Electro-hydraulic servovalves are used for engine fuel control, brake and steering control and primary flight controls (actuation of (elevators, ailerons and rudders). These critical components dictate every movement of an aircraft and number about 40 on a typical airliner. Mostly, they control the flow of hydraulic fluid from high-pressure pumps to hydraulic cylinders, giving precise positioning of the piston within each cylinder. In turn, each piston is linked to a flight control surface or to the steering or braking systems, for example. In a servovalve, which is a precision component, the twitching of a miniature electric motor is hydraulically amplified to move a sliding spool, which in turn opens or closes ports (holes) to control the hydraulic flow. The electric motor ‘twitches’ by less than 100 microns - the thickness of a human hair – ultimately dictating the motion of the aircraft, which may have a mass of several hundred tonnes. Manufacturing costs are extremely high for servovalves, because of the large number of parts, tight tolerances and manual set-up processes required for current designs. These manual manufacturing tasks can result in variability between valves and pose potential reliability issues, which are a real concern for these safety-critical components – not to mention their generation of manufacturing waste (scrap). Furthermore, current designs suffer from leakage flows, which amount to a considerable power drain.
An innovative approach
The EU-funded DNSVCFA project introduced a radical change in technology, to enable the production of more efficient and reliable servovalves that are also cheaper. The key technology is the use of piezoelectric actuation. Piezoelectric ceramics change shape when subject to an electric field, presenting a ‘solid state’ actuation method to replace the complex miniature electric motor. “We also introduced an innovative ‘twin flapper’, which means that for most of the time, the power lost through the valve due to leakage could be reduced to nearly zero,” says Andrew Plummer, project coordinator and director of the Centre for Power Transmission and Motion Control at the University of Bath. A simulation using computational fluid dynamics verified the design, including prediction of fluid flow, which enabled a prototype to be built for testing and validating the concept.
The research was undertaken with the support of the Marie Skłodowska-Curie programme, and research fellow Paolo Tamburrano explored the servovalve production process while seconded to a major servovalve manufacturer. “This experience was extremely useful and helped me to better appreciate practical aspects of servovalve design and manufacture and bring that knowledge back into the project,” he noted. The end result is a novel valve design, which performs as well as a conventional servovalve, but with reduced part count and complexity. “We have also shown efficiency improvement – this is very duty- cycle dependent, but typically, we would expect the hydraulic power used by an aircraft actuation system with the new valves to be halved. The next stage is to quantify the manufacturing benefits, but we are very hopeful that these will be significant,” Plummer concludes.
DNSVCFA, servovalve, hydraulic, piezoelectric, aerospace, computational fluid dynamics