After our initial design was finalized in CAD and verified to work with a minimum viable product, we moved on to implementing the motor. We redesigned our spool to allow for two fingers to connect simultaneously, controlled by the same motor. We also designed our system to work for an alternative motor setup controlled by two co-linear motors.
Next, through testing, we iterated on the design of the finger to improve key characteristics such as the ability of the finger to spring back to a neutral 90°, the flexibility of the finger to be controlled by the torque of the motors we ordered, and the cable slot size to ensure accuracy while reducing friction/binding. Straps were sewn in a loop at the end of each finger to ensure a clean and stable connection.
After critical connections were established we designed and fabricated the housing to improve the stability and aesthetics of our designs. We took extra care to manage the wiring to improve the cleanliness and consistency of the electronic portion of our project. The design of the handle allows for easy access to intuitively control the fingers without any complication.
The actuation system was designed to provide controlled tendon displacement using a high-torque servo motor. The primary design uses a 12V 30kg servo motor because it provides sufficient torque to overcome flexure stiffness and tendon friction while maintaining compact packaging. A NEMA 17 stacked stepper motor with co-linear double-shafts was also evaluated as an alternative option for more precise displacement control.
The servo motor is mounted to the PLA fixture and connected to the custom pulley system that winds the nylon tendon. As the motor rotates, the pulley shortens the effective tendon length, causing the compliant joints to bend sequentially. This converts motor rotation into controlled finger curling.
An ESP32 microcontroller was selected as the primary controller because it provides sufficient processing capability while also allows wireless communication through WiFi and Bluetooth for future remote operation. For servo control, a dedicated servo driver board is used to provide stable signal general and power isolation. For the stepper motor option, TMC2209 motor driver boards and breakout boards were selected to convert low-power control signals from the ESP32 into the required motor drive currents.
A 12V 5A regulated power supply provides stable power for the motors and electronics. M3 bolts and nuts are used for structural mounting and securing the actuator assembly. The electronics layout was selected to prioritize reliability, modularity, and ease of future expansion into the full two-finger pinch gripper system.
Potentiometer Variable Control Code
Parameters & Physical Modeling
Stiffness Calculation
Instead of modeling the stiffness gradient across the flexible fingers, we simplified it into a pseudo-rigid-body model. We treat the thin TPU cross-section as a beam based on the Euler-Bernoulli theory. By calculating the Second Moment of Area and applying the Young’s Modulus of the Shore 95A TPU, we can derive a lumped orsional stiffness.
Finding Static Equilibrium
Bisection Method
Kinematics