Question

I will be using the Endevco model 66A11 near an AC motor having a potentially high AC magnetic field. What precautions should I be aware of? How do you test for an accelerometer's electromagnetic sensitivity?

Answer

Electromagnetic sensitivity is the resultant output that is generated when an accelerometer is subjected to external AC magnetic fields. This output is considered noise and therefore contributes to measurement errors. A significant effort is made to avoid the use of magnetic material during the accelerometer product design phase in order to minimize the risk of electromagnetic sensitivity. Despite this, most accelerometers still exhibit some degree of electromagnetic sensitivity. The Endevco model 66A11 has an electromagnetic noise sensitivity of 0.00023 equivalent g rms per Gauss.

A typical electromagnetic sensitivity test setup is shown in Figure 1, below:

Figure 1

Accelerometer output under test is monitored and measured using both an oscilloscope and signal conditioner. It is then subjected to a uniformly distributed AC magnetic field. The applied AC magnetic flux density is controlled by the power supply and is calibrated against a reference Gauss meter. The corresponding accelerometer output is then measured and recorded. It is important that the sensitive axis of the accelerometer is aligned with the flux lines, to ensure that maximum accelerometer response to AC magnetic fields is achieved and measured.

When installing an accelerometer within a high electromagnetic noise environment, such as in the proximity of AC motors, generators and power transformers, it is best to first measure the magnetic field strengths in probable areas where the accelerometer will reside. For best measurement results, it is important to use a high-quality, fully calibrated Gauss meter. It is also recommended to choose a location with the lowest possible magnetic strength reading, as well as to avoid aligning flux lines with the sensitive axis of an accelerometer. Remember that highest electromagnetic accelerometer response usually occurs when magnetic flux lines are in-line with the sensitive axis. In extreme cases where the electromagnetic field intensity is high, it is necessary to shield an accelerometer with high-permeability materials, such as MuMetal.