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Vibration measurements are useful for the measurement of surface vibration. Many types of sensor can be used for this purpose. The most commonly used ones are piezoelectric sensors and IEPE sensors (with integrated signal conditioning). There are other brands for low frequencies (like piezoresistive, capacitive or MEMS sensors) and non contact probes (eddy current probes and laser sensors).

Piezoelectric sensors

Piezoelectric sensors work on the principle that a piezoelectric material is built between the bottom of the sensor housing and the seismic mass. When a sensor is moved, this mass compresses the piezoelectric material which produces very small voltages. To transfer those small electrical values through the cables require lots of knowledge and expensive cabling, therefore lately these sensors have often been replaced with the IEPE sensors with integrated amplifiers.

However, there are still applications areas where these sensors are very useful. These fields are especially high acceleration and high temperatures. The amplitude measurement range of such sensors can be thousands of g. One can find single axis as well as triaxial sensors.

The Dewetron DAQP-CHARGE-A module needs to be used to measure with these sensors.

 

IEPE (integrated electronic piezoelectric) sensors

IEPE sensors need a power supply of 4 or 8 mA and they typically give out a 5 volt signal, thus it is much easier to transfer these signals over longer cables. Also, the amplifiers for those sensors are much easier to build, and therefore cheaper. The amplitude measurement range is quite limited. We can hardly find a sensor which measures more than 100g. We can find single axis as well as triaxial sensors. Lately, really nice sizes have become available - one can find a triaxial sensor as a cube measuring as little as 10 mm, and with the weight as light as 5 grams.

The MDAQ-ACC, DAQP-ACC, DAPQ-ACC-A and also DAQP-CHARGE-A can be used to measure with these sensors.

 

Static acceleration sensors

Both sensor types have a common limitation: they can't measure static acceleration. They usually start to measure from 0.3 Hz to 10 Hz, depending on the sensor. For static or very low frequency measurements, the user  needs to use a different kind of sensor. Lately, a very popular type is the MEMS sensor. This is actually a microchip which has a mechanical structure (a cantilever beam or seismic mass) that changes its electrical property (usually capacitance) related to the acceleration. Not far in the past, such sensors were very special, since they were used to measure earthquakes or any other slow movements. But with the development of airbag technology, there was a big need to make a low cost sensor which measures the static acceleration. Therefore, the single chip solution emerged for this purpose. Lately these sensors are used also in low cost gyro systems and we can find sensors which also have also quite good bandwidth up to several kHz and quite low noise level (though still bigger than IEPE sensors with the same measurement range).

 

Choosing a correct sensor

So what are the key points to consider when choosing the correct sensor?

Low frequency range

Usually for vibration measurements the requirement is that the sensor has a lower high pass cutoff than for the more interesting frequencies in devices currently being tested. With a rotating machine with 50 Hz, we can choose a sensor with a 5 Hz cut off. It is also better not to have too little bandwidth, the lower the bandwidth gets, the longer the recovery times from shocks or overloads. Also, the amplifier should follow the bandwidth of the sensor. It is nice if the amplifier has at least two ranges in order to be more flexible in measurements.
A typical application for low frequency measurements are the paper mill rolls. They have a frequency of 1÷5 Hz, where the user would need a sensor with 0.3 Hz or less bandwidth. For those applications charge or IEPE are the best.
If we need to measure DC, then we need a different sensor technology, like MEMS sensors is needed.

Bandwidth (high frequency range)

Bandwidth also depends on the application. If we are not interested in higher frequencies or we know there is not much going on there, it is alright to choose sensors with lower bandwidth. However, some applications like bearing condition monitoring require higher bandwidth up to 20 kHz or more. IEPE or charge sensors are needed for those kinds of applications. Also note that the sensor mounting affects the bandwidth. Read following section for more information on this subject.

Amplitude range

Charge sensors have the biggest amplitude ranges (up to 100 000 g or more), but IEPE are also fairly high (up to 1000 g). MEMS sensors usually have limited range (up to few hundred g). For general purposes, it is best to use IEPE, whereas  for high levels piezoelectric sensors are better. Sometimes (for example for seismic applications) an accelerometer with high sensitivity is required (2 g or lower range).

Maximum shock level

The charge sensors are the least sensitive to shock. They can sustain up to 100 000 g of shock, while IEPE can usually take not more than 5 000 to 10 000 g. MEMS sensors are even more sensitive to shock.

Noise level

The residual noise level defines the lowest amplitude level of what the sensor will measure. This is also the reason why we should take a sensor with the optimum measurement range, because sensors with higher a range will also have a higher noise level.

Temperature range

All the sensors that include electronics have a limited high temperature range, up to 130 deg C. The temperature range of charge sensors is much higher - even up to 500 deg C. Please note, however, that this would also require high temperature cable.

Weight

In some applications, like modal testing, weight can be a big factor due to the mass loading effect. The added mass to the structure changes the dynamic behavior, so ideally a sensor should have no mass at all.
In other areas of predictive monitoring in harsh environments, a sensor needs to be robust enough to sustain possible damage or, as my good old friend would say, that you can still find it in the dirt.

Mounting consideration

There are sensors available with holes (usually threaded) or with a clip mounting.
Sometimes it is very important to use the isolation. Since IEPE amplifiers are usually not galvanic isolated, we need the isolation on the side of the measurement. There are also sensors readily available with a housing which is isolated to the connector ground.

 

Sensor mounting

The sensors can be mounted in different ways. Especially the bandwidth of the sensor is affected by the mounting.

Clearly, it is the best to drill a hole in the test specimen and affix the sensor to the surface with a screw. This should not affect any sensor property. Obviously, in some cases a customer might not be particularly thrilled to do this, for example, to his brand new prototype of an airplane wing. Another type of mounting, which doesn't affect the bandwidth that much, is thin double sided adhesive tape or bees wax (this is, however, limited in its temperature range).

A very widely used mounting technique for machine diagnostics is to mount the sensor on a magnet. This will still produce a good bandwidth, but of course, the surface must be ferromagnetic (no aluminum or plastic). On sensors where we can use the mounting clip, we can glue the mounting clip up front and then just attach the sensor itself.

A "quick and dirty" solution is also to hold down the sensor with the a hand on a rod. This is useful for some places which are hard to reach, but the bandwidth will be cut to 1÷2 kHz.