Accelerometer Mounting Methods: Types, Effects, and Solutions
In this article, learn about the different methods for mounting accelerometers, their effect on accelerometer frequency response, and solutions for mounting a MEMS accelerometer.
To accurately measure acceleration, it is important to create an appropriate mechanical coupling between the accelerometer and the system being monitored.
In this article, we’ll take a look at different methods of mounting an accelerometer to the unit under test (UUT).
Accelerometer Mounting: Why It Matters?
As mentioned, it's important to consider the mechanical coupling of the accelerometer and the system.
A common source of error in acceleration sensing is the resonance of the mounting fixture.
For example, when working with micro-electromechanical system (MEMS) accelerometers, the PC board on which the accelerometer is mounted, along with any other mechanical interface between the PCB and the object being monitored, can create a mechanical system with multiple resonances in the frequency range of interest.
If the frequency of the acceleration signal is close to the resonant frequency of the mounting structure, the sensor will receive an amplified version of the original acceleration.
On the other hand, if the mechanical coupling exhibits some attenuation due to damping, the sensor will measure a smaller signal than what it really is.
Proper mounting techniques should be applied to take full advantage of the bandwidth provided by the accelerometer. The mechanical mounting becomes particularly important when measuring acceleration signals above 1 kHz.
Accelerometer Mounting Methods: Stud, Adhesive, Magnetic
In general, there are three main mounting methods namely stud mounting, adhesive attachment, and magnetic mounting, which we’ll briefly look at each of these below.
Whenever possible, it is best to drill a hole in the object and fix the sensor to the device under test (DUT) with a screw.
Stud mounting provides a stiff mechanical connection and is capable of transferring the high-frequency vibrations of the object to the sensor.
Figure 1 shows how stud mounting can be used to affix a piezoelectric accelerometer to the device under test.
Figure 1. An example of a stud-mounted piezoelectric accelerometer. Image used courtesy of PCB Electronics
When using stud mounting, the coupling surfaces should be as flat and clean as possible. It is recommended to use a light film of grease, oil, or similar coupling fluid between the coupling surfaces, especially above 2 kHz. Adding a coupling fluid fills small voids in the mounting surface and improves vibration transmissibility and mounting stiffness.
In Figure 2, the graph shows the effect of using a film of grease on the frequency response of a stud-mounted accelerometer.
Figure 2. A graph showing the effects grease has on accelerometer frequency response. Image used courtesy of MEGGITT/Endevco.
As you can see, when no coupling fluid is used (the yellow curve), the resonant frequency occurs at a relatively lower frequency compared to the pink curve that uses a film of grease.
The figure also illustrates the effect of another important factor: tightening the stud to the recommended mounting torque as indicated on the datasheet.
A torque wrench should be used for tightening the stud to the manufacturer’s specifications. An under-torqued stud may not adequately couple the sensor to the object and further lower the resonant frequency of the system (the cyan curve in the figure). Over-torquing may also cause damage to the device.
Sometimes it is not possible to drill a hole into the structure or the design of some accelerometers does not allow us to use stud mounting.
In these cases, we can use an adhesive to secure the sensor to the monitored object.
A suitable adhesive, such as epoxy, glue, or wax, should be chosen to meet the application requirements. Keep in mind that some adhesives work well for temporary mounts while others provide a more permanent mount.
Another alternative is to use an adhesive mounting base, or mounting pad, where one side of the base is adhesively mounted to the test object and the other side provides a quality surface to stud-mount the accelerometer.
This solution can be seen in Figure 3.
Figure 3. An example of using an adhesive mounting base to mount an accelerometer. Image used courtesy of Condition Monitoring Analytics.
Since a mounting pad provides a smooth surface and perpendicular mounting hole for the accelerometer, it can improve the accelerometer's frequency response.
Additionally, adhesive mounting bases also prevent the adhesive from damaging expensive accelerometers by clogging the mounting threads of the sensor.
A final third method for mounting an accelerometer is with magnetics. Magnetic mounting can be used when the object being monitored has a ferromagnetic surface.
In the case of non-magnetic or rough surfaces, we can weld or epoxy a steel pad to accept the magnetic base.
Figure 4 shows the use of a two-footed magnet on a round surface.
Figure 4. Example of a magnetically mounted accelerometer. Image courtesy of Wilcoxon.
Despite the potential implied ease of mounting, magnetic mounting is normally heavy and the added mass lowers the resonant frequency of the measurement system.
Frequency Responses for Accelerometer Mounting Methods
Compared to the stud mounting method, adhesive and magnetic mounting provides a relatively looser connection and consequently, are suitable for measuring lower frequency acceleration signals. These methods are typically used in applications involving acceleration signals below a few kilohertz.
Figure 5 shows the frequency response of a given accelerometer for different types of mounting.
Figure 5. A graph of frequency vs amplitudes for accelerometers depending on the mounting type. Image used courtesy of NI.
With stud mounting (the blue curve), the resonant frequency of the system occurs at relatively higher frequencies.
However, with adhesive (the black curve) and magnetic mounting (the green curve), the frequency response begins to peak at relatively lower frequencies. The table below gives the typical frequency limits when using different mounting methods.
Table 1. Accelerometer mounting frequency limits. Information (modified) used courtesy of NI.
|Adhesive||2,500 to 5,000 Hz|
|Stud||> 6,000 Hz|
Solutions for Mounting MEMS Accelerometers
Piezoelectric accelerometers are designed to support the above well-established mounting strategies. This makes the installation of these devices relatively straightforward.
However, most MEMS accelerometers should be mounted on a PCB. The PCB then should be attached to the monitored object through a sufficiently stiff mounting technique.
Some PCB design factors that can affect the overall bandwidth of the system are: the location of the accelerometer IC on the PCB, the solder chemistry, and the size of the PCB. Besides, any other mechanical interfaces between the PCB and the object should be examined to ensure that high-frequency vibrations are successfully transferred to the sensor.
The accelerometer IC should be placed near a stable mechanical mounting on the PCB. It is recommended to have multiple hard mounting points near the accelerometer IC.
Figure 6 shows examples of incorrectly placed accelerometers.
Figure 6. Examples of incorrectly placed accelerometers. Image used courtesy of Analog Devices.
In the above examples, the accelerometer is placed at unsupported PCB locations and there can be measurement errors due to the undamped vibrations from PCB itself.
Another factor that can affect the system bandwidth is the PCB thickness. A thicker (but costlier) PCB can reduce the effect of the board assembly resonance on our measurements and improve the measurement accuracy.
An additional solution to address the mounting issues of MEMS accelerometers comes from Analog Devices, which has created a mounting cube for affixing the PCB to the asset, as shown in Figure 7.
Figure 7. Example mounting cube from Analog Devices. Image used courtesy of Analog Devices.
The cube has a central mounting hole that allows for a #10 machine screw to be used to securely attach the block to the device under test.
Note that the sensor PCB is designed a little thicker (3 mm).
The test results provided by Analog Devices show that the above mounting structure can successfully capture high-frequency vibrations. For more details, please refer to this application note.
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