Tag Archives: MEMS Accelerometers

How to Effectively Mount Accelerometers

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.

Condition Monitoring with MEMS Accelerometers

In the market today, several condition-monitoring products are available as easy to deploy and highly integrated devices. A vast majority of them contain a microelectromechanical system or MEMS accelerometer as their core sensor. Not only are these economical, they also help in reducing the cost of deployment and ownership. In turn, this expands the facilities and the number of equipment benefitting from a condition monitoring program.

Compared to the legacy mechanical sensors, solid state MEMS accelerometers offer several attractive attributes. So far, their low bandwidth had restricted their application for use in condition monitoring. For instance, the noise performance of MEMS accelerometers was found to be not sufficiently low to cater to diagnostic applications requiring low noise levels over bandwidths beyond 10 KHz and over high frequency ranges.

The above situation is changing. Although still restricted of a few KHz of bandwidth, MEMS accelerometers with low noise are now available allowing the designers of condition monitoring products to use them in their new product concepts. This is because the use of MEMS brings several valuable and compelling advantages to the designer.

For instance, the size and weight of the MEMS accelerometers are of the utmost importance to airborne applications in health and usage monitoring systems, especially as they employ multiple sensors on a platform. MEMS devices in surface mount packages in a triaxial formation provide very high performance, while their footprints are only 6 x 6 mm, and weigh less than one gram. This shrinks the final package, while the interface of a typical MEMS device uses a single supply, which makes it easier to use in digital applications by saving on cost and weight of cables.

The triaxial arrangement is simpler with solid-state electronics and the small size of the transducers. They offer a small form factor enabling mounting on a printed circuit board, with the assembly hermetically sealed in housing suitable for fitting on a machine. MEMS devices require very low levels of power from single voltage supply and simple signal conditioning electronics, suitable for battery-powered wireless products.

Designers are able to use MEMS accelerometers in industrial settings for easy transition to digital interfaces now common. This is because the topology of the signal conditioning circuit for MEMS devices is common with both analog and digital output variations, allowing them to adapt the sensors to a wider variety of situations.

For instance, designers can load open protocols such as the Modbus RTU into a micro-controller, while using them with easily available RS-485 transceiver chips. Using surface mount chips, designers can lay out the complete solution for a transmitter with small footprint and fit them within relatively small board areas. They can insert these assemblies into packages, hermetically sealing them for supporting intrinsically safe characteristics or for conforming to environmental robustness certifications.

Although the current generation of MEMS devices can safely withstand 10,000 g of shock according to their specifications, in reality they can tolerate much higher levels without affecting sensitivity specifications. For instance, automatic test equipment can trim the sensitivity of a high-resolution sensor to remain stable over time and temperature to 0.01°C.