Shopping for a controlled vibration test system can appear to be a daunting task. But, as with most technical assignments, the problem can be broken down into a sequence of smaller questions to be answered. In general, you are looking to select a system (shaker and amplifier) mechanically capable of running all of the various tests that you expect to perform on all of the different test specimens you must qualify. In addition, this shaker/amplifier must be controlled by a vibration controller with sufficient dynamic range to assure robust and repeatable control of every test with sufficient measurement channels to provide full protection and understanding of your tested device. The controller must have the necessary repertoire of testing software to cover every anticipated test type and provide for future testing expansion.
Figure 1: Basic components of an electrodynamic vibration testing system.
Finding a proper system to run a particular test on a particular specimen involves three distinct steps:
Determine the Frequency Range of the test and the extreme Peak motions (Acceleration, Velocity and Displacement) from the Test Specification. Depending upon the type of test (Swept Sine, Random, or Classical Shock for example) you will perform different calculations to accomplish this.
From the Device Specifications determine the mass of the device under test (DUT) and the dimensions of the mounting surface required to bolt the DUT to a shaker table with its center of gravity (CG) over the table’s center. This may require use of a head expander and/or other mounting fixture. Determine the mass of such additional hardware including all mounting fasteners.
Look at the Shaker Specifications of a trial system. Verify that the mass of the DUT and any mounting fixtures are less than the shaker’s rated Maximum Static Payload. Verify that the DUT (or its mounting hardware) can be bolted to the shaker’s armature or head expander. Add the Effective Armature Mass and head expander mass to the DUT mass and the mass of any mounting fixtures and fasteners and any accelerometers mounted to the DUT – this is the Total Moving Mass that must be moved by the shaker during the test. Multiply the Total Moving Mass by the Acceleration determined in step one to calculate the Force required. Verify the determined test Frequency Range, Force, Peak Acceleration, Peak Velocity and Peak Displacement required by the test and DUT are all less than the corresponding performance ratings of the selected shaker system. If this is true, the selected shaker is acceptable. If not, a more powerful shaker is called for.