3.2. Parameter IdentificationIn general, the unknown system parameters of the linear elements selleck MEK162 in a nonlinear system can be identified by the signal compression method (SCM) [24]. In this study, SCM is applied to identify the hydraulic part of the EHA prototype. However, hydraulic servo systems engage in significant nonlinear behavior due to system uncertainties such as pipe losses, leakages, and parameter variations of the working fluid [9�C11, 16, 17, 19]. Some of those uncertainties are sometimes neglected because the SCM can only identify the linear part of the system in the identification process. This study then compensates for the neglected nonlinear behavior of the EHA system by using a robust controller, that is adaptive PID sliding mode control scheme.

As shown in Figure 4, the test signal that has the same amplitude up to 4Hz in the frequency domain is applied to the position control system of EHA by constructing a close-loop proportional control system to obtain a more accurate equivalent impulse response for the precise identification of unknown parameters. Figure 5 shows the comparison of the frequency response of the position control system between the closed-loop nominal model and the closed-loop actual system with a proportional controller, whose gain is 1,000.Figure 4Test signal.Figure 5Bode plots of the closed loop nominal model and the closed loop actual system with a proportional controller.The identified transfer function of a closed-loop actual system with a proportional controller for EHA position control system is as follows:KG(s)1+KG(s)=11.

8��107s3+2231s2+71.8��105s+11.8��107,(13)where K is proportional gain of closed-loop actual system.From the identified nominal model for the closed-loop position control system for EHA with the proportional controller, the parameters of the position control system can be acquired by eliminating the effect of the proportional controller mathematically.After eliminating AV-951 the effect of the proportional controller, the identified transfer function of the hydraulic part of the EHA prototype is as follows:G(s)=11.8��104s3+2231s2+71.8��105s.(14)The identified system parameters were verified on the time domain. Figures 6(a) and 6(b) show the response against test signal and the step responses of the nominal model and actual system, respectively.