Basic Concept V
Vibration Transducers
• Microphone - sound (mechanical) to electrical energy
• Speaker - electrical to mechanical energy
• Thermometer - thermal to electrical energy
• Vibration is mechanical energy
• It must be converted to electrical signal so that it can easily be measured and analyzed.
• Commonly used Vibration Transducers
• Noncontact Displacement Transducer
• Seismic Velocity Transducer
• Piezoelectric Accelerometer
• Transducers should be selected depending on the parameter to be measured.
Proximity Displacement Probes
• They observe the static position and vibration of shaft
• By mounting two probes at right angles the actual dynamic motion (orbit) of the shaft can be observed
Non Contact Displacement Probes
(Eddy Current Proximity Probe)
• Measures gap and nothing else.
• Coil at probe tip is driven by oscillator at around 1.5 MHz
• If there is no conducting surface full voltage is returned
• Conducting surface near coil absorbs energy
• Therefore, voltage returned is reduced
• Proximitor output voltage is proportional to gap
• Proximitor has a nonlinear amplifier to make the output linear over a certain voltage range
• For a 24 Volt system the output is linear from 2.0 to 18.0 volts
• Only probe that can measures shaft position – both radial and axial
• Good signal response between DC to 90,000 CPM
• Flat phase response throughout operating range
• Simple calibration
• Rugged and reliable construction
• Suitable for installation in harsh environments
• Available in many configurations
• Multiple machinery applications for same transducer – vibration, position, phase, speed
Proximity Probe DisAdvantages
• Sensitive to measured surface material properties like conductivity, magnetism and finish
– Scratch on shaft would be read as vibration
– Variation in shaft hardness would be read as vibration
• Shaft surface must be conductive
• Low response above 90,000 CPM
• External power source and electronics required
• Probe must be permanently mounted. Not suitable for hand-holding
• Machine must be designed to accept probes – difficult to install if space has not been provided
Seismic Velocity Pick-Up IRD 544
• Coil of fine wire supported by low-stiffness springs
• Voltage generated is directly proportional to velocity of vibration
• The coil, supported by low stiffness springs, remains stationary in space
• So relative motion between coil and magnet is relative motion of vibrating part with respect to space
• Faster the motion higher the voltage
Velocity Pick-Up - Suspenped Magnet Type
• All velocity pick ups have low natural frequency (300 to 600 CPM)
• Therefore, cannot measure low frequencies in the resonant range.
• Their useful frequency range is above - 10 Hz or 600 CPM
Advantages of Velocity Pick-Up
• Measures casing absolute motion
• It is a linear self generator with a high output
– IRD 544 pick up – 1080 mv 0-pk / in/sec= 42 mv / mm/sec
– Bently pick up – 500 mv 0-pk / in/sec = 19.7 mv / mm/sec
• High voltage Output
– Can be read directly on volt meter or oscilloscope
– Therefore, readout electronics is much simplified
– Since no electronics needed in signal path, signal is clean and undistorted. High signal to noise ratio
• Good frequency response from 600 to 90,000 CPM
• Signal can be integrated to provide displacement
Easy external mounting, no special wiring requiredDisadvantages of Velocity Pick-Up
• Mechanically activated system. Therefore, limited in frequency response – 600 to 90,000 CPM
• Amplitude and phase errors below 1200 CPM
• Frequency response depends on mounting
• Large size. Difficult to mount if space is limited
• Potential for failure due to spring breakage.
• Limited temperature range – usually 120oC
– High temperature coils available for use in gas turbines but they are expensive
• High cost compared to accelerometers
– Accelerometer cost dropping velocity pick up increasing
Note - Velocity transducers have largely been replaced by accelerometers in most applications.
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