## 4 Results
### Evolution of RIP
#### 4.1.1 Technological Advancements  
Due to the dependency of the RIP technique on wearable belts, the design and specifications of the wire woven or sewn along the belt appears to affect the sensor properties of the RIP. The work in [44] investigated the influence of the step size of the sine wave pattern on the characteristics of RIP sensors by building three RIP sensors with the same wire pattern (sine wave), height (2.2 cm), and length (30 cm) but variable step sizes, as shown in Figure 4. The inductance of each of these belts was measured using an Agilent 4285A Precision LCR Meter. It was observed by the authors that the smaller the step size, the greater the sensitivity. There was a tiny variance in linearity between the 1 cm and 1.5 cm step sizes. However, the linearity significantly reduced with a step size of 3 cm. The authors' conclusion directs that it is thus prudent to keep the step size modest in order to maintain linearity. In [10], the researchers conducted benchtop testing of PneuRIPTM to determine the circuitry and instrument design, reliability, responsiveness, and repeatability. The RIP bands were mounted on a laboratory-grade rocker/waver platform to mimic different breathing situations, and the tilt angles and speed were changed. Over two days, the testing ran without taking any pauses. The benchtop testing was performed along with testing of the Respitrace system (Sensormedics), an RIP system from 1991 [71]. With typical breathing frequencies and phase angles, both the PneuRIPTM and Respitrace systems produced similar results, but the phase angle variance was substantially reduced using PneuRIPTM  
Figure 4: RIP belt coil design with three different sine wave patterns with step sizes of (**a**.) 1 cm, (**b**.) 1.5 cm, and (**c**.) 3 cm [44]. for low and high phase angles, as well as low and high breathing frequencies. PneuRIPTM is more precise and reliable at greater and lower phase angles because the mechanical fluctuation is less. Children with respiratory distress frequently have large phase angles, which PneuRIPTM is intended to detect. According to one study [44], the step size should be reduced to attain adequate linearity. Over time, the RIP device has evolved depending on the applications. One important aspect of the RIP device is that it is a piece of oscillator-based hardware that requires calibration over some time. Additionally, it also requires several inductive sensors to obtain mutual inductance between RIP sensors. RIP calibration was studied in both healthy subjects and COPD patients in [74], and it was found that calibrating the RIP device in each posture produced less error in tidal volume measurements. Research investigations [45; 69] suggest adjusting the frequencies of the thoracic and abdominal band oscillators to minimize modulation and prevent frequency locking. Creating a dependable oscillator for each inductive sensor in a respiratory monitoring system while maintaining frequency separation can be challenging. Energy consumption is crucial for battery-powered portable devices; hence, a compact printed circuit board size is necessary for mobility. Thus, using several inductive sensors in a typical RIP architecture with an LC oscillator might lead to an increase in chip count and complexity in power management. To mitigate this problem, the study [68] devised an RIP system that uses traditional Electrical Impedance Plethysmography (EIP) to quantify the coils' self-inductance. Since a steady current travels through the belts, this approach does not require an LC oscillator. [PERSON]'s method incorporates a continuous AC signal which resolves the frequency locking but requires high power, which makes the system non-portable. Despite the use of a variable-frequency oscillator, the authors of [45] used a Pulse Amplitude Modulation (PAM) approach for indirect inductance measurement [68; 69; 72]. During the pulse excitation phase, the RIP sensor is periodically and regularly delivered a high current from a constant-current source, and its inductance is measured.