Inductive Respiration Sensor Circuit Without Frequency Locking
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional inductive respiration sensors face challenges in accurately measuring respiration parameters due to frequency locking issues with external magnetic fields and high power requirements, particularly when using LC oscillators or voltage dividers, which conflict with low power and cost requirements.
Innovation Solution
An inductive respiration sensor employing a transimpedance amplifier with a feedback loop and a synchronous rectifier, coupled with an AC-coupled inductive transducer and a signal generator, reduces input impedance and eliminates the need for high-gain amplification, allowing for lower power consumption and potentially lower production costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an LC oscillator is used to measure inductance changes, then the oscillator frequency indicates the inductance, but the LC oscillator may lock its frequency to an external magnetic field with a frequency close to the tank frequency
Solution Approach 1:
The patent extracts the measurement function from the oscillation frequency itself and separates it into two independent parts: a stable reference oscillator that provides a fixed frequency, and a separate measurement mechanism that uses this reference to detect inductance changes without being affected by external magnetic fields. This separation eliminates the frequency locking problem by removing the vulnerable feedback loop between the tank circuit and external fields.
Solution Approach 2:
The patent introduces a reference oscillator as an intermediary element that mediates between the inductive sensor and the measurement system. Instead of directly measuring oscillation frequency that can be locked by external fields, the reference oscillator provides a stable intermediate signal that is used to modulate or compare against the sensor output, thereby isolating the measurement from harmful external magnetic field interactions.
2Measurement precision
If a voltage divider driven by a high-frequency periodic signal is used, then the amplitude of the divider output voltage is indicative of the inductance, but a large current must be used to generate a usable divider output voltage
Solution Approach 1:
The patent replaces the high-current analog voltage divider approach with a transimpedance amplifier that converts the inductive sensor's current output directly into a voltage signal. This substitution eliminates the need for large currents by using a high-impedance input stage that can measure small sensor currents without requiring significant power, thereby solving the power consumption problem while maintaining measurement precision.
3Measurement precision
If a voltage divider is used, then the amplitude of the divider output voltage is indicative of the inductance, but a very small output voltage must be amplified, conflicting with the low power and low cost requirements
Solution Approach 1:
The patent substitutes the complex multi-stage amplification chain required for small voltage signals with a single transimpedance amplifier stage that directly converts the sensor's current output to a usable voltage. This substitution simplifies the circuit by eliminating the need for high-gain voltage amplification, reducing both device complexity and associated power consumption while maintaining measurement accuracy.
4Measurement precision
If a higher operating frequency is employed to extract a measurable signal from the voltage divider, then the signal becomes measurable, but this conflicts with the low power requirement
Solution Approach 1:
The patent changes the fundamental measurement parameter from voltage amplitude (which requires high frequency or high current) to current measurement. By using a transimpedance amplifier with high input impedance, the system can measure the small currents generated by the inductive sensor at lower frequencies, thereby achieving measurable signals without increasing power consumption or operating frequency.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively measures respiration parameters with reduced power requirements and lower production costs, while minimizing interference from external magnetic fields and harmonics, providing a more efficient and cost-effective sensing mechanism.
Implementation Method 1
an inductive transducer configured to produce a variable inductance when subjected to mechanical deformation (caused by breathing)
Implementation Method 2
the first and second transistors form together a negative feedback control loop, in which the first transistor is the actuator and the second transistor takes the role of the controller
Data Source
AI summary
An inductive respiration sensor (10) comprises an inductive transducer (12) and sensing circuitry. The sensing circuitry comprises a transimpedance amplifier, TIA, (14), the sense input (16) of which is operatively connected to the inductive transducer. The TIA is configured to drive a current from its output (20) into the sense input such that the voltage on the sense input follows the voltage applied to the reference input (18) and to cause a voltage on the output indicative of the current. The TIA comprises a first (34) and a second (30) transistor. The collector or drain and the emitter or source of the first transistor are operatively connected between the sense input and the output of the TIA. The emitter or source of the second transistor is connected to the reference input, the collector or drain of the second transistor is connected to the base or gate of the first transistor, and the base or gate of the second transistor is connected to the sense input.