Keeping Robots on Their Feet: A Wireless Balance System

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Author: Denis Avetisyan

Researchers have developed a cost-effective wireless system that allows a 29-DoF humanoid robot to dynamically maintain balance during complex movements.

During walking in place, the center of pressure dynamically shifts between the robot’s feet - remaining centered when both feet are grounded, and moving to the single supporting foot upon lifting the other - demonstrating a stable gait achieved through continuous balance adjustment.
During walking in place, the center of pressure dynamically shifts between the robot’s feet – remaining centered when both feet are grounded, and moving to the single supporting foot upon lifting the other – demonstrating a stable gait achieved through continuous balance adjustment.

This paper details the design and implementation of a wireless center of pressure feedback system using load cells, PID control, and an ESP32-C3 microcontroller for enhanced humanoid robot stability.

Maintaining dynamic stability remains a central challenge in humanoid robotics, particularly for high-degrees-of-freedom platforms intended for complex locomotion. This paper details the development and experimental validation of a ‘Wireless Center of Pressure Feedback System for Humanoid Robot Balance Control using ESP32-C3’, designed to address this limitation through real-time CoP estimation and feedback. Results demonstrate a 100% success rate in maintaining balance during single-leg lifting on inclined surfaces using optimized PID control, achieved via a low-latency wireless system integrating custom foot units and an ESP32-C3 microcontroller. Could this approach unlock more fluid and robust movement capabilities in future generations of humanoid robots?

The Challenge of Dynamic Equilibrium

Humanoid robots face a significant engineering challenge in replicating the seemingly effortless balance humans achieve. Unlike statically stable robots with wide bases, these machines operate on two legs, demanding constant adjustments to maintain equilibrium, particularly when executing dynamic movements like walking, running, or recovering from external disturbances. This instability arises because the robot’s center of gravity must be continuously positioned over its support polygon – the area defined by its feet – a task complicated by the inertia of moving limbs and the unpredictable nature of real-world environments. Successfully navigating this requires not just strength and precision, but also sophisticated control algorithms capable of predicting and reacting to shifts in weight distribution with incredible speed and accuracy, pushing the boundaries of robotics and control theory.

Humanoid robots often struggle with balance because early approaches heavily relied on inertial measurement units (IMUs) – sensors that detect acceleration and angular velocity. While useful, IMUs provide data relative to the robot itself, lacking crucial information about external forces at play. This creates a fundamental limitation: a robot might accurately know it is tilting, but not why – is it due to an uneven surface, an external push, or a shifting center of gravity? Without understanding these external forces, particularly those arising from ground contact, the robot’s response is slow and imprecise, leading to stumbles or falls. Consequently, systems dependent solely on IMUs struggle to anticipate and react to disturbances, hindering the development of truly agile and robust humanoid locomotion.

For humanoid robots to navigate the physical world with agility, a nuanced comprehension of ground reaction forces is essential. These forces, representing the interaction at the foot-ground interface, provide critical information beyond what inertial measurement units alone can offer. Accurate measurement and interpretation of these forces – encompassing both magnitude and direction – allows a robot to preemptively adjust its center of mass, effectively countering disturbances and maintaining balance during dynamic movements like walking, running, or recovering from unexpected pushes. This understanding transcends simple static stability; it enables a robot to actively control its balance by modulating ground forces, much like humans do instinctively, and is a key element in achieving truly natural and robust locomotion.

The robot foot design integrates a multi-layered structure comprising a force sensor, a compliance layer, and a rigid base to enable stable and adaptable contact with uneven terrain.
The robot foot design integrates a multi-layered structure comprising a force sensor, a compliance layer, and a rigid base to enable stable and adaptable contact with uneven terrain.

Ground Force Measurement: A Foundation for Balanced Control

Direct measurement of the Center of Pressure (CoP) is achieved by integrating load cell sensors into the robot’s feet, providing data critical for balance control by quantifying weight distribution across the support polygon. Individual load cells exhibit measurement errors ranging from 0 to 19 grams, however, the combined error across all load cells within a single foot totals 0 to 50 grams. This error margin is a key consideration in the development of robust balance control algorithms, and necessitates careful calibration of the sensor array to minimize inaccuracies in CoP calculation and subsequent robot adjustments.

The Center of Pressure (CoP) data obtained from foot-mounted load cells is integrated into a Proportional-Integral-Derivative (PID) control loop to actively manage the robot’s balance. This loop continuously calculates the error between the desired and actual CoP position, generating correction signals that are sent to the robot’s servo motors. These signals adjust the servo positions, shifting the robot’s weight distribution and counteracting any deviations from the desired balance point. The PID parameters – proportional, integral, and derivative gains – are tuned to optimize the system’s responsiveness and stability, allowing for dynamic balance adjustments in response to external disturbances or changes in the robot’s center of gravity.

The ESP32-C3 microcontroller is integral to the robot’s balance control system by managing data acquisition from the load cell sensors integrated into the feet. This microcontroller efficiently handles the analog signals produced by the load cells, converting them into digital data suitable for processing. Its processing capabilities allow for rapid sampling rates, critical for accurately tracking the Center of Pressure (CoP) and responding to dynamic shifts in weight distribution. Furthermore, the ESP32-C3’s integrated communication interfaces facilitate seamless data transfer to the PID control loop, minimizing latency and enabling real-time balance adjustments. The microcontroller’s low power consumption is also a benefit for sustained operation during balance maintenance tasks.

A dedicated application is used to configure and calibrate the load cell, enabling accurate force measurements.
A dedicated application is used to configure and calibrate the load cell, enabling accurate force measurements.

VI-ROSE ITS: A Platform for Validating Advanced Balance Control

The VI-ROSE ITS robot is a 29-degree-of-freedom (DoF) humanoid platform specifically designed for the iterative development and validation of advanced balance control algorithms. This physical embodiment allows for testing in a real-world setting, moving beyond simulation. The robot’s multi-DoF configuration provides a complex kinematic structure representative of human movement, enabling the evaluation of control strategies under realistic conditions. Utilizing a full-body humanoid allows for assessment of balance recovery from a variety of disturbances and perturbations, which is critical for the robustness of any balance control system intended for deployment on similar robotic platforms or potentially even prosthetic devices.

The VI-ROSE ITS robot utilizes Dynamixel motors, specifically the MX-28 and XL-320 servo models, to achieve controlled joint movements across its 29 degrees of freedom. These servos provide both positional accuracy and responsiveness, crucial for maintaining balance and executing dynamic motions. An ESP32 microcontroller serves as the central processing unit, managing communication with the servos and implementing the balance control algorithms. The ESP32 facilitates real-time control by processing sensor data and translating it into precise motor commands, enabling coordinated movements throughout the robot’s body.

Static motion testing of the VI-ROSE ITS humanoid robot confirmed a 100% success rate in maintaining balance during single-leg stance. This was achieved by lifting one foot while the robot was stationary and subjected to a 3-degree tilt. Optimal performance was consistently observed using Proportional-Integral-Derivative (PID) control parameters specifically tuned to Kp=0.1 and Kd=0.005. These parameters were critical in ensuring stable recovery from the induced tilt during the foot-lifting maneuver.

The robot's mechanical design incorporates uniquely identified servos to facilitate precise control and system monitoring.
The robot’s mechanical design incorporates uniquely identified servos to facilitate precise control and system monitoring.

From the Lab to the Dance Floor: Demonstrating Real-World Capabilities

The VI-ROSE ITS robot was specifically engineered to contend in the demanding Indonesian Robot Dance Competition (KRSTI), serving as a dynamic proving ground for its innovative balance control system. This competition pushed the robot to execute complex movements and maintain stability under challenging conditions, effectively demonstrating the system’s ability to handle real-world dynamic scenarios. By participating in KRSTI, researchers aimed to showcase not just the robot’s performance, but also the potential of advanced control algorithms in enabling fluid and reliable locomotion for humanoid robots. The design prioritized responsiveness and adaptability, allowing VI-ROSE ITS to navigate the intricacies of the dance competition while highlighting the robustness of its balance control mechanisms.

The VI-ROSE ITS robot’s ability to execute complex dance maneuvers relies heavily on seamless communication and precise state estimation. Wireless connectivity between the robot’s various components-motors, sensors, and the central processing unit-enables real-time data exchange, crucial for maintaining balance during dynamic movements. Complementing this is the integration of a Kalman Filter, an algorithm that optimally estimates the robot’s state – its position, velocity, and orientation – by fusing data from multiple sensors and accounting for inherent noise. This filtered data provides a more accurate and reliable representation of the robot’s condition than relying on any single sensor, allowing for quicker and more effective adjustments to maintain stability and execute precise dance steps, even when faced with external disturbances or imperfect sensor readings.

The VI-ROSE ITS robot’s balance control system demonstrated remarkable precision, achieving a root mean square error (RMS) of just 0.7077 with a derivative gain (Kd) of 0.010 during testing. This level of stability, proven in the demanding environment of the Indonesian Robot Dance Competition (KRSTI), suggests considerable potential beyond the dance floor. Researchers envision applications ranging from sophisticated assistive devices for individuals with mobility impairments to robust humanoid robots capable of navigating complex disaster zones for search and rescue operations, highlighting the broad applicability of this advanced balance technology.

The presented work emphasizes a holistic approach to humanoid robot balance, mirroring the interconnectedness of complex systems. Every new dependency, such as the integration of load cells and the ESP32-C3 microcontroller, introduces hidden costs in terms of calibration and data processing – a point elegantly captured by John von Neumann: “There is no telling what the future holds, but we should always be prepared for the unexpected.” This sentiment resonates deeply with the challenges of dynamic balance control, where unforeseen disturbances and the robot’s 29 degrees of freedom demand a robust and adaptable system. The system’s architecture, prioritizing feedback loops, is not merely a technical implementation, but an embodiment of structural integrity influencing behavioral outcomes.

Beyond Static Stability

The presented system, while demonstrating successful balance maintenance during discrete movements, merely addresses the surface of a far more complex problem. True dynamic stability isn’t achieved through increasingly refined PID loops, but through a fundamental rethinking of robotic structure. The reliance on reactive force control, however elegantly implemented, suggests a continuing adherence to designs that require constant correction. A more fruitful avenue lies in proactive systems – those whose physical architecture inherently resists destabilizing forces. The question isn’t simply ‘how do we prevent falling?’ but ‘how do we build a robot that cannot fall, except through catastrophic failure?’

Scalability, as always, is the crucial metric. This proof-of-concept, focused on a 29-DoF platform, must be considered within the larger ecosystem of robotic design. The current approach, while functional, presents limitations when applied to robots with significantly altered morphologies or operating in unpredictable environments. The challenge lies in developing algorithms and structural principles that are agnostic to specific degrees of freedom, adapting gracefully to a wider range of physical forms and external disturbances.

Future work should therefore prioritize exploration of biomimetic designs – not simply replicating muscle actuation, but understanding the underlying principles of skeletal structure and proprioceptive feedback that enable biological systems to maintain balance with such remarkable efficiency. The system is a component within a larger organism; simplification, not complication, will yield the most robust and scalable solutions.

Original article: https://arxiv.org/pdf/2512.21219.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

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2025-12-27 20:53