FootTile: a Rugged Foot Sensor for Force and Center of Pressure Sensing in Soft Terrain

Abstract: In this paper we present FootTile, a foot sensor for reaction force and center of pressure sensing in challenging terrain. We compare our sensor design to standard biomechanical devices, force plates and pressure plates. We show that FootTile can accurately estimate force and pressure distribution during legged locomotion. FootTile weighs 0.9g, has a sampling rate of 330Hz, a footprint of 10 by 10mm and can easily be adapted in sensor range to the required load case. In three experiments we validate: first the performance of the individual sensor, second an array of FootTiles for center of pressure sensing and third the ground reaction force estimation during locomotion in granular substrate. We then go on to show the accurate sensing capabilities of the waterproof sensor in liquid mud, as a showcase for real world rough terrain use.

• The paper presents FootTile, a novel, rugged, lightweight foot sensor for measuring force and center of pressure in challenging soft terrains where traditional sensors are ineffective.
• Through experiments, FootTile demonstrated accurate center of pressure estimation within 4mm on solid surfaces and ground reaction force estimation within 1N deviation in granular soil.
• FootTile offers advantages over traditional sensors due to its low cost, portability, scalability, and ruggedness, making it suitable for biomechanics and robotics applications in diverse environments.

Overview of "FootTile: a Rugged Foot Sensor for Force and Center of Pressure Sensing in Soft Terrain"

The paper presents FootTile, a novel foot sensor designed to measure reaction force and center of pressure (COP) in challenging terrains. This sensor is particularly significant for its applicability in environments where traditional biomechanical devices, such as force and pressure plates, fall short due to their bulkiness, mobility restrictions, and limited data collection capability over soft and granular substrates.

Sensor Design and Features

FootTile is a small, lightweight device weighing only 0.9 grams with a footprint of 10 x 10 mm and a sampling rate of 330 Hz. The sensor is constructed around a barometric pressure sensor housed in a polyurethane dome, which incorporates an internal air bubble. This configuration allows for flexible adaptation to different load scenarios by modifying the sensor range according to the dome's material stiffness and size. The sensor design highlights cost-effectiveness, with production costs under 10 euros per unit.

Experimental Validation

The research outlines three experiments to validate FootTile's effectiveness:

1. Single Sensor Performance: The sensor was calibrated using a three-axis force sensor to establish a relationship between the sensor output and applied force. The resulting sensor displays a nonlinear output due to the dome's deformation, which is corrected using a third-order polynomial.
2. FootTile Array for Center of Pressure: By implementing a linear array of FootTiles, the authors assessed the COP in a robotic setup on solid surfaces, comparing results against a pressure plate. The deviation in COP estimation was minimal, within 4 mm of the ground truth, demonstrating the array's precision.
3. Ground Reaction Force Estimation in Granular Soil: The sensor's robustness was tested in granular media using poppy seeds and liquid mud. The FootTile array successfully estimated ground reaction forces with less than 1 N deviation from the readings of a traditional force plate.

Implications and Future Directions

FootTile's versatility extends its application to biomechanics and robotics, particularly soft and harsh terrain locomotion studies. Unlike traditional sensors, FootTile is not only portable and less expensive but also scalable and customizable for various biomechanical applications. The rugged design is highlighted, as the sensor maintains functionality even when immersed in and cleaned with water, validating its suitability for real-world applications.

The paper speculates on future enhancements in response frequency and miniaturization, leveraging advancements in MEMS pressure sensor technologies. Potential future research may involve deploying FootTile on robotic and animal feet to study pressure distribution and force dynamics in diverse terrains.

Conclusion

The development of FootTile represents an engineering advancement in sensor technology for dynamic locomotion applications. By enabling precise force and pressure measurements in environments unfit for standard instruments, FootTile expands the analytical toolkit available for the study of biomechanics in natural and complex terrains.