Tag Archives: MEMS sensors

Condition Based Monitoring and MEMS Sensors

Lately, there has been a tremendous improvement in MEMS accelerometer performance. So much so, it can now compete with piezo vibration sensors that are all-pervasive. This is because MEMS sensors offer several advantages including smaller size, lower power consumption, low noise levels, wider bandwidth, and a higher level of integration. Consequently, the industry is now increasingly using MEMS sensors in CbM or condition-based monitoring for facility and maintenance. Engineers find CbM very useful, as it helps in detecting, diagnosing, predicting, and ultimately, avoiding faults in their machines.

The smaller size and ultra-low power consumption of MEMS accelerometers allow for replacing wired piezo sensors which are typically bulky, with wireless solutions. Moreover, it is easy to replace bulky single-axis piezo sensors with small, light, and triaxial MEMS accelerometers. The industry finds such replacements cost-effective for continuously monitoring various machines.

The world over, millions of electric motors are in continuous operation. They account for about 45% of global electricity usage. In a survey across industries, more than 80% of the companies in the survey claimed to experience unplanned maintenance. More than 70% of the companies remain unaware that their assets are due for upgrade and maintenance. With Industry 4.0, or the IoT, the industry is moving towards digitization to improve its productivity and efficiency.

The trend is more toward wireless sensor systems. An estimate finds there will be about 5 billion wireless modules in smart manufacturing by 2030. Although most critical assets require a wired CbM system, there are many, many more that will benefit from wireless CbM solutions.

For the best performance, speed, reliability, and security, it is difficult to surpass a wired CbM system. For these reasons, greenfield sites still deploy them. However, installing wired CbM systems requires routing cables across factory floors. This may be difficult in cases where it is not possible to disturb certain machinery. Industrial wired sensor networks typically use 60 m or 200 ft of cables, which can be substantially expensive depending on the material and labor the process involves. Some deployments may also require wire harnesses and routing through existing infrastructure, thereby increasing the cost, complexity, and time to install.

On the other hand, brownfield sites may not be amenable to wired solution installations. For them, although the wireless systems may initially appear to be more expensive, other factors can lead to significant cost savings. For instance, initial cost savings can come from less cabling, fewer maintenance routes, and lower hardware requirements. Over the lifetime of the wireless CbM installation, substantial cost savings can accrue from the ease of scalability and more effortless maintenance routines.

Wireless installations depend on batteries for powering them. Depending on the level of reporting, batteries may last several years. Deployment of wireless systems based on energy-harvesting techniques can make maintenance of these systems even easier and less expensive. However, once a company decides to go wireless, they must focus on the best technology for CbM that suits their application, of which, there are quite a few to choose from, such as Bluetooth Low Energy, GlowPAN, and Zigbee.

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.