One of the main challenges of the Blade Tip Timing (BTT) measurement method is to be able to determine the sensing position of the probe relative to the blade tip. It is highly important to identify the measurement point of BTT since each point of the blade tip may have a different vibration response. This means that a change in measurement position will affect the amplitude, phase and DC component of the results obtained from BTT data. This increases the uncertainty in the correlation between BTT measurements and Finite Element (FE) modelling. Also, the measurement point should ideally be located to measure as many modes as possible. This means that the probe’s position should not coincide with a node, or a position at which the sensor misses the blade tip. Changes in the sensing position usually arise from the steady state movements of the blades (change in mean displacement). Such movements are caused by changes to the static (thermal and pressure) loading conditions that result from changes in the rotational speed. Such movements usually have a constant direction at normal operating conditions, but the direction may fluctuate if the machine develops a fault. There are three main types of movements of the sensing position that are considered in this paper: (1) axial movement; (2) blade lean; (3) blade untwist. Ideally, the sensing position is known based on the geometries of both the blade and the probe, but due to different types of movements of the blade this position is lost. Very few works have researched the extraction of the sensing position. Such preliminary works have required a pre-knowledge of mode shapes and additional instrumentation. The aim of this paper is to present a novel method for the identification of the BTT sensing position of the probes relative to a blade tip, which can be used to quantify the above movements. The developed method works by extracting the steady state offset from measurements of blade tip displacements over a number of revolutions as the speed changes from zero to a certain value. Hence, that part of the offset that is due to the angular positioning error of the probes (outside the scope of this work) is cancelled out (since it is independent of speed). The change in steady state offset is then processed to identify the three possible movements. The new method is validated using a novel BTT simulator that is based on the modal model of the FE model of a bladed disk (“blisk”). The simulator generates BTT data for prescribed changes to the sensing position. The validation tests show that the novel algorithm can identify such movements within a 2% margin of error.

The correlation of blade tip timing (BTT) measurements against strain gauge (SG) measurements and finite element (FE) predictions includes a number of uncertainties. One of the main ones is the steady movement of the blades (i.e. change in their mean position and orientation). This causes the sensing positions of the probes relative to a blade tip to deviate from their intended (nominal) positions, leading to deceptive results for the BTT amplitude and the corresponding stress levels.Such movements are caused by variations in static loading conditions (thermal and pressure) associated with changes in the operating speed. A novel method is introduced for the determination of three basic types of blade tip steady movements: axial; lean; untwist. The method relies on linking the shift in the averages of the BTT data to a number of geometrical relations, depending on the type of movement. Not more than two probes (to be placed at different axial positions) are needed to measure all three types of movement. The method is validated by simulations using a novel BTT simulator, and by measurements from both a test rig and real engine tests. The validated results demonstrate the great potential of the method for practical applications.

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The advent of tip-timing systems makes it possible to assess turbomachinery blade vibration using non-contact systems. Currently, the most widely used systems in industry are optical systems. However, these systems are still only used on development engines, largely because of contamination problems from dust, dirt, oil, water etc. Further development of these systems for in-service use is problematic because of the difficulty of eliminating contamination of the optics. Hence, alternatives need to be developed that are immune to contamination but have equivalent resolution and bandwidth as the optical system. Experimental measurements have been carried out using alternative sensors. An eddy current sensor has been developed in a series of laboratory and engine tests to measure rotor blade arrival times. Comparisons are made with an industry standard optical blade tip timing system. The results show that it is possible to acquire high quality blade tip timing data for use in engine condition monitoring using an eddy current sensor. This sensor allows measurements to be taken that do not suffer from flow contamination and allow deployment for hotter flow environments.