Apparatus for generating a knock signal for use with an internal combustion engine

An output signal of a vibration sensor mounted on an internal combustion engine is compared with a background noise reference signal to produce a knock signal which can accurately represent the intensity of knocking. The background noise level signal is produced by averaging a component of the vibration sensor output signal which represents the vibration of the engine other than knock-induced vibration. In order to produce a feedback signal for inhibiting the knock induced component of the sensor output signal, a first comparator compares the sensor output signal with a first reference signal having a sufficiently low level. Knock signal is produced by a second comparator which receives a second reference signal, which is proportional to the first reference signal but having sufficiently higher level, thereby enabling a precise determination of knock intensity.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for generating a signal 
indicative of the intensity of knock occurring in an internal combustion 
engine. 
2. Description of the Prior Art 
Strong knocks occurring in an internal combustion engine cause harm to the 
engine. However, it is generally known in the art that the engine output 
performance and the fuel consumption characteristics are optimized when 
the engine is running under a slight knock condition. 
In view of the fact that the occurrence of knocking is greatly dependent on 
the ignition timing, various systems have been proposed wherein the 
ignition timing is retarded upon detection of knocking. 
In order to accurately determine the ignition timing in accordance with the 
intensity of knocking, apparatus is provided for precisely determining the 
intensity of knocking occurring in the internal combustion engine. 
One technique that has been proposed for generating an accurate knock 
signal is amplification discrimination, in which an output signal of a 
vibration sensor mounted on the engine is compared with a noise reference 
signal at a comparator. 
The noise reference signal masks most of the sensor output signal so that 
only peaks in the sensor output signal which are assumed to be mostly due 
to knock-induced vibration rising above the noise reference signal 
contribute to the knock signal. 
This type of knock signal generating apparatus results in an improvement in 
signal to noise ratio of the knock signal. 
However, a difficult problem in such amplification discrimination is to 
eliminate the influence of the peaks of the sensor output signal upon the 
voltage level of the background noise signal. 
In other words, there is a tendency for the peaks of the sensor output 
signal resulting from knocking falsely increase the voltage level of the 
background to noise signal, and undesirably reduce the apparent amplitude 
and the frequency of knock signal which is produced at the output of the 
comparator. 
SUMMARY OF THE INVENTION 
An object of the present invention is therefore to improve the accuracy of 
detection of the intensity of knocking occurring in the engine, by 
employing a background noise signal having a voltage level precisely 
indicating the vibration of the engine other than knock induced vibration 
of the engine. 
According to the present invention, an apparatus for generating knock 
signal, for use with an internal combustion engine having knock induced 
vibration comprises: 
a vibration sensor mounted on the engine and responsive to both 
knock-induced vibrations and other vibrations to generate an output 
signal; 
a reference signal generating means responsive to the output signal of the 
vibration sensor and operative to generate a first and a second reference 
signal each having a level proportional to the average amplitude of a 
component of the output signal of the vibration sensor representing 
vibration of the engine other than the knock-induced vibration, the 
reference signal generating means including switch means responsive to a 
switch control signal and operative to inhibit signals from the vibration 
sensor and hold the level of the signals from the vibration sensor 
directly before the application of the switch control signal, and 
operative to permit the signal from the vibration sensor to pass 
therethrough upon absence of the switch control signal; 
a first comparator means operative for comparing the output signal of the 
vibration sensor with the first reference signal to produce an output 
signal when the output signal of the vibration sensor exceeds the first 
reference signal; 
feedback means connected to the first comparator means and operative for 
producing the switch control signal from the output signal of the first 
comparator and for same to the switch means; and 
a second comparator means operative for comparing the output signal of the 
vibration sensor with the second reference signal to produce the knock 
signal, wherein the first reference signal is so determined as to be 
sufficiently lower than the component of the output signal of the 
vibration sensor representing the knock-induced vibration so that the 
output signal of the first comparator means includes a component due to a 
slight knocking, thereby eliminating the tendency that the peaks of the 
output signal of the vibration sensor due to knocking falsely raise the 
level of the first reference signal, and wherein the level of the second 
reference signal is so set as to be higher than the first reference signal 
level and adjusted at a suitable level so that the magnitude of the knock 
signal does not include the component due to slight knocking, thereby 
accurately represents the intensity of the knock induced vibration.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Before entering into the description of preferred embodiments, reference is 
first made to FIGS. 1 and 2 in which there are illustrated a block diagram 
of a prior art apparatus for generating a knock signal and waveforms that 
appear at various positions of the apparatus. 
As shown in FIG. 1, the apparatus comprises a vibration sensor 1 mounted on 
an internal combustion engine, an amplifier 2 connected thereto, and a 
background noise reference signal generator which receives the output 
signal of the amplifier 2, and a comparator 7 for producing a knock signal 
by comparing the output signal of the amplifier 2 with a background noise 
reference signal generated by the background noise reference signal 
generator. 
The apparatus also includes a feedback means which comprises a monostable 
multivibrator 8 connected to the comparator 7. 
The background noise reference signal generator includes a rectifier 
circuit 3 connected to the amplifier 2, an analog switch 4 connected to 
the rectifier circuit 3 and responsive to the output signal of the 
monostable multivibrator 8, an averaging circuit 5 connected to the output 
of the analog switch 4, and an amplifier 6 connected to the output of the 
averaging circuit 5. 
As shown by the waveform A of FIG. 2, the output signal of the rectifier 
circuit 3 is a rectified signal having an amplitude proportional to the 
vibration of the engine. 
The analog switch 4, whose operation is described hereinbelow, is provided 
for inhibiting the output signal of the rectifier circuit 3 when the 
engine knocks, so as to eliminate the influence of the vibration due to 
knock upon the background noise reference level. 
The averaging circuit 5 receives the output signal of the rectifier circuit 
3 through the analog switch 4 and translates it into a d-c voltage signal. 
The dc voltage level thereof may be adjusted by the amplifier 6 to form 
the background noise reference signal as shown by the waveform B of FIG. 
2, for example. 
The output signal of the amplifier 2 and the output signal of the amplifier 
6, i.e., the background noise reference signal, are supplied to each of 
two inputs of the comparator 7. To indicate knock, the comparator 7 
produces a rectangular pulse signal, as shown by the waveform C of FIG. 2, 
when the voltage level of the output signal of the amplifier 2 exceeds the 
voltage level of the background noise reference signal. 
The output signal of the comparator 7 thus produced, which takes the form 
of a pulse train, is used for triggering the monostable multivibrator 8 
also, and upon receiving the trigger signal it produces a rectangular 
pulse signal having a predetermined time duration, as shown by the 
waveform D of FIG. 2. 
Referring to the operation of the analog switch 4, under a condition that 
the knock signal is not produced by the comparator 7, the monostable 
multivibrator 8 does not produce an output signal and the analog switch 4 
remains closed, allowing the output signal of the rectifier circuit 3 to 
pass therethrough. Thus, during the time when knocking is not present, the 
background noise reference signal is produced by averaging the output 
signal of the rectifier circuit 3 which is assumed to be free from 
components due to knocking vibration. 
On the other hand, when the knock signal is produced by the comparator 7, 
the analog switch 4 is supplied with the output signal of the monostable 
multivibrator 8. At that moment, the analog switch 4 inhibits the output 
signal of the rectifier circuit 3 and holds the level of the rectified 
signal immediately before the application of the output signal of the 
monostable multivibrator 8. 
Accordingly, when knocking is present, the background noise reference 
signal is produced in accordance with the voltage level of the output 
signal of the rectifier circuit 3 which is held by the analog switch 4. 
However, as previously mentioned, this apparatus has suffered from the 
problem that a knock signal produced by the comparator 7 does not alway 
indicate the intensity of knock-induced vibration accurately. 
This is because of the inappropriateness of the voltage level setting of 
the background noise reference signal, which must precisely represent the 
noise level contained in the output signal of the vibration sensor and 
also must have a sufficiently low level for triggering the monostable 
multivibrator 8 in response to the rise in the engine vibration level due 
to knocking at the same time, in order to eliminate the influence of 
knocking upon the background noise signal. 
For example, if the amplification rate of the amplifier 6 is low in order 
to reduce the voltage level of the background noise reference signal as 
shown by the waveform E of FIG. 2, the comparator 7 produces output 
signals, i.e., the knock signal, responsive to a component of the output 
signal of the vibration sensor which is much lower than the vibration 
level of knocking, thus deteriorating the accuracy of determination of 
knock intensity. 
Conversely, if the amplification rate of the amplifier 6 is high in order 
to produce a high level background noise reference signal as shown by the 
waveform F of FIG. 2, the comparator 7 does not produce the output signal, 
i.e., the knock signal, until the voltage level of the output signal of 
the vibration sensor 1 rises to a level much higher than the vibration 
level due to knocking. In this case, the accuracy of determination of 
knock intensity is deteriorated too. 
The present invention is based on the recognition of the above drawback of 
the prior art apparatus. 
The preferred embodiments of the present invention will be explained in 
conjunction with FIGS. 3 to 5. 
Referring to FIGS. 3 and 4, a first embodiment is explained. 
As shown in FIG. 3, the apparatus according to the present invention 
comprises a vibration sensor 1 mounted on the engine, an amplifier 2 
connected thereto, a reference signal generator including a rectifier 
circuit 3 connected to the amplifier 2, an analog switch 4 responsive to 
the output signal of a monostable multivibrator 8, an averaging circuit 5, 
amplifier 6 and a feedback means in the form of the monostable 
multivibrator 8. 
Since the operation of each of these circuit portions is the same as that 
of the prior art apparatus, detailed descriptions thereof are omitted. 
Preferably, the vibration sensor 1 is a resonant type vibration sensor 
which can resonate at a characteristic frequency of engine vibration due 
to knocking. However, the resonant type vibration sensor may be replaced 
by a combination of a non-resonant type vibration sensor and a bandpass 
filter which allows only the passage of a component of the output signal 
of the vibration sensor having a specified characteristic frequency of 
knocking vibration. 
Further, in case a non-resonant type vibration sensor is employed, it is 
preferable that the bandpass filter is incorporated in the amplifier 2. 
The first embodiment is characterized by the provision of a first and a 
second comparator 71 and 10 which receive the output signal of the 
amplifier 2 at one input thereof, and an amplifier 9 for amplifying and 
adjusting the voltage level of the output signal of the amplifier 6. 
The first comparator 71 receives the output signal of the amplifier 6 at 
the other input thereof and produces a trigger signal for the monostable 
multivibrator 8. 
The second comparator 10 receives the output signal of the amplifier 9 
which is sufficiently higher than the output signal level of the amplifier 
6 for producing a pulse train representing the occurence of knocking. 
The output signal of the second comparator 10 is fed to an integrator 11 
which is resetable by an ignition timing signal produced by an ignition 
coil 12 and a monostable multivibrator 13 connected thereto. 
It will be appreciated that according to the present invention, filtering 
of the background which influences accuracy is carried out separately by 
two comparators. 
The operation of the first embodiment will be described hereinbelow with 
reference to FIG. 4. 
The amplification rate of the amplifier 6 which receives the output signal 
of the averaging circuit 5 is set relatively low so as to produce a first 
reference level signal having a relatively low level, as shown by waveform 
G of FIG. 4. 
The first reference level signal produced by the amplifier 6 is applied to 
a second input of the comparator 71 which also receives the output signal 
of the amplifier 2. 
When the voltage level of the output signal of the amplifier 2 exceeds the 
first reference level, the comparator 71 produces a high level output 
signal for triggering the monostable multivibrator 8. Practically, the 
output signal of the comparator 71 appears as a pulse train as shown by 
the waveform H of FIG. 4 and the monostable multivibrator 8 produces a 
rectangular pulse signal as shown by the waveform I in response to the 
output pulse train of the comparator 71. 
It is to be noted that the voltage level of the first reference level 
signal is set sufficiently low by adjusting the amplification ratio of the 
amplifier 6 so that the monostable multivibrator 8 can be triggered even 
when the voltage level of the output signal of the amplifier 2 rises 
slightly with the occurrence of knocking of a very slight level. 
Similar to the prior art apparatus, the analog switch 4 is opened to 
inhibit the output signal of the rectifier circuit 3 and hold the level 
thereof in accordance with the output signal of the monostable 
multivibrator 8. 
Since the first reference signal is set at sufficiently low level, the 
analog switch 4 is immediately opened even if knocking of a very slight 
level is present, thus eliminating the undesirable influence of knocking 
upon the background noise reference signal. 
The operation of the second comparator 10 and the amplifier 9 will now be 
described. 
The amplifier 9 receives the output signal of the amplifier 6, i.e., the 
first reference signal and amplifies the voltage level thereof to produce 
a second reference level signal as shown by the waveform K of FIG. 2. 
The voltage level of the second reference signal level is set to be higher 
than the first reference level but lower than the peaks of the output 
signal of the amplifier 2 due to knocking, and at a level appropriately 
representing the background noise signal component in the vibration sensor 
1 output. 
The output signal of the amplifier 9, i.e., the second reference level 
signal is applied to one of the inputs of the second comparator 10 which 
also receives the output signal of the amplifier 2. 
When the voltage level of the output signal of the amplifier 2 exceeds the 
second reference level, the second comparator 10 produces an output signal 
in the form of a pulse train as shown by the waveform L of FIG. 4. 
Since the voltage level of the second reference level signal is determined 
appropriately, the output signal of the comparator 10 precisely represents 
the intensity of knocking. 
In order to produce a knock signal having a voltage proportional to the 
intensity of knocking, it is preferable to provide an integrator circuit 
11 which receives the output signal of the second comparator 10. The 
integrator circuit 11 is reset by an ignition timing signal as shown by 
the waveform N of FIG. 4, supplied from the monostable multivibrator 13. A 
knock signal as shown by the waveform M of FIG. 4, which precisely 
represents the intensity of knocking, is thus produced. 
Turning to FIG. 5, a second embodiment of the present invention will now be 
explained. 
The second embodiment features the provision of a divider circuit for 
producing the first reference signal from the second reference level 
signal. 
As shown in FIG. 5, the output signal of the amplifier 6 connected to the 
averaging circuit 5, forms the second reference level signal which is 
applied directly to the input of the second comparator 10. This embodiment 
also features that the amplifier 9 utilized in the first embodiment is 
replaced by a voltage divider circuit 14. 
The voltage divider circuit 14 is connected to the output of the amplifier 
6 and divides the output signal of the amplifier 6, i.e., the second 
reference level signal into a suitable voltage level to form the first 
reference level signal. 
The first reference level signal produced by the divider circuit 14 is 
received by the first comparator 71. 
Since the operation of the second embodiment is the same as that of the 
first embodiment, the explanation thereof is omitted. 
The voltage divider circuit 14 can be constructed by a series resistors 
connected to the output of the amplifier 6 and ground for reducing the 
output voltage level of the amplifier 6. 
Therefore, the second embodiment has an advantage that it requires less 
component parts than the first embodiment and the manufacturing cost will 
be reduced.