## 6 Designed Architecture of the Smart Sensory Glove Based on AlN-Based Sensors
### Development and Testing of the Conditioning Section for the AlN-Based Flexible Sensors
After performing two simulations with two different values of \(\mathrm{Ri}\), (one with a value of \(100\) k\(\Omega\) and the second one with a value of \(1\) k\(\Omega\)), the same values of \(\mathrm{f}_{\mathrm{Z}}\) and \(\mathrm{f}_{\mathrm{L}}\) were obtained, while as regards \(\mathrm{f}_{\mathrm{H}}\), the value obtained (with Ri equals to \(1\) k\(\Omega\)) is two decades greater than that found with \(\mathrm{Ri}=100\) k\(\Omega\). The amplitude of the signal generated from the piezoelectric sensor is typically low, as was previously mentioned. Thus, well-designed filtering and amplification are essential within the conditioning chain. The input signal's informative frequencies should pass through a properly constructed filter, whereas other components should be rejected. Moreover, the filter typology and order should be chosen based on the specific application. A Bessel filter should be employed if the piezoelectric pulse response's shape is essential. A Butterworth filter should be deployed if a flat frequency response in the pass-band is necessary, but the pulse response is not crucial. Indeed, the latter is the filter type used in our work since it meets our design's requirements [129]. We deployed an active band-pass filter based on Sallen-Key cells with Butterworth's response (Figure 25). Since the main focus was to obtain a band-pass filtering stage, according to the characteristics of the preamplifier stage of the abovementioned charge amplifier, the filter's bandwidth was defined by f\({}_{\rm L}\)= 159 Hz and f\({}_{\rm H}\)= 1.59 kHz, which are, respectively, the low and high cut-off frequencies. f\({}_{\rm L}\) and f\({}_{\rm H}\) are related to the quality factor Q and the center-band frequency f\({}_{0}\) by these two relations:  
\[\rm{f_{L}}=f_{0}\cdot[\sqrt{1+\frac{1}{4Q^{2}}}-\frac{1}{2Q}] \tag{8}\]  
\[\rm{f_{H}}=f_{0}\cdot[\sqrt{1+\frac{1}{4Q^{2}}}~{}+\frac{1}{2Q}] \tag{9}\]  
Applying the frequency conditions to the Equations (8) and (9), the values of the quality factor (Q) and f\({}_{0}\) resulted \(\rm{\frac{\sqrt{10}}{9}}\) and 503 Hz, respectively. We assume that values of all resistors and capacitors are R1 = mR, R2 = R, and C1 = C2 = C = 10 nF. Therefore, considering the calculated value of the quality factor Q and assuming a unitary static gain for the first stage (high-pass filtering), the following values of \(\rm{\sqrt{m}}\) and R are derived:  
\[\rm{Q=}\frac{\sqrt{10}}{9}=\frac{\sqrt{m}}{2+m(1-K)}~{}\Rightarrow\sqrt{m}~{ }\cong~{}0.701 \tag{10}\]  
\[\rm{f_{L}=159~{}Hz=}\frac{1}{2\pi RC\sqrt{m}}~{}\Rightarrow~{}R=\frac{1}{2\pi \cdot 159~{}Hz\cdot C\sqrt{m}}=140~{}K\Omega \tag{11}\]  
Stepping into the development of the next stage (the low-pass filter), some desirable features we wanted for the low-pass filter were a flat-like signal attenuation near the cut-off frequency. This requirement was satisfied by a Butterworth-type filter. Considering the sizing method proposed in [134], the same values for both resistors and capacitors as R3 = R4 = R, C3 = C4 = C = 10 nF were assumed. Since the filtering stage must have an amplifying characteristic, a static gain of K = 1+\(\rm{\frac{R6}{85}}\)= 2 was set, introduced by the resistors R5 = R6 = 10 K\(\Omega\). Therefore, knowing the value previously already defined for f\({}_{\rm H}\)= 1.56 KHz, the following R and Q values were given:  
\[\rm{f_{H}=1.59~{}KHz=}\frac{1}{2\pi RC}~{}\Rightarrow~{}R=\frac{1}{2\pi\cdot 1.59~{}KHz\cdot C}=10~{}K\Omega \tag{12}\]  
\[\rm{Q=}\frac{1}{3-K}=1 \tag{13}\]  
Figure 23:0 Overall schematic of the designed Band-pass filter based on Sallen–Key cells. Due to technical reasons and the independence of the actual stage's quality factor from the previous one, an \(t_{\text{H}}\) value equal to 2 KHz was preferred. Considering this variation, a different value for R was also determined, that is, 8.2 K\(\Omega\) (according to the available commercial resistors' values).