homogeneous strength) is then used to fill the hole in aluminum block. Since it is necessary to sense/transfer signal from rubber component, a piezoelectric disk (with soldered wire) is fully submerged into the liquid rubber in such a way that it remains untouched with both lead ball and aluminum structure. Usual rubber curing time is 6 h. However, it is required to start the second step of fabrication at around 3–4 h after the first step. During the initial steps, cylindrical support was used to hold the lead ball at the middle. Hence, an empty space was open at the bottom of the structure after removing the cylindrical support. In the second step, the new empty space was filled with rubber following the same procedure described above. Since it is required to have a good bonding between the rubber, the second step was started before the full curing time in the first step. Please note that during the first step a piezoelectric sensor is placed vertically and allowed to be embedded once set. Following the outcome of the unit cell, a multi-cell metamaterial model is fabricated to sense multiple frequencies from one structure at the same time. The new model is a combination of five (5) unit cells with linearly varying design parameters (e.g. lead ball diameter, hole diameter etc.). It is hypothesized that each unit cell may idealize a specific fiber in the basilar membrane and can take part in sensing a unique frequency. However, constant thickness is considered for the entire structure. Considering the shape of basilar membrane (which is narrow at base and wide at apex end), width of the main structure is decreased linearly following the function for basilar membrane. To avoid moment imbalance during harmonic excitation, main structure was mirrored to complete the total frame (Ref. Fig. 3.4). Same fabrication procedure is followed to make the multi-cell metamaterial and the piezoelectric sensors were embedded. 3.3.2 Testing A Vibration Exciter (type 4809, from B & K Instruments) is used in testing the harmonic response of the fabricated metamaterial. The exciter is controlled through a Sine-Random Generator (type 1024) and Power Amplifier (type 2706) from Bruel and Kjaer. An aluminum support is manufactured to hold the metamaterial system on the vibration exciter. Schematic diagram of test setup with a unit cell is shown in Fig. 3.5a. Experimental setup with the multicellular structure is shown in Fig. 3.5b. The test job is clamped in two opposite sides and a simple harmonic displacement excitation (identical with simulation input) was applied using the Sine-Random Generator (SRG). A frequency sweep operation is not available on used SRG, however, it is only capable of generating a single frequency excitation at one time and excitation frequency can be tuned with the SRG knob. Tuning range of SRG is limited to 20 Hz to 20 kHz. An Oscilloscope (TDS 2004C, Tektronix) was connected to the piezoelectric wafer to sense the vibrational response from the rubber component. Fig. 3.3 Fabrication steps of unit cell metamaterial. (a) Fabrication setup (b) Liquid rubber placement (c) Fabricated final form with embedded piezoelectric sensors to capture the vibration in the rubber only Fig. 3.4 Fabricated multi-cell metamaterial with linearly varying geometric parameters for each cell 24 R. Ahmed and S. Banerjee
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