# A Novel Feature Selection Based on VMD and Information Gain for Pipe Blockages
## 3 Experimental Setup and Experiment Conditions  
An experimental platform was built in the laboratory [37], as shown in Figure 2, for acoustic detection of pipe blockages. In this experiment, the blocked substances were simulated with clay semi-cylinders, and sinusoidal sweep signals were used as the excitation signals were given their adjustable frequency bands as per demands. The use of sinusoidal sweep signals for exciting multi-DOF systems allows energy concentration of acoustic signals within the frequency range sensitive to the pipe thus that the required information in such frequency band can be easily elicited. They are often used as excitation signals for outdoor detection.  
The clay pipeline had a diameter of 150 mm and a length of 14.4 m. During detection, a computer with LabVIEW software was used to control the virtual instrument for generating sinusoidal sweep signals within a 100-6000 Hz frequency range. Then, the analog output port of NI PXIe-636 was controlled via the DAQ assistant of LabVIEW to output analog voltage signals. After amplification of the signals with a power amplifier, the loudspeaker was driven to generate audio signals, which were then transmitted into the pipeline to serve as an excitation signal source. Such sound wave signals undergo complex interactions with the discontinuous interface of acoustic impedance in the pipe interior. The echo signals were received by the microphone placed at the pipe head end, which was then uploaded to the computer for storage. A sampling frequency of 44,100 Hz. Changes in the acoustic performance of pipe interior were identified by analyzing the received signals, during which the LM4950 power amplifier (Texas Instruments in Dallas, TX, USA) was used, as well as the FR874 OH loudspeaker (Visaton in Germany) and the SPM0208 HE5 microphone (Knowles Acoustics in Illinois, USA).  
For the sake of practical simulation, experiments were conducted at varying degrees of blockages on the experimental platform shown in Figure 2. Water flow was provided in the pipe for simulating the pipe condition, whose rate was dependent on the water pump. In the present experiments, the maximum rate of simulated water flow was 7 L/s, and other simulated water flow rates were 0.42 L/s, 1 L/s, 1.8 L/s, 4.25 L/s, and 6.1 L/s, respectively,  
Figure 2: Experimental platform for pipe blockage detection.  
which were used to form intra-pipe water levels of different heights. The severity of pipe blockage was defined by the laboratory as the percentage of blockage height in the cross-sectional pipe height. The simulated blockages with heights of 20 mm, 40 mm, and 55 mm were placed separately in the pipe of 150 mm in diameter. The heights of these rigid, non-porous blockages accounted for 13%, 26%, and 37% of the cross-sectional pipe area, respectively, which were approximately considered to be slightly blocked, moderately blocked, and moderately to severely blocked. The detailed data about the experiments are shown in Table 1.  
In this study, the pipe condition was set as a normal running empty state. There were conventional parts, i.e., lateral connections (LC), inside the empty pipeline that was in a normal operating state. The pipe conditions with the presence of single blockages included a 20 mm blockage; a 40 mm blockage; and a 55 mm blockage. Meanwhile, the pipe conditions with multiple blockages included coexistence of a 40 mm blockage and a 55 mm blockage that were placed in different positions inside the pipeline; coexistence of a 40 mm blockage and a lateral connection; coexistence of a 55 mm blockage and a lateral connection; and coexistence of a 40 mm blockage, a 55 mm blockage, and a lateral connection. This comes to a total of 9 pipe conditions. There were 100 sets of samples for each condition, with a total of 900 sets.