Trustworthy and reliable accelerometers, the minimum requirement for industrial site monitoring

Effective Condition Monitoring requires the installation of a set of accelerometers on the equipment and machines that are under surveillance. Indeed, the appearance of abnormal vibration is in most cases the first detectable sign and can be used to alert those in charge of maintenance and operation. Other signs like rising temperature, smoke, louder noise come later and do not give time to plan a repair in good conditions.

In many cases, these accelerometers are installed on large rotating machines operating in various harsh industrial environments. This is the case in the heavy metal industry, the petroleum and chemicals industry, etc. Windmill farms are also an example, where special attention must be paid to the quality and reliability of accelerometers. In the examples listed above, the cost of production loss due to replacement of defective parts is very high. 

Fatri, a recent entrant in the Conditioning Monitoring market, considered all these requirements when starting the design of accelerometers. 

First step, use only mechanical parts easy to assemble

The first goal of Fatri’s engineers was to design an accelerometer that does not require glue to assemble the various mechanical parts around the central piezoelectric element. This central element is made of a piezoelectric material which is fixed and held between a base fixed on the machine under test and a seismic mass. When it vibrates, the seismic mass compresses the piezoelectric material, which produces electric charge on its surface.

However, many piezoelectric accelerometers known on the market are assembled with glue. But as a chemical element glue may have different aging behavior, the incoming inspection and assembly process require special attention.

To prevent the aging effect and the need for regular calibration, Fatri designed the accelerometer around the "Wedge self-locking structure". This innovative mechanical design brings many advantages: 

?       Behavior is stable over time and does not require additional calibration

?       Solid mechanical bond brings higher sensitivity compared to glue

?       Assembly process is simple, which enables shorter manufacturing time, more stable yield and better efficiency

To learn more on the benefits of the patented Wedge self-locking structure ?, you can read this article "New patented Wedge Self-locking Structure brings performances and reliability of piezoelectric accelerometers at new high levels"

2nd step : make sure that built-in electronics is reliable

Most accelerometers now come with built-in electronics, referred to as Integrated Electronic Piezoelectric (IEPE) accelerometers. Indeed, the electric charges that are measured on the surface of the piezoelectric element are very low, a few picofarads in most cases. The direct reading of this very weak signal is a real problem, especially when the analyzer or recording system must be far from the system under study. The transmission cable adds noise, weakens the signal and makes the measurement impossible to process.

The solution is to add an electronic charge amplifier system. It is composed of a few parts, a FET amplifier and a set of resistors and capacitors. This electronic board brings the advantage of sending a correct and higher signal on the transmission cable, much less sensitive to noise.

This is more convenient, as it allows a longer cable to be used between the measured point and the analyzer or recorder. But with the advantages come some drawbacks, experience shows that the main reason for the failure of the piezoelectric acceleration sensor is the failure of the built-in charge amplifier. 

This can be easily understood, the charge amplifier is a circuit board with several components. Failures can come from different sources: poor soldering and connection of the pads, failure of electronic components, prolonged operation beyond the specified temperature limits, abnormal power supply voltage…

Improper pad connection will affect the performance of the built-in charge amplifier and cause the accelerometer to fail. Historically, there have been countless cases in which electronic products (including for civilian and military use) have failed due to welding and caused accidents. Many factors affect the soldering, such as the loss of weldability of the metal surface, the offset of the protective coating and the solder, and a too thin protective layer. Cold welding is mainly caused by improper welding temperature. Staff training, standard operation, most welding problems can be avoided.

Failure of electronic components can also cause the accelerometer to fail. After a long period of work, lower-grade electronic components are prone to failure under the long-term influence of temperature, pressure and voltage. The choice of high-quality screened electronic components can reduce the occurrence of such failures and reduce the cost of the accelerometer use.

The failure of the accelerometer is also affected by the abnormal power supply. The power supply mode of IEPE accelerometers is current power supply and common DC power supplies are voltage sources. When the accelerometer is assembled and used, inattention to the power supply may cause damage to the accelerometer. Similarly, the positive and negative connections of the power supply (that is, the positive terminal of the power supply is connected to the negative terminal of the accelerometer, and the negative terminal of the power supply is connected to the positive terminal of the accelerometer) may also cause the failure of the accelerometer. A qualified accelerometer should take these issues into consideration when designing, and there will be anti-reverse protection and overcurrent protection to avoid such failures.

Conclusion :

Again, Fatri took particular attention when designing the electronic interface, minimizing the number of electronic component and selecting the best suppliers.

In line with common standard MIL-HDBK-217, we calculate the MTBF of our The AYDV00 series accelerometer

MTBF λP    = 240 000 hrs at 60 °C

This result places these accelerometers in the Best in class category, and make these sensors the preferred choice for Industrial Sites running in harsh environment.