  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.
