Photo credit: Vibration Research
Do we need to re-calibrate our accelerometer? How often do we need to re-calibrate it?
These are most common questions raised by test engineer during training session after the installation of new vibration testing systems. But before going to more details regarding accelerometer calibration, it is better to understand what role is played by accelerometer during vibration testing.
Photo credit: Vibration Research
Nowadays, most of vibration testing (if not all of them) are performed in a close loop-controlled system. What we need to do is "just" setting the test specification (demand) at the vibration control software and let the vibration controller to control the rest. Since it is close loop-controlled system, then a feedback signal will be needed to be compared to the demanded specification. And based on this feedback signal, vibration controller will adjust the drive signal to make the actual vibration signal aligned with the test specification. Looking at how this close loop control system works, then it is obvious that accelerometer and analogue to digital (ADC) processing at the vibration controller is playing a very important part to achieve the testing accuracy needed.
Generally, our accelerometer will send analogue voltage/charge to the vibration controller and the vibration controller will perform the ADC process. By using the accelerometer sensitivity value that we key into the controller software, the vibration controller will convert the voltage/charge into acceleration value. This is why the sensitivity value is very important for the testing accuracy.
Then why do we need to re-calibrate the accelerometer?
Because the sensitivity value may change over period of time, depending on the utilization. The sensitivity value which is commonly used during the testing is called Reference Sensitivity. This reference sensitivity is measured commonly at 100 Hz or 160 Hz. It is called reference sensitivity since the deviation of the sensitivity values over the frequency range are measured based on this value.
But if my reference sensitivity value is relatively constant after so many years of calibration, do I still need to perform re-calibration?
It is true that commonly, reference sensitivity may not drift more than 2 percent every one-year period. But unfortunately, the accuracy of our testing is not only depending on the reference sensitivity, unless that we are running constant sine test at the frequency where the reference sensitivity was measured. But if we are running sine sweep or broadband random test, then accuracy of our testing or measurement will be depending on the deviation of the sensitivity values over our test frequency range.
How is the accelerometer calibration performed?
One of the methods of performing accelerometer calibration is back to back comparison to a reference accelerometer as mentioned at ISO 16063-21. In this method, the calibrated accelerometer will be mounted on top of reference accelerometer (back to back). By comparing the voltage reading, the sensitivity of calibrated accelerometer can be calculated.
Tips:
Some of test engineer may just easily use reference sensitivity during their testing or measurement. But if we really want to achieve higher level of accuracy, we can choose sensitivity at other frequency depending on our test frequency range.
Some of calibration laboratory will measure the bias voltage of calibrated IEPE accelerometer. We can check this value to see if it is still within the acceptable range according to the manufacturer specification. If the bias voltage falls under the acceptable range, there is a chance that our accelerometer already started to deteriorate.
