*5.1. Experimental Setup*

The grinding tests were performed in a surface grinding machine from Sulmecânica manufacturer, model RAPH 1055 (Porto Alegre, Brazil), equipped with an aluminum oxide grinding wheel, model 38A150LVH, from NORTON company (Worcester, MA, USA). AISI 4340 steel workpieces with 150 mm length × 7 mm width × 43 mm height were machined under three different depth of cuts (*a*). Each test consisted of three grinding passes across the workpiece surface at a given grinding condition. The depth of cut, grinding passes, cutting speed (*vs*) and workpiece speed (*vw*) are shown in Table 1. The cutting fluid used in the tests was an oil-water emulsion with a concentration of 4%. Prior to test 1, the grinding wheel was dressed in order to restore its topography and cutting potential. In addition, some grinding passes were necessary to standardize the contact between the grinding wheel and the entire length of the workpiece surface. It is worth mentioning that the workpiece surface standardization operation is commonly used in the grinding process since the workpiece was undergone other manufacturing processes prior to the grinding process. These processes, such as milling and turning, hardly achieve the surface finish required by the specifications, thus making the surface of the workpiece uneven to the full contact of the grinding wheel, justifying the need of the surface standardization operation.



Regarding the monitoring of the process, two piezoelectric diaphragms (buzzers) were attached to the workpiece holder on opposite sides. Prior to the tests, the sensors and data acquisition system were properly calibrated for good signal sensitivity and saturation. The piezoelectric diaphragms used in the tests consisted of a disc-shaped lead zirconate titanate (PZT) ceramic (active element) with a diameter of 10 mm concentrically adhered to a brass disc with a diameter of 20 mm. In addition, the transducers were placed in a metallic case that allowed for a magnetic attachment to the workpiece holder. The experimental setup is shown in Figure 3. Some sensors besides the piezoelectric diaphragm (PZT) can be seen in Figure 3, however, such sensors were not considered in this work. It can also be observed in Figure 3 the metallic case and its connections of the emitter PZT 1 and receiver PZT 2; the latter was attached on the opposite side of the workpiece holder. In order to prevent displacement, a silicone layer was applied around both sensors. Regarding the position of the sensors, both transducers were fixed at a distance of 95 mm from the end of the holder. The transducers have a capacitance of 10 nF and a resonance impedance of 350 Ω, according to the manufacturer's data. Finally, the interface between the surfaces of the transducer case and the workpiece holder was filled with oil. It is worth mentioning that a thin layer of silicon was placed around the piezoelectric transducer in order to protect it from the cutting fluid and prevent displacements during the grinding process.

The raw signal acquisition was carried out by an oscilloscope, model DL850, from Yokogawa company (Tokyo, Japan), at a sample rate of 5 MS/s. An amplifier model AD8606 (Analog Devices, Norwood, MA, USA), with simple input and gain of 50, was used to amplify the signals from the piezoelectric diaphragm set as receiver. A data acquisition system (DAQ), USB-6361 model, from National Instruments (Austin, TX, USA), was employed to generate the excitation signal of the PZT diaphragm set as emitter. The excitation signal consisted of a chirp interval of 500 ms with a frequency variation from 1 to 250 kHz. In addition, five equal packages containing the chirp signal were sent to the emitter PZT. Thus, the excitation signal was composed of five equal chirp packages sent at 250 ms intervals. The PZT excitation by various chirp signals allows the verification of the repeatability of the results of the digital signal processing. Finally, the emitter PZT was connected to the DAQ which generated the excitation signals (chirp) and the receiver PZT was connected to the signal conditioner (amplifier module). Then, the signal conditioner and the DAQ were connected to the oscilloscope in order to collect the emission and reception raw signals.
