On-Line Monitoring of Fluid Effective Bulk Modulus Using Piezoelectric Transducer Impedance

The fluid effective bulk modulus plays a significant role in hydraulic control systems due to its effect on the system response time and performance. In general, the fluid effective bulk modulus is a function of the amount of entrapped air, pressure, and temperature variations. Therefore, it has been recognized that monitoring of the effective bulk modulus is essential for the control of hydraulic actuation system. Measuring of the effective bulk modulus is a very challenging task. Current methods normally require precision testing equipments. Furthermore, the required equipment usually consists of many complex components that will affect the bulk modulus. Their size is in general large and thus makes on-line measurement impractical. In this research, we develop a new on-line effective bulk modulus measuring technique based upon the impedance of piezoelectric transducers. Using the piezoelectric impedance equation, numerical simulation for predicting the peak resonance frequency is performed to identify its dependency on the variation of the fluid bulk modulus. In order to verify the results, the effective bulk modulus of an ATF (automatic transmission fluid) is examined experimentally. The proposed method and the conventional method are also compared. The experimental results show the validity of the proposed method.Copyright © 2007 by ASME