JP2008064497A - Load cell unit, weight sorting machine, and electronic balance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a load cell unit capable of satisfactorily weighing objects to be weighed even in the case where ambient temperature has changed and provide a weight sorting machine and an electronic balance using the load cell unit. <P>SOLUTION: A strain body has a plurality of strain parts. Each strain gauge 41 is a temperature sensitivity compensating type gauge and arranged at a corresponding strain part. A bridge circuit 40 is made of the strain gauges 41. A zero-point compensating element 42 compensates for a zero point of the bridge circuit 40 according to the temperature of the strain body and roughly corrects output of the bridge circuit 40. A temperature-sensitive resistor 45 is a temperature sensor for detecting the temperature of the strain body. The zero-point compensating element 42 and the temperature-sensitive resistor 45 are provided for a thick-walled part between adjacent strain parts. A signal processing part 50 accurately corrects roughly corrected output of the bridge circuit 40. It is thereby possible to further apply software-based zero-point compensation to output of the bridge circuit 40 to which circuit-based zero-point compensation has been applied. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a load cell unit that measures the weight of an object to be weighed based on a load signal, and a weight sorter and an electronic balance using the load cell unit, and more particularly to zero compensation of a bridge circuit using a strain gauge. About.

  Conventionally, for load cells that have a bridge circuit composed of strain gauges, the zero point change amount is calculated based on the temperature detected by the temperature sensor, and the bridge circuit output is corrected based on the calculated zero point change amount. The technique to do is known (for example, patent document 1). Here, the zero point means a voltage (load signal) output from the bridge circuit of the load cell unit when a voltage from the power source is applied to the load cell unit in an unloaded state.

  In addition, a technology (sensitivity correction) that reduces the amount of change in the output of the bridge circuit due to changes in the ambient temperature and compensates for the sensitivity of the load cell unit by connecting a temperature sensitive resistor to the input side of the bridge circuit. Known (Patent Document 2). Furthermore, there is also known a technique for calculating the change amount of the zero point based on the temperature of the load cell unit and correcting the output of the bridge circuit by diverting the temperature-sensitive resistor for sensitivity correction as a temperature sensor (Patent Document). 2).

JP-A-11-125555 Japanese Patent Publication No. 07-069232

  Here, it is known that when the amount of change of the zero point due to temperature variation is large, the output from the bridge circuit varies greatly even if the ambient temperature of the load cell unit slightly changes. As a result, the apparatus of Patent Document 1 has a problem that the weight of the object to be weighed cannot be detected stably. In particular, in a device (for example, a weight sorter) in which weighing processing is executed continuously, zero point correction cannot be executed for each weighing processing, and this zero point variation becomes a problem.

  In addition, in a weighing machine (for example, an electronic scale) that measures the weight of a stationary object to be weighed, when a weighing mode with a smaller minimum scale is added, this zero point change amount becomes a problem. In other words, if the minimum scale for the rated capacity (the maximum load that can be measured with the load cell unit being kept in specification) is further reduced, the measured value may not be stable due to the influence of the ambient temperature where the load cell unit is placed. There was a problem.

  Therefore, the present invention provides a load cell unit that can accurately weigh the object to be weighed even when the ambient temperature changes, and a weight sorter and an electronic balance using the load cell unit. With the goal.

  In order to solve the above-mentioned problem, the invention of claim 1 is a load cell unit for measuring the weight of an object to be weighed based on a load signal, and is a metal block provided with a through hole, and an inner wall surface of the through hole. And a strain generating body having a plurality of thin strain generating portions sandwiched between the outer peripheral surface of the metal block, a strain gauge disposed in a corresponding strain generating portion among the plurality of strain generating portions, and the strain gauge. The formed bridge circuit is connected in series with a strain gauge disposed on one side of the bridge circuit, and compensates for the zero point of the bridge circuit, which fluctuates due to a temperature change, according to the temperature of the strain generating body, A zero compensation element that roughly corrects the output of the bridge circuit, a temperature sensor that detects the temperature of the strain generating body, and a change in the zero after the coarse correction based on a detection result by the temperature sensor A signal processing unit that calculates the amount and finely corrects the output of the coarsely corrected bridge circuit based on the amount of change, and the strain generating body includes a free end to which a load is applied and a fixed end. A Roverval mechanism is configured through a plurality of strain generating portions provided therebetween, and the zero compensation element and the temperature sensor are provided in a portion sandwiched between adjacent strain generating portions. .

  According to a second aspect of the present invention, in the load cell unit according to the first aspect of the present invention, the temperature sensor is a temperature sensitive resistor.

  According to a third aspect of the present invention, in the load cell unit according to the second aspect of the present invention, the temperature sensitive resistor is connected in series to the input side of the bridge circuit.

  According to a fourth aspect of the present invention, in the load cell unit according to the third aspect of the present invention, the strain gauge is a self-temperature compensated strain gauge capable of compensating for thermal expansion of the strain generating body due to temperature change. It is characterized by that.

  According to a fifth aspect of the present invention, in the load cell unit according to the first or second aspect of the present invention, the strain gauge is a temperature that can compensate for the amount of change in the longitudinal elastic modulus of the strain generating body caused by the temperature change. It is a sensitivity compensation type strain gauge.

  According to a sixth aspect of the present invention, in the load cell unit according to any one of the first to fifth aspects of the present invention, adjacent first and second strain generating portions of the strain generating portions are formed on the outer peripheral surface. Among them, it is substantially perpendicular to the load direction received from the object to be weighed, and is provided on the first surface side that bears the load from the object to be weighed. It is substantially perpendicular to the load direction and is provided on the second surface side opposite to the first surface across the through hole, and the strain gauges are disposed in the first to fourth strain generating portions, respectively. It is characterized by.

  The invention according to claim 7 is the load cell unit according to claim 5, wherein the bridge circuit is formed by first and second strain gauges and first and second fixed resistors, The first strain gauge is between the first input terminal and the first output terminal, the second strain gauge is between the second input terminal and the first output terminal, and the first fixed resistance is The second fixed resistor is connected between the first input terminal and the second output terminal, and is connected between the second input terminal and the second output terminal. .

  The invention according to claim 8 is the load cell unit according to claim 7, wherein the first and second strain generating portions adjacent to each other among the strain generating portions are on the object-to-be-measured side of the outer peripheral surface. Is substantially perpendicular to the load direction received from the first object, and is provided on the first surface side that handles the load from the object to be weighed, and the adjacent third and fourth strain generating portions are substantially perpendicular to the load direction. The first and second strain gauges are either one of the first surface and the second surface, and are provided on the second surface side opposite to the first surface across the through hole. Are arranged in each of the strain-generating portions provided on the one surface.

  According to a ninth aspect of the present invention, in the load cell unit according to any one of the first to eighth aspects, the signal processing unit calculates the amount of change of the zero point by a second-order or higher order approximation. It is characterized by that.

  Further, the invention of claim 10 is a weight sorter, which is a weighing device for weighing the weight of the sorting object to be conveyed, and each sorting object based on the weighing result of the sorting object by the weighing device. The distribution device includes a distribution device, and the weighing device is supported by the load cell unit according to any one of claims 1 to 9, and a weighing conveyor that supports each sorting object. It is characterized by having.

  An eleventh aspect of the invention includes the load cell unit according to any one of the first to ninth aspects of the invention, and an upper plate part that is supported by the load cell and loads an object to be measured. Features.

  In the invention of any one of claims 1 to 9, the thin-walled strain-generating portion has a larger thermal resistance than other portions of the strain-generating body, and the portion sandwiched between adjacent strain-generating portions is the surrounding heat. It is hard to be affected. That is, the temperature sensor provided in the portion sandwiched between adjacent strain-generating portions can accurately and stably detect the temperature of the strain-generating body.

  As a result, the zero compensation element can perform rough correction by compensating the zero of the bridge circuit while accurately reflecting the temperature of the strain generating body. The signal processing unit calculates a change amount of the zero point that is coarsely corrected based on the temperature of the strain generating body that is accurately and stably detected, and outputs the output of the bridge circuit that is coarsely corrected based on the change amount. Fine correction can be made. That is, it is possible to further perform software zero compensation on the output of the bridge circuit that has been zero compensated in terms of circuitry (hardware). Therefore, the output accuracy of the bridge circuit can be further improved, and the reproducibility of the weighing process by the load cell unit can be further improved.

  In particular, according to the second aspect of the present invention, the temperature of the strain generating body can be detected based on a change in the resistance value of the temperature sensitive resistor.

  In particular, according to the invention described in claim 3, the temperature sensitive resistor is connected in series to the input side of the bridge circuit. As a result, the temperature-sensitive resistor can be used not only as a sensitivity compensator that compensates for the change in the longitudinal elastic modulus of the strain generating body due to temperature fluctuations but also to correct the output of the bridge circuit, but also as a temperature sensor for the strain generating body Can do. Therefore, the number of parts of the load cell unit can be reduced, and the manufacturing cost of the load cell unit can be reduced.

  In particular, according to the invention described in claim 5, a temperature-sensitive compensation type strain gauge is used. Thereby, the output change of the bridge circuit due to the change of the longitudinal elastic modulus of the strain generating body can be compensated without using a sensitivity compensating element such as a temperature sensitive resistor. Therefore, the circuit configuration near the bridge circuit can be simplified.

  In particular, according to the invention described in claim 9, the amount of change in the zero point after the coarse correction can be easily calculated by a second-order or higher order approximation, and the calculation cost required for fine correction can be reduced. .

  According to the invention described in claim 10, the bridge circuit of the load cell unit is subjected to circuit (hardware) and software zero compensation. Thereby, the weight of each selection object can be calculated based on the output of the bridge circuit controlled with high accuracy. Therefore, even when the weighing interval is short and zero point correction cannot be performed for each weighing as in the case of weight weighing of the sorting object conveyed by the weighing conveyor, the sorting object can be weighed with good reproducibility. .

  According to the invention described in claim 10, the weight of each selection object can be calculated based on the output of the bridge circuit controlled with high accuracy, and the minimum scale for the rated capacity of the load cell unit can be reduced. It becomes possible to set. Therefore, even if the load of the weighing conveyor (tare), which is heavier than the sorting object, is applied to the load cell unit, and the rated capacity of the load cell needs to be set larger than the sorting object, this It is possible to improve the weighing accuracy (weighing resolution) of the selection object.

  According to the eleventh aspect of the present invention, circuit and software zero compensation is performed on the bridge circuit of the load cell unit. Thereby, the weight of each measurement object can be calculated based on the output of the bridge circuit controlled with high accuracy. Therefore, it is possible to set the minimum scale for the rated capacity of the load cell unit small, and as a result, it is possible to further improve the measurement accuracy (measurement resolution) of the measurement object.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

<1. First Embodiment>
<1.1. Configuration of weight sorter>
FIG. 1 is a front view showing an example of the entire configuration of the weight sorter 1 according to the first embodiment. For example, the weight sorter 1 measures the weight of a product brought in from a previous process such as a bagging process, and distributes the product based on the measurement result. As shown in FIG. 1, the weight sorter 1 mainly includes an intake device 10, a weighing device 20, and a sorting device 60. 1 and subsequent figures are attached with an XYZ orthogonal coordinate system in which the Z-axis direction is a vertical direction and the XY plane is a horizontal plane, as necessary, in order to clarify the directional relationship. .

  The take-in device 10 has a take-in conveyor 11 as shown in FIG. The take-in device 10 conveys the sorting object 5 delivered from the upstream device as viewed from the conveying direction AR1 of the sorting object 5 to the weighing device 20.

  The weighing device 20 measures the weight of the sorting object 5 while conveying the sorting object (object to be weighed) 5 delivered from the take-in device 10. As shown in FIG. 1, the weighing device 20 mainly has a weighing conveyor 21 and a load cell unit 30.

  The weighing conveyor 21 is a transport unit that transports each sorting object 5 delivered from the capture device 10 to the sorting device 60. Further, as shown in FIG. 1, the weighing conveyor 21 is supported by the strain body 31 of the load cell unit 30 via the attachment member 25. Therefore, the strain body 31 receives the load of the weighing conveyor 21 and the sorting object 5 conveyed on the weighing conveyor 21.

  The load cell unit 30 is a unit that measures the weight of the sorting object 5 that is an object to be weighed based on a load signal. The detailed configuration of the load cell unit 30 will be described later.

  The sorting device 60 sorts the sorting objects 5 based on the weighing result of the sorting objects 5 by the weighing device 20. As shown in FIG. 1, the sorting device 60 mainly includes a transport conveyor 61 and a lever 62. The transport conveyor 61 is a transport unit that transports the weighing target object 5 passed from the weighing device 20 along the transport direction AR1. The lever 62 is pivotable about a pivot shaft 62a that is substantially perpendicular to the transport direction AR1, and sorts the sorting objects 5 transported by the transport conveyor 61.

  For example, when the measurement result by the weighing device 20 falls within the non-defective range, the lever 62 is rotated so that the length direction thereof is substantially parallel to the transport direction AR1 and is positioned to the side of the transport conveyor 61. . Thereby, the sorting object 5 is delivered to a non-defective product line (not shown) arranged on the extension of the transport direction AR1.

  On the other hand, when the measurement result by the weighing device 20 is out of the non-defective range, the lever 62 is rotated so as to block the conveyance of the selection target object 5. As a result, the sorting object 5 that has reached the lever 62 moves along the lever 62 and is delivered to a defective product line (not shown) provided on the side of the transport conveyor 61.

<1.2. Configuration of load cell unit>
2 and 3 are a front view and a plan view showing an example of the hardware configuration of the load cell unit 30, respectively. FIG. 4 is a surface view showing an example of the configuration of the strain gauge 41. Further, FIG. 5 is a block circuit diagram of the load cell unit 30. As shown in FIGS. 2 to 5, the load cell unit 30 mainly includes a strain generating body 31, a plurality of strain gauges 41 (41a to 41d), a zero compensation element 42, a temperature sensitive resistor 45, and a signal processing unit. 50.

  As shown in FIG. 2, the strain body 31 is a metal block provided with a through hole 32, and is an elastic body formed of a metal such as an aluminum alloy or stainless steel, for example. The strain generating body 31 includes a thin strain generating portion 33 (first to fourth strain generating members) sandwiched between the inner wall surface 32a of the through hole 32 and the outer peripheral surface 35 (including the first and second surfaces 35a and 35b). A plurality of (in this embodiment, four) portions 33a to 33d).

  Here, two adjacent strain generating portions 33a and 33b among the plurality of strain generating portions 33 are substantially perpendicular to the load direction AR2 received from the selection target (object to be weighed) 5, and from the selection target 5 side. It is provided on the first surface (upper surface) 35a side that handles the load. Further, two adjacent strain generating portions 33c and 33d among the plurality of strain generating portions 33 are substantially perpendicular to the load direction AR2, and the second surface (lower surface) opposite to the first surface 35a across the through hole 32. ) 35b.

  Further, a first thick portion 34a thicker than the first and second strain generating portions 33a and 33b is formed at a portion sandwiched between the first and second strain generating portions 33a and 33b. Further, a second thick portion 34b having a thickness greater than those of the third and fourth strain generating portions 33c and 33d is formed in a portion sandwiched between the third and fourth strain generating portions 33c and 33d.

  Thus, when the fixed end 31a of the strain generating body 31 is fixed in the weighing device 20 and a load is applied to the free end 31b side, the first surface (upper surface) 35a side of the first strain generating portion 33a and the first end Compressive stress is applied to the second surface (lower surface) 35b side of the fourth strained portion 33d, and tensile stress is applied to the first surface 35a side of the second strained portion 33b and the second surface 35b side of the third strained portion 33c. Will occur. Therefore, mechanical strain occurs in each of the strain generating portions 33a to 33d.

  In addition, when a load is applied to the free end side 31b, the strain body 31 is deformed into a substantially parallelogram shape and functions as a Roverval mechanism. Therefore, the moment acting on the strain generating body 31 is canceled, and as a result, the weight of the selection object 5 is detected as substantially the same value regardless of the position on the weighing conveyor 21. Thus, the strain body 31 constitutes a Roverval mechanism via the plurality of strain portions 33 provided between the free end 31b to which a load is applied and the fixed end 31a.

  As shown in FIGS. 2 and 3, strain gauges 41 (41 a to 41 d) are arranged on the outer peripheral surface 35 of the corresponding strain generating portion among the plurality of strain generating portions 33. That is, the first and second strain gauges 41a and 41b are provided on the first surface 35a of the first and second strain generating portions 33a and 33b, and the third and fourth strain generating portions 33c and 33d are provided on the third and fourth strain surfaces 33a and 33b. Strain gauges 41c and 41d are bonded to each other.

  Here, as shown in FIG. 4, each strain gauge 41 (41 a to 41 d) places a resistor 40 b such as a photo-etched resistance foil on a thin electric insulator (for example, polyimide resin) base member 40 a. It is formed in a meandering shape. Accordingly, when strain occurs in each strain generating portion 33 (33a to 33d) and the strain is transmitted to the resistor 40b via the base member 40a, a resistance change corresponding to the strain amount occurs in the resistor 40b. .

  In the present embodiment, a temperature sensitivity compensation type strain gauge is used as the strain gauge 41. In other words, in the present embodiment, the strain body 31 thermally expands as the temperature rises according to its linear expansion coefficient. In addition, the longitudinal elastic modulus (Young's modulus) of the strain body 31 decreases as the temperature increases, and the amount of strain under the same stress increases.

  On the other hand, the resistor 40b of the strain gauge 41 of the present embodiment is made of a metal such as a nickel chromium (Ni—Cr) alloy. In addition, with respect to the resistor 40b of the strain gauge 41, the mixing ratio of nickel and chromium is adjusted so that the strain sensitivity temperature coefficient becomes a negative value and the longitudinal elastic coefficient increases as the temperature rises. Then, a predetermined heat treatment is performed on the resistor 40b.

  Therefore, even if the strain body 31 expands due to a temperature rise and the strain amount of the strain body 31 with respect to the same load increases due to the temperature rise, the resistance of the strain gauge 41 bonded to the strain body 31 is increased. The values are substantially the same within a predetermined temperature range. As described above, the temperature-sensitive compensation type strain gauge 41 compensates for the thermal expansion of the strain generating body 31 that expands and contracts due to a temperature change, suppresses the zero point change of the bridge circuit 40, and longitudinally occurs due to the temperature change. The change in the output of the bridge circuit 40 can be suppressed by compensating the amount of change in the elastic coefficient.

  The bridge circuit 40 outputs the weight of the selection object 5 that is an object to be weighed as a load signal. As shown in FIG. 5, the bridge circuit 40 is mainly formed by first to fourth strain gauges 41a to 41d. That is, (1) the first strain gauge 41a is between the plus input terminal 44a and the plus output terminal 43a, and (2) the second strain gauge 41b is between the plus input terminal 44a and the minus output terminal 43b. (3) A third strain gauge 41c is disposed between the plus output terminal 43a and the minus input terminal 44b, and (4) a fourth strain gauge 41d is disposed between the minus output terminal 43b and the minus input terminal 44b. These strain gauges 41 (41a to 41d) constitute a full bridge. The voltage between the output terminals 43a and 43b is detected as the output of the bridge circuit 40.

  The zero point compensation element 42 (42a to 42d) is formed by winding a resistor (copper wire or nickel wire) having a large temperature coefficient of resistance, and is disposed on any one side of the bridge circuit 40. That is, any one of the zero compensation elements 42a to 42d shown in FIG. 5 is used and connected in series with the corresponding strain gauge 41 (41a, 41c, 41b, 41d). The first zero compensation element 42a is provided on the first thick part 34a on the first surface 35a side (see FIG. 2), and the second zero compensation element 42b is provided on the second thick part 34b on the second surface 35b side (illustrated). (Omitted), the third zero compensation element 42c is disposed on the first thick part 34a on the first surface 35a side (not shown), and the fourth zero compensation element 42d is disposed on the second thick part 34b on the second surface 35b side (see FIG. 2), one of which is closely attached. The zero compensation element 42 is not limited to a wound resistor, and may be, for example, a chip resistor having the same resistance value and resistance temperature coefficient as the resistor.

  Here, as described above, the strain gauge 41 is a temperature sensitivity compensation type strain gauge. Accordingly, the resistance value of each strain gauge 41 when no load is applied to the strain generating body 31 (no load state) becomes substantially the same value even if the ambient temperature of the strain generating body 31 changes. . Therefore, when the physical properties (for example, resistance value and resistance temperature coefficient) of each strain gauge 41 are substantially the same, the zero point of the bridge circuit 40 is substantially the same even when the ambient temperature changes.

  However, when each strain gauge 41 has an individual difference or when the adhesion state of each strain gauge 41 is different, the zero point of the bridge circuit 40 fluctuates according to a change in the ambient temperature, and as a result, the weighing by the weighing device 20 is performed. The result will be affected by the ambient temperature.

  On the other hand, in the present embodiment, the zero compensation element 42 corresponding to the amount of change of the zero due to the temperature change is disposed on one side of the bridge circuit 40 and is connected in series with the corresponding strain gauge 41. Therefore, the zero point that fluctuates due to the temperature change is compensated according to the temperature of the strain generating body 31, and the output of the bridge circuit 40 is roughly corrected so as to be within a certain range.

  Further, the thin-walled strain-generating portion 33 has a larger thermal resistance than the other portions of the strain-generating body 31, and the portion sandwiched between the adjacent strain-generating portions 33 (thick wall portion 34) Not easily affected. Therefore, the zero compensation element 42 provided in the thick portion 34 can compensate the zero of the bridge circuit 40 while accurately reflecting the temperature of the strain generating body 31.

  The arrangement of the zero compensation element 42 is not limited to any one side of the bridge circuit 40. For example, the zero compensation element 42 may be disposed on one side of the bridge circuit 40 and on a diagonal side when viewed from the one side. That is, as the zero compensation element 42, the first and fourth zero compensation elements 42a and 42d may be used, or the second and third zero compensation elements 42b and 42c may be used. That is, the zero compensation element 42 only needs to be arranged on at least one side of the bridge circuit 40.

  Similar to the zero compensation element 42, the temperature sensitive resistor 45 is a resistor having a large resistance temperature coefficient. For example, a nickel (Ni), copper (Cu), or platinum (Pt) material is used for the temperature sensitive resistor 45. As shown in FIG. 2, the temperature-sensitive resistor 45 is attached in close contact with the first thick portion 34a on the first surface 35a side.

  As shown in FIG. 5, one end of the temperature sensitive resistor 45 is electrically connected to the temperature measuring terminal 43c, and the other end is electrically connected to the temperature measuring terminal 43d. The temperature measurement terminal 43c is electrically connected to the positive side of the power supply 46, and the temperature measurement terminal 43d is electrically connected to the negative side of the power supply 46 via precision resistors 46a and 46b, respectively. Further, the temperature coefficient of resistance of the temperature sensitive resistor 45 is larger than that of the precision resistors 46a and 46b.

  Therefore, the temperature of the strain generating body 31 can be calculated based on the voltage applied to the temperature sensitive resistor 45 (voltage between the temperature measuring terminals 43c and 43d) and the resistance value-temperature characteristic of the temperature sensitive resistor 45. it can. Thus, in the present embodiment, the temperature of the strain generating body 31 is detected based on the change in the resistance value of the temperature sensing resistor 45, and the temperature sensing resistor 45 is used as a temperature sensor.

  Further, the temperature-sensitive resistor 45 is in close contact with the thick portion 34 that is not easily affected by the surrounding heat, and the resistance value of the temperature-sensitive resistor 45 varies while accurately reflecting the temperature of the strain-generating body 31. . Therefore, the temperature of the strain generating body 31 is accurately and stably detected based on the change in the resistance value of the temperature sensitive resistor 45.

  Here, the load cell unit 30 used in the weight sorter 1 is loaded with the load of the sorting object 5 and the load of the weighing conveyor 21 (tare). In general, the weight of the weighing conveyor 21 is larger than the weight of the sorting object 5. Therefore, a method of increasing the voltage supplied from the power supply 46 is conceivable as a method for improving the weighing accuracy of the selection object 5 by improving the S / N ratio of the load signal output from the bridge circuit 40.

  In addition, the temperature sensing resistor 45 generates heat when the supply voltage increases and the amount of power consumed by the temperature sensing resistor 45 increases. As a result, the temperature sensing resistor 45 itself generates heat. It becomes difficult to accurately measure the temperature of the distortion body 31.

  However, when a temperature sensitivity compensation type strain gauge is used as in this embodiment, the precision resistors 46a and 46b, the temperature sensitive resistor 45, and the bridge circuit 40 are in parallel to the power supply 46. It is connected. As a result, the supply voltage from the power source 46 is divided by the precision resistors 46 a and 46 b and the temperature sensitive resistor 45.

  Therefore, by adjusting the resistance values of the precision resistors 46a and 46b and the temperature sensitive resistor 45, the heat generation of the temperature sensitive resistor 45 can be reduced. As a result, it is possible to improve the weighing accuracy of the selection object 5 while suppressing the problem of heat generation of the temperature sensitive resistor 45 itself.

  The signal processing unit 50 executes signal processing based on a signal (for example, a load signal) from the bridge circuit 40. As shown in FIG. 5, the signal processing unit 50 mainly includes an amplification A / D conversion unit 51, a memory 53, and a CPU 54. These elements 51, 53, and 54 are each a signal processing board 50a. It is provided above.

  The amplification A / D converter 51 outputs the output voltage (voltage between the output terminals 43a and 43b) of the bridge circuit 40 detected as an analog signal and the voltage applied to the bridge circuit 40 (voltage between the input terminals 44a and 44b). Convert to digital signal. The converted digital signal is input to the CPU 54.

  The memory 53 is a storage unit configured by, for example, a nonvolatile memory, and stores programs, variables, and the like. Further, the CPU 54 executes control according to a program stored in the memory 53. Therefore, the CPU 54 can execute the conversion process by the amplification A / D conversion unit 51, the zero point compensation calculation process, the weight calculation process, and the like at a predetermined timing according to this program.

  Here, as shown in FIG. 5, the CPU 54 of the signal processing unit 50 mainly includes a zero compensation operation function (corresponding to the zero compensation operation unit 52) and a weight computation processing function (based on a program stored in the memory 53). The weight calculation unit 55 corresponds).

  The zero compensation unit 52 of the CPU 54 calculates the temperature of the strain generating body 31 based on the detection result by the temperature sensing resistor 45, and calculates the zero point change amount of the bridge circuit 40 after rough correction based on this temperature. Further, the zero compensation unit 52 finely corrects the output of the bridge circuit 40 subjected to the coarse correction based on the amount of change.

  The weight calculation unit 55 of the CPU 54 calculates the weight of the selection target object 5 based on the output of the bridge circuit 40 that has been precisely corrected. The sorting device 60 executes the sorting process based on the calculation result of the weight calculation unit 55.

  FIG. 6 is a diagram illustrating the relationship between the zero value V of the bridge circuit 40 and the temperature T of the strain generating body 31. The vertical axis in FIG. 6 indicates that when the excitation voltage (supply voltage) from the power supply 46 is set to the voltage value Vs (in this embodiment, Vs = 12 V), the strain generating body 31 is separated from the selection object 5. The zero value V (unit: μV / 12V) when no load is received is shown. 6 represents the temperature T (unit: ° C.) of the strain body 31. The horizontal axis of FIG. Further, the curve C1 in FIG. 6 shows the relationship between the zero value V when the zero compensation element 42 is not connected and the temperature T of the strain generating body 31, and the curve C2 shows the zero value V when the zero compensation element 42 is connected. The relationship with the temperature T of the strain body 31 is shown respectively.

  Here, it is known that the zero compensation element 42 can effectively compensate for the zero when the variation of the zero value due to the temperature change is linear (primary). However, as shown in FIG. 6, when the temperature characteristic of the zero value V in a predetermined temperature range Ra (for example, −10 ≦ T ≦ 40) has a constant curvature, the zero compensation by the zero compensation element 42 is not performed. The fluctuation range ΔV of the zero value is merely improved from ΔV1 to ΔV2, and cannot be made smaller than ΔV2. As a result, it is difficult to further improve the reproducibility of the measurement processing by the signal processing unit 50 only by using the zero compensation element 42.

  Therefore, in the present embodiment, the zero point change amount of the bridge circuit 40 roughly corrected by the zero point compensation element 42 is calculated by a second-order or higher order approximation, and the output of the bridge circuit 40 is finely corrected based on the change amount. To do. Specifically, (1) a parameter necessary for fine correction of the output from the bridge circuit 40 is calculated, and (2) the output of the bridge circuit 40 is finely corrected based on this parameter.

  First, parameter calculation will be described by taking the case of quadratic approximation as an example. Prior to attaching the load cell unit 30 to the weighing device 20, the temperature characteristic of the zero point change amount after the coarse correction is obtained. That is, with respect to the bridge circuit 40 using the zero compensation element 42, the temperature T of the strain generating body 31 is changed to temperatures T1, T2, and T3 (all within the temperature range Ra) while being in a no-load state, and each temperature T1, The outputs V1, V2, and V3 of the bridge circuit 40 at T2 and T3 are obtained.

  Subsequently, by substituting (T, V) = (T1, V1), (T2, V2), (T3, V3) into Equation 1 and solving these three simultaneous equations, parameters a, b, c Is calculated. Then, the obtained parameters a, b, and c are stored in the memory 53.

    V = a * T * T + b * T + c ... (Formula 1)

  Note that the parameters a to c have different values depending on individual differences of the strain gauges 41 and the adhesion state of the strain gauges 41 even between the load cell units 30 using the same type of strain gauges 41. That is, the parameters a to c need to be obtained for each load cell unit 30.

  Next, precise correction using parameters will be described. First, prior to weighing the weight of the sorting object 5, the output of the bridge circuit 40 and the temperature of the strain generating body 31 when the sorting object 5 is not loaded on the weighing conveyor 21 are obtained. Subsequently, the zero point at the obtained temperature is calculated. The calculated zero point is stored in the memory 53.

  Subsequently, when possible, the zero compensation unit 52 performs zero measurement every time when weighing is performed, after a predetermined time has elapsed since the time when the previous zero was stored, or when a predetermined number of measurements have been performed from the time when the previous zero was stored. Find the amount of change. In other words, the temperature T of the strain generating body 31 is detected based on the temperature sensitive resistance 45, and the zero value at the present time is obtained by substituting Equation 1 for the detection result. Then, the zero compensation unit 52 compensates and finely corrects the output of the bridge circuit 40 based on the amount of change of the zero from the time when the previous zero was stored.

<1.3. Advantages of Load Cell Unit of First Embodiment>
As described above, in the first embodiment, software-like zero compensation can be further applied to the output of the bridge circuit 40 that has been zero-compensated in a circuit (hardware manner) by the zero compensation element 42. . Further, the temperature T of the strain generating body 31 used for this software-like zero compensation is accurately and stably detected by the temperature sensitive resistor 45 in close contact with the thick portion 34.

  Thereby, the output accuracy of the bridge circuit 40 can be further improved. Therefore, the reproducibility of the weighing process by the bridge circuit 40 can be further improved. As a result, even when the weighing interval is short and the zero point correction cannot be performed for each weighing as in the case of weighing the sorting object 5 to be conveyed, the weight of the sorting object 5 is weighed with good reproducibility. can do.

  In the first embodiment, the weight of each sorting object 5 can be calculated based on the output of the bridge circuit 40 controlled with high accuracy, and the minimum scale for the rated capacity of the load cell unit 30 is set to be smaller. It becomes possible. Therefore, the load of the weighing conveyor 21 (tare), which is heavier than the sorting object 5, is applied to the load cell unit 30, and it is necessary to set the rated capacity of the load cell larger than that of the sorting object 5. However, the measurement accuracy (measurement resolution) of the selection object 5 can be improved.

  Further, in the first embodiment, a temperature sensitivity compensation type strain gauge is used, and the precision resistors 46 a and 46 b, the temperature sensitive resistor 45, and the bridge circuit 40 are parallel to the power supply 46. It is connected to the. Therefore, the heat generation of the temperature sensitive resistor 45 can be suppressed by adjusting the resistance values of the precision resistors 46 a and 46 b and the temperature sensitive resistor 45. As a result, accurate temperature detection becomes possible, and it is possible to shorten the time until the voltage across the temperature sensing resistor 45 (that is, the output from the temperature sensing resistor 45) stabilizes when the power is turned on. The measurement accuracy of the object 5 can be improved.

<2. Second Embodiment>
Next, a second embodiment of the present invention will be described. Here, the weight sorter 100 of the second embodiment is the same as the weight sorter 1 of the first embodiment except that the configuration of the bridge circuit 140 in the load cell unit 130 is different. Therefore, in the following, this difference will be mainly described.

  In addition, in the following description, the same code | symbol is attached | subjected about the component similar to the component in the weight sorter 1 of 1st Embodiment. Since the components with the same reference numerals have already been described in the first embodiment, description thereof will be omitted in the present embodiment.

<2.1. Configuration of load cell unit>
2 and 3 are a front view and a plan view showing an example of the hardware configuration of the load cell unit 130, respectively. FIG. 4 is a surface view showing an example of the configuration of the strain gauge 141. Further, FIG. 7 is a block circuit diagram of the load cell unit 130. As shown in FIGS. 2 to 4 and 7, the load cell unit 130 mainly includes a strain generating body 31, a plurality of strain gauges 141 (141 a to 141 d), a zero compensation element 42, and a temperature sensitive resistor 145. And a signal processing unit 50.

  As shown in FIGS. 2 and 3, strain gauges 141 (141 a to 141 d) are arranged on the outer peripheral surface 35 of the corresponding strain generating portion among the plurality of strain generating portions 33. That is, the first and second strain gauges 141a and 141b are provided on the first surface 35a of the first and second strain generating portions 33a and 33b, and the third and fourth strain generating portions 33c and 33d are provided on the third and fourth strain surfaces. Strain gauges 141c and 141d are bonded to each other.

  Here, as shown in FIG. 4, each strain gauge 141 (141 a to 141 d) places a resistor 140 b such as a photo-etched resistance foil on a thin electric insulator (for example, polyimide resin) base member 140 a. It is formed in a meandering shape. Accordingly, when strain is generated in each strain generating portion 33 (33a to 33d) and the strain is transmitted to the resistor 140b via the base member 140a, a resistance change corresponding to the strain amount occurs in the resistor 140b. .

  In the present embodiment, a self-temperature compensated strain gauge is used as the strain gauge 141. That is, the resistor 140b of the strain gauge 141 is made of a metal such as a copper nickel (Cu—Ni) alloy, for example. In addition, the resistor 140b of the strain gauge 141 is heat-treated so that its resistance temperature coefficient becomes a predetermined negative value corresponding to the linear expansion coefficient of the strain generating body 31.

  Therefore, when the temperature of the strain generating body 31 rises, the strain generating body 31 expands, the gauge length of the strain gauge 41 bonded to the strain generating body 31 is extended, and the resistance temperature coefficient of the resistor 40b is decreased. Therefore, the resistance value of the strain gauge 141 adhered to the strain generating body 31 is substantially the same within a predetermined temperature range. Thus, the self-temperature compensation type strain gauge 141 can compensate for the thermal expansion of the strain generating body 31 that expands and contracts due to a temperature change, and can suppress the zero point change of the bridge circuit 140.

  The bridge circuit 140 outputs the weight of the selection object 5 that is an object to be weighed as a load signal. As shown in FIG. 7, the bridge circuit 140 is mainly formed by first to fourth strain gauges 141a to 141d. That is, (1) the first strain gauge 141a is between the plus input terminal 144a and the plus output terminal 143a, and (2) the second strain gauge 141b is between the plus input terminal 144a and the minus output terminal 143b. (3) A third strain gauge 141c is disposed between the plus output terminal 143a and the minus input terminal 144b, and (4) a fourth strain gauge 141d is disposed between the minus output terminal 143b and the minus input terminal 144b. These strain gauges 141 (141a to 141d) constitute a full bridge. Then, the voltage between the output terminals 143a and 143b is detected as the output of the bridge circuit 140.

  The zero point compensation element 42 (42a to 42d) is formed by winding a resistor (copper wire or nickel wire) having a large resistance temperature coefficient, and is disposed on any one side of the bridge circuit 140. That is, any one of the zero compensation elements 42a to 42d shown in FIG. 7 is used and connected in series with the corresponding strain gauge 141 (141a, 141c, 141b, 141d). The first zero compensation element 42a is provided on the first thick part 34a on the first surface 35a side (see FIG. 2), and the second zero compensation element 42b is provided on the second thick part 34b on the second surface 35b side (illustrated). (Omitted), the third zero compensation element 42c is disposed on the first thick part 34a on the first surface 35a side (not shown), and the fourth zero compensation element 42d is disposed on the second thick part 34b on the second surface 35b side (see FIG. 2), one of which is closely attached. The zero compensation element 42 is not limited to a wound resistor, and may be, for example, a chip resistor having the same resistance value and resistance temperature coefficient as the resistor.

  Here, as described above, a self-temperature compensated strain gauge is used for the strain gauge 141. As a result, the resistance value of each strain gauge 141 when no load is applied to the strain generating body 31 (the unloaded state) becomes substantially the same value even if the ambient temperature of the strain generating body 31 changes. . Therefore, when the physical properties (for example, the resistance value and resistance temperature coefficient of the resistor 140b) of each strain gauge 141 are substantially the same, the zero point of the bridge circuit 140 is substantially the same even when the ambient temperature changes.

  However, when each strain gauge 141 has an individual difference, or when the adhesion state of each strain gauge 141 is different, the zero point of the bridge circuit 140 fluctuates according to a change in the ambient temperature. The result will be affected by the ambient temperature.

  On the other hand, in the present embodiment, the zero point compensation element 42 corresponding to the amount of change of the zero point due to the temperature change is disposed on one side of the bridge circuit 140 and is connected in series with the corresponding strain gauge 141. Therefore, the zero point that fluctuates due to a temperature change is compensated according to the temperature of the strain generating body 31, and the output of the bridge circuit 140 is roughly corrected so as to be within a certain range.

  Further, the thin-walled strain-generating portion 33 has a larger thermal resistance than the other portions of the strain-generating body 31, and the portion sandwiched between the adjacent strain-generating portions 33 (thick wall portion 34) Not easily affected. Therefore, the zero compensation element 42 provided in the thick portion 34 can compensate the zero of the bridge circuit 140 while accurately reflecting the temperature of the strain generating body 31.

  The arrangement of the zero compensation element 42 is not limited to any one side of the bridge circuit 40. For example, the zero compensation element 42 may be arranged on one side of the bridge circuit 140 and on a diagonal side when viewed from the one side. That is, as the zero compensation element 42, the first and fourth zero compensation elements 42a and 42d may be used, or the second and third zero compensation elements 42b and 42c may be used. That is, the zero compensation element 42 may be disposed on at least one side of the bridge circuit 140.

  Similar to the zero compensation element 42, the temperature sensitive resistor 145 (145a, 145b) is a resistor having a large resistance temperature coefficient. For example, a nickel (Ni) based material is used for the first temperature sensitive resistor 145a, and a copper (Cu) based material is used for the second temperature sensitive resistor 145b.

  As shown in FIG. 2, the first temperature sensing resistor 145a is on the first thick part 34a on the first surface 35a side, and the second temperature sensing resistor 145b is on the second thick part 34b on the second surface 35b side, respectively. It is attached closely. As shown in FIG. 7, the first temperature sensing resistor 145a is a terminal 144a on the plus input side 147 of the bridge circuit 140, and the second temperature sensing resistor 145b is a terminal 144b on the minus input side 148 of the bridge circuit 140, respectively. Electrically connected.

  Here, when the stress acting on the strain generating body 31 is σ and the longitudinal elastic modulus of the strain generating body 31 is E, the strain amount ε 1 generated in the strain generating section 33 can be obtained by Equation 2.

    ε1 = σ / E (Equation 2)

  When the strain amount of the strain gauge 141 due to stress is ε2, the resistance value of the strain gauge 141 is R, and the change amount of the resistance value of the strain gauge 141 due to stress is ΔR, the gauge factor of the strain gauge 141 is Can be expressed as:

    K = ΔR / (R · ε2) (Equation 3)

  Then, the strain gauge 141 is bonded to the outer peripheral surface 35 (35a, 35b) of the strain generating portion 33, and if the strain amount 33 of the strain generating portion 33 and the strain amount ε2 of the strain gauge 141 are substantially the same, the gauge 141 is provided. The change amount ΔR of the resistance value can be expressed as shown in Equation 4.

    ΔR = R · σ · K / E (Equation 4)

  The longitudinal elastic modulus E usually decreases as the ambient temperature increases. That is, even if the load applied to the strain generating body 31 is the same, the strain generated in each strain generating portion 33 increases as the temperature rises.

  Therefore, when the longitudinal elastic modulus E decreases as the ambient temperature of the strain generating body 31 increases, the change amount ΔR of the resistance value of the gauge 141 increases as can be seen from Equation 4.

  On the other hand, the resistance value of the temperature-sensitive resistor 145 increases as the ambient temperature increases, and the voltage applied to the bridge circuit 140 (voltage between the input terminals 144a and 144b) decreases.

  As described above, when the temperature-sensitive resistor 145 is connected to the plus and minus input sides 147 and 148 of the bridge circuit 140, the applied voltage of the bridge circuit 140 is adjusted according to the ambient temperature of the strain generating body 31. Therefore, the output change of the bridge circuit 140 due to the temperature change is suppressed, and the output sensitivity of the bridge circuit 40 is electrically corrected.

  Further, the temperature coefficient of resistance of the temperature sensitive resistor 145 used in the present embodiment is larger than that of each strain gauge 141. Thereby, when compared with the temperature sensitive resistor 145, each strain gauge 141 can be regarded as a precision resistor. Therefore, based on the voltage applied to the bridge circuit 140 (voltage between the input terminals 144a and 144b), the excitation voltage of the power supply 46, and the resistance value-temperature characteristic of the temperature sensitive resistor 145, the temperature of the strain generating body 31 is Calculated. That is, in the present embodiment, the temperature of the strain generating body 31 is detected based on the change in the resistance value of the temperature sensitive resistor 145.

  As described above, the temperature-sensitive resistor 145 is used not only as a sensitivity correction unit that compensates the output of the bridge circuit 140 due to a temperature change but also as a temperature sensor of the strain generating body 31. Therefore, the number of parts of the load cell unit 130 can be reduced, and the manufacturing cost of the load cell unit 130 can be reduced.

  Further, the temperature sensitive resistor 145 is in close contact with the thick portion 34 that is not easily affected by the surrounding heat. Thereby, the resistance value of the temperature-sensitive resistor 145 varies while accurately reflecting the temperature of the strain generating body 31. Therefore, the temperature of the strain generating body 31 is accurately and stably detected based on the change in the resistance value of the temperature sensitive resistor 145.

  The signal processing unit 50 performs signal processing based on a signal (for example, a load signal) from the bridge circuit 140. As shown in FIG. 7, the signal processing unit 50 mainly includes an amplification A / D conversion unit 51, a memory 53, and a CPU 54. These elements 51, 53, and 54 are each a signal processing board 50a. It is provided above.

  The amplification A / D converter 51 outputs the output voltage of the bridge circuit 140 (voltage between the output terminals 143a and 143b) detected as an analog signal and the voltage applied to the bridge circuit 140 (voltage between the input terminals 144a and 144b). Convert to digital signal. The converted digital signal is input to the CPU 54.

  The memory 53 is a storage unit configured by, for example, a nonvolatile memory, and stores programs, variables, and the like. Further, the CPU 54 executes control according to a program stored in the memory 53. Therefore, the CPU 54 can execute the conversion process by the amplification A / D conversion unit 51, the zero point compensation calculation process, the weight calculation process, and the like at a predetermined timing according to this program.

  Here, as shown in FIG. 7, the CPU 54 of the signal processing unit 50 is mainly based on a program stored in the memory 53, and a zero point compensation calculation function (zero point compensation calculation unit 52 corresponds) and a weight calculation processing function ( The weight calculation unit 55 corresponds).

  The zero compensation unit 52 of the CPU 54 calculates the temperature of the strain generating body 31 based on the detection result by the temperature sensing resistor 145, and calculates the zero point change amount of the bridge circuit 140 after the coarse correction based on this temperature. The zero compensation unit 52 finely corrects the coarsely corrected output of the bridge circuit 140 based on the amount of change.

  That is, in the present embodiment, the zero point change amount of the bridge circuit 140 roughly corrected by the zero point compensation element 42 is calculated by a second-order or higher order approximation, and the output of the bridge circuit 140 is finely corrected based on the change amount. To do. For example, (1) parameters a to c (see Equation 1) required for fine correction of the output from the bridge circuit 140 are calculated, and (2) the output of the bridge circuit 140 is finely corrected based on the parameters ac. To do.

  The weight calculation unit 55 of the CPU 54 calculates the weight of the selection target object 5 based on the output of the bridge circuit 140 that has been finely corrected. The sorting device 60 executes the sorting process based on the calculation result of the weight calculation unit 55.

<2.2. Advantages of Load Cell Unit of Second Embodiment>
As described above, in the second embodiment, as in the first embodiment, the output of the bridge circuit 140 that has been zero-compensated circuit-wise (in hardware) by the zero-compensation element 42 is further reduced. Software zero compensation can be performed. Further, the temperature T of the strain generating body 31 used for the software zero compensation is detected accurately and stably by the temperature sensitive resistor 145 in close contact with the thick portion 34.

  Thereby, the output accuracy of the bridge circuit 140 can be further improved. Therefore, the reproducibility of the weighing process by the bridge circuit 140 can be further improved. As a result, even when the weighing interval is short and the zero point correction cannot be performed for each weighing as in the case of weighing the sorting object 5 to be conveyed, the weight of the sorting object 5 is weighed with good reproducibility. can do.

  In the second embodiment, as in the first embodiment, the weight of each sorting object 5 can be calculated based on the output of the bridge circuit 140 controlled with high accuracy, and the rating of the load cell unit 30 can be calculated. It is possible to set the minimum scale for the capacity to be smaller. Therefore, the load of the weighing conveyor 21 (tare), which is heavier than the sorting object 5, is applied to the load cell unit 30, and it is necessary to set the rated capacity of the load cell larger than that of the sorting object 5. However, the measurement accuracy (measurement resolution) of the selection object 5 can be improved.

  Furthermore, in the second embodiment, the temperature sensitive resistor 145 (145a, 145b) is connected in series to the plus and minus input sides 147, 148 of the bridge circuit 140. Accordingly, the temperature-sensitive resistor 145 not only serves as a sensitivity compensation unit that compensates for the change in the longitudinal elastic modulus of the strain generating body due to temperature fluctuations and corrects the output of the bridge circuit 140, but also as a temperature sensor for the strain generating body 31 Can be used. Therefore, the number of parts of the load cell unit 130 can be reduced, and the manufacturing cost of the load cell unit can be reduced.

<3. Third Embodiment>
Next, a third embodiment of the present invention will be described. Here, the weight sorter 200 of the third embodiment is the same as the weight sorter 1 of the first embodiment except that the configuration of the bridge circuit 240 in the load cell unit 230 is different. Therefore, in the following, this difference will be mainly described.

  In addition, in the following description, the same code | symbol is attached | subjected about the component similar to the component in the weight sorter 1 of 1st Embodiment. Since the components with the same reference numerals have already been described in the first embodiment, description thereof will be omitted in the present embodiment.

<3.1. Configuration of load cell unit>
2 and 3 are a front view and a plan view showing an example of the hardware configuration of the load cell unit 230, respectively. FIG. 4 is a surface view showing an example of the configuration of the strain gauge 241. Further, FIG. 8 is a block circuit diagram of the load cell unit 230. As shown in FIGS. 2 to 4 and 8, the load cell unit 230 mainly includes a strain generating body 31, a plurality of strain gauges 241 (241 a to 241 b), a zero compensation element 242, a temperature sensitive resistor 45, and the like. And a signal processing unit 50.

  As shown in FIGS. 2 and 3, strain gauges 241 (241 a to 241 b) are arranged on the outer peripheral surface 35 of the corresponding strain generating portion among the plurality of strain generating portions 33. That is, the first and second strain gauges 241a and 241b are bonded to the first surfaces 35a of the first and second strain generating portions 33a and 33b.

  Note that the arrangement of the strain gauges 241 is not limited to this. For example, the second and first strain gauges 241b and 241a may be bonded to the second surfaces 35b of the third and fourth strain generating portions 33c and 33d. That is, each of the first and second strain gauges 241a and 241b has a strain generating portion provided on one of the first surface 35a (upper surface) and the second surface 35b (lower surface). It suffices if they are arranged.

  Here, as each strain gauge 241 (241a, 241b), a temperature sensitivity compensation type similar to that of the strain gauge 41 of the first embodiment is used. Therefore, the strain gauge 241 compensates for the thermal expansion of the strain generating body 31 that expands and contracts due to a temperature change, suppresses the zero point change of the bridge circuit 240, and compensates the amount of change in the longitudinal elastic modulus due to the temperature change. Thus, the output change of the bridge circuit 240 can be suppressed.

  The bridge circuit 240 outputs the weight of the selection object 5 that is an object to be weighed as a load signal. As shown in FIG. 8, the bridge circuit 240 is mainly formed by first and second strain gauges 241a and 241b and first and second fixed resistors 246c and 246d. That is, (1) the first strain gauge 241a is between the plus input terminal 44a and the plus output terminal 243a, and (2) the second strain gauge 241b is between the plus output terminal 243a and the minus input terminal 44b. (3) A first fixed resistor 246c is arranged between the plus input terminal 44a and the minus output terminal 243b, and (4) a second fixed resistor 246d is arranged between the minus output terminal 243b and the minus input terminal 44b. These strain gauges 241 (241a, 241b) constitute a half bridge. The voltage between the output terminals 243a and 243b is detected as the output of the bridge circuit 240.

  The zero point compensation element 242 (242a, 242b) is formed by winding a resistor (copper wire or nickel wire) having a large resistance temperature coefficient, like the zero point compensation element 42 of the first embodiment. As shown in FIG. 2, one of the first and second zero compensation resistors 242a is attached in close contact with the first thick portion 34a on the first surface 35a side. Further, as shown in FIG. 8, the zero compensation element 242 (242a, 242b) is connected in series with one of the strain gauges 241 (241a, 241b) arranged on one side of the bridge circuit 240. The zero compensation element 242 is not limited to a wound resistor, and may be, for example, a chip resistor having the same resistance value and resistance temperature coefficient as the resistor.

  Here, as described above, the strain gauge 241 is a temperature sensitivity compensation type strain gauge. However, when each strain gauge 241 has an individual difference, or when the adhesion state of each strain gauge 241 is different, the zero point of the bridge circuit 240 fluctuates according to a change in the ambient temperature. The result will be affected by the ambient temperature.

  On the other hand, in the present embodiment, the zero compensation element 242 corresponding to the amount of change of the zero due to the temperature change is arranged on one side of the bridge circuit 240 and is connected in series with the corresponding strain gauge 241. Therefore, the zero point that fluctuates due to the temperature change is compensated according to the temperature of the strain generating body 31, and the output of the bridge circuit 240 is roughly corrected so as to be within a certain range.

  Further, the thin-walled strain-generating portion 33 has a larger thermal resistance than the other portions of the strain-generating body 31, and the portion sandwiched between the adjacent strain-generating portions 33 (thick wall portion 34) Not easily affected. Therefore, the zero compensation element 242 provided in the thick portion 34 can compensate the zero of the bridge circuit 240 while accurately reflecting the temperature of the strain generating body 31.

  The temperature sensitive resistor 45 is a resistor having a large resistance temperature coefficient, like the zero compensation element 242. As shown in FIG. 2, the temperature-sensitive resistor 45 is attached in close contact with the first thick portion 34a on the first surface 35a side. Moreover, the temperature coefficient of resistance of the temperature sensitive resistor 45 is larger than that of the precision resistors 46a and 46b.

  Therefore, the temperature of the strain generating body 31 can be calculated based on the voltage applied to the temperature sensitive resistor 45 (voltage between the temperature measuring terminals 43c and 43d) and the resistance value-temperature characteristic of the temperature sensitive resistor 45. it can. Thus, in the present embodiment, the temperature of the strain generating body 31 is detected based on the change in the resistance value of the temperature sensing resistor 45, and the temperature sensing resistor 45 is used as a temperature sensor.

  Further, the temperature-sensitive resistor 45 is in close contact with the thick portion 34 that is not easily affected by the surrounding heat, and the resistance value of the temperature-sensitive resistor 45 varies while accurately reflecting the temperature of the strain-generating body 31. . Therefore, the temperature of the strain generating body 31 is accurately and stably detected based on the change in the resistance value of the temperature sensitive resistor 45.

  Here, the bridge circuit 240 of this embodiment and the bridge circuit formed by the strain gauge 141 used in the second embodiment instead of the strain gauge 241 will be compared and examined. As described above, the strain gauge 141 is a self-compensating strain gauge, and it is necessary to connect the temperature-sensitive resistor 145 (145a, 145b) for sensitivity correction to the input side of the bridge circuit (FIG. 7). reference).

  However, the resistance temperature characteristic of the strain gauge 141 is about 3 (ppm / ° C.), the resistance temperature coefficient of the fixed resistors 246c and 246d is 25 (ppm / ° C.), and the relative resistance temperature coefficient is about 5 (ppm / ° C.). It is. Therefore, the voltage applied to the bridge circuit (that is, the voltage between the input terminals) is affected not only by the resistance value fluctuation of the temperature sensitive resistor 145 due to the temperature change but also by the resistance value fluctuation of the fixed resistors 246c and 246d. It will be.

  Therefore, in the half-bridge type bridge circuit formed by the two self-temperature compensation type strain gauges 141 and the fixed resistors 246c and 246d, the output sensitivity is improved even if the temperature-sensitive resistor 145 connected in series is used. It may not be possible to correct.

  On the other hand, in the present embodiment, a temperature sensitivity compensation type strain gauge 241 is used as a strain gauge, and it is not necessary to provide a temperature-sensitive resistor for sensitivity correction on the input sides 47 and 48 of the bridge circuit 240. . Therefore, even when the half-bridge type bridge circuit 240 is formed, the output sensitivity can be corrected favorably.

  In the present embodiment, the precision resistors 46 a and 46 b, the temperature sensitive resistor 45, and the bridge circuit 240 are connected in parallel to the power supply 46. As a result, the supply voltage from the power source 46 is divided by the precision resistors 46 a and 46 b and the temperature sensitive resistor 45. Therefore, the heat generation of the temperature sensitive resistor 45 can be suppressed by adjusting the resistance values of the precision resistors 46 a and 46 b and the temperature sensitive resistor 45. As a result, accurate temperature detection becomes possible, and it is possible to shorten the time until the voltage across the temperature sensing resistor 45 (that is, the output from the temperature sensing resistor 45) stabilizes when the power is turned on. The measurement accuracy of the object 5 can be improved.

  The signal processing unit 50 executes signal processing based on a signal (for example, a load signal) from the bridge circuit 240. As shown in FIG. 8, the signal processing unit 50 mainly includes an amplification A / D conversion unit 51, a memory 53, and a CPU 54. These elements 51, 53, and 54 are each a signal processing board 50a. It is provided above.

  The memory 53 is a storage unit configured by, for example, a nonvolatile memory, and stores programs, variables, and the like. Further, the CPU 54 executes control according to a program stored in the memory 53. Therefore, the CPU 54 can execute the conversion process by the amplification A / D conversion unit 51, the zero point compensation calculation process, the weight calculation process, and the like at a predetermined timing according to this program.

  Here, as shown in FIG. 8, the CPU 54 of the signal processing unit 50 is mainly based on a program stored in the memory 53, mainly with a zero point compensation calculation function (corresponding to the zero point compensation calculation unit 52) and a weight calculation processing function ( The weight calculation unit 55 corresponds).

  The amplification A / D converter 51 outputs the output voltage of the bridge circuit 240 detected as an analog signal (the voltage between the output terminals 243a and 243b) and the voltage applied to the bridge circuit 240 (the voltage between the input terminals 44a and 44b). Convert to digital signal. The converted digital signal is input to the CPU 54.

  The zero compensation unit 52 of the CPU 54 calculates the temperature of the strain generating body 31 based on the detection result by the temperature sensing resistor 45, and calculates the zero point change amount of the bridge circuit 240 after rough correction based on this temperature. The zero compensation unit 52 finely corrects the coarsely corrected output of the bridge circuit 240 based on the amount of change.

  That is, in the present embodiment, the zero point change amount of the bridge circuit 240 roughly corrected by the zero point compensation element 242 is calculated by a second-order or higher order approximation, and the output of the bridge circuit 240 is finely corrected based on the change amount. To do. For example, (1) a parameter necessary for fine correction of the output from the bridge circuit 240 is calculated, and (2) the output of the bridge circuit 240 is finely corrected based on this parameter.

  The weight calculation unit 55 of the CPU 54 calculates the weight of the selection target object 5 based on the output of the bridge circuit 240 that has been precisely corrected. The sorting device 60 executes the sorting process based on the calculation result of the weight calculation unit 55.

<3.2. Advantages of Load Cell Unit of Third Embodiment>
As described above, in the present embodiment, as in the first embodiment, the output of the bridge circuit 240 that has been zero-compensated circuit-wise (hardware-like) by the zero-point compensating element 242 is further software-like. Zero compensation can be performed. Further, the temperature T of the strain generating body 31 used for this software-like zero compensation is accurately and stably detected by the temperature sensitive resistor 45 in close contact with the thick portion 34.

  Thereby, the output accuracy of the bridge circuit 240 can be further improved. Therefore, the reproducibility of the weighing process by the bridge circuit 240 can be further improved. As a result, even if the weighing interval is short and zero point correction cannot be performed for each weighing as in the case of weighing the sorting object 5 conveyed by the weighing conveyor 21, the weight of the sorting object 5 is reduced. Can be measured with good reproducibility.

  In the present embodiment, the weight of each sorting object 5 can be calculated based on the output of the bridge circuit 240 controlled with high accuracy, and the minimum scale for the rated capacity of the load cell unit 230 can be set smaller. It becomes possible. For this reason, the load of the weighing conveyor 21 (tare), which is heavier than the sorting object 5, is applied to the load cell unit 230, and it is necessary to set the rated capacity of the load cell larger than that of the sorting object 5. However, the measurement accuracy (measurement resolution) of the selection object 5 can be improved.

  Furthermore, in this embodiment, a temperature sensitivity compensation type strain gauge is used, and the precision resistors 46a and 46b, the temperature sensitive resistor 45 and the bridge circuit 240 are connected in parallel to the power supply 46. ing. Therefore, the heat generation of the temperature sensitive resistor 45 can be suppressed by adjusting the resistance values of the precision resistors 46 a and 46 b and the temperature sensitive resistor 45. As a result, accurate temperature detection becomes possible, and it is possible to shorten the time until the voltage across the temperature sensing resistor 45 (that is, the output from the temperature sensing resistor 45) stabilizes when the power is turned on. The measurement accuracy of the object 5 can be improved.

<4. Modification>
Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications can be made.

  (1) In the second embodiment, the temperature-sensitive resistor 145 (145a, 145b) has been described as being connected in series to both the positive and negative input sides 147, 148 of the bridge circuit 140. It is not limited to.

  For example, the temperature-sensitive resistor 145 may be connected only to the plus input side 147 of the bridge circuit 140 or may be connected only to the minus input side 148 of the bridge circuit 140. Thereby, the number of parts can be reduced and the manufacturing cost of the load cell unit 30 can be reduced. As described above, the temperature-sensitive resistor 145 only needs to be connected in series to at least one of the plus and minus input sides 147 and 148 of the bridge circuit 140.

  (2) In the first to third embodiments, the load cell units 30, 130, and 230 have been described as being used for the weighing process in the weight sorters 1, 100, and 200. The application target of is not limited to this. Any device that uses any one of these load cell units 30, 130, and 230 may be used. For example, an electronic balance may be used.

  FIG. 9 is a front view showing another example of an apparatus (electronic balance 300) to which the load cell unit 30 (130, 230) is applied. The electronic balance 300 is a device that measures the weight of the weighing object 305 in a stationary state. As shown in FIG. 9, the electronic balance 300 mainly includes a load cell unit 30 (130, 230) and an upper plate part 310. In the following, the case where the load cell unit 30 of the first embodiment is used will be mainly described.

  The upper plate part 310 is used as a loading part for loading the weighing object 305. As shown in FIG. 9, the upper plate 310 is supported on the free end 31 b side of the strain body 31 via an attachment member 325. Further, the fixed end 31 a of the strain body 31 is attached to a fixed portion 321 in the main body 320. Therefore, the strain body 31 is responsible for the load in the load direction AR3 (substantially Z-axis direction). Then, the signal processing unit 50 calculates the weight of the weighing object 305 based on the load signal output from the bridge circuit 40 (see FIG. 5).

  Thus, also in the electronic balance 300, the signal processing unit 50 can calculate the weight of the measurement object 305 with high accuracy. Thereby, it is possible to set the minimum scale for the rated capacity of the load cell unit even smaller. Therefore, the weighing resolution of the weighing object can be further improved, and the weighing mode with the same rated capacity and the smaller minimum scale can be mounted on the electronic balance 300.

  For example, an electronic scale that has implemented a weighing mode with a maximum weighing of 600 grams and a minimum graduation of 0.2 grams, and a weighing mode with a maximum weighing of 300 grams and a minimum graduation of 0.1 grams, It is possible to implement a weighing mode in which the maximum weighing is 300 grams and the minimum scale is 0.05 grams.

  In addition, since the measurement resolution of the measurement object 305 can be further improved, the measurement object 305 can be accurately measured even if the voltage value applied from the power supply 46 is set to be small. As a result, it is possible to provide the electronic balance 300 with high weighing accuracy even in the case of dry battery driving.

  In order to drive the electronic balance 300 with a battery, it is necessary to suppress the amount of power consumed by the electronic balance 300. For example, in the case of the load cell unit 130 of the second embodiment, the temperature sensitive resistor 145 and the bridge circuit 40 are connected in series to the power supply 46 so that the resistance value of the bridge circuit 40 is 1 kΩ or more. A temperature sensitive resistor 145 and a strain gauge 141 are selected. However, as described above, a Cu-based or Ni-based material is used for the temperature-sensitive resistor 145, and it may be difficult to set the resistance value of the temperature-sensitive resistor 145 even larger.

  On the other hand, in the case of the load cell units 30 and 230 of the first and third embodiments, the precision resistors 46a and 46b, the temperature sensitive resistor 45, and the bridge circuits 40 and 240 are parallel to the power supply 46. It is connected to the. Thereby, the electric energy consumed by the electronic balance 300 can be suppressed regardless of the value of the temperature sensitive resistance 45. Therefore, the load cell units 30 and 230 in which the temperature sensitive resistors are not connected in series to the input sides 47 and 48 of the bridge circuit can easily constitute the battery-driven electronic balance 300.

It is a front view which shows an example of the apparatus to which the load cell unit in the 1st thru | or 3rd Embodiment of this invention is applied. It is a front view which shows an example of the hardware constitutions of the load cell unit in the 1st thru | or 3rd Embodiment of this invention. It is a top view which shows an example of the hardware constitutions of the load cell unit in 1st thru | or 3rd Embodiment. It is a surface view which shows an example of a structure of the strain gauge in 1st thru | or 3rd Embodiment. It is a block circuit diagram of the load cell unit in a 1st embodiment. It is a figure which shows the relationship between the zero of a bridge circuit, and the temperature of a strain body. It is a block circuit diagram of the load cell unit in 2nd Embodiment. It is a block circuit diagram of the load cell unit in 3rd Embodiment. It is a front view which shows the other example of the apparatus to which the load cell unit in embodiment of this invention is applied.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,100,200 Weight sorter 5 Sorting object 10 Take-in device 20 Weighing device 21 Weighing conveyor 30, 130, 230 Load cell unit 31 Straining body 31a Fixed end 31b Free end 32 Through hole 32a Inner wall surface 33 Straining portion 34 Thick part 35 Outer peripheral surface 35a First surface 35b Second surface 40, 140, 240 Bridge circuit 40a Base member 40b Resistor 41, 141, 241 Strain gauge 42, 242 Zero compensation element 45, 145 Temperature sensitive resistor 50 Signal processing unit 50a Signal processing board 60 Sorting device 300 Electronic scale 305 Weighing object 310 Upper plate part AR2, AR3 Load direction

Claims (11)

A load cell unit that measures the weight of an object to be weighed based on a load signal,
(a) a metal block provided with a through hole, a strain generating body having a plurality of thin strain generating portions sandwiched between an inner wall surface of the through hole and an outer peripheral surface of the metal block;
(b) a strain gauge disposed in a corresponding strain generating portion among the plurality of strain generating portions;
(c) a bridge circuit formed by the strain gauge;
(d) It is connected in series with a strain gauge arranged on one side of the bridge circuit, and compensates for the zero point of the bridge circuit that fluctuates due to a temperature change according to the temperature of the strain generating body. A zero compensation element for coarse correction of the output;
(e) a temperature sensor that detects the temperature of the strain body;
(f) a signal processing unit that calculates a change amount of the zero point after the coarse correction based on a detection result by the temperature sensor and finely corrects the output of the coarsely corrected bridge circuit based on the change amount; ,
With
The strain body constitutes a Roverval mechanism via a plurality of the strain portions provided between a free end to which a load is applied and a fixed end,
The load cell unit, wherein the zero compensation element and the temperature sensor are provided in a portion sandwiched between adjacent strain generating portions. In the load cell unit according to claim 1,
The load cell unit, wherein the temperature sensor is a temperature sensitive resistor. The load cell unit according to claim 2,
The load cell unit, wherein the temperature sensitive resistor is connected in series to the input side of the bridge circuit. In the load cell unit according to claim 3,
The load cell unit according to claim 1, wherein the strain gauge is a self-temperature compensated strain gauge capable of compensating for thermal expansion of the strain generating body due to temperature change. In the load cell unit according to claim 1 or 2,
The load cell unit according to claim 1, wherein the strain gauge is a temperature sensitivity compensation type strain gauge capable of compensating for a change amount of a longitudinal elastic modulus of a strain generating body caused by a temperature change. In the load cell unit according to any one of claims 1 to 5,
The first and second strain generating portions adjacent to each other among the strain generating portions are substantially perpendicular to the load direction received from the object to be weighed in the outer peripheral surface, and are responsible for the load from the object to be weighed. It is provided on one side,
Adjacent third and fourth strain generating portions are substantially perpendicular to the load direction, and are provided on the second surface side opposite to the first surface across the through hole,
The load cell unit, wherein the strain gauge is disposed in each of the first to fourth strain generating portions. The load cell unit according to claim 5,
The bridge circuit is formed by first and second strain gauges and first and second fixed resistors,
The first strain gauge is between the first input terminal and the first output terminal,
The second strain gauge is between a second input terminal and the first output terminal,
The first fixed resistor is between the first input terminal and the second output terminal,
The second fixed resistor is between the second input terminal and the second output terminal,
Each load cell unit is connected to each other. The load cell unit according to claim 7,
The first and second strain generating portions adjacent to each other among the strain generating portions are substantially perpendicular to the load direction received from the object to be weighed in the outer peripheral surface, and are responsible for the load from the object to be weighed. It is provided on one side,
Adjacent third and fourth strain generating portions are substantially perpendicular to the load direction, and are provided on the second surface side opposite to the first surface across the through hole,
The first and second strain gauges are arranged on either one of the first surface and the second surface in each of the strain-generating portions provided on the one surface. Load cell unit. The load cell unit according to any one of claims 1 to 8,
The load cell unit, wherein the signal processing unit calculates the amount of change of the zero by a second-order or higher-order approximation. A weight sorter,
A weighing device for weighing the object to be sorted,
A sorting device that sorts each sorting object based on the weighing result of the sorting object by the weighing device;
With
The weighing device is
The load cell unit according to any one of claims 1 to 9,
Supported by the load cell unit, a weighing conveyor for conveying each sorting object,
A weight sorter characterized by comprising: The load cell unit according to any one of claims 1 to 9,
Supported by the load cell, and an upper platen portion on which a weighing object is loaded;
An electronic balance comprising:

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