Patent Application
20080236273
Embodiments are generally related to sensing devices and components. Embodiments are also related to mass fluid flow sensors. Embodiments are additionally related to resistive temperature sensors used to detect mass airflow.
Sensors are used in a variety of sensing applications, such as, for example, detecting and/or quantifying the composition of matter, detecting and/or quantifying the presence of a particular substance from among many substances, and detecting and/or quantifying a mass flow rate of fluid (e.g., in air liquid form). The industrial, commercial, medical, and the automotive industries in particular require many ways to quantify the amount of gaseous and liquid mass flow rates. For example, in the medical industry, an airflow sensor is often employed to monitor and/or control a patient's breathing. Two examples of this include sleep apnea devices and oxygen conserving devices. Similarly, airflow sensors are often employed in microcomputer cooling units to detect the presence and amount of local airflow in, through, and around the cooling units.
Historically, mass flow sensors have been constructed with one temperature-sensing resistor “upstream” and one temperature sensing resistor “downstream,” where “upstream” and “downstream” generally indicate the direction of mass flow. One advancement in mass flow sensors in microchip environments, the “Wheatstone bridge” circuit, is often configured with external, off the chip, resistors. This historical configuration can be improved as described by the inventors by implementing a full Wheatstone bridge, all four resistor branches, each having a temperature sensing resistor, and can be formed on a sensing chip, to allow for an increase in sensitivity, increase the sensitivity to offset ratio of the signal and can be measured from the circuit, and decrease the bias voltage needed to be applied to the mass airflow sensor.
A Wheatstone bridge can be used to detect mass flow. For example, in a “full” Wheatstone bridge configuration, all four legs comprise variable resistors. In one configuration, resistive temperature detectors-resistors that vary in resistance with temperature are used in each leg. A heating element situated between the two sides creates a roughly even thermal distribution about the heating element. As air, for example, passes from one side to the other side of the bridge, heat is conducted away from the “upstream” side of a unit to the “downstream” side of the unit, cooling the upstream side and heating the downstream side.
As the resistance of the two sides varies with temperature, the resultant temperature differential between the two sides causes a measurable voltage difference between the two sides. This voltage difference can be correlated to the difference in temperature. As the temperature change is a function of the air mass flow rate, the voltage difference can also be correlated to the mass flow rate.
Previous full Wheatstone bridge configurations, however, also often incur a low signal to noise ratio, particularly for very high or very low flow rates. A low signal to noise ratio reduces the accuracy and resolution of the bridge measurements and can cause difficulties in quantifying the mass flow rates under investigation.
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Therefore, what is required is a system, apparatus, and/or method that provides an improved sensitivity to high and/or low flow rates that overcomes at least some of the limitations of previous systems and/or methods. The present invention will increase the sensitivity of the mass airflow sensor, increase the sensitivity to offset ratio of the signal, and decrease the bias voltage needed to be applied to the sensor.
The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments disclosed and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is, therefore, one aspect of the present invention to provide for an improved mass airflow sensing device.
It is another aspect of the present invention to provide for a sensor with an increased sensitivity.
It is another aspect of the present invention to provide for a sensor with increase in the sensitivity to offset ratio of the signal.
It is further aspect of the present invention to provide for a sensor to decrease the bias voltage needed to be applied to the sensor.
The aforementioned aspects and other objectives and advantages can now be achieved as described herein. A mass airflow sensing apparatus is disclosed that includes a heating element comprising an upstream side and a downstream side. Two resistive temperature sensors are placed on each side of the heating element and assuming mass air/liquid flows in a direction from the upstream side to the downstream side of the unit. The resistors are configured electrically in a Wheatstone bridge configuration. A regulated voltage is applied across the mass flow sensing, Wheatstone bridge. The regulated voltage is set high enough to produce self-heating effects on the sensing bridge. The central heating element located within the Wheatstone bridge configuration between upstream and downstream resistors, will also be heated. As mass air/liquid flows across the temperature sensors and the heating element, the upstream (RU1 and RU2) resistors are cooled by incoming fluid flow and the downstream (RD1 and RD2) resistors are heated by the flow over the heating element. The resistance in the resistive temperature sensors changes with temperature creating a differential voltage signal proportional to the regulated voltage applied to the sensing Wheatstone bridge and rate of mass air/liquid flow.
The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description, serve to explain the embodiments disclosed herein.
The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof.
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As described at block 506, the low-level analog signal from the resistive temperature sensing Wheatstone bridge is converted to digital form at A/D converter. Temperature compensation of the signal occurs at digital core, as indicated at block 507. The D/A converter convert the signal to analog ratiometric output which is the ratio of D/A converters input to its reference voltages, as illustrated at block 508. The process can then terminate, as indicated at block 509.
It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.
