Constant Temperature Difference Mode Research Articles

A Smart Dual-Mode Calorimetric Flow Sensor

A smart thermal flow sensor system is presented. It makes use of a novel heater control circuit, which can automatically set the operating mode to either constant power or constant temperature difference. It overcomes the limitations of single-mode thermal flow sensors, such as temperature overshoots at low flow rates at constant power mode, or excessive power consumption at high flow rates at constant temperature difference mode. The system is especially useful for temperature sensitive and portable applications, such as respiratory monitoring for medical diagnostics. In this article, a detailed description of the sensor’s design, implementation, and experimental validation are presented. The proposed dual-mode flow sensor achieves an overtemperature reduction up to 9.5% compared with thermal flow sensors operating in constant power mode alone, and a power reduction up to 13.6% compared with thermal flow sensors operating in constant temperature difference mode alone for the flow range of 0 to 50 slm while offering an improved overall sensitivity.

Effect of Insulation Trenches on Micromachined Silicon Thermal Wind Sensors

In this paper, the effect of insulation trenches on micromachined silicon thermal wind sensors is explored. The sensor is composed of a central heater and four thermistors. It measures wind speed and direction through detecting the airflow-induced temperature difference on the chip surface. Three different forms of insulation trenches are designed and fabricated to explore their effects on the performance of wind sensors. The experimental results show that when the sensors work in constant voltage mode, the center and arch trenches enhance the performance, while the arch trenches have the most significant effect on the sensitivity improvement. Moreover, all the three kinds of insulation trenches reduce the power consumption for the sensors operated in constant temperature difference mode. The insulation trench design in this paper can provide a valuable reference for the design and fabrication of micromachined silicon thermal wind sensors.

Fabrication of a Micromachined Two-Dimensional Wind Sensor by Au–Au Wafer Bonding Technology

The design, fabrication, and performance of a micromachined 2-D wind sensor are presented. The sensor operates based on the detection of temperature and flow-dependent heat distribution on a hot sensing surface. It consists of a silicon sensing chip and a ceramic packaging substrate, in which the sensing chip is bonded to the front side of the ceramic packaging substrate through wafer-level gold bumps. The backside of the ceramic substrate provides a smooth surface for the sensor exposed to the wind flow. A silicon diaphragm was fabricated by wet etching to minimize its heat capacity, resulting in the improvement of the power consumption, response time, and resolutions. Experimental results show that the measurement of wind flow speed is demonstrated in the range from 0.5 to 40 m/s with the sensitivity more than 2.73 mW/ms-1. The sensor requires only 2 mW initial heating power, and in constant-temperature difference mode, the response time less than 1.4 s is obtained. By measuring temperature difference in two directions perpendicular to each other, the detection of direction in a full range of 360 has been achieved. The errors in the measured wind speed and direction after calibration are ±4% and ±2°, respectively.

실시간 제어기를 이용한 마이크로 열식 질량공기 유량센서의 열특성 측정

A thermal mass air flow sensor, which consists of a micro-heater and thermal sensors on the silicon-nitride thin membrane structure, is micro-fabricated by MEMS processes. Three thermo-resistive sensors, one for the measurement of microheater temperature, the others for the measurement of membrane temperature upstream and downstream of the micro-heater respectively, are used. The micro-heater is operated under the constant temperature difference mode via a real time controller, based on inlet air temperature. Two design models for microfabricated flow sensor are compared with experimental results and confirmed their applicabilities and limitations. The thermal characteristics are measured to find the best flow indicator. It is found that two normalized temperature indicators can be adopted with some advantages in practice. The flow sensor with this control mode can be adopted for wide capability of high speed and sensitivity in the very low and medium velocity ranges.

A system-level model for a silicon thermal flow sensor

A system-level model with lumped parameters for a thermal flow sensor is presented. The model is built with 13 circuit cells consisting of thermal resistors and thermal capacitors in SPICE. The circuit cell originates from the heat conduction equation using the Finite Differential Method, including the 2-D thermal conduction cell, the convection cell, and the thermal capacity in the chip. Based on the thermal model of the flow sensor, the 2-D temperature distribution of the chip can be calculated with SPICE in both the constant power mode (CP) and constant temperature difference mode (CTD). As an example, the system level model of the thermal anemometer in the CTD mode was established in PSPICE. Wind tunnel test was carried out to verify the system model, and show a reasonable agreement with the simulation results, with an error less than 8%.

A self-heated termistor flowmeter for small liquid flow in microchannels

A self-heated thermistor flowmeter, based on two tiny, glass-encapsulated, NTC thermistor probes, for measuring liquid flow as small as tenths of a millilitre per minute with long-term stability and fast dynamic response has been developed. A new circuit configuration, mainly consisting of a multiplier as well as an active bridge for computing the power dissipated within the measurement probe, which is taken as the output of the thermistor flowmeter, is presented. The circuit configuration ensures that the thermistor flowmeter really operates in constant temperature-difference mode, and combining the temperature characteristic linearization method for NTC thermistors derived here, allows an easy, accurate fluid temperature-dependence compensation to be made. The compensation network needs only one adjustable parameter, the optimal value of which can be found through a simple computation or experimental adjustment. Thermistor probes with matched temperature coefficients are unnecessary. The flowmeter will be used to measure the liquid flow in microchannels in flow injection analysis.