Simulating mechanical stress on a micro Unmanned Aerial Vehicle (UAV) body frame for selecting maintenance actions

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Procedia Manufacturing 00 (2017) 000–000

www.elsevier.com/locate/procedia

* Paulo Afonso. Tel.: +351 253 510 761; fax: +351 253 604 741

E-mail address: psafonso@dps.uminho.pt

Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017.

Manufacturing Engineering Society International Conference 2017, MESIC 2017, 28-30 June

2017, Vigo (Pontevedra), Spain

Costing models for capacity optimization in Industry 4.0: Trade-off

between used capacity and operational efficiency

A. Santana

a

_{, P. Afonso}

a,*

_{, A. Zanin}

b

_{, R. Wernke}

b

a_{University of Minho, 4800-058 Guimarães, Portugal} b_{Unochapecó, 89809-000 Chapecó, SC, Brazil}

Abstract

Under the concept of "Industry 4.0", production processes will be pushed to be increasingly interconnected, information based on a real time basis and, necessarily, much more efficient. In this context, capacity optimization goes beyond the traditional aim of capacity maximization, contributing also for organization’s profitability and value. Indeed, lean management and continuous improvement approaches suggest capacity optimization instead of maximization. The study of capacity optimization and costing models is an important research topic that deserves contributions from both the practical and theoretical perspectives. This paper presents and discusses a mathematical model for capacity management based on different costing models (ABC and TDABC). A generic model has been developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity optimization might hide operational inefficiency.

Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017.

Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency 1. Introduction

The cost of idle capacity is a fundamental information for companies and their management of extreme importance in modern production systems. In general, it is defined as unused capacity or production potential and can be measured in several ways: tons of production, available hours of manufacturing, etc. The management of the idle capacity

This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/)

Peer-review under responsibility of the scientific committee of the 7th International Conference on Through-life Engineering Services. 10.1016/j.promfg.2018.10.160

10.1016/j.promfg.2018.10.160 2351-9789

Peer-review under responsibility of the scientific committee of the 7th International Conference on Through-life Engineering Services. Available online at www.sciencedirect.com

ScienceDirect

* Corresponding author. Tel.: +31(0)534896609; fax: +31(0)53 489 2513

E-mail address: a.martinetti@utwente.nl

Peer-review under responsibility of the scientific committee of the 7th International Conference on Through-life Engineering Services.

7th International Conference on Through-life Engineering Services

Simulating mechanical stress on a micro Unmanned Aerial Vehicle

(UAV) body frame for selecting maintenance actions

Alberto Martinetti*

a

_{, Mihran Margaryan}

b

_{, Leo van Dongen}

a

a_{Department of Design, Production and Management, University of Twente, Drienerlolaan 5, Enschede 7522 NB, The Netherlands} b_{National Polytechnic University of Armenia, 0009 Yerevan, Armenia}

Abstract

The Unmanned Aerial Vehicles (UAVs) are part of our society. According to recent reports from retail research, sales of drones have been tripled over the last few years to about $200 million. The aim of the paper is to highlight the most relevant maintenance actions to take in order to properly maintain commercial UAVs. Describing the main components and characteristics of a UAV, the research wants to provide quick guidelines and suggestions for effectively maintaining micro UAVs. Based on a structure analysis and on the most common flying modes, firstly the paper simulates mechanical stresses on the UAV quadcopter body frames using Finite Element Methods (FEM). Secondly, it analyses the results highlighting the weak aspects of the structure in order to predict possible failure mechanisms and to create effective maintenance approach for guaranteeing high level of product reliability and availability. Finally, it discusses the results for ensuring constant mechanical properties and performance for the entire lifetime of the product.

Keywords:, UAV, Qaudcopter, Finite Element Methods (FEM), Mechanical Stress, Maintenance 1. Introduction

Nowadays along with development of modern technologies the design and development of Unmanned Aerial Vehicles (UAV) is getting more and more popular, generating technical and operational issues and safety concerns related to their application in non-segregated airspace [1]. Many companies are being involved in the design of UAVs. One of the most common UAV structures is the quad-rotor layout. The reason is that quad-rotor aerial

Available online at www.sciencedirect.com

ScienceDirect

* Corresponding author. Tel.: +31(0)534896609; fax: +31(0)53 489 2513

E-mail address: a.martinetti@utwente.nl

7th International Conference on Through-life Engineering Services

Simulating mechanical stress on a micro Unmanned Aerial Vehicle

(UAV) body frame for selecting maintenance actions

Alberto Martinetti*

a

_{, Mihran Margaryan}

b

_{, Leo van Dongen}

a

a_{Department of Design, Production and Management, University of Twente, Drienerlolaan 5, Enschede 7522 NB, The Netherlands} b_{National Polytechnic University of Armenia, 0009 Yerevan, Armenia}

Abstract

The Unmanned Aerial Vehicles (UAVs) are part of our society. According to recent reports from retail research, sales of drones have been tripled over the last few years to about $200 million. The aim of the paper is to highlight the most relevant maintenance actions to take in order to properly maintain commercial UAVs. Describing the main components and characteristics of a UAV, the research wants to provide quick guidelines and suggestions for effectively maintaining micro UAVs. Based on a structure analysis and on the most common flying modes, firstly the paper simulates mechanical stresses on the UAV quadcopter body frames using Finite Element Methods (FEM). Secondly, it analyses the results highlighting the weak aspects of the structure in order to predict possible failure mechanisms and to create effective maintenance approach for guaranteeing high level of product reliability and availability. Finally, it discusses the results for ensuring constant mechanical properties and performance for the entire lifetime of the product.

Keywords:, UAV, Qaudcopter, Finite Element Methods (FEM), Mechanical Stress, Maintenance 1. Introduction

Nowadays along with development of modern technologies the design and development of Unmanned Aerial Vehicles (UAV) is getting more and more popular, generating technical and operational issues and safety concerns related to their application in non-segregated airspace [1]. Many companies are being involved in the design of UAVs. One of the most common UAV structures is the quad-rotor layout. The reason is that quad-rotor aerial

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62 Alberto Martinetti et al. / Procedia Manufacturing 16 (2018) 61–66

2 Author name / Procedia Manufacturing 00 (2018) 000–000

vehicles are versatile, easy to construct, and they have vertical take-off and landing feature. Several studies [2-11] were carried out to determine mechanical stresses on different components of quadcopters. However, there are only few studies that establish a relation between mechanical stresses and necessary maintenance actions for UAVs.

Figure 1 summarized the steps performed during the research.

Fig. 1. Flow diagram of the methodology used

In essence, the classic structure of quadcopter is a frame with four arms and brush-less electric motors positioned at the end of each arm. The main parts of a copter are the frame, motors, propellers, electronic speed controller (ESC) and batteries. The angle between the four arms may vary, but the most common configurations adopt angular diameter of 90 and 120 degrees. Propellers on each rotor are creating vertical thrust. The propellers positioned on the same diagonal have same direction of rotation to prevent the spin of the copter in the air. Regulating the round per minute (RPM) of each motor is possible to change the moving direction, to hoover the copter in the air, etc.

The quadcopter movement can be divided in four different states (Fig.2):

 by creating an equal rotational speed of the propellers on each motor the quadcopter generates enough lift force to move upward/downward (a);

 by increasing the rotational speed of the propellers on motors on the same diagonal, the quadcopter produces yaw motion (b);

 by increasing the rotational speed of the propellers on motors on the same side –front motors or rear motors- the quadcopter achieves pitch (c) and roll motion (d);

Fig. 2. (a) Upward/Downward Motion (Z direction); (b) Yaw Motion; (c) Pitch Motion; (d) Roll Motion.

Author name / Procedia Manufacturing 00 (2018) 000–000 3

As mentioned, to investigate the possible maintenance actions suitable to maintain UAV systems it is necessary to analyse the mechanical stresses acting during the flight using FEM methods in order to create reliable maintenance approaches for guaranteeing a safe and available machine.

1.1. Quadcopter frame: dimensions and materials

The frame is an essential component of a quadcopter (and in general of an UAV), and perhaps the most affected by mechanical stresses generated during taking off, flying and landing [3, 5, 7-10]. The torque generated by the motor system, landing impacts and other external forces make the frame vital in terms of design and maintenance. Moreover, the frame must be at the same time as light (to increase the possible payload) and robust (to be able to face shocks) as possible for facing high vibrations [2, 4].

To analyse stresses and evaluate its performance, the authors chose for one of the most common and used frame designs according to the retrieved sources. After a short literature investigation [2-16] in order to find possible states that generate the most mechanical stresses on the quadcopter, as discussed, the analysis has been performed on a micro UAV; the UAV frame is 183 mm wide and it is symmetric about axis Z (Fig. 3).

Fig. 3. Geometrical shape (a) and 3D rendering (b) of the UAV.

Two main materials have been used during the mechanical simulations: carbon fiber and Acrylonitrile Butadiene Styrene (ABS) plus plastic. The mechanical properties of those materials are summarised in Table 1.

Table 1. Mechanical properties of carbon fiber and ABS plus plastic.

Carbon Fiber ABS plus plastic Young's modulus (GPa) 250 2,5 Ult. Tensile Strength 90° (MPa) 1000 110 Density (g/cm3₎ _1,60 _1,04

Poisson ratio 0,39 0,1

According to [7], the authors decided to choose carbon fiber and ABS plus plastic as materials to use in the simulation due to their extensive usage in the micro and small UAV market. Indeed, this choice is affected by their interesting mechanical properties in terms of strength and lightness that make them suitable for being used as body frame.

2. FEM Simulations

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Alberto Martinetti et al. / Procedia Manufacturing 16 (2018) 61–66 63

vehicles are versatile, easy to construct, and they have vertical take-off and landing feature. Several studies [2-11] were carried out to determine mechanical stresses on different components of quadcopters. However, there are only few studies that establish a relation between mechanical stresses and necessary maintenance actions for UAVs.

Figure 1 summarized the steps performed during the research.

Fig. 1. Flow diagram of the methodology used

In essence, the classic structure of quadcopter is a frame with four arms and brush-less electric motors positioned at the end of each arm. The main parts of a copter are the frame, motors, propellers, electronic speed controller (ESC) and batteries. The angle between the four arms may vary, but the most common configurations adopt angular diameter of 90 and 120 degrees. Propellers on each rotor are creating vertical thrust. The propellers positioned on the same diagonal have same direction of rotation to prevent the spin of the copter in the air. Regulating the round per minute (RPM) of each motor is possible to change the moving direction, to hoover the copter in the air, etc.

The quadcopter movement can be divided in four different states (Fig.2):

 by creating an equal rotational speed of the propellers on each motor the quadcopter generates enough lift force to move upward/downward (a);

 by increasing the rotational speed of the propellers on motors on the same diagonal, the quadcopter produces yaw motion (b);

 by increasing the rotational speed of the propellers on motors on the same side –front motors or rear motors- the quadcopter achieves pitch (c) and roll motion (d);

Fig. 2. (a) Upward/Downward Motion (Z direction); (b) Yaw Motion; (c) Pitch Motion; (d) Roll Motion.

As mentioned, to investigate the possible maintenance actions suitable to maintain UAV systems it is necessary to analyse the mechanical stresses acting during the flight using FEM methods in order to create reliable maintenance approaches for guaranteeing a safe and available machine.

1.1. Quadcopter frame: dimensions and materials

The frame is an essential component of a quadcopter (and in general of an UAV), and perhaps the most affected by mechanical stresses generated during taking off, flying and landing [3, 5, 7-10]. The torque generated by the motor system, landing impacts and other external forces make the frame vital in terms of design and maintenance. Moreover, the frame must be at the same time as light (to increase the possible payload) and robust (to be able to face shocks) as possible for facing high vibrations [2, 4].

To analyse stresses and evaluate its performance, the authors chose for one of the most common and used frame designs according to the retrieved sources. After a short literature investigation [2-16] in order to find possible states that generate the most mechanical stresses on the quadcopter, as discussed, the analysis has been performed on a micro UAV; the UAV frame is 183 mm wide and it is symmetric about axis Z (Fig. 3).

Fig. 3. Geometrical shape (a) and 3D rendering (b) of the UAV.

Two main materials have been used during the mechanical simulations: carbon fiber and Acrylonitrile Butadiene Styrene (ABS) plus plastic. The mechanical properties of those materials are summarised in Table 1.

Table 1. Mechanical properties of carbon fiber and ABS plus plastic.

Carbon Fiber ABS plus plastic Young's modulus (GPa) 250 2,5 Ult. Tensile Strength 90° (MPa) 1000 110 Density (g/cm3₎ _1,60 _1,04

Poisson ratio 0,39 0,1

According to [7], the authors decided to choose carbon fiber and ABS plus plastic as materials to use in the simulation due to their extensive usage in the micro and small UAV market. Indeed, this choice is affected by their interesting mechanical properties in terms of strength and lightness that make them suitable for being used as body frame.

2. FEM Simulations

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2.1. Mesh Definition and flying time moments

The main variables introduced in the creation of the mesh for simulating the states of the quadcopter are summarised in Table 2

Table 2. Mesh Properties

Property Value Mesh Representation Mesh type Solid Mesh

Jacobian points 4 points Max Element Size 0.0112011 m Min Element Size 0.00224022 m

Total nodes 90553 Total elements 51865 Maximum Aspect Ratio 47,189 with Aspect Ratio > 10 1,26

Quadcopter flying time has been divided in two main moments characterised by different values of mechanical stress; lifting/hovering and landing on rigid surface. Analysing UAV life cycle, it has been discovered that around 80% of the quadcopter flight time will be in lifting and hovering mode, and around 20% in downward motion for landing from a defined height. However, after a thorough discussion with UAV manufactures and an exhaustive literature analysis, it appeared clear that most of the mechanical stresses arise during the landing moments. Consequently, the focus of the research has been oriented to understand this particular situation.

For both materials (carbon fiber and ABS plus plastic) Von Mises stresses and displacements were tracked during the simulation in order to find under which conditions the body frame of the micro UAV would have been heavily stressed.

2.2. Landing simulations

Fig. 4. Simulation results for landing: (a) Von Mises stresses for ABS plus plastic (Max 35 MPa); (b) Von Mises stresses for carbon fiber (Max 38 MPa).

Fig. 5. Simulation results for landing: (a) displacements for ABS plus plastic (Max 3,2 mm); (b) displacements for carbon fiber (Max 6,5 mm).

After the simulations (run both with SolidWorks Simulation Tool and Abacus), it appears clear that the most vulnerable part of the quadcopter in terms of mechanical forces is the arm, and, specifically, the connection point between the arm and the body frame.

3. Discussion

Based on FEM results, it is possible to highlight that the highest stresses in landing mode occur, as said, on the arms near to frame center.

Fig. 4 shows the landing test results and how stresses are expanding after contact with ground. Simulation test pinpoints that while for ABS plus plastic frame the landing drop from a height of 1 meter (with a velocity value most likely of 4.43 m/s) on a rigid surface cannot cause cracks, for the carbon fiber, the generated stresses of 38 MPa (beyond the tensile strength of 27 MPa [7]) can bring the material to plastic deformations and cracks. Fig. 5 shows that due to different Young’s modulus the calculated stresses generate possible displacements in the surrounding of the connection between the arms and the body frame, respectively of 3,2 and 6,5 mm for ABS plus plastic and carbon fiber.

These considerations lead to agree that there is a consistent difference between usage of carbon fiber and ABS plus plastic during hard landing events in terms of mechanical strength. Due to its mechanical properties, carbon fiber is a stronger material with a high Young modulus, which is generating lower elastic strain and fragile behavior beyond the tensile strength. On the other hand, ABS plus plastic has higher tensile strength, which can reduce probability of sudden cracks in a collision with rigid surfaces during emergency landing.

According to the presented results, continuous maintenance and special monitoring actions should be taken into account by micro UAV users and operators for:

_{Checking after every landing on rigid surface bolt and connectors between frame and arms (specifically} looking for possible cracks), widely considered as critical to flight functional failure;

 Designing or equipping rubber nozzles or shock absorption devices on the landing gears to reduce stresses and vibrations from the ground.

4. Conclusions

Using SolidWorks Simulation Tools and Abacus for FEM analysis, an estimation of the mechanical stresses generated by impacts on simple quadcopter frame has been studied. The analysis was conducted for two types of materials: carbon fiber and ABS plus plastic. Von Mises stresses on quadcopter arms have been highlighted. Those stresses represent a critical factor (in relation with tensile strength of ABS plus plastic and carbon fiber 3D printing material) creating possible plastic deformations and cracks in case of drop landing on rigid surface. The study finally gave suggestions for correcting checking and maintaining the quadcopter frame.

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Alberto Martinetti et al. / Procedia Manufacturing 16 (2018) 61–66 65

2.1. Mesh Definition and flying time moments

The main variables introduced in the creation of the mesh for simulating the states of the quadcopter are summarised in Table 2

Table 2. Mesh Properties

Property Value Mesh Representation Mesh type Solid Mesh

Jacobian points 4 points Max Element Size 0.0112011 m Min Element Size 0.00224022 m

Total nodes 90553 Total elements 51865 Maximum Aspect Ratio 47,189 with Aspect Ratio > 10 1,26

Quadcopter flying time has been divided in two main moments characterised by different values of mechanical stress; lifting/hovering and landing on rigid surface. Analysing UAV life cycle, it has been discovered that around 80% of the quadcopter flight time will be in lifting and hovering mode, and around 20% in downward motion for landing from a defined height. However, after a thorough discussion with UAV manufactures and an exhaustive literature analysis, it appeared clear that most of the mechanical stresses arise during the landing moments. Consequently, the focus of the research has been oriented to understand this particular situation.

For both materials (carbon fiber and ABS plus plastic) Von Mises stresses and displacements were tracked during the simulation in order to find under which conditions the body frame of the micro UAV would have been heavily stressed.

2.2. Landing simulations

Fig. 4. Simulation results for landing: (a) Von Mises stresses for ABS plus plastic (Max 35 MPa); (b) Von Mises stresses for carbon fiber (Max 38 MPa).

Fig. 5. Simulation results for landing: (a) displacements for ABS plus plastic (Max 3,2 mm); (b) displacements for carbon fiber (Max 6,5 mm).

After the simulations (run both with SolidWorks Simulation Tool and Abacus), it appears clear that the most vulnerable part of the quadcopter in terms of mechanical forces is the arm, and, specifically, the connection point between the arm and the body frame.

3. Discussion

Based on FEM results, it is possible to highlight that the highest stresses in landing mode occur, as said, on the arms near to frame center.

Fig. 4 shows the landing test results and how stresses are expanding after contact with ground. Simulation test pinpoints that while for ABS plus plastic frame the landing drop from a height of 1 meter (with a velocity value most likely of 4.43 m/s) on a rigid surface cannot cause cracks, for the carbon fiber, the generated stresses of 38 MPa (beyond the tensile strength of 27 MPa [7]) can bring the material to plastic deformations and cracks. Fig. 5 shows that due to different Young’s modulus the calculated stresses generate possible displacements in the surrounding of the connection between the arms and the body frame, respectively of 3,2 and 6,5 mm for ABS plus plastic and carbon fiber.

These considerations lead to agree that there is a consistent difference between usage of carbon fiber and ABS plus plastic during hard landing events in terms of mechanical strength. Due to its mechanical properties, carbon fiber is a stronger material with a high Young modulus, which is generating lower elastic strain and fragile behavior beyond the tensile strength. On the other hand, ABS plus plastic has higher tensile strength, which can reduce probability of sudden cracks in a collision with rigid surfaces during emergency landing.

According to the presented results, continuous maintenance and special monitoring actions should be taken into account by micro UAV users and operators for:

_{Checking after every landing on rigid surface bolt and connectors between frame and arms (specifically} looking for possible cracks), widely considered as critical to flight functional failure;

 Designing or equipping rubber nozzles or shock absorption devices on the landing gears to reduce stresses and vibrations from the ground.

4. Conclusions

Using SolidWorks Simulation Tools and Abacus for FEM analysis, an estimation of the mechanical stresses generated by impacts on simple quadcopter frame has been studied. The analysis was conducted for two types of materials: carbon fiber and ABS plus plastic. Von Mises stresses on quadcopter arms have been highlighted. Those stresses represent a critical factor (in relation with tensile strength of ABS plus plastic and carbon fiber 3D printing material) creating possible plastic deformations and cracks in case of drop landing on rigid surface. The study finally gave suggestions for correcting checking and maintaining the quadcopter frame.

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Further researches have to be conducted to move from a simulation contest to a real condition monitoring system for micro UAVs in order to prove the results obtained with FEM methods. Furthermore, it has to be evaluated the financial feasibility to equip, at least, micro UAVs deployed in urban non-segregated area with specific monitoring sensors to avoid functional collapses during flights.

Acknowledgements

The authors acknowledge the European Union to make a first exchange mobility actions between a Dutch and an Armenian University possible, granting the ERASMUS+ project 2016-2-NL01-KA107-034906 to the University of Twente.

References

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