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The Accelerometer

 

 

Piezo-Electric Accelerometer

The compression-type accelerometer, diagrammed here, was the first type to be developed. The shear type, which is arranged so the active element is subjected to shear forces, is generally preferred.

There are also other designs for accelerometers

The piezo-electric accelerometer can be considered the standard vibration transducer for machine vibration measurement. It is made in several different configurations, but the illustration of the compression type serves to describe the principle of operation.

The seismic mass is clamped to the base by an axial bolt bearing down on a circular spring. The piezo-electric element is squeezed between the mass and the base. When a piezo-electric material experiences a force, it generates an electric charge between its surfaces. There are many such materials, with quartz being one of the most commonly used. There are also synthetic ceramic piezo materials that work well, and in some cases, work at higher temperatures than quartz is able to do. If the temperature of a piezo material is increased, finally the so called "curie point", or "curie temperature" is reached, and the piezo-electric property is lost. Once this happens, the transducer is defective and not repairable.

When the accelerometer is moved in the up and down direction, the force required to move the seismic mass is born by the active element. According to Newton's second law, this force is proportional to the acceleration of the mass. The force on the crystal produces the output signal, which is therefore proportional to the acceleration of the transducer. Accelerometers are inherently extremely linear in an amplitude sense, meaning they have a very large dynamic range. The smallest acceleration levels they can sense are determined only by the electrical noise of the electronics, and the highest levels are limited only by the destruction of the piezo element itself. This range of acceleration levels can span an amplitude range of about 108, which is 160 dB! No other transducer can match this performance.

The piezo-electric accelerometer is very stable over long periods of time, and will maintain its calibration if it is not abused. The two ways that accelerometers can be damaged are subjecting them to excessive heat and dropping onto a hard surface. If dropped more than a few feet onto a concrete floor or steel deck, the accelerometer should be re-calibrated to be sure the crystal is not cracked. A small crack will cause the sensitivity to be reduced and also will greatly affect the resonance, and thus the frequency response. It is a good idea to calibrate accelerometers about once a year if they are in service with portable data collectors.

The frequency range of the accelerometer is very wide, extending from very low frequencies in some units to several tens of kilohertz. The high-frequency response is limited by the resonance of the seismic mass coupled to the springiness of the piezo element. This resonance produces a very high peak in the response at the natural frequency of the transducer, and this is usually somewhere near 30 kHz for commonly used accelerometers. A rule of thumb is that an accelerometer is usable up to about 1/3 of its natural frequency. Data above this frequency will be accentuated by the resonant response, but may be used if the effect is taken into consideration.


When using an ICP accelerometer, care must be
taken not to subject it to acceleration levels where the output voltage will exceed several volts. Otherwise, the internal preamplifier will be overloaded and data distortion will result!

 

Most accelerometers used in industry today are of the "ICP" type, meaning they have in internal integrated circuit preamplifier. This preamp is powered by a DC polarization of the signal lead itself, so no extra wiring is needed. The device the accelerometer is connected to needs to have this DC power available to this type of transducer. The ICP accelerometer will have a low-frequency roll-off due to the amplifier itself, and this is usually at 1 Hz for most generally available ICP units. There are some that are specially designed to go to 0.1 Hz if very low frequency data is required.

When an ICP accelerometer is connected to the power source, it takes a few seconds for the amplifier to stabilize, and during this time, any data the unit is collecting will be contaminated by a slowly varying voltage ramp. For this reason, there must be a time delay built into data collectors to assure the unit is stable. If the delay is too short, the time waveform will have an exponentially shaped voltage ramp superimposed on the data, and the spectrum will show a rising very low-frequency characteristic sometimes called a "ski slope". This should be avoided because the dynamic range of the measurement is compromised.

The resonant frequency of an accelerometer is strongly dependent on its mounting. The best type of mounting is always the stud mount -- anything else will reduce the effective frequency range of the unit.

When mounting an accelerometer, it is important that the vibration path from the source to the accelerometer is as short as possible, especially if rolling element bearing vibration is being measured.

 





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