GB2113497A

GB2113497A - Waveform shaping circuit

Info

Publication number: GB2113497A
Application number: GB08236118A
Authority: GB
Inventors: Hiromitsu Makita
Original assignee: Alps Electric Co Ltd
Current assignee: Alps Alpine Co Ltd
Priority date: 1981-12-19
Filing date: 1982-12-20
Publication date: 1983-08-03
Also published as: DE3246243A1; GB2113497B; JPH0211047B2; JPS58106913A

Abstract

A waveform shaping circuit includes a low pass filter (2) for eliminating high amplitude, high frequency noise components from a periodic signal, a reference voltage generator (4) for generating a reference voltage which varies in magnitude in dependence upon the average level of the filtered signal and a comparator (3) for comparing the filtered signal with the reference voltage and slicing the filtered signal to remove low frequency, low amplitude noise components from the filtered signal. The circuit is particularly useful in electronic internal combustion engine analyses which analyse the periodic pulse signal produced by the ignition circuit. Such signals are subject to noise due to contact set chatter. The level of this noise is reduced by the above described circuit. The reference voltage generator (4) includes voltage clamping means (14, 15) to reduce the noise sensitivity of the comparator. <IMAGE>

Description

SPECIFICATION Waveform shaping circuit This invention relates to a waveform shaping circuit suitable for those periodic pulse signals which are accompanied with the waveform distortion due to noise, such as pulse signals representative of opening and closing of the point in an ignition coil of an engine.

In an apparatus for measuring engine characteristics such as an engine analyzer, a reference signal representative of the reference of the time axis, for example, is necessary in order to represent the engine operating condition. A signal which is generated upon opening and closing of a point and appears on the negative side of an ignition coil is used as such a signal.

Due to an impulsive signal for igniting each cylinder of the engine , this signal (hereinafter referred to as an "ignition coil signal") involves the waveform distortion in either of the cases in which a mechanical switch is used as the point (hereinafter referred to as a "point type"), in which transistors are partly used (hereinafter referred to as a "semi-transistor type") and in which transistors are fully used (hereinafter referred to as a "full transistor type").

Figure 1(A) shows the ignition coil signals in the point type and in the semi-transistor type and Figure 1(B) shows the ignition coil signal in the full transistor type.

In Figure 1(A), when the point is closed, the impulsive signal a first appears with vibration of the mechanical switch and when the mechanical switch becomes stable, a predetermined signal bthen appears. Next, when the point is opened, the mechanical switch vibrates and the impulsive signal cappears. This also holds true of Figure 2(B) and the impulsive signal a appears after the stable signal b with opening and closing of the transistor.

The impulsive signal a has an extremely high level and is used for igniting each cylinder. If the ignition coil signal shown in Figures 1(A) and 1(B) is used as the aforementioned reference signal, however, the impulsive signals a and b become rather noise and distort the waveform of the desired ignition signal.

Accordingly, the ignition coil signal must be subjected to waveform shaping to remove the noise.

Figure 2 is a block diagram showing an example of the conventional waveform shaping circuit, in which reference numeral 1 is an input terminal; is a low band-pass filter; 3 is a comparator; 4 is a reference voltage generator; and 5 is an output terminal.

Next, the operation of this prior art device will be described.

In the drawing, the ignition coil signal shown in Figure 1(A) or 1(B) is supplied from the input terminal 1. The low band filter 2 damps the nosies a and cfrom the ignition coil signal.

The output signal of the low band-pass filter 2 is applied to the comparator 3 and its level is compared with the level of the reference voltage from the reference voltage generator 4. Accordingly, the output signal of the low band-pass filter 2 is sliced with respect to the reference level and a waveform shaped ignition coil signal is obtained at the output terminal 5.

In Figures 1(A) and 1(B), when the engine operates normally, the impulsive signals a and c have frequency components above 6 KHz; hence, the cut-off frequency of the low band-pass filter 2 is set to about 6 KHz.

When the ignition cord is broken or is pulled off, for example, the engine operates abnormally and the noises a and sometimes have frequency components of about 1.3 KHz; hence, the cut-off frequency of the low band-pass filter 2 must be set as low as about 1.3 KHz.

However, in the case of the engine having the number of revolution of 100 to 6,000 rpm and two to eight cylinders, the repetition frequency of the resulting ignition coil signal is changeable within a range of 1,6 to 400 Hz. If the low band-pass filter 2 having the cut-off frequency of as low as about 1.3 KHz such as described above is employed, the amplitude level of the low band-pass filter 2 when the ignition coil signal at the time of high speed revolution is passed through the filter 2 becomes smaller than the amplitude level ofthe ignition coil signal at the time of low speed revolution of the engine. In other words, the amplitude level of the output signal of the low band-pass filter 2 changes in accordance with the repetition frequency of the ignition coil signal applied to the low band-pass filter 2.

Besides the repetition frequency, the ignition coil signal is likely to change with the change of the duty ratio and hence, the level of the output signal of the low band-pass filter 2 also changes with the change in the duty ratio.

In Figures 1(A) and 1(B), the duty ratio of the ignition coil signal is expressed as follows:

In the cases of the ignition devices of the point type and semi-transistor type, the duty ratio of the ignition coil signal is approximately 40% and takes a substantially constant value. In the case of the full transistor type ignition device, however, the duty ratio is 90% at the low speed revolution of the engine but it drops down to 50% at the high speed revolution of 2,000 rpm or more.

When the duty ratio of the ignition coil signal changes in the manner described above, the average level of the ignition coil signal naturally changes, too. The average level of the ignition coil signal at the high speed revolution is lower than that at the low speed revolution and the amplitude level of the output signal of the low band-pass filter 2 becomes also lower. Accordingiy, as shown Figure 3(B), the output signal of the low band-pass filter 2 at the high speed revolution exhibits the lower amplitude level and its level becomes lower as a whole in comparison with the output signal of the low band-pass filter 2 at the time of the low speed revolution which is shown in Figure 3(A). In Figures 3(A) and 3(B), the levels Ea and E'a represent the average levels, respectively.

Since the average level of the output signal of the low band-pass filter 2 at the low speed revolution (Figure 3(A)) is high, for example, the reference voltage Eb from the reference voltage generator 4 (Figure 2) must be set so that the slice level becomes high, in order to make accurate waveform shaping. If the reference voltage Eb is set to the high level, however, the following problem occurs. The slice level of the output signal of the low band-pass filter 2 having a low amplitude level (Figure 3(B)) must rather be low at the low average level at the time of the high speed revolution; hence, the reference voltage Eb becomes unsuitable as the reference voltage at the high speed revolution.

If the reference voltage is kept constant in an attempt to make possible the waveform shaping of the ignition coil signal during the abnormal engine operation, too, slicing with a suitable level can not be made for ail the ignition coil signals at all the speeds of revolution.

It is therefore an object of the present invention to provide a waveform shaping circuit which eliminates the problems with the prior art and which can accurately shape the waveform of periodic pulse signals accompanied with the level change.

In order to accomplish this object, the present invention is characterized in that the reference voltage for slicing and waveform-shaping the periodic pulse signals is changed in accordance with the periodic pulse signals.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

Figure 4 is a block diagram showing the waveform shaping circuit in accordance with one embodiment of the present invention. Like reference numerals are used in this drawing to identify like constituents as in Figure 2.

Next, the operation of this embodiment will be described.

In the drawing, the ignition coil signal from the input terminal 1 is applied to the low band-pass filter 2 having a low cut-off frequency of 1.3KHz, for instance. The low band-pass filter 2 damps the noises a and cshown in Figure 1. Its output signal is applied to the comparator 3 and also to the reference voltage generator 4.

The reference voltage generator 4 generates a reference voltage in response to the input signal.

The reference voltage has a level representative of the average level of the input signal, for example.

Even if the level of the output signal of the low band-pass filter 2 changes with the frequency or duty ratio of the ignition coil signal from the input terminal 1, the reference voltage also changes in accordance with such a change. In the comparator 3, therefore, the output signal of the low band-pass filter 2 is always sliced by the reference voltage and a waveform-shaped ignition coil signal is obtained at the output terminal 5.

Figure 5 is a circuit diagram showing a definite example of the embodiment shown in Figure 4 and like reference numerals are used in figure 5 to identify like constituents as in Figure 4.

Next, the operation of this embodiment will be described.

In the drawing, afterthe noise a (Figure 1) is limited below a predetermined level by zenor diode 6, the ignition coil signal from the input terminal 1 is applied to the low band-pass filter 2.

The low band-pass filter 2 consists of resistors 7, 8, capacitors 9, 10 and an operational amplifier 11 and the values of the resistors and capacitors are selected so that the cut-off frequency becomes 1.3 KHz.

The output signal of the low band-pass filter 2 is applied to the positive (+) terminal of the comparator 3 and also to the reference voltage generator 4.

The resistor 12 and the capacitor 13 of the reference voltage regenerator4togetherform an integration circuit and generate a reference voltage representative of the average level of the output signal of the low band-pass filter 2.

This reference voltage is applied to the negative (-) terminal of the comparator 3. When the reference voltage is above the set level at the point P1, a diode 14 becomes conductive and sets the reference voltage to the level of this point P, lest the reference voltage becomes higher than this level.

In other words, the potential at the point P1 and the diode 14 form a clamp circuit and sets the maximal changeable level of the reference voltage to the potential of this point P1.

The function of this clamp circuit will now be explained: Figure 6 shows the output signal of the low band-pass filter 2 (shown in Figure 5). The low band-pass filter 2 damps the noise a (Figure 1(B)) but the residual noise a' is contained in the output signal of the low band-pass filter 2.

As described already, the reference signal Eb represents the average level of the output signal of the low band-pass filter 2. If the duty ratio at the time of low speed revolution is 90% as described already, for example, the reference voltage Ea becomes sufficiently high and exceeds the minimum level of the noise a'. If such a reference voltage Ea is applied to the comparator 3 (Figure 5), the output signal of the comparator 3 contains an undesired signal due to the noise a'.

The clamp circuit shown in Figure 5 limits the maximal level of the reference voltage Eb to the level of the point P1 so as to provide an ignition coil signal which is waveform-shaped without being affected by the noise, from the comparator 3. For example, if the zenor diode 6 limits the amplitude of the noise a (Figure 1) to about 5V and the cut-off frequency of the low band filter 2 is a 1.3 KHz, the minimal level of the noise a' (Figure 6) at low speed revolution drops down to about 85% of the maximal level of the output signal of the low band-pass filter 2. Hence, the clamp circuit of the reference voltage generator 4 sets the maximal level of the reference voltage to about 70%, for example, of the maximal level of the output signal of the low band filter 2 at the time of low speed revolution.

In the case of the ignition devices of the point type and semi-transistor type, the clamp circuit does not operate because the diode 14 is non-conductive.

In Figure 5, the diode 15 of the reference voltage generator 4 becomes conductive when the output voltage of the integration circuit consisting of the resistor 12 and the capacitor 13 is lower than the potential of the point P2 and sets the potential of the negative (-) terminal of the comparator 3 to the potential of the point P2.

When the ignition coil signal is not applied to the input terminal 1, the output voltage of the integration circuit becomes low, the diode 15 becomes conductive, the potential of the negative (-) terminal of the comparator 3 is set to the potential of the point P2 and the output level of the comparator 3 is always kept at low level.

In this manner, the waveform-shaped ignition coil signal can be always obtained at the output terminal 5 even if the frequency and duty ratio of the ignition coil signal change.

Incidentally, Figure 5 shows one embodiment of the present invention which is merely illustrative for the easy understanding of the invention. The signal which is to be subjected to waveform shaping is not limited to the ignition coil signal, in particular, but can be obviously applied to arbitrary periodic pulse signals accompanied by waveform distortion resulting from noise.

As described in the foregoing, the present invention makes it possible to change the reference voltage for slicing the periodic pulse signal in accordance with the periodic pulse signal so that even if the level change occurs in the periodic pulse signal, the pusle signal can be accurately waveformshaped. Thus, the present invention can provide an excellent waveform shaping circuit which can eliminate the problems with the prior art.

Figures 1(A) and 1(B) are signal waveform charts showing the ignition coil signals; Figure 2 is a block diagram showing an example of the conventional waveform shaping circuit; Figures 3(A) and 3(B) are signal waveform charts showing the output signal of the low band-pass filter of Figure 2; Figure 4 is a block diagram showing the waveform shaping circuit in accordance with one embodiment of the present invention; Figure 5 is a circuit diagram showing a definite example of the circuit of Figure 4; and Figure 6 is a diagram useful for explaining the function of the clamp circuit in the reference voltage generator in Figure 5.

2 . . . low band-pass filter, 3 . . . comparator, 4 . . .

reference voltage generator.

Agent: Kenjiro Take, Patent Attorney ( & one other)

Claims

CLAIMS 1. In a waveform shaping circuit which applies a periodic pulse signal accompanied by waveform distortion due to noise to a filter to damp the noise and the level of the output signal of said filter is compared with that of a reference voltage, the improvement including a reference voltage generatorwhich receives said output signal and generates said reference voltage so that said reference voltage can change in accordance with said output voltage. 2. The waveform shaping circuit as defined in claim 1 wherein said reference voltage generator includes a clamp circuit and the maximum changeable level of said reference voltage can be set to predetermined level below the level of said output signal. New claims filed on 18th Jan 83 Superseded claims 1 and 2 CLAIMS

1. A waveform shaping circuit comprising an input terminal for receiving a periodic pulse signal subject to distortion by noise, a filter connected to the input terminal to receive said signal, a reference voltage generator for generating a reference voltage, a comparator for comparing the output of the filter with the reference voltage, to provide an output signal, the reference voltage generator being also connected to the input terminal and being arranged to vary the level of the reference voltage in dependence upon the general level of the periodic pulse signal.

2. A circuit, according to claim 1, wherein the reference voltage generator includes a clamp circuit arranged to set the maximum level of the reference voltage at a level below the maximum level of the output signal from the filter.

3. A waveform shaping circuit for shaping a periodic pulse signal to remove the effects of noise therefrom, comprising a filter for removing relatively large amplitude relatively high frequency noise components from the pulse signal, a reference voltage generator for generating a reference signal, a comparator for comparing the output of the filter with the reference signal to slice the signal at a level to remove relatively low frequency, relatively low amplitude noise from the pulse generator and control means for varying the level of the reference signal in dependence upon the general level of the periodic pulse signal fed through the filter.

4. A circuit, according to claim 3, wherein the control means is arranged to vary the level of the reference voltage in dependence upon the average level of the output from the filter.

5. A circuit, according to claim 3 orto claim 4, wherein the control means comprises a clamp for limiting the magnitude of the reference voltage to a pre-determined maximum.

6. A waveform shaping circuit substantially as hereinbefore described, with reference to Figures 5 and 6 of the accompanying drawing.

7. A waveform shaping circuit substantially as hereinbefore described, with reference to Figure 7 of the accompanying drawing.