VIBRATION SENSOR

Abstract: In a vibration sensor (10a) including a base member (11) shaped in the form of a sleeve having a structure for installation on an object of measurement, a piezoelectric conversion portion (13) put onto the periphery of the same, and lead wire member (18) connecting the same with the outside of the sensor, the present invention is characterized in that the piezoelectric conversion portion and a portion of the lead wire member are integrally covered with polyamide resin (19a) and it is thereby formed into a case member. The present invention with the above described arrangement makes most of the characteristics of the polyamide resin having a strong crystalline polymer structure, whereby the frequency response characteristic of the vibration sensor is improved against thermal stress.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a vibration sensor for use in various technical fields. 2. Description of the Prior Art Piezoelectric vibration sensors used for detecting vibration from various vibration sources are generally classified into two large types, resonant type and nonresonant type. In the former type, the resonant frequency of the sensor is set up to be virtually the same as the frequency of the object of measurement, and thus, the sensor is arranged to have a sensitivity characteristic such that it is highly sensitive only in the set frequency band. On the other hand the latter type is arranged to have a flat sensitivity characteristic such that it is evenly sensitive over a wide range of frequency band. As the introduction to explanation of the present invention, a construction of a nonresonant vibration sensor utilizing a piezoelectric element will be described below. What is shown in FIG. 3 is a first example of a vibration sensor (knocking sensor) used for detecting vibration in an engine for an automobile. The vibration sensor 10 is constructed basically of a base member 11 in the form of a sleeve with a flange portion 11a provided at its lower part and a screw portion 11b formed at its upper part, a fixing hole 12 bored in the center of the base member 11 for fixing the sensor, for example, to an engine, a piezoelectric element 13 and a mass member 14 put onto the periphery of the base member 11 in order of mention, a nut 16 for tightening a piezoelectric element 13 and a mass member 14 through a suitable washer 15, electrode portions 17 placed in contact with upper and under sides of the piezoelectric element 13, respectively, insulating members 17a, and lead wires 18 connected to the electrode portions 17. A vibration sensor of the described structure is known as the structure of an accelerometer set forth, for example, in U.S. Pat. No. 3,307,054. The use of a resin 19 as a covering as shown in FIG. 3 is also known as a simple housing forming method. In the case of FIG. 3, the resin integrally covers the members 11 to 17 and a portion of the lead wires 18 and constitutes a case member. For example, a housing is provided by polyurethane resin 19. SUMMARY OF THE INVENTION From the principle of detection of vibration, it is easily presumed that such a housing of a resin mold as in FIG. 3 has a large adverse effect on the sensor characteristics. In the case of a nonresonont vibration sensor dealt with in the present invention, when an equation of motion with a simplified model such as a combination of a piezoelectric element and a mass member placed on the same is solved, an output voltage V of the sensor which is put under a sinusoidal vibration, for example, is given, within the range of frequencies sufficiently lower than the resonant frequency of the sensor, by where A: vibrational displacement of the object of measurement ω: angular velocity of the vibration d₃₃: piezoelectric constant of the piezoelectric element Ey: Young's modulus of the piezoelectric element S E / 33: strain of the piezoelectric element C: electrostatic capacity of the sensor (inclusive of the piezoelectric element and wiring) m: mass of the mass member t: time  : the initial point for deriving the above equation, expressed as when k representing the modulus of the spring and c representing the viscous damping coefficient in the equation of motion with tile simplified model are used. The output voltage V is thus proportional to A and ω², or, in other words, it is proportional to the acceleration. In the described case, the sensor characteristics are dependent only on the characteristics of the piezoelectric element and the mass member. In the actually constructed vibration sensor, however, such factors as a bias forth exerted when the mass member is placed on the piezoelectric element or the spring factor of the components (such as a belleville spring) are considered in addition to the above mentioned factors and thereby the sensor is carefully designed. Therefore, in putting the theory into practice, it is advantageous to add a minimum of other factors than those mentioned above that are absolutely necessary for assembling, installing or manufacturing a sensor. Hence, In designing the sensor housing, it becomes important to consider not only the objective of it to protect the piezoelectric conversion portion composed of the piezoelectric element, the mass member, etc. from environmental effects, but also the arrangement of its structure not adversely affecting the piezoelectric conversion portion at all. In this regard, there is proposed a vibration sensor without using the covering resin. The vibration sensor 20 of this second example is constructed as shown in FIG. 5, in which the structure of the sleeve-formed base member 11 and the nut 16 in the first example (FIG. 3) has been changed into a structure of an enclosed cylindrical box. More particularly, it has a structure of a cylindrical box made up of a base member 21 in the form of a cylindrical box including therein a post 21a in a sleeve form, a nut 26 with a flange 26a provided at its lower part, and a suitable packing material 26b interposed therebetween. On the other hand, there is also proposed a vibration sensor which uses a covering resin but designed so that the resin portion exercises no effect on the piezoelectric conversion portion. In the vibration sensor 30 of this third example, such a design is employed as shown in FIG. 6 that a separating member 40 is interposed between a resin portion 39 composing the case body and the piezoelectric conversion portion 13, 14, and thereby both parts are isolated. Now, the covering made of a resin as shown in the first example (FIG. 3) is put in direct contact with the piezoelectric element and the mass member, and therefore there arise various problems, that is: 1 ○ the housing exercises an effect as a mass load other than the intrinsic mass member; 2 ○ the covering for the entire piezoelectric element causes deterioration in the transverse sensitivity; 3 ○ the vibrational noise picked up by the lead wires is easily transmitted to the piezoelectric conversion portion through the housing; 4 ○ the resonant characteristic of the housing is detected by the piezoelectric element; 5 ○ mechanical characteristics of the resin (especially, its rigidity) are unstable with change in temperature; and 6 ○ molding stress is exerted on the piezoelectric conversion portion, and since the housing has a larger coefficient of expansion than the material of the piezoelectric conversion portion, the stress exerted on the piezoelectric conversion portion varies with variations in temperature. If use of such a vibration sensor is limited, for example, to detection, not measurement, of knocking out of various vibrations in an internal-combustion engine, and it is arranged to function as a knock sensor, strict designing for transverse sensitivity as with the sensor for measurement is not required therefor, and as the frequency band to be used, that exceeding 1 kHz or thereabout is considered sufficient, and hence, the effect of lead wires becomes small. Thus, for this limited use, if effects of the above mentioned factors excepting 2 ○ and 3 ○ can be minimized or canceled with each other by some way or other, the design of the example 1 is considered to become practicable. Conventionally, great importance has been attached to the structural advantages of the examples 1 to 3 such that the sensor can be installed at any position and lead wires or a connector can therefore be located at its side making the installation space small, and that the stress from fixing bolts can be prevented from affecting the piezoelectric conversion portion thanks to the structure of the base member in the sleeve form. Thus, as to the first example, any serious attention has been paid to the effect of the covering resin. The present inventor investigated the effect of the materials of the covering resin on the sensor characteristics with regard to the factors 1 ○, 4 ○, 5 ○, and 6 ○ mentioned above. Examples using polyurethane as the material for the covering resin were mentioned above. Use of the polyurethane was considered advantageous since the stress from the resin can be made smaller because it has a smaller coefficient of expansion and also is softer than other highly heat-resistant resins, and further, since the resonance characteristic of the housing can be restrained because it is soft and therefore has a good damping characteristic. That is, polyurethane was considered to exercise only little effect as to the factors 4 ○ and 6 ○. In fact, the frequency response curve at normal temperature was flat and hence there was no problem. However, when the frequency response characteristic of the sensitivity at high temperature was measured, it was found that the sensitivity suddenly tended to lower at the frequency exceeding a point. The point of the frequency where the lowering in the sensitivity started was gradually shifted to the lower frequency side with the increase in temperature. In FIG. 4 is shown a frequency response curve of the vibration sensor 10 of FIG. 3 at normal temperature and a frequency response curve of the same at high temperature around 140°C. (The axis of ordinates in the graph represents the output (mV) under a constant acceleration.) The operating temperature of a knock sensor attached to an internal-combustion engine generally rises at the highest to the point as given in FIG. 4, and the knocking frequency of the internal-combustion engine of an automobile in general is present in the range from 5 to 10 kHz. Hence, when a nonresonant vibration sensor is used as a knock sensor, a flat frequency response curve is required thereof under these two conditions (of temperature and frequency band). Thus, it is apparent that the characteristic as shown in FIG. 4 has a considerable significance in practical use of the sensor. Taking these circumstances into consideration, it is an object of the present invention to provide a vibration sensor having a simple housing only using a covering resin as described in the first example (FIG. 3) and yet capable of maintaining a flat frequency response characteristic, the same as that obtained at normal temperature, under the operating conditions dealing with the frequency band and at high temperature as mentioned above, and, when used for detection of knocking in an internal-combustion engine, capable of detecting the knocking vibration with high accuracy. In order to achieve the above mentioned object, there is provided a vibration sensor according to claim 1. The present invention with the described arrangement make the most use of the polyamide resin having a strong crystalline polymer structure, thereby improving the frequency response characteristic of the vibration sensor against thermal stress. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view showing structure of a vibration sensor whose housing is made of polyamide resin according to the present invention; FIG. 2 is a sensitivity - frequency characteristic of the vibration sensor of FIG. 1; FIG. 3 is a sectional view showing structure of a first example of prior art vibration sensors which is equivalent to that in FIG. 1 but its housing is made of polyurethane resin; FIG. 4 is a sensitivity - frequency characteristic of the vibration sensor of FIG. 3; FIG. 5 is a sectional view showing structure of a second example of prior art vibration sensors; and FIG. 6 is a sectional view showing structure of a third example of prior art vibration sensors. DESCRIPTION OF THE PREFERRED EMBODIMENTS Below will be given detailed description of the present invention with reference to illustrated embodiments. In manufacturing vibration sensors for detecting knocking in an automobile engine, for example, a substance with excellent heat resistance is first selected as the material for the covering resin constituting the case body so that the sensor can stand the use under the condition at high temperature (around 130 to 150°C). Further, in molding the case member inclusive of the cables when cables are attached to the vibration sensor, the bonding property of the resin to the cable material is also considered in the selection. Hence, the above described polyurethane resin, for example, is used as a material meeting these requirements. However, when polyurethane resin is used for the housing, it is difficult to maintain a flat sensitivity - frequency characteristic (frequency response curve) at high temperature. At first, the polyurethane was considered advantageous in view of 4 ○ and 6 ○ of the above described six factors, but in connection with the factors 1 ○ and 5 ○, it is not satisfactory since it brings about deterioration in sensitivity in the domain exceeding a particular frequency point at high temperature. The following points can be regarded as the main causes of the deterioration. Polyurethane is soft and has a good damping characteristic and this means that components of vibration at high frequencies are damped. In the present example of structure in which the housing acts as a mass load on the piezoelectric conversion portion as stated in the factor 1 ○, the housing functioning as the mass load at a low frequency band becomes unable to follow the vibration as the mass load in a domain of higher frequencies, thereby contributing to deterioration in the sensitivity. At normal temperature, the critical frequency is higher than the bandwidth required of a knock sensor. But, as the temperature rises accompanying further softening of the resin, the frequency domain in which the housing remains a vibrational load is shifted to the lower frequency side. Further, polyurethane has no good bonding property with a metallic or ceramic material, and besides, while its pressing force against such a material is kept at a certain value at normal temperature thanks to its molding shrinkage, the pressing force is released when it is expanded at high temperature, and thereby it becomes unable to remain a vibrational load. As a consequence of the foregoing, the phenomenon of lowering of the sensitivity at the higher frequency side at high temperature is considered to occur. In the present invention, after investigation of resin materials, polyamide resin was selected as the material highly heat-resistant and capable, even at the upper limit of operating temperatures, of maintaining mechanical characteristics exhibited at normal temperature and keeping a good bonding property to the piezoelectric conversion portion. By mixing glass fibers in polyamide resin, its heat resistance can be enhanced and its rigidity and coefficient of expansion can be changed. The polyamide resin is a resin having a strong crystalline polymer structure. It is known as a material being high in both strength and elongation and excellent in elasticity and extremely resistant to bending, friction, or the like. It is a material used for various applications, not only as the fiber but also for covering material for electric wires or as substitutes for metallic materials for such as gearwheels and bearings. It is generally known as Nylon (Tradename of duPont) and the use of it for the housing material is fit for its rightful purpose. The structure of the vibration sensor of the present invention is shown in FIG. 1. Although detailed description thereof is omitted here because it is of the same structure as that shown in FIG. 3 except the material used for the covering, this vibration sensor 10a is structured of a sleeve-formed base member 11 with a fixing hole 12 made therein, a piezoelectric conversion portion composed of a piezoelectric element 13 and a mass member 14, a nut 16 tightening the piezoelectric conversion portion 13, 14 through a suitable washer 15, lead wires 18 connected with the electrode portions 17 in contact with the upper side and the under side of the piezoelectric element 13, and a covering resin 19a of polyamide covering the members 11 to 17 and a portion of the lead wires 18 and constituting the case body. In the present embodiment, Nylon (Tradename of duPont) with 30% of glass fibers mixed therein is used as the polyamide resin 19a for the covering resin. FIG. 2 shows a frequency response curve of the vibration sensor using this material. The purpose of using the glass fibers mixed in the resin is, based on the investigation carried out as mentioned above, to raise the rigidity and lower the coefficient of expansion to some degree and to provide heat resistance against a temperature exceeding 150°C. As clearly seen from FIG. 2, the vibration sensor of the described arrangement provides a flat frequency response characteristic throughout the domain from the low vibration frequency to the high vibration frequency under the conditions of changed environmental temperatures. It is apparent from the above description that the covering resin originally designed to be a housing acts as the factor influencing the main function of the vibration sensor. The present invention is aimed at reducing or offsetting such influence by selecting the right material for the covering resin thereby making it possible to maintain a good performance of a vibration sensor (a knock sensor in the present example). As clearly understood from the above description, the present invention makes the most of the polyamide resin having a strong crystalline polymer structure, and hence, its composition is not to be particularly defined. It may be suitably determined at the stage of planning and designing of the sensor considering such factors as range of variations of ambient temperature, the domain of frequencies to be detected and purpose and use of the detection.

Claims: A vibration sensor including a base member (11) shaped in the form of a sleeve with a structure (12) for installation on an object of measurement, a piezoelectric conversion portion (13) put onto the periphery of the base member, a lead wire member (18) connecting the piezoelectric element with the outside of the sensor, the piezoelectric conversion portion and a portion of the lead wire member being integrally covered with a polymer resin (19a), thereby forming it into a case member, characterized in that the polymer is polyamide. A vibration sensor according to claim 1, wherein said case member is formed of polyamide resin with glass fibers mixed therein. A vibration sensor according to claim 1 or 2, wherein the content of the glass fibers is 30%. A vibration sensor according to claim 1, 2, or 3, wherein said vibration sensor is a knock sensor for detecting vibration in an internal-combustion engine for an automobile engine or the like.

Inventors: MOCHIZUKI, KAZUO
Applicants: TDK CORPORATION
Application year: 1988
Main classification: G01H11
IPC codes: G01H11, G01H17