The automotive industry in recent years has recognized the advantages of using electronic fuel management systems to improve vehicle performance over mechanically based fuel control systems. It has been predicted in view of the success of such electronic fuel management systems that in the not too distant future all major automobile manufacturers will turn to electronic control systems for monitoring and controlling major automobile subsystems.
To increase fuel efficiency and to meet tighter emission requirements such next generation electronic control systems will need more advanced and highly sophisticated sensors that can be cost effectively manufactured. The microprocessor, which is the heart of such an electronic control system, is capable of executing instructions on the order of magnitude of one million per second. A need has therefore arisen for mechanically rugged and reliable sensors which have an extremely fast response time. Prior to the present invention, such sensors have been performance limiting factors which have caused delay in the development and implementation of cost effective integrated vehicle control systems.
In electronic fuel management control systems, to provide the required fuel-to-air ratio, it is necessary for the control system to be fed mass airflow rate data. With such data, the controlling microprocessor calculates the amount of fuel needed under the then existing operating conditions to generate a fuel injection control signal.
Prior art mass airflow sensors typically are of the thin-wire or thin-film type. The thin-wire type of sensor is fabricated with a fine resistive wire such as platinum or tungsten wound on a ceramic bobbin. In operation, a predetermined current flows through the wire to heat the resistive wire to a preset temperature. Any airflow alters the rate of heat transfer from the heated wire, thereby causing a wire temperature/resistance change. Readout electronic circuitry converts this temperature/resistance change into current or voltage changes from which airflow rate may be determined in a manner well known to those skilled in the art.
The thin-wire type of sensor shows critical limitations in electronic fuel management control applications. In this regard, due to the sensor's significant thermal mass, its speed of response is too slow for effective microprocessor-based real time flow control. Additionally, the use of such thin-wire type sensors renders the overall sensor more bulky than desired. Under some noisy environments, the thin-wire type of sensor transmits noise to the external circuit to thereby limit the sensor's flow resolution and accuracy.
An exemplary prior art thin-film type of sensor is manufactured by Honeywell and referred to as the microswitch and mass airflow sensor. This sensor includes a "bridge" on the front side of the device which is fabricated by undercutting the wafer substrate from the front side of the wafer.
This "bridge" type of thin-film sensor has a number of disadvantages. The sensor is very sensitive to the direction of airflow over the bridge and the manner in which the sensor device is mounted. Accordingly, it is difficult to achieve precisely reproducible results from sensor to sensor rendering the sensor difficult to calibrate. Furthermore, the bridge structure is not as structurally strong or as rugged as the sensor of the present invention. Additionally, the "bridge" thin-film sensor includes an air channel which is built into the silicon wafer. This tiny air channel (which is required due to the design of the "bridge" type sensor) limits the dynamic range of the sensor such that very high airflow rates cannot be accurately detected.
The present exemplary embodiment is a silicon-based mass airflow sensor which has a high flow sensitivity, high speed of response and sufficient mechanical ruggedness and reliability to be fully compatible with automobile and other industrial fluid flow control systems (e.g., where sensed gas flow rate is used to control gas flow). The mass airflow sensor of the present invention is fabricated using silicon micromachining and integrated circuit techniques which allow the sensor to be reliable, compact and cost-effectively manufactured.
The present invention is a thin-film type of sensor having significant advantages over prior art sensors of the nature discussed above. The present invention uses a small, thin dielectric diaphragm providing good thermal isolation for thin-film heating and temperature sensing elements, resulting in high flow sensitivity and low current operation of the heating element. The dielectric diaphragm is bounded by a p-etch-stopped silicon rim. The thermal mass of the diaphragm is so low that the speed of the sensor response to airflow change is much faster than prior art sensor response times. The heating and temperature sensing thin-film elements are advantageously configured and controlled to generate readings which are accurate notwithstanding variations in ambient air temperature.
In contrast to the "bridge" type sensors, the present invention has a wide dynamic range of airflow which can be accurately detected (in part because it does not require such a small airflow channel). Additionally, the present invention is not nearly as sensitive to airflow direction as the "bridge" type sensor.
Mass airflow sensors operate in environments in which the ambient air temperature may vary over a wide range. It is, of course, important for a mass airflow to generate accurate airflow readings notwithstanding whether the ambient temperature is 0.degree. C. or 100.degree. C.
Prior art mass airflow sensors are highly dependent on the ambient air temperature. Such sensors typically use a heated resistance element. Additionally, such sensors may utilize temperature sensing elements disposed adjacent to the heated resistance element. In such sensors, the sensing elements detect the heat loss or transfer due to heat flow through the air. Such sensors are highly dependent on ambient air temperature and require additional circuitry to compensate for variations in ambient air.
In contrast, the present invention, due to its unique design is relatively independent of ambient air temperature variations and, typically requires no additional ambient air temperature compensation circuitry. In the present invention, a primary sensor circuit maintains a heating or heated element and an ambient air temperature sensing element at a constant temperature difference. A slave sensor circuit which also includes temperature sensing elements monitors heat loss due to flow at a particular location on the diaphragm. This slave circuit temperature sensing element does not monitor the amount of heat transferred from the heating element through the air, but rather is used to monitor temperature difference as a function of airflow in a manner explained in detail below.
The primary circuit, by maintaining a predetermined constant temperature difference between the heating or heated element and an ambient air temperature sensing element, keeps the heating or heated element at a fixed temperature (T.sub.FIXED) above the ambient air temperature. At the same time, the primary circuit functions to keep the temperature of the slave circuit temperature sensing element at a fixed temperature offset (related to T.sub.FIXED) above the ambient air temperature reduced by a temperature change due directly to airflow.
The slave circuit utilizes an operational amplifier whose inverting and non-inverting inputs each receive ambient air temperature related signals to thereby cancel the effect of ambient air temperature. The output voltage of the slave circuit is a signal which is indicative primarily of airflow.
The primary sensor circuit does not monitor the current through the resistance heating element (as was typical of many prior art sensors) but rather monitors and maintains a predetermined temperature difference between the primary ambient temperature sensing element and the heated element. This configuration helps to render the circuit relatively immune to long term sensor drift problems arising from the thermal characteristics of the diaphragm, dust build-up or material changes in the heating resistance over time. Sensors which monitor the current through the heating resistance are in contrast highly sensitive to such dust build-up problems or the change in the heating element resistance over time.
Additionally, in the present exemplary embodiment, the circuit elements built into the sensor structure have been selected to establish common-mode rejection of temperature dependence. In this regard, identical thin-film temperature sensing resistors are used which have the same "cold" resistance values. These well-matched elements will, therefore, react to changes in ambient temperature in a uniform manner. In the present sensor, such well-matched elements yield accurate airflow measurements whether the ambient temperature is 0.degree. C. or 100.degree. C.