Lichten Runner-up Paper: Semi-Empirical Modeling of Group 1 UAS Electric Powertrains

Abstract

This paper describes the development of performance prediction models for the electric powertrains of group 1 unmanned aerial systems (G1UAS) that use sensorless brushless DC (BLDC) motor architecture consisting of a BLDC motor, electronic speed controller, and a battery. Per US Army definitions, G1UAS are platforms that weigh less than 20 lb. (9 kg). The resulting semi-empirical models for the motor, power electronics, and battery use high-level component specifications to enable pre-conceptual design space exploration and mission-based design optimization of G1UAS without a library of test data. The models also enable tradeoffs analysis between existing and/or conceptual designs without a series of flight tests. To develop, tune, and validate the models, a custom dynamometer test setup was designed and built to measure torque, speed, and electrical power data of small-scale motor drive systems. The validated models reveal that popular claims of high efficiency for electric powertrains are only valid in a narrow band of high speed/low torque operation. This is a critical finding for vehicle designers in the VTOL industry who are increasingly transitioning to electric powertrains in low speed/high torque applications which may decrease the overall system efficiency. A traditional rotor hover test stand was also developed to generate data with a traditional rotor load. The integrated motor and electronic speed controller model was able to predict the total efficiency of the hover stand tests within 5 percent of experimental values. The electrical models presented in this work can be immediately applied to design G1UAS given the torque and speed requirements of the rotors/propellers, the operating voltage of the vehicle system, and certain high-level component specifications for the motor, electronic speed controller, and battery. The models can also be used to BLDC powertrains for small terrestrial or aquatic electric vehicles.