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Respiratory Inductance Plethysmography (RIP) sensor functioning is based on concepts of electromagnetism, such as Inductance, like the name suggests.
Plux’s RIP sensor is formed by an elastic band with an electric conductor inside. This electric conductor is rolled, acquiring a sinusoidal morphology and forming a coil/inductor. The “core” of this inductive system will be the human body.
When an electric current pass through the inductor a magnetic field (B) is generated around it. The magnetic field is dependent of inductor core cross-sectional area (A) and the angle (θ) formed between the magnetic field vector and our cross-section ☍, like demonstrated in the following expression. For our case the cross-sectional area is the cross-section formed by ribcage.
being Φ the magnetic flux.
During inhalation/exhalation of the volunteer, the ribcage cross-sectional area varies, which modifies the magnetic flux value.
A change in magnetic field (B), produced by a current I, causes a variation of the magnetic flux (Φ) and, consequently, in our coil inductance (L), described mathematically by the next expression, a phenomenon that is called self-induction ☍.
Inductance is “…the property of a component that opposes the change of current flowing through it…” ☍.
The inductor/coil is connected to an oscillator circuit, composed by an odd number of current inverters ☍. Due to the change in inductance, the current that enters in the oscillator circuit will change, causing a characteristic variation in the oscillation frequency ☍.
Finally, the oscillation frequency of the circuit will be converted in an electric voltage, being formed our typical RIP signal during a respiratory cycle (the signal is proportional to the oscillation frequency).