Analysis of liquid-cooled Brushless Motor Actuators for Space Robotics

Abstract

IntroductionBrushless DC (BLDC) motors and, in general, Permanent Magnet Synchronous Motors (PMSM), are ubiquitously used in space for robotics as well as other fine motion-control applications [1]. BLDC motors have a high torque density that is practically only limited by overheating. High torques require high currents, which heat up the windings through ohmic effects and, in addition, the increased amplitude of the alternating magnetic field causes the magnets to heat up due to eddy currents. The resulting excessive temperatures can permanently damage the isolation system or demagnetize the permanent magnets beyond the Curie temperature [2]. As robotic motion systems are usually optimized for torque density and accuracy instead of speed, commonly used actuators usually end up having a large gearbox and a small motor. This combination leads to large mass and inertia, and, as a result, sluggish performance. In fact, for a high-performance motion system, one would want to have large motors and small gearboxes instead. In high performance terrestrial robotics, liquid cooled actuators have demonstrated a previously unattained level of performance by avoiding the high temperatures associated with high currents [3] [4]. This study examines the possibility to have the same level of performance also for space robots, as pumped fluid loop cooling systems in general have extensive heritage in space engineering and thermal management [5] [6]. A liquid cooled drive has numerous possible applications in planetary exploration, for example as joints in robotic arms or as wheel or steering drives in rovers. Furthermore, an active thermal controlled actuator can ensure that the motion system remains in its operational temperature range, even in extreme temperature environments, for example permanently shaded regions at the lunar poles, enabling extended mission operations in such places.Drive ConceptThe proposed system consists of a custom BLDC motor with a single-phase liquid cooling system consisting of a motor-fluid heat exchanger, a pump, an accumulator, and a radiator. The motor can be complemented by several sensors and other functional components depending on the application. The thermal management system is flexible, expandable and allows for several actuators to be cooled by one fluid loop. A schematic of this architecture is depicted in the following figure.Methodology and ResultsThis work presents a design study for a liquid-cooled robotic actuator by performing coupled multi-physics simulations that can model the drive system in the thermal, electrical, and mechanical domains. This holistic approach allows for an optimal design of the main components considering the complete coupled domain and size the motor and controller according to dynamic requirements (response time, step response, stiffness etc.). The results are used to analyze the influence of active thermal control on the motor performance (torque, power, short-time as well as continuous) on component level and design an exemplary system for a specific application. The simulation model was verified by sub-scale thermal-vacuum tests of a commercially available BLDC motor.ConclusionIn summary, active thermal controlled robotic joints have the potential to increase the performance of space robotics systems and can ensure optimal performance in exploration missions with adverse temperature ranges. This study investigates how this proven concept in terrestrial robotics can be applied to in-space applications and presents a simulation-based design methodology to perform preliminary design of such a system.