Unmanned Aerial Vehicles (UAVs) have been widely used to carry cameras, sensors or products for applications such as mapping, frame monitoring, goods delivery, entertainment and more. The most common UAVs are powered by battery only, which limits the duration of operation. However, current batteries of the system depending on vehicle, payload, and wind conditions enable only flights up to 30 min for quadrotors, which can limit the usage of these UAVs for long time missions and experiments.

Parallel Mechanism

Battery powered electric UAVs suffer from uncertainties in estimating the remaining charge and hence, most flight plans are highly conservative in nature. Batteries also decrease in capacity with time and usage during operating. The output current plays a big role in determining the losses inside a battery and is an important parameter to consider when analyzing battery performance. For a long-time mission like large farm observation or border monitoring, a UAV must return to its recharging station, which limits its operation range. A technology of energy supply which makes a UAV operate longer and be able to recharge anywhere during its mission would be an advantage for continuous operation.

Solar energy systems have emerged as a viable source of renewable energy over the past two or three decades and are now widely used in a variety of industrial and domestic applications. Such systems are designed to collect the energy from the sunlight and convert it into electrical power. Solar trackers are defined as devices which have the role of improving efficiency by keeping the solar panel perpendicular to the sun rays. The first automated solar tracking system was proposed by Mcfee.

There are two types of solar tracker: single-axis trackers and dual-axis tacker. Solar trackers with one axis have much better performance than fixed systems, but two-axis systems allow optimal tracking of the sun’s path, since they keep the orientation of the collectors perpendicular to the solar radiation at any time in any season. The main challenge of these kinds of devices is that they have to consume a certain amount of energy in order to move the collectors following the sun trajectory.

Dual axis solar trackers have been of interest for many researchers, and a number of techniques have been developed to obtain better efficiency with the desired optimal power consumption of the system. This led to the introduction of a parallel mechanism. Development and testing of a two-axis decoupled solar tracking system based on a parallel mechanism showed that the tracker requires less driving torque, thus less power consumption than is found with a conventional serial tracker. Complexity and weight of the system are also reduced.

A parallel mechanism is a good one for saving energy in the system. Parallel mechanisms have a large payload to mass ratio and high stiffness. It is possible to reduce the driving torque, scale down the dimension of the mounting and reduce the complexity of the system. When a parallel mechanism is attached to the fixed platform, the dynamic effect would not have much impact on the system. Nevertheless, dynamic effect should be considered when a parallel mechanism is attached to a moving platform. The moving platform can be a boat, an aerial vehicle, a land vehicle etc.

The primary purpose of this work is to develop a dynamic model that can describe and simulate the change in orientation of a solar panel subjected to external forces. The solar panel is attached to a 2DOF parallel mechanism upon which loads are externally applied to change the panel orientation. The parallel mechanism was chosen because it has been proven to be energy efficient. The equation of motion of the system will be derived by determining the constraint reaction force exerted by each kinematic link so that the behavior of the configuration can be investigated. Due to the effect of external forces acting on the linear actuator, the movement of the solar panel would be observed. The result from this work lays the groundwork for controlling the orientation of a solar tracker to obtain optimal solar energy.

Another purpose of this work is to study the dynamics behavior of the parallel mechanism mounted UAV. The attitude of a hexacopter is simulated at the hover point and during its mobility when the parallel mechanism has motion. First, the inverse kinematic equations for a parallel mechanism were derived, which produced the stroke of the linear actuators as a driver to movability of the parallel mechanism. To describe the behavior of the whole system, dynamic modeling was derived. Simulation for a pararllel-mechanism-mounted UAV was used to validate the dynamic motion.

This work was done by Sarot Srang of the Institute of Technology of Cambodia for the Air Force Office of Scientific Research. For more information, download the Technical Support Package (free white paper) below. AFOSR-0011

This Brief includes a Technical Support Package (TSP).
Investigation of Flight Dynamics and Controls for a Solar-Tracker-Mounted UAV

(reference AFOSR-0011) is currently available for download from the TSP library.

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This article first appeared in the May, 2021 issue of Aerospace & Defense Technology Magazine.

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