An appropriate coupling between the accelerometer and the system it is monitoring is essential for accurate measurements. Engineers use different methods for mounting MEMS accelerometers, and this affects their frequency response.
The resonance of the mounting fixture plays an important role, as it can introduce an error in the measurement. Accelerometers using MEMS sensors typically use a printed circuit board or PCB for mounting the sensor, and there may also be other mechanical interfaces between the PCB and the surface of the object it is monitoring. This creates a mechanical system that can have multiple resonances within the frequency range of interest.
For instance, the resonant frequency of the mounting structure may be close to the frequency of the acceleration signal. This will cause the sensor to receive an amplified signal in place of the original acceleration.
Again, if the mechanical coupling causes damping, the sensor will likely receive an attenuated signal.
That means, unless applying proper mounting techniques, it is not possible to take full advantage of the accelerometer’s bandwidth. This is especially so when the measuring acceleration signals are above 1 kHz. Engineers apply three types of accelerometer-mounting techniques such as stud, adhesive, and magnetic mountings.
Stud mounting requires drilling a hole in the object and fixing the sensor to the device under test with a nut and a bolt or a screw. This method of mounting provides an immobile mechanical connection. But it is capable of effectively transferring vibrations of high frequencies from the object to the sensor.
Proper stud mounting requires the coupling surfaces to be as clean and flat as possible. Using a thin film of some type of coupling fluid like oil or grease between the coupling surfaces aids in improving the coupling. The fluid fills small voids between the surfaces, thereby improving transmissivity. It also helps to use a torque wrench to tighten the stud to the manufacturer’s specifications.
Where it is not possible to drill a hole in the device, engineers use an adhesive to couple the sensor to the object it has to monitor. Depending on the nature of the object, engineers use glue, epoxy, or even wax for the coupling. They select the adhesive depending on whether the mounting is temporary or permanent. In case the surface of the object is not smooth, engineers sometimes use an adhesive mounting pad or mounting base. While adhesives fix the mounting pad to the test surface, a stud mounting fixes the sensor to the mounting base.
Engineers have an alternative method of fixing accelerometers, that is, by using magnetics. However, this method is only suitable for ferromagnetic surfaces. If the surface is non-magnetic metal or very rough, engineers often weld a ferromagnetic pad to it to act as a magnetic base.
As the stud mounting method offers a relatively firm connection as compared to the adhesive and magnetic methods, it is suitable for higher frequency signals for measuring acceleration. The adhesive and magnetic methods of mounting accelerometers are suitable for applications where the acceleration signals are below a few kilohertz.
