Fluid Metering Using Active Materials

Abstract

Servovalves are compact, accurate, fast flow modulating valves widely used in aerospace, defence, industrial and marine applications. However, cost reduction pressures exist due to tight tolerances required, particularly in the first stage of the valve. In this research novel servovalve concepts are investigated. In particular, a new first stage actuator assembly is developed to move a servovalve spool using the jet principle.

The conventional torque motor assembly in the first stage was replaced by a multilayered bimorph actuator. A feedback wire was used to facilitate proportional flow control via mechanical feedback. The bimorph was directly coupled to the feedback wire for submerged operation. A steady state analytical model of the bimorph feedback wire assembly was developed to derive the stiffness constants influencing the deflector and the valve spool. The derived stiffness constants were compared to FEA predictions. The flow forces acting on the deflector were determined using CFD analysis. The flow force was found to be proportional to the pressure drop across the deflector and the deflector displacement.

A high order nonlinear model of the valve was developed and used to simulate valve dynamic characteristics. The high order model was linearised and reduced to a first order lag to identify the system parameters that determined the first break frequency and the steady state gain of the valve. Two ‘Mark1’ prototypes were built and tested. The measured frequency responses of the prototypes were in good agreement with the simulation results. At 140bar supply pressure and maximum applied voltage amplitude the 3dB bandwidth of the valve was measured at approximately 41Hz. The frequency response of the valve spool was reasonably consistent for varying applied voltage amplitudes at a fixed supply pressure. The hysteresis of the second stage spool was approximately ±4%. The stroke of the second stage spool was approximately 0.42mm.

The bandwidth and steady state gain of the valve were expressed in terms of ratios between the forward and feedback path variables of the valve system. Performance plots were developed using these variable ratios. A ‘Mark2’ prototype valve was developed and tested, intended to possess a higher bandwidth. The 3dB bandwidth of the ‘Mark2’ valve spool at 140bar supply pressure and maximum applied voltage amplitude was measured at approximately 60Hz. At this voltage amplitude the spool stroke was approximately 0.24mm and the valve hysteresis was approximately ±2%. Bandwidth and stroke were consistent with the predictions.

Date of Award	1 Sept 2011
Original language	English
Awarding Institution	University of Bath
Supervisor	Andrew Plummer (Supervisor)