When installing an accelerometer measurement setup on test structures, I have been advised to avoid ground loops. Please tell me, what exactly are ground loops, and how can I prevent them?
The term "ground loop" refers to the presence of an undesirable electrical current present within an accelerometer installation. Worse still, the noise often shows up at "line" frequencies of 50 or 60 Hz, a range usually within the frequency of interest for most vibration test plans.
Grounds loops come about when the measurement system installation has more than one ground point (see Figure 1). In electrical systems, we often assume that all "grounds" are at the same potential. In reality, this is often not the case, especially with installations outside of the laboratory environment, or in which long distances are involved between the positioning of the accelerometer and signal conditioner. If the grounds are at different potentials, a current flow will result, completely unrelated to measurement signal current.
Diagnosing ground loop problems can be a difficult and frustrating exercise, depending upon actual installation conditions. The best proactive approach for ground loop prevention is to utilize the single-point ground system approach. The idea behind the single-point ground system is to ensure that throughout the entire measurement system (accelerometer, cable and signal conditioner), only one ground point exists. In general, Meggitt recommends that the designated single-point ground be located at the signal conditioner.
Most signal conditioners are grounded via their power supply. The trick, then, is to ensure that the rest of the system is not additionally grounded at any point. The accelerometer must be examined. In particular, is the transducer case isolated, or case grounded?
If case isolated, there should not be a ground loop problem. If the accelerometer is case grounded, however, (i.e., where the signal return is routed through the accelerometer housing; see TP328 for more information) the accelerometer may need to be isolated through the use of an insulated mounting stud. An insulated mounting stud electrically isolates the test structure side of the stud from the accelerometer side, effectively breaking the ground loop (see Figure 2).
To further assist in preventing ground loops, Meggitt offers a number of different insulated mounting studs for use with our case grounded accelerometers. There are other more advanced methods for dealing with ground loop problems beyond the scope of this article.