Measuring and testing – Volume or rate of flow – Thermal type
Patent
1990-10-05
1992-09-29
Goldstein, Herbert
Measuring and testing
Volume or rate of flow
Thermal type
7320416, G01F 168
Patent
active
051506114
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to apparatus for measuring the mass flow rate of air and, in particular, to such apparatus which emit output signals indicative of the mass flow rate of air flowing over a probe.
BACKGROUND INFORMATION
Apparatus for measuring the mass flow rate of air are used for various applications, such as in fuel injection systems. Known apparatus typically include a probe and a bridge circuit, wherein the diagonal voltage of the bridge circuit is applied to a switching control element. The switching control element is in turn supplied with voltage pulses for heating the probe. The switching control element thus regulates the heat output conveyed to the probe. The apparatus emits an output signal proportional to the mass flow rate of air over the probe. A typical apparatus for measuring the mass flow rate of air is shown in DE-PS 24 48 304.
In known circuits for controlling apparatus for measuring the mass flow rate of air, or flow sensors, the heating current typically has a direct current component and an alternating current component. The alternating current component is superimposed on the direct current component and consists of a series of current pulses, each of constant duration, for heating the sensor's probe. The pulse repetition rate, which is the pulse frequency, changes depending upon the mass flow rate of air over the probe.
In a typical known circuit for controlling a flow sensor, an operational amplifier, whose output voltage is changed to a frequency proportional to the mass flow rate of air over the probe, is connected to resistance bridge diagonals. The frequency of the voltage controls the alternating current supplied to the bridge circuit and serves as a controlled variable for use in a fuel injection system. For this purpose, the output voltage of the operational amplifier is changed to a frequency by means of a voltage to frequency converter, and the operational amplifier's output voltage is subjected to a linearization function.
For many purposes, it is desirable to obtain an analog output voltage as the output signal of a switching controlled flow sensor. One problem with known circuits for controlling flow sensors, however, is that the output signal is dependent on the battery supply voltage. Thus, because the analog output voltage is dependent on the battery supply voltage, changes in the battery supply voltage can affect the level of the flow sensor's analog output signal.
It is the object of the present invention, therefore, to overcome the problems of known apparatus for measuring the mass flow rate of air.
SUMMARY OF THE INVENTION
The present invention is directed to an apparatus for measuring the mass flow rate of air. The apparatus includes a resistance bridge circuit, wherein the diagonal voltage of the resistance bridge circuit is applied to a switching control element. The apparatus includes a probe, and the switching control element is supplied with voltage pulses for heating the probe. The switching control element regulates the heat output conveyed to the probe.
The voltage pulses that heat the probe and the measured signal from the resistance bridge circuit diagonals are applied to a first closed loop control circuit. The first control circuit generates a first output signal which is a function of the voltage signal measured from the resistance bridge diagonals, and is proportional to the mass flow rate of air around the probe. A second closed loop control circuit is coupled to the first control circuit and is adapted to receive the first output signal therefrom. The second closed loop control circuit is also adapted to receive an input signal based on the voltage pulses heating the probe, a reference variable, the first output signal, and a desired analog output signal.
One advantage of the apparatus of the present invention is that there is a lack of dynamic response problems when it is switched on. Another advantage of the apparatus of the present invention is that the output remains accurate even when there is a low
REFERENCES:
patent: 4807470 (1989-02-01), Kleinhans
patent: 4934188 (1990-06-01), Tanimoto et al.
Goldstein Herbert
Robert & Bosch GmbH
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