EP0386431B1 - Measurement circuitry for the primary voltage of an ignition coil - Google Patents

Description

The invention relates to a circuit arrangement for measuring the primary voltage of an ignition coil according to the preamble of the main claim.

Circuits for measuring the primary voltage of ignition coils in motor vehicles are known which require transistors and Zener diodes to detect the spark duration of the spark. At the output of this circuit, only a digital signal is provided, which characterizes the spark duration. The required transistors and Zener diodes are unfavorable for use in hybrid circuits, since these components represent a high manufacturing outlay.

The circuit arrangement for measuring the primary voltage of an ignition coil with the features of the main claim has the advantage that the spark duration can still be evaluated digitally and there is also the possibility of analogue detection of the primary voltage on two different scales. When using a so-called static ignition voltage distribution With a plurality of ignition coils, all of the ignition coils can be monitored with regard to their primary voltage using the circuit arrangement according to the invention. For this purpose, a plurality of input diodes are arranged on the input side, which form an input-side OR gate.

The analog primary voltage is divided by means of a voltage divider and reduced primary voltage is evaluated analog and digital. This means that both analog burning voltage detection and digital spark duration detection are possible. Transistors and Zener diodes are not required, which is why the circuit arrangement is well suited for implementation in hybrid technology.

In order to prevent falsification by voltage peaks, the capacitor is averaged at the divider tap. The following IC inputs can also be protected from overvoltages by means of a clamp circuit.

The differential amplifier is a comparator with switching hysteresis, the output voltage of which is used for digital spark duration detection. The output signals of the circuit arrangement can be evaluated in the computer of the existing motor control and taken into account for control functions.

• Figur 1 eine erfindungsgemäße Schaltungsanordnung für analoge und digitale Primärspannungserfassung,
• Figur 2 den zeitlichen Verlauf einer an einer Zündspule anliegenden Primärsspannung,
• Figur 3 den zu der in Figur 2 dargestellten Primärspannung gehörenden Verlauf des analogen Ausgangssignals der erfindungsgemäßen Schaltung und
• Figur 4 den zugehörigen Verlauf des digitalen Ausgangssignals der erfindungsgemäßen Schaltung.

The invention is explained below with reference to the drawing. Show it:

• 1 shows a circuit arrangement according to the invention for analog and digital primary voltage detection,
• FIG. 2 shows the time course of a primary voltage applied to an ignition coil,
• 3 shows the course of the analog output signal of the circuit according to the invention belonging to the primary voltage shown in FIG
• Figure 4 shows the associated course of the digital output signal of the circuit according to the invention.

reduziert. Um eine Verfälschung durch Spannungsspitzen zu verhindern, wird mittels eines dem Teilerwiderstand R2 parallel geschalteten Kondensator C1 eine Mittelwertbildung der reduzierten Primärspannung durchgeführt, die als analoge Ausgangsspannung UA einer Auswerteschaltung 1 zugeführt wird.The circuit arrangement shown in FIG. 1 has two protective diodes D1, D2 at its two inputs for the primary voltages UP1, UP2 of two primary voltages, not shown, which form an input OR gate. The respectively applied primary voltage UP1 or UP2 is multiplied by a factor using a voltage divider with divider resistors R1, R2 $\text{R2 / (R1 + R2)}$

reduced. In order to prevent falsification by voltage peaks, the reduced primary voltage is averaged by means of a capacitor C1 connected in parallel with the divider resistor R2, which is supplied to an evaluation circuit 1 as an analog output voltage UA.

, $\text{R2 = R4}$

Zur Verschiebung der Schaltschwelle des Differenzverstärkers 2 wird eine Diode D3 mit der Diodenspannung UD zum Widerstand R4 in Reihe geschaltet.The battery voltage UB is divided by a second voltage divider with the resistors R3, R4, the divided battery voltage being supplied to the negative input of a differential amplifier 2 as a reference voltage UR. For reasons of symmetry, the resistances of both dividers are dimensioned as follows: $\text{R1 = R3}$

, $\text{R2 = R4}$

To shift the switching threshold of the differential amplifier 2, a diode D3 is connected in series with the diode voltage UD to the resistor R4.

The divided primary voltage is present at the positive input of differential amplifier 2 via an input resistor R5. Between positive input of the differential amplifier 2 and its output, a feedback resistor R6 is effective. The differential amplifier 2 connected in this way works like a comparator with switching hysteresis and outputs a digital output voltage UD, the duration of which corresponds to the spark duration. The digital output voltage UD is also evaluated in the evaluation circuit 1.

In order to protect subsequent IC inputs, the divided primary voltage occurring at the divider tap between the divider resistors R1 and R2 is clamped by means of a diode D5 and another low-resistance voltage divider, which consists of the resistors R7, R8. The brackets are dimensioned such that primary voltages above the operating voltage are bracketed, the divider resistors R1 and R7 acting in parallel with R8 for these voltages. Voltages above the operating voltage can thus also be evaluated analogously in the evaluation circuit 1 on a changed scale. The capacitor C1 must be chosen to be sufficiently small so that the analog evaluation of overvoltages is possible.

The diode D4 has the same function in relation to the battery voltage UB as the diode D5 in relation to the primary voltages UP1 and UP2.

The comparator designed as a differential amplifier 2 compares the divided primary voltage UA with the reference voltage UR, which is derived from the battery voltage UB by means of a voltage divider. The comparator has a hysteresis which is determined by the resistors R5 and R6.

In the evaluation circuit 1, which can be part of the motor control (not shown here), there is an analog primary voltage detection 3 and a digital one Spark duration detection 4. The values determined can be taken into account for control functions in the engine control. The output voltages UA, UD can also be evaluated for error detection.

The time profile of a primary voltage UP is shown in FIG. The associated output voltages UA and UD are shown in FIG. 3 and FIG. 4.

FIG. 3 shows the course of the analog output voltage UA with a broken line as it occurs without capacitor C1. The capacitor C1 thus has the capacitance 0nF. If, on the other hand, a capacitor C1 is used as shown in FIG. 1, the curve of the voltage UA smoothes out in accordance with the voltage curve shown with a solid line.

The signal shown in Figure 4 indicates the spark duration T.

Claims (6)

1. Circuitry for measuring the primary voltage of an ignition coil with a differential amplifier (2) to which a reduced signal which is derived from the primary voltage and a reference voltage (UR) are fed on the input side and which outputs on the output side a firing time signal (UD) whose length corresponds to the firing time of the ignition spark, characterized in that the primary voltage (UP1, UP2) is tapped off via a protective diode (D1, D2) connected in the conducting direction and a voltage divider (R1, R2) and is fed to the one input of the differential amplifier (2) via a resistor (R5), and in that the divided primary voltage (UA) which is obtained at the voltage divider (R1, R2) is fed to an analog evaluation device (3).
2. Circuitry according to Claim 1, characterized in that mean-value generation is performed at the divided primary voltage (UA) by means of a capacitor (C1).
3. Circuitry according to one of Claims 1 or 2, characterized in that the divided primary voltage is clamped at primary voltage values which lie above the firing voltage of the ignition spark.
4. Circuitry according to Claim 3, characterized in that a clamping circuit is used to clamp the divided primary voltage, which circuit consists of a diode (D5) which is connected to the tap-off of the voltage divider (R1, R2) and a further low-impedance voltage divider (R7, R8) which is connected downstream of the diode.
5. Circuitry according to one of the preceding claims, characterized in that the differential amplifier (2) has a switching hysteresis which is determined by an input resistor (R5) which is connected upstream of its measuring input and a feedback resistor (R6) which is effective between the measuring input and its output.
6. Circuitry according to one of the preceding claims, characterized in that a plurality of protective diodes (D1, D2) form an input OR gate at the input of the circuitry.

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