Lasse Schmidt

Robust Non-Chattering Observer Based Sliding Control Concept for Electro-Hydraulic Drives

Abstract This paper presents an observer-based sliding mode control concept with chattering reduction, generally applicable for position tracking control of electro-hydraulic valve-cylinder drives (VCDs). The proposed control concept requires only common data sheet information and no knowledge on load characteristics. Furthermore the proposed scheme only employ ***piston-and valve spool positions- and pressure feedback, commonly available in industry. The main target is to overcome problems with linear controllers deteriorating performance due to the inherent nonlinear nature of such systems, without requiring extensive knowledge on system parameters nor advanced control theory. In order to accomplish this task, an integral sliding mode controller designed for the control derivative employing state observation is proposed, based on a generalized reduced order model structure of a VCD with unmatched valve flow- and cylinder asymmetries. It is shown that limited attention can be given to bounds on parameter estimates, that chattering is reduced and the number of tuning parameters is reduced to the level seen in conventional PID schemes. Furthermore, simulation results demonstrate a high level of robustness when subjected to strong perturbations in supply pressure and coulomb friction force, and that tracking accuracy may be reduced to the level of noise. Furthermore, the proposed controller tolerates significant noise levels, while still remaining stable and accurate.

On Application of Second Order Sliding Mode Control to Electro-Hydraulic Systems

This paper discusses the application of second order mode controls to hydraulic valve-cylinder drives with a special focus on the limitations resulting from nonlinear dynamic effects in flow control valves. Second order sliding mode algorithms appear highly attractive in the successive implementation of sliding mode control, achieving continuous control inputs, while maintaining the main properties of sliding modes. Under certain model assumptions, some of these controllers may even be applied as output feedback controllers. However, intrinsic nonlinear dynamic effects of hydraulic valves such as slew rates and time delays arising in the amplification stages, limits the applicability of such methods, and may lead to partial losses of robustness and limit cycles. These properties are analyzed and experimentally verified, and compensation methods are proposed. The application of the second order sliding algorithm known as the super twisting controller is considered for output feedback control and compared with conventional first order sliding mode control. The controllers under consideration are applied for position tracking control of a hydraulic valve-cylinder drive exhibiting strong variations in inertia- and gravitational loads. Results demonstrate that the super twisting algorithm may be successfully applied for output feedback control of hydraulic valve-cylinder drives, with modifications guaranteeing robust control performance in a small vicinity of the control target.© 2014 ASME

2-SMC of Electro-Hydraulic Drives Using the Twisting Algorithm

In this paper a controller utilizing second order sliding modes, generally applicable for position tracking control of electro-hydraulic valve-cylinder drives (VCD), is proposed. The proposed controller requires pressure measurements, and only the signs of the valve spool position and piston position- and velocity. The main objective is to introduce a control concept that provide for increased performance compared to linear controllers, in the presence of the inherent nonlinear nature characterizing such systems. To accomplish this task, a controller based on the twisting algorithm and knowledge of system gain variations is proposed. Results demonstrate strong robustness when subjected to parameter perturbations and that control chattering is eliminated.

Modeling of movement-induced and flow-induced fluid forces in fast switching valves

Fast switching fluid power valves set strict requirements on performance, size and energy efficiency and simulation models are therefore needed to obtain good designs of such components. The valve moving member is subject to fluid forces depending on the valve flow rate and movement of the valve member itself. These fluid forces may be accurately simulated using Computational Fluid Dynamics (CFD) analysis, but such models suffer from being computationally expensive and is not suited for optimization routines. In this paper, a computationally inexpensive method for modeling the fluid forces is proposed, which includes both the flow-induced fluid forces and the movement-induced fluid forces resulting from movement of the valve moving member. The movement-induced fluid force model is based on a known solution to the linearized Navier-Stokes equations. A method for accurately simulating the flow-pressure relationship of a switching valve based on CFD results is presented along with the fluid force model, to constitute a complete valve fluid model. The parameters needed for the proposed model are determined based on CFD analyses, and the process of finding these parameters are described based on a reference valve design. Simulated results of the total fluid force are presented showing the movement-induced fluid force to be significant for a reference application. The model form established is useful for valve designers during development and for accurate operation simulation.

Experimental validation of modelled fluid forces in fast switching hydraulic on/off valves

A prototype of a fast switching valve for a digital hydraulic machine has been designed and manufactured. The valve is composed of an annular seat plunger connected with a moving coil actuator as the force producing element. The valve prototype is designed for flow rates of 600 l/min with less than 0.5 bar pressure drop, and the models predicts a switching time in the region of a millisecond with a travel length of 3.5 mm using an average power of 250 W. The total machine efficiency when neglecting losses not related to the valves is above 98 %. The objective of this paper is to experimentally validate a transient computational fluid dynamics (CFD) model of the fluid forces that oppose the valve plunger when moving rapidly through the surrounding oil during switching. Due to the fast switching of the valve, the fluid forces which oppose plunger movement increases drastically as the plunger approaches the closed position. Fast switching is essential for digital hydraulic machines to achieve a high efficiency. As the fluid forces influences the response obtaining an accurate model is important. To validate the model tests are carried out on the prototype where the valve is closed, both with and without oil surrounding the valve plunger. The transient CFD model is then verified by comparing measurements with simulation results.

Speed-Variable Switched Differential Pump System for Direct Operation of Hydraulic Cylinders

Efforts to overcome the inherent loss of energy due to throttling in valve driven hydraulic systems are many, and various approaches have been proposed by research communities as well as the industry. Recently, a so-called speed-variable differential pump was proposed for direct drive of hydraulic differential cylinders. The main idea was here to utilize an electric rotary drive, with the shaft interconnected to two antiparallel fixed displacement gear pumps, to actuate a differential cylinder. With the design carried out such that the area ratio of the cylinder matches the displacement ratio of the two gear pumps, the throttling losses are confined to cross port leakage in the cylinder and leakage of the pumps. However, it turns out that the volumetric pump losses and the pressure dynamics of the cylinder and connecting pipes may cause pressure increase- or decrease in the cylinder chambers, which may seriously influence the dynamics and hence the performance during operation. This paper presents an analysis of these properties, and a redesign of the hydraulic system concept is proposed. Here the area- and displacement ratios are deliberately mismatched, causing inherent pressure build-up or cavitation in the return chamber, depending on the direction of motion. In order to avoid cavitation, a third gear pump is introduced, which provides a flow in the relevant cylinder chamber in one direction of motion, while operating in idle mode in the opposite motion direction. Together with two 2/2 way proportional valves, this design allows to control the lower chamber pressure levels, throttling excess compression flow to tank. The resulting design introduces additional losses due to throttling of excess compression flow, but also improves the dynamic properties of the system significantly. The proposed features are verified by comparison with the original pump concept and a conventional valve concept. Furthermore, significant improvement in energy efficiency is demonstrated under certain load conditions.Copyright

Experimental evaluation of control strategies for hydraulic servo robot

In this paper different linear and non-linear controllers applied to a hydraulically driven servo robot are evaluated and validated. The task is to make the actuators of the manipulator track a position reference with minimum error. Hydraulic systems are intrinsically non-linear and using linear control techniques typically results in conservatively dimensioned controllers to obtain stable performance. Non-linear control techniques have the potential of overcoming these problems and in this paper the focus is on applying simple nonlinear robust and adaptive controllers feasible for implementation in industrial servo drives. The different controllers are compared and evaluated from simulation and experimental results.

Application of second order sliding mode algorithms for output feedback control in hydraulic cylinder drives with profound valve dynamics

The application of second order sliding mode algorithms for output feedback control in hydraulic valve-cylinder drives appear attractive due to their simple realization and parametrization, and strong robustness toward bounded parameter variations and uncertainties. However, intrinsic nonlinear dynamic effects of hydraulic valves such as slew rate limitations and time delays arising in the electrical and mechanical amplification stages limits the applicability of such methods, and may lead to partial losses of robustness and limit cycles/oscillations in the outputs, internal states and the valve input signals. The application of some popular second order sliding mode controllers and their smooth counterparts are analyzed and experimentally verified. The controllers are considered for output feedback control and compared with a conventional PI control approach. The controllers under consideration are applied for position tracking control of a hydraulic valve-cylinder drive exhibiting strong variations in inertia- and gravitational loads, and furthermore suffer from profound valve dynamics. Results demonstrate that both the twisting- and super twisting algorithms may be successfully applied for this purpose, when continuous approximations of discontinuous are utilized, and furthermore that excellent performance may be achieved when applying their smooth counterparts directly.ZusammenfassungDie Anwendung von Sliding-Mode-Algorithmen zweiter Ordnung für die Ausgabesteuerung in hydraulischen Zylinder-Antrieben mit tiefgreifender Ventildynamik erweist sich aufgrund folgender Eigenschaften als attraktiv: einfache Realisierung und Parametrisierung und eine starke Robustheit gegenüber begrenzten Parametervariationen und Unsicherheiten. Allerdings begrenzen intrinsische nichtlineare dynamische Effekte von Hydraulikventilen wie Einschränkungen der Anstiegsgeschwindigkeit und Zeitverzögerungen in den elektrischen und mechanischen Verstärkungsstufen die Anwendbarkeit solcher Verfahren. Dies kann zu partiellen Verlusten an Robustheit führen und Grenzzyklen/Schwingungen in den Ausgaben, internen Stadien und die Ventileingangssignale begrenzen. Die Anwendung von einigen populären Sliding-Mode-Steuerungen zweiter Ordnung und deren ebenen Pendants sind analysiert und experimentell verifiziert. Die Regler sind für die Regelung der Ausgaben bestimmt und werden mit einem herkömmlichen PI-Regelungsansatz verglichen. Die zu betrachtenden Regler werden zur Kontrolle der Positionsverfolgung eines hydraulischen Ventil-Zylinder-Antriebs verwendet, der starke Schwankungen in Trägheits- und Gravitationslasten aufweist. Außerdem treten Probleme mit profunder Ventildynamik auf. Die Ergebnisse zeigen, dass sowohl die Twisting- als auch Super Twisting-Algorithmen erfolgreich für diesen Zweck eingesetzt werden können, falls Folgendes gewährleistet ist: eine kontinuierliche Annäherung von Diskontinuierlichem. Darüber hinaus besteht die Aussicht, eine ausgezeichnete Leistung zu erzielen, wenn deren ebene Pendants direkt Anwendung finden.

An Approach for Second Order Control With Finite Time Convergence for Electro-Hydraulic Drives

Being a second order sliding algorithm, the super twisting algorithm is highly attractive for application in control of hydraulic drives and mechanical systems in general, as it utilizes only the control error while driving the control error as well as its derivative to zero for properly chosen algorithm parameters. However a discontinuous term internally in the control structure may excite pressures of transmission lines in hydraulic drives as the control structure strives to maintain the control error and its derivative equal to zero. In this paper a modified version of a controller based on the super twisting algorithm is proposed, with the focus of eliminating the discontinuous term in order to achieve a more smooth control operation. The convergence properties of the proposed controller are analyzed via a conservative phase plane analysis. Furthermore, homogeneity considerations imply finite time convergence of states to the origin under certain model assumptions. Results demonstrate the smooth control operation compared to the direct super twisting control approach while maintaining robustness properties in relation to position tracking of a hydraulic drive, under parameter perturbations, uncertainties and un-modeled dynamics.Copyright

Robust Position Tracking for Electro-Hydraulic Drives Based on Generalized Feedforward Compensation Approach

This paper presents the development of a robust tracking control concept based on accurate feedforward compensation of hydraulic valve-cylinder drives. The proposed feedforward gain is obtained by use of a generalized description of the valve flow that takes into account any asymmetry of valves and/or cylinders, not assuming these asymmetries to be matched, and utilizes pressure measurements. Hence the proposed compensator is generally applicable to any such drive. Simulations show that the proposed compensator poses superior performance and robustness when subjected to strong perturbations in supply pressure and coulomb friction, compared with a conventional constant gain type feedforward compensator.