System identification has importance in modeling and control of industrial systems. The main task of system identification is to build a suitable model that represents the relationship between input, output and disturbances of a real system. The thesis presents identification and discrete time linear modeling of a hydraulic actuator. This thesis demonstrates how to formulate hydraulic functions for both normal and faulty conditions with internal leakage using both offline and on-line measurements. Least square and recursive least square methods are used to estimate the model parameters based on the Auto Regressive technique with Exogenous input (ARX) model. For the offline case, square and sine wave signals are used as input control signals. For the online case, random input control signal is applied. Prediction error criterion is used for model validation based on experimental data. It is shown that the ARX model is capable of representing a valve-controlled hydraulic system dynamics.

System identification has importance in modeling and control of industrial systems. The main task of system identification is to build a suitable model that represents the relationship between input, output and disturbances of a real system. The thesis presents identification and discrete time linear modeling of a hydraulic actuator. This thesis demonstrates how to formulate hydraulic functions for both normal and faulty conditions with internal leakage using both offline and on-line measurements. Least square and recursive least square methods are used to estimate the model parameters based on the Auto Regressive technique with Exogenous input (ARX) model. For the offline case, square and sine wave signals are used as input control signals. For the online case, random input control signal is applied. Prediction error criterion is used for model validation based on experimental data. It is shown that the ARX model is capable of representing a valve-controlled hydraulic system dynamics.

Diagnosis of faults associated with hydraulic actuators is essential to avoid accidents or loss of system functionality. This thesis focuses on internal leakage fault diagnosis in valve controlled hydraulic actuation systems (VCA) as well as electrohydrostatic actuation systems (EHA). For the VCA, the hydraulic actuator is driven in a closed loop mode to track a pseudorandom input signal whereas for the EHA, an actuator is driven in an open loop mode to track a sinusoidal input. Motivated by developing a method that does not rely on the model of the system or type of fault, signal processing techniques based on the ratio of metric lengths of pressure signals, autocorrelation of pressure signal, cross correlation between chamber pressure signals, and cross correlation between control signal and piston displacement is employed for internal leakage diagnosis. For the VCA, autocorrelation of pressure signals performed well at lower lags (less than 4) and at a window size of 200 data points; both cross correlation between pressure signals and cross correlation between control signal and piston displacement performed well at higher lags (higher than 8) and at a window size of 100 data points; ratio of metric lengths of pressure signals was found to be more effective at higher lag ratios (more than 16:3). All methods were sensitive to the lowest simulated leakage of 0.047 L/min, though with different level of success; ratio of metric lengths produced 84% sensitivity, autocorrelation 19% sensitivity, cross correlation between pressure signals 25% sensitivity and cross correlation between piston displacement and control signal 20% sensitivity. For the EHA, all methods were capable of identifying small leakage of 0.98 L/min. The ratio of metric lengths produced 6.7% sensitivity, autocorrelation 2.59% sensitivity, cross correlation between pressure signals 9.4% sensitivity and cross correlation between piston displacement and control signal 31.9% sensitivity. The low leakage detection achieved without requiring a model of the actuator or leakage type make these methods very attractive for industrial implementation

Two - stage servovalves, despite being widely used in aircraft and industry because of their reliability and high performance, have a few disadvantages that are still unsolved at the state of the art, such as the power consumption caused by the quiescent flow (internal leakage) in the pilot stage, and the complexity and high number of parts of the torque motor assembly of the pilot stage. The solution to these problems can help to reduce costs, weight, power consumption, and to enhance the reliability and producibility as well as the performance of these valves. This paper presents a novel configuration of servovalve, which has the potential to overcome the above-mentioned issues. The proposed servo-valve includes a novel architecture for the pilot stage by using two piezo-electric actuators (ring benders). In this paper, the performance of this novel pilot stage is assessed. To this end, a valve prototype has been constructed and tested; the experimental results are also used to validate a numerical model obtained with the software Simscape Fluids. The results show that, acting on specific parameters, the performance of the piezo-valve can be very competitive, while ensuring very low internal leakage and complexity.