Measuring and testing – Volume or rate of flow – Thermal type
Patent
1987-10-07
1989-02-28
Goldstein, Herbert
Measuring and testing
Volume or rate of flow
Thermal type
G01F 168
Patent
active
048074705
DESCRIPTION:
BRIEF SUMMARY
PRIOR ART
A known fuel metering system for an internal combustion engine operates with a flow-measuring probe, which is operated in a bridge circuit, for determining the accurate air volume. The flow-measuring probe is constructed as a hot-wire air volume meter which operates in accordance with the principle of constant temperature. In this arrangement, the hot wire is heated up by a heating current flowing through it, the heating current being adjusted by an electronic regulator to such a level that the operating temperature of the temperature-dependent resistance of the hot wire remains virtually constant. The level of the heating current then supplies reliable and accurate information on the mean time value of the intake air volume. However, the output quantity of such a measuring device has a non-linear relationship to the air volume taken in. In the known circuit arrangement, the output signal supplied by the flow-measuring probe is evaluated by a digital control device. However, this is only possible after a previously performed analog/digital conversion. During this conversion, an accurate voltage reference element is again required as reference source of the absolute voltage. However, this known circuit arrangement, which also largely compensates any dependence of the output signal of the flow-measuring probe on the supply voltage, is very complex and expensive.
ADVANTAGES OF THE INVENTION
The circuit arrangement according to the invention, comprising a flow-measuring probe operated in a bridge circuit and having the features of the main claim, has the advantage that it is designed to be paticularly simple and cost-effective and therefore also appears to be suitable particularly for cases of application which require the air volume to be determined as accurately as possible with the least possible expenditure. The circuit arrangement is distinguished by the fact that the output signal of the flow-measuring probe is proportional to the supply voltage applied. For this reason, a highly accurate voltage reference source can be omitted in the control device evaluating the output signal.
DRAWING
An illustrative embodiment of the invention is shown in the drawing and explained in closer detail in the description following.
FIG. 1 shows a circuit diagram of the circuit arrangement and
FIG. 2 shows the variation with time of the output voltage of an integrator provided in the circuit arrangement.
DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT
FIG. 1 shows the circuit diagram of the circuit arrangement. A bridge circuit consisting of resistors R.sub.K, R.sub.2, R.sub.M and R.sub.H is provided through which current I.sub.K of a constant current source flows after closing of a switch S1. In this arrangement, R.sub.H is the resistance of the flow-measuring probe which is constructed as hot wire and which is arranged in the flow cross-section of the intake duct of the internal combustion engine. One bridge diagonal is connected to input connections of a comparator K1 the output connection of which is carried to the R input (reset) of a bistable flipflop FF1. The S input of the bistable flipflop FF1 can be triggered by an oscillator O having the output frequency f.sub.o. The Q output of the bistable flipflop FF1 actuates switch S1 which connects the bridge circuit R.sub.K, R.sub.2, R.sub.H, R.sub.M to the constant current source I.sub.K whilst the Q output of the bistable flipflop FF1 is connected to the base connection of a transistor T.sub.1 the collector connection of which is connected via load resistor R.sub.L to the positive terminal of supply voltage U.sub.B. The input connection of the integrator I supplying the output voltage U.sub.A is also connected to the collector connection of transistor T.sub.1.
At time t 1, the bistable flipflop FF1 is set by the output signal of oscillator O with frequency f.sub.o and switch S1, which connects the bridge circuit R.sub.K, R.sub.2, R.sub.H, R.sub.M to the constant current source I.sub.K, disclosed via the Q output. At the same time, transistor T1 is cut off
REFERENCES:
patent: 3603147 (1971-09-01), Dorman
patent: 3803913 (1974-04-01), Tracer
patent: 4043195 (1977-08-01), Hunting
patent: 4334186 (1982-06-01), Sasayama et al.
patent: 4665745 (1987-05-01), Ina et al.
Goldstein Herbert
Robert & Bosch GmbH
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