Traditional robotics applications such as CNC and welding have relied on the idea that robots should be extremely rigid, with very precise mechanics and encoders. This leads to robots that are very bad at interacting with the real world. Animals, which are very good at interacting with the real world, are full of wiggly, springy, imprecise actuators. A new paper (PDF format) from the CMU Robotics Institute looks at the benefits of adopting this principle in robot leg design. Using compliant, spring-like mechanisms, the robot can recycle energy and exert higher mechanical power when walking or running. The researchers examine a range of compliance using a variable stiffness leg, called the Electric Cable Differential (ECD) Leg.