Combined Effort

Electrical power systems are critical to a UAV's design and operation but don't operate in a vacuum. Their weight, operating temperature, and power components' efficiency significantly influence performance parameters such as flight duration, payload, or range.?

Below is a short introduction to?power systems, what's involved, and why it all matters.??

The basics?

Electrical power in a #UAV generates with a?starter generator,?which is a critical component for ensuring the electrical system can meet the design objectives of the aircraft. The starter generator is mounted on the engine so that the drive shaft turns the rotor to produce electricity.??

The electrical machine generates power for the onboard systems, known as an?alternator. A starter generator can also start the?internal combustion engine?(ICE) itself if paired with an?electronic engine starter?(EES). The EES is a power electronics component that provides the commutation to the starter generator to rotate the ICE shaft to the cranking speed and torque.?

A?battery pack, which can be used to start the #engine, stores #energy to ensure its continuous supply over the required minimum operating time.??

Power generation?

An electrical generator converts mechanical energy to electrical energy. Its essential parts are a?rotor?and a?stator. The rotor is the magnetic field source, while the stator contains the conductor where the electric motive force is induced (generally, the conductor is a coiled wire called windings). All generators operate on the same basic principle: a magnetic field cutting through conductors, or conductors passing through a magnetic field, to produce electricity. An electrical generator is the same basic design as an electric motor because its construction and working principles are based on the mutual induction between the rotor and the stator windings.?

Windings?are the wire wound on the stator armatures to work as a conductor. The only physical difference between a single-phase generator and a multiphase generator is the stator's additional coils with accompanying parts. But a multiphase generator is smaller, lighter, and less expensive than a single-phase generator with equal power.??

The rotor houses a?permanent magnet?made from a hard #ferromagnetic material that is magnetized in manufacturing, creating a persistent magnetic field. Permanent magnets do not require any power source and usually produce a strong magnetic field compared to their size. Most starter generators use rare earth magnets for high power density.??

The performance of a generator is a complex combination of the material used, the physical design, and the manufacturing processes. An ideal generator will be lightweight but able to produce power reliably and efficiently to meet the mission parameters.?

Electronics?

Given the increased demand for electrical power for more complex missions, system designers must provide higher currents and multiple supply rails. New and innovative power supply topologies are necessary to improve efficiency, reduce weight, minimize heat dissipation, and lower overall costs.?

UAVs can have onboard or outboard starters that must create enough torque and speed for the #ICE, while the battery must provide enough voltage and current. An onboard?electronic engine starter?is an excellent option to solve this.??

Some designers rely on ground-based starters to minimize onboard requirements and thus keep the overall weight down. However, an onboard system has the advantage of being self-contained and may provide greater flexibility in the field.??

Batteries?

For platforms that run on internal combustion engines, energy storage is necessary to ensure a reliable supply of electrical power for onboard systems. This means batteries. Designers need to understand their options in battery technology, such as LiHV, LiFePO4, Li-ion, NiCad, NiMH, and lead-acid batteries.??

Regardless of the battery used in the UAV, its charge and use will influence mission parameters and final performance. Therefore, the importance of a battery management system cannot be overstated when designing UAVs.??

A reliable battery management system will:?

• continuously monitor essential battery parameters,??
• handle the varying power demands of the many aspects of the UAV's operation, and??
• optimize battery usage.?

The battery management system can also protect the battery during charging, which safeguards against #overcurrent or #overvoltage.?

Hybrid operation?

Most large UAVs are still powered by internal combustion engines, while electric propulsion systems are more common in civilian micro and mini UAVs. Increasingly, high-performance electric motors make electric propulsion possible for larger commercial UAVs.??

However, electric propulsion systems may be limited by a reasonably-sized battery pack. What's more, the noise produced by internal combustion propeller-driven UAVs can be a drawback, which can be partially solved by installing an electric motor to drive the propeller.??

Below are some hybrid applications:?

• Internal combustion engines can be sized for steady-state flight rather than sized exclusively for peak power requirements at takeoff. We refer to this as power assist.?
• Battery power can be used to extend flight time. This provides a mechanism to use the battery power to land the aircraft in an emergency.?
• Converting to electrical power temporarily for reduced noise.?

The choices involved in selecting an electric power system are mainly dictated by the duration of continuous and peak power requirements. Unlike many other UAV subsystems, the power system supports the platform and the payload. Depending on the mission, the payload will require electrical power while in flight. This power demand can be tens or hundreds of watts for sensors or communications or tens of kilowatts for more complex payloads. This is why so many components need to be considered when it comes to electric power systems for UAVs.?