Junhui Zhang

Investigation on the radial micro-motion about piston of axial piston pump

The limit working parameters and service life of axial piston pump are determined by the carrying ability and lubrication characteristic of its key friction pairs. Therefore, the design and optimization of the key friction pairs are always a key and difficult problem in the research on axial piston pump. In the traditional research on piston/cylinder pair, the assembly relationship of piston and cylinder bore is simplified into ideal cylindrical pair, which can not be used to analyze the influences of radial micro-motion of piston on the distribution characteristics of oil-film thickness and pressure in details. In this paper, based on the lubrication theory of the oil film, a numerical simulation model is built, taking the influences of roughness, elastic deformation of piston and pressure-viscosity effect into consideration. With the simulation model, the dynamic characteristics of the radial micro-motion and pressure distribution are analyzed, and the relationships between radial micro-motion and carrying ability, lubrication condition, and abrasion are discussed. Furthermore, a model pump for pressure distribution measurement of oil film between piston and cylinder bore is designed. The comparison of simulation and experimental results of pressure distribution shows that the simulation model has high accuracy. The experiment and simulation results demonstrate that the pressure distribution has peak values that are much higher than the boundary pressure in the piston chamber due to the radial micro-motion, and the abrasion of piston takes place mainly on the hand close to piston ball. In addition, improvement of manufacturing roundness and straightness of piston and cylinder bore is helpful to improve the carrying ability of piston/cylinder pair. The proposed research provides references for designing piston/cylinder pair, and helps to prolong the service life of axial piston pump.

A Hybrid Displacement/Pressure Control Scheme for an Electrohydraulic Flow Matching System

The electrohydraulic flow matching system is one of the most promising fluid power technologies for the improvement of the control performance and system efficiency of mobile machinery. However, pressure impacts and low efficiencies caused by flow excess are significant drawbacks that hinder its development. To solve this problem, this paper proposes a hybrid displacement/pressure control scheme by utilizing a switching rule according to the pressure margin and the candidate Lyapunov function. The mathematical model of an electrohydraulic flow matching system with primary pressure compensators was established, and the energy efficiency performance was analyzed. Experimental tests in single-actuator and dual-actuator systems were carried out on a test rig with a hydraulic excavator. The experimental results indicated that the pump pressure could be reduced when the supplied flow was excessive, and also the system efficiency was improved. The proposed controller is simple and pragmatic so that it can be easily implemented in the current mobile machinery.

Multiphysics-Coupled Modeling: Simulation of the Hydraulic-Operating Mechanism for a SF6 High-Voltage Circuit Breaker

The paper describes a novel method for modeling the high-voltage circuit breaker (HVCB) with the hydraulic operating mechanism (OM) by mainly using lumped models. It incorporates electrical, magnetic, dynamic motion, fluid, mechanical, pneumatic, and thermal model into a multiphysics-coupled model. The aim of this study is to build a design model of an HVCB for studying the hydraulic OM. Simulation and experimental validations are implemented in a 550-kv HVCB. It proves that the coupled model can be applied to capture their dynamics with satisfying accuracy. This paper describes the main potentials of the coupled mode including quantifying important features and optimizing multiple objectives associated with the optimization tool. By including their coupling properties, the model is able to accurately predict and optimize the dynamic characteristics of the hydraulic OM.

Pump-Based Compensation for Dynamic Improvement of the Electrohydraulic Flow Matching System

The electrohydraulic flow matching (EFM) system is a competitive alternative for the traditional load sensing system to improve the operability and efficiency of mobile machinery. The energy efficiency of the EFM system can be further improved by fully opening the control valve, but the system tends to oscillate since the damping ratio is reduced dramatically. To address this problem, a pump-based compensation method is proposed to improve the dynamic performance under different working conditions. A performance indicator is defined to evaluate the dynamic behavior under the load condition with wide variation range. Using the defined indicator as the fitness function, a parameter searching method based on stability analysis and particle swarm optimization is designed to determine the optimized control parameters for dynamic improvement. A test rig with a 2-ton hydraulic excavator was built up and used to validate the proposed compensator. Boom lifting motions under different loads and speeds were carried out. The results indicated that the proposed compensator reduced velocity and pressure oscillations under different working conditions, with achieving better dynamic improvement than the traditional compensator.

Impact of typical steady-state conditions and transient conditions on flow ripple and its test accuracy for axial piston pump

The current research about the flow ripple of axial piston pump mainly focuses on the effect of the structure of parts on the flow ripple. Therein, the structure of parts are usually designed and optimized at rated working conditions. However, the pump usually has to work in large-scale and time-variant working conditions. Therefore, the flow ripple characteristics of pump and analysis for its test accuracy with respect to variant steady-state conditions and transient conditions in a wide range of operating parameters are focused in this paper. First, a simulation model has been constructed, which takes the kinematics of oil film within friction pairs into account for higher accuracy. Afterwards, a test bed which adopts Secondary Source Method is built to verify the model. The simulation and tests results show that the angular position of the piston, corresponding to the position where the peak flow ripple is produced, varies with the different pressure. The pulsating amplitude and pulsation rate of flow ripple increase with the rise of pressure and the variation rate of pressure. For the pump working at a constant speed, the flow pulsation rate decreases dramatically with the increasing speed when the speed is less than 27.78% of the maximum speed, subsequently presents a small decrease tendency with the speed further increasing. With the rise of the variation rate of speed, the pulsating amplitude and pulsation rate of flow ripple increase. As the swash plate angle augments, the pulsating amplitude of flow ripple increases, nevertheless the flow pulsation rate decreases. In contrast with the effect of the variation of pressure, the test accuracy of flow ripple is more sensitive to the variation of speed. It makes the test accuracy above 96.20% available for the pulsating amplitude of pressure deviating within a range of ±6% from the mean pressure. However, with a variation of speed deviating within a range of ±2% from the mean speed, the attainable test accuracy of flow ripple is above 93.07%. The model constructed in this research proposes a method to determine the flow ripple characteristics of pump and its attainable test accuracy under the large-scale and time-variant working conditions. Meanwhile, a discussion about the variation of flow ripple and its obtainable test accuracy with the conditions of the pump working in wide operating ranges is given as well.

Research on a novel flow rate inferential measurement method and its application in hydraulic elevators

Measuring and controlling the flow rate is a widely concerned problem in engineering fields. The direct flow rate measurement employing conventional flow meters and the indirect flow rate measurement using speed/position transducers or other particular techniques would result in inevitable pressure drop in hydraulic circuits, more energy consumption for pumping fluid, and higher cost of hydraulic systems. This paper presents a novel flow rate inferential measurement method and its application in hydraulic elevators. Mathematical modeling of the proposed method is deduced. The key component of the hydraulic elevator circuit, a two-stage proportional flow rate valve, is verified by experiments as one of the contributions of this paper. Based on the mathematical modeling and the valve validation test, the feasibility and validity of the proposed method are verified by the experiments performed on a test rig which is designed to imitate work situations of a hydraulic elevator. Moreover, sensitivity analyses of the proposed flow rate inferential measurement method are carried out to find the ways how to improve the accuracy of the proposed method. It is believed that this method can be applied in various engineering devices.

Design and Experimental Research of a Miniature Digital Hydraulic Valve

A digital hydraulic valve is an important component of the digital hydraulic system, and its performance is directly related to the system function. In order to make the valve system more competitive in dimension, digital valve miniaturization is an important research point. A new micro digital valve is designed, which is analyzed from the mechanical structure and magnetic circuit mechanism, and the design difficulties are also expounded. The four subsystems and switching characteristics of the valve are theoretically analyzed and simulated. In order to test the performance of the valve, a test system is designed, and performance of the new micro valve is tested. The test results show that the switch characteristic analysis of the valve is correct. The comparison between the test curve and the simulation curve is carried out, which demonstrates that the accuracy of the simulation model is reasonable. The theoretical analysis of the new micro digital valve is consistent with experiments.