I. Why Peak Tracking
During a Sine Resonance Track and Dwell (SRTD) test, resonances shift as material fatigues. For the test to be valid, the test frequency also needs to shift. An engineer needs insight into these shifts; ideally, the engineer should learn the details of multiple resonances during an SRTD initial sweep, then select a resonance for a Dwell, and run that test with confidence that the frequency will stay on the resonance.
The traditional way to adjust SRTD testing for shifting resonance frequencies is Phase Tracking, implemented by the vibration test controller software. At the end of the Sine sweep, the phase difference between the control channel and the response channel at the resonance frequency is measured. While the theoretical resonant peak in the response is 90 degrees out of phase from the control, the real-world phase difference will usually be close to, but not exactly, 90 degrees because of material imperfections, the mounting locations of accelerometers, and non-linear shaker motion.
Phase Tracking generates an output at the amplitude and frequency selected after the initial sine sweep, presumably the frequency at which peak resonance is occurring on the response channel. The test holds at that frequency until, due to material fatigue, the resonant frequency of the response begins to shift. The frequency of the drive output then shifts to maintain the originally measured phase difference between the control and response sensors.
Phase tracking for SRTD tests offers a clear advantage over tests that do not adjust to resonance shifts. However, there are three limitations to the phase tracking methodology:
- Phase relationships measured during an initial sweep are imprecise.
- If the product has a non-linear amplitude response, a change in amplitude between the detection portion of the test and the dwell portion of the test may result in both a different resonant frequency and a different phase relationship.
- Fatigue may behave in a non-linear fashion, causing the phase difference at peak transmissibility to change as fatigue effects progress.
Peak Tracking is another option for adjusting to resonance shift during the dwell portion of a test. This option allows the controller to adjust both the output frequency and the phase difference between the two channels to maintain peak transmissibility.
Peak Tracking minimizes the need for precise detection during the sine sweep, so users can sweep faster. More importantly, it ensures that peak transmissibility is maintained throughout the dwell portion of the test, even when phase relationships shift along with the resonance frequency.
Improving SRTD Testing
II. How Peak Tracking Works
Peak Tracking works by constantly oscillating the phase difference between the control and the response channels and observing if the transmissibility increases or decreases. As more data is collected, the algorithm controlling the oscillations learns the shape of the transmissibility graph and seeks the peak. As the algorithm learns the shape of the transmissibility graph, the amount of phase change becomes narrower in frequency and slower in time, limiting the amount of oscillation while still accurately tracking the peak.
There is, of course, a 1-to-1 relationship at any point in time between frequency and phase difference during a test. Saying that ‘the software is oscillating the phase’ really means that it is using an algorithm that changes the frequency in a way that achieves the desired phase change. For frequencies close to the resonant frequency, a very small change creates a significant change in the phase difference. See Figure 1.
At the same time, the software tracks the transmissibility to see if it increase or decreases. In this way, it knows if it is approaching the peak or moving away from the peak.
Peak Tracking is a powerful feature that minimizes the need for precise detection during the sine sweep, so users can sweep faster. Most importantly, it ensures that peak transmissibility is maintained throughout the tracked dwell portion of the test.
III. Peak Tracking Parameters in VibrationVIEW
Examining the controlling parameters in VibrationVIEW provides some insights into Peak Tracking. Open the Resonance tab while creating a Sine test to see Figure 2, below.
A user must first select Enable under SRTD Peak Tracking Parameters. Figure 2 shows the default values for those parameters, which can all be modified. Looking at those parameters:
The Delta Starting Frequency parameter sets the frequency where the phase oscillations begin, defined as a percentage of the initially identified resonance frequency. For example, an initial target frequency of 1000 Hz with a 1% Delta Starting Frequency will start the oscillations at 1010 Hz. 0% won’t shift the frequency and a negative percentage will shift down. This shift will only occur when Peak Tracking Mode is enabled.
The Initial Range parameter sets the initial oscillation range in degrees for Peak Tracking mode. The algorithm controlling the test will try to reduce the sweep range from this Initial Range as it oscillates around the peak in transmissibility.
The Min Range parameter sets a lower limit on the phase oscillation range. The controlling algorithm will try to minimize the degree range of oscillation as much as possible until the estimation of the peak is affected too much by the noise in the transmissibility. However, the range in degrees that the Peak Tracking mode searches through will go no lower than the Min Range parameter.
The Max Range parameter sets an upper limit on the phase oscillation range. The range, in degrees that the Peak Tracking mode searches through, will go no higher than this parameter. If this parameter is set lower than the noise level, the phase will drift randomly. This value can normally be left at the default level.
The Initial Period parameter sets the initial time in seconds for a phase oscillation. This variable directly affects the amount of data the test collects between each calculation of the peak of the transmissibility. Longer periods will produce better estimates but will take longer to adjust to the peak. The oscillation will slow down when it begins to oscillate around the peak in transmissibility until it reaches the Max Period value.
The Max Period parameter sets the upper limit on the time for an oscillation. The oscillations will gradually slow down until they reach this value, then stay at this value for the remainder of the test.