The underlying objective for the work as described in this section was to outline the developed method of measuring vibrations with the aim to evaluate the self-damping in the form of a hysteresis loop. Firstly, the hysteresis loop formed represents the lagging between the loadings and displacements. Based on the concept that is explained in the previous section, a LabVIEW program was developed to generate the hysteresis loop. The LabVIEW program was developed in such a way to send signals to the conductor and also receive data from the test conductor.
6.7.1 Test Procedures and Data Acquisition using LabVIEW
In the experimental methodology, the tests conducted were done with the following procedures using the LabVIEW program. The LabVIEW program was developed and designed to send,
157
process and acquire signals between different hardware, the measuring instrument includes the computer, function generators, A/D converters, and power amplifier. The function generator was used to generate a signal with a given frequency and amplitude in the form of a sinusoidal signal.
This signal was then amplified by the amplifier which is send to the shaker to excite the conductor.
The output signals from the accelerometer was in a voltage form and was received by the NI USB- 6210 (National Instruments data acquisition device with USB PC connection). The accelerometers were connected to the NI 9234 dynamic signal acquisition (DSA) module. The NI 9234 DSA module has a built-in antialiasing filter that automatically adjusts to the sampling rate. The DSA module was connected to a desktop computer, which houses the program that controls the function generator and the power amplifier that drives the shaker. A signal acquisition module with signal conditioning function, type NI USB 9234, was connected to a desktop and accelerometers. This digitizes the incoming signals to the analog output signals. The LabVIEW program was developed to work as both input and acquisition of data and also display the acquired data. Measurement of the hysteresis loop for the vibration conductor was done using the LabVIEW DAQ max.
For data input, acquisition, and display, the LabVIEW program that was developed was divided into three parts. For the purposes of these measurements, and the hysteresis loop formed, the graphical user interface (GUI) was created, see figures (6.8) to (6.10). The program was made to operate in any of these three GUI environments using the toggle buttons.
6.7.1.1 Data Input
The first part of the LabVIEW program was developed sending input signal to the conductor system. This program sends signal via the function generator to drive the shaker. The GUI for this aspect is shown in figure (6.8). The input data includes the frequency and amplitude at which the shaker was driven. This aspect was designed to send a specific frequency to drive the conductor.
This specific frequency from the function generator can be changed without having to interfere with values in the program block diagram. This gives the ease during experiment to change the frequency while the shaker was still running. Also, this aspect of the program was also used to calibrate the accelerometers in order to verify the values that are being received from the accelerometers are reliable.
158
Figure 6.8: LabVIEW GUI for input signal 6.7.2 Data Acquisition and Display
For the aspect of data acquisition and display, two forms of data are received from the conductor.
The first is the signal from the force transducer placed on the shaker. During measurement, a specific constant frequency value is fed to drive the shaker, while changing the position of the accelerometers using the clip to accelerometer is attached, to locate the position, which should coincide with the antinode of the vibration.
The second signal is obtained from the accelerometers. This outward signal is made to pass through a DAQ card whose function is to convert digital signals to analog signals by an A/D converter which is built inside it. The GUI for the program shown in figure (6.9), signals from the vibrating conductor were generated as sine waveforms, from the five different accelerometers placed on the line.
Each accelerometer was placed to coincide with an antinode. The experiments were conducted by continuously increasing the input frequency with the tension kept at constant value, the amplitude of the generated signal is then measured. Another experiment is done by changing the value for the axial tension, with another input frequencies.
The third aspect of this LabVIEW program was the generation of the hysteresis loop see figure (6.10). The program for the data acquisition, two forms of signal are received: one signal from force transducer placed between the shaker and the conductor. The second is the signals each from
159
the five accelerometers. The formation of the hysteresis loop was obtained by plotting the force transducers against any of the accelerometers signal. The hysteresis loop was obtained by plotting the signals from the force transducer with any of the accelerometer. This plot represents the force- displacement curve.
Figure 6.9: LabVIEW GUI for signal acquisition
Figure 6.10: LabVIEW GUI for experimental formation of the hysteresis loop
160
All acquired data together with input variables were saved in a data file, from where they can be used for further analysis in the future. Matlab program was then developed to calculate the area for each loop obtained in order to determine the damping. Analysis of the experimental results is presented in the next chapter.