Sine Resonance Track & Dwell (SRTD)

March 29, 2018

A sine dwell test may be run on resonances once they have been determined by a sine sweep. In a sine dwell test, the controller runs a single sine tone at the resonant frequencies instead of sweeping through the frequency range. A product experiences the most fatigue when exposed to its natural resonance. The sine dwell test is used in the design and validation of a product in almost every industry that uses vibration testing.

Figure 21: Sketch of a shift in resonant frequency

Figure 5.1. A shift in resonant frequency.

A Shift in Resonant Frequency

While running a sine dwell test on the resonant frequency of a product, the resonance might shift. See the example in Figure 5.1; it shows that a sine sweep was run and found resonance at 35Hz. If we ran a sine dwell test at 35Hz (if we had the controller dwell on 35Hz) for an extended period of time, the resonance might drift down to 33Hz (the resonance shown in dashes), due to heat or fatigue or something else. This means that if we continue to dwell at 35Hz (if we were to keep the frequency fixed at 35Hz), we will not actually be testing the product at its resonance, nor applying the accelerations to the product that we would be applied at its resonant frequency. Continued dwelling at 35Hz results in an acceleration of only 2G instead of the 10G value at its resonance.

Thus, a fixed-frequency dwell (a sine dwell which does not adjust frequency) has shortcomings when it comes to dwelling at a product’s resonance. However, there are ways to track a drifting resonant frequency and to stay on the resonance. One way is to track the phase difference between the control and response. When a product begins to fatigue, the frequency values of its resonances will decrease, but its phase value for the resonance will not.

Phase-Tracking SRTD

A phase-tracking SRTD takes advantage of this, allowing the frequency value of the test to adjust with the changing resonant frequency of the product. See the technical paper “Improving SRTD With Resonance Phase Settings,” Van Baren.

Figure 22: Frequency vs time graph for an SRTD test on an (already fatigued) aluminum beam. Notice the drift in resonant frequency, which became more severe as the beam approached failure.

Figure 5.2. Frequency vs. time graph for an SRTD test on a fatigued aluminum beam. The drift in resonant frequency became more severe as the beam approached failure.