When selecting accelerometers, the vibration specialists must consider three main areas: amplitude range, frequency range, and environmental considerations
Accelerometers used in predictive maintenance applications are internally amplified, ICP® sensors. These sensors are powered with a constant current DC supply. Supply voltage is regulated between 18 and 28 volts DC and current limited, via a constant current diode, between 2 and 20 mA. The signal output of ICP® sensors is a DC biased AC signal. The vibration signal, typically 100 mV/g AC, is superimposed on the DC bias. This DC bias is normally blocked by a decoupling capacitor so the read-out equipment can be AC coupled. If a normal bias level of 12 VDC is used with an 18 volt DC power supply and the accelerometer signal is 100 mV/g, the maximum measurable signal would be 50 g's or 5VAC. This maximum level can be increased by either increasing the supply voltage level or decreasing the sensitivity of the accelerometer. By using a 10 mV/g accelerometer with the same 5VAC maximum output, the vibration limit increases to 500 g's.
The other criteria to consider when reviewing the amplitude range is the lowest measurable vibration level. This is specified as either the noise floor or the resolution of the sensor. The resolution of the sensor is determined by two factors: electrical noise of the internal amplifier and mechanical gain of the mass/piezoelectric system. The larger the seismic mass, the larger the output of the sensor prior to amplification. This high mechanical gain improves low level measurements by producing substantial electrical signals without the use of amplifier gain. Ceramic sensing elements typically provide greater signal to noise ratios, allowing small levels of vibration to be measured without electrical noise interfering with analysis.
The frequency response of an internally amplified ICP® accelerometer is described as the frequency range over which the sensor will provide a linear response. The upper end of the frequency response is governed by the mechanical stiffness and the size of the seismic mass in the sensing element while the low frequency range is controlled by the amplifier roll-off and the discharge time constant. Figure 4 shows a typical frequency response.
High End Frequency Response
The upper end frequency response is determined by the formula w=Ök/m, where w is the resonant frequency (2pf), k is the stiffness of the sensing structure, and m refers to the size of the seismic mass. With a given stiffness a sensor with a large seismic mass will have a low resonance. A large seismic mass will also produce higher mechanical gain and thereby result in a lower noise accelerometer with greater sensitivity. A smaller seismic mass will produce less signal but will result in a sensor with a higher resonant frequency. Output signal may be low using a smaller seismic mass but the frequency range will be wider allowing for measurements to be made at higher frequency.
Stiffness, the second variable in the w=Ök/m, equation is dependent on the sensing structure. Flexural designs as stated earlier provide significant mechanical gain, but the stiffness is very low. Flexural designs typically have high output, low resonance and limited shock resistance. Compression accelerometers, by virtue of the pre-load compression screw, exhibit a higher stiffness than flexural units and therefore have a higher resonance and a wider frequency range. As stated earlier, other environmental factors such as base strain and thermal transients may limit their use. Shear mode sensors, when mechanically secured, exhibit a high stiffness and thus a high resonance. Insensitivity to environmental factors of strain and thermal shifts places the shear design at the top of the list.
Low End Frequency Response
The low end is governed electrically by a resistive capacitive circuit that determines the discharge time constant (t=R*C). The higher the DTC, the slower the signal is bled off and hence the better the low end frequency response (see table 1). The DTC can be compared to a funnel. The smaller the opening in the bottom of the funnel (or the higher the time constant), the less water (signal) flows out. A sensor with a higher DTC means a better low end frequency response. A low frequency application will often times be unmanageable without a sensor which has the proper DTC. The DTC however not only determines low end frequency response, but is a major factor in determining settling time as well. The higher the DTC, the longer the settling time. (Note: A conservative rule of thumb to follow is that a settling time of 10 times the discharge time constant will allow the signal to decay to within 1% of the output bias.) A settling time of a few seconds or more might not seem to be significant to someone working in a laboratory environment with one or two points, but a person taking point to point data out in the field will certainly think otherwise. Therefore, a compromise must often be made between low frequency response and settling time.