WO1997014025A1

WO1997014025A1 - Linear type high output knock sensor

Info

Publication number: WO1997014025A1
Application number: PCT/US1996/011403
Authority: WO
Inventors: Dale Hall
Original assignee: Motorola Inc.
Priority date: 1995-09-27
Filing date: 1996-07-11
Publication date: 1997-04-17

Abstract

A linear type knock sensor (100) is disclosed. It has: a housing assembly (126) adapted to being connectable with an engine and having output leads for providing an output signal to an electronic interface; a transducer assembly having a plurality of piezoelectric ceramic wafers (110, 116) configured to provide a knock signal substantially greater than a noise signal; the piezoelectric ceramic wafers (110, 116) being electrically connected in parallel to provide a high voltage sensitivity at a predetermined capacitance, and wherein the piezoelectric ceramic wafers (110, 116) are placed with a polarized surface of a first wafer facing an opposite polarized surface of a second wafer with both surfaces having a substantially same electrical charge adapted to minimize noise.

Description

LINEAR TYPE HIGH OUTPUT KNOCK SENSOR

FIELD OF THE INVENTION

This invention relates to acceleration detectors and more specifically to a linear type high output knock sensor for detecting knocking in an internal combustion engine.

BACKGROUND OF THE INVENTION

Knock sensors are well known in the art. Typically, the acceleration or the vibration of the intemal combustion engine produces the movement of an internal weight relative to a housing which causes piezoelectric elements to be stressed by the inertial weight, whereby an electrical signal indicative of the movement of the inertial weight relative to the engine is generated from the piezoelectric elements. The electrical signal is provided through the washer terminal and the lead to be analyzed to determine as to whether or not a knocking signal, which is generated upon knocking of the internal combustion engine, is present.

Knock sensors can be either resonant or non-resonant in form. Resonant or Bender-Type sensors inherently have a higher sensitivity at one frequency. A problem inherent in resonant type sensors is the fact that the resonant frequency of the knock sensor will not always align with the knock frequency in the relevant range of frequencies. When this occurs, the engine knock will not be detected possibly resulting in engine damage. To avoid this problem non¬ resonant (linear) type knock sensor can be used. Non-resonant or linear-type knock sensors, resonate at much higher frequencies (approximately 50 KHz for example) and are useful over a broader range of frequencies. However, it is a common problem among non-resonant type knock sensors to have a lower voltage output than resonant type sensors.

In the frequency range of interest (approximately 5 KHz to approximately 20 KHz), a non-resonant (linear) type knock sensor has a flat response profile which enables most if not all engine knocks to be detected. It is known that non¬ resonant (linear) type knock sensors require a thicker piezoelectric element to achieve a higher voltage sensitivity. Thicker piezoelectric elements, however, create another problem which is low capacitance to drive the cable and electronic parts of the sensing circuit. This problem can be solved by placing two piezoelectric elements of predetermined thickness in parallel. By placing the piezoelectric elements in parallel, two distinct results are achieved. First, the capacitance of the sensor is doubled. Second, the overall thicker part provides a higher knock sensitivity.

With prior art linear knock sensor technology, the resultant electrical signal described above is often at an output level which is close to the background noise level of the engine. As a result, an electrical interface which is designed to detect the signal cannot differentiate between an engine knock and the background noise.

The present invention describes an improved high- output automotive knock sensor which allows an actual engine knock to be clearly distinguished from the background noise. It would be considered an improvement, if a linear type high output knock sensor could be used over a broad range of frequencies, and the linear type knock sensor could provide an electrical response of greater amplitude which uniquely allows an electronic interface to clearly distinguish an engine knock from other background noise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a high output automotive knock sensor in accordance with the present invention.

FIG. 2 shows an exploded view of the high output automotive knock sensor shown in FIG. 1 , in accordance with the present invention.

FIG. 3 shows an alternate embodiment of a high output automotive knock sensor having a through-hole, in accordance with the present invention.

FIGs. 4A and 4B show graphs of the electrical signal generated by prior art knock sensors in comparison to the electrical signal generated by the high output automotive knock sensor of the present invention, respectively.

FIG. 5 shows a simplified electrical schematic of the high output automotive knock sensor, in accordance with the present invention.

FIG. 6 shows a graph of the Net Sensitivity versus Combined Thickness (Frequency) of the high output automotive knock sensor, in accordance with the present invention.

DESCRIPTION OFTHE PREFERRED EMBODIMENT FIG. 1 shows a cross sectional view of a high output automotive knock sensor (100), in accordance with the present invention. This knock sensor (100) contains an elongated stud (101 ) having an electrically insulating base plate (102). Two piezoelectric ceramic wafers (110 and 1 16) are shown inside a housing (126) of the knock sensor (100). The housing (126) is suitably bonded to the base plate (102) with a circular ring of adhesive (104), as shown in FIG. 2. Also, a weighted component (120) is held in place with a hexagonal nut (124). Two lead wires (109 and 114) are also shown in FIG. 1. The use of custom sized piezoelectric ceramic wafers (1 10 and 1 16) connected in parallel, provide the high output characteristics of this knock sensor. The invention shown in the figures is particularly adapted to sensing knocks or (acceleration) in a range of about 5 KHz to about 20 KHz, where most knocks for internal combustion engines occur. A linear type high output knock sensor is advantageous, because it resonates at a frequency in the 50 KHz range, which is far away from the frequency of interest (5-20 KHz region) for knock sensing.

In use, when both wafers are compressed, a knock is occurring, and the result is a high output voltage (current) as shown for example in FIG. 4B. Conversely, when noise (or other vibrations) occur, one ceramic wafer is in tension and the other is in compression, which signals from each wafer substantially cancel each other out, thus minimizing background noise, as also shown in FIG. 4B. Applicant is not aware of any prior art knock sensors which have a structure and function as detailed herein. FIG. 2 shows an exploded view of the high output automotive knock sensor (100). This linear type knock sensor contains an elongated stud (101 ) having an electrically insulating base plate (102). A transducer assembly is defined by the piezoelectric ceramic wafers (1 10 and 1 16) and conductive electrical contacts (108 and 1 12). The conductive electrical contacts (108, 1 12) have lead wires (109, 114) extending therefrom which connect to terminals (128) on an outer surface (130) of the housing (100). The lead wires (109 and 1 14) provide and transport an output signal from the transducer assembly to an electronic interface.

In one embodiment, the electrical contacts (108) shown in FIG. 2 are in the form of a wrap-around type electrical contacts, to eliminate at least one manufacturing step and reduce the part count of the knock sensor (100), while maintaining a reliable electrical contact.

The knock sensor (100) also has insulator washers (106 and 118) as well as an insulator sleeve (122). A weighted component (120) provides compression to the transducer assembly, and a fastening member in the form of a hexagonal nut (124) secures all the components of the assembly in place. The entire assembly can be encapsulated inside a housing (126). The housing may be made from a metal, high temperature plastic, or any other material that can withstand an engine compartment environment and will minimize the possibility of helping to cause an electrical short with the internal components. FIG. 3 is an alternate embodiment of the knock sensor

(100) in FIGs. 1 and 2, with a through-hole version connectable to an engine block, for example. In this embodiment, the elongated stud (301 ) containing a bore (302) of smaller diameter than the stud (301 ) running its entire length. In one embodiment, the bore (302) contains internal threading to facilitate suitable mounting of the sensor. The other components which make up the high output knock sensor are all present in this embodiment. The alternative technique of mounting the sensor shown in FIG. 3 provides yet another design configuration and another technique for mounting the sensor to an engine block. When in use, the knock sensor (100 or 300) is securely mounted to an internal combustion engine. The acceleration or vibration of the intemal combustion engine causes the piezoelectric elements (110 and 116) to be stressed thereby generating an electrical signal. This electrical signal then passes through the electrical contacts and the lead wires where it is analyzed by an electronic interface to determine whether an engine knock has occurred. Once it has been determined that the electrical signal contains a knocking signal, the operating parameters of the engine can be adjusted to prevent engine damage.

In a preferred embodiment, the knock sensor (100) includes structure to create an electrical signal which has a greater amplitude when an engine knock has occurred. This becomes significant when the signal reaches the electronic interface because at this point the knock sensor (100) clearly distinguishes the background noise from an engine knock. An electrical signal (knock signal) with a greater amplitude is achieved by combining two or more piezoelectric elements of predetermined thickness in parallel to increase the overall capacitance of the sensor.

It is submitted that known prior art knock sensors have a poor signal to noise ratio due to cable and circuit capacitance, as shown by FIG. 4A, for example. Stated another way, the cable and the circuit capacitance attenuate the voltage produced by the sensor. In one prior art knock sensor in FIG. 4A (Bosch's single element sensor known as Volkswagen part no. 037905377-A), the capacitance in the cable inherently transmits noise from circuit ground, and the noise level is actually increased. Further, the signal level is actually decreased by the capacitance in the cable.

The high output knock sensor (100 and 300) of the present invention uses piezoelectric wafer elements (1 10 and 1 16) in parallel to increase the sensor capacitance relative to the cable and circuit capacitance, thereby improving the signal to noise ratio of the sensor. To applicant's knowledge, no other knock sensors in the industry combine two or more piezoelectric elements in parallel for the purpose of maximizing the net sensitivity when loaded by the cable and sensing circuit, as detailed herein.

In order to explain the relationship between piezoelectric element thickness and net sensitivity, one must first understand the role of the g33 piezoelectric voltage coefficient in piezoelectric knock sensors. As known in the art, every piezoelectric element has its own g33 piezoelectric voltage coefficient which is a function of the piezoelectric material from which the element is made. In general terms, the piezoelectric voltage coefficient is a measure of the electrical field over the stress on the wafer. Since the electrical field can be defined as the voltage over the thickness, one can conclude that the net sensitivity (or voltage) equals the stress on the wafer multiplied by the g33 piezoelectric voltage coefficient multiplied by the thickness of the wafer. In a knock sensor, the stress and the g33 piezoelectric voltage coefficient values are fixed for given conditions. Thus, it is thought that perhaps the best means by which the voltage sensitivity can be controlled is by controlling the thickness of the piezoelectric wafers.

FIG. 5 provides a simplified electrical schematic which shows loss of current through the cable and measuring circuit. The effect on the measured voltage due to the loss of current can also be expressed in the form of an equation:

Vout=i_sr_s-(icl+lc2)rm where Vout is the measurement of the voltage at the measurement electronics; is the current produced by the sensor; id is the current shunted by the cable; ic2 is the current shunted by the measurement electronics; and rm is measurement electronics input impedance. Returning to the schematic, it can be seen that cs is the capacitance of the sensor; cl is the cable capacitance; c2 is the capacitance of the measurement device; vs is the voltage produced by the sensor; and Rs is the impedance of the sensor. Thus, the equation shows that the current which reaches the measurement electronics (Vo) is that which leaves the sensor less that which is lost in the cable and measurement electronics.

In a preferred embodiment, the knock signal B is at least three times that of the noise signal, for an improved and more reliable knock detection. With this feature, more freedom is involved in selecting a threshold voltage (Vpit). Also in a preferred embodiment, the threshold voltage (signal) is about twice as much as the noise voltage (signal), to minimize false knock detections.

FIG. 6 shows how a maximum net sensitivity is achieved by using only two elements in parallel over the desired frequency range (i.e., 50 KHz to about 100 KHz). The graph in FIG. 6 shows a net sensitivity (measured in volts) versus the Combined Thickness (measured in inches) The horizontal axis of the graph also shows the corresponding frequency (measured in hertz). Note that the frequency is inversely related to the thickness. As the thickness of the piezoelectric increases, its corresponding frequency decreases. When the piezoelectric wafers are very thin or approximately less than .100 inches, net sensitivity is maximized using a single piezoelectric element. However, at this thickness, the net sensitivity level is still very low and close to the noise level.

Over the resonant frequency range of interest for linear knock sensors of approximately 50 KHz to approximately 100 KHz, the greatest net sensitivity is achieved using two piezoelectric elements in parallel, as shown in FIG. 6. This resonant frequency range is desired because it is far away from the knock detection frequency range of about 5-20 KHz. In this range, piezoelectric wafers are approximately 0.15 inches to approximately 0.30 inches thick. As can be seen from the graph in FIG. 6, a single element has a much lower voltage than two or more elements combined in parallel.

It is only at very low frequencies, approximately less than about 40 KHz, that it becomes desirable to place three or more piezoelectric elements in parallel to achieve maximum net sensitivity. By combining more elements of predetermined thickness in parallel, it is possible to achieve ever increasing net sensitivity. But when this is done, the resonant frequency is reduced because the mechanical stiffness is decreased. Eventually, the resonant frequency becomes too low for knock sensor applications. Thus, the present knock sensors (100 and 300) disclose piezoelectric elements, connected in parallel, having optimum thicknesses for knock sensor applications.

The voltage level provided by a thickness mode piezoelectric sensor is directly related to the thickness of the piezoelectric element below resonance. Unfortunately, the capacitance is inversely related to the thickness of the piezoelectric element. As a result, a low current is provided to the measuring circuit. Another factor which makes the design of a knock sensor difficult is the fact that the cable capacitance and the circuit capacitance act as a voltage divider, as shown in FIG. 5. As a result, the remaining voltage is often below the noise level found in a conventional engine compartment. This can make it very difficult to electronically determine when an engine knock occurs. As is shown in FIG. 6, when the Net Sensitivity is reduced to about 50% of the combined cable and circuit capacitance, the increased voltage produced by the thicker piezoelectric elements results in no net gain to the electronics. Thus, increasing the thickness of the piezoelectric wafers to increase the open circuit voltage sensitivity is effective until capacitance is reduced to about 50%. At this point, the Net Sensitivity curve flattens. Conversely, increasing capacitance only by combining more elements in parallel results in a loss in Net Sensitivity if the overall thickness is constrained by package size limitations and/or resonant frequency limitations. Thus, in a preferred embodiment, the wafer thickness is chosen to maximize the output of the knock sensor.

Also in a preferred embodiment, the placement of the piezoelectric wafers within the knock sensor assembly is important. By placing a polarized surface of a piezoelectric wafer having a first charge in a position such that it faces another polarized surface of a second piezoelectric wafer having the same first charge (i.e., positive in FIG. 1 and negative in FIG. 3), a vibration cancelling effect can be achieved. Thus, the placement of the polarized surfaces with predetermined charges of the piezoelectric wafers relative to each other can be critical to cancel noise and to provide large knock detection signals, as shown in FIG. 4B.

There are three major requirements that need to be dealt with to adequately sense knocks in automotive engines. These are, that sensors need to be able to sense equally engine knock over a frequency range of from about 5 to about 20 KHz, secondly, that these sensors need to provide a sufficient signal or voltage level that exceeds the voltage generated by other electromagnetic fields (noise) in an engine compartment and third, that the sensor provide adequate current to maintain the knock voltage (or signal output) level, when shunted by a cable and electronic losses. The instant knock sensor is unique in that it meets all three of the above requirements by providing multiple piezoelectric elements of a predetermined thickness with certain charges in parallel, to provide an adequate output signal which is substantially greater than the background noise in connection with the engine compartment.

As previously detailed, the resonant frequency of the knock sensor was selected at at least 50 KHz. This is important because then the sensitivity is relatively flat up to 20 KHz, which is the knock sensing frequency of interest. Selecting this resonant frequency, translates into an overall sensing element thickness of approximately .3 inches, as shown in FIG. 6 (since the resonant frequency is inversely related to the ceramic thickness with this construction). Next, as can be seen from FIG. 6, at the desired frequency, the net voltage provided by a single element sensor (denoted by the filled square) in FIG. 6, is lower than that provided by the two ceramic elements (unfilled square) in FIG. 6, of the same combined heighth (thickness) connected in parallel electrically. The parallel connection provides a sufficient current to achieve the third above described requirement (of having adequate current to maintain a predetermined voltage level when shunted by cable and other electronic losses).

When the losses and noise contributed by the connecting cable and electronics are considered, there is a limit to the amount of voltage sensitivity that can be achieved by increasing piezoceramic thickness alone (note the flattening of the curve in FIG. 6). The net benefit of an increased current provided by two or three elements in parallel is, in this case, more than offset by the loss and voltage sensitivity of the resulting thinner individual ceramic elements. In summary, all three of the above requirements need to be considered for providing an improved linear type high output knock sensor. The net sensitivity of a knock sensor built using the principles and structure as detailed herein, is improved over known prior art single element knock sensors, as detailed below in connection with the examples. As is seen in the following examples, the signal to noise ratio for the prior art sensor was on the order of about 2 (in FIG. 4A) while that of the two elements sensor of the instant invention is about 10, as shown in FIG. 4B). In use, the piezoceramic wafers will produce a current when a compressive force is applied to them due to vibration in many directions. The current produced by the two wafers in this invention, when compressed along the cylinder axis by vibration transmitted through the engine block by cylinder knock, is thus added constructively, thereby providing a large output signal (knock signal). The polarity of the top and bottom wafers are opposite, and a center electrode connects the wafers at one pole while the outer contacts are connected at the other. A noise signal or extraneous vibration, can be produced by a lead wire attachment putting one wafer in tension and the other in compression. In this instance, the current adds destructively, and is not then seen as an undesired noise on the output signal, because the destructive addition and subtraction of the wafers in tension and compression substantially cancel each other out, thereby minimizing the noise signal.

COMPARATIVE EXAMPLE A

FIGs. 4A and 4B show graphs of a prior art single element knock sensor compared to the high output knock sensor (100) in accordance with the present invention, respectively. The prior art knock sensor is built by Bosch and is known as Volkswagen part number 037905377-A. It detects engine knocks by creating a signal which is shown in FIG. 4A. In this single element prior art knock sensor, the amplitude of the engine knocks is shown as item "A", and is very close to the amplitude of the background noise. In order to detect engine knocks, an electronic interface has to be carefully programmed to detect any deviation above a threshold level as an engine knock. However, for prior art knock sensors, this threshold level would have to be set very close to the level of ordinary background noise. In FIG. 4A, the prior art electronic interface would have to be programmed to detect any deviation above V-prior-art- threshold (Vpat). In that instance, there exists a very strong possibility that the electronic interface would not reliably distinguish between background noise and an engine knock. If the deviation threshold were set too low, the electronic interface may incorrectly diagnose background noise as an engine knock. On the other hand, if the deviation threshold level were set too high, the electronic interface may fail to detect real engine knocks altogether.

EXAMPLE 1 The present invention as shown in FIGs. 1 and 2 was tested in hopes of solving this problem. With a high output knock sensor, engine knocks were detected by an increased output signal "B" in FIG. 4B. With the high output knock sensor (100), the amplitude of the electrical signal when engine knocks occur, is much greater than the amplitude of the electrical signal where there is background noise. Thus, an electronic interface can be more easily programmed to differentiate between an engine knock and background noise. With the present invention, as shown FIG. 4B, the electronic interface can be programmed to detect any deviation above V- present-invention threshold (Vpit). As should be understood, most if not all engine knocks can be more clearly detected, and substantially none of the background noise was detected as a knock. FIG. 5 details the problem solved by the present invention in electrical terms. The low current produced by a typical single piezoelectric ceramic wafer sensor is shunted by the cable and circuit capacitance leaving little electrical voltage potential to measure. The present invention combines two or more piezoelectric ceramic wafers in parallel to create a higher capacitance and more electrical current at the measurement point.

Although the present invention has been described with reference to certain preferred embodiments, numerous modifications and variations can be made by those skilled in the art without departing from the novel spirit and scope of this invention.

Claims

What is claimed is:

1. A linear type knock sensor, comprising:

a housing assembly adapted to being connectable with an engine and having output leads for providing an output signal to an electronic interface;

a transducer assembly having a plurality of piezoelectric ceramic wafers configured to provide a knock signal substantially greater than a noise signal;

the piezoelectric ceramic wafers being electrically connected in parallel to provide a high voltage sensitivity at a predetermined capacitance, and wherein the piezoelectric ceramic wafers are placed with a polarized surface of a first wafer facing an opposite polarized surface of a second wafer with both surfaces having a substantially same electrical charge adapted to minimize noise.

2. The sensor of claim 1 , wherein the knock signal includes both ceramic wafers being in compression for a predetermined period of time and a noise signal includes at least one ceramic wafer in tension and the other in compression at a predetermined period of time.

3. The sensor of claim 2, wherein the noise signal includes a first charge being triggered from one of the ceramic wafers and a second charge being triggered from the other ceramic wafer as substantially the same time, such that the noise signal is substantially minimized.

4. The sensor of claim 1 , wherein the predetermined capacitance is sufficient to have a low enough impedance to drive a cable connected to an output lead.

5. The sensor of claim 1 , wherein the polarized surface of the first wafer having a first charge and faces the opposite polarized surface of the second wafer having a second charge which is the same as that of the first wafer.

6. The sensor of claim 1 , wherein the polarized surfaces of the first wafer and the second wafer face each other and include a conductor therebetween, whereby an output signal is obtainable therefrom.

7. The sensor of claim 1 , wherein the housing includes:

an elongated stud having a base plate attached at one end; a weighted component to provide a predetermined compressive force for providing a desired characteristic; and

an attachment device for attaching the housing assembly to an engine.

8. A linear type knock sensor, comprising:

an elongated stud having a base plate attached at one end,

a transducer assembly having a plurality of piezoelectric ceramic wafers separated by conductive electrical contacts,

the piezoelectric ceramic wafers being electrically connected in parallel to provide high voltage sensitivity at high capacitance,

a weighted component to provide sufficient compression for desired electrical results,

a fastening member to hold the transducer assembly in place,

a housing assembly having an attachment means for attaching the housing assembly to an engine, and

a plurality of lead wires for providing an output signal from the transducer assembly to an electronic interface.

9. The sensor of claim 8, wherein the transducer assembly has two or more piezoelectric ceramic wafers each having a thicknesses in the range of about .150 inches to about .200 inches.

10. The sensor of claim 8 wherein the piezoelectric wafers are placed with a polarized surface of a first wafer facing a polarized surface of a second wafer adapted to provide a noise signal cancelling effect.