Thermal Flow Sensor Bridge Circuit Stress Isolation
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Solution Overview
Problem
Thermal type flow sensors face issues with thermal deterioration of resistive elements, leading to measurement precision degradation over time, due to varying temperature conditions and stress applied by adhesive and sealant expansion/contraction, which affects the bridge balance and response speed.
Innovation Solution
The sensor employs multiple heat resistive elements with a driver circuit that senses resistance changes to control heating current, optimizing temperature distribution and reducing stress impact by positioning resistive elements on a semiconductor substrate with a thin-walled portion and using a bridge circuit configuration to maintain precision and response speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a heat resistive element is used at high temperature to measure flow rate, then the measurement capability is improved, but the element undergoes thermal deterioration over time
Solution Approach 1:
The patent changes the material parameter of the resistive element from conventional metal to semiconductor material. Semiconductor materials can operate at high temperatures required for flow measurement while exhibiting different thermal degradation characteristics compared to metals, thereby maintaining measurement precision over extended periods.
Solution Approach 2:
The patent employs composite material structures where semiconductor resistive elements are integrated with ceramic substrates or other heat-resistant materials. This composite approach allows the resistive element to withstand high temperatures for accurate flow measurement while the composite structure provides thermal management and structural stability.
2Strength
If adhesive and sealant are used to mount the sensor element, then the mechanical fixation is improved, but stress is transmitted to the resistive element causing resistance variation
Solution Approach 1:
The patent applies local quality by creating stress-isolated zones around the resistive elements. The mounting structure is designed so that adhesive and sealant are applied in locations that do not directly transmit stress to the resistive elements, while still providing adequate mechanical fixation. This localized stress management maintains both mounting strength and measurement precision.
Solution Approach 2:
The patent introduces intermediate structural layers or mounting configurations that act as mediators between the adhesive/sealant and the resistive elements. These intermediate structures absorb or distribute the thermal expansion stress, preventing direct stress transmission to the sensitive resistive elements while maintaining secure mechanical attachment.
3Strength
If the sensor element is fixed with adhesive and protected with sealant, then the mechanical protection is improved, but the measurement precision deteriorates due to stress-induced resistance variation
Solution Approach 1:
The patent implements local quality by differentiating the mechanical protection requirements in different areas of the sensor element. Critical areas with resistive elements receive minimal or stress-free protection, while non-critical areas receive full adhesive and sealant protection. This selective approach maintains overall mechanical protection while preserving measurement precision in sensitive regions.
Solution Approach 2:
The patent uses intermediate stress-relief structures between the protective adhesive/sealant layers and the sensor element. These intermediates include compliant mounting layers or isolated fixation points that provide mechanical protection and environmental sealing while decoupling thermal expansion stresses from the resistive elements, thereby maintaining measurement precision.
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 reduces thermal deterioration, maintains measurement precision over time, and enhances response speed by controlling the temperature of heat resistive elements while minimizing the influence of stress, resulting in improved long-term performance and accuracy.
Implementation Method 1
a heat resistive element (temperature sensitive resistive element) having a temperature dependency
Implementation Method 2
the thermal type flow sensor controls a current flowing through a heat resistive element so as to maintain a temperature difference between the heat resistive element whose heat is taken away with the fluid to be measured and a temperature compensation resistor
Data Source
AI summary
A thermal type flow sensor measures a flow rate of a fluid by means of a heat resistive element having a temperature dependency. The sensor is comprised of: plural heat resistive elements used for a flow rate measurement; and a driver circuit for controlling a current applied to these heat resistive elements to cause their heating. The driver circuit is configured to sense a resistance change of a lower-temperature side heat resistive element among the plural heat resistive elements and to control the current to be applied to the plural heat resistive elements in accordance with a sensed value of the lower-resistance's variation.