Blade Tip Deflection
The basic principle of operation of NSMS (also commonly referred to as "blade tip timing") is that the blade tip times of arrival at probes mounted in the case are compared with a fast, very stable system clock referenced to an accurate 1/rev signal. Blade vibration results in either a retarding or advancing of the blade passage time relative to the expected nominal blade passage time. From the advancement or retarding of the blade tip passage time an NSMS/blade tip timing system is able to infer the blade tip deflection. When this blade tip deflection is then used as an input to a finite element analysis of the blade, it is then possible to infer the blade stresses. NSMS/blade tip timing has the unique capability to measure the stresses of all of the blades in an instrumented stage.
NSMS/blade tip timing systems have evolved through several generations (see Table 1) with the first systems being capable only of blade flutter monitoring. As part of an Air Force funded effort to develop a generation 4 NSMS system to address high cycle fatigue (HCF) needs a NSMS/blade tip timing sensor specification was tabulated which lists sensor requirements for both the cold and hot sections of turbine engines (see Table 2).
Current Sensor Needs
NSMS/Blade tip timing systems commonly use optical probes. An advantage of optical probes is that they are capable of providing a high degree of spatial resolution. A disadvantage of optical probes is that they require frequent cleaning of the probe tip. For blade health monitoring and flight test applications non-optical probes are preferred in order to avoid the cleaning requirements inherent in optical probes. The sensor specifications given in Table 2 provide a benchmark against which the performance of non-optical NSMS/blade tip timing probes may be compared.
Strategic Advisory Board (SAB) Members Addressing This Need
Aerodyn Engineering, Inc.