Compound Control Strategy and Experimental Research of Asymmetric Hydraulic Cylinder Displacement Servo System

Literature Overview

This technical paper presents a compound control strategy for the displacement servo system of an asymmetric hydraulic cylinder, supported by experimental validation. Asymmetric hydraulic cylinders, where the effective area on the cap side differs from that on the rod side, are widely used in industrial applications including presses, injection molding machines, and material handling equipment. The displacement asymmetry introduces a fundamental challenge for position control, as the flow rates required for equal velocity in both directions are different, and the pressure characteristics differ significantly between the cap and rod sides.

Asymmetric Cylinder Dynamics and Control Challenges

The dynamics of an asymmetric hydraulic cylinder are governed by the force balance equation and the fluid continuity equations for both the cap and rod chambers. The area ratio, defined as the ratio of the cap-side effective area to the rod-side effective area, is typically between 1.5 and 2.5 for common industrial cylinders. This asymmetry means that for a given flow rate, the velocity on the cap side is lower than on the rod side, and the pressure on the rod side must be higher to achieve the same force output.

The control challenge is exacerbated by the presence of external load disturbances, friction forces that vary with velocity and direction, and the nonlinear pressure-flow characteristics of the control valves. The traditional proportional-integral-derivative control approach often results in poor tracking accuracy during direction reversal and under high-speed operation, where the nonlinear effects are most pronounced. The compound control strategy presented in this paper addresses these limitations through a multi-layered control architecture.

Compound Control Strategy Design

The compound control strategy integrates four elements: a sliding mode controller for robust disturbance rejection, a feedforward compensator for velocity and acceleration prediction, a friction compensator that models the Stribeck effect, and a pressure equalization compensator that addresses the asymmetric pressure characteristics. The sliding mode controller provides the fundamental robustness against model uncertainties and external disturbances, while the feedforward and compensation elements improve tracking accuracy and reduce control effort.

The sliding mode controller is designed with a reaching law that minimizes chattering while maintaining finite-time convergence. The switching gain is determined through Lyapunov stability analysis to ensure that the sliding surface is reached and maintained despite parameter variations. The feedforward compensator uses a kinematic model of the cylinder to predict the required flow rates and pressures for the commanded trajectory, reducing the burden on the feedback controller.

The friction compensator is particularly important for asymmetric cylinders because the friction characteristics differ between the cap and rod sides due to the different seal pressures and contact forces. The Stribeck friction model, which captures the transition from static to kinetic friction, is implemented as a nonlinear compensator that adjusts the control signal based on the instantaneous velocity.

Experimental Validation and Results

The experimental setup consists of a 50-millimeter bore asymmetric hydraulic cylinder with an area ratio of 2.0, driven by a proportional valve-controlled hydraulic power unit. The control system is implemented on a real-time industrial controller with a sampling rate of 10 kilohertz. The experimental results demonstrate significant improvements in tracking accuracy compared to conventional PID control.

The compound control strategy achieves position tracking errors below 0.3 millimeters across the full stroke, which represents a tenfold improvement over the baseline PID control. The direction reversal error, which is typically the most challenging aspect of asymmetric cylinder control, is reduced to below 0.8 millimeters, making the system suitable for high-precision applications such as metal forming and material handling.

Engineering Practice Connections

From the perspective of pressure vessel fabrication, the principles of compound control are relevant to the automation of welding and cladding processes. In automated submerged arc welding or plasma transferred arc cladding, the control of welding parameters such as travel speed, torch height, and wire feed rate requires precise servo control of the actuator systems. Asymmetric hydraulic cylinders are commonly used in welding manipulators and cladding equipment, and the compound control strategies presented here can be directly applied to improve the precision and reliability of automated welding processes.

Furthermore, the experimental methodology employed in this paper, including the systematic comparison of control strategies and the quantitative evaluation of performance metrics, provides a template for the qualification and optimization of automated manufacturing equipment. The rigorous approach to control system validation is consistent with the quality assurance requirements of pressure vessel fabrication, where process qualification and control are essential for ensuring product integrity.

Conclusion

This research presents a comprehensive compound control strategy for asymmetric hydraulic cylinder displacement servo systems that achieves significant improvements in tracking accuracy and disturbance rejection. The integration of sliding mode control, feedforward compensation, friction modeling, and pressure equalization provides a robust and high-performance control solution that is applicable to a wide range of industrial automation applications. For engineers involved in pressure vessel fabrication and cladding manufacturing, the principles of compound control and the experimental validation methodology offer valuable tools for enhancing the precision and reliability of automated manufacturing processes.