4.1 - Project Proposal
Table of Contents:
Introduction
Problem Statement
Proposed Mechanism
Proposed Scope of Work
Preliminary Design Concepts
References
1. Introduction:
Hand function is critical for performing daily activities such as grasping objects and manipulating tools. These tasks can become difficult for individuals recovering from neurological injuries, including stroke, spinal cord injury, and nerve damage, primarily due to reduced finger strength and coordination. Wearable robotic devices that assist finger motion have the potential to restore partial functionality or provide rehabilitation support for these individuals. However, replicating the natural motion of a human finger presents a significant challenge because finger movement does not occur about a single fixed joint axis; instead, it involves coordinated motion across multiple joints [1]. Developing a compact planar linkage capable of reproducing natural finger flexion therefore represents an interesting and meaningful mechanism design problem, combining biomechanical understanding with mechanical innovation [2].
2. Problem Statement:
The goal of this project is to design a planar mechanism that assists finger flexion by coordinating motion across multiple finger joints, potentially requiring multiple torque inputs. Human finger flexion involves coordinated rotations at several joints that follow specific angular relationships during grasping. The metacarpophalangeal (MCP) joint contributes one independent rotation and torque input that largely determines the overall finger trajectory. The proximal interphalangeal (PIP) joint provides another independent rotation, while the distal interphalangeal (DIP) joint typically rotates in a coupled manner with the PIP joint during free grasp but adjusts independently to conform to objects during practical grasping tasks. Together, these motions create a curved fingertip path that enables the finger to wrap naturally around objects.
Achieving this coordinated motion using simple joints is difficult because each joint must follow task-dependent angular relationships while maintaining a constrained fingertip trajectory and efficiently transmitting forces required for grasping. A simple linear chain of revolute joints cannot enforce these relationships or maintain the desired motion path unless mounted along the side of the finger, which would restrict adduction and therefore is not a viable solution. Simply lining up a robot's joints with a human's joints does not work well because everyone's fingers are different lengths [3]. Consequently, a more sophisticated linkage mechanism is required to coordinate the motion of multiple finger segments.
Figure 1: Schematic of Hand Bones and Joints
Figure 2: Types of Hand Movements
3. Proposed Mechanism
The proposed solution is to explore the use of multiple planar four-bar linkages arranged in series to coordinate motion between finger segments [4]. These linkages will be mounted along the dorsal side of the finger to allow natural motion while providing structural support. Although the DIP joint is often coupled with the PIP joint during free motion, true mechanical coupling between them is difficult to achieve within the limited available space. Additionally, most gripping strength is generated by the MCP and PIP joints, and in practical grasping scenarios the DIP joint adjusts its angle to conform to the object being grasped. Therefore, the DIP joint will remain mechanically uncoupled from the PIP joint but will still be included to guide the distal phalanx and support the overall mechanism [3], [4].
The MCP four-bar linkage will include a base link fixed to the hand to provide a stable anchor and will receive an independent torque input to actuate the proximal phalanx. Its geometry must allow approximately 90 degrees of counterclockwise flexion, with possible clockwise motion, while avoiding toggle positions. The PIP four-bar linkage will receive a separate torque input to actuate the middle phalanx and will share a joint with the proximal phalanx while remaining mechanically independent of MCP actuation [5]. Its geometry must also avoid collisions with the MCP linkage throughout its motion range. The DIP four-bar linkage will remain passive, guiding the distal phalanx while allowing approximately 90 degrees of motion and enabling conformity to grasped objects without interfering with the other linkages.
Across all linkages, link ratios must prevent near-collinear configurations to avoid mechanical lockup and ensure smooth motion. Proper geometry will also enable effective force transmission and mechanical advantage, allowing input torques to generate sufficient gripping force while minimizing required actuation effort.
Figure 3: Dorsal vs Ventral Side of Hand and Arm
4. Proposed Scope of Work
During the semester, the project will focus on the design, analysis, and prototyping a functional finger exoskeleton mechanism. The linkage components will be manufactured using 3D printing, while bearings, motors, cables or ropes for torque transmission, and electronic components for motor control will be purchased. The intended outcome for the final project is a fully functional single-finger exoskeleton capable of independently actuating the MCP and PIP joints and producing a perceptible gripping force sufficient to provide noticeable resistance when closing around an object.
Prior to fabrication, several analyses will be conducted. Positional analysis will determine appropriate link lengths and joint locations to achieve the required ranges of motion while preventing collisions. Toggle-point and singularity analyses will ensure that no linkage approaches near-collinear configurations that could cause instability or mechanical lockup. Mechanical advantage and force transfer analyses will evaluate how actuator torques translate into joint torques and optimize geometry for efficient grip force generation. Velocity and acceleration analyses will also be performed to ensure safe operation under dynamic motion conditions.
Additional steps required for full realization beyond the course scope may include force sensing to regulate grip forces, ergonomic adjustments for different hand sizes, material selection and stress analysis to ensure structural integrity, and integration with a user interface or adaptive control system capable of implementing assist-as-needed torque profiles.
5. Preliminary Design Concepts
Following project approval, the final proposal will include initial design concepts and kinematic considerations. The MCP four-bar linkage will provide one degree of freedom and is not required to satisfy Grashof conditions, with a target of approximately 90 degrees of counterclockwise motion and limited clockwise motion. Preliminary analysis suggests that a standard four-bar configuration may restrict motion range, and an inverted slider four-bar mechanism described in prior research may be explored to achieve the desired motion [6].
The PIP four-bar linkage will also provide one degree of freedom and aim to achieve approximately 90 degrees of counterclockwise flexion. Similarly, the DIP four-bar linkage will provide one degree of freedom with approximately 90 degrees of flexion and limited hyperextension capability while remaining passive. Initial design documentation will include kinematic diagram drafts and evaluations using tools such as the Gruebler equation and Grashof criteria. These preliminary designs are expected to evolve throughout the project as analysis, prototyping, and testing refine the final mechanism prior to demonstration and reporting.
Figure 4: Inverted Slider-Crank 4-Bar Mechanism
Figure 5: MCP 4-Bar Linkage - Rest Configuration
Figure 6: MCP 4-Bar Linkage - Flexion Configuration
Figure 7: PIP 4-Bar Linkage - Rest Configuration
Figure 8: PIP 4-Bar Linkage - Flexion Configuration
Figure 9: DIP 4-Bar Linkage - Rest Configuration
Figure 10: DIP 4-Bar Linkage - Hyper-Extension Configuration
Figure 11: DIP 4-Bar Linkage - Flexion Configuration
6. References
[1] P. Braido and X. Zhang, “Quantitative analysis of finger motion coordination in hand manipulative and gestic acts,” Human Movement Science, vol. 22, no. 6, pp. 661–678, 2004.
[2] M. Deng, “Design and simulation of an underactuated finger rehabilitation exoskeleton based on bionic kinematics,” Highlights in Science, Engineering and Technology, vol. 160, pp. 756–757, 2025.
[3] T. Dickmann et al., “An adaptive mechatronic exoskeleton for force-controlled finger rehabilitation,” Frontiers in Robotics and AI, vol. 8, 2021.
[4] E. T. Wolbrecht, D. J. Reinkensmeyer, and A. Perez-Gracia, “Single degree-of-freedom exoskeleton mechanism design for finger rehabilitation,” IEEE International Conference on Rehabilitation Robotics, pp. 1–6, 2011.
[5] I. Jo and J. Bae, “Design and control of a wearable hand exoskeleton with force-controllable and compact actuator modules,” IEEE International Conference on Robotics and Automation (ICRA), pp. 5596–5601, 2015.
[6] Y. Yun, P. Agarwal, J. Fox, K. E. Madden, and A. D. Deshpande, “Accurate torque control of finger joints with UT hand exoskeleton through Bowden cable SEA,” IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), pp. 4011–4016, 2016.