40 Focus | Power management The mathematical behaviour required to calculate the electrical energy production from solar energy on a UAV from known UAV angles of rotation, the position of the sun in the sky, solar irradiance measurements, the solar module area and the solar modules efficiency has been integrated into a custom-made maximum power point tracker (MPPT) with an integrated perturb and observe (P&O) algorithm. Aerial mapping mission flights show that the energy efficiency is more than 96.27% for the MPPT, which extends the flight time by up to 21.25%, showing the importance of power management. The MPPT was installed on a Bramor UAV with a parallel connection between two, four-cell lithium polymer batteries as the primary source of electrical energy. Each battery had a nominal charge capacity of 11 Ah, an energy capacity of 162.8 Wh and a mass of 820 g. The parallel connection provided a total nominal charge capacity of 22 Ah, a total energy capacity of 325.6 Wh and a total mass of 1.64 kg. This amount of energy was enough for up to 3 hours of flight time in good weather. The solar module was connected to the UAV battery via the MPPT. The tracker ensured that the maximum available electrical power was transferred from the solar modules to the battery under given conditions. The maximum power from the module can be obtained if the module works at the maximum power point (MPP). There are many types of trackers with different MPP searching algorithms, although there are practically no offthe-shelf trackers intended for use on small UAVs. The MPPT function can be performed in analogue, digital or most often digital-analogue mode, where the core component of the MPPT’s functionality is a microcontroller that manages the duty cycle of the DC-DC converter. Changes to the DC-DC cycle should be reflected in the different electrical power levels generated by the solar module, and the goal of changing it is to find the MPP. The main task of the MPPT is to find the MPP rapidly, with less convergence time and less oscillation. Finding the MPP can be achieved via searching or calculation. In such cases, an algorithm is run on a device intended to track the MPP. The efficiency of this type of MPPT depends on the algorithm’s efficiency as well as that of the efficiency of the hardware components. The algorithms differ mainly in the speed of tracking the MPP, the complexity of implementation and the type of sensors, whose measurements act as input data into the algorithm. The algorithms are divided into direct and indirect. Direct algorithms require current and voltage measurements of the solar modules, meaning the determination of the MPP position is independent of non-electric quantities such as solar irradiation and temperature. Indirect algorithms determine the position of the MPP by known nonelectric quantities. They are more complex and expensive in terms of integration, and the most widespread indirect algorithms are P&O and incremental conductance. Both are very effective under stationary conditions, although with faster changes in irradiance, their effectiveness can be lowered owing to the loss of the tracking direction. However, most algorithms introduce a certain loss under stationary conditions, which depends on the voltage oscillation around the MPP. Each MPPT input line and output line involves current and voltage monitoring. This raises the possibility of monitoring the current and voltage to calculate the power on each chain with solar modules and the MPPT output during the flight. These measurements are performed using a current, voltage and power monitor. A core component of the MPPT is a DC-DC buck-boost controller with a P&O-type MPPT algorithm. The controller tracks the MPP every 180 seconds, but if a new MPP with a higher yield is found, the controller starts tracking around the new point. The buck–boost controller can provide a voltage above, equal or below the input voltage, and is appropriate for a range of input and output voltages up to a maximum of 80 V. The internal analogue-digital (A/D) conversion value of the controller is 10 bits of resolution, although the A/D resolution of the pulse width August/September 2023 | Uncrewed Systems Technology The C4Eye solar-power UAV (Courtesy of Bramor)
RkJQdWJsaXNoZXIy MjI2Mzk4