Study On The Speed Control Of The Variable-speed Controlled Closed-circuit Hydraulic Elevators

Energy-saving is one of the most important research areas of hydraulic systems.The variable-speed control is applied in the hydrostatic system for its high efficiency.The variable-speed control system is developed for large inertia hydraulic lift equipment,such as hydraulic elevators,hydraulic pumping units.In addition to the discussion of energy-saving, the system performance should be investigated also.In the velocity performance research of the system,there were two problems existed:the open-loop control method had the limitations in achieving high precision control of velocity and the hysteresis occurred in the startup of the system.Therefore,this research focused on developing the appropriate velocity control algorithm and speed startup method for the system.Besides these two aspects,the new energy-saving idea need be discussed because of the requirement of developing higher efficiency approach for hydraulic lift equipment.In this thesis,the behavior of the variable-speed controlled hydrostatic system was investigated on a hydraulic elevator test rig.To achieve high precision control of the velocity,the stability of the system was analyzed.Because the hydraulic elevator is a type 0 system with the feature of low frequency,the steady-state error of velocity still existed by unity-feedback control without compensation,and the velocity lag occurred with the integral compensation.A compound algorithm of PD & feedforward-feedback control was proposed to deal with the steady-state error.In order to overcome the hysteresis in the speed startup control of hydraulic elevator,the relationship between the startup performance and the nonlinear friction in the system was investigated.The speed startup method,based on expert control and model predictive strategy was developed,and was validated by the experiments under different working conditions.In addition,the vibration characteristics of the hydraulic elevator were analyzed.The dynamic model with 8 degree-of-freedom was established for describing the vertical dynamics of the hydraulic elevator system.The results of theoretical and experimental modal analysis on the system showed the validity of the dynamic model.And the results indicated that the frequency coverage of the main response of the system were within 100~150 Hz.Therefore,to avoid the resonance vibrations of the cabin in hydraulic elevator,the vibration source with frequencies range of 100~150 Hz should be isolated.Based on the requirement of higher efficiency in hydraulic lift equipment,an innovative idea of variable counterweight was developed.Compared with other variable-speed controlled hydraulic systems,the efficiency of the variable counterweight system was higher.In general,the results from this research can be applied to not only the variable-speed controlled hydraulic elevator,but also other variable-speed controlled hydrostatic systems.The thesis is outlined as follows.In chapter 1,the variable-speed controlled hydrostatic system and its typical application in hydraulic elevator are introduced.The development of the hydraulic elevator is expatiated and its existed problems are analyzed.The necessity of research on variable-speed control hydrostatic system and the markets requirement of hydraulic elevator are pointed out.Consequently,the contents and difficulties of the research are proposed.In chapter 2,the velocity behavior of the variable-speed controlled hydraulic elevator by feedback control is analyzed.The mathematic models of the system and the transfer function are established.Frequency-domain analyses show that it is a type 0 system with the feature of low frequency and poor damping.The steady-state error still exists in the system by the unity-feedback control without compensation.And the velocity lag occurs if the approach of integral compensation is adopted to eliminate the steady-state error.The compound algorithm of PD & feedforward-feedback control is proposed,which satisfies the requirement of the system against steady-state error and nonlinearities.In chapter 3,the method of speed startup control for the system is discussed.The speed startup behavior of the system by pressure pre-balanced control and speed feedback control is poor because of its friction and large inertia.The influence of nonlinear friction on the speed startup behavior of the system is analyzed,and the friction model is established through experiments.The speed startup method is developed based on the expert control and model predictive strategy,and experiments are conducted under different working conditions,namely variable loads and variable friction.The results show that the speed startup behavior of the improved system is excellent.In chapter 4,the cabin vibration characteristics of the variable-speed controlled hydrostatic hydraulic elevator are discussed.The 8 degree-of-freedom dynamic model of vertical direction for the system is established and the modal analyses are conducted.The experiments of modal analyses for the system are also carried out,which show that the dynamic model is correct and the frequency coverage of the main response of the system are within 100-150 Hz.And so,if the frequencies of outside vibration source are within 100-150 Hz,it must be avoided.In chapter 5,an innovative energy-saving idea of variable counterweight is proposed, and two different kinds of principles of the idea are introduced,namely variable hydraulic-counterweight and mechanical-counterweight with variable speed.The hydraulic elevator system applying the variable hydraulic-counterweight is discussed in detail,and the characteristic of energy consumption of the system is studied based on AMEsim simulation. The features of the system are discussed,which mainly involve efficiency,storage energy in counterweight and dynamic response,and the development of the variable counterweight technology is prospected.In chapter 6,all the research contents are summarized,and some new views are put forward in the future.