| Recently, hydraulic power steering system (HPS) has been the dominant steering system equipped in heavy-duty vehicles, which are likely to result in high-speed vehicles "quilted" and make drivers lack of steering road feeling. Furthermore, there are large amounts of reactive power losses. Therefore it is necessary to improve the steering system of heavy-duty commercial vehicles. Electro-hydraulic power steering system (EHPS) and electric power steering system (EPS) widely used in the field of passenger car and light commercial vehicles are not suitable for heavy commercial vehicles due to power limitations. Electronically controlled hydraulic power steering system (ECHPS) regulates flow or pressure of the hydraulic system via electronically controlled device on the basis of HPS, which is combined with the advantages of HPS great effort and EPS variable assist characteristics, thus ECHPS will be the feasible development tendency of heavy-duty commercial vehicles’ steering system for the next few years, nevertheless existing ECHPS technology can’t perfectly cope with the problems of HPS. So the novel electronically controlled hydraulic power steering system (E-ECHPS) based on electromagnetic slipping clutch (ESC) was proposed. This paper is concerned with the design theory and control strategy of E-ECHPS system, the main work is as follows:To meet the requirement of steering portability, maximum pressure of the hydraulic system is matched and calculated, in order to meet the following performance of steering assistance, maximum flow of the hydraulic system is also matched and calculated depending on the steering angle velocity and steering effort; Combined with maximum flow of the hydraulic system and the engine speed range, the flow characteristic of E-ECHPS steering pump is designed, accordingly the speed and torque scope of ESC is matched. According to the technical requirements of the ESC from the vehicle, the ESC prototype machines is designed, which is acted as the research carrier for ESC control.The ESC mathematical model is established, including flux equation, voltage equations and torque equation and equation of motion. In view of the time-varying winding inductance, the ESC model in dq coordinate system is deduced. The results of simulation and experiment of the mechanical characteristic, input-output characteristic, excitation current and output characteristic verify the correctness of the mathematical model. The model is the basis of the ESC control research.The control strategy of energy claiming type ESC is studied based on field orientation, namely the direct axis and quadrature axis components of current from the AC side of PWM rectifier are controlled separately. In order to realize the consistent phase with the ac voltage and current, the expectation of direct axis component is set to zero. For the stabilization of the dc side voltage, the PI controller of dc side voltage is design, whose output is acted as the expectation of quadrature axis component of AC current. Output speed of the ESC is adjusted using PID closed-loop control. Simulation results show that the proposed control strategy of the ESC based on field orientation is conducive to improve the dynamic response of ESC output speed under load torque disturbance and the recovery efficiency of ESC slipping loss. The control strategy is theoretical basis for on-demand power matching of E-ECHPS system.Based on the theory of on-demand power matching, the matching method of hydraulic system’s flow and pressure is put forward. The pressure of hydraulic system is determined by the steering resistance moment and pilot steering torque, so the variable assist characteristics with vehicle speed are designed based on the drives’ preferred steering torque. The flow of hydraulic system is determined by steering velocity and vehicle speed, so the flow control curves of E-ECHPS depending on steering velocity and vehicle speed are designed. Provided that the flow of hydraulic system is proportional to the steering pump speed, ultimately it is turned into the control of ESC output speed. Consequently the essence of on-demand power matching of E-ECHPS system is to reclaim the slipping loss as well as to match steering power, thus the control strategy of E-ECHPS is made.By comparing the power consumption of HPS and E-ECHPS, the energy-saving mechanism of E-ECHPS is revealed. By simulation of HPS and E-ECHPS under synthesis conditions, including steering at low vehicle speed, steering at high-middle vehicle speed, driving straightly, and parking, the energy consumption is compared quantitatively between HPS and E-ECHPS, the results show that power consumption of E-ECHPS reduced by about 30% compared to HPS. On the aspects of the steering portability, steering transient response and high-speed steering stability, the maneuverability and handling stability of the vehicle equipped with E-ECHPS system is investigated. By lemniscate steering simulation, steering portability at low speed between HPS and E-ECHPS is compared, the results show that the steering portability of E-ECHPS is improved by about 43.6% compared to HPS. The simulation results of step steering and high-speed steering between HPS and E-ECHPS suggest that the high-middle speed steering’road feeling’of E-ECHPS is significantly enhanced.In order to improve high-speed emergency steering stability of the vehicle equipped with E-ECHPS, the feedback tracking linear reference model control strategy is proposed based on differential geometry. The nonlinear dynamics models are established, including vehicle dynamics model, tire model, steering system model and ESC model. The affine nonlinear system state equations are deduced, considering the nonlinearity of tires, hydraulic system, and ESC. Differential geometry theory is applied to implement precise linearization of nonlinear system, then the input and output pseudo linear system is gained. The linear reference model with steering system under consideration is built. The step steering simulation and single lane changing simulation with steering torque as input signal are carried out under controllable and uncontrollable conditions respectively, which verifies the validity of the control strategy.For realization of the control strategy, the controller of E-ECHPS is developed and the test system of energy reclaiming type ESC is constructed. The measurements such as output speed of energy reclaiming type ESC, PWM rectifier AC side voltage/current, DC side voltage and super capacitor voltage/current are carried out, the test results show that the proposed control strategy improves the ESC output speed dynamic response under the load torque disturbance and ESC slipping loss recovery efficiency.Consequently, the E-ECHPS system solves the existing problems of heavy-duty commercial vehicle steering system, it helps to improve the driving safety of heavy-duty commercial vehicles, reduce energy consumption of the vehicle furthermore, so it will come into being social benefits and economic benefits. The findings of the research provide important theoretical and technical basis for further research and development of energy reclaiming type ESC and E-ECHPS. |
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