JP6472708B2 - Weighing device - Google Patents

Description

  The present invention relates to a weighing device including a load detection device that detects a load signal having a magnitude proportional to a load applied to a metal elastic body, and more particularly to a load detection device including a Wien bridge oscillation circuit.

  The Wien bridge oscillation circuit uses a Wien bridge composed of a low pass filter LPF and a high pass filter HPF shown in FIG. The low-pass filter LPF has a cutoff frequency fc composed of a resistor R1 and a capacitance body C1 connected in parallel, and the high-pass filter HPF is a resistor R2 and a capacitance body connected in series. C2 and has the same cutoff frequency as the low pass filter LPF as shown in FIG. As shown in FIG. 7, the high-pass filter HPF and the low-pass filter LPF are connected in series, and the high-pass filter HPF is connected between the output terminal and the non-inverting input terminal of the operational amplifier OP shown in FIG. And a reference potential, a low-pass filter is connected, and a connection point of series-connected resistors R1 and R2 connected in parallel to the low-pass filter and the high-pass filter is connected to the inverting input terminal. The Wien bridge oscillation circuit passes only the sine wave oscillation component of the cutoff frequency fc common to both filters LPF and HPF without being attenuated, and obtains a sine wave oscillation signal of the frequency fc as an output of the operational amplifier OP. R and R define the gain of the positive feedback circuit. For example, Patent Documents 1 and 2 show load detection devices using the Wien oscillation circuit.

Japanese Patent Laid-Open No. 10-30971 No. 2000-199723

  In the first configuration example of the strain sensor, which is the load detection device of Patent Document 1, a conductive wire resistor pattern and a conductive wire counter electrode type capacitive body pattern are formed on a strain sensor substrate attached to the strain detection sensor unit. Two sets are formed. The resistor R1, R2, C1, and C2 are configured by the conductor resistor pattern and the conductor resistance electrode type electrostatic capacitance pattern. A Wien bridge oscillation circuit as shown in FIG. 7 is configured by using R1, R2, C1, and C2 based on the conductive wire resistor pattern and the conductive wire counter electrode type capacitive body pattern. When a load is applied to the strain detection sensor unit and an extension strain is generated on the strained substrate sensor substrate, the conductive wire resistor pattern expands and the resistance components of R1 and R2 increase, and at the same time, the conductive wire resistive electrode capacitance pattern And the capacitance of C1 and C2 increases. When the applied load is changed from 0 to a certain magnitude, the values of R1, R2, C1, and C2 correspondingly change at the same time, and the output frequency f and period T of the Wien bridge oscillation circuit change. For example, by measuring the amount of change between the period when the applied load is 0 and the period when the applied load is w, the amount of change in the load, that is, the applied load can be detected.

  In the configuration example of the second strain sensor of Patent Document 1, a conductive wire counter electrode type capacitive body pattern is formed so that only R1 and R2 change and C1 and C2 do not change. That is, a conductive counter electrode type capacitive body pattern is provided in each of the strain sensor substrate in the strain direction and a direction perpendicular to the strain direction, and the distance between the counter electrodes is increased in the conductive counter electrode type capacitive pattern in the strain direction. As a result, the capacitance decreases, and in the counter electrode type capacitive body pattern perpendicular to the strain direction, the capacitance increases as the counter electrode plate becomes longer. By connecting both of the conductive wire counter electrode type capacitive body patterns in parallel, the amount of change in the electrostatic capacity of both sides is offset and offset to maintain a constant value.

  Contrary to the configuration example of the second strain sensor, only the capacitances C1 and C2 are changed, and the longitudinal direction of the resistor pattern is set to the strain direction on the strain detection sensor substrate so that R1 and R2 do not change. Even if it is provided in the direction perpendicular to the strain direction, when the strain occurs, the resistor pattern changes its resistance value by receiving a strain corresponding to the Poisson's ratio, and the result is the same as in the first configuration example.

  In the technique of Patent Document 1 in which the resistor pattern and the counter electrode type capacitive body pattern are provided on the strain sensor substrate in which distortion occurs in this way, only the resistor can be changed, but only the capacitance is changed. It is not possible.

In order to detect the load change with high resolution, it is necessary to detect the change in the period of the oscillation signal with high resolution. For that purpose, it is necessary to obtain a large change amount with respect to the load change. Period T is
T = 1 / f = 2π (C1, C2, R1, R2) ^1/2 (1)
In order to obtain a large amount of change, the value of R1, R2, C1, C2 when the applied load before the occurrence of strain is 0 is increased, or the application of these R1, R2, C1, and C2 is performed. The rate of change with load must be increased.

  However, if the values of R1, R2, C1, and C2 before the occurrence of distortion are increased, the period T becomes longer. When the measurement interval of one cycle for load detection becomes longer, the load detection interval becomes longer, and the response characteristic as a weighing device is lowered. Therefore, in order to increase the period change amount, the values of R1, R2, C1, and C2 before the occurrence of distortion cannot be increased, and the value of the rate of change with respect to the load change must be increased.

  However, in a load sensor for a weighing device, it is important to speed up the transient response of the load sensor, and it is necessary to increase the spring constant of the strain body to which the strain sensor is attached. Therefore, the strain amount of the strain generating portion of the strain generating body with respect to the applied load must be reduced. If the strain amount of the strain generating portion is small relative to the magnitude of the applied load, the change amount of the resistance value of the strain sensor becomes small, and the resistance change rate becomes small. Further, the gauge factor, which is the ratio of the resistance change rate to the elongation rate of the metal pattern, is about 2 in the case of a metal material. Therefore, a load sensor for a weighing device with an adequately fast transient response cannot obtain a larger resistance change rate or a larger resistance change amount with respect to an applied load. Since the amount of change in capacitance is also determined in proportion to the expansion / contraction material of the conductor counter electrode type capacitance pattern, the rate of change is as small as the rate of change in resistance.

  As described above, the strain sensor of the technique of Patent Document 1 cannot be configured to change only the capacitance body, and the change rate of both the resistance and capacitance is small. The change amount of the period T cannot be increased, it is difficult to detect the period change amount with high resolution, and an accurate load output with high resolution cannot be obtained.

  In the technique of Patent Document 2, only the capacitance on one side of the Wien bridge oscillation circuit is changed according to the amount of change in load. That is, the constant of either the low-pass filter or the high-pass filter of the Wien bridge oscillation circuit is changed according to the load change amount. For example, when the constant of the low-pass filter is changed, only the cut-off frequency of the low-pass filter moves to the left and right on the frequency axis of FIG. Since many frequency components are fed back with a gain of 1, the oscillation waveform of the sine wave is distorted according to the amount of load change, so that a load that varies widely cannot be detected accurately. Moreover, although a configuration with a large change rate is used as the capacitance body, only the capacitance body on one side of the bridge circuit is changed. Therefore, only a load with a small change width can be accurately detected.

  It is an object of the present invention to measure a load that changes over a wide range with a high resolution, a high transient response speed, and a short load signal detection period in a load sensor using a Wien bridge oscillation circuit.

  The weighing device according to one aspect of the present invention is applied to the load sensor by a load sensor having a Wien bridge oscillation circuit that oscillates a sine wave, a measurement unit for an output signal of the Wien bridge oscillation circuit, and the measured output signal. Load signal calculation means for detecting the load. The Wien bridge oscillation circuit has a bridge circuit, and a high-pass filter is provided on one side of the bridge circuit, and a low-pass filter is provided on the other side. Both of these filters have the same cut-off frequency, and both have a capacitance body. The load sensor includes a parallelogram load cell made of a metal elastic body. This parallelogram type load cell has a fixed part for attaching the parallelogram type load cell to the base surface, and a movable part to which a load is applied. The parallelogram load cell connects the upper and lower sides of the quadrilateral with beams between the fixed part and the movable part, and both the upper and lower beams are closer to the fixed part and the movable part. And a strain generating part. The reason for using a parallelogram load cell as a load sensor is that even if the object to be weighed is placed at any position on the weighing platform connected to the movable part, the movable part is displaced in the vertical direction relative to the fixed part. This is because it is desirable to adopt the configuration. Moreover, it is desirable that the fixed portion and the movable portion have surfaces that face each other. This facing surface is desirably provided in the horizontal direction or the vertical direction. For example, the fixed portion is provided with a fixed portion electrode on the facing surface, and the movable portion is provided with a movable portion electrode on the facing surface, for example. The fixed portion electrode and the movable portion electrode are opposed to each other in a non-contact manner. When a load is applied to the movable part by the fixed part electrode and the movable part electrode, the area of the opposing part of the fixed part electrode and the movable part electrode, or the distance of the opposing part, As a result of changing in accordance with the amount of change in the load, at least two capacitors whose capacitance changes are configured. These at least two capacitors form part of the low-pass filter included in one side of the Wien bridge oscillation circuit and the high-pass filter included in the other side. The electrode plates on the fixed part side and the movable part side of these at least two capacitors are preferably formed in the same shape.

  In the weighing device according to this aspect, as described above, it is difficult to configure a resistor having a large change rate. Therefore, at least two capacitors having a large change rate are configured by the fixed portion electrode and the movable portion electrode. These at least two capacitors form part of the high-pass filter and low-pass filter of the Wien bridge oscillation circuit. By applying a load to the movable part, the movable part electrode is accurately displaced in the vertical direction with respect to the fixed part side electrode, so that the distance between the two electrodes is proportionally displaced with respect to the applied load or between the two electrodes. The overlapping area changes proportionally to the applied load. In this way, since the capacitances of at least two capacitors used in the low-pass filter and the high-pass filter of the Wien bridge oscillation circuit are changed at the same time, even if the load change has a large change width, The oscillation waveform can be accurately measured without distortion.

  In addition, in at least two capacitance bodies, when the overlapping area of both facing electrodes changes, the width of both electrodes is D, the overlapping length of both electrodes before applying the load is Lo, and after the load is applied Let Lx be the amount of displacement of both electrodes, the rate of change in capacitance can be determined by Lx / Lo. In order to speed up the excessive response, Lx cannot be increased, but Lo can be increased in advance. Accordingly, the rate of change of the capacity can be increased while the transient response is made faster, and the period can be measured in a short time. Further, since Lx is the same in at least two capacitors, at least two capacitance changes can be made substantially the same value, and the cut-off frequencies of the low-pass filter and the high-pass filter that are part of these at least two capacitors are almost the same. It is possible to measure the load change with high resolution and high accuracy without interruption of oscillation or distortion of the sine wave oscillation waveform due to a wide range of applied load changes.

  Alternatively, when the distance between the counter electrodes changes, the capacitance of the capacitor changes in proportion to the reciprocal of the distance. Therefore, if the distance between the two electrodes before the load application is appropriately adjusted, the transient response can be accelerated. However, the rate of change of the capacity can be increased, and the period can be measured in a short time. Further, since the distance between the electrodes of at least two capacitors changes by the same distance, the capacitance change can be made substantially the same value, and the cut-off frequency of the low-pass filter and the high-pass filter in which these at least two capacitors form a part. The change in load can be measured with high resolution and high accuracy without interruption of oscillation or distortion of the oscillation waveform due to changes in a wide range of applied loads.

  The measuring means may measure the period of the output signal of the Wien bridge oscillation circuit. In this case, the load signal calculation means calculates the load by a secondary or higher order load calculation formula regarding the load based on the measured period.

Hereinafter, the high-order load calculation formula of the load signal calculation means will be described. As described above, when a load is applied to the movable part, the movable part side electrode is accurately displaced in the vertical direction with respect to the fixed part side electrode. The overlapping area between both electrodes changes. When at least two capacitance bodies formed by both electrodes are C1 and C2, capacitances when no load is applied to the load sensors of the capacitance bodies are Co1 and Co2, and volume change rates are β1 and β2, respectively. When the load change due to the load applied to the movable part is wx,
C1 = Co1 (1 + β1 · wx)
C2 = Co1 (1 + β2 · wx)
It becomes. Assume that the resistors R1 and R2 of the low-pass filter and the high-pass filter are both fixed resistors and their values do not change.

If at least two capacitance bodies are formed in different parts, it is difficult to create the same counter electrode plate area and opposite distance, and the no-load capacitances Co1 and Co2 are slightly different from each other. It is assumed that the change rates β1 and β2 are slightly different from each other. Substituting the above Co1 and Co2 into the formula (1) of the period T of the Wien bridge oscillation circuit described above, where To is the period when no load is applied and Tx is the period when a certain load is applied, To , Tx is To = 2π (Co1, Co2, R1, R2) ^1/2
Tx = 2π {Co1 · Co2 (1 + β1 · wx) · (1 + β2 · wx) · R1 · R2} ^1/2
It is.

  The periods To and Tx are sinusoidal waves whose oscillation signal is centered on the common potential COM. Therefore, for example, a comparator is used as a part of the measuring means, and the output signal of the Wien bridge oscillation circuit is supplied to the comparator. Is converted to a rectangular wave signal with a continuous high level or low level of 1/2 period of the oscillation signal when a larger sine wave signal is input, and the duration of this periodic signal is set to a period sufficiently shorter than this periodic signal. The periods To and Tx are measured by counting with a pulse signal.

By the measured periods To and Tx,
Tx ² -To ²
= 4π ² · R 1 · R ² · Co 1 · Co 2 {(1 + β1 · wx) · (1 + β2 · wx) −1}
= 4π ² · R 1 · R ² · Co 1 · Co 2 {β 1 · β 2 · wx ² + (β 1 + β 2) · wx}
Is calculated. from now on,
4π ² · R 1 · R ² · Co 1 · Co 2 · β 1 · β 2 · wx ² + 4π ² · R 1 · R ² · Co 1 · Co 2 · β 1 · β 2 (β 1 + β 2) · wx} − (Tx ² −To ² ) = 0. (2)
A quadratic expression regarding the load change amount wx is obtained.

In equation (2),
4π ² · R1 · R2 · Co1 · Co2 · β1 · β2 = a1
4π ² · R 1 · R ² · Co 1 · Co 2 · β 1 · β 2 (β 1 + β 2) = a 2
(Tx ² −To ² ) = b
Then,
Equation (2) is
a1 · wx ² + a2 · wx + b = 0 (3)
Each time a load having various different magnitudes is applied to the movable part, it is known by measuring the period of the oscillation signal. Therefore, the coefficients a1 and a2 are known in advance in order to obtain wx.

  The coefficients a1 and a2 can be obtained by applying three different types of known loads to the movable part, measuring the period, and substituting them into the equation (3). When Tx and To are measured to determine b, noise components are included in the measured value, and when it is considered that the determination of the coefficients a1 and a2 is affected, several types of known loads are used. It is possible to prepare and measure Tx and To each time each known load is applied, and determine a1 and a2 by the least square method.

  At the time of adjustment of the weighing device, the period To is measured in an unloaded state, and this period To is stored as an initial value in, for example, a nonvolatile memory included in the load signal calculation unit. At this time, the coefficients a1 and a2 are already stored in the nonvolatile memory. During operation of the weighing device, an unknown load is applied to the movable part, the period Tx is measured by the output signal measuring means of the Wien bridge oscillation circuit, b is obtained from the period Tx and the known period To, and the equation (3 ) To obtain a solution of the quadratic equation, wx is obtained in the load signal calculation means. In order to accurately calculate the load, it is desirable not to simplify the expression (3) using an approximate expression.

In the above description, the two resistors are fixed values irrespective of the load application. However, the resistor whose resistance value changes according to the load change by attaching a strain gauge to the strain generating portion as a resistor. Can also be used in the Wien bridge oscillation circuit. However, as described in the section of the problem to be solved by the invention, it is difficult to change the resistance value at a large change rate like the capacitance. If the resistance value of the resistor is also changed even if the change rate is small, in order to obtain an accurate weight, if the change rate of the resistance value is α1, α2,
R1 = Ro1 (1 + α1 · wx)
R2 = Ro2 (1 + α2 · wx)
It is necessary to obtain the change amount wx, and the equation for obtaining the period in Equation (2) is represented by a quartic equation relating to wx, and there are five coefficients to be determined. Therefore, it is possible to calculate the load by a secondary or higher order load calculation formula.

  However, whether to determine 5 coefficients of the quartic equation at the time of adjustment or to determine the 5 coefficients, at least 5 known loads with different values Must be used, which is a heavy work load in mass production of load sensors. Therefore, in order not to complicate the load calculation formula, it is desirable that only the capacitance of the capacitance body is changed according to the load change, and the resistor has a fixed resistance value that does not change due to the load change.

  As described above, in the weighing device according to the present invention, the capacitance of the capacitor body of the low-pass filter and the high-pass filter constituting the two sides of the Wien bridge oscillation circuit is changed at a large change rate with respect to the load change amount. In addition, since the cut-off frequencies of both filters change almost the same, the change in load can be measured with high resolution and high accuracy without interruption of oscillation or distortion of the oscillation waveform due to a wide change in applied load.

It is the side view of the parallelogram type load cell used for the weighing | measuring apparatus of the 1st Embodiment of this invention, and the enlarged view of the electrode plate vicinity provided in this. FIG. 2 is a block diagram of the weighing device in FIG. 1. 1 is a side view of a weighing device according to a first embodiment of the present invention. It is explanatory drawing of the capacity | capacitance change in the measuring apparatus of FIG. It is a frequency characteristic figure of the Wien bridge oscillation circuit used in the measuring apparatus of FIG. It is a figure which shows the side view of the parallelogram type load cell used for the measuring apparatus of the 2nd Embodiment of this invention, and the electrode plate provided in this. It is a circuit diagram of a low pass filter and a high pass filter used for a Wien bridge oscillation circuit. FIG. 8 is a frequency characteristic diagram of the low-pass filter and the high-pass filter of FIG. 7. FIG. 8 is a circuit diagram of a Wien bridge oscillation circuit using the low pass filter and the high pass filter of FIG. 7.

  The weighing device according to the first embodiment of the present invention has a load sensor which is a parallelogram type load cell, and this load sensor has a Wien bridge oscillation circuit 2 as shown in FIG. The Wien bridge oscillation circuit 2 has a bridge circuit 4. This bridge circuit 2 has four sides, and a low-pass filter LPF and a high-pass filter HPF are arranged on two of them in series. Fixed resistors R are respectively arranged on two serial sides connected in parallel to the two serial sides. The low-pass filter LPF is a capacitor body C1 and a resistor R1 connected in parallel, and the high-pass filter HPF is a capacitor body C2 and a resistor R2 connected in series. The low pass filter LPF and the high pass filter HPF have the same cut-off frequency fc.

  The connection point between the low-pass filter LPF and the high-pass filter HPF is connected to the non-inverting input terminal of the operational amplifier 6, and the connection point between the fixed resistors R is connected to the inverting input terminal of the operational amplifier 6, and is fixed to the high-pass filter HPF. The connection point with the resistor R is connected to the output terminal of the operational amplifier 6, and the connection point between the low-pass filter LPF and the fixed resistor R is connected to a reference potential, for example, the common potential COM. Thus, a low-pass filter LPF and a high-pass filter HPF having the same cutoff frequency fc are provided in the positive feedback circuit between the output terminal and the non-inverting input terminal of the operational amplifier 6. The fixed resistors R and R determine the gain of the positive feedback circuit. With such a configuration, a sinusoidal oscillation signal having a frequency fc is obtained as an output of the operational amplifier 6. Resistors R1 and R2 also have fixed resistance values. By applying a load as will be described later, the capacitances of the capacitance bodies C1 and C2 change, and the oscillation frequency changes from fc to fx as shown in FIG.

  The oscillation output of the operational amplifier 6 is shaped by the waveform shaping circuit 8 when the measuring device is in the operation mode, and then the period Tx (= 1 when the load wx is applied by the measuring means, for example, the period judgment circuit 10. / Fx) is measured and supplied to a load signal calculation means, for example, a load signal calculation circuit 12. The load signal calculation circuit 12 uses the cycle Tx in the same manner as described in the section of means for solving the problem based on the cycle Tx and the cycle To (= 1 / fo) at the initial load when no load is applied. Then, the coefficient b is obtained from the known period To and substituted into the equation (3) to calculate the load as the solution of the quadratic equation. The calculation result is displayed on a display means, for example, the display 14. When the weighing apparatus is in the adjustment mode, the coefficients a1 and a2 are determined and the period To is measured in a state where no load is applied as described in the section for solving the problem. It is stored in the storage means of the calculation circuit 12, for example, a nonvolatile memory.

  Capacitance bodies C1 and C2 of the Wien bridge circuit 4 are provided in a parallelogram load cell 16 made of a metal elastic body shown in FIG. In the parallelogram load cell 16, the upper circular holes 20 a and 20 b and the lower circular holes 22 a and 22 b are penetrated in the thickness direction of the metal block 18 at the four corners of the parallelogram metal elastic block 18. Is provided. A narrow upper horizontal gap 24 is horizontally provided so as to penetrate the metal block 18 in the thickness direction so as to connect the upper circular holes 20a and 20b. A narrow lower horizontal gap 26 is horizontally provided so as to penetrate the metal elastic body block 18 in the thickness direction so as to connect the lower circular holes 22, 22. A narrow vertical gap 28 is formed through the metal elastic body block 18 in the thickness direction so as to connect the centers of the upper horizontal gap 24 and the lower horizontal gap 26.

  An upper beam 30 is formed by the upper circular holes 20 a and 20 b and the upper horizontal gap 24, and a lower beam 32 is formed by the lower circular holes 22 a and 22 b and the lower horizontal gap 26. The upper circular holes 20a and 20b and the lower circular holes 22a and 22b are such that part of their peripheral edges are in contact with the upper and lower sides of the metal elastic body block 18 by thin portions 34, 34, 34, and 34 having a width d. These thin-walled portions 34, 34, 34, and 34 constitute a link mechanism with the parallel upper beam 30 and lower beam 32. A portion between the vertical gap 28 and one side and a portion between the other side of the vertical gap 28 are a fixed portion 36 and a movable portion 38. Either side may be the fixed portion 38, but in this embodiment, the left side in FIG. 1 is the fixed portion 36 and the right side is the movable portion 38.

  For example, a weighing device as shown in FIG. 3 is configured using the load cell of FIG. The fixed portion 36 is attached to a foundation mounting bracket 42 provided on the foundation surface 40, the weighing platform support bracket 44 is attached to the movable portion 38, and the weighing platform 46 is horizontally placed on the weighing platform support bracket 44. It is attached. An object to be weighed is placed on the weighing platform 46, and the weight of the object to be weighed is measured. When the load is applied in the direction indicated by the arrow in FIGS. 1 and 3, that is, in the vertical direction with respect to the base surface 40, the position of the object to be measured is changed from the position indicated by the solid line to the position indicated by the broken line on the weighing table 46. Regardless of the movement or the magnitude of the load of the object to be weighed, the movable portion 38 moves vertically downward. Therefore, the end surface 50 on the movable portion 38 side moves downward in parallel with respect to the end surface 48 on the fixed portion 36 side that faces the vertical gap 28 in FIG.

  If electrodes, for example, electrode plates are provided on the end surfaces 48 and 50 and face each other, the electrode plate on the end surface 50 on the movable portion 38 side is opposed to the electrode plate on the end surface 48 on the fixed portion 36 side. It moves parallel to the end face 48 in the vertical direction at a distance proportional to the applied load. As a result, the opposing areas of the two electrode plates change in proportion to the load change. Therefore, when the two electrode plates are made to be capacitive bodies, the capacitance of the capacitive body is accurately adjusted to the magnitude of the applied load. Proportionally changes.

  FIGS. 1B and 1C show an example of an electrode plate. As shown in FIG. 2B, resin-made insulating substrates 52 and 54 are bonded to the end surface 48 on the fixed portion 36 side and the end surface 50 on the movable portion 38 side, respectively. For example, four metal patterns are arranged on these insulating substrates 52 and 54 so as to face each other, and each of the four fixed portion side electrode plates 56a, 56b, 56c and 56d, and the movable portion side electrode plates 58a, 58b and 58c. , 58d are formed. The fixed portion side electrode plate 56a and the movable portion side electrode plate 58a form one capacitance body. Similarly, the fixed portion side electrode plates 56b, 56c, and 56d and the movable portion side electrode plates 58b, 58c, and 58d are the same. Three electrostatic capacity bodies are formed. Two of these four capacitance bodies are connected in parallel, for example, and used as the capacitance body C1 of the low-pass filter LPF, and the remaining two are connected in parallel to form the capacitance body of the high-pass filter HPF. Used as C2.

  As shown in FIG. 5C, the electrode plates 56a, 56b, 56c, and 56d all have a vertical dimension La, and lead wires 60 are soldered to both ends so as to cover the solder connection portions. An insulating coating 62 is applied to both ends of the insulating substrate 52 with an interval of a width D. That is, the electrode plates 56a, 56b, 56c, 56d all have a width dimension (dimension in the thickness direction of the metal elastic body block 18). The same applies to the electrode plates 58a, 58b, 58c, and 58d.

  FIG. 4 shows an enlarged view of the electrode plates 56a and 58a. Hereinafter, the electrode plates 56a and 58a will be described as representatives, but the same applies to the other electrode plates 56b, 56c, 56d, 58b, 58c, and 58d. When an object to be weighed is not loaded on the weighing platform 46 (when only the load of the weighing platform 48 is applied to the parallelogram load cell 16), the fixed portion side electrode plate 56a and the movable portion side electrode The plate 58a is arranged so that there is a difference of Lb in the vertical direction. That is, with respect to the vertical direction, the position of the movable part electrode plate 58a is arranged to be higher than the position of the fixed part electrode plate 56a. When an object to be weighed is loaded on the weighing table 48, and the heavier object to be weighed is loaded, the position of the movable part electrode plate 58a is displaced downward, and the opposing area of the two electrode plates 56a and 58a becomes larger. Increasing the capacitance increases.

  Electrode plates 56a to 56d and 58a to 58d are formed in advance on the insulating substrates 52 and 54, respectively, and four capacitance bodies are formed by bonding the insulating substrates 52 and 54 to the fixed portion end surface 48 and the movable portion end surface 50. However, if the insulating substrates 52 and 54 have the same size and the positional relationship between the electrode plates 56a to 56d and 58a to 58d formed in advance is different so as to satisfy the above relationship, the solid portion side end face 48 is formed. By simply bonding the insulating substrate 52 and the insulating substrate 54 to the movable part side end surface 50, the above-mentioned preferable conditions at the initial load can be obtained.

The length Lo of the portion where both electrode plates 56a, 58a face in parallel when the initial load is applied is:
Lo = La-Lb
It is. Since the width of the electrode plates 56a and 58a is D as described above, the area So where the electrode plates 56a and 58a face when the initial load is applied is
So = Lo · D
It is. Then, assuming that an object to be weighed wx is loaded on the weighing platform 46 and the movable part side electrode 58a is displaced by Lx, the distance in which the electrode plates 56a and 58a are parallel is Lo + Lx
Therefore, the area Sx that the electrode plates 56a, 58a face when the object to be weighed is placed on the weighing platform 46 is
Sx = (Lo + Lx) · D
It becomes. When the distance between the electrode plates 56a and 58a is H, and the dielectric constant of the substance between the electrode plates 56a and 58a is ε,
The capacitance Co of the set of capacitance bodies when the initial load is applied is
Co = ε · So / H
And the capacitance Cx of a set of capacitance bodies when an object to be weighed is loaded,
Cx = ε · Sx / H
It becomes.

The rate of change of capacitance is (Cx-Co) / Co = Lx / Lo
Since the displacement Lx is proportional to the weight wx of the object to be weighed, if Lx = k · wx,
Rate of change in capacitance (Cx-Co) / Co = Lx / Lo = (k · Lo) · wx = β · wx
Since k and Lo are constant, the coefficient β = k / Lo is also constant. Therefore,
Cx-Co = Co. (Lx / L) = Co. (K / Lo) .wx
And Cx is
Cx = Co + Co. (K / Lo) .wx = Co {1+ (k / Lo) .wx}
= Co (1 + β · wx)
It is expressed.

When two capacitance bodies are connected in parallel, the capacitance Cx when an object to be weighed in one capacitance body is applied is expressed as described above, and another capacitance body. Cx ′ is Cx ′ = Co ′ + Co ′ · (k ′ / Lo ′) · wx, where Cx ′ is the capacitance when the object to be weighed is applied
It is represented by The combined capacitance of these two capacitance bodies connected in parallel is
Cx + Cx ′ = Co + Co ′ + Co (k / Lo) wx + Co ′ (k ′ / Lo ′) wx
= Co + Co '+ (Co + Co') [Co / (Co + Co '). (K / Lo) + Co' / (Co + Co '). (K' / Lo ')] wx
And
Co / (Co + Co ′) · (k / Lo) = K = constant Co ′ / (Co + Co ′) · (k ′ / Lo ′) = K ′ = constant,
Cx + Cx ′ = (Co + Co ′) + (Co + Co ′) (K + K ′) · wx
= (Co + Co ′) {1+ (K + K ′) · wx}
And if K + K ′ = β,
Cx + Cx ′ = (Co + Co ′) {1 + β · wx}
It can be expressed.

  In this way, whether only one capacitance body is used or connected in parallel, the capacitance after applying the load is expressed in the same manner as described in the section for solving the problem. Is done.

  As described above, the two capacitance bodies connected in parallel are used as the capacitance body C1 of the low-pass filter LPF, and the remaining two capacitance bodies connected in parallel are the capacitance bodies of the high-pass filter HPF. Used as C2.

The output signal of the operational amplifier 6 of the Wien bridge oscillation circuit 2 is supplied to the waveform shaping circuit 8 as described above. The waveform shaping circuit 8 is a circuit that outputs a high-level or low-level logic signal in a region where a sine wave signal oscillating around the common potential COM of the bridge circuit 4 continues at the common potential COM or higher. This logic signal is read as a cycle value by the cycle determination circuit 10. In the adjustment mode, the cycle To is read in the initial load state, stored in the nonvolatile memory of the load signal calculation circuit 12, and in the operation mode, the cycle Tx is read in a state in which an object to be weighed is loaded on the weighing platform 46. The period Tx is supplied to the load signal calculation circuit 12. In the load signal calculation circuit 12, b (= Tx ² −To ² ) is calculated in the same manner as described in the section for solving the problem, and is set in the nonvolatile memory in advance in the adjustment mode with b. The load wx is calculated using the coefficients a1 and a2 and is displayed on the display unit 14.

In order to measure the value of b (= Tx ² −To ² ) corresponding to the change amount of the period with high resolution, a large difference between Tx and To is required. This requires a large rate of change in capacitance. The rate of change of capacitance is (Cx-Co) / Co = Lx / Lo
Therefore, when an object to be weighed is loaded on the weighing platform 46, the value of Lx / Lo increases as the value of Lx with respect to Lo increases. Lx is, for example, about 150 μm, where Lx is the amount of displacement in the vertical direction when the rated load is applied to the parallelogram load cell 16. Although it is not easy to change this value, the value of Lo can be easily changed. Since Lo is the length of the portion where the fixed portion electrodes 56a to 56d and the movable portion electrodes 58a to 58d face each other in parallel with the initial load applied, it is easily set to about several mm, for example, about 3 mm. Can do. Accordingly, since the rate of change can be easily set to, for example, about 150/3000 = 1/20, periodic measurement can be performed with high resolution, and the weight wx of the object to be weighed can be obtained with high resolution accordingly. . In addition, if Lo is reduced, a plurality of sets of counter electrodes (fixed part electrodes and movable part electrodes) can be arranged, and the coefficient β, that is, the rate of change can be increased as an added value for each counter electrode.

  The fixed resistors R1, R2, R, R and the operational amplifier 6 of the Wien bridge oscillation circuit 2 shown in FIG. 2 are mounted on a resin printed board 64 as shown in FIG. It is attached to 36 side surfaces.

  The resistors R1 and R2 constituting the low-pass filter LPF and the high-pass filter HPF are set so that R1≈R2 is always established, and C1 and C2 are set so that C1≈C2 is established when the initial load is applied. The fixed portion side electrodes 56a to 56d and the fixed portion side electrodes 58a to 58d have the same initial position and displacement amount due to load application in the four capacitance bodies. Therefore, even when an arbitrary load is applied, C1≈C2 holds, and the cutoff frequency determined by R1, R2, C1, and C2 in the low-pass filter LPF and the high-pass filter HPF is an arbitrary load even when the initial load is applied. Even when the voltage is applied, it is almost the same. Therefore, in the state where the cutoff frequency of the low-pass filter LPF and the high-pass filter HPF always matches the arbitrary applied load, the value of the cutoff frequency changes according to the magnitude of the load change amount. Therefore, an oscillation signal having an accurate shape can always be obtained, an accurate period measurement can be performed, and a load change amount can be accurately obtained.

  FIG. 6 shows a parallelogram load cell 161 used in the weighing device of the second embodiment. Since this weighing device is configured in the same manner as the weighing device of the first embodiment except that the configuration of the parallelogram load cell 161 is different, only the configuration of the parallelogram load cell 161 will be described.

  Also in this parallelogram type load cell 161, like the parallelogram type load cell 16 of the first embodiment, upper circular holes 20a and 20b and lower circular holes are formed at the four corners of the parallelogram type metal elastic body block 18. 22a and 22b are provided. A narrow upper horizontal gap 24 is horizontally provided so as to connect the upper circular holes 20a, 20b, and a narrow lower horizontal gap 26 is provided horizontally so as to connect the lower circular holes 22, 22. Yes. A saddle-shaped gap 281 is formed so as to penetrate in the thickness direction of the metal elastic body block 18 so as to connect the lower part of the upper circular hole 20b and the upper part of the lower circular hole 22a.

  The gap 281 includes an upper vertical portion 281a extending vertically from the lower portion of the upper circular hole 20b, a horizontal portion 281b extending horizontally from the lower end of the upper vertical portion 281a toward the lower circular hole 22a, and an end of the horizontal portion 281b. A lower vertical portion 281c extending vertically from the lower circular hole 22a side. The elastic metal block 18 is divided into a fixed part 361 and a movable part 381 by the saddle type gap 281. Further, similarly to the first embodiment, thin portions 34, 34, 34, 34 are formed, and a parallel link mechanism having an upper beam 30 and a lower beam 32 is formed. When an electrode plate is formed on the surface 481 on the fixed portion 361 side and the surface 501 of the movable portion 381 facing each other across the horizontal portion 281b of the saddle-shaped gap 281, a capacitance body can be configured.

  Although not shown, a weighing platform is coupled to the movable portion 381 via a weighing platform support fitting in the same manner as shown in FIG. 3, and the fixed portion 361 is attached to the foundation surface via a foundation mounting fitting. It is attached. When a load in the vertical direction is applied to the movable portion 381 as indicated by an arrow, the movable portion 381 moves downward in the vertical direction. Accordingly, since the distance between the electrode plates of the surface 481 on the fixed portion 361 side and the surface 501 of the movable portion 381 is changed by the application of the load, the reciprocal of the electrostatic capacitance of the capacitive body is accurately determined by the magnitude of the applied load. Changes in proportion to

  As shown in FIG. 4B, the electrode plate is formed on resin insulating substrates 521 and 541 attached to the surface 481 on the fixed portion 361 side and the surface 501 of the movable portion 381. Two electrode plates 561a and 561b are provided on the insulating substrate 521 along the thickness direction of the metal elastic block 18 as shown in FIG. Further, two electrode plates 581a and 581b (not shown) are provided on the insulating substrate 541 so as to face the electrode plates 561a and 561b. Similar to the first embodiment, the electrode plate 561a, 561b, 581a, 581b is provided with a lead wire 60 and an insulating coating 62.

The opposing area of the electrode plates 561a and 581a, the opposing area of the electrode plates 561b and 581b are S, respectively, the dielectric constant of a substance such as air existing between the electrode plates 561a, 561b and 581a, 581b is ε, and the electrode plate Assuming that the distance between 561a, 561b and 581a, 581b is Ho when an initial load is applied and Hx is when an object to be weighed is applied, the capacitance Co between the pair of electrode plates 561a and 581a when the initial load is applied is, for example, ,
Co = ε · S / Ho
The capacitance Cx when an object to be weighed is expressed as
Cx = ε · S / Hx
It is represented by Since Hx changes in proportion to the applied weight, the reciprocal 1 / Cx of the capacitance is
1 / Cx = Hx / ε · S
And changes in proportion to the applied load. The electrostatic capacitances of the electrode plates 561b and 581b are the same. Then, the electrostatic capacity body formed by the electrode plates 561a and 581a is used as the electrostatic capacity body C1 of the low-pass filter LPF of the Wien bridge oscillation circuit 2, and the electrostatic capacity body formed by the electrode plates 561b and 581b is used as the electrostatic capacity of the high-pass filter HPF. Used as body C2.

Assuming that the period of the oscillation signal of the Wien bridge oscillation circuit 2 at the time of initial load application is To and the period when a certain load wx is applied is Tx, Tx ² −To ² in Expression (2) is (1 / Tx ² − If it is replaced with 1 / To ² ), as described in the section of means for solving the problem, a1 and a2 are obtained in advance, and b = (1 / Tx ² −1 / To ² ). The load wx can be obtained by (3).

  If the vertical dimension d1 (see FIG. 6) of the horizontal portion 281b of the saddle-shaped gap 281 at the time of initial load application is appropriately selected with respect to the amount of displacement of the movable portion 381 due to the load change, a large capacitance change Rate can be obtained freely. For example, the horizontal portion 281b is machined in advance so that d has a preferable initial load application dimension.

  In the first embodiment, four capacitance bodies are configured, and in the second embodiment, two capacitance bodies are configured. However, any number of capacitances may be used as long as the number is two or more. You can also make up your body. However, since it is desirable to match the cut-off frequencies of the low pass filter and the high pass filter HPF of the Wien bridge oscillation circuit, it is desirable to provide an even number of capacitance bodies.

  In the first and second embodiments, only the capacitance values of the low-pass filter and the high-pass filter of the Wien bridge oscillation circuit are changed in accordance with the application of the load, but the values of the resistors of the low-pass filter and the high-pass filter are changed. Alternatively, it can be changed according to the application of a load. In that case, for example, a strain gauge is used as a resistor, and the strain gauge is attached to the strain generating portion 34 shown in FIGS. However, when a strain gauge is affixed so that the resistance value increases with the application of a load, it is affixed to the strain generating portion 34 on the upper circular holes 20a, 20b, and these strain gauges are fixed as shown in FIG. Used in place of resistors R1 and R2. However, as described above, in this case, as a high-order equation for obtaining the load, a quartic equation is used as described above.

  The Wien bridge oscillation circuit shown in the above two embodiments shows a basic form, and various known modified Wien bridge oscillation circuits can also be used. For example, a variable resistor is connected in series to the fixed resistor R connected to the inverting input terminal of the operational amplifier 16, and the variable resistor is changed according to the output level of the operational amplifier 16 to provide automatic gain control. It can also be a Wien bridge oscillation circuit

2 Wien bridge oscillation circuit 10 Period determination circuit (measurement means)
12 Load signal calculation circuit (load signal calculation means)
16 Parallelogram load cell 36 Fixed portion 38 Movable portion 56a to 56d Fixed portion electrode plate (fixed portion electrode)
58a to 58d Movable part electrode plate (movable part electrode)
LPF Low pass filter HPF High pass filter

Claims (3)

A load sensor having a Wien bridge oscillation circuit; a means for measuring an output signal of the Wien bridge oscillation circuit; and a load signal calculation means for detecting a load applied to the load sensor based on the measured output signal. In the weighing device,
The load sensor includes a parallelogram load cell made of a metal elastic body,
The parallelogram load cell has a fixed portion for attaching the parallelogram load cell to a base surface, and a movable portion to which a load is applied, and the movable portion is displaced in a vertical direction when the load is applied. And
The fixed part is provided with a fixed part electrode, the movable part is provided with a movable part electrode, and the fixed part electrode and the movable part electrode are opposed to each other in a non-contact manner. Provided in
When a load is applied to the movable part by the fixed part electrode and the movable part electrode, the area of the opposing part of the fixed part electrode and the movable part electrode, or the distance of the opposing part However, the low-pass filter includes at least two capacitance bodies that change in accordance with the amount of change in the load and change in capacitance, and these at least two capacitance bodies are included in one side of the Wien bridge oscillation circuit. And a weighing device that forms part of each high-pass filter included on the other side.   2. The measuring device according to claim 1, wherein the measuring means measures a period of the output signal of the Wien bridge oscillation circuit, and the load signal calculating means is based on the measured period, and the second or higher order related to the load. A weighing device that calculates a load using a load calculation formula.   3. The weighing apparatus according to claim 2, wherein each of the low-pass filter and the high-pass filter has a fixed resistor having a fixed value.

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