Onboard charging solution from Millswood Engineering

Australian-based Millswood Engineering (www.millswoodeng.com.au) has developed a new onboard generator for small to medium-sized UAVs.

Onboard electrical power generation is useful for extending the range and endurance of unmanned aircraft. Coupled with a suitable brushless DC motor, the new Power Management Unit (PMU) performs all of the functions necessary to assemble a high-performance and full-featured power generation system. The PMU can handle up to 500W, although the power available in any given installation is dependent on the avionics bus voltage. At 28V the PMU’s main output can deliver up to 420W continuously.

The PMU’s key features are:
– Main avionics output user-programmable from 10 to 30V. 15A continuous.
– Universal battery charger with user-programmable current limit from 2 to 5A.
– 6V auxiliary output for ignition systems. 5A continuous.
– Input voltage range of 10 to 70V.
– Efficiency greater than 90% at 28V at rated load.
– User programmable input current and power limiting.
– Choice of CAN or RS232 interfaces (custom CAN protocols available on request).

Maximising efficiency was one of the key design goals. An analysis of the losses in a typical generator system showed that the input rectifier was an area where significant improvements could be made. The use of an active rectifier instead of a diode bridge saves nearly 30W under maximal loading conditions. This is particularly important when a UAV is heat-stressed, such as during the pre-flight period when all systems are running but without the benefit of any significant airflow. This often represents the worst case situation for internal temperatures, and increased efficiency translates directly into lower temperatures.

Another innovation is the use of a Buck-boost converter (step-up as well as step-down), rather than a Buck converter (step-down). This allows the PMU to continue to charge the battery even when the input voltage is less than the output voltage, such as during periods of low engine RPM. A user-programmable input current limit prevents the possibility of the generator stalling the engine.

The Generator PMU weighs in at 720g excluding the BLDC motor.