Amplitude modulation enhances obstacle negotiation for sidewinders

Sidewinding snakes cyclically lift portions of their body as they move. This gait can be modeled as two coupled head-to-tail traveling waves: a horizontal wave of body undulation and a vertical wave controlling substrate contact. The changing contact pattern introduces a lateral component to movement that creates a large projection of the body onto the direction of motion and makes obstacle negotiation challenging. Biological experiments revealed that the sidewinder moves through a row rigidly-anchored vertical posts by squeezing parts of its body, which we hypothesized was achieved by increasing the amplitude of the horizontal wave. To test this, we created a sidewinding robot from a chain of 14 servo motors with alternating direction of actuation. Horizontally and vertically oriented sets of motors were each driven sinusoidally in time and position along the body. When in contact with a rigid post (detected via capacitive touch sensors), the robot either increased its horizontal amplitude for one full cycle or continued with its nominal waveform. The robot successfully moved beyond the post in 80% of the trials with increased amplitude, compared to never succeeding when maintaining the nominal waveform.