SBIR-STTR Award

The power required during take-off in eVTOL aircraft can be 3-10X the average power during cruise. Thus, electric propulsors capable of meeting the peak power requirement on a continuous basis may not be ideal. We propose a commercial tool that can safely extract the transient capability of high-power electric powertrains to offer operational flexibility and increased payload. A model-based control scheme that keeps track of the state of the electrical components through the mission profile will be used to establish the actual capability of the system instead of a fixed steady state rating. Since the power rating of many of the devices are based on temperature limits which are themselves set by life projections, a high fidelity, multi-physics model, complemented with appropriate sensor data, will be used to project a safe operating region as a function of time and risk of failure. The current/torque limits of the powertrain ‘saturation’ blocks will be adjusted based on the state of the system, and mode selected. Three operating modes are envisioned: Regular mode – with traditional limits on current/power based on ratings. This is the current baseline. Enhanced mode – with current/power limits based on data from ‘digital twin’ that tracks complete state of the powertrain. Can be implemented with data from qualification tests of components. Emergency mode – relax limits further under emergency situations, consuming some life of hardware. Needs digital twin to also estimate of remaining useful life and/or risk of failure. This is future work needing a lot more data.

The proposed work of this Phase II STTR is hardware demonstration and system application of the digital tools and controls developed in Phase I. The technology package extends the performance range of electrified powertrains under highly transient loading conditions, such as those experienced by eVTOL vehicles, by taking advantage of finite thermal rise times seen by embedded electric motors/generators throughout the flight profile. With detailed electro-thermal models of the electric machines and controls, thermal states are projected forward and used to inform mission-level decisions and component-level controls in order to extend the vehicle performance and ensure safe operation during emergency situations. Studies in Phase I have shown that over a 60% reduction in electric machine mass is achievable for extreme flight profiles compared to machines sized to continuously output the maximum power demand. In Phase II, Hinetics will partner with VerdeGo Aero to demonstrate the controls developed in Phase I as applied to their existing series hybrid-electric powertrain, Iron Bird. Models will be developed for VerdeGo’s 200 kW powertrain including the COTS electric machines used in the generator stack. By impeding the generator’s cooling loop, we will push temperatures seen by the armature windings close to their rating and simulate deep thermal cycles by running various load profiles via a programmable electronic load. For the designed flight profile of a representative vehicle, supplied by VerdeGo’s customers, a sizing methodology built in Phase I will be implemented to quantify how oversized the COTS generators are in the current Iron Bird and inform the design of a replacement generator using Hinetics’ patented machine topology. A prototype of this advanced generator will be built and qualified both at POETS/Hinetics’ facilities and on an AFRL drive-stand to emulate deep load profiles. In a parallel task, accelerated insulation lifetime tests will be run on Hinetics’ form-wound litz coils used in the slotless generator topology. The insulation temperature will be forced above its rating via a heat pad to accelerate insulation failure according to standard overtemperature-lifetime associations and will be cycled to generate thermal stresses. In past work, members of the Hinetics team have correlated partial discharge inception voltage to failure imminency and results from these tests will be used to generate a probability of failure model as a function of the cumulative temperature profiles. This model will feed a set of controller rules that adjust the maximum rated torque or power that the drive can command from the machine, effectively temperature flight inputs to maintain safety margin. Finally, Hinetics will develop detailed plans and budgets for a flight demonstration on an existing vehicle platform through one of several companies identified and contacted in Phase I.