What is a vibration?

Vibration is a mechanical phenomenon where a structure oscillates around some equilibrium point. The oscillations can be periodic, such as the vibration from engines, random such as vibrations from a car transversing a gravel road. Vibrations are in general measured as the instantaneous acceleration of the structure, given in m/s^2 or standard gravity g = 9.80665 m/s^, but it is in many cases also worthwhile to observe the instantaneous velocity and amplitude of the vibrations.

Measured vibration is more often than not a combination arising from random vibration sources, periodic vibration sources and all other accelerations affecting the system such as impacts, centrifugal forces or any type of motion.

Periodic vibration is characterized by its frequency spectrum (amplitude and phase vs frequency)[1], and random vibration is often characterized by the acceleration spectral density (ASD), (amplitude squared vs frequency) [2].

In many cases, vibration is unwanted as it leads to noise and might in the long term compromise the integrity of the vibrating structure. Many things can be said when characterizing vibrations. A shift in amplitude or frequency might indicate that the structure is beginning to malfunction and needs replacement.

How can you measure vibrations? 

To start from the beginning:

When measuring vibrations you will need some type of accelerometer. There are multiple types of accelerometers (see the section below) that can be used and all of them have their pros and cons. We will cover this topic in a structured way a bit further down in this article.

The accelerometer shall be mounted directly on the element or object that is vibrating to make sure that the accelerometer is fully exposed to the vibration. When mounted this way, the accelerometers (or sensors) can be used for a wide range of application that ranges from high-frequency measurements, such as gearboxes and rolling elements, to low-frequency measurements which often are shock measurements where an object is examined to see the shock levels it has been exposed to.

Before mounting the accelerometer, it’s very important to understand if you have an accelerometer that only measures in one direction or three axes. If you’re using a single axis accelerometer you will only get results on the same axis as you have mounted the sensor and will have to readjust the sensor to measure in all directions. We prefer simplicity and have made sure that the ReLog comes with a tri-axis accelerometer that measures in all directions to make it as easy as possible for you. After all, not all of us are measurement experts and we’re here to help you become, or at least look like, one 🙂

The way you mount the accelerometer is a very important factor so we suggest that you have a look at our article on suggested mounting methods.

How does MEMS acceleromters actually work? 

If you’ve made it this far you are most likely as interested in measurements as we are, fantastic! In all simplicity, MEMS accelerometers are Microelectromechanical systems that use a small mechanical element (a spring/cantilever/similar) and a small proof mass that when being offset by vibration from its initial position. It is this offset that is being measured in a very precise way. Such a movement of the proof mass is consistent with Newton’s second law of motion (F = ma).

If you would like to read a bit more on the topic, we would be more than happy to recommend our friends at Analog Devices and their article on how MEMS accelerometers can be used in condition monitoring.