Shahram Tafazoli

Tracking control of an electrohydraulic manipulator in the presence of friction

Analysis and estimation of friction and compensation for its effects in the control of an electrohydraulic manipulator is addressed. The specific hydraulic manipulator is an integral part of an automated fish processing machine which has been developed in our laboratory. The analysis reveals that considerable static and dynamic friction exists in the system. An available nonlinear observer for Coulomb friction, is modified to simultaneously estimate friction, velocity, and acceleration. A novel observer-based friction compensating control strategy is developed for improved tracking performance of the manipulator. The approach is based on acceleration feedback control. Experimental investigations show that this controller significantly outperforms the conventional PD controller. The general approach presented in this paper, may be applied to compensate for friction in any servomechanism, particularly when the actuator dynamics is not negligible.

Impedance control of a teleoperated excavator

Earth-moving machines such as hydraulic excavators are usually used for carrying out contact tasks. Impedance control can be employed as an approach for achieving compliant motion in such tasks. This paper describes a position-based impedance controller that has been developed in our laboratory for excavator-type manipulators, and presents the supporting experimental results. First, the problem of impedance control for a single hydraulic cylinder is addressed and a method is presented to analyze the system stability. The steady-state position and force tracking accuracy of the closed-loop system is also studied. Next, the problem of impedance control for a multi-link hydraulic excavator is addressed and the arm Jacobian and accurate estimates of the arm inertial terms are employed to map the desired impedance of the end-effector (bucket of the excavator) onto the hydraulic cylinders. Various contact experiments carried out using an instrumented mini-excavator demonstrate that the proposed impedance controller has very good performance for both single-link and multilink cases.

Identification of inertial and friction parameters for excavator arms

A novel yet simple approach for experimental determination of the link (mass and inertia-related) parameters and friction coefficients is developed for a typical excavator arm. The parameters are needed for indirect measurement of the external forces, compensation for the link weights in the operators hand in a force-feedback teleoperation setup, impedance control of the arm, simulation of the manipulator dynamics, and model-based fault detection. Treating the machine arm as an open kinematic chain, its dynamic equations are presented symbolically. The static torque equations are derived from these equations and the gravitational parameters are defined accordingly. A new method for decoupled estimation of the gravitational parameters from static experiments is presented. Furthermore, the arm dynamics are expressed in a form which is linear in the inertia and friction-related parameters. The results obtained show that the identified model predicts the joint torques, in both static and dynamic conditions, with a very good accuracy.

Impedance control of a teleoperated mini excavator

A position-based impedance controller for excavator-type manipulators has been developed in our laboratory. This paper describes the proposed impedance controller and presents supporting experimental results. First, the problem of impedance control for a single hydraulic actuator is addressed and a method is presented for stability analysis. Steady-state position and force tracking of the closed loop system is also studied. Then, the desired impedance of the end-effector (bucket of the excavator) is mapped onto the hydraulic cylinders using the arm Jacobian and accurate estimates of the arm inertial and gravity terms. A nonconservative method is presented for predicting stability of the multivariable closed loop system. Experiments with an instrumented mini excavator (for the single cylinder case) show that the designed impedance controller has a good performance.

A virtual excavator for controller development and evaluation

In order to facilitate the testing and evaluation of control strategies and operator environments designed for heavy duty hydraulic machines, an excavator simulator has been developed and is described in this paper. The simulator comprises an impedance model of the excavator arm, a model for the bucket-ground interaction forces, a graphical environment and a haptic interface. This paper describes the simulator components and their integration.

Friction estimation in a planar electrohydraulic manipulator

An automated machine for industrial fish head cutting has been designed and built in our laboratory. A planar electrohydraulic manipulator is used to quickly move a sharp pneumatically actuated blade to its desired position. There are two lubricated metal guideways under the blade. The objective of this experimental investigation is to understand the nature of friction present in the system and corresponding effects. At first, a simple open-loop procedure is used to obtain the steady-state friction-velocity behavior at very low velocities. Online estimation of dynamic friction (in the closed-loop system) is considered afterwards. A nonlinear reduced-order observer, introduced in the literature, for simultaneous estimation of friction and velocity, is applied to the system. Experimental results indicate that this estimator does not work well. Thus, in its original form, it is not suitable. A modification is proposed herein, to obtain good performance. Experiments with the modified observer show satisfactory estimation of velocity and friction.

Bilateral matched impedance teleoperation with application to excavator control

This paper addresses issues of transparency and implementation of dual hybrid teleoperation. A method for automatically adjusting the master and slave impedances to match stiff and soft environments and to interpolate in between them is presented and evaluated using simulations. The application of this technique to the force-feedback control of a mini-excavator is also presented and discussed with supporting experimental results.

Evaluation of Impedance and Teleoperation Control of a Hydraulic Mini-Excavator

A position-based impedance controller has been implemented on a mini-excavator. Its performance and an approach to evaluate its stability robustness for given environment impedances are discussed. A dual hybrid teleoperation controller is proposed for machine control. Issues of transparency are discussed.

An accelerometer-based joint angle sensor for heavy-duty manipulators

An indirect, self-calibrating, easy to install, and robust joint angle sensing method for heavy-duty manipulators is presented in this paper. This method is suitable for the harsh working environment of these machines where conventional contact-type angle sensors cannot be deployed, or problems are associated with their use. The approach is based on processing the outputs of a pair of biaxial accelerometers placed very close to the joint axis on the adjacent links. In the proposed technique, joint angles are obtained without integrating the accelerometer outputs to avoid measurement error accumulation over a long period of time. Two calibration procedures are also described for accelerometers to ensure the accuracy of their measurements. The experimental results attest to the efficiency and accuracy of the new angle sensing mechanism.

Design and Calibration of an Integration-Free Accelerometer-Based Joint-Angle Sensor

A methodology has been developed for indirect, noncontact, and dynamic sensing of angles for robotic applications. Two accelerometers are placed on the adjacent links close to the joint axis, and their outputs are processed to estimate the joint angle. In the proposed technique, the joint angles are obtained without integrating the accelerometer outputs. To ensure accuracy of accelerometer readings, two calibration procedures for the accelerometers are presented, which can easily be implemented in place. Both these methods solve a nonlinear least squares problem to adjust the offset parameters of the accelerometers. The accelerometer-based angle sensor is particularly suitable for the harsh working environment of heavy-duty manipulators, where conventional contact-type angle sensors cannot be deployed or problems are associated with their use. The performance of the new sensor is studied and compared with the performance of digital resolvers in two applications, involving the position control and dynamic payload measurement of a miniexcavator. The experimental results attest to the efficiency and accuracy of the new angle-sensing mechanism.