On-line Gravity Compensation Research Articles

Rapid and restricted swing control via adaptive output feedback for 5-DOF tower crane systems

Crane swing & relevant vibration are critical safety and efficiency issues to be addressed in construction automation, involving various disturbance and uncertainties. This paper presents for the first time a controller designed for 5-DOF tower crane systems with no velocity signals, and its effectiveness is experimentally verified. Compared to previous controllers, the new design utilizes an adaptive output feedback control method with accurate online gravity compensation, which solves the problems of unavailability velocity, noise amplification by numerical difference, and steady state error of cable in tower crane control. Additionally, the saturated control inputs are guaranteed by employing bounded functions. The results of this study unveil that the controller proposed in this paper can simultaneously achieve accurate trolley/jib/cable positioning, rapid payload swing suppression and elimination, as well as precise payload mass estimation, and guarantee control inputs constraints. This study provides a new approach for accurate control of tower crane systems.

An improved IDA-PBC method with link-side damping injection and online gravity compensation for series elastic actuator

Elastic elements in series elastic actuator (SEA) will cause residual vibration in position control. Incorporating link-side damping injection and friction compensation, we propose an improved interconnection and damping assignment passivity-based control (IDA-PBC+) method to suppress residual vibration. Damping on the motor side and link side can be adjusted simultaneously. In addition, the desired motor-side trajectory planning and online gravity compensation are also introduced to improve control performance and steady-state accuracy. The effectiveness of the proposed method in suppressing residual vibration is experimentally verified with a two-degree-of-freedom SEA device.

New Adaptive Control Methods for $n$-Link Robot Manipulators With Online Gravity Compensation: Design and Experiments

For robot manipulators subject to unmeasurable/uncertain plant parameters, this article designs a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">new</i> adaptive motion controller, which ensures positioning errors to converge to zero and provides accurate gravity compensation. Meanwhile, specific motion constraints are also satisfied during the entire control process. Additionally, the proposed controller is further extended to address output feedback control without velocity measurement/numerical differential operations. A useful feature of this article is that <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">neither</i> complicated gain constraints <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nor</i> the upper/lower bounds of model parameters/matrices in the dynamics are required in controller design and analysis, which greatly facilitates practical applications. Meanwhile, by introducing a nonlinear auxiliary term (related to motion constraints and error signals) into the proposed controllers, all links accurately reach their desired positions <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">without</i> exceeding the preset constraints, while the gravity vector is estimated online to eliminate static errors. Additionally, the asymptotic stability of the system equilibrium point is strictly proven; more importantly, the difficulty of stability analysis is significantly decreased based on the elaborately constructed Lyapunov function candidate. Compared with existing controllers, the main merits of the designed control schemes include <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fewer</i> control gain conditions, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">more concise</i> closed-loop stability analysis, and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">higher</i> safety satisfying specific constraints. Finally, some hardware experiments are carried out to validate the performance of the presented controllers.

Position Control for Flexible Joint Robot Based on Online Gravity Compensation With Vibration Suppression

This paper presents a position control scheme for flexible joint robot based on online gravity compensation (OGC). After concluding some drawbacks of OGC-based PD control law, a nonlinear state feedback controller is designed, where analysis for overshoots of motor side and residual vibration of link side is provided. It is shown that asymptotic stability is guaranteed, and each signal of closed loop system is proved to be bounded via Lyapunov method. Comparing experiment results illustrate effectiveness of the proposed approach.

PD control with on-line gravity compensation for robots with elastic joints: Theory and experiments

A proportional-derivative (PD) control with on-line gravity compensation is proposed for regulation tasks of robot manipulators with elastic joints. The work extends a previous PD control with constant gravity compensation at the desired configuration. The control law requires measuring only position and velocity on the motor side of the elastic joints, while the on-line gravity compensation torque uses a biased measure of the motor position. It is proved via a Lyapunov argument that the control law globally asymptotically stabilizes the desired robot configuration. A simulation study on a two-joint arm reveals the better performance that can be obtained with the new scheme as compared to the case of constant gravity compensation. Moreover, the proposed controller is experimentally tested on an eight-joint cable-driven robot manipulator, in combination with a point-to-point interpolating trajectory, showing the practical advantages of the on-line compensation.