The correct mounting for the Control or Reference transducer seems to be more art than science to a great many shaker lab personnel. There are a multitude of factors to consider in conducting accurate shaker tests. Here are some basics to get started.

Ideally, both Test and Reference transducers should weigh 1/10th or less of what they are mounted to. The Reference transducer often is mounted directly to the shaker armature in tests where the mounting adapter is not reactive to the shaker output. This armature is usually heavy enough that the Reference transducer can weigh anything less than a pound or two (about 1/2 to 1KG). In applications where a mounting adapter is used between the shaker and the article under test it is important to understand if the mounting block may react to the excitation of the shaker within the frequencies being studied. If this may occur it is recommended to mount the reference to the mounting adapter itself to gain a true reference of the energies being transferred in the article under test. Various mounting methods may be employed but most experts prefer fastener mounting over adhesives or wax. For a detailed discussion of this topic you can refer to Endevco Technical Paper #312.

As for the Test transducer, this should be mounted as close to the area of interest as possible to obtain the most accurate characterization of motion. To reduce any load effects that the test accelerometer may impose on the article under test it is recommended that a sensor with the highest output to weight ratio be used. If multiple points need to be studies it is recommended that multiple transducers are employed. This not only saves testing but also considerable set-up time for complex tests. In multiple transducer tests care must be taken to reduce unnecessary loading since additional weight that is not the test article causes a shift in characteristics from what the test article alone would exhibit. The object here is to make that shift negligible if possible, while getting the necessary data during the test. The same rule (1/10th the article weight), applies in this case to the total Test accelerometer(s) weight. This can be a major restriction, especially at low (<50g) g levels. Often, for small test items like cell phones and the like, a miniature transducer such as the Endevco model 22, 25 or the new model 71 are the only devices capable of meeting the criteria.

Most light weight accelerometers (1oz (27gm), or less) are capable of attaining a flat frequency response to ≤ 4000 Hz. This is sufficient for most random profiles and sine-sweeps. Lighter transducers usually have higher flat-frequency values.

This is of course just a few of the standard practices of Reference and Test transducer mounting but certainly provides a good start toward getting accurate test data from most shaker testing. For additional insights and tip on shaker testing we recommend reading "Random Vibration & Shock Testing" by Wayne Tustin.

Your Endevco Applications Engineering Team.


[ Return to front page ]