Torsional Vibration Measurement Research Articles

Compensation for encoder geometry and shaft speed variation in time interval torsional vibration measurement

Digital time interval techniques for the measurement of torsional vibrations in rotating shafts have been used in a variety of applications. The techniques use high-speed timers to detect the passage times (often referred to as “zero crossings”) in a carrier signal from a multiple pulse per revolution encoder on the shaft. Next, a reference array is created by computing the passage times for an encoder with geometrically uniform segments rotating at a constant speed. The measured times are compared against the reference array and the time difference processed to yield the shaft torsional vibration. If the reference array differs from the encoder passage times, the calculated torsional vibration will be subject to bias errors manifested as high-level order content. There are two potential causes for inconsistency in the reference signal: (1) geometric variation in angular encoding device and (2) a non-constant shaft speed. Procedures to handle these situations are addressed. An in situ encoder calibration method is presented that creates a reference array representative of the actual encoder angular intervals. The method uses time synchronous averaging of the encoder zero crossing over many shaft revolutions. The process causes the time passage variations induced by the torsional vibration to average to zero, ultimately yielding the time stamps caused by the passage of each respective encoder segment. The synchronous averaged encoder passage times are then used to correct the reference array to calculate the torsional vibration. Variation of the encoder geometry and changes in shaft running speed cause the torsional vibration be sampled on a non-uniform time interval basis. The non-uniform time sampling would cause errors if used with fixed sampling interval algorithms such as the DFT. A resampling algorithm is applied to create an array sampled with a constant time interval basis to minimize the error. The background and implementation details of time interval torsional vibration measurement enhancements are first presented. The capabilities are then demonstrated with experimental results from several pieces of rotating equipment. The tests show the successive improvement in the torsional vibration data as the proposed compensation methods are sequentially applied.

Measurement of torsional vibrations by using strain gages

In this work the measuring method of torsion vibrations is presented according to the measurement of torsion stress using strain gages. This method is particularly suitable in checking the system elements strain level that transfers changeable torsion moments and oscillate torsionally. Besides that, the system elements critical velocity rotation can be estimated, folio-wed by the resonant strain and problems in the function of that system such as: amplified noise, -wearing-out of gears, fatigue crack, damage and break of shafts and junctions etc. The measuring chain is proposed in -which the central part is a contemporary mobile system Spider 8, -which enables measurement, processing and displays measured results on a computer.

Measurement and Control of Torque Ripple-Induced Frame Torsional Vibration in a Surface Mount Permanent Magnet Machine

A sensor to measure the stator torsional vibration due to torque ripple produced by a surface mount permanent magnet machine is first described. The sensor is relatively inexpensive and is straight forward to incorporate into a drive system. Experiments are performed to validate that the voltage produced by the sensor is linearly related to torque ripple amplitude. Closed-loop controllers are then described that adjust the stator current harmonics applied to the machine to achieve a commanded average torque while mitigating measured torsional vibration. Simulation and experimental results are used to demonstrate the effectiveness of the control techniques.

Measurement system of torsion vibration for reactor internal pump

Object of this invention is to provide an axial torsional vibration measurement device of the reactor internal pump capable of measuring the axial torsional vibration easily. The vibration detector 2 that detects vibration in the radial direction, and the sound detector 3 that detects vibration in the circumferential direction are fitted on cylindrical motor casing 6 , which contains the motor of a reactor internal pump. The dimension of both signals of a vibration waveform signal detected by the vibration detector 2 , and a sound waveform signal detected by the sound detector 3 are matched in vibration calculating device 15 , to thereby obtain an axial torsion vibration of a motor from the difference of both signals.

Torsional damping measurements for a gearbox

Torsional vibration measurements on rotating machinery under working conditions have been limited. This is particularly the case for measurements made under controlled excitation conditions. In previous papers, the authors have presented experimental results obtained for a back-to-back gearbox rig with a controlled excitation. This paper is concerned with methods of extracting the torsional characteristics of a single gearbox from the measurements made on the complete test rig that included a servo-motor and a second gearbox.

Hot topics in structural acoustics: Optical imaging of surface vibrations—Some new developments and applications

Laser Doppler vibrometry is routinely used for a vast range of applications to measure surface vibrations in the nanometer range. Some new developments include the use of laser vibrometers with continuous tracking on moving targets, the detection of the vector displacement of a surface (either with multiple beam interferometry or with a photo-emf probe), and the direct measurement of torsional vibrations on rotating shafts. Electronic speckle pattern interferometry (ESPI) is a full-field optical technique that is also commonly used to image surface vibrations. An interesting development of ESPI is electronic shearography. Some applications of these optical techniques will be discussed. These include calibrations of transducers and MEMS devices, imaging of rotating parts, mapping of structural intensity, characterization of the dynamic response of viscoelastic materials, as well as some more exotic applications such as damage assessment of frescoes and painting, three-dimensional vibration imaging of the ossicular chain in cats, and dynamic characterization of human teeth.

Blade and Shaft Crack Detection Using Torsional Vibration Measurements Part 2: Resampling to Improve Effective Dynamic Range

The primary goal of the development project was to demonstrate the feasibility of detecting changes in blade bending natural frequencies (such as those associated with a blade crack) on a turbine using non-contact, non-intrusive measurement methods. The approach was to set up a small experimental apparatus, develop a torsional vibration detection system, and maximize the dynamic range and the signal to noise ratio. The results of the testing and analysis clearly demonstrated the feasibility of using torsional vibration to detect the change in natural frequency of a blade due to a change in stiffness such as those associated with a blade crack. However, it was found that harmonics of shaft operating speed, created as an unwanted artifact of the measurement method, resulted in spectral regions in which the effective dynamic range was inadequate to detect low-level torsional vibration associated with the natural frequencies. The loss of effective dynamic range was due to the “skirts” created by the sampling window. Application of order resampling, followed by frequency resampling, to the torsional vibration waveform increased the effective dynamic range and improved the ability to identify shaft torsional and blade bending natural frequencies.

ROTATIONAL VIBRATION MEASUREMENTS USING LASER DOPPLER VIBROMETRY: COMPREHENSIVE THEORY AND PRACTICAL APPLICATION

This paper builds on a previous study of the velocity sensed by a single laser vibrometer beam incident on a target in an arbitrary direction to predict the velocity sensed by multiple laser beams with arbitrary orientations. The usefulness of parallel beam arrangements is demonstrated and a concise new theory for the difference velocity sensitivity for two parallel beams is presented. Parallel beam arrangements for virtually unambiguous measurement of torsional vibration are presented along with arrangements that allow measurement of the pitch and yaw vibration to be derived. Resolution of individual pitch and yaw motions is shown not to be possible by any geometrical arrangement of the beams but a post-processing technique that enables the genuine pitch and yaw vibrations to be estimated from the measurements of the pitch and yaw vibration sets is demonstrated and validated experimentally in the laboratory. Measurements of pitch and yaw vibration are, for the first time, obtained from the crank-shaft of a running diesel engine, enabling identification of the first natural frequency of the crank-shaft in bending.

LASER VIBROMETERS AND CONTACTING TRANSDUCERS, TARGET ROTATION AND SIX DEGREE-OF-FREEDOM VIBRATION: WHAT DO WE REALLY MEASURE?

This paper presents a comprehensive analysis of the velocity sensed by a single laser vibrometer beam incident in an arbitrary direction on a target that is of substantial interest in engineering — a rotating shaft requiring three translational and three rotational co-ordinates to describe its vibratory motion fully. Six separate “vibration sets”, each a combination of motion parameters, appear in the full expression for vibration velocity sensitivity and it is shown not to be possible to resolve individual motion components within each set by arrangement, or even manipulation, of laser beams. To place this non-contact transducer velocity sensitivity model in its proper context, the velocity sensitivity of a contacting transducer under identical conditions is also derived and comparison is made between non-contacting and contacting transducer performance. Specific applications of the laser vibrometer theory to radial and axial vibration measurements are set out and it is shown how estimation of radial vibration components is only possible by post-processing. The theory is easily extended to include measurements made with multiple beams, underlining the model's versatility in enabling determination of the vibration component sensitivity of a measurement with any beam orientation or combination of beam orientations. This will prove beneficial in devising further optical configurations for the measurement of torsional, pitch and yaw vibrations.

Identification of Gear Errors by Vibrations of a Spur Gearpair

A new method of identifying modal parameters (natural frequency and damping) and the equivalent gear error of a spur gear pair is developed. The equivalent error is a function of the gear position and is related to the errors of the driving and the driven gears and to the non-dimensional stiffness of the teeth. The method is based on the measurement of the gear torsional vibrations. The test bench is modelled as a single degree of freedom system and must be assembled by using stiff bearings and torsionally compliant shafts.

TORSIONAL AND BENDING VIBRATION MEASUREMENT ON ROTORS USING LASER TECHNOLOGY

Based on the principles of laser Doppler velocimetry, the laser torsional vibrometer (LTV) was developed for non-contact measurement of torsional oscillation of rotating shafts, offering significant advantages over conventional techniques. This paper describes comprehensive theory to account for the sensitivity of the LTV's measurements to shaft motion in all degrees of freedom. The optical geometry of the LTV offers inherent immunity to translational motion of the target shaft, either axial or radial. However, its measurements are sensitive to angular lateral vibration of the shaft. The significance of this sensitivity is compared with the instrument noise floor and typical torsional and lateral vibration levels. Optimum alignments of the instrument are then specified to ensure effective immunity to all lateral motion in typical applications. To overcome this problem more reliably, a new technique is proposed permitting unambiguous measurement of pure torsional vibration in situations where use of a single LTV demonstrates unacceptable sensitivity to angular lateral vibrations. Practical application of this technology is demonstrated with torsional vibration measurements from a diesel engine crankshaft. Simultaneously, previously unattained measurements of shaft bending vibration measurements are made. The first bending mode of the crankshaft was identified and its vibration amplitude and damping estimated. This application of laser vibrometry for non-contact measurements of shaft vibration represents a further step forward in the use of this technology for machinery diagnostics.

Quantitative material characterization by ultrasonic AFM

In an atomic force microscope equipped with a micromachined cantilever tip, the cantilever vibration spectra in contact with the sample were found to be strongly dependent on the excitation power. However, if the excitation power is small enough, the resonance peak width decreases and the peak frequency increases to a certain limiting value. In this condition the tip–sample contact is kept linear, and satisfactory agreement between the measured and calculated frequency is obtained, assuming a constant contact stiffness; the agreement is further improved by taking into account the lateral stiffness. More quantitative information on the elasticity of the sample is obtained from the contact load dependence of the frequency, where contact stiffness of a non-spherical tip shape is derived from the Sneddon–Maugis formulation, and the tip shape index is estimated by an inverse analysis of the load–frequency relation. A further advantage of evaluating not only the vertical but also the lateral stiffness is demonstrated on a ground silicon wafer by simultaneous measurement of deflection and torsional vibration. Copyright © 1999 John Wiley & Sons, Ltd.

Calculation and Measurement of Torsional Vibrations in Large Steam Turbosets – New Technique

ABB has been carrying out torsional vibration test on steam turbine sets for many years, with sizes ranging from 350MW coal fired steam turbines to 1000MW nuclear steam turbines. Rated speeds have been 1,800 rpm and 3600 rpm. Torsional shaft vibrations are excited in large steam turbine generators sets by electrical disturbance torques occurring in the active part of the generators. Due to the low torsional damping of the shaft trains, resonance can cause high torsional vibration amplitudes. Some thermal power plant operators require experimental verification of the natural torsional shaft frequencies. ABB has developed a new measurement technique which allows torsional vibration to be measured in a selected measurement plane along the shaft. With today's highly sensitive sensors and modern measurement and data analysis equipment, the excitation due to normal random disturbances in the electrical grid provides sufficiently good results. All natural torsional frequencies in the range of interest are obtained with an accuracy of ± 0.2 Hz and identified using the calculated natural frequencies. The new measurement technique, places no restrictions on normal power plant operation.

A METHOD TO IDENTIFY MODAL PARAMETERS AND GEAR ERRORS BY VIBRATIONS OF A SPUR GEAR PAIR

A new method to identify modal parameters (natural frequency and damping) and the equivalent gear error of a spur gear pair is introduced. The equivalent error is a function of the gear position and is related to the errors of the driving and the driven gears and to the non-dimensional stiffness of the teeth. The method is based on the measurement of the gear torsional vibrations. The test rig is modelled as a single-degree-of-freedom system and must be assembled by using stiff bearings and torsionally compliant shafts. The solution of the equation of motion is obtained through the harmonic balance method. The proposed approach has some advantages with respect to traditional metrological methods. The effect of noise on the accuracy of the identification is also investigated and discussed. Applications of the method to the identification of natural frequency, damping and profile errors are shown.

Practical performances of high-speed measurement of gear transmission error or torsional vibrations with optical encoders

We evaluated in this paper an improved technique for measuring gear transmission error (GTE) at high speed, by using low pulse per revolution optical encoders. The originality of this technique lies in the fact that highly precise, completely digital measurements of torsional vibration or transmission error (TE) at high speed are achievable by the use of low-price, basic optical components. The lengths of encoder pulses are estimated with a high-frequency timer (100 MHz): thus, it appears that the theoretical precision of this device depends only on the angular speed of shafts, not on the number of pulses per revolution of the encoder. In practice, the intrinsic encoder accuracy (namely the grating or electronic signal processing precision) directly affects precision measurements. Alternatively, the number of pulses per revolution of the encoder specifies the resolution. We examined the possibility of calibrating encoders through using a specific test rig. The determination of corrective data assigned to each grating leads to an insignificant improvement of the precision measurement. The coherence from one revolution to another does not present any significant deterministic component. The overall precision achieved is less than 0.03 second of arc for each frequency of the power spectral density. This calibration device only gives a good assessment of eccentricity induced by mechanical mounting of optical discs on a shaft, compared with the direct measurement on grating discs. The correlation between the two measurements is less than 3% of the magnitude of the relative eccentricity. Thus, the encoder technique seems to be a cheap and easy way to implement transmission error measurement on real mechanical systems with high precision and sufficient reliability.

REMOVAL OF SPECKLE HARMONICS IN LASER TORSIONAL VIBROMETRY

Torsional vibration measurements on rotating machinery have traditionally been undertaken using techniques such as mechanical torsiographs, strain gauging with slip rings or telemetry systems, and encoders (slotted disks, gears, etc.). These techniques have the common limitation that the test machine has to be stopped in order to attach the transducer. The application of a cross-beam laser vibrometer to torsional vibrometry by Simpson and Lamb [1] and further development by Halliwellet al.[2] provided a non-contact torsional measurement technique, resulting in significant time savins. However, this design was restricted to use on shafts with a circular cross-section and had a very small beam crossing zone (<1 mm in depth), which had to be focussed carefully on the shaft surface. This cross-beam design was also unable to distinguish between torsional and transverse shaft vibrations.The invention of the modern (parallel beam) laser torsional vibrometer (LTV) by researchers at the ISVR in Southampton [3,4] overcame the main limitations of the cross-beam device. Their elegant design responds inherently to torsional vibrations only and may be used with shafts of arbitrary cross-section. In addition, the laser probe may be hand-held and there is a significant tolerance in the allowable distance between the laser source and the target surface (typically between 5 and 45 cm). A recent review paper on the development and application of laser torsional vibrometers has been published by Halliwell [5].

Torsional Vibration Measurements on a Diesel Engine and Dynamometer

Torsional vibration measurements were performed on a diesel engine coupled to an induction dynamometer. The motivation for this work wasan unexpected failure of a flexible coupling between the engine and dynamometer. Excessive torsional vibration is not easily observed; quite often, little warning is give before coupling failure. The objective of this testing was to confirm that the likely cause of the coupling failure was torsional vibration and to demonstrate the feasibility of a new, dual-laser technique for measuring torsional vibration.

THE LASER TORSIONAL VIBROMETER: A STEP FORWARD IN ROTATING MACHINERY DIAGNOSTICS

This paper describes the development of the laser torsional vibrometer since its invention at the Institute of Sound and Vibration Research, University of Southampton in 1983. The theory of laser vibrometry measurements is first introduced together with a description of early attempts to harness the technology for torsional vibration measurement through use of a cross-beam laser Doppler velocimeter. This is followed by the theory for the laser torsional vibrometer itself. A measurements section then describes results taken by the first prototype built at ISVR and compares these with the cross-beam velocimeter in a controlled experiment. Further tests with the commercial version of the instrument produced by Brüel and Kjaer Ltd as the Type 2523 Laser Torsional Vibration Meter, are described, involving checks with the well known Hooke's joint. Results from an investigation into torsional vibration damper performance are also presented. The sensitivity of the instrument is dealt with in detail before practical considerations are described. The laser torsional vibrometer has solved a particularly difficult metrology problem and is now used on a worldwide basis. It is rapidly gaining acceptance as the standard means of measuring torsional vibration.

A torsional vibration measurement system

At the development stage of automobile construction, many power-train configurations may be considered. Due to complicated coupling mechanisms, the natural frequencies of the combined system are difficult to predict. It is therefore necessary to run tests on automotive subsystems under realistic operating conditions. To fully understand the dynamic behavior, a study of both torsional and translational motions along the driveshaft is necessary. The computerised torsional measurement system described has been designed to measure translational and torsional motion simultaneously at points along a rotating shaft. It overcomes the drawbacks of existing systems that measure torsional vibrations, which are either expensive, require shaft modifications, or do not account for the coupling between torsional and translational motions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurement of Torsional Vibration in Rotating Machinery

Five techniques for measurement of torsional vibration in a drive train are described, along with the advantages and disadvantages of each. Two laboratory test rigs, designed and constructed for experimental evaluation of various torsional vibration measurement techniques, are described. The measurement techniques investigated fall into three categories: a) those using signals from strain gages, b) those using signals from a magnetically transduced gear, and c) using signals optically transduced from a special tape applied to the shaft. One of the techniques described requires no specialized instrumentation other than a spectrum analyzer, which is now typically available in plants with rotating machinery. Another technique is capable of producing a detailed time history of torsional oscillations during acceleration of the drive train (i.e., startup), using a time interval measurement system. A comparison of results from the various techniques is given, and recommendations are made for several types of applications.