Selection and operation of the vibration testing system

Question

We have a laboratory used for electrical fixtures testing. Sometimes we have to test mechanical rigidity of items. Below are the test impact specifications. Please provide us with your recommendations concerning selection of the shaker and shaker controller.

Test 1: random vibration impact in the banwidth of  – 20 Hz with vibrational acceleration spectral density of 0,96 m²/s³ and in the banwidth of 20 – 500 Hz  with vibrational acceleration spectral density of 0,96 m²/s³ at the frequency of 20 Hz, then– 3 dB/oct.

Test 2: sinusoidal vibration, acceleration 4 g, frequency 5 – 80 Hz

Test 3: sinusoidal vibration, acceleration 2 g, frequency 25 Hz, duration – 30 min. в

Test 4: sinusoidal vibration, acceleration 3 g, frequency up to 15 Hz

Test 5: sinusoidal vibration, acceleration 1 g, frequency 1 – 100 Hz

Test 6: sinusoidal vibration, displacement with the amplitude of 0,76 mm and full sweep of 1,52 mm; impact frequency: from 10 up to 55 Hz at the speed of 1 Hz/min (and backwards).

Test 7: sinusoidal vibration, acceleration amplitude of 3 g in the frequency range of 10 – 100 Hz.

 Answer

Based on the specified test profile, we have calculated the following top values of the vibration parameters:

Frequency range – from 1 up to 500 Hz.

Acceleration – up to 4 g for sinusoidal vibration, and  0,9 g for random vibration.

Displacement– up to 0,76 mm.

It is noteworthy, that in the low frequency range (1-5 Hz) the shaker systems have displacement limits, and in the high frequency range (above 50 Hz) – limits by acceleration.

Acceleration and displacement of the shaker system in the sinusoidal vibration mode are mutually related in accordance with the following formula:

A[m/s^2] = (2 * pi * F[Hz] ) ^2 * D[mm] / 1000

or, in approximate fashion, A = D * F^2 / 25

and vice-versa: D = 25 * A / F^2

Thus, at the frequency of 1Hz and at the maintained acceleration level of 1g, the displacement value will be 250 mm.

At the frequency of 10 Hz and at acceleration of 3 g, the displacement value will be 7,5 mm.

In the case if we maintain the displacement value of 0,76 mm at the frequency of 55 Hz, the resulting accceleration value will be 9,3 g.

The same calculations are to be applied to the test profiles 5, 6 and 7. For the convenience of operator, and for safety of tests performance, the program “Sinusoidal vibration” performs calculation of the relevant vibrational characteristics and verifies their applicability.

As an intermediate conclusion, we recommend you to consider a shaker system with the displacement more than 7,5 mm and the maximal acceleration level over 10g. In this case, you will have to re-calculate only one test profile (without taking into consideration the weight of the specimen).

 Question

Could you please help us to select a shaker system?

We are going to test the equipment with the mass from 100gr up to 20kg, however, 90% of the tests will be conducted with the load below 1kg. Probably, we should select relatively cheap model and produce the test impact in the horizontal direction? In the case if the shaker system price increase, we are ready to limit the weight of the tested equipment with 3kg.

 Answer

The shaker systems suitable for testing of specimen up to 20kg, have the minimal impelling force of 20kN, their weight is more than 1,5 tons and they are quite expensive (for instance, TIRA TV59320/340).

In the case if the weight to be tested will be limited by 3 kg, you can select ashaker with the impelling force of 2 kN (for instance, TIRA TV50303-120).

The maximal load, that can be used on the shaker, is limited by the parameters of the shaker system, its suspension system, amplifier, and the weight of the shaker.

In the case of vertical load, in accordance with Newton’s third law of motion, the shaker applies to the floor a force, which is equal to the force applied to the specimen. The heavier is the shaker, the less load is applied to the floor.

Vibration testing in horizontal direction allows to avoid the limitations concerning the weight of the specimen in the case if the weight of the specimen will be applied to another object: slip table or suspension. The limitations in impelling force and acceleration will remain the same.

 Question

Is it possible to estimate the available peak acceleration, that can be produced by the shaker, based on the characteristics of the slip table? What is the dependence on the weight of the specimen and location of the specimen (i.e., on the table or in the case of lateral mounting)?

 Answer

In the case of shock impact testing (and random vibration testing) the formulas are the same, the only difference is the impelling force, that can be produced by the shaker. The more is the mass of the specimen, the lower is the available peak acceleration. In the case of testing in horizontal direction, the dependence is the same.

In the case of powerful shaker systems, the manufacturers specify three values of the impelling force: for sinusoidal vibration, random vibration and shock impact.

 Question
Am I right to think that the test system consists of the shaker, shaker controller with the relevant software, and the vibration transducer, which is mounted on the specimen?

 Answer

The components included into vibration testing system depend on the particular model of the shaker system. Some of the models require the use of compressor (in the case of calibration shakers), water cooling (in the case of powerful shakers), etc. The control systems and control transducers are universal instruments, that are compatible with almost all electrodynamic shakers. We can provide you with the shaker controller ZET 017-U, besides, we have a wide range of transducers and shaker systems.

Question

We have to test a compicated and expensive specimen, so, what is the safest way of tests performance?

 Answer

If by danger you mean even minor excess of vibration level in the test profile, then the shaker controller system should be configured in compliance with the applicable limitations.

The easiest option is to reduce the relevant parameters of the shaker in the settings: displacement, velocity, and acceleration. In this case, the vibration testing will be suspended in the case of parameters exceeding.

More complicated option implies correction of the test profile. In order to edit profile, you have to measure the AFR of the specimen to be tested. Depending on the measured AFR value, you can reduce the sweep rate of sine vibration, or add an additional point for random vibration in the resonance area.

The best possible option is the control with the use of several transducers. In order to provide the feedback, select the control by the maximal values. In this case, there will be no overload, and the more transducers are used, the higher will be the reliability of the control process.

 Question

How fast do the VCS programs react to the excess of the set acceleration level?

 Answer

The program “Sinusoidal vibration” reacts to the excess of acceleration level depending on the set signal frequency. The response time is in the range form 330ms (at the frequency of 3Hz) up to 20ms (at the frequency of 50 Hz and above).

The program “Random vibration” reacts to the excess of acceleration level within 1 second.

The programs “Classical shock” and “Vibrational shock” respond to the excess of acceleration level within the duration of the shock pulse.

 Question

What are the requiements to the base of the shaker system and the relevant recommendations concerning installation of the system

 Answer

Small shakers intended for calibration of the transducers (e.g., TV 50018) can be placed on tables or cabinets for convenience of the operator. However, it is necessary to use a special furniture with solid foundation, or place the shaker above the legs of the table, so that to avoid strong resonance. Such shakers have rubber foundations.

Shakers of medium size are to be placed on the floor, or onto a foundation.

Large shakers are placed on the floor only. Large shakers may have pneumatic support, which compensates oscillations of the shaker, caused by oscillations of the specimen. For large shakers without pneumatic load it is recommended to use vibration isolating foundation.

 Question

In your video-lessons I have seen that the shaker was placed on an ordinary table. But when I placed my TIRA 51120 on office desk, at low frequencies there appeared noise and vibration (the shaker has rubber shock absorbers). What object can you recommend as a foundation for such a small shaker?

Answer

In accordance with Newton’s third law, “For every action, there is an equal reaction”, which means, that the force applied to the specimen by the shaker, is equal to the force applied to the shaker by the specimen. However, the shaker is much heavier, than the specimen, hence amplitude of oscillations is relatively small. Thus, the shaker oscillates the table as well.

Normal office desk has a slat closer to the edge, and not in the middle. Thus, the table has low intrinsic oscillations frequency, and in the center of the table there is the anti-node area (here the wave achieves its maximal amplitude). Hence, if we install the shaker in the middle of the table, the table will be oscillating. Due to this reason, we use an additional stand. The top surface of the stand is much smaller, which means that the frequencies of the intrinsic oscillations will be higher. In this case, we also have a resonance frequency at 24-27 Hz. Certain level of vibration is still observed, but the displacement amplitude is relatively small and does not distort the results of the vibration testing.

The best possible foundation for the shaker is a concrete cube (the shaker is mounted on it with the use of bolt connection). At the office, you can place the shaker on the floor (on the rubber legs). But in this case, it will not be convenient to work with the shaker, thus, the best possible option is to use an additional stand without shelves, made of thick slabs, and with the strengthening rib located in its center.

 Question

Is it possible to establish connection with the shaker via  Ethernet network?

 Answer

Most of our instruments have a hardware option of Ethernet connection (FFT Spectrum analyzers also have this option). In order to establish connection, it is necessary to have this option in the software of the FFT spectrum analyzer. This option is added in the course of order processing, which means, that you should inform us if you need this option.

The first thing to do is to configure the network address of the FFT spectrum analyzer, and the sub-net mask. To do that, connect the FFT Spectrum analyzer via USB, activate the program “Device manager”. In the properties of FFT Spectrum analyzer there should be “Ethernet” tab (in the case, if this option has been added to the spectrum analyzer). In this tab, you should set the address and sub-net mask.

As the parameters are saved, switch off the analyzer, disconnect it from PC, connect Ethernet cable, and switch on the analyzer. The PC will not find the aalyzer automatically, it is necessary to establish the connection manually. To do that, start the program “Connection of devices via Ethernet”, set the number of the devices to be connected in the tab “Network programs”, enter the network address of FFT Spectrum analyzer, then click the keys “Check” and “Activate”. As the indicator flashes green, and there appears the name of the device, you can close the program “Connection of devices via Ethernet” and start using the analyzer.

 Question

Is it possible to set auto-saving of the reports in VCS software?

Answer

Auto-saving of the test reports is already implemented in VCS programs. Upon completion of the vibration testing, the program generates reports in DTU format, which are saved to the specified directory. In the case if you want to select a new directory for saving of the results, click ZETLAB icon at the top left section of ZETLAB panel, in the drop-down menu, go to “User configuration path”->”Processing results”, and set the required directory.

 Question

How should I convert the test results files, so that I could use them in other programs?

 Answer

All test results, as well as results of other ZETLAB programs operation, are saved in DTU format. DTU-file is a text file, in the beginning of which there is a text data about the program and operation mode. Then follow the tags (labelled by the symbol “#”), which specify the design of the graph. After the tags, there is a string contating the names of chart columns. Then there is a string specifying the measurement units for each of the columns. Then there is a chart with test results. The first column contains information about X axis and can be recorded in the format “date-time”. The numbers in the columns are separated from each other with the tab character.

DTU-files can be easily converted in EXCEL format. To do that, open the file in  EXCEL program, and select the tab character as the symbol to be used for separation of the columns from each other.