An optimized self-sensing piezoelectric cantilever for micro-robotic applications

Quasi-static piezoelectric actuators are used in micro-robotics due to the small displacements they can achieve, and they have been proven to be suitable for Self-Sensing Actuation (SSA) applications. The morphology and material of an actuator, and therefore its characteristics (i.e. stiffness, capacitance), all have an impact on the performance of the SSA implementation. This article presents a methodology that can be used to optimize the design parameters for a piezoelectric quasi-static bimorph cantilever for micro-robotic applications with sub-mN manipulation forces and sub-µm positioning resolutions in mind. A model is established for this type of actuator, from which an objective function aiming to maximize β, the charge accumulation coefficient, is defined. Boundary conditions are set on the optimization study by imposing constraints on the design. An analytical optimization approach is then adopted to study the evolution of the objective function across the design space. The study evidences the existence of a subset of solutions satisfying the constraints and maximizing the objective function, and the partial derivatives of this subset are studied, leading to conclusions for an optimized design strategy. Experimental results show that inaccuracies in the prediction of the β parameter lie within the self-sensing actuator model due to the omission of the thin electrode layers inserted between each piezoelectric layer. Nonetheless, the presented optimization methodology has proven to be suitable to maximize 1 the potential SSA performance of a design, with the fabricated optimized actuator having a higher β coefficient than a commercial actuator of similar external dimensions by a factor of approximately 7.

Design of an Optimized Self-Sensing Piezoelectric Cantilever for Micro-Robotic Applications

Louis Antoine Masson, Yves Perriard

Quasi-static piezoelectric actuators are used in micro-robotics due to the small displacements they can achieve, and they have been proved suitable for Self-Sensing Actuation (SSA) applications. The fabrication parameters of an actuator (i.e. morphology, stiffness, capacitance) will have an impact on the performance of the SSA implementation, and this study intends to examine how to optimize those parameters for a piezoelectric cantilever. A model is established for this type of actuator, from which an objective function is defined. Boundary conditions are set on the optimization study by imposing constraints on the design. An analytical optimization approach is then adopted to study the evolution of the objective function across the design space. The study evidences the existence of a subset of solutions satisfying the constraint and maximizing the objective function, and the monotonicity of this subset is demonstrated. It is concluded that to obtain an optimal solution, one must maximize the length, minimize the number of layers and their thickness, and derive the appropriate width from these parameters applied to the constraint imposed on the objective function.

IEEE2018

Uncooperative Rendezvous in Space: Design of an Electronic Architecture for High Performance Avionic with Multi Sensor Input and Intensive Data Rate.

Michaël Yannick Juillard

Active Debris Removal missions consist of sending a satellite in space and removing one or more debris from their current orbit. A key challenge is to obtain information about the uncooperative target. By gathering the velocity, position, and rotation of the desired object, the satellite is able to plan its trajectory and define the sequence of approach. It requires the use of a variety of sensors with often a high data rate. For this task, a dedicated payload computer is envisioned with the responsibility of processing the information from the various rendezvous sensors. This component has the goal to provide meaningful information to the main satellite computer about the targeted debris. The focus of this work is on the data processing, the number of elements, and the electrical energy with constraints due to internal communication.First, an avionic testbench was built at the EPFL Space Center to assess early hardware and software architectures. The goal is to enable Hardware-In-the-Loop testing while developing the payload computer. A communication data bus has been implemented between the Platform and the Payload On-Board Computer. Emphasis was placed on the reliability of the high-level protocol and multiple concepts for improvement were analyzed.In a second phase, the testbench has been extended to support other data buses. The aim was to develop a reliable and efficient backup or fall-back data bus in case of failure in the main link. Four data buses have been tested with extensive analyses on their resilience to error and the efficiency of their data exchange.In parallel, extensive work has been performed to develop a simulation and optimization tool. Its goal is to support the design of payload avionic architectures for ADR missions by providing trade-offs and analyses. In the first iteration, a simulator has been created with instances of various high-level elements modeled.The second iteration introduced the additional dimension of optimization. It is trying to determine the best set of instruments and algorithms to use. With this capability, numerous analyses have been conducted on the influence of various parameters linked to the general optimizer behavior. It allows us to better understand the strength and limitations of the tool.The third step regarding the tool was to start its verification. The task has been to develop an Hardware-In-the-Loop architecture to compare its behavior with the results of the optimizer. The implementation of various mock-up algorithms enables the verification of their models in the tool. These analyses guarantee the feasibility of the outputted solution.The last part was dedicated to the creation and analysis of a realistic payload avionic architecture. The goal was to test the capability of the tool with hardware elements inspired by actual components. This work has shown the procedure to efficiently use the tool in the design phase of a mission.In conclusion, the work on the communication between the Payload and the Platform On-Board Computer has brought valuable lessons and experiences to the projects. They can now be used for the establishment of the high-level protocol into ClearSpace-1 flight software. In addition, the optimizer created allows tackling the design of complex payload avionic architecture where mass saving and processing resources are crucial. It is an essential point to develop a highly efficient payload computer for an Active Debris Removal satellite.

EPFL2022

Enabling Collective Operation of Indoor Flying Robots

James Roberts

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.