Measuring vibrations with accuracy
What are vibrations?
Let us start at the top: A vibration is in all its simplicity some form of movement that is either random or continuous.
The more tech-savvy way to describe it: Vibration is a mechanical phenomenon where a structure oscillates (moves) around an equilibrium point. The oscillations can be periodic, such as the vibration from engines, random such as vibrations from a car driving on a gravel road. But it can also be more continuous, as the vibrations created by a compressor or a pump in an industrial setting would be.
Vibrations are defined by their strength or, more correctly put, by the immediate acceleration of the structure mentioned above. This is generally expressed in m/s^2 or standard gravity (g), where g is defined as “g = 9.80665 m/s^2”. And it can in many situations also be worthwhile to observe the immediate velocity and amplitude of the vibrations.
If you prefer a more deep-dive than this we do recommend taking a look at Wikipedia and their two great articles on Random vibrations and how to perform a Fourier Transform calculations.
So really, how can I measure vibrations?
Good question! Let us begin!
When measuring vibrations you will need some type of accelerometer. Multiple types of accelerometers can be used and all of them have their pros and cons, as everything has. We will to make it easy for you, cover this topic in a structured way a bit further down in this article.
But before we get into the details, let us start with the basics around accelerometers and how to use them:
1) Before you even start to mount the accelerometer, it’s very important to understand if you have an accelerometer that only measures in one direction or in three different 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.
2) Proceed to ensure that the accelerometer is mounted directly on the element or object that is vibrating to make sure that the accelerometer is fully exposed to the vibration. If not, you’ll risk getting sloppy measurements. And to make sure that that doesn’t happen, we suggest that you have a look at our article around our suggested mounting methods.
3) When the accelerometer has been mounted properly, the accelerometers can be used for a wide range of application that ranges from high-frequency measurements of gearboxes and rolling elements to shock measurements where an object is examined to see the shock levels it has been exposed to.
And lastly, 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 an expert!
So, are there different types of accelerometers?
Well yes! This piece of the article could without a doubt be extended to the length of a book or two (which we very well may do in the future) but for now, we will try to keep it as light and easy as possible for you.
Firstly, there are three different types of accelerometers: 1) Piezoelectric accelerometers, 2) Piezoresistive accelerometers and 3) Capacitive accelerometers. Let’s go through them one by one.
1) The piezoelectric version utilizes the fact that piezoelectric materials produce electricity when being put under physical stress to be able to sense a change in acceleration/movement. The main positive sides of using this type are their recognition of being able to sense a wide frequency range, having no internal moving parts and that the acceleration signal can be used to sense velocity and displacement. This leads to piezoelectric accelerometers being used in a wide range of applications.
2) The piezoresistive accelerometer is way less sensitive and can’t pick up details in the vibration as the piezoelectric version manages. The principle behind it can be found in the piezo element that increases its resistance to being in proportion with the amount of pressure/stress that it’s being exposed to.
3) The capacitive version is actually the type of accelerometer that is most commonly used across most applications. And the principle behind it can best be described by that when the accelerometer is being subjected to vibration, the distance between the capacitor plates changes and the distance is being measured.
What type of accelerometer should you use?
Good question! This is, of course, on of those times where our answer will be: “It depends…”. As it actually does depend on the situation. But what we would like to do is to recommend you to take a look at the ReLog vibration data logger offered by us at ReVibe Energy. We have included two different accelerometers: One primary and one secondary, to be able to measure with high resolution and low noise from 0 Hz to 32,000 Hz.
How can I analyse the vibrations?
We asked ourselves the same question! And to answer it, we developed the VibInspect analysing software (take a look here) which you can download for free and analyse the vibrations you have measured with the ReLog, or your own accelerometers. And if you are unsure about the actual theory behind the analysis, take a look at our article “How to analyse vibration measurements?”