Downloaded from https://www.everyspec.com ADS-79D-HDBK G.7.1.2 Data Acquisition. The sampling rate for sensors (Figure G-5) should be high enough to detect all modulations of the signal and to prevent aliasing. The sampling rate should be high enough to capture higher order harmonics (typically up to 4th order), which for turboshaft engines could mean sampling rates above 20-30KHz, depending on the fault characteristic of interest and specific geometry and rotation speeds. The sampling window (Figure G-5) should be on the order of one second at quasi- steady state conditions (e.g., in a hover or at level cruise).
The sampling interval (Figure G-5), or time between samples, is dependent on the type of fault/failure mode of interest. If the sampling and subsequent signal processing detects an anomaly or a possible fault (with low/moderate confidence), the sampling interval should be adjusted to capture more data and improve the fault detection confidence. FIGURE G-5. HUMS time scale Update Time intervals associated with data collection impact the on board software setup significantly. If the fault(s) to be monitored is expected to occur over a significant time horizon (i.e.
50 or more flight hours), the Update Time interval should be sized accordingly, along with the amount of data stored by the Digital Source Collector (DSC). Likewise, if the fault(s) is expected to occur in a short time (i.e. 1 or 2 flight hours) the Update Time Interval should be shortened appropriately and the data stored by the DSC (per acquisition) should be minimized. How often a measurement is taken is referred to as the update rate or the iteration rate. This rate is usually determined by the component being monitored. Typical turboshaft EMS update rates for vibration are provided in Table G-X.
As noted in Appendix E.2.3.4, the sampling frequency of the sensor must be high enough to capture the highest frequency of interest in the component without aliasing. Also, the duration of the measurement should be long enough to capture the lowest frequency of interest. 125