Wireless Center of Pressure Feedback System for Humanoid Robot Balance Control using ESP32-C3
Abstract
Maintaining stability during the single-support phase is a fundamental challenge in humanoid robotics, particularly in dance robots that require complex maneuvers and high mechanical freedom. Traditional tethered sensor configurations often restrict joint movement and introduce mechanical noises. This study proposes a wireless embedded balance system designed to maintain stability on uneven surfaces. The system utilizes a custom-designed foot unit integrated with four load cells and an ESP32-C3 microcontroller to estimate the Center of Pressure (CoP) in real time. The CoP data were transmitted wirelessly to the main controller to minimize the wiring complexity of the 29-DoF VI-ROSE humanoid robot. A PID control strategy is implemented to adjust the torso, hip, and ankle roll joints based on CoP feedback. Experimental characterization demonstrated high sensor precision with an average measurement error of 14.8 g. Furthermore, the proposed control system achieved a 100% success rate in maintaining balance during single-leg lifting tasks at a 3-degree inclination with optimized PID parameters (Kp=0.10, Kd=0.005). These results validate the efficacy of wireless CoP feedback in enhancing the postural stability of humanoid robots, without compromising their mechanical flexibility.
Summary
This paper addresses the challenge of maintaining balance in humanoid robots, particularly dance robots, which require complex movements and high mechanical freedom. The authors propose a wireless embedded balance system that utilizes a custom-designed foot unit integrating four load cells and an ESP32-C3 microcontroller to estimate the Center of Pressure (CoP) in real time. This wireless CoP data is then transmitted to the main controller, minimizing wiring complexity in their 29-DoF VI-ROSE humanoid robot. A PID control strategy is implemented to adjust the torso, hip, and ankle roll joints based on the CoP feedback. The system aims to improve postural stability, especially during single-support phases and on uneven surfaces. The methodology involves both hardware and software design. The hardware consists of the foot unit with load cells, the ESP32-C3 for data acquisition and wireless transmission, and the VI-ROSE humanoid robot. The software comprises a GUI for load cell calibration, CoP calculation algorithms, and a PID control system. The system was tested by lifting the robot's foot at a 3-degree inclination, both with and without PID control, and by analyzing the impact of different PID parameter values on balance. The key findings include high sensor precision with an average measurement error of 14.8 g. Furthermore, with optimized PID parameters (Kp=0.10, Kd=0.005), the proposed control system achieved a 100% success rate in maintaining balance during single-leg lifting tasks at a 3-degree inclination. This research is significant because it demonstrates the effectiveness of wireless CoP feedback for enhancing humanoid robot stability while preserving mechanical flexibility, a crucial aspect for dance robots and robots operating in unstructured environments.
Key Insights
- •The system employs a wireless communication architecture using ESP32-C3 to transmit CoP data, reducing wiring complexity and improving the robot's range of motion.
- •The custom-designed foot unit integrates four load cells to provide real-time CoP estimation, enabling the robot to react quickly to balance disturbances.
- •Experimental characterization revealed a sensor precision with an average measurement error of 14.8 g, which is adequate for balance control. This measurement error was derived from tests using reference masses.
- •The PID control strategy successfully maintained balance during single-leg lifting tasks at a 3-degree inclination with optimized parameters (Kp=0.10, Kd=0.005), achieving a 100% success rate.
- •High values of the derivative gain (Kd) in the PID controller can amplify sensor noise, leading to instability and causing the robot to fall.
- •Empirically determined gain coefficients for torso, hip, and ankle roll joints (0.8, 1.0, and 0.4, respectively) optimize the balance recovery strategy by prioritizing hip movements for CoM shifts.
- •While increasing Kp to 0.25 maintained a 100% success rate, the RMS error was higher than that with the best Kp = 0.10, indicating that higher proportional gains cause overshoot and low-frequency oscillations around the setpoint.
Practical Implications
- •The wireless balance system can be directly applied to humanoid robots designed for dance, entertainment, or other applications requiring dynamic movements and stability on uneven surfaces.
- •Researchers and engineers working on humanoid robot control can use the proposed system as a blueprint for developing similar wireless CoP feedback systems.
- •The study provides insights into the selection and tuning of PID parameters for balance control in humanoid robots, specifically highlighting the importance of balancing responsiveness (Kp) with noise amplification (Kd).
- •The findings suggest future research directions, such as incorporating Zero-Moment Point (ZMP) calculations based on CoP data to further improve balance accuracy and robustness.
- •Replacing the servos with more robust models, such as MX-64T, for the motors that control the robot’s roll position is recommended.