System Overview

Our team meticulously designed the mechanical, electrical, and software components of this project within just five months, ultimately developing a novel approach to human-robot devices that offers distinct advantages over existing systems. Unlike other haptic or robotic hand systems that struggle to accurately translate forces between the robot and the user—often causing rigid objects grasped by the robot to feel squishy to the operator—our design ensures precise force feedback. This precision is crucial when operators need to work with delicate equipment to manipulate small objects.
Traditional power transfer solutions fall short in this application: gear-train systems exhibit noticeable backlash, linkages are overly complex and hinder a low-profile design, and pneumatic or hydraulic systems cannot match the force exerted by the human hand. To overcome these challenges, we chose a cable-driven design powered by brushless DC motors positioned near the operator's palm. This approach eliminates system play while maintaining a slender and robust mechanical structure. The use of cables enabled us to focus on a high-gain controller, providing operators with an accurate representation of the robot’s interactions.
The system comprises distinct haptic and robotic components for both fingers and thumbs, each featuring two actuated degrees of freedom and a third passive degree of freedom. This configuration allows for natural and precise motion while ensuring the design remains feasible within the project’s timeframe.
I developed a modular electrical system for these robotic and haptic devices, creating three different PCBs to manage power, communication, and control for each subsystem. The 'power board' supplies and regulates power across four distinct domains, ensuring stable voltage with multiple user and system protections. The 'data board' supports real-time communication through a dual CAN bus architecture, controlled by a Teensy microcontroller and equipped with fault-tolerant transceivers for both intra-system and inter-system communication. High-resolution magnetic encoders deliver essential feedback for motion control, while integrated safety mechanisms, including emergency stops and system monitoring, protect both the operator and the hardware.

Mechanical System Overview

Robot Finger

The robot finger subsystem is engineered to closely replicate human finger curling motions, featuring two actively actuated joints that deliver smooth, natural articulation. Rather than relying on a traditional gear train, it uses a pulley-driven transmission with a 4:1 ratio at each joint, enhancing torque delivery while maintaining a compact profile. High-performance Maxon EC 45 flat motors supply the requisite torque density without adding bulk. To ensure stability under load, stainless steel cables are tensioned using simple screws and nuts, and magnetic encoders provide precise positional data for fine-tuned control. Structural components are fabricated through both water jet cutting and CNC milling for accuracy and durability. While performance and torque efficiency meet design goals, refining linkage interfaces and streamlining the assembly process remains an ongoing priority.

Robot Thumb

Similar to the robot finger, the robot thumb is tailored to replicate the dexterity and form factor of the human thumb, offering two actuated degrees of freedom to produce both 'waggle' (abduction/adduction: motion that moves the thumb away from the palm) and 'curl' (flexion/extension: curling or extending your thumb) motions. The 'waggle' joint operates at a 7.33:1 transmission ratio, driven by Maxon ECX Torque 22XL motors for steady, responsive motion. In contrast, the 'curl' movement uses a 7.94:1 ratio powered by Maxon EC 45 Flat motors, chosen for their compact design and strong torque-to-size ratio. Multiple cable wraps, tensioning blocks, and CNC-machined pulleys work together to maintain tension. High-precision machining, including turning and milling, ensures tight tolerances and secure mechanical coupling. Similar to the finger, there are still some issues regarding tensioning which would likely be resolved by refining the pulley system and having an easier method for tensioning.

Haptic Finger

The haptic finger subsystem recreates human finger dynamics, offering two actively controlled joints (MCP and PIP) plus an additional passive degree of freedom for a more natural range of motion. Stainless steel cables, routed through machined pulleys, maintain precise motion transfer. Cables are tensioned using vented screws and wing nuts, allowing convenient adjustments and steady performance over time. Similar to other designs Maxon ECX Torque 22XL motors drive the system and are mounted near the palm. The motors are placed in such a way that they do not interfere with the mounting of the haptic thumb.

Haptic Thumb

The haptic thumb subsystem attempts to emulate the range of motion for the human thumb with only 2 driven degrees of freedom. Similar to the robot thumb it offers 'waggle' and 'curl' motions enhanced by an additional passive degree of freedom. Cable tension is managed via a series of idler pulleys and vented fasteners to achieve smooth, consistent engagement. Maxon ECX motors drive the both the 'waggle' and 'curl' motions, and a rotating fitting allows your thumb to freely swivel within the mechanism. A flexible TPU thimble interface accommodates various thumb sizes while preserving a secure, ergonomic fit. CNC-machined elements support a sturdy yet lightweight structure. Friction in the cable system occasionally emerges during operation, but we believe this issue could be fixed with higher tolerance pulleys, and a less complex way to tension the cables.

Mechanical System: Haptic Thumb

Haptic Thumb Overview

The goal of the haptic thumb system (pictured above) was to create a haptic interface for the robotic thumb with two actuated degrees of freedom while providing a comfortable workspace for the user’s thumb. Given this constraint, we broke down the thumb motion into two principal functional motions that we defined ourselves: CMC planar rotation ‘waggling’ of the thumb and MCP/IP 'curling'. These two degrees of freedom make up two subsystems of our design as well as a third subsystem consisting of the thimble holster interface to the user’s thumb. The MCP/IP subsystem and the thimble subsystem each contain an unactuated passive degrees of freedom to provide greater comfort to the user during our curl motion. The mechanical system block diagram above depicts the subsystems and their major components and the decomposed motions are also depicted in the figure below

Cable Routing

The haptic thumb system makes use of three vented screws to tension the cables: one in the CMC subsystem and two in the MCP/IP subsystem. The configuration shown below creates a 1.4:1 coupling
The CMC cable is wrapped around the CMC Motor Pulley 3 times and is terminated through a wire-lockable screw at one end of the system. The other side of the cable is wired through the CMC plate (illustrated by the dashed pink line below) and is then wired through a vented screw and terminated with a copper crimp. The cable is tensioned off of the MC plate by unscrewing the vented screw in the counterclockwise direction against a wingnut.
The MCP/IP cable is wrapped around the MCP/IP Motor Pulley 3 times, traversing through a drilled hole in the center of the pulley, and is wired across the CMC system through the use of 3 idler pulleys/shafts. The CMC idler pulley sits on the CMC shaft and allows the motor pulley to be remotized to the same location as the CMC motor pulley. This pulley has 2 grooves that allow for 1 wrap in each direction. The horizontal idler shaft is located between the CMC idler and the vertical idler shaft and guides one side of the cable to the location of the MCP/IP pulley, allowing the cable to approach the pulley perpendicularly to its surface. The vertical idler shaft is located between the horizontal idler shaft and the MCP/IP pulley, and guides one side of the cable towards the MCP/IP surface in a way that allows it to wrap tangentially in both directions. The cable is terminated at each end using copper crimps with a vented screw and wingnut that create tension in the cable by rotating in the counterclockwise direction off of a tensioning block and a hex nut underneath to lock the configuration.

CAD Model

The full CAD model of the haptic thumb system is shown below The motors and connection points between the different subsystems are outlined. The CMC Side Plates provide a mounting point for the haptic thumb system to connect to the rest of the haptic system. The MCP Side Plates are attached to the CMC Plate to connect the CMC and MCP/IP systems together. The Phalanx Proximal Plates connect to the compliant Phalanx Distal Plate to connect the Thimble system to the rest of the haptic system. The Inner and Outer Thimble connect the user to the haptic system. Different views of the subsystems are shown below to highlight the different parts and features.
All bearings within the system are designed to be push fit with both the plates and the shafts. All plates should be manufactured using CNC milling and reaming, and shafts/pulleys were turned on the manual lathe. All components were designed to be made out of metal, with the exception of the inner thimble, which is designed to be made out of 3D printed TPU.

CMC Subsystem

The CMC subsystem is depicted below, with joint limits set at 0° and 70° by a spacer separating the top and bottom plates. This subsystem utilizes a ~5:1 cable drive transmission ratio, determined by the radii of the CMC plate and CMC motor pulley.
The assembly process begins with the top plate sub-assembly, which includes the CMC top plate, ECX 22 XL motors, 2-piece collar shafts, and motor pulleys. These components are assembled together to ensure alignment and secure attachment. Next, the CMC plate is assembled with the bottom plate, incorporating the CMC shaft and idler pulley. This step may require a clamp or a second set of hands to maintain alignment. At this point, the cable routing for the CMC is performed. The CMC cable is wrapped around the CMC motor pulley three times, terminated through a wire-lockable screw, and routed through the CMC plate, vented screw, and copper crimp. Once the cable is properly tensioned, the spacers between the top and bottom plates can be added. Finally, the horizontal idler shaft is installed on the CMC plate, completing the mechanical assembly of the CMC subsystem.

MCP/IP Subsystem

The components of the MCP/IP subsystem are depicted below The active MCP joint is driven at a 3.5:1 transmission ratio due to the diameter of the MCP pulley and two sets of spacers contact the edge of the MCP side plates to create the mechanical joint limits providing 77 degrees of rotational motion. The MCP pulley is clearance fit on the MCP shaft and loctite was used to constrain the pulley rotation to the shaft rotation. As a result, the pulley as well as all of the proximal phalanx plates rotate together. This subsystem can be assembled separately from the CMC subsystem and attached at a later time, meaning that the CMC joint does not need to be re-tensioned if repairs or adjustments need to be made to the MCP/IP subsystem.
Assembly for this subsystem involves locating all of the components (spacers, bearings, and plates) on the MCP shaft with the exception of the phalanx proximal plates. At the same time, the vertical idler shaft needs to be located within the bearings in the MCP side plates. Once the inner MCP pulley joint has been assembled, the outer phalanx prox and phalanx prox L plates can be attached to the MCP shaft. The shaft and plates for the passive IP joint can be assembled before or after the assembly of the MCP.

Thimble

The thimble subsystem is designed to provide comfortable but secure attachment between the user’s thumb tip and the haptic system. In order to accommodate multiple thumb sizes, multiple diameters of the thumb tip were printed out of TPU, a flexible polymer that allows for a more comfortable experience. The inner thimble piece can be easily removed and swapped for a different size by unscrewing the bolt that clamps the inner thimble. Multiple iterations of the thimble design were created, including a version that uses an elastic band that wraps around the proximal phalanx of the user’s thumb to secure the thimble on the thumb. Further testing is needed to determine whether the flexibility of TPU is sufficient to prevent slippage or if a more robust thumb attachment strategy is required. Additionally, the system lacks the roll degree of freedom for the thumb tip, which results in a moment around the thumb that could be potentially uncomfortable. Designing and implementing a lightweight but secure roll degree of freedom should be a focus in future iterations.