Luis Govinda García-Valdovinos

Observer-based sliding mode impedance control of bilateral teleoperation under constant unknown time delay

Sliding mode control has been used extensively in robotics to cope with parametric uncertainty and hard nonlinearities, in particular for time-delay teleoperators, which have gained gradual acceptance due to technological advancements. However, since the slave teleoperator is in contact with a rigid environment, the slave controller requires a free of chattering control strategy, thus making first order sliding mode teleoperation control unsuitable. As an alternative, chatter free, higher-order sliding mode teleoperator control is proposed in this paper to guarantee robust tracking under unknown constant time delay. Moreover, complete order observers are proposed to avoid measurement of velocity and acceleration, along with a formal closed-loop stability proof of the observer-based controller. Experimental results are presented and discussed, which reveals the effectiveness of the proposed teleoperation scheme.

Neural Network-Based Self-Tuning PID Control for Underwater Vehicles

For decades, PID (Proportional + Integral + Derivative)-like controllers have been successfully used in academia and industry for many kinds of plants. This is thanks to its simplicity and suitable performance in linear or linearized plants, and under certain conditions, in nonlinear ones. A number of PID controller gains tuning approaches have been proposed in the literature in the last decades; most of them off-line techniques. However, in those cases wherein plants are subject to continuous parametric changes or external disturbances, online gains tuning is a desirable choice. This is the case of modular underwater ROVs (Remotely Operated Vehicles) where parameters (weight, buoyancy, added mass, among others) change according to the tool it is fitted with. In practice, some amount of time is dedicated to tune the PID gains of a ROV. Once the best set of gains has been achieved the ROV is ready to work. However, when the vehicle changes its tool or it is subject to ocean currents, its performance deteriorates since the fixed set of gains is no longer valid for the new conditions. Thus, an online PID gains tuning algorithm should be implemented to overcome this problem. In this paper, an auto-tune PID-like controller based on Neural Networks (NN) is proposed. The NN plays the role of automatically estimating the suitable set of PID gains that achieves stability of the system. The NN adjusts online the controller gains that attain the smaller position tracking error. Simulation results are given considering an underactuated 6 DOF (degrees of freedom) underwater ROV. Real time experiments on an underactuated mini ROV are conducted to show the effectiveness of the proposed scheme.

Higher-order sliding mode impedance bilateral teleoperation with robust state estimation under constant unknown time delay

Time delay teleoperation systems have gained gradual acceptance due to technological advancements, in particular in its communication channel, however, it is difficult to measure in real time the time delay. In this schemes, slave teleoperators are in contact to rigid environments, wherein slave control requires fast, robust and free of chattering control, thus making first order sliding mode teleoperation control unsuitable. As an alternative, chattering free, higher-order sliding mode teleoperator control is proposed in this paper to guarantee robust tracking under constant, but unknown time delay. Moreover, complete order sliding mode observers are proposed to avoid measurement of velocity and acceleration. Simulations are presented and discussed, which show robust impedance tracking

Cartesian sliding PID force/position control for transparent bilateral teleoperation

This article presents a novel, model-free, Cartesian sliding PID control scheme for obtaining transparency in teleoperation systems with parametric uncertainties and hard nonlinearities. A condition has been found, which establishes that it is equivalent to get transparency in dynamically similar teleoperation systems by simply implementing the same proposed controller at the master and the slave sites. In this manner, fast exponential convergence of position and velocity tracking errors, and an acceptable force tracking are attained, without acceleration measurements, nor any knowledge of the master/slave robot dynamics, nor estimating the impedance of human and environment. Moreover, computation of inverse kinematics and/or Cartesian robot dynamics are not necessary. Simulation results validate the proposed teleoperation scheme.

Uncalibrated image-based position-force adaptive visual servoing for constrained robots under dynamic friction uncertainties

Visual servoing of constrained dynamical robots has not yet met a formal treatment. Also, notices that due technological constraints, this task is done slowly at velocity reversals, thus dynamic friction arises, which complicates even more the problem. In this paper, a new adaptive scheme for visual servoing of constrained robots subject to dynamic friction is proposed. An image-based control is introduced to produce simultaneous convergence of the constrained visual position and the contact force between the end-effector and the constraint surface. Camera and robot parameters are considered uncertain. This new approach is based on a new formulation of the orthogonalization principle used in force control, coined here visual orthogonalization principle. This allows, under the framework of passivity, to yield a synergetic scheme that fuses camera, encoder and force sensor signals. Simulation results are presented and show that image errors and force errors converge despite uncertainties of friction model.

Observer-based Higher-Order Sliding Mode Impedance Control of Bilateral Teleoperation under Constant Unknown Time Delay

Sliding mode control has been used extensively in robotics to cope with parametric uncertainty and hard nonlinearities, in particular for time-delay teleoperators, which have gained gradual acceptance due to technological advancements. However, since the slave teleoperator is in contact to a rigid environment, the slave controller requires a free of chattering control strategy, thus making first order sliding mode teleoperation control unsuitable. As an alternative, chattering free, higher-order sliding mode teleoperator control is proposed in this paper to guarantee robust tracking under unknown constant time delay. Moreover, complete order observers are proposed to avoid measurement of velocity and acceleration, along with a formal closed-loop stability proof of the observer-based controller. Experimental results are presented and discussed, which reveals the effectiveness of the proposed teleoperation scheme

Bilateral teleoperation of a commercial small-sized underwater vehicle for academic purposes

This paper describes the platform for a bilateral teleoperation where the master is a haptic device and the slave is a commercial small-sized underwater vehicle. The submarine will be instrumented, so that the electronics architecture can be upgrade at any time. With this new architecture it will be possible to program any advanced control scheme an the embedded. Also, position and velocity commands from the haptic interface to the small-sized underwater vehicle, will be send. Such that, when the force sensor, placed in the nose of the underwater vehicle, interacts with the environment, it will measure the force at the slave site and feed back it to the master robot.

Higher order sliding mode based impedance control for dual-user bilateral teleoperation under unknown constant time delay

This paper presents a dual-user teleoperation scheme to perform a collaborative task using n-DOF nonlinear manipulators as masters and slave. Impedance controllers for the manipulators are implemented in order to achieve a desired dynamic behavior depending on the users necessities. Furthermore, a sliding mode controller is introduced to cope with the time delay in the communication channels and the uncertainty in the slave. Since the slave teleoperator is in contact with a rigid environment, the slave controller requires a free of chattering control strategy, which makes first order sliding mode teleoperation control unsuitable. Then a higher order sliding mode based impedance controller is proposed to guarantee robust impedance tracking under constant, but unknown time delay. Therefore, a position scaling factor is incorporated to deal with the different workspaces among masters and slave. The validity of the proposed control scheme is demonstrated via experimentation on a 3-DOF dual-user teleoperation system. While the haptic devices Phantom Premium 1.0A and a Phantom Omni are used as masters, a virtual industrial manipulator Catalyst-5 is used as slave. The dual-user system is tested not only in presence of constant unknown time delay in each of the communication channels but also in free and constrained motion regime.

Control of ROVs using a Model-free 2nd-Order Sliding Mode Approach

Remotely Operated Vehicles (ROVs) have had significant contributions in the inspection, maintenance and repair of underwater structures, related to the oil industry, especially in deep waters, not easily accessible to humans. Two important capabilities for industrial ROVs are: position tracking and the dynamic positioning or station-keeping (the vehicles ability to maintain the same position with respect to the structure, at all times). It is important to remember that underwater environment is highly dynamic, presenting significant disturbances to the vehicle in the form of underwater currents, interaction with waves in shallow water applications, for instance. Additionally, the main difficulties associated with underwater control are the parametric uncertainties (as added mass, hydrodynamic coefficients, etc.). Sliding mode techniques effectively address these issues and are therefore viable choices for controlling underwater vehicles. On the other hand, these methods are known to be susceptible to chatter, which is a high frequency signal induced by control switches. In order to avoid this problem a High Order Sliding Mode Control (HOSMC) is proposed. The HOSMC principal characteristic is that it keeps the main advantages of the standard SMC, thus removing the chattering effects. The proposed controller exhibits very interesting features such as: i. a model-free controller because it does neither require the dynamics nor any knowledge of parameters, ii. It is a smooth, but robust control, based on second order sliding modes, that is, a chattering-free controller is attained. iii. The control system attains exponential position tracking and velocity, with no acceleration measurements. Simulation results reveal the effectiveness of the proposed controller on a nonlinear 6 degrees of freedom (DOF) ROV, wherein only 4 DOF (x, y, z, ψ) are actuated, the rest of them are considered intrinsically stable. The control system is tested under ocean currents, which abruptly change its direction. Matlab-Simulink, with Runge-Kutta ODE45 and variable step, was used for the simulations. Real parameters of the KAXAN ROV, currently under construction at CIDESI, Mexico, were taken into account for the simulations. In Figure 1 one can see a picture of KAXAN ROV. For performance comparison purposes, numerical simulations, under the same conditions, of a conventional PID and a model-based first order sliding mode control are carried out and discussed.

Design Parametrization for Dynamically Similar Delayed Teleoperation Systems

Bilateral teleoperation system are prone to instability coming out from the time-delay introduced by the indeterministic communication channel. This problem has been subject of intensive research under the assumption of non-equal master-slave teleoperators, however, dynamical similarity of master-slave robots can always be induced by proper tuning of feedback gains in the controllers, based on the physical ratio of both robots When we consider a linear Dynamically Similar Teleoperation System (DSTS), there arises advantages, observed heuristically in several experiments [1] or analytically in [10]. In this paper, a simple yet important parametrization trade-off of feedback control gains and performance teleoperation parameters is presented that allows an easy way to achieve stability or even also passivity. The analysis is based on an an impedance control scheme reported in [2] when the time-delay in the communication channel is considered constant but unknown. Stability and passivity limits are obtained based on Llewellyn’s and Raisbeck’s criteria.