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AMERICAN UNIVERSITY IN CAIRO MECHANICAL ENGINEERING DEPARTMENT MENG 475: Applied Vibration Measurement, Analysis and Control Lab 1: Vibration Transducers & Signal Analysis Objective The objective of this lab is to measure vibrations using different types of vibration sensors and to analyze their signals in the time and frequency domains. The transducers used are: accelerometer, velocity probe, and displacement sensor. A brief description of the theory of operation, advantages and limitations of each type is given. You will then be instructed to use these instruments for vibration measurements in a lab setup. The measurements will be recorded to be discussed in a written report. Equipment Function generator Power amplifier Electrodynamic shaker Multi-channel data acquisition platform VIBROPORT general vibration measurement system Accelerometer Velometer Proximity probe Background The essentials elements of measurement: Physical behavior Sensor Transducer Signal conditioner Display unit or data acquisition system Data analysis Sensor: device which detects and responds to physical quantity to be measured Transducer: converts quantity to be measured to an analog signal that can be manipulated (ex: Volt) Signal conditioner: amplify, filter, integrate, differentiate, etc. DAQ: data acquisition hardware, software, calibration, processing, displaying Data analysis: extracting useful information from measurements Vibration Transducers 1. Accelerometer: an instrument that measures the acceleration of a vibrating body. The accelerometer uses a piezoelectric transducer. Piezoelectric materials (e.g. quartz) generate an electrical charge when subjected to deformation or mechanical stress. The charge disappears when the loading is removed. A small mass is preloaded against a piezoelectric crystal. When the base vibrates, acceleration of the mass generates a force on the piezoelectric crystal, which produces a proportional electric charge. A charge amplifier is then required to amplify this output. Velocity and displacement can be obtained by integration. A basic accelerometer measures the acceleration in one direction. Triaxial accelerometers exist to measure accelerations in 3 perpendicular directions simultaneously. Piezoelectric accelerometer Advantages Compactness Ruggedness High sensitivity Good response at high frequencies Can be used at high temperatures Disadvantages Represents a dynamic load on the vibrating structure. Therefore there is a minimum vibrating mass constraint. Accelerometer output requires signal conditioning (charge amplification) Contact measurement pickup Cannot be used at very low frequencies Fastening: 1. The magnet can be used on a clan plain ferromagnetic surface. 2. The threaded pin/stud is screwed into the mounting area (clean and plain) and secured. The accelerometer is screwed onto the threaded pin 3. The sensor probe can be used when other methods are not possible. The sensor probe should be kept perpendicular to the machine surface. The contact force and angle should be kept constant. 4. An internal threaded part can be bonded onto the surface using an adhesive. The screw can be removed upon completion of the measurement Using a threaded pin would be best because it can withstand higher dynamic forces. 2. Velometer or velocity meter: an instrument that measures the velocity of a vibrating body When an electrical conductor (coil winding) is moved in a magnetic field, an emf is induced in the conductor. This emf is proportional to the velocity of the coil relative to the magnetic field. The magnetic field may be produced by a permanent magnet or an electromagnet. Velocity pick-up Laboratory Schenck velocity pickup: It is used in the lower frequency range. The upper frequency limit is 2kHz. Minimum vibrating mass: 3 kg. Advantages Measures relative velocity Sensor output does not require any signal conditioning Relatively cheap Disadvantages Heavy and bulky; therefore dynamically loads the vibrating structure which imposes a constraint on the minimum vibrating mass Life time is shortened by wear since the pickup contains parts which are moving relative to each other Contact measurement pickup. 3.Vibrometer or displacement meter: an instrument that measures the displacement of a vibrating body. Proximity probe: used for non-contacting displacement measurement (shaft vibration and shaft displacement). It operates in accordance with the eddy current principle. The displacement transducer coil, extension cable, and circuit elements of the oscillator form an oscillating circuit. The transducer produces a magnetic field around the coil. If an electrically conducting material is present within the range of this magnetic field, eddy currents will be generated within this surface and they will attenuate the oscillator circuit. This attenuation is converted into a gap proportional to the output signal by the oscillator (note that the larger the gap, the weaker the eddy currents generated, the smaller the attenuation, and the higher the voltage output across the elements of the oscillator circuit.) Eddy current probe The gap (displacement) and gap voltage are linearly related over a certain range (0.2mm to 2.2mm for the proximity probe available in the laboratory). Typical frequency response allows measurements of vibrations with frequencies up to 2kHz. The output of the proximity probe is fed into the VIBROPORT, which is a general vibration measurement device that performs the necessary signal conditioning. Advantages Non contact probe No dynamic load on the vibrating mass, therefore no minimum limit for the vibrating mass Measures relative displacement (could be disadvantageous) Disadvantages Free space and minimum distance limitations from electrically conducting surfaces other than the vibrating surfaces Minimum shaft diameter (25mm for available proximity probe) Measures relative displacement (could be advantageous) Requires additional electronics and additional signal conditioning Experimental Setup Charge amplifier Conditioning circuit Spectrum analyzer P A Shaker V Power amplifier Function generator Function Generator: used to generate an electrical signal of the desired frequency and amplitude. The available model generates sine, square and triangle wave functions from 1mHz to 19.99 MHz, with amplitudes up to 10V. Power Amplifier: used to amplify or attenuate the signal generated by the function generator. It can supply a large current, while the function generator has little power. Electrodynamic Shaker: Used to provide the mechanical excitation. Multi-channel data acquisition platform: used to view the output signal in the frequency and time domain. Required 1. Start by feeding a signal from the function generator to the analyzer. Analyze the spectrum of different waveforms (sine, square and triangular). Comment on the harmonics. 2. Drive the shaker at different frequencies and observe the outputs of the three sensors used (accelerometer, velocity sensor and proximity probe). Tabulate your results as shown below. Knowing that the sensitivity of the proximity sensor is 8 mV/m, determine the sensitivity of the velometer and accelerometer. Function generator Frequency [Hz] Amplitude [V] Channel 1: Proximity probe reading [V] Channel 2: Velometer reading [V] Channel 3: Accelerometer reading [V]