Hot-Wire Fluid Sensing Circuit With Hysteretic Pulsed Current
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Solution Overview
Problem
Existing hot wire fluid sensors require expensive amplifiers and large packaging due to high power dissipation, necessitating a more efficient method for measuring fluid level and flow.
Innovation Solution
A fluid sensing system utilizing a hysteretic controller with a MOSFET switch, inductor, diode, and current sensor to manage current thresholds, reducing power dissipation and eliminating the need for heat sinking and precision amplifiers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a constant current supply is used to measure voltage across the sensing element, then measurement precision is improved, but power dissipation increases and requires heat sinking
Solution Approach 1:
The patent uses periodic pulsed current instead of continuous constant current. The current is applied in short pulses only when needed for measurement, allowing the sensing element to cool down between pulses. This reduces average power dissipation while maintaining measurement precision during the pulse periods.
Solution Approach 2:
The sensing element itself serves as the heating element, eliminating the need for separate heating components. By using the sensing element's resistance to generate heat directly during measurement pulses, the system achieves both measurement function and heating function with a single component, reducing overall power requirements.
2Measurement precision
If expensive precision amplifiers are used to condition the signal, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent replaces expensive precision amplifiers with simpler, lower-cost electronic components. By using pulsed measurements and differential voltage reading techniques, the system achieves adequate precision without requiring high-end amplification circuitry, effectively substituting complex expensive components with simpler alternatives.
Solution Approach 2:
The system uses feedback through the hysteretic controller to maintain stable operating conditions. The controller monitors the voltage across the sensing element and adjusts the pulse timing and duration to keep the sensing element within optimal temperature ranges, providing automatic stabilization without complex amplification.
3Speed
If the sensing element is continuously heated, then measurement responsiveness is improved, but power dissipation increases
Solution Approach 1:
The sensing element is heated periodically with short pulses rather than continuously. The pulse duration and frequency are optimized so that the element reaches sufficient temperature for accurate measurement quickly, then cools down during idle periods. This periodic heating maintains measurement responsiveness while dramatically reducing average power consumption compared to continuous heating.
Solution Approach 2:
The system performs preliminary heating with short pulses before measurements are needed, allowing the sensing element to reach optimal temperature quickly. The pulse timing is arranged so that measurements are taken when the element is already warmed up, eliminating the need for continuous heating while maintaining measurement speed.
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 system achieves cost and space savings by minimizing power dissipation, allowing for accurate fluid depth and flow measurements without the need for heat sinking or precision amplifiers.
Implementation Method 1
a conductive material can dissipate heat more easily when submerged in a liquid having a specific heat that is lower than the specific heat of air than compared to when the conductive material is exposed to air
Implementation Method 2
the conductive material of the hot wire sensor has a positive temperature coefficient of resistance, and consequently, as electrical energy is applied to the conductive material, the conductive material increases in temperature and as the temperature of the conductive material increases, the resistance of the conductive material also increases
Data Source
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AI summary
A fluid sensing system (110) includes a sensing element (112) having a positive temperature coefficient of resistance. The system also includes a switch (121) which is switchable between an open position which prevents current from passing and a closed position which permits current to pass. An inductor (136) is connected electrically in series between the switch electric output (121b) and the sensing element (112) and a diode (138) is connected electrically in series between the sensing element (112) and the inductor (136) such that an output (138b) of the diode (138) is between the switch (121) and the inductor (136). A hysteretic controller (122) changes the switch (121) from the open position to the closed position when current passing to the sensing element (112) falls to a lower current threshold (IL) and changes the switch (121) from the closed position to the open position when current passing to the sensing element (112) rises to an upper current threshold (IL). A voltmeter (126) measures voltage across the sensing element (112).