Measuring and testing – Test stand – For engine
Patent
1989-07-21
1991-07-02
Raevis, Robert
Measuring and testing
Test stand
For engine
G01M 1500
Patent
active
050276450
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a measuring device to determine the distance between an electrically conducting part, which can be moved in a defined chamber, and a reference point, in particular to determine the upper dead point of the piston of an internal combustion engine, with an electronic evaluation circuit in which a capacitive sensor can be positioned in a holding device, held in the wall of the chamber, and is connected to an evalution circuit.
2. The Prior Art
In a known measuring device of the aforementioned kind one electrode is isolated in the combustion chamber of an internal combustion engine, the piston or the cylinder block forming the second electrode. Depending on the distance of the piston from the electrode, the capacitance of this system changes. In well-known cases, the change in capacitance is now evaluated in such a manner that said evaluation influences the modulation of a carrier frequency for which a relatively complicated evaluation circuit is necessary.
Due to this principle of measurement, the well-known system has proven to be relatively insensitive to small changes in capacitance and also exhibits a poor signal to noise ratio. In addition to this, one must work with relatively large sensors, resulting in the necessity of providing matching measurement openings, which permit the electrode to be inserted.
The object of the invention is to avoid the above drawbacks and to propose a measuring device of the aforementioned kind, which in particular can be used in a simple manner in conventional internal combustion engines and is characterized by a high degree of sensitivity.
SUMMARY OF THE INVENTION
The invention achieves this object in that the evaluation circuit has a measuring bridge, fed by an oscillator via output potential dependent switching elements, with at least two capacitors, that the sensor head of the sensor and the movable part, in particular the piston of the internal combustion engine form one of the two capacitors, that there is a differential amplifier to which the transverse voltage, which can be picked off the measuring bridge, can be led, and that the outputs of the differential amplifier control the output potentials of the potential-dependent switching elements. A measuring device with the evaluation circuit of the invention contributes significantly to the improvement of the measurement system, primarily with respect to the evaluation of small, rapid changes in capacitance. Without any loss in sensitivity, smaller, thinner sensors can be used that can be inserted into existing openings provided for measuring sensors, fuel injection nozzles, spark or heater plugs.
In the case of the evaluation circuit of the invention, a measuring bridge, for example, a capacitance bridge, is fed by an oscillator with square wave pulses via output potential dependent switching elements. When a capacitance changes, the result is a changed ratio of voltage division in the series connection of the capacitances in question. The result in turn is a change in the transverse bridge voltage. Said change causes, via a differential amplifier, a change in the output voltage and a change in the potential that adjusts the size of the incoming pulses via the switching elements to the new capacitance ratio. Thus the smallest differences in the bridge arms lead to relatively large changes during the charge transfer during the discharging of the capacitors of the bridge, the result of which is a high sensitivity of the evaluation circuit. The evaluation circuit can be designed by correspondingly sizing the capacitors of the bridge in such a manner that said capacitors rapidly recharge themselves and can, thus, function at a relatively high frequency of the signals of the oscillator so that the sensor can determine the exact point in time when the closest approach occurs. This is very important, for example, in determining the upper dead point of a piston of an internal combustion engine.
Another advantage of the proposed evaluation circui
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Leitmeier Klaus
Woss Gerhard
Zeiringer Rudolf
AVL Gesellschaft fur Verbrennungskraft-maschinen und Messtechnik
Raevis Robert
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