NSMS Fundamentals

• Overview
• Externally Mounted Senors
• Various Measurement Capabilities
• Event Time Determination
• Arrival Times to Deflection Conversion
• Deflection to Stress Conversion
• Complete System Characterization
• Flexible Test Scheduling
• Synchronous Realtime Analysis
• Non-Synchronous Realtime Analysis
• High Cycle Fatigue

Non-Synchronous Realtime Analysis (Traveling Wave Analysis)

Observed Engine Order

• Engine Order is the vibration frequency of individual blades
• Nodal Diameter is the relative phase from one blade to the next, it is always an integer, from 0 to ½ the number of blades
• Observed Engine Order = Engine Order + Nodal Diameter

• Observed Engine Order is what a probe (or probe difference) observes from a fixed location on the case when looking at all blades.
• Since we are sampling all blades as a system or single entity the engine order and nodal diameter are observed as one or a summation. Until one of the two is known, the other cannot be uniquely defined.

Nodal Diameter

Observed Engine Order

Converting Frequency to Engine Order

Vibrations are described in their field of study by their engine order.

Aliasing

Aliasing is a type of distortion that occurs when digitally recording high frequencies with a low sample rate. If a periodic motion (vibration) is occuring at a frequency off, Then to positively identify the signal, it must be sampled at a frequency of at least 2*f (Nyquist Frequency). If the signal is not sampled at this rate, aliasing occurs, and the vibration can appear to be at a much lower/higher frequency.

Each probe records only one blade passing time for each blade for each revolution. Most of the observed signal is therefore aliased. Understanding how aliasing effects data is critical when designing analysis tools.

The AMS Non-Synchronous Realtime analysis allows the user to view both individual blades and the entire system in 3 dimensional FFT plots. These functions allow the user to track Mode buffet responses, monitor for crack detection and flutter, and confirm resonant responses, all in real time. The graphical displays and functions are similar to that found in the Offline software and their descriptions can be found on the AATAS OFFLINE link