6 EXPERIMENTAL VERIFICATION OF THE MCDLA
6.2.1 Experimental Setup
The set-up used for the experimentation is shown in Figure 6.1. A mild steel shaft of circu- lar cross-section is used for the experimentation. The shaft is supported by deep-groove ball bearings at both ends as shown in Figure 6.2. The bearings are accommodated in specially fab- ricated split-type bearing housings (made from rectangular mild steel plates), which provided a firm support and simple means for mounting/dismounting of bearings as well as of the shaft. A stringer is used to connect the modal exciter with the shaft. It prevents the shaft from rotation while application of the transverse force. The shaft is excited by the sine-sweep force with the help of the exciter. Transverse cracks are introduced in the shaft artificially by using a saw.
Cracks made by this procedure are open cracks, i.e., the cracks will remain open for any crack orientation angle (Figure 6.3).
Figure 6.1: The test rig for the experimental crack detection.
Figure 6.2: A close view of one end of the shaft, supported by a deep-groove ball bearing.
Figure 6.3 A close view of the slit-type crack.
The modal exciter is placed under the shaft at a suitable location. The exciter is placed away from locations of cracks since it is expected that effect of crack on responses would be more if the exciter is near to the crack and accordingly crack identification would be expected to be better. However, in real situation it is less likely that placement of exciter happens to be near the unknown crack location. A sensor stand is designed and fabricated to mount proximity sen- sors. Provisions were made to slide the sensor stand in the axial direction of the beam at vari- ous locations over the beam length.
6.2.2 The Excitation Unit
An electromagnetic exciter (B&K, Type 4808) is used in the present experiment (Figure 6.4). In an electromagnetic exciter the supplied input signal is converted to an alternating mag- netic field by means of a coil which is attached to the drive part of the device (Sujatha, 2009).
A force sensor is used to measure the force applied by the exciter. The exciter has force rating of 112 N sine peak (or 187 N with the cooling). The working frequency range is 5 Hz to 10 kHz and the first axial resonance is 10 kHz.
Figure 6.4: An electromagnetic exciter connected to the shaft through a stringer via a trans- ducer.
6.2.3 Signal Generator and Power Amplifier
The necessary excitation to the shaft system is given by a modal exciter, which is operated by a signal generator module of the B&K Pulse software through a power amplifier. The gen- erator module (B&K, Type 3107) can be used for generating various kinds of signals. The gen- erator is designed around a digital signal processor and a 16-bit D/A converter. The frequency range is 0 to 102.4 kHz. Output levels are adjustable from 1 mV to 5V. In terms of waveforms, single and dual superimposed sine waves can be generated. Sweep-sine (continuously changing
the frequency of the signal) can be used for both the sine and dual sine signals. The power am- plifier - B&K, Type 2719 - is used in the present experimentation to drive the vibration exciter.
It has 180VA power output, adjustable RMS output-current limit and low or high output im- pedance.
6.2.4 Force Transducer
The force transducer is installed directly between the exciter and the test structure being ex- cited. The piezoelectric type of transducers is suited for measuring tensile and compressive forces. The transmitted force, or a known fraction of it, is applied directly across the crystal, which generates a corresponding charge, proportional to the force. The following are features of the force transducer (B&K, Model 2311-1), which is used in the present experiment. It is de- signed for the modal exciter, and to measure the compression and tension excitations. It has low impedance output and its force sensitivity is 0.227 mV/N and the maximum frequency range is 75 kHz.
6.2.5 Proximity Transducers
Bently-Nevada® 3300 eddy current probes (along with amplifiers and condition monitor- ing) are used for measuring the shaft displacements (Figure 6.5). They are non-contact type of sensors. They provide a voltage signal proportional to the gap between the probe tip and the shaft. It comprises of a probe, a length of extension cable and an oscillator demodulator. The proximity probe sensitivity is 7874 V/m.
Figure 6.5: Measurement of shaft displacements using proximity sensors.
The probe stand is a specially fabricated by Perspex sheet, which had threaded holes. Prox- imity sensors are screwed in these holes. The stand can be fixed at any required axial position of the shaft for transverse displacement measurements.
6.2.6 Laser Vibrometer
In the second attempt to improve the accuracy of measurements, rotational laser vibrometer (RLV) is used to measure transverse vibrations of the shaft. Rotational laser vibrometer is a la- ser optical instrument for non-contact acquisition of rotational vibrations on rotating parts. It can be used to measure vibrations of a non-rotating part also. The RLV is made up of the con- troller RLV-5000 and the sensor head RLV-500. The optical measurement principle for the ro- tational vibrometer is based on laser interferometry. Dynamic acquisition of rotational vibra- tions is possible in a frequency range from 0Hz to 10 kHz.
For rotation measurements, the sensor head of the rotational vibrometer contains two inter- ferometers aligned in parallel which acquire the translational velocity components in the direc- tion of the respective laser beam. For translation measurements, only one channel of the rota- tional vibrometer is used. Also, it needs reflecting film to be applied on the surface of vibrating body. In the present work, translational vibrations are measured with single channel of the vibrometer. The rotational laser vibrometer used in the experiment is shown Figure 6.6.
Figure 6.6: Laser head of the rotational laser vibrometer.
6.2.7 Data Acquisition System
Pulse analyzer® (B&K, 3560C) is used for the purpose of data acquisition of measured forced responses. It is a PC-based sound and vibration analysis system and it consists of a PC with LAN interface, software, Microsoft® Windows® operating system, and data acquisition front-end hardware. The system has both the time capture and the FFT analyzer for recording the real-time data. The algorithm is tested for both single-cracked shafts and double-cracked shafts. Stages for conducting the experiments are discussed in the following section.