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

Arrival Times to Deflection Conversion

The arrival times of each blade are converted to deflections by knowing the rotational velocity and the radius of the measurements.

X = Vt=2*pi*R(t/T)

Rotational Velocity (V) = revolutions / minute

Radius to blade tip (r) = inches

t = time from measured blade event to measured blade event

T = time for one revolution of all the blades

Blade Tip Timing

In the blade tip timing example, the green line is the actual arrival time for mid-blade edge (blue dot) of a particular blade as measured by an AMS probe. The red dashed line is the center-time for a blade passing. The center time is calculated using the arrival times of all the blades and averaging them over 3 revolutions. The time difference (black arrow) represents the difference between when the blade should pass and when it actually does. Following the blade is a 5E (five vibration/one rotation) Sine wave. The difference value is converted into blade deflections and used in the overall blade analysis.

Blade Vibrations

Data from multiple probes at a common axial location are used to determine the mode frquency of a vibration. Once fit, amplitude and phase data can be calculated along with engine order, peak response rpm, data fit, absolute phase, damping, and other vibratory characteristics. 2N + 2 probes are required to analyze N simultaneously occurring blade vibrations.

Probes are placed in specific locations around the case of the given application to optimize on certain (prespecified) engine orders. This means that the locations of the probes allows for more accurate data around the regions where resonances are bound to occur.

When solving for a resonant frequency as mapped as a sinusoidal oscillator, it is necessary to view the response at three points along the phase of the vibration; Therefore, 2N + 2 probes are required to analyze N simultaneously occurring blade vibrations.

Simultaneous Engine Vibrations

The Campbell diagram shows two modes coming into resonances at the same time. (2N+2)