Derivations of more precise mathematical models:
Accepted mathematical models of hydraulic spool valves and amplifiers do not provide accurate enough results to use existing models to predict the dynamics of piezoelectric/hydraulic hybrid actuators. Therefore, an improvement to the mathematical models is integrated into this program. Ultimately, the use of improved models will allow for simulation of actuator designs. This simulation will provide a means of refinement and optimization before developing any actuator hardware. Refinement of the existing mathematical models will focus on the inclusion of a 2nd order function to simulate the observable motion of a piezoelectric stack and a variable flow-by rate for hydraulic fluid within a hydraulic amplifier. Models will be developed in accepted state space format and tested in a variety of formats: Matlab/Simulink, C++, and VTB.
In conjunction with improvements to the actuator models, a mathematical relationship between the piston position and the engine valve position will be established. It is anticipated that an equivalent frequency can be calculated. This equivalent frequency takes into account the interrupted motion of engine valves within an engine cycle. For example, a typical intake valve is only open for 270 degrees out of every two engine revolutions (720 degrees). Therefore, the actuator must open and close the engine valve during this 270 degree portion of crank rotation. This results in a minimum equivalent frequency that the actuator must use as input for a given engine speed. Shown in Figure 1 is a graphical interpretation of equivalent frequency.
Figure 1 Equivalent Frequency
The development of mathematical models that address the observable 2nd order motion of piezoelectric stacks combined with traditional hydraulic actuator relationships will lead to a more accurate mathematical model of hybrid actuator systems. Such a model will improve the accuracy of actuator simulations, lead to shorter development time for future designs, and provide a foundation for optimizing performance of the actuators being developed.