Downloaded from https://www.everyspec.com ADS-79D-HDBK imply a rectangular window which can induce high levels of unwanted signal leakage, a redirection of the data into other spectral lines. The rectangular window can be used in situations where the Fourier Assumptions are not violated by the signal being measured (e.g. Exactly 360° of a shaft signal average). E.3.3 Diagnostic algorithm guidance. Vibration-Based Diagnostic Algorithms perform two basic functions: anomaly detection and fault isolation. Anomaly detection is the process of classifying the signal as either normal or anomalous.
Fault isolation is the process of determining the root cause of an anomalous signal down to the component level. As an example, if a diagnostic algorithm is intended to detect a crack of 10 mm or larger in a gear tooth, the accelerometer monitoring the transmission and its associated signal processing algorithms should be sensitive enough to measure the vibration caused by a 10 mm crack at the location at which the sensor is mounted. The following paragraphs provide the guidance for Vibration-Based Diagnostic Algorithms. E.3.3.1 Computational efficiency.
In systems employing onboard fault state estimation the detection technique should be sufficiently computationally efficient so that all required algorithms can be executed without incurring system latencies which preclude execution of minimum system requirements or flight critical functions. In systems where processing is performed off-board, the algorithms should be efficient, so that results are available in a timeframe acceptable to the maintainers making repair decisions. If the computational expense is too high for a particular algorithm, then an alternative technique should be used in order to arrive at a realizable implementation to meet the time requirement.
E.3.3.2 Physical description . The mathematical system of equations that describe the CI should be based on the Physics of Failure Modeling. In addition, the ―signature feature‖ to which the matched filter is ―tuned‖ for extraction should be describable with the physics of failure. The designed CI behavior should be firmly based on the Physics of Failure Characterization of the device or system. A CI selected in an ad hoc fashion based simply on historical observation without being grounded in the theoretical analysis can be risky and will frequently lead to an implementation that is less than robust.
For example, simply stating that, when a particular phenomenon is observed, it has been found experimentally that ―X‖ is the fault and ―Y‖ is the time to failure may not be stringent enough to yield an implementation that will work reliably in the field. The physical science behind the effect should typically be understood in order to develop a robust detection technique. E.4 EXISTING VIBRATION BASED DIAGNOSTICS Army aircraft mechanical systems are predominantly grouped in the engine, the drive system, the accessory subsystems, and the rotor systems. In the engine and drive system the critical faults typically include gear, bearing, and shaft failures.
Accessory subsystems, such as electrical and hydraulic systems, also include components typically consisting of gears, shafts, and bearings that derive power from the drive system through auxiliary gearing and shafts. The rotor system consists of main and tail rotor smoothing, or tail rotor smoothing (a.k.a. track and balance). The following paragraphs list the CIs that have been developed for the various mechanical system components. 105