# Design of a Two-DOFs Driving Mechanism for a Motion-Assisted Finger Exoskeleton
###### Abstract  
This paper presents a novel exoskeleton mechanism for finger motion assistance. The exoskeleton is designed as a serial 2-degrees-of-freedom wearable mechanism that is able to guide human finger motion. The design process starts by analyzing the motion of healthy human fingers by video motion tracking. The experimental data are used to obtain the kinematics of a human finger. Then, a graphic/geometric synthesis procedure is implemented for achieving the dimensional synthesis of the proposed novel 2 degrees of freedom linkage mechanism for the finger exoskeleton. The proposed linkage mechanism can drive the three finger phalanxes by using two independent actuators that are both installed on the back of the hand palm. A prototype is designed based on the proposed design by using additive manufacturing. Results of numerical simulations and experimental tests are reported and discussed to prove the feasibility and the operational effectiveness of the proposed design solution that can assist a wide range of finger motions with proper adaptability to a variety of human fingers. b pacs: 12.30.-p, 11.15.  
linkage between the exoskeleton and finger has one DOF. Each linkage consists of four links and four joints with one DOF each. The exoskeleton allows flexion and extension of all phalanxes of the finger as well as passive abduction and adduction of the first phalanx. This exoskeleton is quickly adjustable and has a low resistance against finger movement when the motors are not activated. The device applies 80 g to the finger, and it has five angle-sensors to monitor link orientations. The index-finger exoskeleton uses a closed-loop torque-control with maximum torque at the first phalanx equal to 250 Nmm. The maximum torque at the second phalanx is 50 Nmm [4]. The exoskeleton reported by [PERSON] et al. [5] is optimized by an evolutionary synthesis algorithm. The synthesis determines the link length of the mechanism design to fulfill a given coupler curve with high accuracy. The kinematic structure of the device is set before the synthesis. It consists of seven links that are fixed on the back of the hand and on each phalanx, resulting in one DOF in total. A video analysis of the healthy finger motion acquires the coupler curve. The phalanx lengths of a patient's finger are acquired by an X-ray. The proposed exoskeleton is manufactured for each patient individually by 3D-printing and it has one servo motor. Amadeo is a commercial hand and finger rehabilitation device that has been developed by Tyromotion GmbH [6]. The fingertips can be connected to the caps of the exoskeleton while being adjustable to different finger sizes. Each cap is attached to an automated linear slide. The fingertips can be pushed with a predefined force, and the device can move the finger while they apply no force. In this way, a grasping activity is simulated. The arm and wrist are fixed on the device frame, and the finger caps are connected to a slider. All fingers and the thumb can be treated. The device also allows for several measurements, such as force and range of motion (ROM). In [7], it is reported that 70% of the ROM of a healthy finger is sufficient for rehabilitation motions. The Script SPO-F exoskeleton [8] is a passive device with no motor but a spring as the actuator. It consists of six links and seven joints with only one DOF. Earlier versions of this exoskeleton have been bulky, complex, and reached a weight of 1.5 kg. The Script SPO-F is actually an exoskeleton for both the wrist and the hand. It is designed for home-usage during rehabilitation treatment. The finger is flexed by a user, and a spring exerts a force on the finger due to the deflection. In contrast to other exoskeletons, it has no predetermined trajectory. The finger can be moved due to a cable connection between the fingertip and the exoskeleton. As the fingertip is fixed, only first and second phalanxes can move. A torque of about 125 Nmm is needed for a 90\({}^{\circ}\) flexion of the second phalanx.