Vibration and frequency measuring instruments
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Vibration measuring instruments are used to assess the displacement, velocity, frequency, phase distortion, and acceleration of vibrating bodies. Key types include vibrometers for low frequency measurements and accelerometers for high frequency measurements, with each having specific applications and disadvantages. Frequency measuring instruments, such as Fullarton and Frahm tachometers, determine the natural frequency of vibrations using resonant principles.
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Vibration and frequency measuring instruments
- 1. Vibration Measuring Instruments • The instruments or equipments which are used for measure the displacement, velocity, frequency, phase distortion and acceleration of a vibrating g body are called vibration measuring instruments. • The displacement, velocity and acceleration is identify by the relative motion of the suspended mass of the instruments with respect to its case which is placed on the vibrating body. • Vibration measuring devices having mass , spring , dashpot etc. are called seismic instruments .
- 2. • The quantity which is measure by the instruments are displayed on the screen in the form of electric signal which can be readily amplified and recorded. • The output of electric signal of the instrument is proportional to the quantity which is measured. The input is reproduced as output very precisely.
- 3. Types of Vibration Measuring Devices 1. Vibrometer 2. Accelerometer Figure : 1
- 4. Vibrometer • A Vibrometer or seismometer is used for measuring displacement of a vibrating body. Basically it is design under the condition of low natural frequency due to this reason it is also called as low frequency transducer. This is used to measure high frequency ω of a vibrating body. • Its natural frequency has ranged between 1 Hz to 5 Hz and useful natural frequency range of 10 Hz to 2000 Hz. • The sensitivity of this instruments is in the range between the 20 to 350 mV/cm/s. The maximum displacement is ranged between the 0.5 peaks to one peaks.
- 5. Application & Disadvantages of Vibrometer Application •The instrument is used to record building vibrations. also used for measuring vibration of the huge structure like railway bridge. Disadvantage •It is large in size because of its relative motion of the seismic mass must be of the same arrangement of the magnitude as that of the vibration to be measured.
- 6. Accelerometer • Accelerometer is a instrument used for measuring the acceleration of the vibrating body. The accelerometer is design with the high natural frequency and it is said to be high frequency transducer. Figure : 2
- 7. Figure : 3
- 8. Types of Accelerometer • Electromagnetic type of accelerometer use damper to extend the useful natural frequency range. it is also using for the prevents phase distortion. • Piezoelectric crystal accelerometer having zero damping is operating without distortion. It is used for measuring high frequency. • Seismic mass accelerometer is used for low frequency vibration. The supporting springs are four electric strain gauges wires which is connected with the bridge circuit.
- 9. Frequency Measuring Instruments Most frequency-measuring instruments are of the mechanical type and are based on the principle of resonance. There are two types of frequency measuring instruments 1.Fullarton tachometer 2.Frahm tachometer.
- 10. Screw Single Reed Instrument or Fullarton Tachometer • This instrument consists of a variable length cantilever strip with a mass attached at one of its ends. • The other end of the strip is clamped, and its free length can be changed by means of a screw mechanism. Figure : 4
- 11. • Since each length of the strip corresponds to a different natural frequency, the reed is marked along its length in terms of its natural frequency. • In practice, the clamped end of the strip is pressed against the vibrating body, and the screw mechanism is manipulated to alter its free length until the free end shows the largest amplitude of vibration. • At that instant, the excitation frequency is equal to the natural frequency of the cantilever; it can be read directly from the strip.
- 12. Multi Reed Instrument or Frahm Tachometer • Frahm’s reed tachometer is used to measure the frequency of vibrations. • The instrument consists of a number of reeds in the form of cantilever carrying small masses at their free ends. • The reed with natural frequency rear to that of excitation frequency to be measured will vibrate at resonance to produce large amplitude of the vibration. Cantilever reeds Figure : 5
- 13. • Each reed has a different natural frequency and is marked accordingly. Using a number of reeds makes it possible to cover a wide frequency range. • When the instrument is mounted on a vibrating body, the reed whose natural frequency is nearest the unknown frequency of the body vibrates with the largest amplitude. • The frequency of the vibrating body can be found from the known frequency of the vibrating reed. Disadvantage • The main disadvantage of these instruments is the small range of frequencies which can be measured.