What is a Fixture?
In the vibration industry it is common to attach a fixture to a shaker table top to assist in properly mounting a Device Under Test (DUT). The fixture is a specially designed item that is used to fasten a DUT to the shaker in the orientation of its end use environment.
Suppose you have a vehicle braking assembly that you want to test with road-like vibrations. The shaker vibrates in the vertical direction. Without a fixture it would be very difficult to orientate and fasten the brake assembly in the orientation it would have in an automobile. It would not make much sense to lay the brake assembly down on the shaker table and attach it to the shaker table. With such an orientation the brake assembly would vibrate in a direction perpendicular to its normal real-life vibrations.
With a specially designed fixture, a test engineer could mount the brake assembly to the fixture so that it is orientated in the direction it will vibrate in real-life road vibrations (Figure 1).
The same principle holds for whatever must be tested. Engineers need to test a product while in the same orientation as it will be used during its serviceable time. Another example is a navigation system used in the auto industry. The instrument panel and navigation system must be orientated during vibration testing in the same orientation (including tilt) that it will have in the vehicle (Figure 2).
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| Figure 1: A brake disc is mounted on a fixture in the orientation of its end use environment. |
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| Figure 2: A navigation system/radio system mounted on a fixture in the orientation (including tilt) of its end use environment. |
What is the Purpose of a Fixture?
A fixture serves the important purpose of mounting the DUT to the shaker. But a fixture accomplishes this task in a unique manner so that the DUT is orientated in exactly the same way as it will be used in its final environment.
The fixture must be designed in such a way as to accomplish this purpose without damping any of the vibrations from the shaker system. The fixture must not interfere with (or add to) the vibrations that the DUT is to experience.
In addition, a fixture serves an important purpose of speeding up the amount of testing an engineer can accomplish and adds an element of consistency to that testing. A fixture has a set design. An engineer can readily remove one DUT only to replace it with another DUT in order to confirm testing results. Without the fixture, an engineer would spend much time and effort trying to set the second DUT in exactly the same orientation as the first one. A fixture helps accomplish that task much more easily. And, therefore, a fixture increases the consistency and reliability of the tests.
To summarize, a fixture serves three purposes:
- Mount the DUT to the shaker system in the orientation of end use
- Transmit the forces from the shaker to the DUT
- Serve to quickly mount and test other samples of the DUT
What Features Should Characterize a Fixture?
As briefly mentioned previously, a fixture must not interfere with the vibrations that are designed to test the DUT. Therefore, the DUT must be engineered in such a manner that it does not have any natural resonances within the frequency range of the DUT’s test.
Additionally, the fixture must be constructed so that it does not experience breakages itself. However, engineers must be careful not to “over-engineer” the fixture by designing a very massive fixture. Additional mass will require the shaker to work harder in order to vibrate not only the DUT but also a massive fixture. So, engineers must wisely design a fixture that is both light weight but rigid enough to endure vibrations.
Finally, a fixture must not have any motion of its own in any axis, so as not to disrupt the testing of the DUT. This means the fixture must be well mounted to the shaker table and not have any parts that are free to move and vibrate.
To summarize, a fixture should meet the following four characteristics:
- Designed so that it does NOT have a natural frequency within the test range of the DUT
- Designed to have minimal mass so that the shaker need to provide only minimal force to move it
- Designed to have rigid construction to avoid easy breakages
- Designed to have minimal motion in any axis
