Apparatus for measuring the mass of a flowing medium

#The invention is based on an air flow rate meter. An apparatus is already known in which the hot wire comes into contact with each support point only one time in the distance from one to the other end. This results in a relatively large size of the apparatus.

#The contrast to the foregoing, the apparatus according to the present invention has the advantage that the apparatus is of small size, so that it can be placed into very narrow flow diameters while still having a high degree of measuring sensitivity.

#By means of the steps described hereinafter, advantageous variations and improvements of the apparatus can be realized. It is especially advantageous to twist the hot wire coils together and to encase the hot wire coils with a common protective coating.

#The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawing.

#FIG. 1 is the switching diagram of an apparatus to measure the mass of a flowing medium;

#FIG. 2 is a first exemplary embodiment of an apparatus according to the invention;

#In FIG. 1, there is shown a flow cross-section 1, for instance the air induction pipe of an internal combustion engine (not shown), through which a medium, for instance the air aspirated by the internal combustion engine, flows in the direction of the arrows 2. In the flow cross-section 1 there is shown as part of an apparatus to measure the mass of the flowing medium a temperature-dependent precision resistor 3 for measuring purposes, for instance a hot wire, through which flows the output variable of a regulator and which simultaneously delivers the input variable for the regulator. The temperature of the temperature-dependent measuring resistor 3 is set to a fixed value by the regulator, which is higher than the mean temperature of the medium. If the flow speed, i.e. the amount of medium flow mass per unit of time, is increased by a flow-through value of 0, the temperature-dependent measuring resistor 3 is cooled to a greater degree. This cooling-off is fed to the input of the regulator, so that the latter increases its output variable to the extent that the fixed temperature value is again achieved in the temperature-dependent measuring resistor 3. The output variable of the regulator adjusts the temperature of temperature-dependent resistor 3, based on changes in the flow-through values 0 of the medium, to the pre-set value and, at the same time, constitutes a measure of the flowing medium mass. This value can be fed to the metering circuit of an internal combustion engine in the form of a flow-through measuring value U.sub.S in order to adjust the necessary amount of fuel to the amount of air aspirated per unit of time.

#The temperature-dependent resistor 3 is arranged in a resistance-measuring circuit, for instance in a bridge circuit and, together with a reference resistor 4, it forms a first bridge arm to which a second bridge arm, formed by the two fixed resistors 5 and 6, is switched in parallel. The pick-up point 8 is located between the resistors 3 and 4 and pick-up point 8 between the resistors 5 and 6. Both bridge arms are switched in parallel at points 9 and 10. The diagonal voltage of the bridge occurring between points 7 and 8 is fed to the input of an amplifier 11, to the output terminals of which points 9 and 10 are connected, so that its output variable supplies the bridge with operational voltage or operational current. As indicated, the flow-through measuring value U.sub.S, which serves as the regulated variable, can be read between terminals 12 and 13.

#The temperature-dependent measuring resistor 3 is heated by the current flowing through it to a value at which the input voltage of the amplifier 11, the bridge diagonal voltage, becomes zero or achieves a pre-set value. A defined current flows from the output of the amplifier 11 into the bridge circuit. If the temperature of the temperature-dependent measuring resistor 3 is varied because of changes in the amount 0 of the flowing medium, the voltage at the bridge supply voltage or the bridge current to a value at which the bridge is balanced again or is imbalanced in a pre-set manner. The output variable of the amplifier 11, that is, the regulated variable U.sub.S, forms a flow-through measuring value for the amount of flowing medium, just as does the current in the temperature-dependent measuring resistor 3, for instance the amount of air aspirated by an internal combustion engine, and this can be fed to an electronic regulator or control unit 29, which, for instance, triggers at least one fuel injection valve 30.