More electric aircrafts are the future of modern personnel and cargo transportation. To achieve this goal, the conventional centralized hydraulic flight control system, which is responsible for the operation of the aircraft, needs to be replaced by decentralized electro-hydraulic actuators (EHA). The conventional hydraulic system consists of a large pump within the airplanes fuselage. To meet safety requirements, the entire system needs to be redundant resulting in additional heavy components. To further reduce fire risk, expensive and hazardous oils are used. To overcome the drawbacks of present systems individual small piston pumps powered by high dynamic electric motors are installed. Two actuators can be installed in close proximity to achieve redundancy and the limited oil volume in each EHA decreases the fire risk and making the need for large volumes of hazardous oil obsolete. Therefore, compared to the conventional system, the complexity can be reduced and safety can be increased at the same time.Seeing as the displacement volume of the piston pump is fixed due to a static swash plate angle, the actuation of the cylinder is controlled by the rotary speed of the pump. Due to the high dynamic properties of the small electric motor, fast changes in the direction of rotations can occur. Additionally, pressure-holding states of the EHA cause the pump to operate in operating points of solid friction. This internally results in temperature spikes and immense wear on the pistons as well as the swash plate, cylinder block and valve plate of the pump. The temperature spikes cannot be neglected as the small oil volume is unable to transfer the heat away, resulting in temperature increase of the entire EHA. Because of the forementioned problematic operation points, EHA are currently only used as safety. Yet, recent studies aim to increase the robustness of EHAs.In parallel, condition monitoring (CM) and predictive maintenance (PM) approaches are investigated to increase the system safety and to reduce costs by extending the maintenance intervals. Only via seamlessly monitoring the actual health status of a system, conclusions about the remaining life expectancy can be drawn. While common monitoring signals in hydraulic systems, such as temperature and pressure are widely implemented, vibration and oil degradation sensors are now on the rise. Yet, especially optical particle sensors are expensive and extremely fragile, resulting in an unreliable application in aircrafts. Other technics such as oil conductivity are more robust and therefore better suited in harsh working environments. Monitoring the increase of particles due to wear is a viable method of determining the current health status, as the particle directly reflects the degradation of the working surfaces, thus enabling the approximation of the remaining operational life span.In the contribution, the effectiveness of measuring the oil conductivity as a condition monitoring technique to determine the wear and therefore its current health status is examined. Different conductivity measurement methods are discussed and compared to alternatives such as physical, chemical and optical monitoring techniques. Reference measurements with standard oil (HLP46) are conducted and gradually contaminated with particles of different materials as well as water and dust. Latter are included to determine, whether the actual wear particles can be separated from water or dust or whether the impact is too limited to register. The metallic particles include ferro magnetic materials such as iron as well as non-magnetic materials such as bronze, magnesium, chrome, nickel and aluminum.&nbsp;

Investigating the Condition Monitoring Potential of Oil Conductivity for Wear Identification in Electro Hydrostatic Actuators

Paper Proceedings: https://asmedigitalcollection.asme.org/FPST/FPMC2021/volume/85239

2021 ASME/BATH Symposium on Fluid Power and Motion Control (FPMC2021)

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