A review of electro-hydraulic servovalve research and development

This paper provides a review of the state of the art of electro-hydraulic servovalves, which are widely used valves in industrial applications and aerospace, being key components for closed loop electrohydraulic motion control systems. The paper discusses their operating principles and the analytical models used to study these valves. Commercially available units are also analysed in detail, reporting the performance levels achieved by current servovalves in addition to discussing their advantages and drawbacks. Adetailed analysis of research that investigates these valves via computational fluid dynamic analysis is also provided. Research studies on novel control systems and novel configurations based on the use of smart materials, which aim to improve performance or reduce cost, are also analysed in detail.</p

Electrohydraulic servovalves – past, present, and future

In 2016 it is 70 years since the first patent for a two-stage servovalve was filed, and 60 years since the double nozzle-flapper two-stage valve patent was granted. This paper reviews the many alternative servovalve designs that were investigated at that time, focusing on two-stage valves. The development of single-stage valves – otherwise known as direct drive or proportional valves – for industrial rather than aerospace application is also briefly reviewed. Ongoing research into alternative valve technology is then discussed, particularly focussing on piezoelectric actuation and the opportunities afforded by additive manufacturing

A lightweight, low leakage piezoelectric servovalve

We describe a new two-stage aerospace servovalve which has an additively manufactured titanium body, and a small piezoelectrically actuated spool as its first stage, with electrical main stage position feedback. The approach promises to provide low weight, low leakage, and more accurate spool positioning. In addition, it enables increased manufacturing automation thereby reducing cost, increasing repeatability, and generates less waste material. Valve design, operation and measured performance are discussed in this paper

A lightweight, low leakage piezoelectric servovalve

We describe a new two-stage aerospace servovalve which has an additively manufactured titanium body, and a small piezoelectrically actuated spool as its first stage, with electrical main stage position feedback. The approach promises to provide low weight, low leakage, and more accurate spool positioning. In addition, it enables increased manufacturing automation thereby reducing cost, increasing repeatability, and generates less waste material. Valve design, operation and measured performance are discussed in this paper

A review of electro-hydraulic servovalve research and development

This paper provides a review of the state of the art of electro-hydraulic servovalves, which are widely used valves in industrial applications and aerospace, being key components for closed loop electrohydraulic motion control systems. The paper discusses their operating principles and the analytical models used to study these valves. Commercially available units are also analysed in detail, reporting the performance levels achieved by current servovalves in addition to discussing their advantages and drawbacks. Adetailed analysis of research that investigates these valves via computational fluid dynamic analysis is also provided. Research studies on novel control systems and novel configurations based on the use of smart materials, which aim to improve performance or reduce cost, are also analysed in detail.</p

Electrohydraulic servovalves – past, present, and future

In 2016 it is 70 years since the first patent for a two-stage servovalve was filed, and 60 years since the double nozzle-flapper two-stage valve patent was granted. This paper reviews the many alternative servovalve designs that were investigated at that time, focusing on two-stage valves. The development of single-stage valves – otherwise known as direct drive or proportional valves – for industrial rather than aerospace application is also briefly reviewed. Ongoing research into alternative valve technology is then discussed, particularly focussing on piezoelectric actuation and the opportunities afforded by additive manufacturing

A dual lane piezoelectric ring bender actuated nozzle-flapper servo valve for aero engine fuel metering

Amongst many other high performance flow control applications, servo valves are used to control aero engines by metering the fuel delivered from the fuel pump. Conventionally, a fuel metering servo valve has a pilot stage with an electromagnetic torque motor moving a flapper which differentially restricts a pair of nozzles to create a hydraulic signal (i.e. a pressure difference). These valve pilot stages use mature, optimised technology such that to achieve improvements requires a novel approach. Torque motors in particular present reliability and manufacturing difficulties, and news solutions should ultimately allow a reduction in manual assembly and set-up, improve repeatability, and eliminate failures associated with fine wire devices. In this paper, a pilot stage actuated by piezoelectric ring benders is proposed, designed, built and tested, and test results are compared with a model used to predict pressure-flow characteristics. A particular challenge is the need to include redundancy, and thus a pair of ring benders is used, allowing isolation between duplicated electrical control channels. Another challenge is the mounting of the ring bender, which has to flex to allow the outer edge of the ring bender to deform, yet be stiff enough to adequately react against generated forces. O-ring mounts made from three different elastomer materials are compared in this study. In aerospace, an added complication is the large range of fuel temperature; F70 fluorosilicone O-rings have been chosen with this in mind, and successfully demonstrated in the range -50 C to +180 C. With one active and one inactive ring bender to simulate a failure condition, the new dual lane pilot stage achieves +/-50 μm displacement under test, giving control port flows up to +/-0.6 l min -1, and a control port pressure variation of 40 bar using a 100 bar supply pressure difference (supply minus return pressure). This research establishes that a piezoelectric aero engine fuel valve is feasible, and in particular, that piezoelectric ring bender actuators with elastomeric mountings are highly suited to this application. </p

Non-linear Control of a Piezoelectric Two Stage Servovalve

This paper describes an algorithm to control a two stage hydraulic servovalve designed for aerospace applications. The valve has a piezoelectric ring bender actuating a first stage spool with a significant amount of overlap to reduce internal leakage. The piezoelectric ring bender is a less complex and lighter alternative to a conventional torque motor. The second stage has electrical instead of the conventional mechanical feedback. The control algorithm includes compensation for the first stage spool overlap, piezoelectric hysteresis compensation and a feed forward term. The hysteresis compensation is based on a relatively simple Bouc-Wen hysteresis model that is able to significantly reduce the amount of first stage hysteresis. The overlap compensation, increasing the gain in the overlap region, reduces the impact of amplitude change and increases performance. It can also reduce any asymmetry in the system. The controller has a superior performance compared to a PI controller, as demonstrated experimentally using step and frequency responses

Non-linear control of a hydraulic piezo-valve using a generalized Prandtl-Ishlinskii hysteresis model

The potential to actuate proportional flow control valves using piezoelectric ceramics or other smart materials has been investigated for a number of years. Although performance advantages compared to electromagnetic actuation have been demonstrated, a major obstacle has proven to be ferroelectric hysteresis, which is typically 20% for a piezoelectric actuator. In this paper, a detailed study of valve control methods incorporating hysteresis compensation is made for the first time. Experimental results are obtained from a novel spool valve actuated by a multi-layer piezoelectric ring bender. A generalized Prandtl-Ishlinskii model, fitted to experimental training data from the prototype valve, is used to model hysteresis empirically. This form of model is analytically invertible and is used to compensate for hysteresis in the prototype valve both open loop, and in several configurations of closed loop real time control system. The closed loop control configurations use PID (Proportional Integral Derivative) control with either the inverse hysteresis model in the forward path or in a command feedforward path. Performance is compared to both open and closed loop control without hysteresis compensation via step and frequency response results. Results show a significant improvement in accuracy and dynamic performance using hysteresis compensation in open loop, but where valve position feedback is available for closed loop control the improvements are smaller, and so conventional PID control may well be sufficient. It is concluded that the ability to combine state-of-the-art multi-layer piezoelectric bending actuators with either sophisticated hysteresis compensation or closed loop control provides a route for the creation of a new generation of high performance piezoelectric valves.<br/