Acceleration sensor having a self test feature

Abstract: A fixed base plate (10) having a rigidity and a flexible base plate (20) are so provided as to face each other. The peripheries of both the base plates are fixed on a casing (40) of a device. An acting body (30) is bonded to the lower surface of the flexible base plate. Testing electrodes (11t, 13t, 15t) are formed on the lower surface of the fixed base plate and further fixed electrodes (11, 13, 15) are formed under the testing electrodes via an insulation layer (16). Displacement electrodes (21, 23, 25) are formeed on the upper surface of the flexible base plate. When an acceleration acts on the acting body, the flexible base plate bends and the distances between the fixed electrodes and the displacement electrodes changes, causing the capacitance between the electrodes to change. The acted acceleration can be measured by measuring the capacitance changed. For testing the performance of the device, a Coulomb force is made to act on the testing electrodes and the displacment electrodes by applying voltages across both the electrodes. The flexible base plate is bent due to the Coulomb forces and there is the same state as the one when the acceleration acts. The testing of the performance becomes possible by measuring the change of the capacitance between the fixed electrodes and the displacement electrodes in this state.

Description: This invention relates to a detector for a physical quantity such as force, acceleration, or magnetism, etc.. In the automobile industry or the machinery industry, there has been increased demand for detectors capable of precisely detecting a physical quantity such as force, acceleration or magnetism. Particularly, it is required to realize small detectors capable of detecting physical quantities every respective two dimensional or three dimensional components. To meet such a demand, there has been proposed a force detector in which gauge resistors are formed on a semiconductor substrate such as silicon, etc. to transform a mechanical distortion produced in the substrate on the basis of a force applied from the external to an electric signal by making use of the piezo resistance effect. When a weight body is attached to the detecting unit of the force detector, an acceleration detector for detecting, as a force, an acceleration applied to the weight body can be realized. Further, when a magnetic body is attached to the detecting unit of the force detector, a magnetic detector for detecting, as a force, a magnetism exerted on the magnetic body can be realized. For example, in US-A-4,905,523, US-A-4,967,605, US-A-4,969,366, detectors for force, acceleration or magnetism according to the invention by the inventor of this application are disclosed. In place of the above described detectors utilizing the piezo resistance effect, detectors utilizing changes in distance between electrodes are proposed. For example, in the Japanese Patent Application JP-A-274299/90 specification, there is disclosed a detector of a simple structure including two substrates oppositely arranged and electrodes formed on respective substrates. In this detector, one substrate is allowed to be subjected to displacement on the basis of a physical action such as force, acceleration, or magnetism, etc. to be detected. An applied force changes the distance between electrodes formed on both the substrates. By this displacement the applied force is detected as a change of an electrostatic capacity between both the electrodes. Alternatively, there is also disclosed a method in which a piezo electric element is put between both electrodes to detect a change of the distance between the electrodes as a voltage produced from the piezo electric element. If an external force is directly exerted to one substrate, this detector functions as a force detector for detecting an external force exerted. Further, if a weight body is connected to one substrate so that the substrate is subjected to displacement on the basis of an acceleration exerted on the weight body, this detector functions as an acceleration detector for detecting an acceleration exerted. In addition, if a magnetic body is connected to one substrate so that the substrate is subjected to displacement on the basis of a magnetism exerted on the magnetic body, this detector functions as a magnetic detector for detecting a magnetism exerted. Generally, in the case of providing apparatuses for detecting any physical quantity as commercial products, there takes place the necessity of carrying out an operation test as to whether or not this detector outputs a correct detection signal. As such an operation test, there is adopted a method of actually exerting a physical quantity to be detected on that detector to examine a detection signal at that time. For example, in the case of an acceleration detector, there is employed a method of actually applying, from a predetermined direction, an acceleration of a predetermined magnitude to the detector to judge whether or not a detection signal at that time is a correct one corresponding to the applied acceleration. However, in order to carry out such an operation test, a test equipment exclusively used therefor is required, and the test work is complicated or troublesome, requiring much time. Especially, the quantity of detectors which can be tested by an exclusive test equipment is limited, resulting in lowered productivity. Accordingly, such a conventional test method is not suitable as the method of testing the operation of detectors mass produced. From an article of V. Allen et al.: "Beschleunigungsmesser mit Selbsttest", UND- ODER- NOR + STEUERUNGSTECHNIK, Band 23 (1990) Nr. 3, pages 30 to 33, an acceleration sensor having a self test feature has come to be known. This known sensor is a linear acceleration sensor of the piezoresistive type having a self test feature, comprising a sensor casing, a working body flexibly supported by said sensor casing, fixed test electrode means placed on said sensor casing, and voltage application means for applying a predetermined voltage to move the working body by a Coulomb force. From document US-A-4,950-914 a collision detection system for a vehicle exhibiting a multi-acceleration sensor of the piezoelectric type has come to be known. This known sensor comprises a sensor casing, a working body flexibly supported by said sensor casing so that a displacement is produced between the working body and the sensor casing when an acceleration is applied to the working body, detection means for detecting said applied acceleration based on said displacement, fixed test electrode means placed on said sensor casing, displacement test electrode means placed on said working body, and voltage application means for applying a predetermined voltage between said fixed test electrode means and said displacement test electrode means to move the working body. An object of this invention is to provide an acceleration sensor, having a self diagnostic function by this simple operation test method capable of conducting a simple operation test of apparatuses for detecting a physical quantity utilizing changes in distance between electrodes. This object is solved according to the invention by an acceleration sensor exhibiting the features as set out in independent patent claim 1. Dependent claims 2-6 exhibit further improvements of the subject-matter of claim 1. Subject-matter of patent claim 1 is related to a method for testing the operation of a detector including a displacement electrode supported so that it can be subjected to displacement by application of an external force, a fixed electrode fixed to a detector casing at a position opposite to the displacement electrode, and detection means for taking out, as an electric signal, a change of a distance between both the eletrodes, thus to detect a physical quantity corresponding to the external force as an electric signal. This operation test can be grasped in a manner classified into four methods as described below: The first method of the operation test resides in a method of applying a predetermined voltage across the fixed electrode and the displacement electrode, allowing the displacement electrode to be subjected to displacement by a coulomb force produced on the basis of the applied voltage, and comparing an electric signal detected by the detection means in this displacement state with the applied voltage to thereby test the operation of this detector. The second method of the operation test resides in a method in which there is further provided a test electrode fixed to the detector casing at a position opposite to the displacement electrode, the method comprising the steps of applying a predetermined voltage across the test electrode and the displacement elcctrode, allowing the displacement electrode to be subjected to displacement by a coulomb force produced on the basis of the applied voltage, and comparing an electric signal detected by the detection means in this displacement state with the applied voltage to thereby test the operation of this detector. The third method of the operation test resides in a method in which there is further provided a test electrode supported so that it can be subjected to displacement together with the displacement electrode, the method comprising the steps of applying a predetermined voltage across the test electrode and the fixed electrode, allowing the displacement electrode to be subjected to displacement by a coulomb force produced on the basis of the applied voltage, and comparing an electric signal detected by the detection means in this displacement state with the applied voltage to thereby test the operation of this detector. The fourth method of the operation test resides in a method in which there are further provided a first test electrode supported so that it can be subjected to displacement together with the displacement electrode and a second test electrode fixed to the detector casing at a position opposite to the first test electrode, the method comprising the steps of applying a predetermined voltage across the first and second test electrodes, allowing the displacement electrode to be subjected to displacement by a coulomb force produced on the basis of the applied voltage, and comparing an electric signal detected by the detection means in this displacement state with the applied voltage to thereby test the operation of this detector. In some relationship to this invention, by applying the above described operation test method to a physical quantity detector of the type for detecting changes in the distance between electrodes as changes in the electrostatic capacitance, three detectors having a self diagnostic function which will be described below can be realized. A first detector comprises: a flexible substrate including a fixed portion fixed to a detector casing, a working portion adapted to receive a force based on a physical action such as force, acceleration or magnetism from the external, and a flexible portion having flexibility formed between the fixed portion and the working portion; a fixed substrate fixed to the detector casing so as to oppose or face the flexible substarte; a displacement electrode formed at a position where a displacement is produced by bending of the flexible substrate; a fixed electrode fixed to the fixed substrate and formed at a position opposite to the displacement electrode; a test electrode fixed to the detector casing at a position opposite to the displacement electrode and electrically insulated from the fixed electrode; detection means for outputting, as an electric signal, a change of an electrostatic capacitance produced between the displacement electrode and the fixed electrode; and voltage application means for applying a predetermined voltage across the test electrode and the displacement electrode, to detect a force exerted on the working portion on the basis of an electric signal outputted from the detection means, and to compare the electric signal outputted from the detection means with the applied voltage applied by the voltage application means, thus making it possible to carry out the operation test. The second detector comprises: a flexible substrate including a fixed portion fixed to a detector casing, a working portion adapted to receive a force based on a physical action such as force, acceleration or magnetism, etc. from the external, and a flexible portion having flexibility formed between the fixed portion and the working portion; a fixed substrate fixed to the detector casing so as to oppose or face the flexible substrate; a displacement electrode formed at a position where a displacement is produced by bending of the flexible substrate; a fixed electrode fixed by the fixed substrate and formed at a position opposite to the displacement electrode; a test electrode subjected to displacement together with the displacement electrode at a position opposite to the fixed electrode, and electrically insulated from the displacement electrode; detection means for outputting, as an electric signal, a change of an electrostatic capacitance produced between the displacement electrode and the fixed electrode; and voltage application means for applying a predetermined voltage across the test electrode and the fixed electrode, to detect a force exerted on the working portion on the basis of an electric signal outputted from the detection means, and to compare the electric signal outputted from the detection means with the applied voltage applied by the voltage application means, thereby making it possible to carry out the operation test. The third detector comprises: a flexible substrate including a fixed portion fixed to a detector casing, a working portion adapted to receive a force based on a physical action such as force, acceleration or magnetism, etc. from the external, and a flexible portion having flexibility formed between the fixed portion and the working portion; a fixed substrate fixed to the detector casing so as to oppose or face the flexible substrate; a displacement electrode formed at a position where a displacement is produced by bending of the flexible substrate; a fixed electrode fixed by the fixed substrate and formed at a position opposite to the displacement electrode; a first test electrode formed so that the flexible substrate is bent on the basis of displacement of the first test electrode itself; a second test electrode fixed to the detector casing at a position opposite to the first test electrode and electrically insulated from the fixed electrode; detection means for outputting, as an electric signal, a change of an electrostatic capacitance produced between the displacement electrode and the fixed electrode; and voltage application means for applying a predetermined voltage across the first and second test electrodes, to detect a force exerted on the working portion on the basis of the electric signal outputted from the detection means, and to compare the electric signal outputted from the detection means with the applied voltage applied by the voltage application means, thereby making it possible to carry out the operation test. Further, a further detector relating to this invention comprises: a flexible substrate including a fixed portion fixed to a detector casing, a working portion adapted to receive a force based on a physical action from the external, and a flexible portion having flexibility formed between the fixed portion and the working portion; a first fixed substrate fixed to the detector casing so as to oppose or face a first plane of the flexible substrate; a second fixed substrate fixed to the detector casing so as to oppose or face a second plane of the flexible substrate; a first displacement electrode formed on the first plane of the flexible substrate; a second displacement electrode formed on the second plane of the flexible substrate; a first fixed electrode fixed by the first fixed substrate and formed at a position opposite to the first displacement electrode; a second fixed electrode fixed by the second fixed substrate and formed at a position opposite to the second displacement electrode; and detection means for outputting, as an electric signal, a difference between a change of an electrostatic capacitance produced between the first displacement electrode and the first fixed electrode and a change of an electrostatic capacitance produced between the second displacement electrode and the second fixed electrode, to detect a force exerted on the working portion on the basis of the electric signal outputted from the detection means. FIG. 1 is a side cross sectional view showing the basic structure of an acceleration detector of the electrostatic capacitance type to which a method of testing the operation according to this invention is applied. FIG. 2 is a bottom view of a fixed substrate 10 of the detector shown in FIG. 1. The cross section cut along the X-axis of the fixed substrate 10 of FIG. 2 is shown in FIG. 1. FIG. 3 is a top view of a flexible substrate 20 of the detector shown in FIG. 1. The cross section cut along the X-axis of the flexible substrate 20 is shown in FIG. 1. FIG. 4 is a side cross sectional view showing a bent state of the detector when a force Fx in an X-axis direction is exerted at the working point P of the detector shown in FIG. 1. FIG. 5 is a side cross sectional view showing a bent state of the detector when a force Fz in a Z-axis direction is exerted on the working point P of the detector shown in FIG. 1. FIG. 6 is a circuit diagram showing a circuit for operating the detector shown in FIG. 1 and a circuit for implementing an operation test thereto. FIG. 7 is a circuit diagram showing an example of an actual circuit configuration of the CV converting circuit shown in FIG. 6. FIG. 8 is a side cross sectional view showing an actual test method with respect to the detecting operation in a positive Z-axis direction. FIG. 9 is a side cross sectional view showing an actual test method with respect to the detecting operation in a negative Z-axis direction. FIGS. 10a and 10b are side cross sectional views of an acceleration detector of the electrostatic capacitance type in which the operation test method according to this invention is carried out to thereby provide a function to carry out a self diagnosis. FIG. 11 is a circuit diagram showing a circuit for operating the detector shown in FIG. 10a and a circuit for implementing an operation test thereto. FIG. 12 is a side cross sectional view of another acceleration detector of the electrostatic capacitance type in which the method of testing the operation according to this invention is carried out to thereby provide a function to carry out a self diagnosis. FIG. 13 is a side cross sectional view of a force detector of the electrostatic capacitance type in which the method of testing operation according to this invention is carried out to thereby provide a function to carry out a self diagnosis. FIG. 14 is a side cross sectional view of an acceleration detector of the type to take differences in detection of acceleration components in all directions. FIGS. 15a and 15b are plane views showing an electrode arrangement in the detector shown in FIG. 14. FIG. 16 is a circuit diagram of a circuit for carrying out detection of an acceleration in a Z-axis direction in the detector shown in FIG. 14 and for carrying out the operation test thereof. FIG. 17 is a graph showing the general relationship between the distance d between electrodes of the capacitance element and the capacitance value C. FIG. 18 is a side cross sectional view showing another embodiment of the detector shown in FIG. 14. FIG. 19 is a side cross sectional view showing a further embodiment of the detector shown in FIG. 14. FIG. 20 is a side cross sectional view showing an embodiment in which the detector shown in FIG. 14 is constituted with metal. FIG. 21 is a side cross sectional view showing an embodiment in which the detector shown in FIG. 14 is applied to a force detector. Prior to the description of an operation test method relating to this invention, the structure of a detector to which this invention is applied and the principle thereof will be briefly described. FIG. 1 is a side cross sectional view showing the basic structure of an acceleration detector to which this invention is applied. This detector comprises, as the major component, a fixed substrate 10, a flexible substrate 20, a working body 30, and a detector casing 40. The bottom view of the fixed substrate 10 is shown in FIG. 2. The cross section cut along the X-axis of the fixed substrate 10 in FIG. 2 is shown in FIG. 1. The fixed substrate 10 is formed as a disk shaped substrate as shown, and is fixed at the peripheral portion thereof to the detector casing 40. On the lower surface thereof, fan shaped fixed electrodes 11 to 14 and a disk shaped fixed electrode 15 are formed. On the other hand, the top view of the flexible substrate 20 is shown in FIG. 3. The cross section cut along the X-axis of the flexible substrate in FIG. 3 is shown in FIG. 1. The flexible substrate 20 is also formed as a disk shaped substrate as shown, and is fixed at the peripheral portion thereof to the detector casing 40. On the upper surface thereof, fan shaped displacement electrodes 21 to 24 and a disk shaped displacement electrode 25 are formed. The working body 30 is columnar as the upper surface thereof is indicated by broken lines in FIG. 3, and is coaxially connected to the lower surface of the flexible substrate 20. The detector casing 40 is cylindrical, and fixes and supports the peripheral portions of the fixed substrate 10 and the flexible substrate 20. The fixed substrate 10 and the flexible substrate 20 are arranged with a predetermined spacing therebetween at positions in parallel to each other. While both substrates are a disk shaped substrate, the fixed substrate 10 is a substrate having high rigidity such that bending is difficult to occur, whereas the flexible substrate 20 is a substrate having flexibility such that when a force is applied, bending occurs. In the example shown in FIG. 1, the fixed substrate is caused to have high rigidity by allowing the thickness thereof to be thick, and the flexible substrate 20 is caused to have flexibility by allowing the thickness thereof to be thin. In addition to the above, they may be caused to have rigidity or flexibility by changing material thereof. Alternatively, by forming a groove in the substrate, or forming a through hole therein, such members may be caused to have flexibility. As long as the fixed substrate 10, the flexible substrate 20 and the working body 30 can perform their primary functions, they may be constituted with any material. For example, they may be constituted with semiconductor or glass, etc., or may be constituted with metal. It is to be noted that in the case where the fixed substrate 10 and the flexible substrate 20 are constituted with metal, it is necessary to adopt a method of forming insulating layers between the metal and the respective electrodes in order not to short circuit the electrodes, or similar methods. In addition, respective electrode layers may be constituted with any material as long as it has conductive property. It is now assumed that the working point P is defined at the center of gravity of the working body 30 as shown in FIG. 1, and that a XYZ-three-dimensional coordinate system having the working point P as the origin is defined as shown. Namely, the X-axis is defined in a right direction of FIG. 1, the Z-axis is defined in an upper direction thereof, and the Y-axis is defined in a direction perpendicular to the plane surface of paper and extending toward the back side of the plane surface of paper. If the central portion to which the working body 30 is connected, the peripheral portion fixed by the detector casing 40, and the portion therebetween of the flexible substrate 20 are respectively called a working portion, a fixed portion, and a flexible portion, when an acceleration is applied to the working body 30, bending is produced in the flexible portion, so the working portion is subjected to deviate from the fixed portion. Assuming now that the entirety of this detector is mounted, e.g., in an automotive vehicle, an acceleration will be applied to the working body 30 on the basis of running of the automotive vehicle. By this acceleration, an external force is exerted on the working point P. In the state where no force is exerted on the working point P, as shown in FIG. 1, the fixed electrodes 11 to 15 and the displacement electrodes 21 to 25 are maintained in a parallel state with a predetermined spacing therebetween. It is now assumed that combinations of the fixed electrodes 11 to 15 and the displacement electrodes opposite thereto are called capacitance elements C1 to C5, respectively. Now, when a force Fx in the X-axis direction is exerted on the working point P, this force Fx allows the flexible substrate 20 to produce a moment force. As a result, as shown in FIG. 4, bending will be produced in the flexible substrate 20. By this bending, the spacing between the displacement electrode 21 and the fixed electrode 11 is increased, but the spacing between the displacement electrode 23 and the fixed electrode 13 is decreased. When a force exerted on the working point P is assumed to be -Fx in an opposite direction, bending having the relationship opposite to the above will be produced. As stated above, when a force Fx or -Fx is exerted, any change appears in the electrostatic capacitance of the capacitance elements C1 and C3. Accordingly, by detecting this change, it is possible to detect the force Fx or -Fx. At this time, although respective spacings between displacement electrodes 22, 24 and 25 and fixed electrodes 12, 14 and 15 partially increase or decreace, their spacings may be assumed to be unchanged as a whole. On the other hand, in the case where a force Fy or -Fy in the Y direction is exerted, changes similar to the above are produced only in connection with the spacing between the displacement electrode 22 and the fixed electrode 12 and the spacing between the displacement electrode 24 and the fixed electrode 14. Further, in the case where a force Fz in the Z-axis direction is exerted, as shown in FIG. 5, the spacing between the displacement electrode 25 and the fixed electrode 15 becomes small. In contrast, in the case where a force -Fz in an opposite direction is exerted, this spacing becomes large. At this time, the spacings between the displacement electrodes 21 to 24 and the fixed electrodes 11 to 14 also become small or large. In this case the spacing between the displacement electrode 25 and the fixed electrode 15 most conspicuously varies. In view of this, by detecting the change of the electrostatic capacitance of the capacitance element C5, it is possible to detect the force Fz or -Fz. When it is assumed that the electrode area, the electrode interval and the dielectric constant are represented by S, d and ε , respectively, the electrostatic capacitance C of the capacitance element is generally determined by the following equation: C = ε S/d. Accordingly, when the electrode interval becomes short, the electrostatic capacitance C becomes large, while when it becomes broad, the electrostatic capacitance C becomes small. This detector utilizes the above mentioned principle to measure changes in the electrostatic capacitance between respective electrodes, thus to detect an external force exerted on the working point P, i.e., an acceleration exerted. Namely, an acceleration in an X-axis direction is detected on the basis of changes in the capacitance between capacitance elements C1 and C3, an acceleration in a Y-axis direction is detected on the basis of changes in the capacitance between the capacitance elements C2 and C4, and an acceleration in a Z-axis direction is detected on the basis of changes in the capacitance of the capacitance element C5. This invention relates to a method of testing the operation of the detector based on the above mentioned principle. It is to be noted that while the above described detector is of the electrostatic capacitance type, in the case of a detector of the piezo electric type, a piezo electric element is inserted between the flexible substrate and the fixed substrate. In this case, in place of detecting changes in the electrostatic capacitance, a voltage produced from the piezo electric element is detected. A method of testing the operation related to this invention will now be described. FIG. 6 is a circuit for operating the detector shown in FIG. 1 and a circuit for implementing the operation test thereto. Here, capacitance elements C1 to C5 correspond to capacitance elements formed at the above described acceleration detector. For example, the capacitance element C1 is comprised of combination of the fixed electrode 11 and the displacement electrode 21. Further, CV converting circuits 51 to 55 connected to respective capacitance elements C1 to C5 have a function to convert electrostatic capacitance values C of respective capacitance elements to corresponding voltages V. Accordingly, voltage values V1 to V5 outputted from the CV converting circuits 51 to 55 take values proportional to electrostatic capacitance values of the capacitance elements C1 to C5. A differential amplifier 71 outputs a difference between voltage values V1 and V3 to the terminal Tx, and a differential amplifier 72 outputs a difference between voltage values V2 and V4 to the terminal Ty. Further, a voltage value V5 is outputted to the terminal Tz. By making reference to the explanation of the structure and the operation of the detector described in §1, it can be understood that a voltage (V1 - V3) obtained on the terminal Tx becomes an acceleration detection value in an X-axis direction, a voltage (V2 - V4) obtained on the terminal Ty becomes an acceleration detection value in a Y-axis direction, and a voltage V5 obtained on the terminal Tz becomes an acceleration detection value in a Z-axis direction. It is to be noted that an acceleration in an X-axis or a Y-axis direction is detected by taking a difference between two voltages by the differential amplifier as described above. Such a detection based on difference can advantageously cancel an error (e.g., temperature error) resulting from the external environment. It is to be noted that while detection based on difference is not conducted in connection with an acceleration in a Z-axis direction, this will be described in the succeeding §4. The CV converting circuits 51 to 55 and the differential amplifiers 71 and 72 which have been described are eventually circuits necessary for allowing this detector to carry out the detecting operation. The operation test according to this invention can be carried out by further adding voltage generating circuits 61 to 65 and test switches S1 to S5 to the above mentioned circuit. Any voltage generating circuits 61 to 65 may be employed as long as they are circuits capable of generating a desired voltage. For example, there may be employed such a circuit to convert digital data outputted from a microcomputer to an analog signal at a D/A converter. Let now consider the case where test switches S1 and S2 are turned ON in the circuit shown in FIG. 6. At this time, if there is employed a circuit configuration such that charges having polarities opposite to each other are delivered from the voltage generating circuit 63 to both the electrodes of the capacitance element C3, one electrode is charged positive and the other electrode is charged negative between the fixed electrode 13 and the displacement electrode 23. For this reason, an attractive force based on coulomb force is exerted therebetween. Further, if there is employed a circuit configuration such that charges having the same polarity are delivered from the voltage generating circuit 61 to the both electrodes of the capacitance elenent C1, a repulsive force based on coulomb force is exerted therebetween. Thus, as shown in FIG. 4, the flexible substrate 20 will produce displacement. It is seen that this is the same state as the state where a force Fx in an X-axis direction is exerted on the working point P. In this state, detected voltages outputted to respective terminals Tx, Ty and Tz are examined to determine whether or not they are correct detected values indicating that the force Fx is exerted. Eventually, the operation test is carried out by applying voltages from the voltage generating circuits 61 and 63 to the capacitance elements C1 and C3 to thereby create the same state as the state where a force Fx in an X-axis direction is exerted on the working point P to examine detected voltages. If a correct relationship between an applied voltage and a detected voltage is determined in advance, a quantitative operation test can be conducted. In this operation test, there is no necessity of using vibrator, etc. to actually apply an acceleration to the working body 30. Namely, only by observing an electric signal outputted when an electric signal is inputted, the operation test is completed. Accordingly, the work becomes simple as compared to the conventional operation test, and is thus suitable for mass production. The test of the detecting operation of an acceleration in an -X axis direction may be conducted by applying voltages to capacitance elements C1 and C3 by using the switches S1 and S3. Further, the test of the detecting operation of an acceleration in a Y-axis direction may be conducted by applying voltages to capacitance elements C2 and C4 by using the switches S2 and S4. In addition, the test of the detecting operation of an acceleration in a Z-axis direction may be conducted by applying a voltage to the capacitance element C5 by using the switch S5. In the circuit diagram shown in FIG. 6, it is preferable to employ a voltage generating circuit such that the capacitance values of circuits 61 to 65 viewed from the external are constant irrespective of voltages generated. Alternatively, a voltage generating circuit having a predetermined correlation between a voltage generated and a capacitance viewed from the external may be adopted. Since the voltage generating circuits 61 to 65 are connected in parallel with capacitance elements C1 to C5 when viewed from the circuit arrangement, if the capacitance values of the voltage generating circuits 61 to 65 vary at random, a correct operation test cannot be carried out. Further, it required for the CV converting circuits 51 to 55 to detect electrostatic capacitance values of the capacitance elements C1 to C5 without undergoing the influence of voltages applied by the voltage generating circuits 61 to 65. An example of a CV converting circuit having such a function is shown in FIG. 7. The CV converting circuit 50 shown here has a function to convert an electrostatic capacitance C of a capacitance element C0 to a voltage V to output it to the terminal Tout. It is here noted that a voltage is applied from the voltage generating circuit 60 to the capacitance element C0. The CV converting circuit 50 is comprised of an oscillator circuit 56 and a rectifier circuit 57. The oscillator circuit 56 generates an a.c. signal of a predetermined frequency to apply it to the capacitance element C0. The rectifier circuit 57 is comprised of resistors R1 and R2, capacitors C6 and C7, and a diode D1, and serves to convert an electrostatic capacitance C of the capacitance element C0 supplied with the a.c. signal to a voltage V to output it. The CV converting circuit 50 thus constructed can convert an electrostatic capacitance C of the capacitance element C0 to a voltage V without undergoing the influence of an applied voltage from the voltage generating circuit 60. It is to be noted that the circuit of FIG. 7 is illustrated as an example, and various circuits can be therefore applied for the CV converting circuit in addition thereto. The operation test according to this invention will now be described in connection with a further actual embodiment. FIG. 8 is a side cross sectional view showing an actual test method with respect to the operation for detecting a force in a positive Z-axis direction. This detector is comprised of a fixed substrate 80 and a flexible substrate 90. At the bottom surface of the flexible substrate 90, a doughnut shaped groove G is dug. The thickness of the portion where the groove G is dug is thinner than those of other portions. Thus, the flexible substrate 90 is caused to have flexibility at this portion. The fixed substrate 80 is connected or bonded to the upper surface of the flexible substrate 90 with a predetermined space being kept therebetween in a manner to cover the upper surface of the flexible substrate 90. A plurality of fixed electrodes 81 and a plurality of displacement electrodes 91 are formed at positions opposite to each other on the lower surface of the fixed substrate 80 and the upper surface of the flexible substrate 90, respectively. In this embodiment, the fixed substrate 80 and the flexible substrate 90 are comprised of a glass substrate and a silicon substrate, respectively. The fixed electrodes 81 and the displacement electrodes 91 are comprised of aluminum layers formed on the respective substrates. Further, between the flexible substrate 90 and the displacement electrodes 91, an insulating layer 92 such as a silicon oxide film or a silicon nitride film is formed. In such a detector, in order to make a state which is equivalent to the state where a force in a positive Z-axis direction is exerted on the working point P, it is sufficient to exert an attractive force based on coulomb force between the fixed electrodes 81 and the displacement electrodes 91. FIG. 8 shows a method of applying a voltage in this case. Namely, when positive charges and negative charges are respectively given to the fixed electeodes 81 and the displacement electrodes 91 by means of power supply V, an attractive force is exerted therebetween. As a result, the operation test in the state where a force in a positive Z-axis direction is exerted on the working point P can be conducted. It is to be noted that, in order to make a state which is equivalent to the state where a force in a negative Z-axis direction is exerted on the working point P, it is required to change somewhat the structure of the detector itself. Namely, as shown in FIG. 9, a plurality of auxiliary electrodes 82 are formed on the upper surface of the fixed electrode 80. Here, by using the power supply V, positive charges are given to the auxiliary electrodes 82 and the flexible substrate 90 (silicon substrate) and negative charges are given to the fixed electrodes 81 and the displacement electrodes 91. Thus, polarization takes place between the auxiliary electrodes 82 and the fixed electrodes 81, and polarization takes place between the displacement electrodes 91 and the flexible substrate 90. As a result, respective portions are charged so that they have polarities as shown. Eventually, a repulsive force based on coulomb force is exerted between the fixed electrodes 81 and the displacement elctrodes 91, resulting in the state where a force in a negative Z-axis direction is exerted on the working point P. In the operation test in the state where an attractive force is exerted between both members, this test can be sufficiently conducted only by two electrodes which directly exert coulomb force. However, in the operation test in the state where a repulsive force is exerted thetebetween, it is required to additionally form an auxiliary electrode. It is to be noted that since the description of the structure becomes complicated, the description of this auxiliary electrode will be omitted in the following respective embodiments. In the above described embodiment, the electrode pair for generating coulomb force and the electrode pair for constituting a capacitance element are the same electrode pair. Namely, the electrode pair for generating coulomb force is a pair of the fixed electrode (11 to 15, 81) and the displacement electrode (21 to 25, 82), and the electrode pair for constituting a capacitance element is exactly the same electrode pair. If the same pairs are commonly used in this way, although there is the advantage that it is unnecessary to form another electrode for the operation test, there is the drawback that there occurs limit in the degree of test and the circuit for test becomes complicated. Particularly, in order for such equipment to circulate as goods on the market, it is convenient that a detection terminal for outputting a detection signal of a physical quantity and a test terminal for applying a voltage for test are separately provided. In the detector disclosed below, test electrodes are formed in advance, and an electrode pair for generating coulomb force and an electrode pair for constituting a capacitance element are constructed by separate electrode pairs. FIG. 10a is a side cross sectional view of an acceleration detector provided with a test electrode for carrying out an operation test method according to this invention. The basic structure is the same as the acceleration detector shown in FIG. 1 wherein a fixed substrate 10 and a flexible substrate 20 are oppositely provided, and respective substrates are fixed at their periphery thereof to a detector casing 40. The fixed substrate 10 is a substrate having rigidity, but the flexible substrate 20 is thin in thickness thus to have flexibility. On the upper surface of the flexible substrate 20, five displacement electrodes 21 to 25 as shown in FIG. 3 are formed. On the other hand, on the lower surface of the fixed substrate 10, five test electrodes 11t to 15t are formed, and five fixed electrodes 11 to 15 are further formed through an insulating layer 16. The planar arrangement of the five fixed electrodes 11t to 15t and the planar arrangement of the five fixed electrodes 11 to 15 are the same as the electrode arrangement shown in FIG. 2. In such a structure, the electrode pairs for constituting capacitance elements C1 to C5 is an electrode pairs of the fixed electrodes 11 to 15 and the displacement electrodes 21 to 25, but the electrode pairs for generating coulomb force are pairs of the test electrodes 11t to 15t and the displacement electrodes 21 to 25. In these electrode pairs, displacement electrodes are commonly used. FIG. 10b is an embodiment in which five test electrodes 21t to 25t are formed on the flexible substarate 20. Between the test electrodes 21t to 25t and the displacement electrodes 21 to 25, an insulating layer 26 is formed. In this case, the electrode pairs for generating coulomb force are pairs of the test electrodes 21t to 25t and the fixed electrodes 11 to 15. FIG. 11 is an example of a circuit diagram in which this invention is applied to a detector of a structure as shown in FIG. 10a. A voltage generating circuit 60 applies a voltage across the test electrode llt and the displacement electrode 21 by allowing the test switch S to be turned ON to exert coulomb force therebetween. Thus, the flexible substrate 20 produces bending, resulting in the equivalent state where an external force is exerted. On the other hand, the CV converting circuit 50 detects an electrostatic capacitance of the capacitance element C1 comprised of the fixed electrode 11 and the displacement electrode 21 to output it as a voltage V. Since the fixed electrode 11 and the test electrode 11t are electrically insulated by the insulating layer 16, the CV converting circuit 50 can carry out detection of the electrostatic capacitance without being affected by any means by an applied voltage produced by the voltage generating circuit 60. As the external connection terminal of this detector, it is sufficient to provide a common terminal conducting to respective displacement electrodes 21 to 25, detection terminals conducting to respective fixed electrodes 11 to 15, test terminals conducting to respective test terminals 11t to 15t. By confirmining whether or not a predetermined output is provided on the detection terminal when a predetermined voltage is applied to the test terminal, it is possible to easily carry out self diagnosis. FIG. 12 shows an embodiment in which electrodes on the flexible substrate 20 side are separately provided without being commonly used. Namely, first test electrodes 21t to 25t are formed on the upper surface of the flexible substrate 20, and displacement electrodes 21 to 25 are formed through an insulating layer 26 thereon. Further, the fixed substrate 10 side is similar to that in the previously described embodiment, i.e., fixed electrodes 11 to 15 are formed through an insulating layer 16 on second test electrodes 11t to 15t. In such a structure, the electrode pairs for constituting capacitance elements C1 to C5 are pairs of fixed electrodes 11 to 15 and displacement electrodes 21 to 25, but the electrode pairs for generating coulomb force are pairs of the first test electrodes 21t to 25t and the second test electrodes 11t to 15t. The both electrode pairs completely separated from each other are used as the electrode pair. FIG. 13 is a side cross sectional view of a force detector related to the invention and described for illustration purposes. Within a detector casing 100, a fixed substrate 110, a flexible substrate 120, a working body 130, and an auxiliary substrate 140 are provided. A detection piece 131 extending from the working body 130 is protruding through a hole portion 101 provided in the detector casing 100 to the outside. In this detector, the above described respective components are all comprised of metal. An insulating layer 116 is formed on the lower surface of the fixed substrate 110, and two fixed electrodes 111 and 112 are further formed thereon. Further, an insulating layer 146 is formed on the upper surface of the auxiliary substrate 140, and two test electrodes 141 and 142 are further formed thereon. At the outside on the left side of the detector casing 100, a detection terminal 151, a common terminal 152, and a test terminal 153 are conducted out (although respective single terminals are only illustrated in the figure, terminals corresponding to the number of electrodes are actually prepared). As a matter of course, respective terminals 151 to 153 are electrically insulated from the detector casing 100. Further, the respective terminals 151 to 153 are connected to the respective electrodes and the flexible substrate 120 by means of bonding wires 161 to 163. In this detector, although no displacement electrode is provided on the flexible substrate 120, since the flexible substrate 120 is comprised of metal as previously described, the flexible substrate itself has a function of the electrode. In this detector, when a force is applied to the front end of the detection piece 131, the flexible substrate 120 produces bending. As a result, the distance between the flexible substrate 120 (functioning as a displacement electrode) and the fixed electrodes 111 and 112 varies. Accordingly, it is possible to detect a force exerted on the basis of changes in the electrostatic capacitance between the detection terminal 151 and the common terminal 152. However, since two fixed electrodes are only provided in the detector of this embodiment, it is only possible to carry out detection of force components in a two dimensional direction (force components in left and right directions and in upper and lower directions). In order to detect force components in a three-dimensional direction, it is enough to provide four fixed electrodes at the minimum. Now, in order to carry out the operation test of this detector, it is sufficient to carry out the previously described detection processing in the state where a predetermined voltage is applied across the test terminal 153 and the common terminal 152. By action of coulomb force based on an applied voltage, it is possible to create the same state as the state where an external force is exerted on the detection piece 131. It is to be noted that it is sufficient to similarly provide four test electrodes at the minimum in order to carry out the operation in a three-dimensional direction. As illustrated in the circuit diagram shown in FIG. 6, the basic acceleration detector shown in FIG. 1 takes a difference for detection of acceleration components in an X-axis and a Y-axis directions, but does not take a difference for detection of an acceleration component in a Z-axis direction. Since detection based on difference advantageously cancels an error due to the external environment such as temperature, etc., it is preferable to take a difference for detection of an acceleration in a Z-axis direction as well. An embodiment fot realizing this is shown below. The embodiment showing the side cross section in FIG. 14 is an acceleration detector for carrying out detection of acceleration components in all X, Y and Z directions by taking differences. On the lower surface of the fixed substrate 310, five fixed electrodes 11 to 15 are formed in accordance with the layout shown in FIG. 15a. In a flexible substrate 320, a doughnut groove G is dug at the lower surface thereof, a working portion 321 is formed at the central portion thereof, a flexible portion 322 is formed therearound, and a fixed portion 323 is formed therearound. On the upper surface thereof, five displacement electrodes 21 to 25 are formed in accordance with the layout shown in FIG. 15b. The above mentioned configuration is the same as the basic configuration shown in FIG. 1. This detector is characterized in that a second fixed substrate 330 is further provided, and that a displacement electrode 326 and a fixed electrode 336 are respectively formed on the lower of the working portion 321 and the upper surface of the second fixed substrate 330 in a manner that they are opposite to each other. Detection of an acceleration components with respect to the X-axis and the Y-axis directions by this detector and the operation test thereof are the same as those of the detector of FIG. 1. However, detection of an acceleration with respect to the Z-axis direction and the operation test thereof are carried out by the circuit shown in FIG. 16. Here, a capacitance element C5 is comprised of a fixed electrode 15 and a displacement electrode 25, and a capacitance element C6 is comprised of a fixed electrode 336 and a displacement electrode 326. When compared with the circuit with respect to the Z-axis direction shown in FIG. 6, this circuit differs from the former in that a voltage generating circuit 66 and a CV converting circuit 56 with respect to the capacitance element C6 are added, and that a difference between voltage values V5 and V6 is determined by a differential amplifier 73 to output it as a detected value with respect to the Z-axis direction. Thus, detected values with respect to X,Y and Z-axis directions are determined on the basis of differences, thus making it possible to cancel the influence of temperature, etc. Further, such a detection based on difference is advantageous also in the following points. The relationship between distance d between electrodes and the capacitance value C of the capacitance element is generally as in the graph shown in FIG. 17. When the distance is contracted by Δd from the state where the capacitance value indicates C0 at the distance d0 so that d0 - Ad results, the capacitance value is increased by ΔC1, resulting in C0 + ΔC1. In contrast, when the distance is widened by Δd so that d0 + Δd results, the capacitance value is decreased by ΔC2, resulting in C0 - AC2. Here, ΔC1 > ΔC2. Accordingly, when detection of an acceleration component in a Z-axis direction is made on the basis of only the capacitance value of a set of capacitance elements C5, even in the case of acceleration components of the same absolute, the degree of changes in the capacitance value in a positive Z-axis direction and that in a negative Z-axis direction differ from each other. To cope with this, it is required to provide any correction circuit. However, when detection based on difference is carried out as shown in FIG. 16, such a problem does not arise. For example, in the detector shown in FIG. 14, when an acceleration is exerted in a positive Z-axis direction (in an upper direction in the figure), the capacitance value of the capacitance element C5 changes from C0 to C0 + ΔC1, and the capacitance value of the capacitance element C6 changes from C0 to C0 - AC2. Thus, an output from the differential amplifier 73 becomes equal to a value corresponding to ΔC1 + ΔC2. On the contrary, when the same acceleration is exerted in a negative Z-axis direction (in a lower direction in the figure), the capacitance value of the capacitance element C5 changes from C0 to C0 - ΔC2, and the capacitance value of the capacitance element C6 changes from C0 to C0 + ΔC1. Thus, an output from the differential amplifier 73 becomes equal to a value corresponding to -(ΔC1 + ΔC2). In this way, an output of the same absolute is provided with respect acceleration components of the same absolute. The embodiment shown in FIG. 18 is characterized in that a working body 345 and a pedestal 340 are further added to the embodiment shown in FIG. 14. A displacement electrode 326 is formed on the lower surface of the working body 345. In addition, the embodiment shown in FIG. 19 is characterized in that the position of the electrode of the embodiment shown in FIG. 18 is varied: respective fixed electrodes 11 to 15 are formed on the fixed substrate 330, respective displacement electrodes 21 to 25 are formed on the lower surface of working body 345; a fixed electrode 336 is formed on the lower surface of fixed substrate 310; and a displacement electrode 326 is formed on the upper surface of flexible substrate 320. The above described respective embodiments of FIGS. 14, 18 and 19 are suitable for constituting respective substrates with glass or semiconductor (an insulating layer is formed between the substrate and the electrode in this case). On the other hand, the acceleration detector shown in FIG. 20 is directed to the example suitable for constituting the substrate with metal. In this embodiment, members 410, 420, 425, 430, 440 and 450 are all comprised of metal. On the lower surface of the member 410, five fixed electrodes 11 to 15 are formed through an insulating layer 418. Further, five displacement electrodes 21 to 25 are formed through an insulating layer 428 on the upper surface of the member 420, and a doughnut shaped displacement electrode 426 is formed through an insulating layer 429 on the lower surface of the member 420. A member 425 is connected on the lower surface of the member 420, and the lower end of the member 425 is connected to a member 450 through a diaphragm 435. Further, a doughnut shaped fixed electrode 436 is formed through an insulating layer 438 on the upper surface of the member 430. When an acceleration is exerted on the member 450, the diaphragm 435 is bent, so the member 420 is subjected to displacement through the member 425. The principle of detection of displacement is as described above. In the embodiment shown in FIG. 21, the acceleration detector shown in FIG. 20 is applied to a force detector wherein the configuration of the lower half is replaced by a metal member 431 (flexible substrate). The member 420 is subjected to displacement on the basis of an external force exerted on the front end of the detection piece 432. Thus, detection of this external force is carried out. While this invention has been described in connection with the several embodiments shown, this invention is not limited to such embodiments, but may be carried out in various forms in addition thereto. For example, as the arrangement of respective electrodes, various arrangements may be taken in addition to the above described embodiments. Further, the numbers of respective electrodes are not limited only to those in the above described embodiments. How many number of fixed electrodes, displacement electrodes and test electrodes are formed and/or where they are formed are the matter suitably changeable in design. As compared to an employment of the structure in which two electrode layers are stacked as in the detector shown in FIG. 10a, 10b or 12, the arrangement of a single electrode layer as shown in FIG. 15 becomes simple in the manufacturing process. This arrangement is rather preferable in the mass production. While, in the above described embodiments, the method of forming a metal layer such as aluminum on a semiconductor substrate to use it as an electrode is mainly disclosed as an example, such an electrode may be formed by using any method. For example, there may be employed a method of forming an impurity diffused region in a semiconductor substrate to use it as an electrode. Further, if the fixed substrate or the flexible substrate is formed with metal, this substrate itself may be used as an electrode. Accordingly, in this invention, it is not necessarily required that the electrode is separate from the substrate. In addition, while, in the above described embodiments, the acceleration detectors and the force detectors have been described, if the working body is constituted with a magnetic material, it is possible to carry out detection of a force based on magnetism. Namely, this invention is similarly applicable to detectors for magnetism. A method of testing the operation according to this invention is widely applicable to force detectors, acceleration detectors or magnetic detectors for detecting a physical quantity by making use of changes in distance between electrodes. Further, detectors having a function to carry out this operation test can carry out the operation test by a simple method, and can be therefore utilized with high reliability in practical use. Accordingly, it can be expected that this invention is applied to automotive vehicles or industrial robots.

Claims: An acceleration sensor having a self test feature which comprises: a) a sensor casing (40); b) a working body (30) flexibly supported by said sensor casing so that a displacement is produced between the working body and the sensor casing when an acceleration is applied to the working body; c) detection means (50) for detecting said applied acceleration based on said displacement; d)fixed test electrode means placed on said sensor casing; e) displacement test electrode means placed on said working body; and f) voltage application means (60) for applying a predetermined voltage between said fixed test electrode means and said displacement test electrode means to move the working body by a coulomb force; characterized in that g) the sensor has a feature to detect a first component of acceleration along an X-axis and a second component of acceleration along a Y-axis in an XYZ three-dimensional coordinate system; h) a flexible substrate (20) arranged parallel with an XY-plane of said coordinate system, said flexible substrate including a working portion to which the working body is attached, a fixed portion fixed to the sensor casing and a flexible portion located between said working portion and said fixed portion wherein said flexible portion having flexibility to produce mechanical deformation when an acceleration is applied to the working body; and i) a fixed substrate (10) fixed to the sensor casing and arranged parallel with the XY-plane so as to face said flexible substrate with a predetermined distance; j) wherein the detection means (50) comprises four capacitance elements (C1-C4), each of said capacitance elements comprising a fixed electrode (11-14) fixed to said fixed substrate on a first plane parallel with the XY-plane and a displacement electrode (21-24) fixed to said flexible substrate on a second plane parallel with the XY-plane, each displacement electrode facing each fixed electrode, a first capacitance element (C1; 11, 21) being located in a negative field of the X-axis, a second capacitance element (C2; 12, 22) being located in a negative field of the Y-axis, a third capacitance element (C3; 13, 23) being located in a positive field of the X-axis, a fourth capacitance element (C4; 14, 24) being located in a positive field of the Y-axis and the detection means detecting the first component of acceleration along the X-axis depend on a capacitance of the first capacitance element and the third capacitance element and the second component of acceleration along the Y-axis depend on a capacitance of the second capacitance element and the fourth capacitance element; k) wherein the fixed test electrode means comprises four fixed test electrodes (11t-14t) fixed to said fixed substrate on a third plane parallel with the XY-plane, a first fixed test electrode (11t) being located in a negative field of the X-axis, a second fixed test electrode (12t) being located in a negative field of the Y-axis, a third fixed test electrode (13t) being located in a positive field of the X-axis and a fourth fixed test electrode (14t) being located in a positive field of the Y-axis, l) wherein the displacement test electrode means comprises four displacement test electrodes (21t-24t) fixed to said flexible substrate on a fourth plane parallel with the XY-plane, a first displacement test electrode (21t) being located in a negative field of the X-axis to face the first fixed test electrode (11t), a second displacement test electrode (22t) being located in a negative field of the Y-axis to face the second fixed test electrode (12t), a third displacement test electrode (23t) being located in a positive field of the X-axis to face the third fixed test electrode (13t) and a fourth displacement test electrode (24t) being located in a positive field of the Y-axis to face the fourth fixed test electrode (14t); and m) wherein the voltage application means (60) has a first function to apply a predetermined voltage between the first fixed test electrode (11t) and the first displacement test electrode (21t) and between the third fixed test electrode (13t) and the third displacement test electrode (23t) to move the working body in the X-axis direction and a second function to apply a predetermined voltage between the second fixed test electrode (12t) and the second displacement test electrode (22t) and between the fourth fixed test electrode (14t) and the fourth displacement test electrode (24t) to move the working body in the Y-axis direction. An acceleration sensor as set forth in claim 1, further having a feature to detect a third component of acceleration along a Z-axis in the coordinate system: a) wherein the detection means (50) further comprises a fifth capacitance element (C5) located around the origin of the coordinate system, the fifth capacitance element comprising a fixed electrode (15) fixed to the fixed substrate (10) on the first plane and a displacement electrode (25) fixed to the flexible substrate (20) on the second plane, and the detection means detecting the third component of acceleration along the Z-axis depend on a capacitance of the fifth capacitance element; b) wherein the fixed test electrode means further comprises a fifth fixed test electrode (15t) fixed to the fixed substrate on the third plane and the displacement test electrode means further comprises a fifth displacement test electrode (25t) fixed to the flexible substrate on the fourth plane, the Z-axis crossing through said fifth fixed test electrode and said fifth displacement test electrode; and c) wherein the voltage application means (60) further has a third function to apply a predetermined voltage between the fifth fixed test electrode (15t) and the fifth displacement test electrode (25t) to move the working body in the Z-axis direction. An acceleration sensor as set forth in claim 1 or 2; wherein the first plane and the third plane are identical with each other and the respective fixed electrodes (11-15) and the respective fixed test electrodes (11t-25t) consist of identical common electrodes (11-15) respectively. An acceleration sensor as set forth in claim 1 or 2: wherein the second plane and the fourth plane are identical with each other and the respective displacement electrodes (21-25) and the respective displacement test electrodes (21t-25t) consist of identical common electrodes (21-25) respectively. An acceleration sensor as set forth in any one of claims 1 to 4, characterized in that each of the displacement electrodes (21-25) consists of a singular conductive layer. An acceleration sensor as set forth in any one of claims 1 to 5, characterized in that each of the displacement test electrodes (21t-25t) consists of a singular conductive layer.

Inventors: OKADA KAZUHIRO, OKADA, KAZUHIRO
Applicants: OKADA KAZUHIRO, OKADA, KAZUHIRO
Application year: 1991
Main classification: G01L25
IPC codes: G01P15, G01P21, G01L5, G01L25