MEMS Thermal Time-of-Flight Flow Sensor
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Solution Overview
Problem
Conventional gas and liquid flow meters are limited by the consistency of the flow media composition, and traditional thermal time-of-flight sensors are vulnerable to humidity and particle damage, affecting accuracy and reliability.
Innovation Solution
A MEMS thermal time-of-flight silicon flow sensor with serpentine-shape thermistors on a heat-isolated membrane, using a single frequency sinusoidal signal to measure flow speed, and a hydrophobic coating to prevent particle adherence, offering improved accuracy and durability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional platinum wire heater and sensing element are used in time-of-flight sensor, then the sensor can measure flow speed, but the sensor becomes vulnerable to humidity and particle damage affecting accuracy and reliability
Solution Approach 1:
The patent uses a thin film membrane structure to mount the heater and sensing element, replacing the traditional rigid platinum wire configuration. This membrane structure provides mechanical protection while maintaining thermal conductivity, thereby reducing vulnerability to particle impact and humidity effects.
Solution Approach 2:
The patent employs composite material structures combining different materials with complementary properties - including hydrophobic coatings on the membrane surface to repel particles and moisture, and thermally conductive materials to maintain heat transfer efficiency. This composite approach simultaneously improves durability and resistance to environmental factors.
2Measurement precision
If the heater and sensing element are installed in the middle way of the flow path, then the sensor can measure flow speed, but particles with high flow speed could damage the wires and cause failure of operation
Solution Approach 1:
The thin film membrane structure distributes the mechanical stress from particle impact across its surface rather than concentrating it on thin wires. The membrane's flexibility allows it to deform slightly under particle impact without breaking, while still maintaining thermal contact for accurate measurement.
Solution Approach 2:
The patent uses a disposable membrane structure that can be easily replaced if damaged, rather than using expensive, fragile platinum wires. The membrane is designed to be sacrificial - if particles do manage to damage the sensing area, the entire membrane can be replaced without replacing the expensive electrical components.
3Measurement precision
If conventional time-of-flight sensors are used, then flow speed can be measured, but the sensor performance alters substantially due to humidity affecting thermal conductivity
Solution Approach 1:
The thin film membrane provides a physical barrier between the thermal sensing elements and the humid environment. By mounting the heater and sensor on this membrane rather than using exposed wires, the structure protects the thermal conduction path from humidity-induced contamination while maintaining sufficient thermal conductivity for accurate measurement.
Solution Approach 2:
The patent uses composite material structures including hydrophobic coatings on the membrane surface. These coatings repel water and humidity, preventing moisture from reaching the thermal conduction path. The composite structure combines the thermal conductivity needed for measurement with the humidity resistance needed for stability.
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 MEMS sensor provides accurate, composition-independent flow measurements with enhanced durability and reduced power consumption, achieving high reproducibility and dynamic range, suitable for applications like city gas custody transfer and medical flow rate monitoring.
Implementation Method 1
both heater and sensing element are having electric current passed through
Implementation Method 2
the amount of heat that has been carried away by the flow media
Implementation Method 3
both heater and sensing element are disposed on a heat-isolated membrane
Implementation Method 4
a hydrophobic coating to prevent particle adherence
Implementation Method 5
it measures the time span for heat wave that is carried away by the flow fluid to travel from upstream of heater position to the downstream sensing element position
Data Source
AI summary
An apparatus comprising a micromachined (a.k.a. MEMS, Micro Electro Mechanical Systems) silicon flow sensor, a flow channel package, and a driving circuitry, which operates in a working principle of thermal time-of-flight (TOF) to measure gas or liquid flow speed, is disclosed in the present invention. The micromachining technique for fabricating this MEMS time-of-flight silicon thermal flow sensor can greatly reduce the sensor fabrication cost by batch production. This microfabrication process for silicon time-of-flight thermal flow sensors provides merits of small feature size, low power consumption, and high accuracy compared to conventional manufacturing methods. Thermal time-of-flight technology in principle can provide accurate flow speed measurements for gases regardless of its gas compositions. In addition, the present invention further discloses the package design and driving circuitry which is utilized by the correlated working principle.