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Design and Analysis of a Variable Pitch UAV Propeller
No full textThe interest in UAV technology and applications has skyrocketed in recent years establishing a growing trend in research and development in the field. One of the main areas of interest in UAVs is the energy efficiency of the aircraft with better energy efficiency meaning longer flight durations or larger payload capacity. Hence, a lot of effort is invested in the aerodynamic and structural optimization of UAVs, however, the principal contributor to energy savings would be the increase in efficiency of the propulsion system. Most of UAVs today employ constant pitch propellers driven by an electric or an internal combustion engine for thrust generation.
Constant pitch propellers have become a standard due to their simplicity and reliability, however, they have limitations that can be exceeded by the use of variable pitch propellers. This is especially emphasized for fixed wing UAVs, VTOL (Vertical Take-Off Land) and UAVs requiring higher maneuverability where different operational ranges and high agility are expected. Two different methods were considered for the design of the variable pitch propeller: active and passive variable pitch. In the active method, the pitch is controlled via a servo actuator, providing the possibility of optimal matching with the propeller driver and advanced control in different operating regimes but with the introduction of higher weight and additional complexity in the system.
In the passive variable pitch propeller, there is no need for an actuation system since the propeller is designed in such a way as to use the aerodynamic moments as a mean to change the pitch. This reduces the operational complexity of the system, however, it increases the design complexity since more complex aero-structural optimization methods need to be utilized. By considering the design and analysis of both variable pitch methods, a better understanding of the design approach and the advantages and disadvantages of each is established. The analysis of obtained results provides an insight into the applicability of each system as well
COMBUSTION OF LOW CALORIFIC GASEOUS FUELS AS ENERGY- EFFICIENT TECHNOLOGY
No full textThe reserves of fossil fuels will eventually disappear or be reduced to such quantities that they will no longer be able to cover the sectors of final energy consumption (industry, transport, households, trade). For this reason, technologies are constantly being developed to make efficient and effective use of alternative and renewable energy sources, as well as other types of fuels that have potential but require special technologies to be used. In this sense, the combustion of gaseous fuels with a low calorific value, such as gasses produced as a by-product of the anaerobic decomposition of organic material (biogas or landfill gas) or technological gasses produced in certain production processes (blast furnace gas, gas from cupola furnaces, refinery gas, etc.) or gasses from mines, represents a significant potential that can be used as an alternative fuel. Since the middle of the last century, consideration has been given to the development of technologies that allow gasses that are considered waste gasses to be burned efficiently and safely. The disadvantage of gasses produced by various technological processes is the composition's instability and, thus, the variability of heat output. If the heat output of a gaseous fuel is extremely low, this fuel cannot burn stably in burners of classic design, as the flame is then usually "blown off" due to the low combustion rate of this fuel. This article presents the principle of burning gaseous fuels with extremely low heat output in a porous, inert medium. This technology enables efficient heat recovery between the combustion products and the fuel mixture (gaseous fuel + air), significantly extending the range of stable combustion of low calorific value gaseous fuels in various operating modes, which is impossible with classic designburners. This technology is efficient and enables minimal concentrations of components such asnitrogen oxides due to the uniform temperature distribution in the porous inert media of the burner
THE SIMULATION OF MACHINING FREE-FORM SURFACES ON A MILLING MACHINE WITH A HORIZONTAL ROTARY AXIS
No full textU okviru ovog rada je prikazana je simulacija procesa obrade skulptorskih površina na mašinama sa horizontalnom obrtnom osom. Predmet ovog rada je četvoroosna glodalica LOLA VMC4 iz klase mašina MultiProDesk, strukture A'Y'OXZ. Za potrebe programiranja ove mašine, razvijen je postprocesor baziran na jednačinama inverzne kinematike. Deo za koji se definiše obrada je deo kompleksne geometrije koji služi kako bi se verifikovala obrada složenih površina na razmatranoj mašini, kao i mogućnost softvera za postprocesiranje za proračun putanje alata pri obradi kompleksnih površina. Za generisanje početne putanje alata korišćen je CAD/CAM softverski paket PTC Creo Parametric. Obrada dela je izvršena u okviru virtuelnog tehnološkog okruženja u softverskom paketu Vericut.This paper presents a simulation of machining a part with a complex, freeform surface on a milling machine with a horizontal rotary axis. The machine on which this simulation is based is the four-axis milling machine with a horizontal rotary axis and the kinematic structure A'Y'OXZ, LOLA VMC4 from the machine class MultiProDesk. A custom-made postprocessor with integrated machine kinematics was used to generate the toolpath for this machine. The test piece was chosen with specific free-form surfaces intended to evaluate the performance of the machining system and
the postprocessing program under various operational scenarios The initial toolpath was defined using the CAD/CAM software PTC Creo Parametric. The virtual machining of the said part was done in a virtual machining environment using the Vericut software.Za izdavača: Dekan, dr Vladimir Popović, red. prof.
Tehnički urednici:
Prof. dr Saša Živanović, Doc. dr Miloš Pjević , Doc. dr Dušan Nedeljković, Lazar Matijašević, mast. inž. maš.
Štampa: Planeta prin
Average Velocity Field Downstream Distributions in the Free Turbulent Swirling Jet Generated by the Axial Fan Impeller with Twisted Blades
No full textThis is an extended abstract published in the Booklet of abstracts of the “2nd International Conference on Mathematical Modelling in Mechanics and Engineering”, Belgrade, 12.-14. September 2024
New results on finite-time stability of nonlinear fractional-order multi-time delay systems: Delayed Gronwall inequality approach
No full textIn this paper, finite time stability (FTS) analysis of fractional-order nonlinear multi-time delay
systems is studied. By use of a fractional Gronwall inequality with time delay, new FTS criteria for proposed systems are established. Two numerical examples are given to illustrate the effectiveness of the obtained theoretical results
Research of the draw gear and screw coupling failures of rail vehicles
No full textThe reliability and safety of railway vehicles are significantly influenced by the performance of their draw gear systems and screw coupling elements, which are essential for maintaining stable connections between rail cars. The draw gear component failures, especially coupling links, have raised concerns about their ability to withstand operational loads. Although failures in draw gear components are relatively rare, they can lead to severe consequences for both rail vehicles and infrastructure when they do occur. These failures often result in fractures and breakages, Fig. 1. This research analyses different cases of failure and proposes a detailed study aimed at understanding the underlying causes of draw gear failures. The planned methodology involves a multifaceted approach, combining field data collection, finite element analysis (FEA), and laboratory testing. Initially, field inspections will be conducted to identify common failure points and document the operational conditions under which these components fail. Following this, a forensic approach will be applied, selected samples of failed components will undergo laboratory analysis to assess material properties and identify signs of fatigue or other forms of degradation.
Finite element analysis will be used to simulate operational stresses and predict potential failure points under various loading scenarios. A numerical model will be developed based on experimental research and will be used for more detailed analysis of different load cases and failures of different parts of draw gear and screw coupling assembly. By systematically investigating these issues, this research aims to lay the groundwork for improved draw gear systems that contribute to safer and more dependable railway operations
POSITION CONTROL OF ROBOT MANIPULATOR USING OPTIMAL PID CONTROLLER
No full textDue to increasing demands of productivity and accuracy in today’s robotic applications, scientific community is under constant pressure to deliver more and more efficient control strategies. This is a challenging task considering the nonlinear nature of robotic structure, which is often accompanied with some degree of uncertainty of model parameters. Most of robots work in industrial surrounding and again, most of them are controlled through standard
proportional-integral-derivative (PID) algorithms. The reason for this lies in the relatively
simple structure of PID controller which can be easily implemented in practice, and the fact it
can achieve desired position without steady-state error. However, tuning control gains of the regulator in order to guarantee transient performance is not an easy task, and deserves further research.
In our case, a robotic manipulator is actuated by electric motors with reduction gears of high ratios. The presence of gears tends to linearize system dynamics and nonlinear coupling effects between the joints can be neglected. This way, each robotic joint can be controlled independently by using a linear model of DC motor. In this paper we are revisiting the PID control of robot manipulators in the light of modern control theory, which assumes an inevitable trade-off between the performance and robustness of the closed loop system.
Efficient load disturbance response is of primary concern when designing a control system. Also, system should be robust with respect to model uncertainties, i.e. it should possess a certain degree of relative stability when it comes to modeling errors. It is also important to have a good response to set point changes. More specifically, in this paper we are interested in achieving a set point response without overshoot, so that robot manipulator can approach the manipulated object without damaging it. Last but not least, the closed loop system should be able to cope with negative effects of measurement noise, and this is accomplished by incorporating derivative filter in the control design procedure. All the above specifications are necessary to be captured in an appropriate mathematical form. For example, the efficiency of load disturbance rejection can be expressed in terms of the integrated absolute error (IAE) due to a unit step at the process (motor) input. Sensitivity to modeling errors can be evaluated by the largest value of the sensitivity function. A good set
point response without overshoot can be achieved by placing dominant poles of the closed loop system in predetermined positions on the negative real axis of the complex s plane. Finally, noise attenuation can be adequately addressed by limiting the largest value of the sensitivity to measurement noise. So, having in mind everything stated above, the design specifications can
be formulated as the following optimization problem: find controller parameters that maximize proportional gain subject to the constraints on robustness and sensitivity to measurement noise. In order to successfully cope with the given problem, it is required to solve four nonlinear algebraic equations. Some preliminary results of position control of a robot manipulator with
three degrees of freedom are given in the figure below. Controller gains are determined based on the well-defined constrained optimization problem. Time response of every joint due to the unit step change of the set point value is shown. It can be seen that a zero overshoot set point response is obtained, which under a given robustness and performance constraints provides
optimal load disturbance rejection
Утицај предтретмана и метода сушења на садржај елемената у црвеној зачинској паприци (Capsicum Annuum)
No full textCrvena paprika koja se koristi u proizvodnji začinske paprike je izvor značajnih vitamina i esencijalnih elemenata. Sušenjem paprike dobijaju se različiti proizvodi (sušena paprika, sušene ljuspice ili mlevena paprika). Pre samog procesa sušenja često se primjenjuju fizički i hemijski predtretmani, kao i njihove kombinacije. Cilj predtretmana je postizanje boljeg kvaliteta proizvoda, skraćivanje vremena sušenja, a samim tim i smanjenje troškova proizvodnje. Iako predtretmani imaju brojne prednosti, tokom ovih procesa često dolazi do gubitka određenih nutrijenata. Cilj ovog istraživanja je bio da se utvrdi kao različiti hemijski predtretmani, ultrazvuk i metode sušenja utiču na sadržaj elemenata u začinskoj paprici. Eksperimenti su postavljeni korištenjem frakcionog faktorijskog eksperimentalnog dizajna. Parametri su bili temperatura predtretmana (20, 50, 80 ℃), vreme (1, 3, 5 min), ultrazvuk (off/on), 0,25% aditiv (limunska kiselina, K2S2O5, limunska kiselina/ K2S2O5), pH vrednost (3, 6.5, 10), masa tretiranog uzorka (30, 100 i 170 g u 1L rastvora za predtretman) i metoda sušenja (sušenje u tunelskoj sušati i liofilizacija). Najveći uticaj na sadržaj elemenata imala je metoda sušenja. Klasično sušenje je doprinelo boljem očuvanju K, Ca, Na, P, Fe, Zn, Cu, Co i Cr. Vrsta primenjenog aditiva je uticala na sadržaj Fe, K, Na i Mo. Veći sadržaj Fe, K i Mo postiže se sa K2S2O5, dok je smeša limunska kiselina/K2S2O5 (NaOH je upotrebljen za regulaciju pH) bila efikasnija za veći sadržaj Na. Više temperature dovele su do većih gubitaka svih elemenata, posebno Cr, K i P. Duže vreme predtretmana je doprinelo višem izluživanju većine elemenata. Veća masa tretiranog uzorka obezbedila je manji gubitak većine elemenata, posebno Cr, Co i Fe.The red peppers used in the production of paprika are a source of valuable vitamins and essential elements. By drying peppers, various products are obtained (dried peppers, dried flakes, or ground paprika). Before the actual drying process, physical and chemical pre-treatments, as well as their combinations, are frequently applied. The goal of pre-treatment is to achieve better product quality, shorten drying time, and consequently reduce production costs. Although pre-treatments have numerous advantages, during these processes, there is often a loss of certain nutrients. This study aimed to investigate how different chemical pre-treatments, ultrasound, and drying methods influence element content in red peppers. The experiments were set using fractional factorial experimental design. Parameters were pre-treatment temperature (20, 50, 80 ℃), time (1, 3, 5 min), ultrasound (off/on), 0.25% additive (citric acid, K2S2O5, citric acid/K2S2O5), pH value (3, 6.5, 10), mass of treated sample (30, 100 and 170 g in 1L pre-treatment solution) and drying method (hot air drying and freeze drying). The drying method had the greatest impact on the elemental content. Classic drying was better for the preservation of K, Ca, Na, P, Fe, Zn, Cu, Co, and Cr. The type of applied additive influenced the content of Fe, K, Na, and Mo. Higher Fe, K, and Mo content is achieved with K2S2O5, while the mixture of citric acid/K2S2O5 (NaOH used for pH regulation) was more effective for higher Na content. Higher temperatures led to greater losses of all elements, especially Cr, K, and P. Longer pre-treatment resulted in increased leaching of most elements. Additionally, a larger sample mass ensured better preservation of most elements, particularly Cr, Co, and Fe
OPTIMIZATION OF STANLEY PLUM DRYING PROCESS USING SOLID FUEL BATCH DRYER WITH TRAYS
No full textDrying fruit is a traditional preservation method that has been used for thousands of years. This technique allows for long-term storage of fruit, keeping it fresh and usable under ordinary storage conditions. In this research, the drying process of the selected variety of Stanley plums was analyzed in a batch dryer with trays using solid fuel. The research aimed to calculate the optimal air temperatures at the inlet and outlet of the heater, as well as the temperature at the outlet of the dryer itself. The constructed dryer has double trays, allowing drying capacities from 1550 kg to 3100 kg of Stanley plums. The main focus was on achieving efficient drying with minimal heat loss. The selection of basic equipment, such as a hot water boiler, circulation pump, heat exchanger, and fan, was tailored to the specificities of the dryer to maximize energy efficiency. The heat exchanger utilizes compact lamellar Cu-Al technology, while the fan is equipped with a filter section for a cleaner drying process. The use of solid fuels such as biomass brings significant economic benefits, reducing the costs of dried products. However, manual labor and detailed equipment inspection are necessary for each drying cycle, which is the main drawback of this method