PRE2018 3 Group17

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Group members

Group Members	Student nr.
Diederik Geertsen	1256521
Cornelis Peter Hiemstra	0958497
Joël Peeters	0939193
Benn Proper	0959190
Laila Zouhair	1260529

All Robot Ideas

Below are all ideas that were thought up for the robots everywhere project, they are ordered by the order in which they were thought up. The final idea that was elaborated on was the disaster observation drones.

• Tilting 3D printer to eliminate support material
• Breakfast bot
• Robot to remove microplastics from water
• Clothes folding robot
• Building guidance robot
• Disaster observation drones

Problem Description

The employment of drones in disaster areas is an obvious application which can aid in the fast gathering of information concerning the situation and locating survivors. In recent years, extensive research has been conducted into using drones for this cause. The focus of these studies were, among other topics:

• Computer vision for recognizing survivors [Rivera, Villalobos, Monje, Mari ̃nas OppusRivera .2016].
• Communication among drones [Saha .2018].
• Optimal routing [Mersheeva .2015].

Past research has created a good foundation towards the development of an actual product, however a practical issue that still remains is the limited operation time of these drones. This problem is especially challenging because when applied to disaster areas, a solution must be independent of existing infrastructure which is likely to be damaged. The aim of this project is to develop a solution to extend the operation time of aerial drones, which are very suitable for disaster area monitoring due to their independence of ground infrastructure, but have an especially limited operation time compared to e.g. ground vehicles. Drones can be applied to many kinds of disaster, for the scope of this project we will focus specifically on earthquakes.

Users

Our users require a way to quickly get information about a large disaster. This would mean that we must automate this information gathering on different scales. For example, when there is a very large earthquake. The emergency services have no good way to get to the disaster, they do no immediately know the scale of the disaster and they do not know which parts really require their attention. This all costs a lot of time, which can be greatly reduced. To get all this information really quick, drones are often used. These are manual controlled. This means that they can only gather information at as many places as they have people available. If we can make the robots independent and automated, while communicating with each other and giving important information to the users, this process would become much faster. A major problem in the automation of these search processes is the limited operation time of the drones used. This is a problem for the operators, as it severely limits the range of the drone system, and requires the users to manually make sure drones are recharged. A system that allows these drones to be autonomously brought back into operation can save a lot of time which the people can save elsewhere.

Who are the stakeholders?

There are different stakeholders with different roles in this project involved. They would all take advantage of a solution we provide to their problem. The three stakeholders are Users, Society and Enterprise. We will describe per category why this particular stakeholder is involved with our problem and how our project will contribute to a solution for their problems.

Users

The biggest group of stakeholders are the users, which consists of civilians, government organizations, and private organizations or non-government organizations. These would all take advantage of the solution we provide, in particular, those which are our intended end-user, i.e. the groups which will be involved during a natural disaster. We shall describe how these groups use our solution.

• Government Organizations

Organizations formed by the government to combat natural disasters will take the most advantage of our solution. When a natural disaster will take place on large scale, emergency services or other organizations want to gather information as quick as possible. With our solution, this will be possible over a wider area and with less involvement of personnel necessary.

• Civilians

Civilians struck by natural disasters benefit from our solution. The quicker help comes, the smaller problems arising for civilians will be. This counts for medical care, but also search and rescue and preventing loss of private property.

• Private organizations/non-government organizations

Organizations could also use our solution to work for different purposes. For example as security of property. Next, our solution to the described problem could be used as a good solution for similar problems as government organizations are describing.

Society

The society as a whole would benefit greatly from our solution. Our solution is relatively cheap and would be a great addition or replacement for existing solutions. Our solution would contribute to prevent loss of life, loss of property and would help organizations greatly. Next to that, since it is not an expensive solution, it would be much more cost effective than existing solutions such as the manually controlled drone.

Enterprise

The enterprise would also benefit from our solution. Firstly, the usage of drones would be far greater than before. This would mean that enterprises could cash in into our solutions.

Approach

There are four functions that the system will need to be able to perform, each of which is listed below:

• 1. Drones need to be able to reach the system. This may include something that allows the system to move around in a potentially chaotic and hard to traverse disaster area.

• 2. The system needs to be able to either recharge or swap the batteries of a drone so that the drone can continue its search afterwards.

• 3. The system needs to keep working long enough to be able to increase the effectiveness of the swarm before having to be recharged/refuelled itself.

• 4. The system needs to be able to communicate their location to drones and receive information on battery levels and location of drones.

In order to keep a narrow and well-defined problem, a general solution for the design will be chosen based on the benefits and drawbacks of some chosen concepts. This general design will then be used to choose an optimal design for each function in order. That means, for instance, that the design for function 4 will be chosen to be chosen to work best combined with the designs for the other three already chosen functions. Once each function has been chosen, they will be worked out in greater detail, this time making sure all designs are fully compatible. In short, only one design option will be researched at a certain time, and the design of any option will be constrained by choices already made for the other options.

RPCs

The RPCs for the system will be defined as follows:

Requirements

• Can swap batteries of the drones.
• Needs to be mobile.
• 25 km system range extension.
• Fully autonomous positioning and task execution.
• Return to base after task completion or when in need to service.
• Max. one minute battery swap.
• Can service 10 drones in its operation cycle (about 2 kg payload).
• Can service at least 1 drone at a time.

Preferences

• The system should still function in "bad" weather conditions (rain, wind up to a certain speed).
• The vehicle should be safe for human interaction.
• Low manufacturing and operation costs.
• The system should support a manual override.
• The system should be easy employable.

Constraints

• The system needs to function independently of available infrastructure.
• Can be operational on rough terrain.

Phase 1: Traversal of environment

Here the concepts will be discussed on how the drone moves around the environment, a choice will also be made afterwards.

Concepts

In this section, several different concepts for the transport of the system will be discussed. The benefits and drawbacks of each concept in the given environment will be discussed so a well-informed choice can be made.

Ground vehicle

A ground drone that can move around the disaster area that can serve as a charging platform for drones. This comes with some up- and downsides. First of all the drone could move a bigger payload with the same amount of fuel when compared to a flying drone. It will also be less sensitive to weather conditions than an airborne drone. It does, however, have limited speed and movement depending on the state of the terrain.

This last point could prove troublesome after an earthquake. After such a disaster the rubble will prevent proper movement for smaller drones, larger drones could, however, have less trouble with this but can also need more space to move through. It is clear that a ground vehicle cannot follow the searching drones as well as it could if it were flying, this further complicates its function as a mobile charging platform.

It also operates in a more dangerous environment compared to aerial drones. It must be mindful of unstable areas and if they hit an obstacle. It could, for example, cause some walls or a building to collapse. This then further endangers the lives of both the drone and survivors of the disaster.

Ground drones do need to be given more attention to how they move around a disaster area. In general, some path planning has to be done from beginning to end. Considering that the area will be difficult to traverse and certain locations are destroyed, the drone will need some type of advanced vision analysis [2][4][5]. This would either require a constant connection to a computation server, or a considerable amount of processing power to achieve this.

Balloon UAV

A balloon is added to a quadcopter, this balloon's lift partially cancels the weight of the quadcopter. This allows it to stay in the air for a considerably longer amount of time. The drone also does not have any problem with broken infrastructure due to the ability to fly.

This idea also has some downsides. First of all it would be difficult to keep stable in windy areas [6]. It would, therefore, have difficulty with charging the drones using any method due to the movement of the two drones. The size of the balloon would also have to be large to cancel the weight enough for it to fly considerably longer [7]. This is therefore not an ideal solution for the main drone design.

Continuous flying large drone

A drone that can fly continuously without having to land. This drone uses up a lot of energy especially considering that drones usually only have a battery life of about 30 minutes during operation. The obvious solution to this would be by using alternative fuel sources or additional batteries. Alternative fuel sources could include diesel or petrol, which have a considerably higher energy density and therefore extend its range, even considering the lower efficiency of combustion engines. Such a large drone is however very costly to manufacture and uses a lot of resources during operation, which may be limited in a disaster scenario. More power cells could also be an improvement, but this would also increase the total weight of the drone and therefore decrease the flight time. This drone also suffers from other problems that aerial drones usually face. Its relatively large size and mass do make it more stable in daunting weather conditions [6].

Hopping drone

Drones fly ahead and land at certain spots creating an infrastructure for the observation drones to have their battery swapped and extend operation time. A certain amount of drones can form a grid of charging stations. The observation drones can fly to the nearest station when in need of service. Because the drone lands on its designated location, it can stay relatively simple and cheap compared to a drone that has to stay airborne for a longer time. The system itself takes the form of a larger drone, which should be large enough to carry what is needed to recharge or swap the batteries of the other drones.

Large drone with chain of recharging drones

Fuel powered large drone. Can fly for around 1 hour without being refuelled or recharged itself. With a chain of refuelling drones, the large drone will be refuelled/recharged. The hopping drone is a concept that combines the best of both worlds in regards to aerial and terrestrial drones. It solves the traversal of the environmental problem in the ground drone, but also solves the stability problem of aerial drones, as it can land and charge the drones on the ground.

Choosing

Out of the ideas and concepts discussed, the hopping drone seems to be the best solution. it's hard to keep any of the other designs stable while keeping them in the air and resting on the ground instead of flying in the air allows the system to last much longer. Drones will have an easier time reaching a still target. The hopping drone is a concept that combines the best of both worlds in regards to aerial and terrestrial drones. It solves the traversal of the environment problem in the ground drone, but also solves the stability problem of aerial drones, as it can land and charge the drones on the ground.

Phase 2: Increasing flight time of drones

For the drone to succeed in its function, it would need a method to extend the flight time of the drone. This can be done in a multitude of ways, below several concepts are discussed and a decision is made.

Concepts

Below several concepts for increasing the lifetime of the search drones are discussed. The pros and cons will both be discussed so an informed decision can be made.

Changing Batteries

Changing the batteries of a drone is one of the quickest method to extend the drone's flight time. In this case, the downtime of the drone only consists of the time it takes to change the battery. This method also has zero power loss, which is a considerable advantage compared to charging the battery in the field.

It does, however, have a few drawbacks. Relative positioning of the drone has to be far more precise compared to some charging methods. Multiple solutions have already been made for this problem, the solutions range between a 50% success rate to an almost 100% success rate [9][10][11][12]. These systems are quite similar, however the system of [10] has the biggest success rate and will therefor be the basis for our design.

Refuelling

Refuelling a drone using liquid fuel has its benefits. It is more energy dense, therefore, carrying fuel containing an equal amount of energy as batteries will be lighter. It is also already possible to refuel UAVs in the air, which means that refuelling on the ground would be easier to implement [13]. The major drawback to this idea is the fact that the drones would have to be fuel based drones. These drones usually aren't common and are more difficult to develop, complicating the rescue drone design. Also, the fact that fuel-based vehicles and machines are slowly being phased out, makes an alternative solution more desirable.

Charging

Recharging the drone's batteries is the most obvious choice to increase the flight time of the drones. This would require a way for the drone to be connected to the charging station. This can be approached by using the feet of the drone as charging points by making that and the contact point with the main drone conductive [14], it can also be approached by using wireless charging, a relatively new method of charging electronic equipment[8]. Both of these are relatively easy to implement, but they both have drawbacks. Both of these options suffer from power loss while transferring energy from one drone to the other, wireless charging wireless charging is in this case worse than wired with an efficiency of around 65% [8]. The tradeoff made here is the ease of implementation and the efficiency of the transferal of power.

Replacing Drones

This concept is an extension of the charging concept. This system will consist of a certain amount of fully charged drones that are transported using the main drone. When a currently flying drone is running low on battery, the drone can fly towards one of these 'Hub' drones and essentially get replaced by one of the dormant drones. This new drone will then continue with where the other drone left off in regards to its search for survivors. The drone that was just replaced can then be charged in the meantime.

This idea has the smallest downtime, but will also require a lot of additional weight to be carried by the main drone. The charging time of the drones is also of importance, it should be able to finish charging a drone in the same time it takes for a drone to need a recharge. This means that an additional charging solution needs to be devised for it to function properly. The additional dormant drones are however also a waste of resources in a way, they could be used for searching instead of lying around not doing anything.

Choosing

The final choice for this design is to use the battery replacement concept. The main drawback for these is the added weight and the chance of failing to properly replace a battery. As can be seen in the provided sources it is possible for a 100% success rate with replacement which means that the second point is not a big problem anymore. The added weight is already partially solved by using the hopping drone concept from before. This means that even less power is needed for operation as it won't be flying for long periods of time. In the next section the overal longevity of the design is improved. This means that the problem will be dealt with after choosing the optimal design for that.

Phase 3: Prolonging the systems functioning

Concepts

Kind of fuel/energy it uses

Per subject:

• How long does it take before depleted?
• How easy is it recharge/refuel the energy source?
• How much weight does it add?
• How much does it cost?
• Any other problems, will be added
• Are there any problems with the given solution?

Big interchangeable batteries

The energy density of Lithium metal batteries is 1.8 MJ/kg. 1

Average Prop Diam (inch)	Average Prop Ptich	Average V	Average A	Average Thrust(g)	Average W	Average Theoretical Thrust
5.333	3.4167	13.7667	7.5167	355.4167	96.1683	678.8333

2

From this, we can deduct how much power we need per hour to keep a large UAV of let's say x kilos in the air. And from that, we can give an estimate of how big the battery must be, and how long it will keep in the air.
We now know that we need 96.1683 W(J/s) to keep a drone in the air. This keeps 355.4167 g in the air. So, it takes 96.1683/0.3554167 = 270.579 W/Kg to keep the drone in the air.
So this means that for a drone of 2 kg, the drone would use 541.158W. Let's say this is including a 1 Kg battery. That would mean that the drone has 1.8 MJ energy stored. Using this, we get that the drone would have: 1.8 MJ / 541.158 J/s = 3326.2s of battery power. That is 0.9240 hours of power. Of course, the drone would not have a 1 kg battery, when it is only 2 kg. So let's say that 1/8 is dedicated to its own battery. That would mean that the drone has 0.1155 hours of battery, which is equal to 6.93 minutes. This is only for hovering. We excluded other systems here, for example, the control unit of the drone, the communication module, the refuelling/recharging of other batteries. That is why we came up with 1/8 is dedicated to its own battery.

Big rechargeable batteries

The uptime of a rechargeable battery would be comparable to that of a replaceable one, but the time it takes for the system to be ready again when the battery needs to be recharged is significantly larger. While charging times are heavily dependent on the quality of the charger, but it is generally not recommended to charge a battery above 10 Ampere [3]. The expected battery charge time is about an hour per kg, which means the uptime of the system would be smaller than the downtime of the system, severely reducing usefulness.

Solar energy

The option to instead charge the battery while in action is also available: Solar panels can ensure that the system does not have to return to a charging station to recharge, instead opting to simply land somewhere and wait until the battery is mostly recharged. The idea of making short flights with breaks in-between is already the idea behind the hopper drone, so these concepts might work well together. The downside of solar panels is that they are heavy, which increases the power consumption while in-flight. The question is whether or not the solar panels are feasible in terms of possible flight duration.

Light-weight solar panels are produced by companies such as Flisom [4], and their products will serve as a benchmark for the potential of solar panels in drone applications. According to their datasheet [5], an 87*41 cm solar panel weighs approximately 0.8 kilograms. It has a peak nominal power of 30 Watt and is expected to be able to function properly for ten years when used at 90% of that power. This means that a light-weight solar panel of this kind is expected to deliver 80 Watt per square meter at a weight of 2.35 kgs. Using the previous metric of 270.579 W/Kg to keep a drone flying, a solar panel would increase the net power consumption while in-flight with 554 W/m^2 , while offering an on-ground energy production of 80 W/m^2. The price would be a one time purchase of 70 euros per unit.

Fuel D

energy production per fuel weight and volume

Fuel has a density of 46.4 MJ/kg. This is way more than batteries. Here, you need to include a motor in order to retrieve the energy. The efficiency of these engines are most of the time around 40%. The rest is emitted with movement of the engine itself(vibrations), and heat. Further, an engine also adds mass. We will need to calculate how big of an engine we need to keep the drone hovering, and how efficient this will be. 1: https://en.wikipedia.org/wiki/Energy_density

2: http://www.starlino.com/power2thrust.html

3: http://batteriesbyfisher.com/determining-charge-time

4: https://flisom.com/

5: https://flisom.com/wp-content/uploads/2019/01/Datasheet_eFlex_0.8m_rev.pdf

How refuel/recharge drone

...

• How much time does it take? J
• Can it be autonomous? Does it require manpower? D
• How high is the possibility of danger? J
• Any more items will be added D

• Short work time
• Autonomous recharge/refuel
• Other vehicle recharge/refuel large drone
• Going to station to recharge/refuel
• Swapped

Choosing

• Why chosen idea is the chosen one?
• Perhaps adding interview

Phase 4: Collecting Data from rescue drones

A system consisting of multiple surveillance drones which have to share a limited number of service drones, depends heavily on communication to determine which surveillance drone should be serviced when. To keep the system as low-cost and simple as possible, communication should be limited.

Key information to transmit by surveillance drone: 1. Position 2. Battery status 3. Surveillance data (buiten onze scope maar wellicht rekening mee houden)

Information to submit by service drone: 1. Position? 2. Status (available / empty / damaged)

The communication method adheres to the following requirements: 1. Latency: 1s? één keer per seconde batterij en positie doorgeven snel genoeg? 2. Data rate: Afhankelijk van aantal drones, hoeveelheid informatie en latency 3. Reliability: Moeten we rekening meehouden met package loss? Dan latency omlaag? 4. Independent of ground infrastructure (GPS zou wel kunnen maar mobiel netwerk bijvoorbeeld niet)

Options

RF tracking

Ultra-Wideband RF Signals (licenced by NASA) accurate up to 1% of distance, tested up to 1 km [15].

LoRa: Range: 2-5 km urban areas
15 km suburban

Data rate: about 27 kbs
Airtime: about 40 ms https://arxiv.org/pdf/1607.08011.pdf (LoRaWan, wij moeten een andere hebben)
Latency: Unknown, system goes through some duty cycle and after every transmission there is a downtime.
At least realistic: Small packets every second (about 10 Bytes).
Low duty cycle: Little interference
Legal frequencies in almost all countries

Mobile 3G/4G station: Range: 2-5 km in urban areas up to 50 km in suburban areas https://mobilenetworkguide.com.au/mobile_base_stations.html

Concepts

1. Mobile 3G/4G station to connect to a single observation drone / some leader drones, or use the service drones as communication stations WiFi / ZigBee between observation drones: longer battery life than 3G/4G?

Emergency 3G/4G station can also provide cell service for survivors as added bonus?

Choosing

Other important questions: 1. Which surveillance drone should seek a service drone when? 2. How to enable the surveillance drone to find a service drone.

Planning

Week 4

• Reformulate Idea and specify the exact approach
• Rewrite wiki
• Finish Phase 1
  • Concepts and Choice
• Write State of the art Part 1
  • Supports Phase 1 of the design

Carnaval holiday

• Finish Phase 2
  • Concepts and Choice
• Write State of the Art Part 2
  • Supports Phase 2 of the design
• Finish Phase 3
  • Concepts and Choice
• Write State of the Art Part 3
  • Supports Phase 3 of the design

Week 5

• Finish Phase 4
  • Concepts and Choice
• Write State of the Art Part 4
  • Supports Phase 4 of the design
• Start work on the design

Week 6

• Continue working on the design
• Add finished aspects to the wiki

Week 7

• Finish Design
• Finish sections design wiki
• Write Conclusion, Reflection and Discussion
• Prepare presentation
• Finalize the wiki

Week 8

• Presentation
  
• Hand in report
  

State of the art

Pathfinding through rough terrain

Path planning for robots can be done in multiple ways, but finding the right choice for rescue operations can prove cumbersome. Research has been done to improve the path planning in regards to time planning and determining if the path taken can be completed. One such research is using genetic algorithms to determining a path as shown in source [3]. This does, however, have the drawback as it assumes to know what terrain is difficult to traverse and what isn't. However, this method is able to deal with unexpected situations and plans a new path that is close to optimal to reach its goal.

Another method is to evaluate the chance the robot will tilt when moving through the disaster area [4]. This is done by determining the height of the area using sensors and constructing a height gradient. The robot can then decide on a path through this gradient after nodes have been set, it takes into account the length of the path and the chance of tilting over. This method is ideal for small case areas but would need some considerable computation power to reliably do this continuously.

A variation on this idea is to change the configuration of actuators depending on the terrain [2]. This combines the path planning of the previous idea with additional functionality to further decrease the chance of tipping over. This can, therefore, be added as an extra to existing robots, given that it knows what the path will be like when moving towards it.

Using deep reinforcement learning is also an option for terrain navigation [5]. This method uses an elevation map as well and can learn what route it should take to reach the goal. This can then be applied to a robot and it should select a successful route, it could even learn if it makes a mistake. This aspect of self-improvement is unique to deep learning.

The final option that was researched is the option of using a guidance system that will guide a robot through dangerous areas [1]. This guidance system can use a multitude of lightweight sensors that can be placed all around the area. These will then connect with the main network and determine what areas are hazardous. This system is not ideal to move around obstacles. It is however useful in finding survivors as this is another functionality of this design.

Increasing travel time of a drone

There are multiple ways to charge a drone. The main way to do it would be to allow the feet of the drone to be conductive, and for the plateau where the drone rests to also be conductive and connected to a battery [14]. This allows the drone to charge without trouble. Another option with charging is by using the wireless charging method [8]. This method uses electromagnetic fields to generate a current in a coil present in the drone.

Changing a battery is also a viable option when trying to increase the flight time of a drone. By building a system that can handle the batteries, and have the possibility of a drone landing on top of it. It is possible to have a functioning battery replacement unit [9][10][11][12]. This is, however, dependant on the way the drone is built, and if on the orientation of the drone with respect to the base. If both of these are well suited for the base, the batteries can be replaced with a 100% success rate.

Refuelling is also a possibility, over the last few years more progress has been made for autonomous refuelling in flight [13]. These systems can properly refuel flying UAVs for continued function. This could, however, just as easily be adapted to a ground based vehicle or landed drone.

Reflection

Conclusion

Discussion

Sources

• [1] Aliali, S., & Benchaiba, M. (2018). Safe route guidance of rescue robots and agents based on hazard areas dissemination. Proceedings of the 2017 International Conference on Mathematics and Information Technology, ICMIT 2017, 2018–Janua, 29–37. https://doi.org/10.1109/MATHIT.2017.8259692
• [2] Brunner, M., Bruggemann, B., & Schulz, D. (2012). Motion planning for actively reconfigurable mobile robots in search and rescue scenarios. 2012 IEEE International Symposium on Safety, Security, and Rescue Robotics, SSRR 2012, 00(c), 2–7. https://doi.org/10.1109/SSRR.2012.6523896
• [3] Fries, T. P. (2019). Autonomous Robot Navigation in Diverse Terrain Using a Fuzzy Evolutionary Technique. IECON 2018 - 44th Annual Conference of the IEEE Industrial Electronics Society, 1, 5618–5623. https://doi.org/10.1109/iecon.2018.8591210
• [4] Ohki, T., Nagatani, K., & Yoshida, K. (2010). Safety path planning for mobile robot on rough terrain considering instability of attitude maneuver. 2010 IEEE/SICE International Symposium on System Integration: SI International 2010 - The 3rd Symposium on System Integration, SII 2010, Proceedings, 55–60. https://doi.org/10.1109/SII.2010.5708301
• [5] Zhang, K., Niroui, F., Ficocelli, M., & Nejat, G. (2018). Robot Navigation of Environments with Unknown Rough Terrain Using deep Reinforcement Learning. 2018 IEEE International Symposium on Safety, Security, and Rescue Robotics, SSRR 2018. https://doi.org/10.1109/SSRR.2018.8468643
• [6] Santoso, F., Garratt, M. A., Anavatti, S. G., & Petersen, I. (2018). Robust Hybrid Nonlinear Control Systems for the Dynamics of a Quadcopter Drone. IEEE Transactions on Systems, Man, and Cybernetics: Systems, 1–13. https://doi.org/10.1109/TSMC.2018.2836922
• [7] Turns, S. (2006). Thermodynamics - Concepts and Applications. New York, NY: Cambridge University Press.
• [8] Choi, C. H., Jang, H. J., Lim, S. G., Lim, H. C., Cho, S. H., & Gaponov, I. (2017). Automatic wireless drone charging station creating essential environment for continuous drone operation. 2016 International Conference on Control, Automation and Information Sciences, ICCAIS 2016, 132–136. https://doi.org/10.1109/ICCAIS.2016.7822448
• [9] Fujii, K., Higuchi, K., & Rekimoto, J. (2013). Endless flyer: A continuous flying drone with automatic battery replacement. Proceedings - IEEE 10th International Conference on Ubiquitous Intelligence and Computing, UIC 2013 and IEEE 10th International Conference on Autonomic and Trusted Computing, ATC 2013, 216–223. https://doi.org/10.1109/UIC-ATC.2013.103
• [10] Lee, D., Zhou, J., & Tze, W. (2015). Autonomous Battery Swapping System for Quadcopter. In 2015 International Conference on Unmanned Aircraft Systems (ICUAS) (pp. 118–124). Denver, Colorado: IEEE. https://doi.org/10.1109/ICUAS.2015.7152282
• [11] Liu, Z., Leo, D., Zhao, H., Wang, Z., & Liu, X.-Q. (2017). QUADO: and Autonomous Recharge System for Quadcopter. In 2017 IEEE 8th International Conference on CIS & RAM (pp. 7–12). Ningbo, China. Retrieved from https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8274740
• [12] Swieringa, K. A., Hanson, C. B., Richardson, J. R., White, J. D., Hasan, Z., Qian, E., & Girard, A. (2010). Autonomous Battery Swapping System for Small-Scale Helicopters. In 2010 IEEE International Conference on Robotics and Automation (pp. 3335–3340). Anchorage, Alaska. Retrieved from https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5509165
• [13] Mart, C., Richardson, T., & Campoy, P. (2013). Towards Autonomous Air-to-Air Refuelling for UAVs Using Visual Information. In 2013 IEEE International Conference on Robotics and Automation (ICRA) (pp. 5756–5762). Karisruhe, Germany. https://doi.org/10.1109/ICRA.2013.6631404
• [14] Costea, I. M., & Plesca, V. (2018). Automatic battery charging system for electric powered drones. In 2018 IEEE 24th International Symposium for Design and Technology in Electronic Packaging (SIITME) (pp. 377–381). Iasi, Romania. Retrieved from https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8599208

[15] https://www.nasa.gov/sites/default/files/664352main_MSC-24184-1_UWB-Tracking-System_2012.pdf

Old Sections

• Old State of the art