I have recently embarked on conducting impact tests and Operational Deflection Shape (ODS) analysis on-site using Me'scope and various instruments. Currently, I am not facing any issues with the software or devices themselves. The challenge lies in the setup process and obtaining real-time data from actual structures. For instance, I am interested in conducting an impact test on the baseplate provided to determine the natural frequencies of the structure. I have chosen a frequency span of 200 Hz with 1600 lines for optimal resolution, considering the machine operates at 1500 rpm. However, I am unsure about the type of hammer to use and the locations for measurements. Should I impact the structure in all X, Y, Z directions? Is it better to move the hammer or the accelerometer, or does it not make a difference? Can I be certain that impacting one point will yield accurate results at a distant point? Do I need to assess the structure beforehand? I am finding it challenging to apply practical considerations on-site, despite grasping the software and hardware usage. Any assistance or guidance you can offer would be greatly appreciated. Thank you for your time. - Nicolas
Your point is valid and raises an interesting question about the advantages and disadvantages of focusing on roving the acceleration versus roving the impact. While reciprocity theory suggests that it may not make a difference, practical considerations in certain structures and situations may prove otherwise. If minimal energy is expected in one direction while impact occurs in another, can the test provide accurate results? In your specific scenario, it is likely that the vertical direction is of most interest. An Operational Deflection Shape (ODS) analysis may be a good starting point to identify any problematic motions. Best regards, Jim P.
Reciprocity is an essential property when measuring magnitudes, ratios, and relative phase. Utilizing a roving accelerometer is preferred for ensuring accuracy and minimizing impact on the node before conducting tests. Once you have confirmed that there is no interference and energy is being imparted, you can proceed with the test. Additionally, using a fixed impact location helps ensure that the impact direction encompasses all three axes. This approach helps guarantee accurate and reliable test results.
When conducting impact tests, it is advisable to vary the machine's location and acceleration. Observe the machine's response in all three axes to identify an optimal testing point. Once you find a point that activates all measurement points in each plane, use it as the fixed location. It is recommended to utilize 400 lines for a maximum frequency of 200Hz, providing a frequency resolution of 30 lines. This resolution is sufficient and helps decrease data collection time. Adjust resolution accordingly if natural frequencies are closely spaced based on impact response data analysis.
Hello Nicolas, I found Jim's response to your question very informative, especially regarding the use of 400 lines of resolution. This level of resolution not only saves time but also provides more than enough detail. When it comes to testing baseplates and larger surfaces, I prefer using a small fractional HP variable RPM DC motor. I attach a flywheel type rotor with a specific unbalance weight to create vibrations. By securing the motor in place and adjusting the RPM through different ranges using a rheostat controller, I am able to measure vibrations in various locations and axes. It's common to discover that only one direction is problematic at times. I track the motor's RPM (forcing frequency) using a photo tach or strobe light. The flywheel attachment I have created includes drilled and tapped holes evenly spaced around the diameter for the option to add extra weight if needed. This customization depends on the mass involved and the need to excite any natural frequencies. In some cases, using a "dead blow" sledgehammer may be necessary. By utilizing this method, I consistently apply force without relying on a hammer or other techniques to excite natural frequencies. I have successfully used this technique to identify natural frequencies of objects and perform nodal testing on structures and piping. It is important to securely mount the unit to ensure accurate testing, especially for piping. If there are doubts about the safety of the mounting, reverting back to the "dead-blow" hammer process is recommended. If you are interested, I can share a picture of the unit I assembled some time ago.
Hey Nicolas, I can definitely understand your predicament as I was in a similar situation when I first started doing impact tests. As per your questions about moving the accelerometer or the hammer, it largely depends on the characteristics of the structure. Normally, it's less error-prone to move the hammer. However, depending on the size and complexity of the structure, multiple accelerometers might be more feasible. Always try to cover all XYZ directions since most structures are three-dimensionally complex and not just one-directional. Concerning the accuracy at distant points, you may want to integrate a roving accelerometer to your setup to gauge the behavior at multiple points. Lastly, preliminary structure assessment is a smart move, it can alert you to any anomalies such as cracks or deformations that may affect your measurements. I hope this helps, and blows away some of the fog! Happy testing!
Hi Nicolas, working with ODS and impact tests sure is an interesting but challenging task. To answer some of your questions, the type of hammer you choose depends on the size and the material of the structure being tested–for larger, denser structures, you may need a harder impact source. Impacting a structure in all X, Y, Z directions aids in capturing all the possible modes of vibration and can provide a comprehensive description of your object's behavior. As for moving either the hammer or the accelerometer, it generally doesn't make much of a difference, as long as you're consistent in your methods. However, bear in mind the accessibility of multiple points while deciding. Although ODS analysis allows for some distant inferences, impacting one point and expecting accurate results at a far distant point might not yield accurate results due to the damping characteristics of real structures. Additionally, a preliminary assessment of the structure might provide valuable insights into parameters like material, constraints, the presence of damage, etc., which can affect your results. Keep experimenting and learning! Don't feel discouraged by initial setbacks.
Hey Nicolas, I understand the challenges you're facing. ODS analysis can indeed be tricky in practice. First off, hammer selection is usually based on the mass of the structure you're analyzing. For a baseplate, a small to medium-sized hammer should suffice, but always remember to avoid a contact force that could potentially change the structural modal behavior. As for the measurement locations, impacting the structure in all X, Y, Z directions and observing the ensuing responses probably gives you a more holistic picture. For your question on the hammer and accelerometer, I would advise moving the hammer. The advantage of the roving hammer method is you obtain an independent response at each point tested, which provides clearer and more reliable data. As for the accuracy at distant points, typically, the closer your accelerometer placements are to the impact point, the more accurate the data. Finally, assessing the structure beforehand is always a good practice because it allows you to plan your points of impact and measurements in an efficient manner. Hope this helps and good luck!
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Answer: - Answer: It is suggested to use a frequency span of 200 Hz with 1600 lines for optimal resolution.
Answer: - Answer: Ensure to select an appropriate hammer type and strategically place accelerometers for impact testing in all X, Y, Z directions.
Answer: - Answer: It is important to move the hammer to different points on the structure during testing to gather comprehensive data.
Answer: - Answer: Impacting one point can yield valuable insights, but assessing multiple points on the structure is recommended for a more thorough analysis.
Answer: - Answer: Yes, it is crucial to evaluate the structure's condition and characteristics prior to performing impact tests for accurate and meaningful results.
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