Starting with this issue, our measurement column will address frequently asked questions received by our customer support center and provide answers to those questions.
An impulse hammer is a vibration hammer with a built-in force sensor for measuring the natural frequency and performing modal analysis of structures. Vibration sensors such as acceleration detectors are attached to the object to be measured, and the impulse hammer strikes (hammers) it. The resulting force and vibration signals are then analyzed using an analysis device such as an FFT analyzer to measure the frequency response function and coherence function. From these results, the natural frequency and damping ratio can be determined, and modal analysis can be performed.
In hammering measurements, the force signal is used as the trigger source, and data is acquired in synchronization with the force signal. However, if the analysis device is not properly configured to match the actual signal, the trigger will not activate and data will not be acquired. Therefore, this time we will introduce how to configure the analysis device for hammering measurements.
Initialization of the analysis device settings
The analysis device can be used for various measurements and has various functions tailored to each measurement. If the analysis device used for a specific measurement is still set to the settings used for that measurement, or if a setting not used for hammering is turned on, the correct data will not be acquired unless it is changed to the settings for hammering measurement. In particular, if another user has used it and the settings have been left as they were, it is difficult to know which settings to revert.
Therefore, when using analysis equipment such as an FFT analyzer, it is recommended to reset the analysis equipment settings to their default values and then reconfigure only the functions used for hammering measurement. After completing all the settings, save them to a project file or panel condition, and then load them for measurements the next time.
In particular, the following settings, if unsuitable for hammering measurements, can prevent data from being acquired correctly.
Auto-range function
This function automatically changes the voltage range (input range) according to the magnitude of the input signal. In the case of hammering, the signal is small when no impact is being made, so the voltage range is set to the minimum value. Auto-ranging cannot keep up with instantaneously applied impact signals, so the auto-ranging function should be set to OFF.
A/D Overload Cancellation
This setting is located within the sample condition settings. It prevents data acquisition when an input overload occurs. While this is a useful function during actual measurements, input overloads can occur before the analysis device is fully adjusted, so it should be kept OFF until the adjustments are complete.
Scale of the Y axis of the graph
Set this to "Default". When set to "Default", the Y-axis scale of the graph will be linked to the voltage range value. If the voltage range is not appropriate, or if the impulse hammer or sensor is not working properly and is only outputting a weak signal, the data will not be displayed, making it easier to notice abnormalities.
Calculus Function
This function involves integrating acceleration signals to convert them into velocity and displacement signals. When checking signals from impulse hammers or sensors, keep the calculus function OFF. After the setup is complete and you have confirmed that the raw signals from the hammers and sensors are being measured correctly, enable the calculus function as needed.
Provisional settings for hammering measurement
First, connect the impulse hammer, acceleration detector, and other components to the analysis device.
Next, we temporarily set up the analysis device for hammering measurement. Initially, we set it to conditions that ensure reliable triggering, and then later adjust the settings to match the actual signal and conditions.
Input settings
Set the CCLD (whether to supply power to the preamplifier built into the hammer, etc.) item in the input settings. Set it to ON if the impulse hammer is connected directly to the analysis device as shown in Figure 1-1, and to OFF if it is connected via a power supply unit, etc., as shown in Figure 1-2. Set the channels to which acceleration detectors, etc., are connected in the same way.
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Figure 1-1 Connection of impulse hammer and analysis device (when power supply unit and sensor amplifier are not used)
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Figure 1-2 Connection of impulse hammer and analysis device (when using power supply unit and sensor amplifier)
Unit calibration
Check the impulse hammer's sensitivity in the shipping specifications sheet and set the unit and sensitivity for the channel to which the hammer is connected. Here, enter the sensitivity when using the hardest tip. If an extender is available, enter the sensitivity with the extender attached. If the hammer's sensitivity is 2.3mV/N, set the unit name to "N", the EU type to "V/EU", and enter the sensitivity value "0.0023" converted to V/N as the EU value. Leave the 0 dB reference value at 1 and the offset at 0 dB. For channels to which acceleration detectors, etc., are connected, match the sensitivity of the detector.
Window function
Set the window function for all channels to a rectangular window. If you want to use an exponential window or force window, change it after the initial settings are complete. Hanning windows and flat-top windows are not used in hammering measurements.
frequency range
Initially, set the frequency range to a higher value, such as 20 kHz or 5 kHz. While checking the operation of the impulse hammer and trigger functions, gradually lower the frequency range to the one you will actually use.
Trigger settings (Figures 2-1, 2-2)
Set the channel to which the impulse hammer is connected as the input channel, and set the detection level to 5% and the hysteresis to 2%. The trigger position can remain at the default "-32".
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Figure 2-1 Trigger settings (DS-3000 Sound and Vibration Analysis System) -
Figure 2-2 Trigger Settings (CF-9000 Series FFT Analyzer)
Adjusting the voltage range
Attach the hardest tip to the end of the impulse hammer. If the hammer comes with an extender, attach the extender. If you are using a power supply unit or sensor amplifier, turn on their power.
Set the voltage range of the analysis device to the smallest range. For the DS-3000 series, the smallest range is 10 mVrms. For the CF-9000 series, it is the 1 Vrms range.
The object to be measured is struck with an impulse hammer. The impulse hammer is struck by dropping it onto the object using its own weight. Maintaining a constant drop height ensures a consistent force is applied. If the input overload LED on the analysis device lights up red, increase the voltage range by one step and strike again. Continue increasing the voltage range until the input overload light turns off.
Similarly, adjust the voltage range for other channels connected to accelerometers, etc.
Impulse hammer operation check
The analysis device's screen should display the time-domain waveform and power spectrum of the channel to which the impulse hammer is connected. Turn on the TRIG (trigger) button on the analysis device, press the START button, and then strike the object to be measured with the impulse hammer. The trigger should engage and the waveform should be updated. Once the trigger engages, stop it with the STOP button.
If the trigger does not engage, try striking it harder from a higher position, or try striking it with something harder than the object being measured. If the trigger still does not engage under these conditions, there may be a broken cable or a malfunction in the hammer or other components.
Adjusting the trigger detection level
Open the trigger condition settings dialog and set the trigger detection level. For the DS-3000 series, press the Open button under [Input/Output Settings Menu] ⇒ [Trigger Condition Settings] ⇒ [Internal Trigger]. For the CF-9000 series, go to [HOME] ⇒ [Input] ⇒ [Trigger] ⇒ [Int Trigger].
Set the trigger detection level to approximately 1/2 to 1/3 of the maximum value of the force waveform from the impulse hammer. A hysteresis value of around 2% is usually sufficient, but if the trigger detection level is less than 5%, set the hysteresis value to slightly less than half the detection level.
Clicking the waveform displayed in the trigger condition settings dialog will change the trigger detection level and position. The trigger position is usually set to a position where the force waveform is easily visible, such as -32 or -100, so if the position is changed by clicking the waveform, please revert it to the original value.
Close the dialog box, restart the analysis device, strike it with the impulse hammer, and confirm that the trigger engages.
Adjusting the tip and extender
Adjust the tip that attaches to the end of the impulse hammer.
Check the power spectrum of the impulse hammer force signal. If the spectrum extends to frequencies higher than the frequency band you actually want to measure, change the impulse hammer tip to one step softer and measure again. Check the maximum value of the force waveform on the trigger condition setting screen and readjust the trigger detection level to match the maximum value of the force waveform.
If you have a specific chip in mind, you can change it to that chip. However, if the trigger stops working after changing the chip, change it one step at a time using the method described above, and readjust the trigger detection level each time.
If you are not using an extender, remove it and check if the trigger works. If the trigger does not work, readjust the voltage range. If the trigger still does not work, try lowering the trigger detection level and hysteresis.
If you change the tip or extender you are using, check the impulse hammer's shipping specifications and adjust the sensitivity of the unit calibration settings.
Adjusting the frequency range
The frequency range should also be changed to match the frequency band you want to measure, but if you suddenly lower the frequency range, the trigger may not work. In that case, return the frequency range to its original value, then measure again with the frequency range lowered by 2 to 3 steps (about half of the original range) and readjust the trigger detection level. After that, lower the frequency range by another 2 to 3 steps and repeat the measurement and adjustment. Also, when changing the trigger detection level, be sure to reset the hysteresis as well.
Other adjustments
If the force waveform displayed on the trigger condition setting screen is small, try lowering the voltage range by one step. If the input does not overload when you strike the object, you can measure at that range. After changing the voltage range, set the trigger detection level and hysteresis on the trigger condition setting screen.
If the force waveform displayed on the trigger condition setting screen is small, changing the impulse hammer tip to a softer one may improve performance. Using a softer tip will lower the frequency bandwidth of the generated force, so please check the actual frequency response function and coherence function measurements to determine if changing the tip is appropriate.
summary
The actual procedure for measuring frequency response functions and other parameters continues, but this time we have introduced the setup method to enable triggering during hammering measurements.
Even after the trigger is activated and measurement becomes possible, the trigger often stops working when the object being measured or the conditions are changed. In such cases, I hope this column will be helpful.
(Excerpt from the email newsletter issued on April 23, 2015)
