# A Study on Failure Analysis and High Performance of Hydraulic Servo Actuator
## 1 Introduction  
Hydraulic servo actuators have been widely used in industrial applications because of their high power-to-weight ratio, high stiffness, high payload capability, etc. For example, test devices, aircraft pilot simulators, anti-lock braking systems, hydraulic excavators, press, plate hot rolling, shaking systems and steam control valve actuator of power plant turbines. [1; 2; 3; 4; 5; 6]. To improve servo actuators, techniques applied to control elements were introduced [7], which resulted in good results in field operations. A hydraulic actuated clutch control system for commercial cars was also analyzed [8; 9]. Comprehensive effects coming from measurement noises and matched and mismatched uncertainties make it difficult for electro-hydraulic servo systems to further attain a high-accuracy tracking level. The existing control strategies often consider these control issues one-sidedly. Accordingly, [PERSON] and [PERSON] develop two different control strategies combining integral robust control and direct adaptive control for high-precision position control of double-rod electro-hydraulic systems to account for these control issues concurrently [10]. [PERSON], [PERSON], and [PERSON] present a degradation assessment and life prediction method for electrohydraulic servo valve (EHV). Unlike traditional statistical methods, the work is motivated by the failure mechanism of erosion wear. The degradation of the performance characteristics was related to structure wear in twin flapper-nozzle valve and spool valve, which is also related to the hydraulic servo actuator [11]. The system reliability is most important part in the industrial group because the users need to prevent the failures of the system before failures and collapses occur. Failures and collapses of steel truss towers may be caused by various factors, which often occur simultaneously. The evaluation of truss tower reliability when single elements fail constitutes an important research area [12; 13]. [PERSON] and [PERSON] were concerned with the design of fault-tolerant servo systems against actuator failure. A controller structure consisting of a diagonal servo compensator and full matrix stabilizing controller is proposed for the fault-tolerant servo systems [14]. Two output feedback controllers are proposed for motion control of double-rod electrohydraulic servo actuators with matched and mismatched disturbances rejection [15]. Additionally, [PERSON] and [PERSON] introduce the prognostic and health management system for fly-by-wire electro-hydraulic servo actuators for the detection and tracking of actuator faults [16]. Friction in hydraulic actuators can be described using nonlinear, velocity dependent models. In this paper, the friction is described by an exponential Stribeck friction model [17]. The dynamic equations describing the pressure changes in the chambers cannot be combined into a single load pressure equation. This complicates the controller design since it not only increases the dimension of the system to be dealt with but also brings in the stability issue of the added internal dynamics [18]. In a typical real-time hybrid simulation (RTHS) setup, servo-hydraulic actuators serve as interfaces between the computational and physical substructures. Time delay introduced by actuator dynamics and the complex interaction between the actuators and the specimen has detrimental effects on the stability and accuracy of the RTHS [19; 20; 21]. Dynamic testing of structures and components in the laboratory to determine their mechanical properties is an essential part of engineering research and development. Test apparatus of increasing sophistication has been designed over the past few decades to replicate real-world forces and motions within the laboratory environment [22; 23; 24].