Problems inherent in pressure control circuits are manifest in many common applications such as those of cushion control, and bumpless transfer between displacement and pressure control. Often, solutions involve complex electrical feedback systems to achieve the required performance characteristics. However, in many cases, a thorough understanding of the plant and control circuit should enable fulfilment of these requirements using a simple and inexpensive open-loop system.
In this case the plant is an automotive CVT (Continuously Variable Transmission) which has particular performance requirements. Constraints applied by the plant characteristics dictate that large flows be catered for with a low pressure increase and also that specific frequency response features are attained.
Initial work has involved the development of dynamic models of the powertrain and hydraulic control system to allow simulation in the time domain and linearisation to investigate the frequency response of the components. A test rig has been developed which has enabled validation of the dynamic models of the hydraulic system in the frequency domain.
This paper explains the approach devised to investigate the system characteristics. It will go on to present data both from simulation and testing of the hydraulics over the frequency range of interest. Finally, specific requirements from the hydraulic system to enable efficient operation of the transmission are identified, and methods of achieving this are presented.