Downloaded from https://www.everyspec.com ADS-79D-HDBK FIGURE E-1. Sensor response characteristics E.3.1.6 Installation. Vibration sensors should be mounted as close as practical to the component(s) they are intended to monitor. In addition, they should be oriented such that their sensitive axis is aligned with the predominant axis of vibration. Each proposed mounting location should be tested (during developmental testing) to characterize the natural structural response at the mounting location.
Mounting locations having resonant frequencies near defect frequencies of interest and the use of brackets should be avoided, especially when defect frequencies are narrowband and vary with rotor speed. E.3.1.7 Built-in test capability. The VMS should have a capability for verifying the proper functioning of the sensor circuitry. E.3.1.8 Sensor Reliability. The long term reliability of the sensor is important and information regarding its Mean Time Between Failure (MTBF) should be included in the system documentation. E.3.2 Data acquisition and signal processing guidance. Data acquisition deals with how frequently and under which conditions data sets are acquired.
Signal processing is required to convert the sensor‘s analog signal to a digital signal for computation processing in the diagnostic algorithms. In addition, prior to conversion, the analog signal may require filtering to improve the signal to noise ratio, scaling to improve sensitivity, or adjustments to account for biases due to drift. Care should be taken in signal handling so as not to induce unwanted distortion of the signal. E.3.2.1 Data acquisition conditions. Time series data should be acquired under operating conditions with the greatest signal stationarity. Stationarity denotes the consistency of a signal‘s statistical properties over time.
Conditions with the greatest stationarity may occur when the aircraft is on the ground with the main rotor at full speed and flat blade pitch, or in the forward climb regime. 20 20 Zakrajsek, J., P. Dempsey, E. Huff, H. Decker, M. Augustin, R. Safa-Bakhsh, A. Duke, and P. Grabill, ―Rotorcraft Health Management Issues and Challenges,‖ NASA/TM—2006-214022, February 2006. 103