Blade Tip Timing Measurement Research Articles

Analysis and Calibration of Blade Tip-Timing Vibration Measurement Under Variable Rotating Speeds

Blade tip timing (BTT) vibration measurement is a promising on-line blade monitoring method, but various uncertainties bring great challenge in engineering applications. Most existing works are based on the assumption of constant rotating speeds. However, rotating speed is hardly fixed under variable conditions. In this case, these uncertainties always become more serious. To deal with this problem, this paper proposes to investigate BTT measurement derivations in angular domain, instead of time domain. Firstly, this paper systematically analyzes the effects of variable rotating speed, static angle errors and translational blade motions on the accuracy of BTT vibration measurement. Then the corresponding calibration methods are presented by only using times of arrival (TOAs). In the end, Matlab/Simulink simulations are done to validate the proposed method under linear and quadratic variations of rotating speeds. The results demonstrate that BTT measurement deviation under low rotating speeds is more than those under high rotating speeds and BTT measurement deviation due to static angle errors is independent of rotating speeds. And the proposed TOAs-based calibration method can reduce BTT measurement deviation greatly under variable rotating speeds, compared with traditional methods. BTT measurement deviation due to static angle errors can be calibrated by using low rotating speeds and those due to translational blade motions are difficult to be calibrated under variable rotating speeds. Thus simulation results indicate great potential of the proposed method for practical applications of the BTT method.

Synchronous Vibration Measurements for Shrouded Blades Based on Fiber Optical Sensors with Lenses in a Steam Turbine.

It is important to obtain accurate dynamic vibrations of steam turbine blades for safe operation. Strain gauge (SG) measurements cannot fully obtain vibrations of all blades and nodal diameter (ND) details. The blade tip timing (BTT) technique could resolve this problem because it has the advantage of measuring all blades’ vibrations. This article proposed an improved BTT technique for measuring synchronous vibrations of shrouded blades in a steam turbine based on fiber optical sensors with lenses. The newly developed sensor was equipped with a Plano-convex lens, and the optical path was specifically designed to collimate the beam within a large working distance from 4 to 19 mm and improve the measuring accuracy. A method to search the spectra of all peak vibration amplitudes of all the blades was proposed to obtain the ND details of synchronous vibration. Experimental results validated the efficiency and accuracy of the proposed methods and sensors. Comparison results of BTT measurements with SG measurements showed that the relative errors of normalized frequency and strain were small and acceptable.

Improved Blade Tip Timing measurements during transient conditions using a State Space Model

D.H. Diamond was supported by a Skye Foundation postgraduate scholarship during the first write-up and by a Claude Leon Postdoctoral Scholarship during revisions to this article.

Modelling and experimental validation of active and passive eddy current sensors for blade tip timing

To monitor the vibration of blades in rotating machinery, the contactless method called Blade Tip Timing (BTT) is widely used. blade vibration and clearance are important diagnostic features for condition monitoring, including the detection of blade cracks. To perform the BTT technique, optical sensors were widely used by industry due to their high accuracy, but the main drawback of these systems is their low tolerance to the presence of contaminants. To overcome this downside, eddy current sensors are a good alternative for health monitoring applications in gas turbines due to their insensitivity to contaminants and debris. This type of sensor has been used by many researchers, predominantly on the experimental side to investigate BTT systems and there is a lack of modelling to support the measurement system design. This paper fills the gap between experiments and modelling for the particular case of a blade rotating past eddy current sensors. Hence the novelty of this paper is the simulation of the BTT application using detailed quasi-static finite element models of the electro-magnetic field to estimate the outputs from active and passive eddy current sensors. A test rig composed of a bladed disk with 12 blades clamped to a rotating shaft was designed and manufactured in order to validate the proposed models with experimental measurements. Finally, a parametric study is presented to show the effect of the blade tip clearance and the rotational speed on the accuracy of the BTT measurement. This leads to better understanding of the sources of error in the time of arrival of the blades passing the sensor and hence insight into the blade vibration measurement accuracy.

Calibration of blade tip-timing sensor for shrouded 40″ last stage blade

Abstract Today’s power generation market requests steam turbines of high efficiency, reliability and wide operation range. The last stage moving blades are designed with integral shroud, mid-span, and fir-tree dovetail. The blades are continuously coupled by the blade untwist due to the centrifugal force, and thus the vibration control and increased structural damping are provided. Unfortunately this shroud complicates the blade vibration measurement using contactless methods. This paper describes in detail the Blade Tip-Timing measurement and calibration provided in test rig and in steam power station where the new eddy-current and optical sensors were used together to measure blade vibration in various conditions. Detail calibration procedure for Blade Tip-Timing sensors will be presented as well as the results of the measurements and comparison to FEM Maxwell calculation and to FEM stress analysis.

Wavelet Transform‐Based Damage Identification in Bladed Disks and Rotating Blades

Blade vibration and blade clearance are effective diagnostic features for the identification of blade damage in rotating machines. Blade tip‐timing (BTT) is a noncontact method that is often used to monitor the vibration and clearance of blades in a rotating machinery. Standard signal processing of BTT measurements give one blade response sample per revolution of the machine which is often insufficient for the diagnosis of damage. This paper uses the raw data signals from the sensors directly and employs a wavelet energy‐based mistuning index (WEBMI) to predict the presence and locations of damage in rotating blades. The Lipschitz exponent is derived from the wavelet packet coefficients and used to estimate the severity of the damage. In this study, experiments were conducted to obtain BTT measurements on rotating blades at 100 rpm using three different sensors: an active eddy current sensor, a passive eddy current sensor, and an optical sensor. In addition, hammer excitation experiments were conducted for various added mass (damage) cases to compute the damage severity for a bladed disk. To simulate the damage experimentally in the bladed disk and rotating blades, masses were added to the blades to alter their dynamics and mimic the damage. The results indicate that the WEBMI can detect the presence and location of damage in rotating blades using measurements from common BTT sensors. To check the robustness of the proposed damage severity index, the experimental results were compared with numerical simulation for the bladed disk and showed good agreement.

Sparse Representation Based Frequency Detection and Uncertainty Reduction in Blade Tip Timing Measurement for Multi-Mode Blade Vibration Monitoring.

The accurate monitoring of blade vibration under operating conditions is essential in turbo-machinery testing. Blade tip timing (BTT) is a promising non-contact technique for the measurement of blade vibrations. However, the BTT sampling data are inherently under-sampled and contaminated with several measurement uncertainties. How to recover frequency spectra of blade vibrations though processing these under-sampled biased signals is a bottleneck problem. A novel method of BTT signal processing for alleviating measurement uncertainties in recovery of multi-mode blade vibration frequency spectrum is proposed in this paper. The method can be divided into four phases. First, a single measurement vector model is built by exploiting that the blade vibration signals are sparse in frequency spectra. Secondly, the uniqueness of the nonnegative sparse solution is studied to achieve the vibration frequency spectrum. Thirdly, typical sources of BTT measurement uncertainties are quantitatively analyzed. Finally, an improved vibration frequency spectra recovery method is proposed to get a guaranteed level of sparse solution when measurement results are biased. Simulations and experiments are performed to prove the feasibility of the proposed method. The most outstanding advantage is that this method can prevent the recovered multi-mode vibration spectra from being affected by BTT measurement uncertainties without increasing the probe number.

A Blade Tip Timing Method Based on a Microwave Sensor.

Blade tip timing is an effective method for blade vibration measurements in turbomachinery. This method is increasing in popularity because it is non-intrusive and has several advantages over the conventional strain gauge method. Different kinds of sensors have been developed for blade tip timing, including optical, eddy current and capacitance sensors. However, these sensors are unsuitable in environments with contaminants or high temperatures. Microwave sensors offer a promising potential solution to overcome these limitations. In this article, a microwave sensor-based blade tip timing measurement system is proposed. A patch antenna probe is used to transmit and receive the microwave signals. The signal model and process method is analyzed. Zero intermediate frequency structure is employed to maintain timing accuracy and dynamic performance, and the received signal can also be used to measure tip clearance. The timing method uses the rising and falling edges of the signal and an auto-gain control circuit to reduce the effect of tip clearance change. To validate the accuracy of the system, it is compared experimentally with a fiber optic tip timing system. The results show that the microwave tip timing system achieves good accuracy.

Forced response of rotating bladed disks: Blade Tip-Timing measurements

The Blade Tip-Timing is a well-known non-contact measurement technique currently employed for the identification of the dynamic behaviours of rotating bladed disks. Although the measurement system has become a typical industry equipment for bladed disks vibration surveys, the type of sensors, the positioning of the sensors around the bladed disk and the used algorithm for data post-processing are still not standard techniques, and their reliability has to be proved for different operation conditions by the comparison with other well-established measurement techniques used as reference like strain gauges. This paper aims at evaluating the accuracy of a latest generation Tip-Timing system on two dummy blisks characterized by different geometrical, structural and dynamical properties. Both disks are tested into a spin-rig where a fixed number of permanent magnets excite synchronous vibrations with respect to the rotor speed. A new positioning for the Blade Tip-Timing optical sensors is tested in the case of a shrouded bladed disk. Due to the presence of shrouds, the sensors cannot be positioned at the outer radius of the disk pointing radially toward the rotation axis as in the most common applications, since the displacements at the tips are very small and cannot be detected. For this reason a particular placement of optical laser sensors is studied in order to point at the leading and trailing edges' locations where the blades experience the largest vibration amplitudes with the aim of not interfering with the flow path. Besides the typical Blade Tip-Timing application aimed at identifying the dynamical properties of each blade, an original method is here proposed to identify the operative deflection shape of a bladed disk through the experimental determination of the nodal diameters. The method is applicable when a small mistuning pattern perturbs the ideal cyclic symmetry of the bladed disk.

Turbine Rotor Disk Health Monitoring Assessment Based on Sensor Technology and Spin Tests Data

The paper focuses on presenting data obtained from spin test experiments of a turbine engine like rotor disk and assessing their correlation to the development of a structural health monitoring and fault detection system. The data were obtained under various operating conditions such as the rotor disk being artificially induced with and without a notch and rotated at a rotational speed of up to 10,000 rpm under balanced and imbalanced state. The data collected included blade tip clearance, blade tip timing measurements, and shaft displacements. Two different sensor technologies were employed in the testing: microwave and capacitive sensors, respectively. The experimental tests were conducted at the NASA Glenn Research Center's Rotordynamics Laboratory using a high precision spin system. Disk flaw observations and related assessments from the collected data for both sensors are reported and discussed.

Application of Microwave Sensing to Blade Health Monitoring

This paper discusses the application of microwave sensing to turbine airfoil health monitoring. The proposed microwave system operates at 6- and 24-GHz and is applicable to both blade tip-clearance and blade tip-timing measurements. One of the main advantages of microwave systems, compared to other technology such as capacitive or eddy current, is that it can be installed for long term operations in the harsh environment of the first turbine stages. The monitoring of blade tip-timing and tip-clearance pattern is useful for detecting abnormal blade behavior due to structural damage. Such a sensing system can also be used in actively maintaining optimal blade-to-casing clearance, thereby enhancing turbine efficiency. This paper presents blade tip-clearance pattern monitoring based on microwave measurements. First, a laboratory study shows the ability of the system to consistently measure tip clearance pattern. Then tip clearance pattern measurements from a real engine test are presented. While this paper presents results from system testing on tip clearance, it is expected that this study will be carried forward in the next phase to demonstrate tip-timing measurement and further, to show how such as system can form the basis for a more comprehensive health management system.

Experimental measurements of out-of-plane vibrations of a simple blisk design using Blade Tip Timing and Scanning LDV measurement methods

The study of dynamic properties of rotating structures, such as bladed discs, can be conveniently done using simple bladed discs where the blades do not have staggering angles. Simplified design, although not truly representative of real structures, can be easy and economic to manufacture and, still, very helpful for studying specific dynamic properties. An example of this can be called as mass mistune blisk study. Experimental measurements of vibrations of bladed discs under rotating conditions can be performed using Scanning Laser Doppler Vibrometer (SLDV) systems. However, in the aerospace industry, the vibrations of complex bladed discs must be measured under operating conditions which are more hostile than laboratory simulations. The Blade Tip Timing (BTT) measurement method is a measurement technique, which can be used to measure vibrations of bladed discs of an engine aircraft under operating conditions. However, the BTT technique is ineffective when used with a flat bladed disc whose blade vibrations cannot be measured. This can be detrimental when the use of controlled dynamic parameters, such as those obtained from a simple bladed disc design, can improve the confidence for the validation of post-processing software.This paper presents a work about experimental measurements of a simple bladed disc design whose vibrations were measured synchronously by Scanning LDV and BTT measurement systems. A rotating test rig and its mechanical modifications for the installation of the BTT probes are introduced. Implications of rotating a specimen inconsistently are presented so as solutions to obtained constant revolving speed. The experimental comparisons of forced response vibrations measured synchronously at one blade are presented and explained.

Reconstruction of the Blade Vibration Signal from Rotating Machinery Based on Blade Tip-timing Measurement

The non-contact measurement vibration signals obtained by blade tip-timing sensor mounted on the machinery case are discrete undersampling signals.A method for the reconstruction of rotating blade vibration signal with interpolation technique at even speed is presented.The reconstructive signal is helpful for the identification of the blades vibration amplitude which is one of the features for rotating assemblies fault diagnosis.With the analysis of signal's sampling and reconstruction model,the reconstruction is based on a finite number of samples by timelimited kernels which are constructed with the involvement of B-splines.The reconstruction has lower truncation error and less calculation as a result of the rapid constringency of higher-order B-splines.The simulation shows that the precision of signal reconstruction is highly dependent on the stability of sampling rate,which is related with the rotation rate.Gas excitation experimentation on the blade vibration test bed is introduced to test and verify the performance of this method in practice.

Tubomachinery blade vibration amplitude measurement through tip timing with capacitance tip clearance probes

Turbomachinery blade vibrations can cause high cycle fatigue (HCF), which reduces blade life. In order to observe this vibration a non-intrusive monitoring system is sought. The vibration can be detected by measuring blade tip timing since in the presence of vibration the blade timing will differ slightly from the passing time calculated from rotor speed. Much research and development has gone into investigating the ability of optical probes to achieve this. However, this paper looks at the potential for a dual use capacitance probe sensor to measure both tip timing and tip clearance. This paper provides new insights into the ability of a commercially available capacitance probe tip clearance measurement system for application as a non-intrusive turbomachinery blade tip timing measurement device. This is done by correlating capacitance probe tip timing results with simultaneously measured blade-mounted strain gauge vibration results and precise rotational speeds. Blade tip vibration amplitudes are measured using capacitance probes and compared to strain derived vibration levels. Thus the characterisation and quantification of the performance of the capacitance probe system when measuring blade vibration on a full-sized low-speed research compressor is analysed and reported.