Hardware and Control Co-Design of Compliant Actuators for Energy-Efficient Humanoid Robots

This project will advance the state of the art of humanoid robotics, towards a vision of collaborative partners performing physically demanding tasks alongside humans. A series of gears connects the arms and legs of the robot to the electric motors that act as “muscles.” Conventionally the effect of these gears is to multiply the torque applied by the motor to the limbs by more than one hundred times, which is needed for standard low-torque, high-speed electric motors to meet the demands of robot tasks.

However, this amplification creates a safety hazard, because the effect of external torques on the motor is reduced by the same amount, and therefore the motor does not “feel” contact, such as with a human coworker. The development of “quasi-direct-drive” motors requiring little or no torque amplification is a major advance in human-safe robots, but these new motors use power for static operations, like holding up a heavy weight, leading to low efficiency and reduced operating times.

This project complements state-of-the-art electric motors with innovative spring mechanisms to generate steady or periodic actions with a minimum of added energy. These novel actuators will provide humanoid robots with new capabilities to safely work alongside humans, move naturally, react quickly to their environments, and operate for longer periods.

These features will be experimentally demonstrated using a one-legged hopping robot. Potential societal benefits include better collaborative robots for manufacturing and logistics, and longer lasting and higher performing prostheses and orthoses. To inspire the next generation of roboticists, the project will offer hands-on STEM experience to 7th and 8th-grade students through the University of Michigan WISE GISE summer camp and will host lab tours to students from the Detroit area.

This project will establish a systematic framework for the design and control of humanoid robots with an energy-efficient compliant actuator, which combines a Quasi-Direct-Drive (QDD) motor and Unidirectional Parallel Spring (UPS). This unique combination allows robots to generate high torque with significantly less energy consumption and heat dissipation, without inhibiting the leg swing motions.

Integrating QDD and UPS results in complex interaction between robot hardware and control, since the parallel spring introduces a coupling effect between neighboring robot links. The holistic design framework developed by this project will simultaneously optimize the mechanical and control parameters so that the controller can utilize rather than counteract the robot’s natural dynamics.

By addressing key questions about scalable optimization and bridging the gap between simulation and the real world, the investigators aim to pave the way for humanoid robots with high power autonomy and agility, facilitating prolonged operation in remote areas. This project will answer the following research questions: (1) how to quantify the performance of humanoid robots with QDD motors and UPS, (2) how to simultaneously optimize hardware and control parameters for improved energy efficiency and agility, and (3) how to bridge the simulation-to-reality gap in humanoid control to achieve agile and efficient locomotion.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.