When using modern, high technology measurement devices one can often be tripped up by the simplest things. The most common is the ground loop. Time and again this issue rears its head. So, let’s talk about how to avoid a ground loop.
What is a ground loop?
Most sensors and their cases will be made from metal. Usually the mechanical structures under test are made of metal. These metals are normally ferrous and, almost always, will conduct electricity.
Generally, accelerometers are of the non-isolated type. That is, the case of the accelerometer is electrically connected to the ground of the cable that carries the sensor signal.
This means the electrical potential of a large metal object being tested could be at a different potential to the measurement system. Perhaps the measurement system is running from mains power or a battery. The result is that a current is induced in the cabling between the measurement system and the metal test object. Thus the cables carrying the signals are corrupted by the current flows. The only solution is to correct the issue and stop the current flow. Another ‘easy’ solution will not have the desired effect.
How to avoid a ground loop
Normally, one would ensure that the measurement system and any computers using the measurement system are powered from the same source and so they will be at the same potential. Even this is not as easy as it sounds. Low cost power supply units often omit the common ground path from AC/DC input to DC output.
Good quality measurement systems will always have a ground connection. One could attach an accelerometer to an engine block and then ground the measurement system to the same ground as the block. This would be the vehicle 0V. Then, if there is an issue, the ground cable can make the connection and so any current flowing will not adversely affect the signal path. The 0V should be the same 0V that is powering the measurement system and any laptops.
It is always best to ensure that the piece of metal under test is itself grounded.
Are there any other considerations?
What is the catch?
Sometimes the above will not help, but running the laptop from its internal battery rather than from the common ground will. Technically this should not help or even work. However, experience has shown that although we don’t always understand what is happening, sometimes it makes the difference. That is why we call it the black art of ground loops!
What can be done to help this issue?
It is possible to apply electrical tape between the base of the accelerometers and the metal they are physically connected to. This will isolate the accelerometers and they will still respond in a similar fashion, but it is likely that the tape will change the frequency response of the accelerometer. Using a product like Blu Tack would mean the accelerometers may not respond as expected, as it will act like a spring. Also it will melt easily.
Petro wax is the most commonly used method for affixing accelerometers. Unfortunately, that method does not ensure that there are no electrical connections between the accelerometer and the test piece.
An acceptable solution is to purchase isolated accelerometers or isolation bases for the accelerometers.
What about impact hammers?
When using impact hammers a metal tip is often required to excite at the desired frequency range. These hammers are usually not an isolated design and so when an impact is made the signal can become corrupted. In these cases, it is not possible to apply a material (e.g. tape) to the tip or use a non-metallic tip. This would adversely affect the excitation frequency range. The only solution is to ensure the test piece is properly grounded.
My equipment is connected to the mains power socket, so surely it’s grounded?
Well, not really. It is likely that the earth connector in the wall socket is not actually connected to anything and therefore the potential of the supply is actually floating relative to another supply. In my experience this is a big problem in the Far East. If that is the case there is no choice, but to ground the equipment directly. Usually, with a large pole buried into the ground.
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