Patent Application
20090164163
1. Field of the Invention
The present invention generally concerns thermal mass flow sensors, and more particularly, concerns thermal mass flow sensors made of micro-electro-mechanical systems (MEMS) approach, and methods of manufacturing and operating such mass flow sensors.
2. Description of the Related Art
Heat transfer principle has been widely used for mass flow measurements. Thermal mass flow sensors can be found in many applications in industrial process monitor and control, medical gas flow management, and environmental equipments, to name a few. One of the major concerns on the traditional mass flow sensors is the large power consumption used to heating the fluid flow that leads to an uncertainty in low flow measurement domain and restrains on some applications such as gas trade metrology. The difficulties of making identical sensors also make manufacture cost higher than those for other technology. MEMS technology, on the contrary, allows fabrication of thermal mass flow sensors directly on silicon with excellent reproducibility, low power, and high reliability at low cost. MEMS-based thermal mass flow sensors have been received increasing attentions in flow measurement applications.
Thermal mass flow sensors can be classified into three basic categories: anemometers, calorimetric flow sensors, and time-of-flight sensors. For simplicity, these three types of thermal mass flow sensors are hereinafter abbreviated as A-, C-, and T-type mass flow sensors, respectively. Traditional T-type mass flow sensors, such as that disclosed in U.S. Pat. Nos. 5,339,695 and 5,347,876 (Kang), utilize hotwire set apart at a certain distance to measure the heat pulse flight time between two wires so that the fluid flow speed can be determined. Hariadi et al (I. Hariadi, H.-K. Trieu, W. Mokwa, H. Vogt, “Integrated Flow Sensor with Monocrystalline Silicon Membrane Operating in Thermal Time-of-Flight Mode,” The 16th European Conference on Solid-State Transducers, Sep. 15-18, 2002, Prague, Czech Republic) disclose a time-of-flight flow sensor fabricated on Silicon-On-Insulator (SOI) wafers, in which heat pulse is fed to the fluid by a heater and a temperature sensor located downstream detects its delay. Measuring a flight time, the sensors give the velocity of the streaming fluid. However, the pulse will be deformed by the flow velocity profile and broaden at the same time by heat diffusion when it propagates down the stream. This means that the pulse tends to be too broad to be useful for slow flows and thus become inaccurate. Similar approaches have been adapted by U.S. Pat. Nos. 5,533,412 (Jerman) and 6,234,016 (Bonne).
Calorimetric flow sensors usually consist of a heater surrounded by temperature sensitive elements arranged symmetrically downstream and upstream. A moving fluid will carry away heat in the direction of flow and accordingly change the temperature distribution around the heater. The temperature difference between upstream and downstream is measured by the temperature sensitive elements. The output signal is commonly fetched using a Wheatstone bridge circuit, in which a pair of downstream and upstream sensing elements comprises two of its four branches. The output signal, which is a measure of temperature difference, is proportional to the flow velocity initially until a high flow velocity is reached where the temperature difference saturates and then decreases at higher flow velocity. In general, calorimetric flow sensors can accurately measure flows with extremely low velocities. However, calorimetric flow sensors may saturate at high flow velocities and hence have a difficulty to measure flows above a certain level of flow velocity. Many traditional thermal mass flow meters using capillary approach utilize this principle. Sensors made with this principle are disclosed such as in U.S. Pat. Nos. 5,014,552 (Kamiunten) and 6,550,324 (Mayer).
Jiang et al (F. Jiang, Y. C. Tai, C. M. Ho, and W. J. Li, “A Micromachined Polysilicon Hot-Wire Anemometer,” Digest Solid-State Sensors & Actuator Workshop, Hilton Head, S.C., pp. 264-267, 1994) disclose a micro-machined A-type flow sensor comprising of a single element, which is heated and the heat loss of which is measured. This heat loss is dependent on the flow rate of the fluid. This heat loss increases with the flow velocity, and the signal of an anemometer is proportional to the square root of the flow velocity. In general, A-type mass flow sensors are less sensitive and extremely noisy in small flows and hence cannot measure small flows accurately. Nevertheless, A-type mass flow sensors have demonstrated that they are capable of accurately measuring flows with high velocities. Hinkle disclosed in U.S. Pat. No. 5,461,913 that in a capillary tube configuration, a pair of A-type sensor can be installed for improved performance, but yet this capillary by-pass configuration shall not apply to flow in a large conduit.
In summary, a major concern is how to extend the measurable flow rate range to the low flow rate and at the same time to the high flow rate within a single MEMS mass flow sensor. Specifically, for those of ordinary skill in the art there is still a need to provide a MEMS mass flow sensor to expand the measurable flow rate range to low flow velocities with sufficient accuracy and at the same time to keep its capability of accurately measuring flows with high velocities.
It is an object of the present invention to provide an integrated MEMS mass flow sensor with a large measurable flow rate range. In addition, the present invention further discloses the methods for making and operating the mass flow sensors.
According to the present invention, the single integrated MEMS mass flow sensor in a preferred embodiment consists of two identical A-type mass flow sensors, each of which is composed of a heater and a reference resistor. The two A-type mass flow sensors are placed along the flow path: one is at an upstream location and another is located at a downstream site. Each of the A-type mass flow sensors can provide a flow signal which depends on the flow rate. In particular, the signal from the upstream A-type mass flow is able to accurately and independently measuring flows with high velocities. In addition, a differential signal of the above two A-type flow signals will be able to measure low flow velocities with sufficient accuracy. Such a differential signal will be called the differential A-type signal hereinafter.
According to the present invention, the reference resistor in a preferred embodiment is at least three-time larger than resistance of its heater counterpart. With increased resistance of the reference resistor, the power consumption by heater control circuit shall be reduced by 30% 50%.
In accordance with the present invention, the integrated MEMS mass flow sensor in a preferred embodiment will also include a downstream temperature sensing element and a upstream temperature sensing element placed to surround the heaters to form a C-type mass flow sensor to give an additional flow signal, which is capable of independently measuring low flows with high accuracies.
In accordance with the present invention, all the three signals: one C-type flow signal, one A-type flow signal, and one differential A-type flow signal will be inputted into a microcontroller unit through multiple or multi-channel analog-to-digital converters (ADCs). All these three signals can be processed in the microcontroller unit to give unique indications on flow velocity and flow concentration simultaneously.
In accordance with the present invention, the upstream and downstream heaters and the upstream and downstream temperature sensing elements are formed on top surface of a suspending membrane to achieve good thermal isolation. The suspending membrane is typically made of thin dielectric films such as silicon nitride, silicon dioxide, etc. The suspending membrane structure can be formed by an anisotropic bulk etching from the back side of silicon wafer. The heaters and temperature sensing elements are preferred to be made of high TCR (temperature coefficient of resistance) materials, such as platinum (Pt), gold (Au), silicon carbide (SiC), etc. With the high TCR property, the temperature sensing elements can attain high sensitivity. A thin layer of silicon nitride material is deposited on top of the entire MEMS chip as passivation material.
These and other objects, features and advantages of the present invention will no doubt become apparent to those skilled in the art after reading the following detailed description of the preferred embodiments that are illustrated in the several accompanying drawings.
The present invention can be better understood with reference to the following drawings. The components within the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the principles of the present invention.
In the operation of the flow sensor, the heaters 110-1 and 110-2 are energized to produce temperature elevations at the central area that are considerably higher than the ambient temperature. A temperature gradient thus is produced from the heaters to edges of the membrane area. The sensing resistors 115-1 and 115-2 preferably made of materials, exhibiting a high resistance dependence on temperature, preferably a high thermal coefficient resistance (TCR) (e.g., 3000 ppm/° C.˜8300 ppm/° C.). Accordingly, the resistances of the sensing resistors are caused to change in proportional to the temperature change in the area of the membrane containing the sensing resistors. The heaters 110-1 and 110-2 and reference resistors 120-1 and 120-2 preferably made of similar materials like the sensing resistors, exhibiting a high resistance dependence on temperature, preferably a high TCR. The reference resistors are located on the silicon substrate and thus their resistance changes with the ambient temperature.
As the flow moves along the direction from the upstream toward the downstream, it carries away heat from the heaters in the direction of flow. When the constant-temperature mode is used for controlling the two heaters, both heaters will consume more electrical powers for a larger flow rate. The measurements on the power consumptions in the two heaters will provide two A-type signals proportional to the flow rate. In particular, the A-type signal associated with the upstream heater can be used as an electrical signal for the flow rate. That is the principle of A-type flow sensing.
In addition, the heat carried away from the upstream heater flows down to the downstream heater, which will help the downstream heater to keep up with its temperature and hence reduce the power consumption of the downstream heater. The difference between the power consumptions between the upstream and downstream heaters will also be proportional to the flow rate, and thus provides a differential electrical signal for the flow rate. This is the principle of differential A-type flow sensing.
As the flow moves along the direction from the upstream toward the downstream, it changes the temperature distribution around the heaters. The temperature difference between upstream and downstream temperature sensing resistors can be measured by applying the sensing resistors 115-1 and 115-2. That is the principle of C-type flow sensing.
The circuit shown in
a and
The differential A-type configuration shown in
The C-type flow sensing implemented with the two-branch, Wheatstone circuit as shown in
As mentioned above, the temperature difference between upstream and downstream saturates once a flow reaches a certain velocity level hereinafter this velocity is referred to as the saturation velocity level. When the flow velocity is greater than the saturation speed, the temperature difference is decreased with the increase of the flow rate. Thus, the C-type flow sensing is not applicable for a flow larger than saturation velocity level. In accordance with the present invention, the differential A-type flow sensing or the traditional A-type flow sensing will be employed for high flows beyond the saturation velocity level.
The present invention describes an integrated MEMS mass flow sensor, which is capable of simultaneously combining differential A-type, conventional A-type, and C-Type operations to achieve significantly expanded measurable flow rate range. In addition, the present invention disclosed methods of operating such a high-performance integrated MEMS mass flow sensor.
In preferred embodiments of the present sensor, the three digital signals obtained from the differential (or downstream) A-type, upstream A-type, and C-type sensing signals via amplification, signal processing, and analog-to-digital conversions are processed by a digital processing unit and combined within this digital processing unit to provide a single measurement signal for the mass flow rate. Due to the A-type signals and the C-type signal have different dependencies on the fluid properties such as gas concentration in a gas mixture, an optional output signal can be provided for concentration measurement. Algorithms to generate such a single measurement signal for mass flow rate and/or a concentration measurement signal from the A-type and C-type digital signals are implemented either as a firmware or hardware within the digital processing unit.
In preferred embodiments of the present sensor, the digital processing unit monitors the differential A-type, upstream A-type, and C-type signals. The digital processing unit will, a priori, detect and record the characteristics of all the three signals, and is able to automatically switch between the differential A-type signal, the conventional A-type signal, and the C-type signal as well as seamlessly combine all three signals into a single measurement signal. As a consequence, the digital processing unit will use the differential A-type digital signal or the C-type digital signal for measurement of extremely small to medium flows, whereas employ the differential A-type digital signal or the conventional A-type digital signal for measurement of large flows. The conventional A-type digital signal contains plenty of concentration information, and be combined with the differential A-type or C-type digital signal to provide a concentration measurement signal.
According to
The digital signal processing units are provided with different flow-rate computational programs to compute the flow rates by using the digital signals measured by the flow sensors and transmitted and processed from the three types of controlling circuits. The flow-rate versus the three-signal calibration data table obtained in a calibration process is also available as a database table for the flow rate computations when these flow rate programs are executed.
Referring to
