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Week 13   vibration measurements
 

Week 13 vibration measurements

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    Week 13   vibration measurements Week 13 vibration measurements Presentation Transcript

    • Week 13 Vibration Measurement 1
    • Week 13 Vibration Measurement Scheme x, v, a, F amplified signal electrical signal signal data The type of vibration measuring instrument used will depend on: - The expected range of frequencies and amplitudes - The size of machine / structure involved - Conditions of operation of the machine / structure - Type of data processing used 2
    • Week 13 Dynamic Testing of Machine/Structure Dynamic testing is a valuable complement to dynamic analysis Modal testing: Natural frequencies Damping Mode shapes Operational testing: In-service loads Various cases Often random 3
    • Week 13 Dynamic Testing of Machine/Structure Measurement Hardware 4
    • Week 13 Transducer A transducer translates changes in mechanical quantities (such as displacement, velocity, acceleration or force) into electrical signals (such as voltage or current). The size of the transducer is importatn Especially when measuring small structures Ideally should not influence the structure’s dynamics through added mass or stiffness Analytical models often include effects of transducer mass 5
    • Week 13 Variable Resistance Transducer R ∞ ε ∞ x(t) Mechanical motion produces a change in electrical resistance of the strain gage, which in turn causes a change in the output voltage or current. 6
    • Week 13 Variable Resistance Transducer R ∞ ε ∞ x(t) Wheatstone bridge, sensitive to small changes in R 7
    • Week 13 Piezoelectric Transducer Piezoelectric material, such as quartz and Rochelle salt, E generate electrical charge when given deformation or mechanical stress. crystal Piezoelectric accelerometer 8
    • Week 13 Electrodynamic Transducer Voltage ~ Velocity Current ~ Force The movement of the coil in a magnetic field produces a voltage Vout. 9
    • Week 13 LVDT Linear Variable Differential Transformers: - coils and magnetic core - output voltage depends on the axial displacement of the core - Displacement transducer - Insensitivity to temperature - Mass of core restricts application - Best for low frequency, high displacement application 10
    • Week 13 Vibration Pick-ups Instruments that measure vibrations, e.g. vibrometer, accelerometer. Most commonly used pick-ups are known as seismic instruments - a mass-spring-damper system in a cage inertial sensor - a certain type of transducer is integrated in the instrument. - fastened onto the vibrating body 11
    • Week 13 Vibration Pick-ups 12
    • Week 13 Vibrometer - Measures the displacement of a vibrating body - Works in the right (blue) range where r >= 3 Z/Y = 1 if Recorded z(t) lags from measured y(t) 13
    • Week 13 Accelerometer Measures the acceleration of a vibrating body, from which the velocity and displacement are obtained by integration. recorded z(t)ωn2 lags from the acceleration of the base ytt(t) 14
    • Week 13 Accelerometer Works in this range 15
    • Week 13 Vibration Exciter Application: - in the determination of the dynamic characteristics of machines and structures; - In fatigue testing of material. Types: mechanical, electromagnetic/electrodynamic, hydraulic. Also known as ‘shakers’. 16
    • Week 13 Mechanical Exciter 17
    • Week 13 Mechanical Exciter 18
    • Week 13 Electrodynamic Shaker Also known as electromagnetic exciter. A reverse of the electrodynamic transducer. Apply a force through a range of frequencies (harmonic or random input) 19
    • Week 13 Electrodynamic Shaker - Current (A) passes through coil - Generates force F (N) ~ I (A) - Generates magnetic flux intensity B (Tesla) F = BIl l : length of coil This accelerates the component on the shaker table. AC: F harmonic DC: F constant 20
    • Week 13 Signal Conditioning 21
    • Week 13 Signal Analyzer 22
    • Week 13 Signal Analyzer 23
    • Week 13 Impulse Testing 24
    • Week 13 Relationship between components and vibration spectrum 25