Coulomb Friction Compensation Research Articles

On the Model-Free Compensation of Coulomb Friction in the Absence of the Velocity Measurement

This paper demonstrates that the Coulomb friction, the most difficult part of friction to be compensated because of its discontinuity with respect to the velocity, can be precisely compensated without either its mathematical model or a velocity measurement, as commonly required in the literature. Instead, the necessary information needed in the friction compensation is obtained in real time from an implicit extended observer in the context of a common proportional-derivative motion control system, using the proposed linear reference compensation scheme. The robustness of this particular observer design to the time-delay uncertainty resulting from the model reduction is thoroughly investigated, which illustrates the extent to which a high bandwidth can be employed to achieve the favorable dynamic performance such that the limitation on the bandwidth of the original extended state observer (ESO) can be effectively eliminated. Finally, numerical examples are provided to validate the proposed method.

Friction identification and compensation design for precision positioning

Precision positioning systems driven by linear motors are vulnerable to force disturbances owing to the reduction of gear transmission. The friction, included in the disturbance, can be modeled and compensated to improve the servo performance. This paper proposes a modified Stribeck friction model (SFM) and an optimization algorithm for consistency with the positioning platform. The compensators based on the friction model and disturbance observer (DOB) are simulated. The simulation results show that as compared with the DOB compensator (the velocity recovers by 5.19%), the friction model based compensator (the velocity recovers by 10.66%) exhibits a better performance after adding the disturbance. Moreover, compensation comparisons among the Coulomb friction, traditional SFM, and modified SFM are performed. The experimental results show that the following error with modified SFM compensation improves by 67.67% and 51.63% at a speed of 0.005 m/s and 0.05 m/s, compared with the Coulomb friction compensation. This demonstrates that the proposed model, optimization algorithm, and compensator can reduce the following error effectively.

Cascaded proportional–integral–derivative controller parameters tuning for contour following improvement

Most commercial servo systems utilize cascaded proportional–integral–derivative controller due to its simplicity and robustness. Under this control structure, motion performances such as single-axial tracking and multi-axial contour following are completely determined by the controller parameters. In real applications, current loop controller parameters usually remain unchanged while velocity loop and position loop ones should be tuned as the mechanisms including inertia and resonant frequency vary. This article focuses on such cascaded proportional–integral–derivative controller parameters tuning for contour following performance improvement. It first describes the servo system dynamics under the cascaded proportional–integral–derivative control structure and identifies the model parameters via relay feedback technology. Then, the effects of the closed-loop dynamics on contour following are analyzed to gain that the matched axial dynamics is preferable. In order to match the axial dynamics, the controller parameters are tuned loop by loop through setting all the axes to have the same bandwidths. Based on the identified results, feedforward controller parameters including nonlinear coulomb friction compensation are also tuned to further improve the contour following performance. At last, the proposed tuning method is verified through the ellipse and diamond following experiments carried out on an X-Y motion stage. The results show that it can match the axial dynamics effectively and thus reduce the contour error greatly.

Actuators for Motion Control: Fine Actuator Force Control for Electric Injection Molding Machines

Currently, most plastic products are manufactured using injection molding machines, where the product quality largely depends on the injection force. In a typical injection molding machines force control system, force information from the machine environment is obtained by a force sensor. However, these sensors have a few disadvantages in terms of signal noise, sensor cost, and narrow bandwidth, and thus a sensorless force detection method is desirable. The use of a reaction force observer, based on the two-inertia resonant model, has been proposed, but this method suffers from some estimation errors due to the influence of nonlinear characteristics in the holding process and the screw back-pressure process. This article proposes a new injection force estimation method based on a high-order reaction force observer (HORFO) with Coulomb friction compensation, which is not significantly influenced by the nonlinear friction phenomenon. Moreover, this article evaluates the possibility and versatility of a sensorless force control system using the proposed HORFO with Coulomb friction compensation.

Optimal State Space Control of DC Motor

In comparison to classical cascade control architecture of DC motors, the state feedback control offers advantages in terms of design complexity, hardware realization and adaptivity. This paper presents a methodic approach to state space control of a DC motor. The state space model identified from experimental data provides the basis for a linear quadratic regulator (LQR) design. The state feedback linear control is augmented with a feedforward control for compensation of Coulomb friction. The controller is successfully applied and the closed loop behavior is evaluated on the experimental testbed under various reference signals.

State-periodic adaptive compensation of cogging and Coulomb friction in permanent-magnet linear motors

This paper focuses on the state-periodic adaptive compensation of cogging and Coulomb friction for permanent-magnet linear motors (PMLMs) executing a task repeatedly. The cogging force is considered as a position-dependent disturbance and the Coulomb friction is non-Lipschitz at zero velocity. The key idea of our disturbance compensation method is to use past information for one trajectory period along the state axis to update the current adaptation law. The new method consists of three different steps: 1) in the first repetitive trajectory, an adaptive compensator is designed to guarantee the l/sub 2/-stability of the overall system; 2) from the second repetitive trajectory and onward, a trajectory-periodic adaptive compensator stabilizes the system; and 3) to make use of the stored past state-dependent cogging information, a search process is utilized for adapting the current cogging coefficient. We illustrate the validity of our state-periodic adaptive cogging and friction compensator by actual PMLM-model-based simulation.

Experimental evaluation of model-based controllers on a direct-drive robot arm

This paper describes the experimental comparison among four model-based control algorithms on a direct-drive robotic arm. We present experimental results of the following controllers: Computed-Torque control, PD+ control, PD control with computed feedforward and PD control. All controllers include Coulomb and viscous friction compensation as well as cancellation of gravitational torques. They are tested under common trajectory specification and performance index.

Evaluation of Real-Time Motion Controllers on a Direct-Drive Robot Arm

Motion control of robot manipulators offers inter-esting practical and theoretical challenges to control researchers. This paper describes experimental comparison among four model-based control algorithms on a direct-drive robotic arm. All controllers include Coulomb and viscous friction compensation as well as cancellation of gravity torques, they are tested under common trajectory specification and performance index.

Stable interaction control and coulomb friction compensation using natural admittance control

AbstractA new control system design formulation is presented for achieving high‐performance, guaranteed‐stable impedance control. While the bandwidth of the resulting controller is no higher than alternative techniques, the new formulation significantly improves performance when Coulomb friction is present in the system. The technique requires a careful choice of the target impedance. The resulting feedback compensators are causal and have stable poles, although they are often non‐minimum phase. General rules for controller synthesis are derived. Experimental performance results are presented for a two‐mass system with internal compliance and Coulomb friction. Results demonstrate that the technique is successful in rejecting internal friction force disturbances while maintaining a passive driving‐point impedance. © 1994 John Wiley & Sons, Inc.

Adaptive Velocity Control of DC Motors With Coulomb Friction Identification

A new dc motor control technique for the Coulomb friction compensation is proposed. The technique uses an adaptive velocity control scheme for a dc servo motor with on-line estimated parameters, including a Coulomb friction parameter, which is a combination of the Coulomb friction torque, motor time constant, moment of inertia of the motor, and sampling time of the discrete-time motor model. The estimation model used in the adaptive control process is validated off-line by a pseudo-linear regression algorithm for system parameters in a linear ARMAX model, and by adaptive Kalman filters for the Coulomb friction parameter described as pseudo-random binary sequences. The adaptive controller consists of a friction compensator and a PID controller, whose gains are adjusted adaptively in terms of estimated parameters. The proposed adaptive control law is implemented and tested on a microprocessor-based dc servo motor, and is applicable to many dc-motor-driven precision servo mechanisms. Experimental results are shown to be superior to those of conventional PID controls in terms of parameter fluctuation.

Modelling and Control of an Elastic Robot Arm with Rotary and Prismatic Joints

The application of industrial robots to advanced manufacturing tasks requires highly accurate position and/or force control. Actual limitations to these requirements are mainly caused by elasticity, Coulomb friction and backlash in the system. Conventional control algorithms do not consider these effects and can hardly provide sufficient control accuracy or yield limit cycles. In this paper the common multibody approach of models of industrial robots is extended to derive more realistic models including elasticity of both joints and links, Coulomb friction and backlash. For the rotatory motion of an elastic robot arm with a revolute joint an algorithm for a highly accurate position control is developed based on (i) multi-layer control to damp quickly the elastic motions "separately" from the rigid-body motion of the robot, and (ii) compensation of Coulomb friction and backlash by the method of disturbance rejection control. In contrast to the very successful results in case of rotatory motion, the modelling and a suitable design of a control system for the translational motion of an elastic arm with a prismatic joint is a much more difficult Problem. A complex control system is obtained represented approximately by a set of ordinary linear time-variant differential equations of vanable order depending on the actual arm length driven out. Certam approaches of designing a feedback control are discussed.

Dynamic Influence and Partial Compensation of Coulomb Friction in a Position- and Speed-Controlled Elastic Two-Mass System

Abstract In controlled elastic systems, such as machine tools, robots, antennae, telescopes, pointing and tracking systems, Coulomb friction decreases the dynamic performance considerably. The influence of Coulomb friction with unequal coefficients of static and dynamic friction on a position- and speed-controlled elastic two-mass system is investigated. Stable stick-slip limit cycles at various frequencies can occur. A first-order disturbance observer avoids high-frequency stick-slip completely and reduces the amplitude of low-frequency stick-slip considerably. Moreover it extends the range of application of cascade control towards “soft” mechanical systems and improves its disturbance performance substantially.