Vibration Sensors

Sensors are divided into two general classes:

1. Seismic transducers

(a) Accelerometers

(b) Velocity Transducers

2. Proximity Transducers

Sensors respond either to amplitude or to displacement of the vibration. Seismic sensors are also frequency sensitive. Figure 8-33 shows a comparison of various methods of sensing vibration. Velocity sensors have a sensitivity directly proportional to frequency and amplitude;

Figure 8-33. Three methods for measuring shaft vibration. (Courtesy of Bently Nevada)

whereas, accelerometers show a sensitivity directly proportional to the amplitude as the square of the frequency. It is appropriate to use seismic sensors when the frequency of vibration is high, since high acceleration forces are involved at high frequencies and the sensitivity of seismic sensors increases with the frequency. Conversely, if the frequency is low, the proximity sensor would be favored (see Figure 8-34).

Often, two or more types of sensors may be used in conjunction with each other to give more "visibility" to what is going on. Monitoring of a turbine generator is a good example. Information may be collected about shaft thrust, eccentricity, rpm, bearing wear, gearbox wear, and others for maintenance and protection analysis.

Sensors may be used with a number of monitoring devices. Most sensors give a voltage output signal proportional to the vibration level. The output signal is interpreted as either peak-to-peak voltage, peak voltage, or rms voltage signals. Figure 8-35 illustrates these values on a sine wave.

Seismic Sensors

Accelerometers

Characterized by high frequency response, accelerometers are compact and rugged, ideal for mounting on machinery cases, foundations, piping, etc. Applications to gear trains and rolling element bearings are typical.

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V

FREQUENCY

Figure 8-34. The relationship of displacement, velocity, and acceleration to vibration amplitude and frequency.

Average = 0.63 * Peak Value

Figure 8-35. The relationship between peak, peak-to-peak, and average amplitudes for a sine wave.

Average = 0.63 * Peak Value

Figure 8-35. The relationship between peak, peak-to-peak, and average amplitudes for a sine wave.

A crystal material is excited by the force imposed on it by an internally mounted mass. A voltage is produced by the crystal proportional to acceleration. This voltage is then amplified by a charge amplifier type signal conditioner from whence the signal can be transmitted long distances (1,000 feet is not uncommon) to the monitor/readout unit. It is calibrated in terms of gravitational units (g), which are proportional to force. Force is one of the most reliable indicators of equipment distress.

The accelerometer output is measured in terms of pico coulombs per gravitational unit. The nominal signal level output of the charge amplifier/signal conditioner is 100 mv/g.

The size of mass within the accelerometer determines the self-resonant frequency of the sensors. The smaller the mass, the higher the frequency. Accelerometers are usually operated in a range below this self-resonant frequency.

Velocity Sensors

Since acceleration is the second derivative of displacement, a piezoelectric accelerometer sensor with an integrator becomes a velocity transducer. This arrangement is gradually superseding the self-generating moving-coil velocity sensor (where a coil of wire moves relative to a magnetic field).

The signal levels from these various velocity sensors are comparable An advantage of the self-generating moving coil type is that no excitation voltage is required to drive it. However, it could be affected by high magnetic fields generated around heavy electrical equipment, a problem that the accelerometer type is immune to.

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