characterization. Figure 2 shows a detail of a video-captured motion for a healthy human finger movement referring to a typical human finger motion. In particular, the plot in Figure 4 shows trajectories of PIP, DIP, and FT markers during the motion of an index finger that moves from fully open to fully close. These trajectories will be used as the reference motion trajectories for the linkage mechanism of the proposed novel exoskeleton. However, the desired exercising motion usually does not require the full joint feasible rotation range of the motion range of a healthy human finger but only a subset of such a motion. Figure 3: Experimental setup for finger motion video tracking (**a**) and definition of angles of the finger joints (**b**). Moreover, it is advisable to avoid full finger closing as this would result in an undesired interference with the palm. A specific linkage mechanism has been identified as convenient for mimicking the desired reference finger motion. This proposed mechanism consists of eight links that are arranged as two interconnected four-bar mechanisms. This specific linkage arrangement has been proposed as an Italian patent [34]. The proposed mechanism requires two motors that each drive one of the two four-bar mechanisms. A detailed kinematic scheme is shown in Figure 5. The motors are located at joints D0 and B0. The angle of driving link D0-D is called \(\delta\). The angle of the driving link B0-B is called \(\beta\). Both angles are positive in a clockwise direction. The first four-bar linkage (D0-D-E-MCP) has four links and four revolute joints with one active DOF. The second four-bar linkage (B0-B-A-A0-C) has one active DOF, four links, and four revolute joints. The second four-bar linkage drives the point C that is attached to the fingertip (FT). The exoskeleton is also attached to the palm at the fixed frame link D0-MCP and to the first phalanx with the link E-B0-A D, as shown in Figure 5.  
For the first linkage, the link lengths need to be chosen by considering the dimensional constraints of the motor and the palm of a hand. Accordingly, the link length D0-MCP is close to the vertical dimension of the selected motor. The link length E-MCP is mostly determined by the geometrical constraints for the attachment of E-B0-A to the first phalanx.