Enabling Collective Operation of Indoor Flying Robots

Collective flying robots show great potential in many diverse indoor applications. Their added robustness, parallel operation and redundancy has clear advantages over a single flying robot. However, flying within cluttered and unprepared indoor environments, even for a single flying robot, is extremely challenging. The main reason why collective flying robots have not yet been successful within indoor environments, is due to a combination of several challenges related to the size constraints placed on an indoor hovering platform, which directly limits the available energy, embedded sensing and processing capabilities of a flying robot. The energetic cost of flying, places limits on the flight endurance and practicality of a swarm of flying robots. The current spatial-coordination approaches implement methods that are either too computationally expensive, or impractical for real-world operation, within unknown and unprepared indoor environments. The goal of this thesis was to develop a practical methodology for enabling energy efficient, autonomous indoor flying robots capable of inter-robot spatial-coordination for unprepared indoor environments, without using external aids. In order to enable collective operation of indoor flying robots, several practical methodologies have been proposed and demonstrated. A generalised design strategy has been proposed to dimension a hovering platform for a specific flight endurance, payload capability and robustness criteria. The developed method can be used as a practical design tool for anyone working with hovering platforms. The dimensioning strategy has created a design, which is highly suitable for carrying the necessary sensing and processing required to enable the collective operation of indoor flying robots. A simple sensing and control strategy is proposed for enabling anti-drift control and obstacle avoidance behaviours on an indoor highly dynamic, hovering platform. The approach has enabled one of the first indoor hovering platforms that could achieve such a capability without using any external aids. For the collective operation of indoor flying robots to work in reality, the on-board energy needs to be managed efficiently and conserved in a way that allows a swarm of robots to be useful, extending beyond the individual 10-20 min flight time. A generalised energy model has been developed, allowing for the accurate estimation of the flight endurance and perching time of hovering platforms. The energy model can be used to optimise the battery selection process of a hovering platform, to obtain the highest possible endurance. This is the only model known that is able to predict any combination of flying endurance and perching times. Additionally, a method of attaching to the ceiling has been presented that allows a flying robot to conserve energy and have a stable birds-eye-view while performing static sensing tasks. By applying energy management techniques, through use of energy modelling and behaviours that reduce the flight time, the energetic cost of flying can be mitigated and the mission endurance can be extended over several hours, which is especially useful for collective operation. A new infrared ranging technique has been developed that allows for a high sensing performance, including long range (12 m), high-speed (1 kHz / # robots) and high resolution (better than 1.1 cm up to 6 m). A practical on-board sensing method using this technique, has been developed that can provide spatial-coordination between multiple robots in three dimensions. The developed approach allows for easily adaptation, to suit other robots and applications, depending on a specific sensing speed and coverage requirement. The developed sensor is the worlds first embedded 3-D relative positioning sensor that has the ability to enable inter-robot spatial-coordination in three dimensions, which is necessary for achieving goal-directed flight on highly dynamic flying robots. A practical autonomous flight control methodology has been demonstrated that can provide hovering platform stabilisation, 3-D obstacle avoidance and 3-D waypoint navigation, all using the 3-D relative positioning sensor. Goal-directed flight and collective deployment have been achieved using only the information from the 3-D relative positioning sensing. The developed methodologies within this thesis, has enabled for the first time, the collective operation of highly dynamic indoor flying robots, without using external aids.

3-D Relative Positioning Sensor for Indoor Flying Robots

Dario Floreano, James Roberts, Timothy Stirling, Jean-Christophe Zufferey

Swarms of indoor flying robots are promising for many applications, including searching tasks in collapsing buildings, or mobile surveillance and monitoring tasks in complex man-made structures. For tasks that employ several flying robots, spatial-coordination between robots is essential for achieving collective operation. However, there is a lack of on-board sensors capable of sensing the highly-dynamic 3-D trajectories required for spatial-coordination of small indoor flying robots. Existing sensing methods typically utilise complex SLAM based approaches, or absolute positioning obtained from on-board tracking sensors, which is not practical for real-world operation. This paper presents an adaptable, embedded infrared based 3-D relative positioning sensor that also operates as a proximity sensor, which is designed to enable inter-robot spatial coordination and goal-directed flight. This practical approach is robust to varying indoor environmental illumination conditions and is computationally simple.

Springer Verlag2012

Enabling Large-Scale Collective Systems in Outdoor Aerial Robotics

Severin Leven

For many real-life applications such as monitoring, mapping, search-and-rescue or ad-hoc communication networks, fleets of flying robots are expected to out-perform existing solutions. Robots can join forces to cover larger areas in less time, act as efficient communication relays and overcome difficult terrain more easily than ground-based robots. Compared to a single robot, the main advantages of collective systems are robustness, parallelism, flexibility, and the fact that they enable tasks that could not be solved by a single robot. While a multitude of theoretical models for collective operation have already been developed and tested in simulation, they have rarely been validated with physical robots yet. Transition to reality has so far been inhibited because of strong limitations in scalability. Indeed, the cost, safety risk, and number of operators associated with a collective aerial system increase proportionally with the number of robots. Substituting such systems for single robots is therefore unattractive or even impracticable. In previous experimental approaches, a maximum of five physical robots were operated simultaneously in outdoor scenarios, assisted by human backup pilots on the ground. In contrast, most theoretical models rely on a minimum of ten robots to obtain interesting swarm effects. With respect to this discrepancy, we consider here ten robots to be a large-scale system in aerial robotics. In order to scale real collective aerial systems to at least ten robots, we suggest the following paradigm: flying robots must be low-cost, inherently safe, deal with mid-air collisions and require minimal supervision from an operator on the ground, such that the entire system warrants similar cost as well as similarly safe and easy deployment as a single, classical flying robot. Moreover, this would make swarms of flying robots as accessible as wheeled robots that have already been used successfully in larger collective systems. The above criteria can best be addressed with a global, systematic approach on all major design levels, which are the robot's airframe, low-level control, collective supervision and control as well as mid-air collision avoidance. On each level, we identified the bottlenecks related to scaling and developed methods for compliant robot design. Based on a systematic analysis of airframe configurations, a flying-wing is proposed that is made of low-cost, safe and durable foam material and can be deployed everywhere by hand-launch. A model for the dimensioning of such an airframe is presented. For low-level control, a novel minimalist control strategy is proposed, implemented on an inexpensive, custom-made autopilot and validated in field tests. In order to enable robots for collective operation, we suggest to use WiFi communication links and embedded Linux-computers onto which swarm designers can download distributed controllers for collective behaviors directly from their computer simulation. Furthermore, the idea is put forward to enable collective supervision through an operator interface with the modality for direct group-control. Implemented and field-proven, these solutions can serve as guidelines for other developers. Finally, the problem that robots may collide with each other during collective operation has been addressed with a model assessing the collision risk and a distributed strategy for mid-air collision avoidance, validated on real robots. Applying the proposed methodology allowed us to demonstrate the world's first collective system composed of ten autonomous flying robots in outdoor experiments with several different collective behaviors. Robots are low-cost (about 1/10 the cost of commercially available robots), inherently safe (kinetic energy of a medium-sized bird) and can be configured, deployed and supervised by a single operator on the ground. This represents a significant step for the scaling of flying collective systems with respect to the state of the art as well as an unprecedented operator-to-robot ratio.

EPFL2011

Concept of Modular Kinematics to Design Ultra-high Precision Parallel Robots

Murielle Richard

Miniaturisation will be the key challenge for the next decade in numerous industrial fields, such as microelectronics, optics and biomedical engineering. Although most of their products already achieve footprints of some square millimeters, the trend towards the integration of a maximum number of elements in a minimal volume requires even more compact components. This tendency creates a growing need for industrial robots able to perform micromanipulation and microassembly tasks with a submicrometric precision. Nonetheless, the design of such machines is nowadays costly, both in time and money, mostly because of the twofold complexity of their development: first, from a kinematic standpoint, the use of a parallel structure consists in a particularly interesting approach to build ultra-high precision robots. However, the synthesis of such a kinematics proves especially challenging for machines presenting more than 3 degrees of freedom. Moreover, the resulting robots are scarcely flexible: if the industrial specifications are modified, which for example necessitates to add a degree of freedom or to change the position of a rotation centre, the design process has to be restarted, often from the very beginning. The second challenge consists in the mechanical design of flexure-based mechanisms: flexure hinges are joints which are based on the elasticity of the matter. They allow to perform motions which are without friction, backlash and wear; their use is thus mandatory to achieve the aimed submicrometric precision. Albeit the synthesis of planar and low-degree of freedom structures is now widely investigated, the development of a whole tridimensional flexure-based robot is still infrequent, especially in the industrial context. This thesis thus introduces a modular design methodology which significantly reduces the time-to-market of ultra-high precision robots. This procedure can be compared to a robotic Lego, where a finite number of conceptual building bricks allows to easily design and modify parallel robots. Furthermore, this work shows that the machines resulting from this approach present similar or even improved performances compared to robots developed more traditionally. The key aspect of this thesis consists in the concept of modular kinematics, which aims at facilitating the synthesis of parallel kinematics thanks to solution catalogues. At this step of the methodology, the conceptual building bricks and the kinematics are totally independent from any mechanical design: they can thus be used to synthesise a large variety of robots, from machine-tools to microscale robots. An exhaustive conceptual solution catalogue groups all kinematics generated by the combination of the building bricks. Then, a reduced solution catalogue for ultra-high precision is proposed: based on selection criteria linked with the design and machining of flexure-based mechanisms, it allows to reduce the total number of solutions and thus facilitates the practical use of the concept. The second part of this work details the mechanical design of the building bricks, whose main challenge consists in increasing the ratio between the working ranges of the mechanisms and their overall size. One or more flexure-based solutions have been developed for each motorised brick: a special emphasis is given to the original use of a Remote Centre of Motion, which allows to achieve high rotation angles while drastically reducing parasitic translations. The development of a standardised actuation sub-brick, common to all motorised bricks, introduces a new level of modularity, thus increasing even more the flexibility of the methodology. As for non-actuated bricks, original designs and uncommon uses of well-known mechanisms are proposed. A case study on a 5-degree of freedom robot, Legolas 5, finally illustrates the practical use of the methodology: first, the selection in the solution catalogue of a kinematics adapted to the specifications of the robot is detailed. Then, the development of the Legolas 5 prototype highlights the mechanical design of the necessary building bricks, as well as assembly subtleties, such as force alignment and gravity compensation, which allow to shrewdly design a high-performance robot. The measurements of this machine have shown motion resolution and repeatability of 50 nm in translation and 1.9 µrad in rotation (limited by the sensor resolution). This case study has generated the Legolas family, a new family of ultra-high precision parallel robots, which notably includes the orthogonal version of the Delta kinematics: using only 6 of the conceptual building bricks, one solution can be built for each of the 19 possible robot mobilities. The promising characterisation of the Legolas 5 tends to suggest that the robots from this family will be interesting candidates to fulfill the upcoming need for quickly designed and high-performance industrial ultra-high precision machines.