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Actuators and controls of the pneumatic and hydraulic variety are fast becoming outdated for unmanned aerial vehicles (UAVs)—now being replaced by electrical motors and electronic computers and sensors. Designers working on power distribution systems for unmanned vehicles must realize this trend, and consider five key factors when specifying components for a UAV power system: efficiency and weight; quality; built-in test and control functions; flexibility; and cost. Prioritizing each of these factors is now a critical part of power distribution design.

The efficiency of power converters and bus switches will affect the use of the available fuel load and thermal management in a UAV platform. This is true whether the unmanned vehicle uses MIL-STD-704 400 Hz AC power, a high-voltage DC bus, or a low-voltage DC bus. Ultimately, waste heat energy derived from fuel must be dissipated, which can be quite a problem for a high-altitude UAV where avionics are often located in pressurized compartments and composite construction makes heat transfer extremely challenging.

Reducing system heat load will reduce the weight of the cooling system and extend the range and payload capacity of the UAV, thereby increasing overall efficiency. Therefore, liquid cooling using fuel or polyalphaolefins is sometimes employed in these vehicles.

Furthermore, designing power converters or transformer rectifier units that meet specific system requirements for minimum size and weight will result in more efficient use of fuel and available payload capacity. Advanced thermal design techniques, a lightweight case, potting and connector materials, and the use of more recent semiconductor technologies, such as silicon-carbide (SiC) semiconductors, are typically used to achieve the appropriate balance between weight and efficiency.

As size and weight of power conversion or control equipment decreases, maintaining reliability becomes a growing concern. It becomes essential that an AS9100C-certified manufacturer is chosen for the project, so as to ensure the manufacturing processes are being carefully controlled and monitored to maintain overall reliability. Also, the system designer must work closely with the component manufacturer, ensuring that all aspects of the design are carefully analyzed and components are selected to work reliably in the anticipated application of the system.

Adding test, control, and monitoring functions to an existing system using commercial-off-the-shelf (COTS) power converters can be costly and will occupy greater volume and add to the weight of a system. Therefore, a system designer must determine the needs for specific control and monitoring functions within the power system components as early in the design process as possible.

As UAV missions become more complex, the demand for accurate, real-time system information continues to increase. For example, it will be necessary to test, control, and monitor power converters to include output voltage and current, temperature, and input voltage, as well as control factors such as engagement or battle-short of protective features as well as the enable/disable sequencing of converters. The increasing demand for test, control, and monitoring functions will continue to grow in the future, and it will be necessary to prioritize them early in your power system design process.

To save time and money, system engineers may choose to work with a supplier able and prepared to modify their standard product offering in a timely manner to meet specific system requirements without excessive nonrecurring-engineering (NRE) charges. Flexibility is critical to a UAV design over the long-term. Designing a flexible power distribution system will not only help to minimize fuel waste, but also enable the system to grow with evolving mission requirements.

The final factor is cost, which is important for both commercial and military applications. Reliable but flexible components should be chosen to avoid incurring large expenses in the future, when it becomes necessary to modify a UAV’s power distribution system. Furthermore, a power distribution system should be designed that maximizes efficiency, so that operating costs can be controlled while reducing fuel waste. A flexible and balanced system designed today with high-quality, military-grade components will save money and reduce problems over the long-term.

Kerima Batte, CEO/President

Abbott Technologies, Inc., 8203 Vineland Ave., Sun Valley, CA 91352; (818) 504-0644.

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