A flexured-gimbal 3-axis force-torque sensor reveals minimal cross-axis coupling in an insect-sized flapping-wing robot

TL;DR

This paper introduces a novel flexured-gimbal 3-axis force-torque sensor for insect-sized flapping-wing robots, enabling precise measurement of torque and thrust with minimal cross-axis coupling, thereby improving understanding and control of such miniature flying devices.

Contribution

The paper presents the first sensor capable of simultaneously measuring two torque axes in a tiny flapping-wing robot, revealing minimal cross-axis coupling effects.

Findings

Cross-axis coupling errors are under 17.24% for torque axes.

Thrust deviation from commanded values is up to 5.78%.

Sensor validation confirmed accurate torque and thrust measurements.

Abstract

The mechanical complexity of flapping wings, their unsteady aerodynamic flow, and challenge of making measurements at the scale of a sub-gram flapping-wing flying insect robot (FIR) make its behavior hard to predict. Knowing the precise mapping from voltage input to torque output, however, can be used to improve their mechanical and flight controller design. To address this challenge, we created a sensitive force-torque sensor based on a flexured gimbal that only requires a standard motion capture system or accelerometer for readout. Our device precisely and accurately measures pitch and roll torques simultaneously, as well as thrust, on a tethered flapping-wing FIR in response to changing voltage input signals. With it, we were able to measure cross-axis coupling of both torque and thrust input commands on a 180 mg FIR, the UW Robofly. We validated these measurements using free-flight experiments. Our results showed that roll and pitch have maximum cross-axis coupling errors of 8.58% and 17.24%, respectively, relative to the range of torque that is possible. Similarly, varying the pitch and roll commands resulted in up to a 5.78% deviation from the commanded thrust, across the entire commanded torque range. Our system, the first to measure two torque axes simultaneously, shows that torque commands have a negligible cross-axis coupling on both torque and thrust.