Thermal Flowmeter Sensor Element Design for Particle Adherence
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Solution Overview
Problem
Thermal-type flowmeters used in internal combustion engines face challenges in maintaining measurement accuracy due to contamination from buoyant particles like carbon particles, which adhere to the sensor elements due to the thermophoresis effect, leading to reduced sensitivity and increased power consumption.
Innovation Solution
The flowmeter design includes a semiconductor substrate with a heating resistor and temperature-measuring resistors, where the heating resistor is positioned with specific dimensions relative to the hollow portion to minimize particle adherence, and the heating temperature is raised to enhance sensitivity and reduce power consumption, with the heating resistor extending along longer sides and folded back at ends, and temperature-measuring resistors positioned closer to the center to concentrate heat distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the heating temperature of the heating resistor is raised to enhance sensitivity and reduce power consumption, then measurement sensitivity is improved, but buoyant particles adhere more to the sensor element due to thermophoresis effect
Solution Approach 1:
The patent introduces a new spatial dimension by positioning the heating resistor above a hollow portion (cavity) in the semiconductor substrate. This vertical arrangement creates a temperature gradient primarily in the vertical direction, causing buoyant particles to move vertically away from the sensor surface rather than adhering to it, thereby resolving the contradiction between high heating temperature and particle adherence
Solution Approach 2:
The hollow portion (cavity) acts as an intermediary structure between the heating resistor and the sensor element surface. It mediates the thermal field distribution and particle movement, allowing the heating resistor to operate at high temperature while preventing direct particle-sensor contact through the cavity space
2Speed
If the heating resistor is made small to achieve fast response and low power consumption, then response speed is improved, but the temperature difference for flow detection becomes insufficient
Solution Approach 1:
The patent applies local quality by concentrating the heating function in a small, localized heating resistor with fast response, while the hollow portion extends the thermal influence zone vertically. This allows the heating element itself to remain small for fast response, while the overall system maintains sufficient temperature difference for accurate flow detection
3Speed
If the electric insulating film is made thin to reduce heat capacity and improve response, then response speed is improved, but the film becomes weaker and more susceptible to damage
Solution Approach 1:
The hollow portion introduces a vertical dimension that redistributes mechanical stress away from the thin film plane. By positioning the heating resistor above the cavity, the film experiences reduced in-plane stress concentration, allowing it to remain thin for fast thermal response while maintaining adequate mechanical strength through the structural support provided by the cavity geometry
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
This configuration effectively prevents buoyant particle adherence, enhancing flow-rate detection sensitivity and reducing power consumption by maintaining a high heating temperature while minimizing thermophoresis effects, thus ensuring reliable and accurate measurements in harsh environments.
Implementation Method 1
a sensor element which is provided in the passage and measures a flow rate of the measurement-target fluid by radiating heat from the heating resistor to the measurement-target fluid
Implementation Method 2
buoyant particles such as carbon particles are deposited on the electric insulating film on the surface of the hollow portion of the sensor element due to a thermophoresis effect
Data Source
AI summary
The present invention provides a highly-sensitive thermal-type flow-rate sensor with enhanced reliability. Provided is a thermal-type flow-rate sensor including: a passage into which a measurement-target fluid is introduced; and a sensor element (1) which is provided in the passage (21) and which measures the flow rate of the measurement-target fluid. The sensor element (1) includes: a semiconductor substrate (2); a hollow portion (29) formed in the semiconductor substrate (2); and a heating resistor (5) formed on an electric insulating film (3a, 3b) above the hollow portion (29). The sensor element (1) measures the flow rate of the measurement-target fluid by radiating heat from the heating resistor (5) to the measurement-target fluid. When Lh is the length of the heating resistor (5) in a direction perpendicular to a flowing direction of the measurement-target fluid and Wd is the shortest distance to an upstream-side edge of the heating resistor (5) from an edge of the hollow portion (an outer peripheral edge of a diaphragm) in the flowing direction of the measurement-target fluid, Wd ≥ 0.4 × Lh is satisfied in a relation between Lh and Wd.