Application of Vibration Sensor ©, Dr. Luis San Andrés1APPLICATIONS OF VIBRATION TRANSDUCERS1) Measurements on Structures or Machinery Casings: Accelerometers andVelocity SensorsUsed in gas turbines, axial compressors, small and mid-size pumps.These sensors detect high frequency vibration signals related to bearing supports, casingand foundation resonances, vibration in turbine/compressor vanes, defective roller or ballbearings, noise in gears, etc.2) Displacement measurements relative to rotating shafts: Proximity Probes(capacitance or eddy-current)Used in turbomachinery supported on fluid film bearings, centrifugal compressors, gearsand transmissions, electric motors, large pumps (>300HP), some turbines and fans.These sensors detect shaft static displacements, unbalance response, misalignment, shaftbending, excessive loads in bearings, dynamic instabilities, etc.ACCELEROMETERSAdvantages DisadvantagesSimple to install Sensitive to high frequency noiseGood response at high frequencies Require external powerStand high Temperature Require electronic integration for velocitySmall size and displacementVELOCITY SENSORSAdvantages DisadvantagesSimple to install Low resonant frequency & phase shiftGood response in middle range frequencies Cross noiseStand high temperature Big and heavyDo not require external power Require electronic integration forLowest cost displacementPROXIMITY SENSORSAdvantages DisadvantagesMeasure static and dynamic displacements Electrical and mechanical noiseExact response at low frequencies Bounded by high frequenciesNo wear Not calibrated for unknown metal materialsSmall and low cost Require external powerDifficult to installNovel types: OPTICAL FIBERS and LASER BEAMS. Their performance is not well known yet.Application of Vibration Sensor ©, Dr. Luis San Andrés2From Reference: Harry N. Norton, Handbook of transducers, Prentice Hall, Chap:5,6,7VELOCITY SENSORSElectromagnetic linear velocity transducers : Typically used to measure oscillatory velocity. Apermanent magnet moving back and forth within a coil winding induces an emf in the winding.This emf is proportional to the velocity of oscillation of the magnet. This permanent magnet maybe attached to the vibrating object to measure its velocity.Electromagnetic tachometer generators : Used to measure the angular velocity of vibratingobjects. They provide an output voltage/frequency that is proportional to the angular velocity.DC tachometers use a permanent magnet or magneto, while the AC tachometers operate as avariable coupling transformer, with the coupling coefficient proportional to the rotary speed.ACCELERATION SENSORSCapacitive accelerometers : Used generally in those that have diaphragm supported seismicmass as a moving electrode and one/two fixed electrodes. The signal generated due to change incapacitance is post-processed using LC circuits etc., to output a measurable entity.Piezoelectric accelerometers : Acceleration acting on a seismic mass exerts a force on thepiezoelectric crystals, which then produce a proportional electric charge. The piezoelectriccrystals are usually preloaded so that either an increase or decrease in acceleration causes achange in the charge produced by them. But they are not reliable at very low frequencies.Potentiometric accelerometers: Relatively cheap and used where slowly varying acceleration isto be measured with a fair amount of accuracy. In these, the displacement of a spring masssystem is mechanically linked to a viper arm, which moves along a potentiometric resistiveelement. Various designs may have either viscous, magnetic or gas damping.Reluctive accelerometers : They compose accelerometers of the differential transformer type orthe inductance bridge type. The AC outputs of these vary in phase as well as amplitude. They areconverted into DC by means of a phase-sensitive demodulator.Servo accelerometers : These use the closed loop servo systems of force-balance, torque-balanceor null-balance to provide close accuracy. Acceleration causes a seismic mass to move. Themotion is detected by one of the motion-detection devices, which generate a signal that acts as anerror signal in the servo-loop. The demodulated and amplified signal is then passed through apassive damping network and then applied to the torquing coil located at the axis of rotation ofthe mass. The torque is proportional to the coil current, which is in turn proportional to theacceleration.Strain Gage accelerators: these can be made very small in size and mass. The displacement ofthe spring-mass system is converted into a change in resistance, due to strain, in four arms of aWheatstone bridge. The signal is then post-processed to read the acceleration.ME617 ‐ A Few Sensor Types Used for V ibration Measurements 1A FEW SENSOR TYPES USED FOR COMMON VIBRATION MEASUREMENTS Disclaimer: The material in this document was copied ad‐verbatim from the sources noted. Its accuracy can not be warranted. 1) Accelerometers (piezoelectric) 2) Velocity Sensor 3) Proximity Probes (capacitance or eddy current) 4) Laser displacement sensors 1) ACCELEROMETERS REFERENCE: WWW.OMEGA.COM An accelerometer is a device that measures the vibration, or acceleration of motion of a structure. The force caused by vibration or a change in motion (acceleration) causes the mass to "squeeze" the piezoelectric material which produces an electrical charge that is proportional to the force exerted upon it. Since the charge is proportional to the force, and the mass is a constant, then the charge is also proportional to the acceleration. There are two types of piezoelectric accelerometers (vibration sensors). The first type is a "high impedance" charge output accelerometer. In this type of accelerometer the piezoelectric crystal produces an electrical charge which is connected directly to the measurement instruments. This type of accelerometer is also used in high temperature applications (>120C) where low impedance models cannot be used. The second type of accelerometer is a low impedance output accelerometer. A low impedance accelerometer has a charge accelerometer as its front end but has a tiny built-in micro-circuit and FET transistor that converts that charge into a low impedance voltage that can easily interface with standard instrumentation. Piezoelectric Accelerometers Reference : www.pcb.com ME617 ‐ A Few Sensor Types Used for V ibration Measurements 2Piezoelectric accelerometers rely on the