Study On Effective Fluid Bulk Modulus And Measurement In Hydraulic Systems

Hydraulic systems are widely used in many types of mechanical equipment. Effective fluid bulk modulus taken as the system rigidity plays an important role in the hydraulic systems. Due to its compressibility, the hydraulic fluid is generally regarded as a hydraulic spring. The effective bulk modulus varys with different working conditions and is considered as a soft parameter, and the variations are frequent and very complex. To estimating effective bulk modulus accurately is very important when designing high performance hydraulic systems, especially when the high quality systems with fast response and high stability are concerned. Therefore, researches on the influences of bulk modulus variation, the measurement of bulk modulus and the system behavior analysis based on dynamic bulk modulus are very significant to improve the hydraulic system performance.The concept of dynamic fluid bulk modulus was first introduced and its mathematical model was proposed in this study. The model is taking the effect of working pressure, fluid temperature and air content into account, especially the dynamic pressure oscillation produced by the fluid volume change in the actuator. The model can effectively improve the accuracy of the bulk modulus in description. A measuring device was developed based on the bulk modulus definition. In this device, a servo motor was employed to load the fluid sample and two sensors with high accuracy were used to detect its pressure and volume in real-time. The loading force (0-20 MPa) and speed (0.35-4.45%/min) can be easily conrolled, and the measuring error was expected less than 2.08%. Adopting the vacuum-pumping treatment, the air content of the hydraulic system was reduced from 12.29% to 7.41%. The step response results show that the overdhoot reduces 5-10% as air content decreases 4.88%, and the settling time is also decreased. A method according to the dynamic fluid bulk modulus was proposed to adjust the control parameters of hydraulic systems. The system gain was adjusting with the variation of the fluid bulk modulus in real-time in order to meet the hydraulic nature frequency. The sine response results show that when the control parameters are adjusted following this method, the error of the system output reduces about 25-30% and the phase lag reduces about 3.5°8.6°. The displacement overshoot is also decreased.In chapter 1, the definition, typicality and action of effective fluid bulk modulus in hydraulic systems were discussed. The current research progresses on bulk modulus measurement and the influences of different working parameters were reviewed. The main research subjects were presented.In chapter 2, the air effect caused by the variation of air volume and air bubble state was presented, and its influence on bulk modulus was analyzed. The air movement characterics of both the single air buble and air bubbles in swarm were simulated. The influences of bubble diameter, fluid viscosity, pressure and interaction between different air bubbles on the behaviors of ascending velocity and distortion were analyzed.In chapter 3, the mathematical model of effective fluid bulk modulus was presented, and the bulk modulus variation with pressure under different air content and working temperature was simulated. Based on the analysis, a power unit adopting vacuum pumping and bulk modulus measurement was designed to improve the effective fluid bulk modulus.In chapter 4, a test rig consisting of vacuum pumping treatment, bulk modulus measurement and hydraulic servo motion was designed. The bulk modulus measuring device consists of a pressure sensor, a displacement sensor, a temperature sensor and a test chamber loading by a servo motor.In chapter 5, the air volume measurement, vacuum pumping treatment and bulk modulus measurement were carried out. Based on the experimental results, the bulk modulus variation with pressure under different air contents, working temperature and fluid volume changing rate was analyzed.In chapter 6, Step responses and sine wave responses of a valve-controlled cylinder were tested under different air content. The experimental results show that the bulk modulus varies instantaneously with working pressure. The overshoot and the settling time influenced by dynamic bulk modulus were analyzed. A method for improving the system behavior by regulating the control parameters with the dynamic bulk modulus variation was proposed.In chapter 7, conclusions according to the work were summarized and future research proposals were suggested.