International Peer-Reviewed Journal Papers

Abstract:This paper presents a simple, low-cost teleoperation system. The leader device is a handheld camera integrated with an Inertial Measurement Unit (IMU), making it feasible to use a modern smartphone for this purpose. Existing leader devices require expensive and/or complex hardware, and sensors to measure both the user interactions and to control the follower device. By contrast, the proposed method uses the handheld camera both as a leader device and as a sensor to control the position of the follower device through visual servoing. To the best of the author's knowledge, this visual servoing scenario where the camera is held by a user has not been thoroughly studied. The measurements from the handheld device and the follower are fused together in an Extended Kalman Filter (EKF) to improve further the pose estimation. A Virtual Camera and IMU (VCI) concept is introduced to filter hand tremors for teleoperation efficiency without hindering the bandwidth of the relative pose control loop. The EKF and the VCI performance are assessed experimentally by teleoperating a Suspended Aerial Manipulator (AMES) prototype.

Abstract:Art is one of the oldest forms of human expression, constantly evolving, taking new forms and using new techniques. With their increased accuracy and versatility, robots can be considered as a new class of tools to perform works of art. The STRAD (STReet Art Drone) project aims to perform a 10-meter-high painting on a vertical surface with sub-centimetric precision. To achieve this goal we introduce a new design for an aerial manipulator with elastic suspension capable of moving from one equilibrium position to another using only its thrusters and an elliptic pulley-counterweight system. A feedback linearization control law is implemented to perform fast and accurate winding and unwinding of an elastic cable.

Abstract:This paper presents a robust altitude control that merges the principles of active disturbance rejection control (ADRC) with the in-ground-effect model (IGE). To this end, a nonlinear extended state observer is designed along the vertical axis, taking attitude and altitude measurements. Then, the forces generated by low-level flight, ground effect and other external disturbances are estimated and used (as an anticipation term) together with a non-linear control law (as a feedback term) to reject them. Closed-loop stability is analyzed in the Lyapunov sense. Extensive numerical simulations and real-time experiments validate the proposal. Thanks to its simplicity, the control algorithm is easy to implement. It can be used for various maneuvers that depend on proximity to the ground, obstacles or surfaces, such as take-off, landing, inspection, surveillance and hovering.

Abstract:This article addresses the challenge of attitude synchronization of a group of rigid bodies (considered to be agents) with zero final angular velocity. A collaborative control strategy is proposed to tackle this issue, using decentralized consensus within a leader-follower scheme, and the communication between agents is activated by events. The attitude is parameterized by the unit quaternion. The control law incorporates an event function specifying the moment when the i-th agent should transmit attitude and angular velocity information to its neighbors. The communication topology between agents is modeled using a connected and undirected graph. The event-triggered communication produces asynchronous information exchange, reducing data traffic without compromising system performance. Furthermore, in practical scenarios such as networks of satellites or aerial robots, the proposed strategy could reduce the use of communication channel bandwidth. The Lyapunov method is utilized to analyze the stability of the overall system. Numerical simulation results confirm the proposal's effectiveness.

Abstract:In this work, a macro-mini aerial manipulator with elastic suspension is introduced. The mini is an omnidirectional aerial manipulator suspended from the macro by a spring. The macro is a Cartesian robot that moves the anchoring point of the spring. This design combines the advantages of the large workspace of the macro robot with the high dynamics of aerial vehicles, while reducing energy consumption thanks to gravity compensation. A partitioned control scheme is first implemented to regulate the aerial manipulator and its carrier separately. The redundancy resolution strategy positions the macro robot to minimize the energy consumption of the aerial manipulator at steady state. Then, a nonlinear model predictive controller replaces the partitioned controller to improve further the efficiency of the combined system, notably by anticipating the slow dynamics of the macro robot. A sufficient condition for offset-free tracking has been investigated theoretically. Experiments with a cable-driven parallel robot as macro are carried out to assess the added value of the carrier. Both controllers are validated and compared experimentally.

Abstract:In this letter, we describe a new light-flashing shield to be used at flash-based imaging or event-based vision tasks performed in environments with poor light conditions. The shield incorporates a multiplexer that permits to set the operation mode, the nominal current, the pulse-width and the trigger sensitivity of up-to 2 high-brightness LEDs by means of a single I2C bus. A trigger conditioning circuit permits to synchronize the LEDs light and an Intel RealSense d455 camera to obtain a set of clear images while the LEDs are illuminated, being this feature the main novelty and contribution of this work in order to provide light on-demand in visual-based tasks performed in environments with poor light conditions. Images obtained during real tests prove the correct synchronization between the light-flashing shield and the camera.

Abstract:In this article, thrusters embedded on a cable-driven parallel robot (CDPR) platform are proposed to improve the CDPR dynamics and trajectory tracking performance. On-board thrusters with their short response time can compensate for the reduced bandwidth of the winch actuation due to winding speed limit and \textcolor{blue}{low cable stiffness}. To compute and allocate control signals to winches and thrusters, a nonlinear model predictive controller (NMPC) is designed. A model of a CDPR with its hybrid actuation dynamics and saturation is introduced, including an alternative model of the thrust actuation that removes the need of thrust sensors and reduces the size of the NMPC optimization problem. To achieve a zero steady-state error with the NMPC, which becomes offset-free, an augmented model including additional disturbance states is proposed. The theoretical conditions to achieve offset-free control are checked. The proposed NMPC scheme is validated experimentally on a planar CDPR with three cables and four propeller-based thrusters. Results show that onboard thrusters contribute to a tracking error reduction along complex trajectories and an efficient damping of the platform vibration.

Abstract:This paper proposes an approach for accurate and efficient parameter identification of Lithium-ion Battery Packs using only Drive Cycle Data obtained from the Hybrid or Electric Vehicle. The approach has been experimentally validated using data collected from a BMW i8 hybrid vehicle. The Dual Polarization model is used, and a new open circuit voltage equation is proposed based on a simplification of the Combined model with the aim of reducing the number of parameters to be identified. The parameter identification is performed using NEDC data collected on a rolling road dynamometer and results show that the proposed model improved the accuracy of the terminal voltage estimation, reducing peak voltage error from 2.16% using the Nernst model to 1.28%. Furthermore, the robustness of these models to maintain accuracy when new drive cycles are used was evaluated by comparing WLTC simulations with experimental measurements. The proposed model showed improved robustness, with a reduction in RMS error of more than 50% compared to the Nernst model. These findings are significant because they will improve the accuracy of model-based Battery Management Systems used in electric vehicles, allowing improved performance prediction, without the requirement to recharacterize for different drive cycles or individual cell characterization.

Abstract:This paper addresses an attitude tracking control design applied to multirotor Unmanned Aerial Vehicles (UAVs) based on an ADRC approach. The proposed technique groups into a total disturbance, the endogenous and exogenous disturbances, and then it is estimated online via an extended state observer (ESO). Further, a quaternion-based feedback is developed, which is assisted by a feedforward term obtained via the ESO to relieve the total disturbance actively. The control law is bounded; consequently, it takes into account the maximum capabilities of the actuators to reject the disturbances. The stability is analyzed in the ISS framework, guaranteed that the closed-loop (controller-ESO-UAV) is robustly stable. The simulation results allow validating the theoretical features.

Abstract:Aerial manipulators may replace industrial manipulators for specific tasks involving large workspaces and high dynamics. However, these robots suffer from a lack of accuracy, autonomy, payload capability and a complex regulatory environment. To overcome some of these limitations, we introduce a novel aerial manipulator suspended by a spring to a robotic carrier. A computed torque controller with exteroceptive feedback is used to control the robot, using singular perturbation theory to prove the stability. We carried out various experiments to assess the workspace, accuracy and dynamics of the manipulator.

Abstract:Nowadays, electric vehicles are one of the main topics in the new industrial revolution, called Industry 4.0. The transport and logistic solutions based on E-mobility, such as handling machines, are increasing in factories. Thus, electric forklifts are mostly used because no greenhouse gas is emitted when operating. However, they are usually equipped with lead-acid batteries which present bad performances and long charging time. Therefore, combining high-energy density lithium-ion batteries and high-power density supercapacitors as a hybrid energy storage system results in almost optimal performances and improves battery lifespan. The suggested solution is well suited for forklifts which continuously start, stop, lift up and lower down heavy loads. This paper presents the sizing of a lithium-ion battery/supercapacitor hybrid energy storage system for a forklift vehicle, using the normalized Verein Deutscher Ingenieure (VDI) drive cycle. To evaluate the performance of the lithium-ion battery/supercapacitor hybrid energy storage system, different sizing simulations are carried out. The suggested solution allows us to successfully optimize the system in terms of efficiency, volume and mass, in regard to the battery, supercapacitors technology and the energy management strategy chosen.

Abstract:This article presents the design and implementation of an event-triggered control approach, applied to the leader-following consensus and formation of a group of autonomous micro-aircrafts with capabilities of vertical take-off and landing (VTOL-UAVs). The control strategy is based on an inner-outer loop control approach. The inner control law stabilizes the attitude and position of one agent, whereas the outer control follows a virtual leader to achieve position consensus cooperatively through an event-triggered policy. The communication topology uses undirected and connected graphs. With such an event-triggered control, the closed-loop trajectories converge to a compact sphere, centered in the origin of the error space. Furthermore, the minimal inter-sampling time is proven to be below bounded avoiding the Zeno behavior. The formation problem addresses the group of agents to fly in a given shape configuration. The simulation and experimental results highlight the performance of the proposed control strategy.

Abstract:El trabajo presenta el diseno de una estrategia de control distribuido con comunicación activada por eventos, que resuelve el problema de consenso líder-seguidor, de un conjunto de robots móviles tipo péndulo invertido (RMPI). La linealización de las ecuaciones de movimiento de los RMPI, alrededor del punto de equilibrio, permiten explotar las propiedades de planitud diferencial, dando lugar a una reparametrizacion del sistema mediante la salida plana. Asumiendo que los vehículos se comunican mediante una red, cuya topología es representada por un grafo no dirigido y fuertemente conectado, se disena una ley de control distribuido y una funcion de evento que indica el instante en el que el i-ésimo vehículo debe transmitir informacion (su estado) a sus vecinos. El resultado es un intercambio asíncrono de informacion entre vehículos y donde el tiempo entre eventos no es equidistante. El analisis de estabilidad se lleva a cabo en el sentido de Lyapunov y en el sentido entrada-estado ISS (Input-to-State Stability). Los resultados en simulacion numérica muestran el buen desempeño del consenso de la red de vehículos en dos escenarios representativos: regulacion y seguimiento de trayectoria.

Abstract:Although periodicity simplifies design and analysis in control theory, it is no more adapted for embedded systems because it results in a conservative usage of resources. Indeed, the control signal is computed and updated at the same rate regardless whether is really required or not. On the other hand, event-driven sampling calls for resources whenever they are indeed necessary. Event-based PID controllers are proposed in this paper as an alternative to classical PID approaches, with the same performance but reducing the control updates. The algorithms are built here without safety limit condition contrary to the seminal event-based PID setup that was originally proposed by Arzen (1999), in order to reduce the periodicity even more. Both integral and derivative terms are considered. The different approaches are tested for controlling the position of a real-time mini quadrotor helicopter. A reduction of the computing and communication resources utilization is demonstrated for similar final performance.

Abstract:In this paper, a new event-switched control method is presented for controlling discrete-time linear systems subject to bounded disturbances. The main advantage of the proposed method is that the nominal performance of the controlled system with periodic control updates is kept in a framework that do not require to periodically update the control law. The feedback control loop can be opened as long a state-dependent event condition is satisfied. This condition is obtained using set theory approaches. In particular, the concept of robustly positively invariant sets is used to calculate the nominal performance and the event condition. The simulation presented in this paper confirms the efficiency of the present approach. A reduction of the numerical complexity of the approach is also proposed.

Abstract:Event-triggered control is a resource-aware sampling strategy that updates the control value only when a certain condition is satisfied, which denotes event instants. Such a technique allows a reduction of the control's computational cost and communications demand. In this paper, an asynchronous feedback is developed for event-triggered stabilization of a PVTOL (Planar Vertical Take-Off and Landing) system wherein the control loop is closed through an Internet connection. The proposed feedback ensures asymptotic stability to the desired position. Real-time experiments are carried out in order to show the convergence of the PVTOL to the desired position as well as robustness with respect to external disturbances. Results show that the proposed strategy can reduce the number of control updates and consequently reduce the communication traffic over the network without sacrificing performance of the whole system.

Abstract:Many Processor Systems-on-Chip (MPSoC) have become tremendously complex systems. They are more sensitive to variability with technology scaling, which complicates the system design and impact the overall performance. Energy consumption is also of great interest for mobile platforms powered by battery and power management techniques, mainly based on Dynamic Voltage and Frequency Scaling (DVFS) algorithms, become mandatory. A Globally Asynchronous Locally Synchronous (GALS) design alleviate such problems by having multiple clocks, each one being distributed on a small area of the chip (called island), whereas an Asynchronous Network-on-Chip (ANoC) allow to communicate between the different islands. A robust technique is proposed to deal with a GALS-ANoC architecture under process variability constraints using advanced automatic control methods. The approach relaxes the fabrication constraints and help to the yield enhancement. Moreover, energy savings are even better for the same perceived performance with the obtained variability robustness. The case study is an island based on a MIPS R2000 processor implemented in STMicroelectronics 45nm technology and validated with fine-grained simulations.

Abstract:In this paper, a quaternion-based feedback is developed for event-triggered attitude stabilization of a quadrotor mini-helicopter. The feedback is derived from the universal formula for event-triggered stabilization of general nonlinear systems affine in the control. Event-triggered control is a resource-aware sampling strategy that updates the control value only when a certain condition is satisfied, which denotes event instants. Such a technique allows a reduction of the control computational cost and communications demand. The proposed feedback ensures asymptotic stability to the desired attitude. Real-time experiments are carried out in order to show the convergence of the quadrotor states to the desired attitude as well as robustness with respect to external disturbances. Results show that the proposed strategy can reduce by 80 % the number of control function calls and consequently reduce the communications of the embedded system without sacrificing performance of the whole system. To the best of the authors’ knowledge, this is the first time that a nonlinear event-triggered controller is experimentally applied to the attitude stabilization of an unmanned aircraft system.

Abstract:This paper deals with the attitude estimation of a rigid body equipped with angular velocity sensors and reference vector sensors. A quaternion-based nonlinear observer is proposed in order to fuse all information sources and to obtain an accurate estimation of the attitude. It is shown that the observer error dynamics can be separated into two passive subsystems connected in “feedback”. Then, this property is used to show that the error dynamics is input-to-state stable when the measurement disturbance is seen as an input and the error as the state. These results allow one to affirm that the observer is “robustly stable”. The proposed observer is evaluated in real-time with the design and implementation of an Attitude and Heading Reference System (AHRS) based on low-cost MEMS (Micro-Electro-Mechanical Systems) Inertial Measure Unit (IMU) and magnetic sensors and a 16-bit microcontroller. The resulting estimates are compared with a high precision motion system to demonstrate its performance.

Abstract:Contrary to the classical (time-triggered) principle that calculates the control signal in a periodic fashion, an event-driven control is computed and updated only when a certain condition is satisfied. This notably allows to save computations in the control task while ensuring equivalent performance. In this paper, we develop and implement such strategies to control a nonlinear and unstable system, that is the inverted pendulum. We firstly propose to apply an event-based approach previously developed in Marchand et al. (2011) for the stabilization of the pendulum near its inverted position. We then study the swinging of the pendulum up to the desired position and especially design a low computational cost control law for this second case, based on an energy function. The switch between both strategies is also analyzed for stability reason. A real-time experimentation is realized and notably demonstrates the efficiency of the event-based schemes even in the case where the system has to be actively actuated to remain upright. A reduction of about 98% and 50% of samples less than the classical scheme is achieved for the swing up and stabilization parts respectively, whereas the system behavior remains the same (in terms of balancing and stabilizing time or control amplitude).

Abstract:Embedded devices using highly integrated chips must cope with conflicting constraints, while executing computationally demanding applications under limited energy storage. Automatic control and feedback loops appear to be an effective solution to simultaneously accommodate for performance uncertainties due to the tiny scale gates variability, varying and poorly predictable computing demands and limited energy storage constraints. This paper presents the example of an embedded video decoder controlled by several feedback loops to carry out the trade-off between decoding quality and energy consumption, exploiting the frequency and voltage scaling capabilities of the chip. The inner loop controls the Dynamic Voltage and Frequency Scaling (DVFS) through a fast predictive control strategy. The outer loop computes the scheduling set-points needed by the inner loop to process frames decoding. The feedback loops have been implemented on a stock PC and experimental results are provided.

Abstract:Mobile computing platforms need ever increasing performance, which implies an increase in the clock frequency applied to the processing elements (PE). As a consequence, the distribution of a single global clock over the whole circuit is tremendously difficult. Globally Asynchronous Locally Synchronous (GALS) designs alleviate the problem of clock distribution by having multiple clocks, each one being distributed on a small area of the chip. Energy consumption is the main limiting factor for mobile platforms as they are powered by batteries. Dynamic Voltage and Frequency Scaling (DVFS) in each Voltage and Frequency Island (VFI) has proven to be highly effective to reduce the power consumption of the chip while meeting the performance requirements. Environmental parameters (i.e. temperature and supply voltage) changes also strongly affect the chip performance and its power consumption. Some sensors can be buried in order to estimate via data fusion techniques the supply voltage and the temperature variations. For instance the knowledge of the gap between the temperature and its maximum value can be used to adapt the power management technique. The present paper deals with the design of a voltage and frequency management approach (DVFS) that explicitly takes into account the thermal constraints of the platform.

Abstract:In this paper, a universal formula is proposed for event-based stabilization of general nonlinear systems affine in the control. The feedback is derived from the original one proposed by E. Sontag in the case of continuous time stabilization. Under the assumption of the existence of a smooth Control Lyapunov Function, it is proved that an event-based static feedback, smooth everywhere except at the origin, can be designed so to ensure the global asymptotic stability of the origin. Moreover, the inter-sampling time can be proved not to contract at the origin. More precisely, it is proved that for any initial condition within any given closed set the minimal inter-sampling time is proved to be below bounded avoiding the infinitely fast sampling phenomena. Moreover, under homogeneity assumptions the control can be proved to be smooth anywhere and the inter-sampling time bounded below for any initial condition. In that case, we retrieve a control approach previously published for continuous time stabilization of homogeneous systems.