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

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.

Figures (9)

• Figure 1: Principle of the torque measurement, showing the robot and the counterweight.
• Figure 2: Diagram of the principle of operation of the flexure-based force/torque sensor. The flexures are positioned such that the roll and pitch axes of the device intersect, and intersect with the approximate center of mass of the FIR. The addition of a damping rod, below, whose end is immersed in glycerin, provides damping to eliminate oscillations.
• Figure 3: Image of the flexured-gimbal device with a FIR attached. Not shown in the figure is the glycerin petri dish below. Readout is accomplished in this case using a camera-based motion capture system.
• Figure 4: Flexured-gimbal device sensitivity measurements in the roll and pitch axes, with calculated trend lines.
• Figure 5: (a). Mapping of the pitch voltage offsets in the control signal to the resulting pitch torque measured by the device, with a color map to show the strength of the roll voltages at each data point. (b). Mapping of the roll and pitch voltage offsets in the control signal to the resulting pitch torque measured by the device. Error from the mapping trendline is shown via the colormap at the measurement points. Pitch torque is not significantly impacted by changes in roll control voltage.