In this study, a nonlinear sliding-mode controller is designed for force tracking of a piezoelectric-hydraulic pump (PHP)-based actuation system, which is developed to replace the current electro-hydraulic actuation systems for automatic transmission (AT) friction elements, such as band brakes or clutches. By utilizing the PHP, one can eliminate the various hydraulic components (oil pump, regulating valve and control valve) in current ATs and achieve a simpler configuration with more efficient operation. With the derived governing equation of motion of the PHP-based actuation system integrated with the friction element (band brake), a switching control law is synthesized based on the sliding-mode theory. To evaluate the effectiveness of the proposed control law, its force tracking performance for the engagement of a friction element during an AT 1 → 2 up-shift is examined experimentally. It is shown that one can successfully track the desired force trajectory for AT shift control with small tracking error. This study demonstrates the potential of the PHP as a new controllable actuation system for AT friction elements.

The effective bulk modulus of working fluids plays an important role in the control of hydraulic actuation systems because of its effect on the system response time and performance. Therefore, to ensure good control, monitoring the effective bulk modulus of the working fluids is an important task. Current methods normally require precision test equipment consisting of many complex components. The size of these devices is large and thus makes online measurement impractical. In this research, we develop a new on-line technique to estimate effective bulk modulus of the working fluids based on measurements of the impedance of piezoelectric transducers. The idea is to generate a sensitivity curve characterizing the relationship between the effective bulk modulus and the impedance resonant frequency via either off-line numerical simulation or off-line experimental calibration; the curve can then be used for monitoring the working fluids bulk modulus in an online manner. In this article, a simulation model is utilized to predict the peak resonance frequency of the impedance function and identify its dependency on the variation of the fluid bulk modulus. The new approach is then illustrated and a sensitivity curve is generated through comparing the simulation results with experimental data.

This paper demonstrates that a hydraulically acting Helmholtz resonator can exist in a piezoelectric-hydraulic pump (PHP) based hybrid actuator, which in turn affects the volumetric efficiency of the PHP. The simulation and experimental results illustrate the effect of Helmholtz resonance on the flow rate performance of the PHP. The study also shows how to shift the Helmholtz resonant frequency to a higher value through changing parameters such as the cylinder diameter and the effective bulk modulus of the working fluid, which will improve the volumetric efficiency and broaden the operating frequency range of the PHP actuator.