UAVs on a power trip

AMJune21Features - vicor1
AMJune21Features - vicor1

Using Vicor’s DCM DC-DC converters, FlyFocus reveals how it has developed detachable tethered UAVs that enable continuous mission critical surveillance.

FlyFocus specialises in the design and production of UAVs for the civilian market and law enforcement. Applications include monitoring of sensitive areas, such as borders, power stations or military installations. However, its products are also found in areas as diverse as highway monitoring and vehicle counting, crowd monitoring in stadia, radio jamming, and radio relays.

A typical ground station and tracking system

Although diverse, these applications share a common requirement for continuous flight enduring for extended times. This requirement is met by FlyFocus’s main product CableGuard, a complete system comprising a tethered UAV, ground station and tracking system.

The UAV spends most of its operational life in tethered mode, hovering at altitudes of up to 70m above an area of interest. In this mode, its electric motors and onboard electronics are powered via the tether cable, which delivers 400VDC from the ground station. This DC voltage is derived from a 230VAC input, typically supplied by a portable generator. Accordingly, the UAV can perform its surveillance role virtually indefinitely, and has recorded continuously airborne times of up to 30 days. In practice, however, landing the vehicle every week or so is recommended to allow maintenance inspections of the bearings and other mechanical parts.

In the camera eye

If one of the UAV’s cameras spots a suspicious object, such as a moving car, the tether cable can be immediately detached by a remote command. A parachute deploys and lands the cable close to the ground station.

This demo UAV uses a ZVS buck

Meanwhile, onboard batteries seamlessly take over the vehicle’s power supply, so the UAV – with a single daylight camera payload - can fly and track freely for up to 30 minutes before landing. With a dual camera (thermal plus daylight) setup, 20 minutes’ untethered flight time is possible. These untethered flight times give FlyFocus a competitive edge for their surveillance applications. After the mission, the vehicle may be reconnected to the ground station to continue its surveillance role.

At 5kW, the UAV is rated as the most powerful tethered type available on the market today. FlyFocus has used this extra power to enhance both the vehicle’s performance and its functionality. It has eight motors and can be safely landed even if two motors are disabled. The two cameras provided are a full HD type with 10x optical zoom and 4x digital zoom, and a night vision IR 640p camera with 4x digital zoom.

The power capacity gives FlyFocus latitude to use better quality cameras, such as heavy cooled thermal types or cameras with better zoom. Additionally, it opens a new perspective for adding different payload modules, such as telecommunications equipment or other specific solutions for individual customer requirements.

Vicor isolated, regulated DC-DC converters have been designed into the power delivery network from the tether cable to the UAV’s motors and electronics. These devices have contributed to the CableGuard UAV’s power capacity and competitive edge in several ways. Firstly, their ability to buck from 400VDC down to the UAV’s operating voltages means that power can be delivered through the tether cable at 400VDC; this higher voltage means a lower current, and lower I2R losses. Accordingly, a much smaller and lighter cable can be used.

Secondly, FlyFocus found that the DCMs’ power density far exceeded that of any other supplier. Together with their efficient operation, this meant that eight 600W modules could be packed into a limited-size pod under the UAV, with just two heatsinks/fans, to deliver nearly 5kW total power for the UAV’s eight motors, two cameras, and associated avionics.

FlyFocus can adapt its deployment of this ‘compact power’ to meet each customer’s priorities and preferences. The company can optimise for higher-performance payloads as described above; alternatively, it can design for longer flight times, because of the DCM modules’ weight and volume savings compared with competing manufacturers’ converters. FlyFocus estimates that the modules’ weight savings amount to around 1.3kg, which contributes from a third to a half of the current UAV’s tether-free flight time.

The modules have proven to be robust in performance as well as specification, as FlyFocus hasn’t experienced a single DCM malfunction during its three years of using the products.

Modules make their mark

With up to 1244W/in3 power density, plus a power rating up to 1,300W, 46.43A continuous, the DCM family of isolated, regulated DC-DC converters can operate with up to 96% peak efficiency. This high efficiency is due to its high frequency zero voltage switching (ZVS) topology. Operating from an unregulated, wide range input, it can generate an isolated output, with a fully operational current limit. Over-voltage (OV), over current (OC), under voltage (UV), short circuit and thermal protection is provided.

DCM modules are available in both ChiP (Converter housed in Package) and VIA (Vicor Adapter) packages. Leveraging the thermal and density benefits of the Vicor ChiP packaging technology, the DCM ChiP modules offer flexible thermal management options with very low top- and bottom-side thermal impedances. The VIA is also a ‘thermally adept’ package which eases thermal management; it does not have to be printed circuit board-mounted, although it has pinout options allowing this if required.

The CableGuard UAV power delivery network, showing the eight Vicor DCM isolated, regulated DC-DC converters

Additionally, the DCM VIA module provides EMI filtering, tight output voltage regulation over all line and load conditions, and a secondary-referenced control interface for trim, enable and remote sense operation while retaining the fundamental design benefits of the conventional brick architecture.

The converter provides superior power system performance and connectivity from a variety of unregulated power sources to the point of load. Additionally, arrays of up to eight units can be assembled with no power derating.

As Vicor’s power systems are based on modules, FlyFocus can accommodate such requirements more quickly and easily than if they had to design from component level for each new implementation – or try to modify a component-based design. Yet the DCM specifications were not the only reason why FlyFocus chose Vicor, and continues to work with them.

“Support is a particularly important issue for us,” says Julian Zyromski, vice-president of FlyFocus. “We are always buying in different types of mechanical and electronic components for sophisticated, custom projects; at some point we will always need assistance. In fact, even after identifying the most attractively-specified component on the market for a particular application, we are not ready to buy it unless we are convinced that the quality of its support matches that of its specifications.”

Vicor’s support was not limited to the converters themselves. FlyFocus had to design a circuit to reinforce the DCMs’ voltage stabilisation. This reduces their susceptibility to high inrush currents caused by the operator fully opening the throttle or responding to wind gusts. Vicor documentation, and its engineers’ advice proved useful to FlyFocus in developing this circuit.

Vicor’s DCM modules allow FlyFocus to succeed in this competitive market. Its high DC input voltage facilitates the use of thin, lightweight tether cables, while its high efficiency and power density allows many to be packed into a minimal space. Power module payload weight is minimised, allowing longer untethered flight times, while sufficient room and power is available for lifting and supporting a high-specification payload.



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