Design and Experimental Study of BLDC Motor Immersion Cooling for Legged Robots

Abstract

Legged robots are constrained by the torque density of the actuators, especially with humanoids that have higher degrees of freedom. For a given actuator, better cooling can drastically increase its torque density, pushing the actuator closer to its limit for longer periods. This paper presents a novel design for integrated direct coil immersion cooling of BLDC motors, that also fits the space and weight constraints of legged robots. The immersion-cooled actuator is manufactured and tested for its cooling capabilities. The results are then compared against other currently widely adopted cooling approaches. It is shown that the thermal resistance is 3.7 times better even compared to liquid cooling, and that number increases to 10 times when compared to forced air cooling. In some cases, the gaps between the continuous and peak torque of the actuator are effectively eliminated, allowing the actuator to sustain its peak torque indefinitely. Additionally, the benefits of two-phase cooling against single-phase cooling are evaluated. Finally, guidelines on future actuator design and cooling approaches for humanoids and legged robots using proprioceptive actuators are presented.