Meeting Abstract
Undulatory motion of the torso is a salient feature of locomotion in many vertebrate taxa, particularly in fishes and reptiles. Although important insights into the mechanics of swimming were uncovered in numerous studies of body kinematics and muscle activity patterns, it has been challenging to investigate how the extent of lateral muscle activation affects propulsive performance due to the difficulties associated with modulation of in vivo muscle activation in freely-swimming fishes. To gain insight into co-contraction and body stiffness control we previously built a compliant physical model with soft bending actuators that allows for actively-controlled shape changes on the body. A parameter sweep of cyclic undulation frequencies and co-contraction phasing was performed and self-propelled speed measured. Measurement of thrust as a function of co-contraction phasing between the right and left sides in the undulatory swimming of our model revealed that antagonistic co-activation for a small fraction of the cycle period can increase thrust. Expanding upon this, we sought to determine the extent to which bilateral left-right co-activation has the capacity to alter the stiffness of the body, we carried out experiments to measure the body stiffness directly. When soft actuators were co-activated we measured an increase in stiffness from 18 to 29 N/m for pressure values of 0, 15, 20, 30, and 40 kPa. Moreover, we integrated hyperelastic soft sensors for estimation of body curvature. The soft sensors were mounted laterally along the soft pneumatic actuator to close the loop. Sensors contained microchannels filled with liquid metal eutectic Gallium Indium. This marked the first time an eGaIn-based soft sensor was tested under water. Despite the hydrodynamic pressure, it allowed for measurement of strain changes of body curvature. During bending of the soft actuator the fin curvature increases. The associated length changes correlated with changes in electrical resistance in the liquid metal within microchannels. Resistance increased proportionally with bending in the range of actuator pressurization from 0.13 to 0.66 bar, thus enabling fin displacement amplitude control. Sensory feedback will allow for applying the necessary pressure correction to remain at the desired body-caudal curvature at a range of water flow speeds. This biorobotic platform provides a physical model for testing hypotheses on how swimming performance can be affected by modulation of torso stiffness.
