Compliant Stepper Motor PCB
Custom Robotic Arm Architecture
The Objective
The standard approach to compliant robotics relies on BLDC motors or bulky mechanical Series Elastic Actuators (SEAs). For my thesis in the Tactile Manipulation Lab at UCSC, I am researching a different approach: achieving pure software-defined compliance using standard NEMA-17 stepper motors. Steppers offer a superior torque-to-size ratio and are significantly cheaper than BLDC motors, but documentation on closed-loop compliant stepper control is sparse. The end goal is to build a two-link manipulator and a universal 'boilerplate' PCB that can convert standard stepper motors into compliant actuators. My approach is heavily based on the research presented in the following paper:
IEEE Paper: Torque-Sensorless ControlArchitecture & Control
To achieve compliance without physical torque sensors or mechanical springs, the system relies entirely on embedded control theory. The architecture reads an absolute magnetic encoder to track the physical angle of the rotor and compares it against the electrical angle. By applying the Park transform and reading dedicated current sensors, we calculate the exact phase currents. The firmware executes a high-frequency 20 kHz control loop to actively regulate this current through discrete H-bridge drivers. Controlling this current allows us to estimate and artificially cap the torque output, meaning the joint will dynamically back-drive and behave compliantly when external forces are applied.
More Coming soon
I will be adding more documentation soon, this project is still in development...