| Electro-hydraulic load simulator(EHLS)is directly and mechanically connected with the tested actuator.It is a force or torque control system and widely applied in the testing of actuator system.However,for the passive torque servo control system,the active motion of the actuator is a strong disturbance to the load simulator.This kind of strong disturbance is called surplus torque,which seriously affects the load simulation accuracy of EHLS.It is difficult to improve the load simulation accuracy of EHLS due to surplus torque.To improve the accuracy of load simulation,this paper proposes a friction EHLS(FEHLS)based on friction loading.To explore the method of suppressing the surplus torque of EHLS and a novel load simulation scheme,the mathematical model of EHLS is established,the causes of surplus torque and the limiting factors of synchronous compensation effect are analyzed.The effect of synchronous compensation and robust control to suppress the excess torque is analyzed by simulation,and the influence of surplus torque on the torque simulation of load simulator is verified,which is difficult to eliminate.To eliminate the surplus torque completely,the coupling between the load simulator and the tested actuator can be eliminated only by fundamentally changing the loading scheme.Therefore,this paper proposes a friction electro-hydraulic load simulation method based on friction loading.Working principle of FEHLS is introduced in detail,its mathematical model is established.And it is analyzed that the movement of the actuator has little effect on its loading performance.The basic loading performance of FEHLS are verified by simulations.In the theory,there is no problem of surplus torque for FEHLS.However,on the one hand,friction heat generation and friction and wear problems will occur when using friction to load,on the other hand,friction characteristics will be different due to various external factors,which will affect the loading performance.The friction heat generation and wear in the friction process are analyzed theoretically.The changes of friction coefficient of different friction pairs with pressure,relative speed and temperature are measured by friction tester,and the temperature rise and friction coefficient of different friction pair materials under the same conditions are also measured,which lay the foundation for choosing the suitable material for friction loading.Friction loading is the characteristic of FEHLS.Friction components and friction have great influence on the loading performance of FEHLS.Friction coefficient is usually small so that the maximum load torque from friction loading is generally less than the maximum torque of the EHLS under the same power.Temperature rise,pressure and relative speed change the friction coefficient,which directly affects the performance of FEHLS.The friction components increase the inertia of system,and the motion of actuator produces inertia moment,which affects the loading performance of FEHLS.The thermal stress generated by friction temperature rise is a kind of disturbance for the loading of the friction load simulator.Due to installation accuracy and fricition wear,it is easy to produce clearance between the friction pairs,which leads to the dead zone of torque tracking when torque tracking crosses zero.The effects of friction coefficient and its variation,inertia of friction components,friction temperature rise and friction pair clearance on the loading performance are simulated and analyzed.According to these factors,the key structure and structural parameters of the system are optimized.FEHLS is a typical electro-hydraulic torque servo system,which is a high-order nonlinear system.Compared with the general electro-hydraulic servo systems,FEHLS uses friction to realize torque servo control,and the uncertainty and nonlinearity of friction directly increase the difficulty of its control.Considering the high-frequency fluctuation of friction and the high-order characteristics of the system,a flatness control based on singular perturbation theory is proposed.The high-order system is divided into two low-order systems,and suitable controllers are designed respectively,which reduces the complexity of controller design and the sensitivity to noise and fluctuation.Lyapunov function is used to prove that the closed-loop reduced order system and closed-loop boundary layer system are asymptotically stable.Tikhonov theorem is used to analyze the actual asymptotical convergence of the whole closed-loop system.Simulation results show that the flatness control based on singular perturbation theory is effective.FEHLS has the problems of disturbance inertia moment,parameter uncertainty,unmodeled dynamics and input saturation.A dynamic surface robust disturbance rejection control method is proposed.It is proved that all signals of the system are semi-global and ultimately uniformly bounded under the proposed dynamic surface robust disturbance rejection control.The simulation results show that the dynamic surface robust disturbance rejection control is effective.Experimental system of FEHLS are developed.Load simulator driven by hydraulic motor(LSDBHM)and FEHLS are debugged and their basic performance is tested.The control methods proposed in this paper is verified by experiments. |
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