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Hydraulic cylinder is the actuator in hydraulic system, and its failure directly affects the normal operationof the system. The leakage of hydraulic cylinder is a major failure mode usually caused by failure or dam-age of the seal, and it should be detected as early as possible to avoid worse breakdowns of system. How-ever, the leakage is difficult to be diagnosed in the early stage, so the basic aim of the paper is to seek outthe fault features which could forecast the early leakage of hydraulic cylinder. In this study, inlet and out-let pressure signals and displacement signal of piston rod of hydraulic cylinder have been collected fromaspecial designed test-bed, which simulates different leakage levels of hydraulic cylinder. The result showsthat there are five fault features in total can be extracted by wavelet packet analysis to distinguish differ-ent levels of the leakage obviously. 32491
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1. IntroductionHydraulic systems are widely applied, especially in power,control and execution system of construction machinery. Hydrau-lic cylinder is the actuator in hydraulic system, and its statuscondition affects system performance. Its working load is usuallychanging; the vibration and working speed are also unstable, espe-cially in some special conditions, such as the transient conversionand the inertial impaction, which lead to the failure of hydrauliccylinder and account for a large proportion of the maintenance ofconstruction machinery. Leakage is mostly caused by the seal fail-ure, resulting in crawling of hydraulic cylinder, lack of thrust, fall ofspeed or instability of system. However, leakage is difficult to bediagnosed because of its concealment. Therefore, it is significantto research the leakage failure mechanism of hydraulic cylinderand detect the leakage as early as possible in order to takeappropriate actions timely.In recent years, diagnosis of leakage of hydraulic cylinder getsmore andmore attention.Wenzheng Du proposed the leakage faultdiagnosis method based on the wavelet multi-scale edge detection.The features of the pressure transient signals could be extracted bydetecting the maximum in the appropriate scale of wavelet transform. In this way, it provided a quantitative basis for internalleakage fault diagnosis of the hydraulic cylinder [1]. Yang foundthe leakage could be detected accurately by the pressure rise time[2]. Tang applied wavelet transform and BP neural network tothe diagnosis of hydraulic cylinder leakage [3]. Nonlinear self-adaptive robust observer was used by Garimella for the modelingof the electric hydraulic cylinder system.
The impact of unmoldeddynamic factors could be reduced by robust filtering and theuncertainty of the range of model variables could be reduced bythe controllable online self adaptive parameters. The method wassuitable for diagnosing the initial fault of hydraulic cylinder speedsensor failure, oil pollution and pressure shortage [4]. An, Sepehriapplied the extended Kalman filter and serial test method todiagnose the leakage of fault hydraulic cylinder quantitatively[5].Werlefors applied the nonlinear observation model establishedby static feedback to diagnose the leakage successfully by measur-ing the piston position and piston speed [6]. By using theUnscented Kalman Filter, Sepasi monitored the operation ofhydraulic systems online. The method could quickly and accuratelydiagnose the hydraulic cylinder internal and external leakage, thechanges of dynamic friction load and transient load of the hydrau-lic cylinder. Wavelet analysis showed great advantage in process-ing fault signals of hydraulic pump and motor [7]. Goharriziestablished a wavelet transform based approach to detect internalleakage in hydraulic actuators caused by seal damage [8]. But thehigh frequency part of the origin signal was not well decomposedand analyzed, which contains a lot of detail information. In practical application, the main reason of hydraulic cylinderleakage is the damage of seal, which is a progressive failure.
This gradually process can be described as a potential failuremode, i.e. the system performance declines with the fault devel-opment. If the features of leakage can be extracted, it can bediagnosed timely to avoid worse breakdowns of system. There-fore, this paper focuses on the application of wavelet packetanalysis to reveal the connection between the external faultsymptoms and the internal fault mode and to find out the faultfeatures which can describe their relationships through a theo-retical basis to diagnose and predict the early leakage ofhydraulic cylinder.2. Experimental studyThe leakage of hydraulic cylinder is mainly related with thestructural parameters of the hydraulic cylinder piston rod, sealforms, oil viscosity and wear situation of the piston. It can bepided into two cases: concentric and eccentric, they have differ-ent formula to calculate leakage flow. If the leakage model is sim-plified as concentric rings leak gap flow, and the leakage can becalculated as:Qc ¼ pdh312llDp ð1Þwhere d is the cylinder diameter; h is the height of the gap; l is thegap length; l is the fluid dynamic viscosity; Dp is the pressure dif-ference between the high pressure chamber and the low pressurechamber.However, in the actual simulation study, accurate control ofleakage cannot be achieved according to this model. In literature[3], the leak fault simulation approach is to install a throttle anda flow sensor in parallel between the oil cavity and the oil returncavity of the cylinder, and the leakage can be controlled by adjust-ing the opening degree of throttle. But this simulation method hasthe following disadvantages:(1) Throttle has a minimum stable flow. When the leakage isless than the minimum stable flow, leakage flow is not con-tinuous, causing cylinder crawling phenomenon. Thereforeit cannot simulate the hydraulic cylinder early minor leak-age fault.(2) The leakage changes with the system pressure rising. If theleakage is controlled through the flow sensor, it will be dif-ficult to study the system performance effects with leakagewith pressure change because the leakage changes isunknown.To solve the above problems, the paper proposes a methodusing orifice to simulate the leakage of hydraulic cylinder, i.e.installing orifices with different diameters between the cylinderimport and export in parallel. According to the principle of hydrau-lic cylinder leakage, part of the oil will seep into the outside of cyl-inder or into the oil return chamber, leading to that the actual oilvolume flow into the work chamber is less than no leakage inthe same time. If we put a certain size orifice parallel in thehydraulic cylinder oil circuit, it can not only achieve this effect,and the precisely control the leakage can also be realized by chang-ing different size of orifice. Then, different leakage levels of the cyl-inder can be simulated, and its effects on the system performancecan be quantitatively analyzed.The leakage can be calculated with an elongated hole model: