Non-intrusive stress measurement (system), or NSMS, is a method for determining dynamic blade stresses in rotating

turbomachinery Turbomachinery, in mechanical engineering, describes machines that transfer energy between a rotor and a fluid, including both turbines and compressors. While a turbine transfers energy from a fluid to a rotor, a compressor transfers energy from ...

. NSMS is also known by the names "blade tip timing" (BTT), "arrival time analysis" (ATA), "blade vibration monitoring" (BVM), ''Beruehrungslose Schaufel Schwingungsmessung'' (BSSM), and "blade health monitoring" (BHM). NSMS uses externally mounted

sensor A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...

s to determine the passing times of turbomachinery blades. The passing times after conversion to deflections, can be used to measure each blade's vibratory response characteristics such as amplitude/

stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...

phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform * Phase space, a mathematic ...

, frequency and

damping Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples in ...

. Since every blade is measured, stage effects such as flutter, blade mistuning, and nodal diameter can also be characterized. The measurement method has been used successfully in all stages of the

gas turbine A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...

engine (fan, compressor, and turbine) and on other turbo-machinery equipment ranging from

turbocharger In an internal combustion engine, a turbocharger (often called a turbo) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to pro ...

s to rocket pumps. The ability to apply the technology to a given situation is dependent upon a sensor type that can meet the environmental requirements.

Method

A set of sensors is used to measure the arrival times of rotating blades. These arrival times, in comparison to a baseline, are used to determine blade deflections. The blade deflections over a number of revolutions and/or across a number of sensors can be used to determine vibratory characteristics. This information, in conjunction with a finite element model (FEM), can then be used to determine the dynamic stresses in a rotating part. Spot Probe Example, Radial View

Hardware

Any sensor or probe that can provide a precise indication of a blade's passing can be used in NSMS. Commonly optical probes using fiber optics are used to obtain a high level of spatial resolution. A light source, typically a laser, and fiber optics are used to direct a constant beam of light into the path of a rotating blade. As a blade passes the probe, light is reflected and captured by fiber optic and routed to a photo detector for conversion to an electrical signal. Field type sensors such as Eddy Current, Capacitive, and Microwave sensors are useful in harsh environments and for long duration testing.

History

NSMS technology was patented as early as 1949, with many improvements patented since that date. Some recent US practitioners believe that NSMS was first developed in 1980, by major aircraft engine OEMs (and the US Air Force). It may be envisioned as a replacement technology to rotating

strain gauge A strain gauge (also spelled strain gage) is a device used to measure strain on an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports ...

s, however it is currently used as an essential complement to strain gauges. The technology is widely used for turbo-machinery development and high cycle fatigue (HCF) troubleshooting. Due to the direct blade observation, external mounting and inherent serviceability of the sensors, NSMS has been used since at least 2006 as a long-term health monitoring method. More recently, multiple practitioners are becoming active in this application of the technology.

References

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