Typically used to measure oscillatory velocity. A permanent magnet moving back and forth within a coil winding induces an in the winding. This is proportional to the velocity of oscillation of the magnet. This permanent magnet may be attached to the vibrating object to measure its velocity. Electromagnetic linear velocity transducers emf emf
Used to measure the angular velocity of vibrating objects. They provide an output voltage/frequency that is proportional to the angular velocity. use a permanent magnet or magneto, while the operate as a variable coupling transformer, with the coupling coefficient proportional to the rotary speed
Used generally in those that have diaphragm supported seismic mass as a moving electrode and one/two fixed electrodes. The signal generated due to change in capacitance is postprocessed using LC circuits etc., to output a measurable entity. Capacitive accelerometers
Acceleration acting on a seismic mass exerts a force on the piezoelectric crystals, which then produce a proportional electric charge. The piezoelectric crystals are usually preloaded so that either an increase or decrease in acceleration causes a change in the charge produced by them. But they are not reliable at very low frequencies.
Relatively cheap and used where slowly varying acceleration is to be measured with a fair amount of accuracy. In these, the displacement of a spring mass system is mechanically linked to a viper arm, which moves along a potentiometric resistive element. Various designs may have either viscous, magnetic or gas damping. : They compose accelerometers of the differential transformer type or the inductance bridge type. The AC outputs of these vary in phase as well as amplitude. They are converted into DC by means of a phasesensitive demodulator.
These use the closed loop servo systems of forcebalance, torquebalance or nullbalance to provide close accuracy. Acceleration causes a seismic mass to move. The motion is detected by one of the motiondetection devices, which generate a signal that acts as an error signal in the servoloop. The demodulated and amplified signal is then passed through a passive damping network and then applied to the torquing coil located at the axis of rotation of the mass. The torque is proportional to the coil current, which is in turn proportional to the acceleration.
These can be made very small in size and mass. The displacement of the springmass system is converted into a change in resistance, due to strain, in four arms of a Wheatstone bridge. The signal is then post processed to read the acceleration
Eddy currents are formed when a moving (or changing) magnetic field intersects a conductor, or viceversa. The relative motion causes a circulating flow of electrons, or currents, within the conductor. These circulating eddies of current create electromagnets with magnetic fields that oppose the effect of the applied magnetic field. The stronger the applied magnetic field, or greater the electrical conductivity of the conductor, or greater the relative velocity of motion, the greater the currents developed and the greater the opposing field Eddy current probes sense this formation of secondary fields to find out the distance between the probe and the target material.
Capacitive sensors use the electrical property of“capacitance” to make measurements. Capacitance is a property that exists between any twoconductive surfaces within some reasonable proximity. Changes in the distance between thesurfaces change the capacitance. It is this change of capacitance that capacitive sensors useto indicate changes in position of a target. Highperformance displacement sensors use smallsensing surfaces and as result are positioned close to the targets