Servohydraulic systems are widely used for actuation where high force and fast response are needed, and the servovalve is the type of control valve which provides the highest performance for such systems. In aerospace, servovalves are used for many safety critical systems, such as flight control, steering and braking. However, during the last fifty years the conventional two stage servovalve design for aerospace applications has hardly changed. Due to the desire for increased energy efficiency, there is a need to make valves more efficient and lighter. A new type of servovalve for aerospace applications is therefore investigated. Increased efficiency implies that leakage should be small, and other key requirements are reliability, robustness and low manufacturing cost. The aim of this research is to provide background knowledge so that in the future an improved two stage servovalve can be manufactured. The prototype valve in this research will not be lightweight, but will provide knowledge for a future researcher or manufacturer to create a light weight servovalve. A two stage servovalve was designed with a multi-layer piezoelectric ring bender actuating a low leakage small spool as a first stage to control a second stage spool with electrical position feedback. The valve body was manufactured through Additive Manufacturing. A dynamic model of the complete valve was developed and correlated with experimental data. The model included the drive amplifier for the piezoelectric ring bender, the ring bender with hysteresis, first stage spool with overlap and the second stage spool. This showed that hysteresis and overlap had a significant effect on valve behaviour. A second stage spool position controller was developed to counteract the non-linear behaviour due to the piezoelectric hysteresis and the dead-band due to the first stage overlap. The controller also includes a feed forward path to increase the bandwidth of the valve. This controller outperforms a conventional Proportional-Integral controller and is also less sensitive to amplitude change. Mounting a ring bender sufficiently stiffly so that it can generate a high blocking force, but also in such a way that the deformation and hence the free displacement of the ring bender is not constrained, is a significant challenge. An analytical model to optimize the ring bender mount has been developed. This model can be used to find the optimum mount overlap at the outer edge of the ring bender, and the optimum thickness of the mount. An extensive investigation of the durability of the ring bender in Hyjet has been completed. Hyjet is a tradename for a fire-resistant phosphate-ester hydraulic fluid commonly used in civil aerospace. The ring bender will quickly start to break down if the Hyjet reaches the electrical connections. The Hyjet will penetrate the ceramic and create an electrical circuit between the electrical connection, the Hyjet and the outermost internal electrode. The breakdown of the ring bender can probably be eliminated by protecting the ceramic with an impermeable layer of material. In this research a metallic foil applied to the surface of the ring bender was investigated.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:760941 |
| Date | January 2017 |
| Creators | Persson, Johan |
| Contributors | Plummer, Andrew ; Bowen, Christopher |
| Publisher | University of Bath |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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