All types of UAVs need a portable power source to keep their flight control system operational for long and steady flights and, as a result, the market is faced with a growing need for more efficient and lighter designs.
This need is the next big challenge for engineers, and starter generators (SGs) are a key design element that supports the system throughout the entire flight mission. But there are some questions as to the exact role of the SGs in the UAV system and their function in increasing battery efficiency.
Below is a short overview, adapted from Starter Generators for UAVs, that addresses the more general issues and questions on the subject, while also explaining how ePropelled’s SGs fit within the market needs.
How do starter generators work?
Starter generators are an important source of power for UAVs because they convert mechanical power from onboard engines into electrical power. They are economical for UAVs due to their design’s inherent dual characteristics—they can function as a starter motor until the engine starts and operate as a generator once the engine reaches a certain speed.
The construction of SGs is a typical synchronous DC motor with a stator and rotor capable of operating at high RPMs. SGs are not standalone units and need an electronic system to make them work as a starter or as a generator.
Three major components of an SG system are the electronic engine starter (EES), electronic speed controller (ESC)—both of which make the SG spin—and an intelligent power system (iPS). The iPS is a power conversion unit that provides a clean 28 VDC output for onboard electronic systems.
SGs have two sequences of operation, one is to start the engine as a starter motor and the other is to use the generator mode, which is turned on once the engine has reached a pre-defined acceleration that is enough to generate power.
The fundamental challenge for SGs is keep the flight instruments functional throughout the entire mission. To achieve a longer flight time, SGs must have the following characteristics: lighter weight, higher operating temperatures, high altitude operation, and effective thermal management. All of the above means less energy is used, therefore extending the battery life and flight time.
Fault-tolerant design and thermal management
The design of SGs is limited by its operating temperature and the torque extracted from the engines. The latter plays a critical role in the function of the UAV.
The need for a fault-tolerant design is driven by the customer needs. There are examples of SG failure, where onboard power management system turns off power control units in abnormal operating condition. This stops the power generation process, and the power from back-up batteries is routed to power flight control systems. However, due to a lack of sustained power, the flight systems stop working and the UAVs are then prone to fail.
ePropelled’s SG addresses this problem with an advanced fault-tolerant electronic design that is intelligent enough to send instant feedback, such as voltages, current, and temperature measurements of the SGs through iPS units under any critical flight condition. The UAV systems are then given the control to handle UAV maneuvers at the lowest possible power mode, while the entire flight system remains operational throughout the flight duration.
To control the overheating of iPS systems, a K-type thermocouple (thermal sensor) is used to measure winding temperature. An active cooling system can allow the SG to operate at temperature as high as 180oC. The iPS system also has a thermally efficient housing to dissipate any excessive heat from the internal circuitry during high-speed stepdown power conversion.
For hybrid UAVs, starter generators act like range extenders and power the propulsion motors during eVTOL maneuvers. They can switch to generator mode to extract power from the internal combustion engine to charge the batteries and flight instruments onboard, and also power the propulsion motors for eVTOLs.
The quality of the range extender depends on the power conversion rate i.e., the energy transferred from fuel as electric power with minimum losses. Hybrid UAVs use propulsion motors that drain the battery during eVTOL maneuvers and forward using the pusher propeller fitted to the main ICE that uses oil to power the forward flight.
Range extenders eliminate the need to carry an additional power source, since they extract power from the ICE using direct mounting mechanism.
This blog post was adapted from Starter Generators for UAVs. To learn more about starter generators, their power variants, innovative mounting, and using an integrated CAN bus to improve UAV performance, you can download the whole paper here.