Safety analysis on digital hydraulics : Redundancy study for aviation applications

Digital hydraulic actuators (DHA) are an interesting new technology that couldreplace todays system with inefficient proportional valves. By using an arrayof on/off valves the hydraulic pressures are discretised. This gives a fixed setof force outputs that can be used to control the actuator. DHA systems havebeen proven to drastically reduce the energy consumption at the cost of highersystem complexity. More components and more advanced controllers are neededto maintain an equal system performance.Previous research has been mentioning the fault tolerance of the DHA systemwithout analysing the actual requirements to achieve it. In this thesis a safetyanalysis is made. One first approach of making an active fault tolerant system ispresented and the effects of using this is analysed. In total, over four million failuremodes are analysed and grouped into 2402 system outputs. The thesis is also thefirst within the research of DHA system to present a chamber wise analysis, whereall four chambers are analysed independently.The thesis also presents a method to calculate reliability for the system. Themethod is a new computational way of creating and reducing fault trees. Fromthe fault trees the probability of system failure can be calculated.The conclusion of this thesis is that DHA is not fault tolerant by default butcan be if designed correctly. The thesis also concludes that if the componentsin the DHA system have the same reliability as the components used in today’ssystem the reliability is similar