Indirect Adaptive Robust Control of Electro-Hydraulic Systems Driven by Single-Rod Hydraulic Actuator

This paper presents an indirect adaptive robust control (IARC) of electro-hydraulic systems driven by singlerod hydraulic actuators. Unlike the tracking-performanceoriented direct adaptive robust control (DARC) algorithm, in addition to good output tracking performance, the IARC also focuses on accurate parameter estimates for secondary purposes such as machine health monitoring and prognostics. Accurate parameter estimates are obtained through parameter estimation algorithms based on the plant dynamics rather than the tracking error dynamics, while robust stability and performance are achieved through adaptive robust control. Comparative experimental results show that the proposed IARC achieves good tracking performance and accurate parameter estimation. INTRODUCTION Electro-hydraulic systems have been widely used in industry due to their small size-to-power ratio and the ability to apply very large force and torque. However, precision motion control of hydraulic systems is far from trivial. The dynamics of hydraulic systems are highly nonlinear [1]. Furthermore, the system may be subjected to non-smooth and discontinuous nonlinearities due to control input saturation, directional change of valve opening, friction, and valve overlap. Aside from the nonlinear nature of hydraulic dynamics, electro-hydraulic systems also have large extent of model uncertainties, which can be classified into two categories: parametric uncertainties and uncertain nonlinearities. Examples of parametric uncertainties include the large change in load seen by the actuator and large variations in the hydraulic parameters (e.g., bulk modulus) due to the change of temperature, component wear, etc. Uncertain nonlinearities, such as the external disturbances, leakage, and friction, cannot be modelled exactly and the nonlinear functions that describing them are usually unknown. These model uncertainties may cause the controlled system, designed on the nominal model, to be unstable or have a much degraded performance. Nonlinear robust control techniques, which can de∗The work is supported in part through the NSF grant CMS-0220179. liver high performance in spite of both parameter uncertainties and uncertain nonlinearities, are essential for successful operations of high-performance electro-hydraulic system. In [2] and [3], the adaptive robust control (ARC) technique proposed by Yao and Tomizuka in [4] and [5], was applied to precision motion control of electro-hydraulic systems driven by single-rod actuators. The underline parameter estimation law in ARC controller are based on the direct adaptive control design such as the tuning function based adaptive backstepping, in which the adaptive control law and parameter adaptation law are synthesized simultaneously to meet the sole objective of reducing the output tracking error. Such a design normally leads to a controller whose dynamic order is as low as the number of unknown parameters to be adapted, and achieves excellent output tracking performance even in the presence of model uncertainties and external disturbances as done in [2] and [3]. However, the direct ARC (DARC) approach also has the drawback that the design of adaptive control law and the parameter estimation law cannot be separated and the choice of the parameter estimation algorithm is limited to the gradient type with certain actual tracking errors as driving signals. It is well known that the gradient type of parameter estimation law may not have as good convergence properties as other types of estimation laws (e.g., the least square method). Furthermore, although the desired trajectory might be persistently exciting and of large signal, the actual tracking errors in implementation are normally very small, and thus the parameter adaptation is prone to be corrupted by other factors such as the sampling delay and noise that have been neglected when synthesizing the parameter adaptation law. As a result, in implementation, the parameter estimates in the DARC are normally not accurate enough to be used for secondary purposes such as prognostics and machine component health monitoring, even when the desired trajectory is persistently exciting enough. The paper focuses on the of precision motion control of electro-hydraulic systems driven by single-rod actuators while accurate parameter estimates are needed for secondary purposes. An IARC design will be presented to overcome the poor parameter estimation problem of the DARC designs