PEATTIE, A M; GARCIA, M S; KUO, A D; WANG, PC; FULL, R J: DAMPING ESTIMATES IN INSECT LEGS INDICATE A ROLE IN CONTROL

Experiments and theoretical models suggest that the remarkable stability observed in the death-head cockroach (Blaberus discoidalis) can reside in the preflexive dynamics of the musculo-skeletal system. To determine one parameter often implicated in contributing to stability, we measured the damping of the femur-tibia joint. We perturbed the free tibia and recorded its motion with a high-speed video camera until it returned to an equlibrium position. Subsequently, weights were attached to the tibia to simulate the leg in stance phase and the experiment repeated. When interpreted as linear dampers, legs were overdamped during the �swing� phase (unloaded tibia), but underdamped during the �stance� phase (loaded tibia). Removal of muscle tissue lowered the damping ratio by about 30% for the weighted case and 10% for the unweighted case. The majority of the damping at the femur-tibia joint was due to the exoskeleton. Large, passive damping has implications for control. Perturbations to limbs can be dissipated by passive limb dynamics requiring less neural feedback. Muscle activation could be regarded more as a simple position or velocity command rather than a force command. A self-stabilizing limb favors the use of simple feedforward control strategies for locomotion. In an effort to determine if the large damping was unique to arthropods or a general phenomenon, we characterized the scaling of damping. Scaling arguments imply that as the size of a limb decreases, relative friction and viscosity (damping ratio) increase. We predict that damping should play an increasingly dominant role in the dynamics of small animals. Supported by DARPA/ONR N00014-98-1-0747.