Vibration Monitors

Vibration Monitors
What is Vibration Monitoring? 
Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road. Vibration is wasting energy and creating unwanted sound – noise. 
For example, the vibrational motions of engines, electric motors, or any mechanical device in operation are typically unwanted. Such vibrations can be caused by imbalances in the rotating parts, uneven friction, the meshing of gear teeth, etc. Careful designs and predictive maintenance will substantially minimize unwanted vibrations and down time. 
Vibration of a machine is not usually simple
•          Many frequencies from many malfunctions
•          Total vibration is sum of all the individual vibrations
•          Unfiltered overall amplitude indicates overall condition 
•          Displacement amplitude is not a direct indicator of vibration severity unless combined with frequency
•          Velocity combines the function of displacement and frequency
•          Unfiltered velocity measurement provides best overall indication of vibration severity
Note here that the acceleration is 180 degrees out of phase with the displacement. This means the acceleration of a vibrating object is always in the opposite direction to the displacement!

Vibration Transducers
  Transducer is a device that converts one form of energy into another. 

•          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
•          Proximity probes measure the displacement of shaft relative to the bearing housing
•           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 the distance (or “lift off”) of a conducting surface from the tip of the 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

Proximity Probe Advantages
•          Measures shaft dynamic motion
•          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
Permanent magnet is attached to the case. Provides strong magnetic field around suspended coil
•           Coil of fine wire supported by low-stiffness springs
•           Voltage generated is directly proportional to velocity of vibration
•      When pick up is attached to vibrating part magnet follows motion 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
•          Coil fixed to body, magnet floating on very soft springs
•           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 required

 Disadvantages 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.

Piezoelectric Accelerometers
  Piezoelectric crystal is sandwiched between a seismic mass and outer case.
•          Preload screw ensures full contact between crystal & mass
•           When mounted on a vibrating surface seismic mass imposes a force equal to mass x acceleration
•           Charge output of piezo crystal is proportional to applied force
•           Since mass is constant, output charge is proportional to acceleration 

Piezoelectric Accelerometers 
Converting Charge to Voltage
•          The output of accelerometers is charge. Usually expressed as picocoulomb / g (pc/g)
•          Electronic charge amplifier is required to convert charge signal to voltage signal
–        Impedance of accelerometer is high. Cannot be connected directly to low impedance instruments
–        Charge amplifier has high input impedance and low output impedance so that long cables can be used.
•          Charge amplifier can be external or internal 
–        In bigger accelerometers amplifier can be located inside
–        In small, high frequency units amplifier is located outside

 Accelerometers Resonance & Frequency Response
•          Frequency response depends on resonance frequency
•           Higher the resonance frequency, higher the useful range
•           Maximum useable frequency range is 1/3rd of resonance 
•           Resonance frequency, however, depends on mounting

Frequency Response - Screw Mount
•          Screw mount has the highest resonance and, therefore the highest frequency response
•           This film of silicon grease improves contact. 
•           Make sure bottom of accelerometer contacts measured surface

 Frequency Response - Magnet Mount

•          Weight of magnet determines the mounted resonance  

•           Smaller the magnet higher the frequency response
 Use the smallest magnet that holds the accelerometer without slipping. Use a machined surface for the best grip

Filtering Necessary for Accelerometers
•          Any high frequency vibration in the resonant range will be highly amplified.  
–        Amplification can be up to 30 dB or almost 1,000 times
–        Filtered amplitudes will be highly distorted 
•          Resonant frequency highly depends on mounting
–        By previous example – 32 KHz for screw mount. Only 2 KHz for handholding 
•          Therefore, resonance range should be filtered out 
–        For screw mount low pass filter should be set at 10 KHz
–        For hand holding filter should be set at 1 KHz.
–        Analyst must know frequency response of accelerometer used for different mounting conditions.
Filtering can be done in FFT Analyzer by setting maximum frequency correctly.

 Advantages of Accelerometers

•          Measures casing or structural absolute motion
•          Rugged and reliable construction
•          Easy to install on machinery, structures, pipelines
•          Small size, easiest to install in cramped locations
•          Good signal response from 600 to 600,000 CPM
•          Low frequency units can measure down to 6 CPM
•          High freq units can reach 30 KHz (1,800,000 CPM)
•          Operates below mounted resonance frequency
•          Flat phase response throughout operating range
•          Internal electronics can be used to convert acceleration to velocity – Bently Velometer 
•          Units available from a cryogenic temperature of minus 200oC to a high temperature of > 600oC

 Disadvantages of Accelerometers
•          Sensitive to mounting and surface conditions
•          Unable to measure shaft vibration or position
•          Not self generating – Need external power source 
•          Transducer cable sensitive to noise, motion and electrical interference
•          Low signal response below 600 CPM (10 Hz)
•          Temperature limitation of 120oC for ICP Acceleroms
•          Double integration to displacement suffers from low frequency noise – should be avoided
•          Signal filtration required depending on mounting
•          Difficult calibration check

Machine With Both Shaft and Bearing Housing Vibration Monitoring

No comments:

Post a Comment