Thermal Flow Sensor Startup Control via Dual Temperature Feedback
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Solution Overview
Problem
Conventional thermal flow sensors face challenges in achieving stable startup characteristics and preventing degradation due to variations in heat capacities of heaters and indirect heat resistors, which can lead to longer startup times and increased power consumption.
Innovation Solution
A thermal flow sensor with a heating controller that includes a first and second temperature-sensitive resistor, where the heating controller controls the heating resistor's temperature based on a target and escape temperature, using a second temperature control circuit to stabilize the startup process and prevent overheating, thereby reducing the impact of heat capacity variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a conventional indirect heat control scheme is used with small heaters and indirect heat resistors, then the sensor responds quickly and has small heat capacities, but the startup characteristics become unstable and degradation occurs due to variations in heat capacities
Solution Approach 1:
The patent applies preliminary action by introducing a startup control mode that activates before normal operation. During startup, the heating controller operates in a first control mode that differs from the normal indirect heat control mode, preparing the system in advance to avoid instability. This preliminary control action ensures stable startup characteristics before transitioning to the standard control mode.
Solution Approach 2:
The patent implements dynamics by making the control mode changeable based on operational conditions. The heating controller dynamically switches between a first control mode during startup and a second control mode during normal operation. This dynamic adaptation allows the system to optimize performance for different operational phases, ensuring stability during startup while maintaining quick response during normal operation.
2Speed
If the heater size is reduced to decrease heat capacity, then the response speed increases, but the startup time increases and power consumption increases due to control instability
Solution Approach 1:
The startup control mode performs preliminary heating and stabilization actions before normal operation begins. This preliminary phase prepares the heater and indirect heat resistor by establishing stable temperature conditions, preventing the need for extended startup times even with small heater sizes. The preliminary action ensures that the system reaches operational temperature efficiently without instability-induced delays.
Solution Approach 2:
The patent employs feedback mechanisms in both control modes to monitor and adjust heating based on actual temperature conditions. During startup, feedback from the indirect heat resistor allows the controller to adjust heating power dynamically, preventing overshoot and instability. This feedback control ensures that small heaters reach operational temperature quickly and stably, minimizing startup time while maintaining control precision.
3Measurement precision
If feedback control is implemented to maintain constant heater temperature, then temperature control accuracy improves, but control response delay occurs at startup due to small heat capacities
Solution Approach 1:
The first control mode during startup performs preliminary heating actions that are optimized for rapid temperature rise rather than precise maintenance. This preliminary phase establishes baseline temperature conditions quickly, after which the system transitions to the second control mode for precise temperature maintenance. The separation of preliminary heating and precise control functions eliminates the delay that would occur if precise control were attempted from the very beginning of startup.
Solution Approach 2:
The patent applies dynamics by implementing different control strategies at different operational stages. During startup, the control system uses a first mode optimized for rapid temperature increase, then dynamically transitions to a second mode optimized for precise temperature maintenance. This dynamic control approach allows the system to achieve both fast response during startup and high temperature control accuracy during normal operation, eliminating the trade-off between speed and 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 solution ensures stable and rapid startup characteristics, reduces the risk of sensor element degradation, and allows for a smaller heater size without increasing power consumption, while also serving as a fail-safe mechanism to prevent thermal runaway.
Implementation Method 1
a heating resistor (hereinafter, referred to as the 'heater') is formed on a dielectric film of several microns thick on the cavity portion
Implementation Method 2
a temperature-sensitive resistor (hereinafter, referred to as the 'detection resistor') is formed near a heater, and a flow rate is detected on the basis of the amount of heat transferred from a fluid flowing over the heater to the detection resistor
Implementation Method 3
a flow rate is detected on the basis of the amount of heat transferred from a fluid flowing over the heater to the detection resistor
Data Source
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AI summary
There is provided a thermal flow sensor including a heater temperature controller that realizes stable startup characteristics and preventing degradation of a sensor element and can also accommodate a smaller heater. The thermal flow sensor comprises: a semiconductor substrate 1; a cavity portion 2 provided in the semiconductor substrate 1 ; a dielectric film 4 provided on the semiconductor substrate 1 so as to cover the cavity portion 2; a thin layer area 5 formed as a result of the dielectric film 4 covering the cavity portion 2; a heating resistor 6 provided in the thin layer area 5 on the dielectric film 4; a first temperature-sensitive resistor 7 provided in the thin layer area 5 on the dielectric film 4, a resistance value of the first temperature-sensitive resistor varying depending upon temperature; a heating controller 8 that controls temperature of the heating resistor 6 on the basis of temperature of the first temperature-sensitive resistor 7; a second temperature-sensitive resistor 9 provided near the heating resistor 6, a resistance value of the second temperature-sensitive resistor 9 varying depending upon temperature; and a flow rate detector 10 that detects a flow rate of a fluid on the basis of temperature of the second temperature-sensitive resistor 9. The heating controller 8 controls the temperature of the heating resistor 6 on the basis of a first reference temperature, which is a target temperature of the first temperature-sensitive resistor 7, and a second reference temperature, which is an escape temperature of the first temperature-sensitive resistor 7.