Selection of power sources for agricultural drones

Today, the vast majority of modern UAVs are designed specifically for rechargeable batteries. But the agricultural sector has its own specifics, which must be taken into account, namely:

• The need to survey large areas, which requires a relatively longer flight duration;
• Limited infrastructure for recharging batteries, which requires the purchase of several sets of batteries and specialized chargers, the cost of which is quite high;
• The need for high carrying capacity in view of the introduction of chemical and biological plant protection agents and microfertilizers, etc.

A possible solution is the use of engines internal combustion (ICE), which are devoid of some of the problems inherent in batteries. Their refueling can be organized relatively easily and offline. The duration of the flight is up to several hours, which increases the radius of action, the value of which is relevant in view of the state of domestic dirt road networks, etc. The devices created under DVZ have certain basic disadvantages, namely, significant dimensions, noise during operation and explosiveness, which limits designers when using them. However, some of these shortcomings are unacceptable for the conditions of dense construction of megacities and are not critical for rural areas. Therefore, it is quite possible to consider DVZ as a promising candidate for unmanned agricultural aviation.

Since there is little information about the specifics of the use and intricacies of the operation of drones, the purpose of this article is to try to acquaint the reader with these undeniably important aspects that must be taken into account when choosing a UAV.

UAVs with electric motors

The trend of electrification has been observed in the world in recent decades. transport.

The main advantage of electric transport is the very specificity of electric motors. So, modern models have a high efficiency (up to 95% in the case of a collectorless engine). In the operation of electric valve motors, there are no electric sparks that create powerful electromagnetic interference, which greatly complicates remote radio control of the equipment. Electric motors weigh much less than internal combustion engines with similar characteristics. Moreover, it does not need to be supplied with fuel to ensure functioning, which radically simplifies and reduces the cost of the design. In terms of the number of moving parts, electric motors have no equal, which potentially provides high reliability. The cost of energy required to drive an electric car is several times less than the cost of energy used to drive a fossil fuel car over the same distance.

Проте, якщо з електродвигуном все досить добре по відношенню до ДВЗ, то з енергозабезпеченням все навпаки, АКБ за щільністю енергії програють бензину. Так відносно швидко (за десятки хвилин, а не за кілька годин) можна зарядити АКБ на спеціалізованих зарядних станціях, де вартість електрики в кілька разів вища, що знижує економічну доцільність використання електротранспорту. Тому електромобілі передусім орієнтовані на специфічний сегмент споживачів – мешканців великих міст та передмістя, що можуть заряджати свої автомобілі в нічний час по зниженому тарифу. Орієнтація саме на мегаполіси обумовлена їх специфікою трафіка із постійною наявність заторів – ділянок із призупиненим чи уповільненим рухом. Класичні двигуни внутрішнього згорання в заторах експлуатуються переважно в режимі холостого ходу і споживають біля половини палива від максимального навантаження, тому електродвигуни із мінімальним споживанням є більш прийнятними. Для мешканців міст середня відстань пробігу складає до 50-150 км/добу, яку можна здійснити на одній зарядці АКБ. На великих відстанях використання суто АКБ на сьогодні може бути проблемним через складність забезпечення енергією.

There are many designs of batteries, and new solutions are promising. However, for now, UAVs are mainly using lithium-polymer (Li-Pol) batteries. They have better energy density than lithium-ion batteries, which is critical for UAVs. The development of Li-Pol batteries began relatively recently - in the 1970s. In them, instead of liquid, a dry polymer was used inside. In fact, it is not a conductor of current, but it does not interfere with the movement and exchange that occurs between ions. Electrolyte in the form of a polymer has replaced the traditional porous material impregnated with liquid electrolyte. The new technology allowed manufacturers to focus on the potential advantages of the technology - reliability, safety and the ability to reduce the size of the battery. With a cell thickness of only 1 millimeter, designers are limited only by their imagination when developing such batteries. In fact, such a battery can be given any shape and size. But in the process of development, manufacturers encountered one significant drawback of the technology – it was characterized by poor conductivity, which prevented its use for powering modern mobile devices. It was possible to improve such a characteristic only by heating the cells to 60 degrees, which was unacceptable in view of the durability of the device. Instead, a compromise option was found - adding an electrolyte in the form of a gel. Commercial batteries use an electrolyte membrane made using a porous polyethylene or propylene separator filled with a polymer that turns into a gel upon contact with a liquid electrolyte. As a result, the form has changed, but the essence remains the same as in the case of liquid electrolyte - similar chemistry and capabilities. Thanks to their capabilities, Li-Pol batteries have found their niche in the market – they are used wherever a small size and the ability to change the shape of the battery are required. However, the use of Li-Pol batteries for UAVs, where the batteries are optimized for minimum dimensions and weight, has its own specifics:

• in the process of charging, it is necessary to carry out visual control. If the Li-Pol battery changes shape during charging, deflates, ignites or smells of burning, it is necessary to immediately stop the charging process by disconnecting the battery from the charger;
• for storing and transporting high-capacity Li-Pol batteries, for example for radio-controlled models, special protective bags must be used, due to the possibility of an internal short circuit, which leads to a fire. Spontaneous ignition occurs mainly in fully charged batteries;

Li-Pol Battery Storage and Carrying Bag (https://www.dx.com/)

Photograph of a Li-Pol battery after an internal short circuit

As a conclusion, along with significant advantages, the operation of electric UAVs has its own specifics and in the case of industrial use requires qualified maintenance.

According to the available experience, only electric power is used on UAVs weighing up to 40 kg, in larger devices - internal combustion engines.

UAVs with internal combustion engines

Such devices are deprived of a significant part disadvantages inherent in electric UAVs. Gasoline has a higher energy density - almost 100 times compared to the best commercially available batteries, which gives it a significant power advantage.

The cost of an internal combustion engine is relatively low, especially when it comes to professional equipment, but it is more difficult to maintain compared to electric motors, which in most cases do not require maintenance. It should be noted that its operation does not require specialized education, just like passenger cars. Most two-stroke gasoline engines used for UAVs use widely used brands of fuel and engine oils, meaning there is no dependence on a limited number of suppliers or the location of service centers. In fact, pre-flight preparation consists of a visual inspection of the fuel system for leaks and takes only a few minutes to refuel. Gasoline engines can be used at low temperatures without significant deterioration in performance. Gasoline does not freeze in winter, but you need to warm up the engine before the first start. An additional advantage of gasoline engines is good resistance to difficult weather conditions.

Structurally, the internal combustion engine contains a large number of mechanical and electrical components, among which there is an ignition system and a radiator for cooling, which significantly complicates the design of the power unit, its dimensions and mass. The electromagnetic field produced by the ignition system causes radio interference, which interferes with the reliable operation of control and information transmission channels. The problem is solved with the help of additional equipment and a special layout of the device, in which the radio equipment is placed no closer than 30 cm from the ignition circuits. On the one hand, this requires a significant increase in mass-dimensional characteristics, which has a positive effect on the wind resistance of UAVs, but increases inertia, which complicates control.

For aircraft UAVs, the use of internal combustion engines allows the creation of industrial devices for applying plant protection products. The Aerodrone company presents the DR-60 gasoline unmanned aircraft on the Ukrainian market.

Gasoline UAV DR-60 of the company Aerodrone, intended for processing fields (https://smartdrones.ua/products/dr-60)

Its empty take-off weight is 85 kg, and the payload can reach 60 kg. Since the working height of the flight is 5-10 meters, it allows to spray plant protection products with high precision in automatic mode, with a field treatment productivity of 75 ha/h. The pumps stop the liquid supply to the atomizer when the UAV is outside the required area.

The explosive nature of gasoline engines limits the options for landing vehicles with such engines. If landing on the fuselage or parachute is standard for electric aircraft, then DVZ uses a chassis, which, on the one hand, requires the availability of ground platforms, as well as a certain qualification of pilots.

Compared to electric motors, internal combustion engines have significantly worse speed control dynamics - there is always some delay when changing it. From this point of view, the classic maneuvering scheme for copter UAVs by changing the speed of rotation of the propellers has certain difficulties for diesel engines due to their inertia. In a number of models of copter UAVs with internal combustion engines, the helicopter scheme was borrowed from manned vehicles with an automatic propeller skew. The tilting machine, due to its complex and high-quality mechanics, determines the relatively high cost of such devices. Yes, the price of the Yamaha RMAX is about $100,000 (as of 2018), which is about an order of magnitude higher than the average cost of professional electric UAVs capable of delivering reagents.

The Australian company SOAPdrones offers its own version of a gasoline quadcopter. The Belias model is equipped with a two-stroke engine, and the design of the rotors is made in such a way that the pilot can control the pitch of the propellers. Special servomotors are used, with the help of which there is an instant change in the pitch of the propellers. This makes it possible to quickly change the direction of movement. The drone can stay in the air for about 3 hours, and the weight of the payload is 10 kilograms. One of the promising ways to create industrial unmanned copters, with the ease of control, power and flight duration inherent in electric models, as in vehicles with internal combustion engine, are hybrid models (combination of both engines).

Quadrocopter with gasoline internal combustion engine by the company SOAPdrones (http://www.soapdrones.com)

A good example of a hybrid is the Yeair UAV. The Yeair drone was created by Airstier for cargo delivery. It is capable of carrying a payload weighing up to 5 kilograms at a speed of up to 100 km/h. A tank containing one and a half liters of fuel is installed on board. The flight range in economy mode reaches 55 km. The use of an internal combustion engine and electric motors allows the drone to simultaneously have good traction and remain easy to control. A special two-stroke internal combustion engine with a volume of 10 cm3 and a power of more than 1.5 kW was developed for this model. Each of the four electric motors has a power of 600 W and is powered by a battery with a capacity of 1250 mAh. The batteries practically do not need recharging between flights, as they are fueled during the flight from the internal combustion engine. It is equipped with an autopilot function, can take off and land independently, and supports several flight modes. You can use a smartphone to control the copter. A feature of the model is the use of protective covers for propellers, which significantly improves safety conditions when using this model. The cost of the device as of 2018 is from 1,400 euros, which is competitive for the UAV market.

Yeair hybrid UAV by Airstier (https://yeair.de)

A domestic example of a hybrid UAV is the Cicada Hexacopter manufactured by UMT. Cicada is a UAV that has an internal combustion engine with a generator on board. Thanks to this, the device can stay in the air much longer than traditional electric copters powered by batteries (150 minutes with a weight of 3 kilograms).

The Cicada UAV was developed in Ukraine for military needs and, in addition to sensor equipment, is equipped with a special additional container. In the photo, it can be seen in front of the plastic fuel tank. In the civilian version, the container can be used, in particular, for the biological protection of plants, when trichograms are applied to the identified pest center. That is, in addition to monitoring, such UAVs can become an effective tool for direct crop management capable of differentiated intervention.

Hybrid UAV "Cykada" of the company "UMT"(https://umt.aero/cykada/)

In the end...

Therefore, battery solutions cannot be considered as having no alternative for creating small UAVs for agricultural purpose. Already today, you can get serial devices that run on gasoline, and the cost of annual maintenance of such devices is relatively insignificant, just like that of an ordinary passenger car. Accordingly, when choosing such devices, you can more widely use devices for monitoring and introducing reagents without worrying about the number and duration of flights in view of battery depreciation.

Professional agricultural drone UAS6-50 with DVZ from the Ukrainian company UkrAeroservice. Flight time 120 min., load capacity 130 kg, tank volume 50 l, high productivity (up to 20 ha/h), water consumption 6-7 l/ha. Dense processing of the plant due to a powerful turbulent flow (over 20 ms).