Powering Unmanned Vehicles: DC-DC Converters for Diverse System Power Solutions

One could claim that Montgolfier’s hot-air balloon presented in 1783 was the first unmanned aircraft. However, the flight of an unmanned balloon without a rope to the earth is determined only by the wind.

In 1892 Thomas Edison demonstrated a wire-guided torpedo and Nikola Tesla displayed his radio-controlled boat in 1898. A few decades later the first remote-controlled unmanned vehicles and aircrafts came to the market. Today, applications range from toys to autonomous, guided, or remote-controlled flying, driving, or diving unmanned vehicles.

Figure 1: Examples of unmanned vehicles - Delivery robot, Multicopter UAV, Underwater robot

All these devices have some common challenges. The most important one is a safe operation to prevent collision with people or objects or an uncontrolled air crash. The vehicles should be lightweight but powerful enough to carry as much payload as possible and achieve long operating times.

While in many cases quantities are rather small, each application may require different configurations of sensors, manipulators, or surveillance devices. Manufacturers can handle this challenge by using flexible, modular platforms and off-the-shelf equipment allowing a quick and low-cost adoption to specific needs.

How these unmanned vehicles are powered ?

It depends on the application and the necessary operation time. For shorter-term operation batteries or supercaps are used, modern fuel cells allow extended operation, and for long-term operation combustion engines are used or energy is supplied by a tether.

With all these different parameters, powering the electronics on these vehicles becomes a key element for safety, expected runtimes, and high payload. Source voltages vary, and many loads need their own stable supply voltage. In this article, we will describe how power modules from P-DUKE enabled extremely flexible and power modular solutions.

On-board power architecture for these vehicles whether aerial, ground, or underwater is very similar and consists of a power source, DC-DC converters for the various loads, speed controllers, and propulsion motors (figure 2).

Figure 2: Basic UAV power architectures - simplified block diagram of the basic power architecture

In most applications, the propulsion motor is supplied directly from the power source to avoid additional conversion losses.

In many applications, battery stacks are used and the necessary voltage and capacity depend on the size and maximum power of a system. To avoid additional conversion losses, the motors are normally supplied directly from the battery, but the rest of the system needs stable voltages.

For unmanned aerial vehicles, 3.7V LiPo batteries are connected in series with nominal voltages ranging from 3s = 11.1V up to 16s = 59.2V (s = number of cells in series). For longer flight times fuel cells can be used.

Modern warehouse robots, whether autonomous (UAV) or guided (UGV) are equipped with Li-Ion or LiFePO4 batteries with nominal voltages from around 24V for smaller systems up to over 100V for heavy load applications like forklifts.

What types of DC DC converters power solution are suitable for different power sources in UAV applications?

These wide source voltages are a real challenge for designers looking for flexible, modular power solutions to be used in different applications. P-DUKE offers a large product portfolio of DC/DC converters for these requirements with input voltages ranging from 9 – 75V and 14 – 160V, single or dual output voltages from 3.3V to 48V, and power levels from 10W to 200W. These products are also qualified for the harsh environments in which these vehicles sometimes operate.

To leave as much payload and space as possible for the transported goods and the safety equipment, all other components should be as lightweight and small as possible. Another reason to use highly efficient, small DC/DC converters for the internal power chain. Efficiency has a significant impact on overall weight as the lower the losses, the smaller heatsinks are needed. Using modules instead of a centralized power solution has the advantage, that converters and their dissipated heat can be spread across the system, close to the loads which makes heat management easier.

With the small and lightweight converters from P-DUKE, it is easy to design one single solution suitable for the different power sources in various applications. And if another or additional load voltage is needed? Very simple, adding another DC/DC module or replacing one with a plug-and-play replacement from the same family, and the job is done.

Let’s look at a few examples:

Modern, smart factories must be very flexible and need vehicles navigating and transporting goods autonomously within the production or warehouse facility. These vehicles use lasers and cameras for navigation and communicate directly with the manufacturing system and other robots. Autonomous vehicles can move almost freely, while guided vehicles (UGV) follow a predefined path defined by visual markers, magnetic tapes, object recognition, and odometry data.

The key challenge is safe operation as these powerful and heavy vehicles must detect human workers or other obstacles under all circumstances. This autonomous travel was enabled by significant improvements in camera, sensor, laser, and object recognition technology.

The power architecture is similar to Figure 1, the higher power motors for lifting and moving the transported goods are normally supplied directly from the battery. For battery charging wired or wireless solutions are used.

In the application shown in Figure 3, the manufacturer had the goal to use batteries with 24V or 48V nominal voltages, depending on the necessary power and desired payload. For autonomous driving the onboard computer of the vehicle processes information from off-the-shelf cameras and Lidars. Many industrial devices already have wider supply voltage ranges, but they do not cover 24V and 48V batteries. Here are a few examples of different Lidar systems with power levels ranging from 4 W up to 10W

Therefore a 12V bus for the CPU, cameras, Lidars, and other 12V equipment was selected and where needed, small downstream converters generate 5V or 3.3V voltages. Isolated converters are used to avoid noise being coupled to sensitive sensors.
Please refer to P-DUKE’s HAE150W series for 12V voltages, the UDH03 series / PDL12W series for 5V and the OSR03 series for 3.3V DC DC converters power solutions.

Figure 3: Block diagram of an unmanned autonomous vehicle (driving warehouse robot) for automated factories

In lower power applications, where short, intermittent operation times and quick recharging are possible, supercapacitors might be an option. Energy density is lower and the newest technologies seem to achieve values similar to a NiMH battery. Supercaps offer the advantage that they don’t have any dangerous material and much lower internal resistance, enabling higher peak currents than batteries. The lifetime of batteries is limited to 2000 – 5000 charge cycles while supercaps can be charged up to over 1 million times without significant degradation. They also work at very low temperatures for example in a warehouse for food or frozen goods.

Other than batteries which keep a nearly constant voltage during the discharge process, the voltage of supercaps drops significantly. The following example is for a small robot with in total 100W power consumption and a maximum run time of 60 seconds between charging cycles.

Energy calculation:

E = P * t = 100W * 60s = 6kJ = 1.67Wh

Maximum voltage of the capacitor bank: 48V

Minimum voltage: 12V

Required capacity:  

By connecting seventeen 3V/100F supercaps in series, the required capacity at a maximum rated voltage of 51V can be realized.
Figure 4 shows the discharge curve of this supercar solution vs. a standard 48V battery.

Figure 4: Discharge curves of Supercaps vs Battery

Price is certainly higher than a battery but the supercap bank can be recharged during operation within seconds enabling the robot to work 24 hours without interruption. Considering the difference in lifetime and the cost of replacing defective batteries, the supercap solution is cheaper in the long run.

A converter covering the wide capacitor voltage swing of 12 V to 48 V is needed. No problem, P-DUKE has a converter with 9 – 75V input voltage and 100W output power, please refer to P-DUKE’s QAE100Useries, see Figure 5.

Figure 5: Typical power architecture of a robot using supercaps power solution

The last example is an unmanned small helicopter system with a flight time of over 4 hours requiring a fuel engine. Other than shown in Figure 1, the rotors are directly driven by the engine which also provides the typical aircraft voltage of 28V for the flight control systems. The helicopter can be equipped with infrared/optical gimbals, various sensors, lidars, 3D mapping and geo-tagging systems, and many other devices that need a stable 12V supply voltage with a total power of 170W.

The necessary 28V to 12V DC/DC converter must be light, able to work at ambient temperatures as low as -40°C and withstand high shock and vibration stress from the rotors. IP67 protection and complete EMI shielding required a hermetically sealed chassis for the complete electronics and therefore conduction cooling for the power converter.

The ideal solution was P-DUKE’s half-brick converter from the HAE200W family (figure 6). It works over a 16.5 -75V input range, down to ambient temperatures of -40°C and the metal baseplate enables conduction cooling to the chassis of the helicopter. Due to the silicon potting, it withstands the shock and vibration requirements of MIL standards. Weighing only 105g, this converter represents less than 0.5% of the total weight of the helicopter. With a maximum power of 212W, it also gives room for additional payloads or short load peaks.

Figure 6: Simplified power chain of the unmanned helicopter system

The remote control unit can be powered from a 12V battery or the 28V (16 – 50V) mil grid of a ship or ground vehicle. The RF amplifier needs a powerful 28V source and therefore this voltage was selected as the main bus voltage. A 100W converter from P-DUKE’s QAE100U family with an input range of 9V – 75V generates this 28V bus and downstream, lower power converters from P-DUKE provide the necessary supply voltages within the remote-control unit (figure 7).

Figure 7: Block diagram of the remote control unit

With P-DUKE’s wide range of isolated and non-isolated products, various voltages can be generated depending on load requirements.
Please refer to P-DUKE’s PDL12W series for 12V and 5V voltages, the OSR03 series for 3.3V DC DC converters power solutions.

Each UAV application might have different challenges and requirements. In this article, we wanted to narrow it down to some key elements of the power supply chain which in most cases has common requirements: small space, low weight, high efficiency, and reliability but even more importantly flexibility, modularity, and ease of use. In the end, designers need a competent partner offering the necessary product portfolio and able to support the customer in finding the optimal solution for their specific requirements.