Detector having self-calibration function

Abstract: A detector for detecting a physical quantity as a quan­ tity of electricity has a detection portion (1), a portion (2) for stimulating the detection portion and a signal processing portion (4), wherein a calibrating signal is supplied from the signal processing portion (4) to the detection portion (1) via the stimulating portion (2) so as to measure a specific re­ sponse of the detection portion whereby a self-calibration and a correction of the characteristics are performed in accord­ ance with the amount of the change in the response.

Description: BACKGROUND OF THE INVENTION The present invention relates to a detector for detecting a physical quantity from a value of an electric signal converted from the physical quantity, and, more particularly, to a detector having a self-calibration function and a characteristic corrective function. Related Art Statement Hitherto, a conventional detector has been, as disclosed in, for example, JP-A-61-31952, arranged in such a manner that the measuring operation is stopped so as to start a calibrating operation which is performed as the offline work. Furthermore, there has been disclosed, in JP-A-61-212753, an apparatus capable of diagnosing deterioration by analyzing the characteristics observed in the detector. However. the apparatus of this type also performs, as the offline work, the operation for diagnosing the deterioration. The conventional calibration has been realized for the purpose of automating the offline work. Furthermore, there has been a proposal that the reliability of a detector is improved by observing the line and giving an alarm if necessary as the online work. However, since no measure has been taken for performing the calibration as the online work, a problem takes place that the measurement is stopped for a relatively long time in comparison with the time in which the value of the measurement can be changed. US-A-4 225 851 discloses a detector system for measuring strain on rotating members of a tubrine under operating conditions. A calibration signal is introduced for adjusting an amplifier gain in order to minunize noise effects present within the system. US-A-4 673 870 discloses a field probe which may be calibrated by placing the probe in a known standard field. WO-A-89/03019 relates to a system for performing calibration of a sensor system and storing the calibration history of the sensor in a non-volatile memory. DE-C 3 127 637 discloses a detector system with the features of the first part of claim 1 and a method therefor with the features of the first part of claim 10. It is object of the present invention to provide a method and a detector which is capable of performing a self-diagnosis in order to enable correction of errors especially due to deterioration. It is a further object to provide a compact detector which is capable of self-diagnosis and which can be easily manufactured. These objects are met by the invention as set out in the independent claims. The diagnosis or the corrective operation should be completed in a significantly short time in comparison with the time in which the value of the measurement can be changed. The reason for this lies in that the data of the measurement must be protected from a disorder or an error due to the calibration or the corrective operation performed during the measurement operation. As for the device for processing an electric signal, significantly high speed semiconductor ICs are available recently due to the progress of the LSI technology. Therefore, the thus realized speed of processing the electric signal can cope with the time of several tens to 100 µs which is the value necessary to conduct measurements in automobile in which the values to be measured are varied in a relatively short time. Therefore, unsolved problem is to shorten the time taken to operate the detection means. Accordingly, the present invention employs stimulating means disposed adjacent to the detection means so as to stimulate and operate the detection means. A structure can be realized in which a small sensor or detector, the size of which is, for example, several hundreds of µm, and an actuator, that is, the stimulating device can be integrally formed by utilizing the micromachining technology for silicon or the like which has been remarkably progressed recently. Therefore, a compact and integrally formed stimulating device is able to apply a stimulation signal to the detector without delay. It is further necessary to supply an accurate stimulation signal to the detector and to correctly measure the response of the detector. Therefore, according to an embodiment of the present invention, a structure is employed in which a stimulation signal is supplied to the detector by a signal processing circuit induding an accurate and high resolution analog-to-digital converter. Then, a responding electric signal which is inherent to the detector is then processed. Furthermore, a suitable self-diagnosis algorithm is accurately and quickly performed by a microcomputer. In order to achieve the other object of the invention, a structure is employed according to an embodiment of the present invention, in which the processing device is provided with a communication function, the self-calibration and characteristics correction are instructed from a remote position by another communication device and the result of the self-diagnosis and the characteristic correction are confirmed. The detector according to the present invention is preferably structured such that the stimulating means is formed adjacent to and integrally with the detection means so that the calibration signal can be supplied through the stimulating means. Therefore, the delay of response from the detector can be significantly prevented. Furthermore, a high-speed signal processing circuit can be employed to shorten the time required for completing the self-diagnosis in comparison to the time in which the values to be measured are changed. Therefore, although the self-diagnosis is performed during the measurement operation, the output from the detector can be protected from disorder. Therefore, a so-called "online diagnosis" can be realized. Furthermore, the characteristics obtained during operation are always corrected in accordance with a comparison made with the initial characteristics of the detector based on a diagnosis and corrective algorithm previously prepared in the processing means. Therefore, the initial performance can be maintained to significantly improve the reliability. Other and further objects, features and advantages of the invention will be made more apparent by the following description. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates the basic structure of an embodiment of the present invention; Fig. 2 is a circuit diagram for a signal processing means; Figs. 3 to 6 illustrate the operation of an electrostatic capacity type pressure sensor, Figs. 7A and 7B are flow charts of the operation of a microcomputer, Figs. 8 to 11 illustrate the operations of semiconductor acceleration sensors; and Figs. 12 and 13 illustrate another embodiment of the acceleration sensor. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Fig. 1, the basic structure of an embodiment of the present invention will be described. Reference numeral 1 represents detection means and 2 represents stimulating means disposed adjacent to the detection means 2 and integrally therewith. Reference numeral 3 represents an assembly of the detection means 1 and the stimulating means 2. Reference numeral 4 represents a signal processing means structured as shown in Fig. 2 and arranged to supply a power supply voltage Ex for operating the detection means 1 and the stimulating means 2 and to process a calibrating signal to be supplied to the stimulating means 2. Furthermore, the signal processing means 4 has a so-called "signal adjustment function" capable of amplifying/converting a responding output signal from the detection means 1. In addition, the signal processing means 4 has a function of calibrating the input/output and a characteristic corrective function realized by digital-data processing performed by a microcomputer 44. Reference numeral 5 represents-a detector induding the above-described elements. Usually, the detector converts an input physical quantity such as a pressure, a discharge and an acceleration into a digital quantity of a certain number of bits so as to output it. Reference numeral 7 represents a communication device capable of transmitting/receiving a command signal and an output signal to and from the signal processing means 4, the communication device 7 further having a function of displaying the command signal and the output signal. Fig. 2 illustrates the specific structure of a circuit serving as the signal processing means 4. In response to a command issued from the microprocessor 44 having a memory 45, a multiplexer 411 is operated so as to cause the output signal transmitted from the detection means 1 to be received by an amplifier 412a and an analog-to-digital converter 42. As a result, the output signal thus received is converted into a digital signal. In accordance with the value represented by the digital signal, the signal processing means 4 supplies the power supply voltage Ex or supplies the calibration signal to the stimulating means 2 via another amplifier 412b. As a result, an accurate detection signal capable of correcting an error can be obtained. Then, the principle of a span calibration operation will be described with reference to an electrostatic capacity type pressure sensor so as to specifically describe the invention. As shown in Fig. 3, the electrostatic capacity type pressure sensor is constituted in such a manner that an electrode plate 12, which can be displaced by a differential pressure ΔP, is disposed between electrode plates 11a and 11b each of which has an area k The distance between each pair of the electrode plates is X 0 under no differential pressure. Furthermore, each of the spaces between the electrodes is filled with a substance having a dielectric constant ε. When the differential pressure ΔP is applied, the intermediate electrode is, as shown in Fig. 4, displaced by ΔX. Since the displacement is substantially in proportion to the differential pressure, the relationship expressed by (ΔX = k · ΔP) is held, where symbol k denotes a compliance (the reciprocal of the spring constant). Assuming that k is varied as the time proceeds, it can be expressed as the function of time T, that is, k(T). As a result of the above-described displacement of the intermediate electrode plate 12, there is generated a capacity difference ΔC between capacities C 1 and C 2 , where the capacity C 1 is the capacity between the electrodes 11a and 12 and the capacity C 2 is the capacity between the electrodes 12 and 11b. As a result of the arrangement of a circuit shown in Fig. 5, the capacity difference ΔC due to the differential pressure can be detected by the following equation: e = ΔC C 1 + C 2 E where symbol E denotes excitation voltage, e denotes voltage to be detected and ΔC stands for C 2 - C 1 . As an alternative to this, the voltage can be expressed by the loaded differential pressure ΔP as follows: e = k(T) X 0 ΔPE As can be seen from the above-described equation (2), detected voltage e max , that is, the output span under the maximum differential pressure ΔP max is changed as the time proceeds if the compliance k is changed as the time proceeds. The voltage V of the intermediate electrode is applied so as to generate a displacement Δxv due to the electrostatic force, so that the output voltage e generated at this time is measured for the purpose of calibrating the above-described span change based on the relationships thus obtained. First, calibrating voltage V shown in Fig. 5 is selected in a manner expressed by the following Equation (3): V = E/2 + v However, assuming that (v << E/2), the displacement due to the voltage V is of a value expressed by ΔX shown in Fig. 6 and given by the following Equation 4: Δxv = ε 2 A 3 X 0 2 k(T)E·v The output voltage e at this time is of a level given by the following equation: e = [ k(T) X 0 P + ε 2 A 3 X 0 3 k(T)E·v]·E Therefore, since k(T) can be known by varying V, the span can be calibrated. Then, the procedure will be described. It is defined that the outputs are e 1 and e 2 when calibrating voltages V 1 and V 2 are applied. In order to delete from Equation (5) the term which depends upon the pressure, the following calculation is performed: Δe = e 1 - e 2 = ε 2 A 3 X 0 3 k(T)E 2 (V 1 - V 2 ) Then, the proportion to Δe with the initial T being 0 is given as follows: d ≡ Δe(T = 0) Δe(T) = k(T = 0) k(T) By using d thus obtained, calibrating output can be obtained from the following equation: (where v = 0). Figs. 7A and 7B illustrate flow charts for a process to be performed in the microcomputer. Fig. 7A illustrates the main routine in which the ordinary measurement work flows along a route 1. In the route 1, the detection voltage e at (v = 0) in Equation (5) is measured so as to obtain calibration voltage by multiplying calibration coefficient d. As a result, the calibration voltage which is the final detected value, is obtained. The above-described calibration coefficient d is calculated in a route 2. The program for the microcomputer is provided with a calibration measuring sub-routine shown in Fig. 7B, in which the differential value Δe between the detection voltages e 1 and e 2 is calculated, the detection voltages e 1 and e 2 being obtained from measurements in which the calibrating pulse voltages V 1 and V 2 are applied. The calibration includes an initial calibration and an optional calibration. The initial calibration is performed at the delivery of the product from a manufacturing plant and conducted such that an initial differential value (Δe-init) is calculated and stored by the process of a route 3. The route 2 is performed as the optional calibration in which present differential value (Δe-present) is calculated so as to obtain the calibration coefficient d which is the ratio to Δe-init. The calibration coefficient d thus obtained is stored so as to update the previous value. As described above, the pressure, the sensitivity drift of which is corrected by the calibration voltage obtained by multiplying the detection voltage e by the calibration coefficient d, can be obtained. Figs. 8 and 9 respectively illustrate basic structures of semiconductor acceleration sensors of an electrostatic capacity type and of a piezoelectric resistance type, which are two typical types, the sensors being manufactured by a silicon micromachining technology. The acceleration sensor is for obtaining an acceleration by a measurement of an inertia force acting on a predetermined mass in the case where the acceleration exists. Each of the acceleration sensors shown in Figs. 8 and 9 is structured such that a load 53 and a cantilever 54 for supporting the load 53 are formed on an intermediate silicon substrate 51 by anisotropic etching. When acceleration a is applied, inertia force (E 1 = mα) acts on the load (mass m), causing the load (mass m) to be displaced. On the other hand, the cantilever acts as a spring so that it gives to the load a restoring force expressed by (F 2 = ax) (where symbol a denotes a spring constant and x denotes the amount of displacement), the restoring force being given in the direction reverse to the direction of the displacement. As a result, the load is displaced to the position at which the above-described two forces are balanced. From the relationship expressed by (F 1 = F 2 x), the displacement x is given by: x = md/a Therefore, the acceleration a can be obtained from the displacement x. The electrostatic capacity type acceleration sensor shown in Fig. 8 includes an upper fixed electrode 55a and a lower fixed electrode 55b formed on the surfaces of the upper substrate 52a and the lower substrate 52b which face the intermediate silicon substrate 51. The electrostatic capacity type acceleration sensor acts to measure the acceleration by obtaining the displacement x of the Equation (I) from the electrostatic capacity between the fixed electrodes and the load (movable electrode). On the other hand, the piezoelectric resistance type acceleration sensor shown in Fig. 9 is structured such that a gauge portion 58 comprising an impurity diffusion region is formed on the cantilever 54. When the load 53 is displaced by an acceleration, the cantilever 54 is deformed, causing the electric resistance of the gauge portion 58 to be changed by the piezoelectric resistance effect. The displacement can be obtained from the electric resistance of the gauge portion so that the acceleration is obtained. Thus, an output signal V(α) corresponding to the acceleration can be obtained by the signal processing circuit which processes the electrostatic capacity between the load and the fixed electrode or the electric resistance of the gauge portion. Since the output and the acceleration α are usually processed so as to keep a linear relationship, the output V(α) is expressed by the following equation: V(α) = ρα + q It is assumed that the acceleration sensor is changed as the time proceeds for some reason. If the change takes place with the linear relationship between the acceleration and the output (substantially) maintained, the output becomes the function of the time. Therefore, the output becomes as follows: V(α,t) = P(t)α + q(t) If the span p(t) and the zero point q(t) of the acceleration-output characteristics (III) have been correctly known, the acceleration α can be accurately obtained by measuring the output V(α, t). In the case where p(t) and q(t) are unknown in Equation (III), they can be obtained by generating two different accelerations α 1 and α 2 by some method so as to measure the outputs V(α 1 , t) and V(α 2 , t) which correspond to the two accelerations α 1 and α 2 . Namely, P(t) and q(t) can be obtained from the following simultaneous equation: On the other hand, the acceleration a corresponds to the displacement x of the load in the relationship given by Equation (I). Therefore, determining the acceleration α 1 and α 2 becomes equivalent to determining the displacements x 1 and x 2 which correspond to the accelerations α 1 and α 2 . Thus, the following relationships are obtained from Equations (I) and (IV): where p'(t) = kp(t)/m The predetermined displacements x 1 and x 2 shown in Equation (5) can be relatively easily realized. That is, the structure may be such that the load is forcibly displaced by an actuator and the characteristics of the sensor output V(x, t) sharply varies at the predetermined certain displacements x 1 and x 2 . As an alternative to this, the structure may be such that any further displacement is inhibited. Figs. 10 and 11 illustrate examples of the above-described structures, in which stoppers 60a and 60b are provided for the purpose of preventing any displacement which exceeds a predetermined degree even if an acceleration or an external force acts on the load. If the displacements x 1 and x 2 obtained when the loads are brought into contact with the stoppers 60a and 60b are previously known, p(t) and q(t) can be obtained from Equations (V) and (VI) by measuring the outputs V(x 1 , t) and V(x 2 , t) at this time. In order to displace the load at a desired time so as to bring it into contact with the stoppers in the case of the electrostatic capacity type sensor, voltage is applied between the load and the upper fixed electrode 55a or the lower fixed electrode 55b, which acts to obtain the electrostatic capacity, so as to apply the electrostatic force between them. Also in the case of the piezoelectric resistance type sensor, the upper fixed electrode 55a and the lower fixed electrode 55b are formed and voltage is applied between the load and the upper fixed electrode 55a or the lower fixed electrode 55b. As described above, an advantage can be obtained that the change in the acceleration-output characteristics as the time proceeds can be corrected by a simple calculation from the output obtained by periodically applying a voltage between the fixed electrodes and the load. Furthermore, the correction can be performed even if the acceleration is being applied to the sensor. According to the above-described embodiments, the displacement of the load is measured and the acceleration is obtained from the displacement thus measured. A servo type acceleration sensor is also known as a typical acceleration sensor. The servo type acceleration sensor is designed such that the displacement of the load due to an acceleration is measured and a signal representing the displacement is fed back. Furthermore, in response to the signal, the load is given, by some method, a force in the reverse direction in the sensor so that the load is restored to the original position. Since the quantity of the feedback corresponds to the magnitude of the acceleration, the acceleration is obtained from the quantity of the feedback. According to this method, the displacement is substantially constant regardless of the acceleration. Similarly to the above-described embodiment, the displacement is often measured by the electrostatic capacity method or the piezoelectric resistance method. Furthermore, force is applied to the load in accordance with the quantity of the feedback by using an electrostatic force or a magnetic force. In the servo type sensor, the relationship between the final output signal and the acceleration is usually expressed by Equation (II). It is assumed that a second force F is applied to the load in addition to the force which corresponds to the quantity of the feedback in the servo system. The output from the sensor at this time can be expressed by the following equation: V(α, F) = p(α + F/m) + q In the case where forces F 1 and F 2 of predetermined magnitudes are applied, the following relationships are held: Subtracting the lower equation from the upper equation, the following equation can be obtained: V(α, F 1 ) - V(α, F 2 ) = p(F 1 - F 2 )/m If V(α, F 1 ), V(α, F 2 ), F 1 , F 2 and m are known, p can be obtained. When the sensor device is brought into upside down by an actuator such as a motor with the application of the force F (F may be 0) maintained, the direction of each of the acceleration and the second force applied to the sensor is inverted. Therefore, the output becomes as follows: V(- α, - F) = p(- α - F/m) + q Adding Equations (7) and (10), the following relationship is obtained: V(α, F) + V(- α, - F) = 2q Therefore, q can be obtained from Equation (Xl). Figs. 12 and 13 respectively illustrate specific examples of the structures of the sensor devices. Fig. 12 illustrates the structure for measuring a displacement by using the electrostatic capacity, while Fig. 13 illustrates the structure for measuring a displacement by using the piezoelectric resistance device. In each of the two structures, the force according to the quantity of the feedback and the second force are applied to the load by using the electrostatic force for the purpose of forming a servo system. Reference numerals 61a and 61b represent electrodes for detecting the electrostatic capacity which corresponds to the displacement Reference numerals 62a and 62b represent electrodes for applying the electrostatic force for the servo effect. Reference numerals 63a and 63b represent electrodes for applying the electrostatic force for the purpose of applying the second force to the load. These electrodes can be used in a combined manner by arranging a circuit for the servo system and that for applying the electrostatic force to be of proper structures. Since the displacement of the load is substantially constant in the servo type sensor, the gap between each of the two electrostatic force applying electrodes 63a and 63b and the load is constant Therefore, the constant forces F 1 and F 2 shown in Equation (VIII) can be obtained simply by varying the level of the voltage to be applied to the electrostatic force applying electrodes 63a and 63b. Therefore, the magnitude of each of the forces F 1 and F 2 can be calculated if the areas of the electrodes, the sizes of the gaps and the level of the voltage to be applied are previously known. According to the above-described embodiment, an advantage can be obtained that the change in the acceleration output characteristics of the servo type acceleration sensor as the time proceeds can be corrected.

Claims: A detector system for detecting a physical quantity as a quantity of electricity, comprising detection means (1) having a sensor portion, said physical quantity acting directly on said sensor portion and producing an effect onto said sensor portion which results in an output signal of said detection means, signal processing means (4) for processing the output signal of said detection means and for diagnosing said detection means, and stimulating means (2) for stimulating said detection means (1) under control of said signal processing means (4), said signal processing means (4) performing evaluation of the output signal of said detection means (1) caused by said stimulation means to diagnose on the status of said detection means, characterised in that said stimulating means (2) stimulates said detection means (1) on-line during measurement operation of said detector system by supplying a stimulating signal to the senscr portion of said detection means so as to directly activate the sensor portion of said detection means in an operation condition in which the same effect onto said sensor portion is achieved as produced by the physical -quantity, wherein said processing means (4) evaluates the output signal of said detecting means (1) resulted from the operation condition of said sensor portion caused by said stimulating signal to diagnose on the status of said detection means (1). The detector system of claim 1, wherein said signal processing means (4) provides correction of the output of said detection means (1) in accordance with an amount of change detected during diagnosis. The detector system of claim 1, further including a communication device (7) and means for displaying the results of said diagnosis. The detector system of claim 1, wherein said detection means (1), said stimulating means (2) and said processing means (4) are formed on a silicon substrate. The detector system of claim 3, wherein said communication device (7) communicates with a plurality of detectors each of which has an identification code. The detector system of claim 4, wherein said detection means (1) and said stimulating means (2) are formed on a silicon monocrystal substrate and are fabricated by ani-sotropic etching with a mask formed by an oxide film or a nitride film formed on said silicon monocrystal substrate. The detector system of claim 1, wherein said detection means (1) and said stimulating means (2) are electrostatically coupled via borosilicate glass. The detector system of claim 1, wherein said detection means (1) comprises a strain generation body including a piezoelectric resistance device (58), a calibration voltage signal being applied via an electrostatic capacity formed at the central portion of said strain generation body as said stimulating means (2). The detector system of claim 1, wherein said detection means (1) is a variable electrostatic capacity comprising a supported movable electrode (53) and at least one fixed electrode (55a, 55b), a calibration signal is applied via an electrostatic capacity serving as a stimulating means (2) and formed by said movable electrode (53) and another fixed electrode (55a, 55b) so as to measure the capacity of said variable electrostatic capacity, and the sensitivity is calibrated in accordance with the change in said capacity. A method of processing data in a detector system for detecting a physical quantity as a quantity of electricity, said detector means comprising detection means (1) having a sensor portion, said physical quantity acting directly on said sensor portion and producing an effect onto said sensor portion which results in an output signal of the detection means, signal processing means (4) and stimulating means (2), comprising the step of stimulating detection means (1) via said stimulating means (2) under control of said signal processing means (4) and evaluating the output signal of said detection means (1) caused by said stimulating means (2) with said signal processing means (4) to diagnose on the status of said detection means, characterised in that said stimulating step is carried out by supplying a stimulating signal on-line during measurement operation to the sensor portion of said detection means (1) so as to directly activate the sensor portion of said detection means in an operation condition in which the same effect onto said sensor portion is achieved as produced by the physical quantity, said evaluating step comprises evaluation of the output signal of said detection means (1) resulted from the operation condition of said sensor portion caused by said stimulating signal, and a further step of calibrating output signals of said detection means (1) on the basis of the diagnosis result is provided. The method of claim 10, wherein the output signal of said detection means is corrected in accordance with the amount of change detected during diagnosis. The method of claim 10, wherein said diagnosing is performed in response to a command issued from a communication device.

Inventors: KANEYASU MASAYOSHI, KUROIWA HIROSHI, SHIMADA SATOSHI, SUZUKI SEIKO, TSUCHITANI SHIGEKI, UGAI SEIICHI, YOKOTA YOSHIHIRO, KANEYASU, MASAYOSHI, KUROIWA, HIROSHI, SHIMADA, SATOSHI, SUZUKI, SEIKO, TSUCHITANI, SHIGEKI, UGAI, SEIICHI, YOKOTA, YOSHIHIRO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1990
Main classification: G01D18
IPC codes: G01P15, G01N27, G01P21, G01D18