Before we continue the discussion about relative and absolute vibrations, we need to straighten out this matter first. Relative and absolute vibration is not included in one of the vibration classifications (read the following article to find out the classification of vibrations). Relative and absolute vibration is only a method of viewpoint, for measuring vibration.
As the name implies, relative is a measuring technique that refers to a certain point that is not silent. Then relative vibration is a measurement of vibration (either displacement, speed, or acceleration) measured relative to the position of the vibration sensor.
We take for example in measuring the engine shaft vibration according to the picture above. The shaft vibration measuring instrument in the form of an eddy current sensor is installed very close to the engine bearing position. So that when a vibration arises on the engine shaft, the sensor will read the vibration whose value is relative to the position of the transducer.
If there is a simultaneous vibration between the shaft and the engine bearing, the vibration sensor will read zero. When this happens, the condition of the installation of the engine foundation against the basic foundation must be considered.
Vice versa, if there is vibration on the engine shaft due to bearing failure, for example, the transducer sensor will read high vibration value relative to the sensor position. In other words, if relative vibration occurs then it is estimated that damage occurs on the engine bearing.
There are two ways to use eddy current sensors, first is using one sensor. If this is used, the sensor is placed just above the center of the bearing. Whereas the second method is to use two sensors that are paired at +45° and -45° from the vertical line of the shaft (according to the image below).
Eddy current sensors require a conversion process so that the reading of the sensor output voltage can be changed to a displacement value. If you want to know the velocity value, you need an integration process from the displacement value, and double integration if you want to know the relative acceleration value of vibration.
Absolute is a measuring technique that refers to a stationary point in free space. Then absolute vibration is a measurement of vibration (either displacement, speed, or acceleration) measured against a stationary point in free space.
Generally absolute vibration measurements only use speed transducers or accelerator. The speed transducer sensor is equipped with a coil as a reference point, while the acceleration transducer uses a mass.
Absolute vibration on a machine shaft cannot be measured directly. A real-time measurement method and calculation is needed.
Measuring the absolute vibration of a machine shaft as shown above, it is necessary to measure the relative displacement of the shaft using the same eddy current sensor as in the measurement of relative vibration. It is also necessary to measure absolute vibration on the bearing using an accelerometer sensor or speed sensor, which is set to have the same axis as the position of the eddy current sensor.
The absolute vibration value of the shaft is obtained by calculating the difference between the relative vibration of the shaft, and the absolute vibration of the bearing. The calculated absolute vibration value of the shaft can be in the form of 0-peak displacement, or also peak-peak displacement as shown in the graph below.
As for calculating vibration speed, an integration process of absolute displacement values is needed. While the value of vibration acceleration is obtained by double integration.