Abstract
A typical vibration test often requires running the test up to 2000 Hz or beyond. This is a tremendous challenge because the resonant and anti-resonant frequencies are often only a few hundred Hz. This article analyzes the obstacles to running a sine test over a high frequency range, including the dynamic characteristics of the Unit Under Test (UUT) and fixtures, the control dynamic range of the vibration controllers, and the sensor mounting locations. Several strategies and recommendations are then discussed with results.
Keywords: Vibration Control, Sine, Structural Dynamics, Resonances, Anti-Resonances, Average Control, Damping, Modal Survey, Dynamic Range, Sensor Mounting
Introduction
Executing a sine vibration control test is a common method of verifying the performance of the manufactured product under real world conditions. However, there are various challenges that could hinder a successful sine sweep test. These challenges include system structural dynamics (resonances and anti-resonances present in the system), sensor placement location, measurement dynamic range, and sensor quality and mounting method.
Structure dynamics – The resonances and anti-resonances present in the system can push the drive voltage to its limits. It is necessary to deal with these challenges in order to ensure the safety of the test components. Different control strategies can be implemented to overcome this challenge as discussed in the paper. In addition, sensor location is also an important aspect that must be considered while choosing the location for controlling and monitoring the sine sweep test. Information from a quick modal survey can help optimize the sensor location which further assists in enhancing the test setup. This information can also be used to avoid over testing or under testing the specimen.
Control dynamics – A low dynamic range of the controller not only affects the measurement but also affects the control. The high amplitude signals would be clipped, and the low amplitude signals will be close to the noise floor. This ultimately affects the precision of the output drive voltage at the resonances and anti-resonances which results in bad control of the sine sweep profile. To overcome this challenge, a high dynamic range is used and the dual ADC technology behind this is discussed in the upcoming sections.
Sensor quality and mounting methods – The sensitivity of the sensor and the mounting method affect the usable frequency range of the sensor. A sensor with high sensitivity would be required for a low-level vibration test to ensure a good signal-to-noise ratio. Also, the mounting method for the sensor should be taken into consideration. Care must be taken to ensure that sensitivity of the sensor does not change in the interested frequency range of operation.
To demonstrate the issues mentioned above, a sine sweep vibration control test is executed with the EDM VCS software. A sensor is mounted onto the UUT and a single shaker sine sweep test is executed in the 5-2000 Hz frequency range using a single control channel strategy. The plot below shows that the test aborts abruptly because of the system anti-resonance at 168.5 Hz.
Figure 1. Issues observed in a Sine Vibration Control test
At the anti-resonance at 168.5 Hz, the drive voltage must quickly ramp up to ensure the control follows the reference profile. Even after the drive reaches the maximum limit, the response at the control location is low and continues to drop. This causes the control to drop beneath the lower abort limit, aborting the test.
This paper discusses and illustrates various strategies such as weighted average control, optimal sensor location and high controller dynamic range, all of which help achieve better control for the sine sweep test.
Outline of Sine Vibration Control Test
The digital Vibration Control System (VCS) is a computer-based system that conducts closed-loop control of vibration shaker table systems. It generates an electronic signal for a shaker amplifier that provides the drive signal to a hydraulic or electro-dynamic shaker. The vibration response on the UUT (Unit Under Test) is then fed back to the VCS controller from transducers that measure acceleration, velocity, or displacement. The controller adjusts the drive output such that the control signal conforms to specified characteristics in the time or frequency domain. There are many vibration control test types, including Swept Sine tests.
Figure 2. Hardware connection diagram for a Vibration Control test
While a Random test generates broadband signals over a band of frequencies at once, a Sine test generates one frequency, and sweeps the frequency over a defined range. Feedback from the control channel(s) is then used to adjust the drive channels’ output amplitude such that the response amplitude of the control channel(s) on the shaker table or UUT matches the pre-defined reference profile. The test profile is amplitude spectra defined as peak acceleration versus sweeping frequency.
Figure 3. Sine Control System Structure
The Sine control process consists of generating sine wave output to excite the shaker table or device under test, detecting the control signals’ input amplitudes, comparing the detected levels with the reference profiles, and updating the drive signal appropriately to achieve control.
Figure 4. Single Axis Sine Vibration Control Test in EDM VCS Sine
Usually, a sine vibration test is used to determine the performance of the UUT when subjected to the programmed profile. However, the Sine control tests sweep through the desired range one frequency at a point and hence have a much higher Signal-to-Noise ratio which produces cleaner responses. This process thus excites each of the frequencies and can be used to obtain resonances and anti-resonances for structural analysis. This information about the amplitude relationship at different frequencies can guide the user in measuring or improving the mechanical properties of the structure.
Figure 5. Basic harmonic motion equation for displacement, velocity, and acceleration
The basic equation for harmonic motion is illustrated in the diagram above. This relationship can also be used to convert the input profile values between displacement, velocity, and acceleration for the sine vibration test.
Parameters like sweep range and the sweep duration are pre-determined to test the life cycle of the specimen. Using these parameters, settings for Start and Stop frequency, time duration of sweep, sweep fashion and sweep rate, and acceleration level of the profile are programmed accordingly.
The sine sweep can be executed in a linear, log or octave fashion. To illustrate an example, a sine sweep is executed as shown below.
Figure 6. Sine sweep time signal
The reference profile for the sine sweep vibration control test consists of entering a set of input amplitudes for acceleration, velocity or displacement for the corresponding frequency points and the slope values associated with the segments. The software can automatically calculate and interchange the crossover frequency points or the amplitude values when the slope between the data points is present.
An example of an acceleration profile programmed in “g’s” vs frequency plot is shown below.
Figure 7. Example Profile for Sine Vibration Test
To ensure that the test is safely within the shaker limits, the maximum peak values of acceleration, velocity and maximum peak-peak values of displacement are displayed alongside shaker capabilities. If the programmed profile exceeds the system’s limits, the software will alert the user to adjust accordingly.
Figure 8. Shaker Limit Check
At some measurement points, the reference profile of the sine vibration test will produce some response “g” level on the UUT. The vibration experienced at the resonances would be significantly higher while the anti-resonances would experience little vibration.
Figure 9. Example illustrating input profile and output response of a sine vibration test
Read the remaining sections online: Sine Sweep in High Frequency Range