Mechatronic design of actuation for robots with human-like strength and agility

MSc assignment

The need for more dynamic robots has promoted the development of actuators using high-torque electrical motors combined with lower gear ratios - quasi-direct-drive or proprioceptive actuation. The result has been actuators with reduced reflected inertia, higher speeds, and generally reduced output impedance with better interaction capabilities.

However, this approach also poses significant challenges. The lower gearing ratio results in larger currents, and associated heating. Consequently, torque capacity and density suffer, which is why most applications of quasi-direct-drive actuation have been limited to smaller robots of typically 10-30 kg and a clear gap exists towards application on larger robots with human-like strength.

Using series-elastic actuation allows to alleviate some of these drawbacks. In series-elastic actuators, an elastic element is placed between the motor+gearbox and actuator output. This allows for regulation of the actuator's output force and to increase gear ratio, giving it some of the same benefits as quasi-direct-drive while increasing torque capacity and density. However, design and mechanical complexity increases, control bandwidth is limited, and output impedance tends to be higher than for quasi-direct-drive actuators.

Despite the limitations, many small quadrupedal robots with excellent physical capabilities have been introduced using both approaches, e.g. Spot and Anymal. Compared to traditional actuation, this fundamental change in robot actuation design has produced a step-change in their capabilities. Simply put: You cannot control your way out of a bad robot!

And yet, current approaches do not scale well to robots of human-like size and strength, and therefore their applications are inherently limited. New mechatronic approaches are needed to create robots that are capable of such tasks.

This assignment focuses on addressing this technological gap in actuation, by developing new concepts that use e.g. very large radius electric motors, different types of gearing, redundant actuation, elasticity, and liquid cooling. The goal is to develop one such concept, ideally supported by an experimental prototype. Depending on the background and interest of the student the exact direction may be finetuned.