The objective of this research is to develop a control system for the wheel motors of a mobile robot which presents hardware-induced delay (HID). This mobile robot equips a haptic antenna, designed to perform measurement and identification tasks. This control system must be robust to step-like disturbances at the input, e.g., unknown terrain slopes. To address this, a complex control scheme is presented that is designed from the combination of two well-known schemes from the literature: the active disturbance rejection control scheme, presented in a novel way, and the Smith predictor. In addition to a brief study of the two schemes separately and the proposal of the combined scheme, the paper includes the result of some simulations, differentiating between the ideal and the real versions of the scheme, and experiments carried out to justify the validity of the combined scheme.

This work addresses the adaptive Smith predictor control of a tele-operated robot composed of a flexible link. Measurements of the angle of the motor that moves the arm by using an encoder and the moment at the base of the arm by using two strain gauges are fed back. These strain gauges normally present noticeable offset and high-frequency noise. In order to implement such control, the simultaneous real-time characterizations of the main vibration mode - which changes with the carried payload - and the delay of the tele-operated robot - which is time-varying - are carried out using a new algorithm based on an algebraic identification technique, which is robust to the previous strain gauge disturbances. This algorithm is faster than others previously developed, being especially suited to implement an adaptive version of the Smith predictor. The control of the tip position of the robot is closed using a fractional order controller, which has the advantage of removing the steady state error introduced by the strain gauges offset on the tip position. This adaptive control system is subsequently evaluated on a prototype. Simulated and experimental results are presented demonstrating the speed, accuracy, and robustness of the performed control system.

The development of new measurement systems for mobile robots has been a growing interest in recent years, particularly tactile systems based on bioinspired sensor antennae, such as whiskers and antennae found in animals and insects. This work focuses on studying a mobile robot equipped with such systems. Specifically, a dynamic model is developed for an omnidirectional robot with a sensing antenna, considering the planar motion of the system and taking into account the gravity effect. The extended Hamilton principle is applied to derive the equations of motion for the mobile platform, while the boundary-value problem is formulated for the antenna. Subsequently, modal analysis is employed to obtain a unique solution for the sensor antenna model, which is then validated using experimental data.

Soft pneumatic bending actuators (SPBAs) are commonly employed in soft robotics due to their unique characteristics, including safety, low weight, speed, and load capacity. However, the combination of pneumatics with soft materials causes SPBAs to exhibit nonlinearities and infinite degrees of freedom, complicating their dynamic modeling. In this work, we present how the dynamics of SPBAs can be adjusted to a fractional order model (FOM), showing an approach for their empirical identification. We also present a method for designing fractional order controllers (FOCs) for this type of actuators, based on the inversion of the empirical FOM. This modeling and control is applied to a modular SPBA made of a smart hydrogel, which endows the actuators with self-healing, self-adhesion, and self-sensing capabilities.

In this article, two algorithms are proposed to identify the parameters of elastic mechanical slightly damped systems with one dominant vibration mode, which are robust to sensor offset. The identification algorithms are obtained using the algebraic identification framework. The first one differentiates the input and the output signals in order to remove the sensor offset disturbances from the estimation procedure. The second one is more complex because it includes the offset in the dynamics used in the algebraic estimator. They are applied and evaluated on one degree of freedom flexible robot. Experiments proved the effectiveness of both algorithms but they showed that the second one yields a much faster estimation of the parameters.

This paper studies the design and implementation of a robust non linear fractional-order PI controller for a coupled tank system. The proposed control scheme is formed of two nested loops. The inner-loop consists in a feedback linearization control law closed to the water level of the upstream tank which reduces the effect of the uncertainty of the parameters of the actuator. The outer-loop is a fractional-order PI controller closed to the water level of the downstream tank which is robust to flow changes caused by variations of the cross sections of the outputs of the two tanks. The control signal generated by the outer-loop controller is the reference of the inner-loop. A new frequency-response technique is proposed to tune the robust outer-loop controller in such a way that the closed-loop system is not only robustly stable but also yields a nearly constant damping (overshoot) when the output cross sections change.

Sensing antenna devices, that mimic insect antennae or mammal whiskers, is an active field of research that still needs new developments in order to become efficient and reliable components of robotic systems. This work reports a new result in the area of signal processing of these devices that allows to detect the instant of the impact of a flexible antenna with an object faster than other reported methods. Previous methods require the use of filters that introduce delays in the impact detection. A method based on the Super-Twisting algorithm is proposed here that avoids the use of these filters and reduces such delays improving the impact instant estimation. Experiments show that these delays can be reduced in more than 50%, allowing reliable estimation of the impact instant with an error of less than 5 ms in many cases requiring a limited computational effort.

For delayed fractional order processes, fractional order controllers of the form PIαare designed to fulfil an iso-phase margin constraint and two design frequency specifications: gain crossover frequency and phase margin. In this paper, regions of robust frequency specifications to gain changes are developed and are approximated in function of the fractional orders of the plant and the controller. For the obtained regions, every two frequency specifications could be satisfied and the controller is designed respecting to an added iso-phase margin constraint. Numerical simulations in the time and frequency domains clearly justify the utility of the developed regions and the performances of the designed controllers.

In this paper, we present an experimental study concerning the gravity compensation of flexible link arms based on linear springs. In the field of flexible link robotics, the gravity compensation based on counterweights has been successfully applied in the past, but little effort has been made to examine the potential benefits and difficulties of using spring-based compensation mechanisms. This paper focuses on the modeling and identification of a single link flexible arm compensated with a spring based mechanism. As modeling approach, we followed the lumped-mass methodology to develop a model capable of reproducing the first vibrational frequency of the flexible link arm. Keeping in mind the forces that interact with the flexible link, a combination of sensors is suggested in order to measure and estimate the most important variables of the system. Subsequently, a very simple and reliable identification method based on the time and frequency response of the system is proposed. Finally, the results of the modeling and identification are validated on our experimental platform.

Despite the benefits that the spring based gravity compensation mechanism has brought to the field of rigid robotic manipulators, there have been no substantial efforts toward transferring these developments to the field of flexible link robotics. In this paper, we present an input state feedback linearization controller for the tip positioning of a flexible link arm with a gravity compensation system based on springs. The controller is implemented into a double loop control scheme, in which the inner loop addresses the motor position control in presence of joint friction, and the outer loop deals with vibration cancellation and the tracking of fourth-order trajectories for the tip position of the flexible arm. Taking into considerations the interacting forces between the flexible link and the gravity compensation mechanism, and also the characteristics of the control law, we propose a sensory system to measure all the relevant signals. The proposed controller is tested on an experimental prototype built in our laboratory.

In this paper a fractional order model for an irrigation main canal is proposed. This system has two pools that present a strong interaction between them. Then a multivariable model with two inputs: the pump flow and the opening of an intermediate gate, and two outputs: the water levels in the two pools, is derived. The identification of the model parameters is based on the experiments developed in a laboratory prototype of a hydraulic canal and the application of a direct system identification methodology. The accuracy of the proposed fractional order models is compared with the standard integer-order models of the canal. The parameters of the mathematical models have been identified by minimizing the Integral Square Error Index (ISE) existing between the time responses of the models and the real-time experimental data obtained from the canal prototype. A comparison of the performances of the integer-order and fractional-order models shows that the fractional-order model has significantly lower error: about 30%, and, therefore, higher accuracy in capturing the canal dynamics.

Flexible manipulators have many advantages over rigid robots when used in grasping tasks, owing to the characteristic flexibility of the materials used in their construction, which allow them to absorb energy during impact and make them controllable in order to limit the force applied during contact. In this paper, a two-flexible-finger gripper with a simple sensory feedback system is proposed. A generalized proportional-integral controller is implemented to acquire high control accuracy, while eliminating the need for derivatives of system output. The performance of this force feedback control system is investigated when mounted on a 6-degree-of-freedom (6-DOF) manipulator and grasping different objects. The experimental results indicate that the system proposed is able to manipulate different types of objects (both rigid and flexible) in an industrial environment by perfectly controlling the applied contact torque.

This paper proposes a new control method for a three-degree-of-freedom flexible robot. In the model, the mass is assumed to be concentrated at the end of the robotic arm. The main feature of the method is that it consists of two nested control loops: the inner one controls the angular positions of the motors and the external one the tip's position and cancels the vibration. The outer control loop, as will be shown, is robust to tip mass variations. Simulations are carried out, supposing different masses with succesfull results in trajectory tracking.

The present paper deals with the design of a controller focused to the automatic control of hydraulic canals. The designed controller will be required to be robust to large changes in the time delay as well as in all other parameters of the model, such as static gain and time constants. The methodology developed here is based on fractional order PI controllers (FPI), which have proven to be more robust than classical PI controllers, by virtue of the extra λ parameter (fractional order) that permits it to fulfill additional specifications. The results obtained will be compared with another FPI designed specifically to be robust to changes in all the parameters, except time delay, which is considered constant.

In this paper, we propose a fast online closed-loop identification method combined with an output-feedback controller of the generalized proportional integral (GPI) type for the control of an uncertain flexible robotic arm with unknown mass at the tip, including a Coulomb friction term in the motor dynamics. A fast nonasymptotic algebraic identification method developed in continuous time is used to identify the unknown system parameter and update the designed certainty equivalence GPI controller. In order to verify this method, several informative simulations and experiments are shown.

In this article, we propose a fast on-line closed loop identification method of continuous-time combined with an output feedback controller of the generalized proportional integral type (GPI), for the control of an uncertain flexible robotic arm with unknown mass at the tip, including a Coulomb friction term in the motor dynamics. A fast, non-asymptotic, algebraic identification method is used to identify the unknown system parameter and update the designed certainty equivalence GPI controller. In order to verify this method several informative experiments are shown.

This paper describes the mechanical devices, the movements and the trajectory generation of a novel wheelchair prototype capable of climbing staircases. The key feature of the mechanical design is the use of two decoupled mechanisms for each axle, one used to negotiate steps, and the other to position the axle with respect to the chair to accommodate the overall slope. This decoupling makes many different climbing strategies possible, the overall mechanism becoming extraordinarily versatile from a control point of view. A control system is necessary to synchronize the movements of all the actuators of the wheelchair so that its centre of mass can follow arbitrary spatial trajectories. Different trajectories have been designed in order to keep the seat as erect as possible to guarantee passenger comfort. Experimental results with the real prototype are reported which show the efficiency of our mechanism, the accuracy of the developed kinematic models for control, and the comparison of comfort for two different spatial trajectories

This paper proposes a new method for effective control of single link flexible manipulators. Dynamic models of these arms are complex because their mass is distributed along the link. Then control systems of these arms are usually complicate because they include complex observers, or some kind of inversion of these high order dynamics. We propose here a simple controller for these arms, based on a relatively new control technique called repetitive control. In order to apply this technique here, the dynamics of the arm has to be modified in such way that all the vibration modes have to be multiple of a basic frequency. This can be easily achieved by attaching some small masses at some points of the link. After repetitive control may be applied. The resulting controller is very simple, it is easy to tune, and only needs feedback of two signals: motor angle and torque measured by a strain gauge placed on the base of the arm. Simulations carried on a standard flexible arm show the effectiveness of our method.

This article describes a new control scheme designed for a three-degree-of-freedom flexible arm. This arm has been built with light links, has most of its mass concentrated at the tip, and uses a special mechanical configuration to approximately decouple tip motions in spherical coordinates. This configuration facilitates the dynamic modeling and control of the arm. A compliance matrix can be used to model the oscillations of the structure. A consequence of this simple dynamic is that minimum sensing effort is required (only direct motor and tip measurements), and the use of complex observers is avoided because the state of the system can be very easily obtained from these measurements. A two-nested control-loop scheme is used to control the tip position by using joint position and tip acceleration feedbacks. The stability of the controlled system is analytically proven in the case of perfect dynamic modeling. Then, an analysis of stability in the case of modeling errors is carried out, and conditions for local stability are derived. Experimental results are presented for the cases of perturbation control and trajectory tracking. Comparative results between controlled and noncontrolled tip responses are also shown.

We study a 3D visual servoing system composed of a robot manipulator, a CCD camera mounted on the end effector of the robot and a specific hardware. The use of computer vision as feedback transducer strongly affects the closed loop dynamics of the overall system, so that a visual controller is required for achieving fast response and high control accuracy. Due to the long time delay to generate the control signal, it is necessary to carefully select the visual controller. The objective of this work is to present a well structured design of efficient controllers to keep a desired relative 3D position between the object and the camera. Also an experimental setup has been built and used to evaluate the performance of the position based dynamic look and move system. Several results relative to different types of controller are presented. These results demonstrate the possibility of getting competitive results in real time performance with respect to more closed solutions shown in other works, while maintaining the advantage of easy system adaptability.

This paper analyzes the possibility of carrying out the collision detection as a first step towards the position/force control of three-degree-of-freedom flexible arm by using gauges placed at some distance of the tip (where contact is expected) in such way that they can easily stand the impact. Simulated results over a prototype three-degree-of-freedom flexible arm are presented.

Presents a solution to the problem of determining the orientation of arbitrarily located 3D parts. The proposed algorithm is based on a new data structure specifically designed to store spherical representation models of 3D objects. This data structure called modeling wave set (MWS), allows keeping redundant information about local and global features of an object under a unique framework. It involves a different approach to deal with the modeling process providing new methods for solving generic problems like object recognition or pose determination. The algorithm described in this work is based on the fact that a rotation of a solid can be seen as a change in the wave used to model it, because of the invariance to spatial transformation of the MWS structure. This property makes an algorithm simpler and with lower computational cost than any other method based on spherical representations. The algorithm has been tested over a wide set of polyhedral and free-from shapes. Success ratio and 3D orienting errors obtained with the proposed algorithm can be considered as acceptable.

This article describes the modeling, control scheme and experimental validation of a new three-degree-of-freedom flexible arm. This arm was built with light links, has most of its mass concentrated at the tip, and uses a special mechanical configuration to approximately decouple radial tip motion from angular tip motions. This special configuration facilitates the dynamic modeling and control of the arm. The compliance matrix can be used to model the oscillations of the structure. Due to this simple dynamics, minimum sensing effort is required (only motor and tip measurements) and the use of complex observers is avoided because the state of the system is obtained directly. A two-nested loop controls the tip position. Experimental results are presented.

A new method for controlling single-link flexible manipulators is proposed. The objective is to control the tip position of a flexible manipulator in the presence of changes in its payload. Particularly, a new adaptive control system has been designed. The proposed control system requires calculating only one parameter, the tip load, instead of all the transfer functions as in conventional adaptive control methods. The tip load identification algorithm is based on the equivalence existing between a manipulator with payload and a manipulator submitted to the action of external forces. Moreover, this equivalence provides a new technique for modeling these systems which simplifies the methods usually applied. This results in a simple control law that needs minimal computing effort and, thus, can be used for real-time control of flexible arms.

The control of flexible arms with friction in the joints is studied. A method to identify the dynamics of a flexible arm from its frequency response (which is strongly distorted by Coulomb's friction) is proposed. A robust control scheme that minimizes the effects of this friction is presented. The scheme consists of two nested feedback loops: an inner loop to control the motor position and an outer loop to control the tip position. It is shown that a proper design of the inner loop eliminates the effects of friction while controlling the tip position and significantly simplifies the design of the outer loop. The proposed scheme is applied to a class of lightweight flexible arms, and the experiments show that the control scheme results in a simple controller. As a result, the computations are minimized and, thus, high sampling rates may be used.>

The design of feedforward controllers to control the tip position of single-link flexible arms is developed. The objective is to drive the tip position along a commanded trajectory without any oscillations at the tip. The method is based on the well-known dynamics model inversion technique. Since the controllers are implemented on a computer, the dynamic inversion of the single-link flexible arms is studied from a discrete point of view. A general method to obtain a feedforward controller is developed, even in the case when the system transfer function is of nonminimum phase. The method is general in the sense that it removes oscillation in the arm with any number of vibration modes. It is shown that the input preshaping scheme developed by N.C. Singer and W.P. Seering (1990) is a special case of this method. The design technique is illustrated with numerical examples, and a comparison with the input preshaping method is carried out.>

Tip-position control of a flexible arm with friction in the joints is carried out. The control scheme is based on two nested feedback loops, an inner loop to control the position of the motor and an outer loop that controls the tip position of the flexible arm. The inner loop is controlled by a high-gain controller to remove the effects of friction. A control law partitioning scheme that partitions the control law into a model-based portion and a servo portion is used for the control of the outer loop. To make the arm follow the desired trajectory without any delay, a feedforward term is added to the control law. The control scheme is experimentally evaluated on two very lightweight flexible arms.>

A method for controlling single-link, lightweight, flexible manipulators is proposed. The objective of this method is to control the tip position of the flexible manipulator in the presence of joint friction and changes in the payload. Both linear and nonlinear friction are overcome using a very robust control scheme that is based on two nested feedback loops: an inner loop to control the position of the motor and an outer loop to control the tip position. Changes in the load are compensated by decoupling the dynamics of the system and then applying a very simple adaptive control for the tip position. This results in a quite simple control law that requires minimal computing effort and thus can be used for real-time control of flexible arms.>

The identification of continuous plants from sampled data is a technique commonly used. This method is normally divided into two parts: the determination of the discrete transfer function from the sampled data and the calculation of the continuous transfer function from the discrete one. An original method to perform the second part of this problem is described. The method, which establishes the minimum condition under which the continuous model can be obtained from the discrete one, has shown to have important advantages in the treatment of multiple poles. The method allows one to assume any temporal behavior of the input signal between sampling instants.

Digital regulators for continuous plants are designed today by means of conventionalz-transfer function methods. With thesez-transfer functions the value of the system outputy(t) can be learned only at instants spacedTseconds apart, whereTis the sampling period. A technique is proposed where the value ofy(t) can be learned at instants spacedT/nseconds apart, wherenis any integer. This technique is based on multirate sampling. First a method to calculate the output multirate sampled sequence is described. Then an efficient algorithm to calculate the multirate sampling discrete transfer function from the normal discrete transfer function is proposed.