Table 1. Measurement Capabilities

Table 2. Frequency Ranges Vs. Respective Measurement Resolutions

TM-1-6625-724-13P Aviation Vibration Analyzer Manual
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TM 1-6625-724-13&P 0009 00 0009 00-5 The types of measurements performed for a particular aircraft are completely determined by the aircraft configuration file. If the configuration file does not request a particular measurement it will not be made. The AVA has been designed to provide a flexible system to measure vibration and track for the most sophisticated helicopters. The DAU can interface to the following sensors: (a) 14 Accelerometers of various types (b) 2 Magnetic or optical tachometers (c) 2 Universal Tracking Devices (UTDs) (d) 1 Strobe Light The measurement and diagnostic setups for particular helicopters are controlled by the individual aircraft configuration file. This allows easy customization of measurement, display, and diagnostics to an airframe. The AVA has basic measurement capabilities as shown in table 1, and discussed as follows: Table 1. Measurement Capabilities Sensor Measurements Resolution Range Sample Vibration Power Average (sync): 400 lines 1/8 R to 50 R 128 Spectra Power Average (async): 400 lines 2.0 Hz to 20 kHz 128 Spectra Time Average: 128 lines 1/4 R to 32 R 512 Revs Zoom Displays (power only) 6400 Lines x1 x2 x4 x8 x16 x32 Vibration Amplitude: 2% 68 dB (100 g max.) Vibration Phase: 1° 0 to 360° Tachometer Frequency: 2 to 485 Hz (120 to 29100 RPM) Track: 2 mm 512 Revs Lag: 2 mm 512 Revs (1) Asynchronous Sampled Power Averaged Vibration Spectrum (ASPA) (ASPA Zoom): The conventional power spectrum provides a measurement of vibration amplitude vs. frequency. Normally 400 spectral points are provided, but an optional zoom parameter increases the spectral resolution to 6400 (ASPA Zoom) spectral points. The AVA can collect a single vibration channel at a time using this mode. Table 2 provides a list of the possible frequency ranges vs. their respective measurement resolutions. The aircraft configuration file can setup the frequency range, number of Fast Fourier Transforms (FFTs) to average, window type, channel setup, accelerometer type, and output type. The asynchronous vibration measurements can be collected on any of the available fourteen channels (2) Synchronous Sampled Power Averaged Vibration Spectrum (SSPA)(Signature Ratio): This mode provides a power spectrum measurement with sampling, which is based on the external tachometer frequency. This type of measurement eliminates the spectral smearing commonly found when using a conventional asynchronous spectrum with vibration components that constantly change frequency. Normally 400 spectral points are computed, however there is an optional zoom (SSPA Zoom) capability that increases the resolution to 6400 spectral points. The AVA collects this type of measurement on a single channel at a time. The external tachometer frequency range can vary between 2.0 Hz and 485 Hz. The setup in the aircraft configuration file determines the number of FFTs to average, window type, overlap, output type and acceptable frequency range. This is probably the method of choice for collecting vibration data that does not have a harmonic relationship to the tachometer source, but does have a fixed frequency ratio.

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