JP5339846B2 - Load cell and weight measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately correct the variation of a load signal by the force applied to a strain inducing unit due to atmospheric pressure difference, by detecting the atmospheric pressure difference between the inside and outside of a load cell body by a pressure difference detection means without using a special bellows and by correcting the load signal based on this atmospheric pressure difference. <P>SOLUTION: A part of or a whole strain inducing unit 5 formed on the load cell body 2 consisting of a metal elastic body is disposed in an air-tight chamber 3 which is a space closed to the atmosphere, a differential pressure sensor 12 is placed for detecting the difference between the pressure of the air-tight chamber 3 and the atmospheric pressure, and the load signal generated in accordance with the strain on the strain inducing unit 5 is corrected based on the pressure difference detected by the differential pressure sensor 12. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention corrects that a load signal fluctuates due to a force applied to the strain generating part due to a difference between the pressure and atmospheric pressure of the closed space when a part or all of the strain generating part is in the closed space. The present invention relates to a load cell and a weight measuring device.

  In general, a load cell is configured to connect a strain gauge affixed on a strain generating body that causes expansion / contraction strain by receiving a load to a bridge circuit and take out a voltage signal change proportional to the load.

In order to use this type of load cell in a weighing device or the like that is placed in an inferior environment where the temperature changes greatly, it is attempted to improve moisture resistance. For example, in the load cell 50 disclosed in Patent Document 1, as shown in FIG. 7, the load cell main body (elastic body) 51 has a thin strain-generating portion 52 at the upper portion thereof, and a closed space 53 is formed inside. And the gas flow is completely blocked. The reason for adopting such a blocking structure is that the outside air containing water vapor enters the closed space 53 and is strained by moisture on the strain gauges G _{1 to} G ₄ and wiring terminals attached to the strain-generating portion 52. This is to prevent a decrease in the insulation resistance of the gauge and corrosion. In the load cell 50, in order to make the closed space 53 airtight, the open hole in the bottom of the load cell main body 51 is closed by welding the metal disk 54, and the open hole in the side wall is supplied with power supply and load signal. The airtight terminal 55 for connecting the lead-out wiring is closed by welding.

  Also, pores (not shown) are provided in the side wall portion of the load cell main body 51, and dry nitrogen gas is injected from the pores to replace the air in the closed space 53. After the nitrogen gas is injected, the pores are welded. Measures are taken to fill the area with air and keep the outside air blocking. In this case, the atmospheric pressure in the closed space (airtight chamber) 53 formed inside the load cell main body 51 is maintained at a constant pressure P1 of about 1 atm.

  In the load cell manufactured in this way, if the atmospheric pressure is P2 at the time of installation, the strain generating part receives a force corresponding to the pressure difference of P1−P2, and according to the magnitude of this pressure difference (differential pressure). It is as if the initial load Wi has been increased or decreased. Since the atmospheric pressure changes during use of the load cell, the magnitude of the pressure difference always varies, and the initial load Wi also always varies according to the magnitude of the pressure difference. Furthermore, this pressure difference is also caused by the temperature of the gas in the airtight chamber changing when the air temperature changes even if the atmospheric pressure is constant, and the gas volume expanding or contracting. Further, the load cell receives a load, and the strain difference is pushed down to reduce the volume of the hermetic chamber, whereby a pressure difference is generated.

  In any case, if a pressure difference is generated, it appears in the load signal as a change in the apparent tare load, and an error is given to the load signal. Zero error due to tare load fluctuations can be resolved by adjusting the zero point immediately before use.However, when a load cell is used and weight measurement is performed while the object to be weighed stays on for a long time. Since zero adjustment is not possible, this zero point change causes a weight measurement error.

  Next, as another example, a load cell shown in FIG. 8 will be described. The load cell 60 includes a load cell main body (elastic body) 61 and a load receiving portion 62 that is formed to protrude from the central portion on the upper surface side of the load cell main body 61, and has a small spring constant in the direction in which the load load f acts vertically. A strain generating portion 66 is provided in an airtight chamber 65 formed by the bellows 63, the bottom lid 64, and the load cell main body 61. In the case of this example as well, as with the load cell shown in FIG. 7, it has a wiring outlet, an airtight terminal, a nitrogen gas injection hole, etc., but is not shown.

  In this example, when the load f is received from the load receiving portion 62, the strain generating portion 66 generates a bending strain. At this time, a force acts on the bellows 63 due to a pressure difference between the hermetic chamber 65 and the external atmospheric pressure, and the force is transmitted from the bellows 63 to the strain generating portion 66, causing an error in the load signal.

  As for the load cell structure in which the influence of the pressure difference as described above is excluded, there is one proposed in Patent Document 2. In the method disclosed in this document, bellows are provided above and below a load receiving portion that transmits a load to the strain generating portion, and when the air pressure in the hermetic chamber is greater than the atmospheric pressure, the upper bellows pushes up the load receiving portion. The lower bellows acts as a force that pushes down the load receiving portion, so that both forces cancel each other out so that the load receiving portion is not affected by the force due to the pressure difference.

JP 2005-140646 A JP-A-5-209796

  By the way, the bellows described in Patent Document 2 cannot be mounted on a load cell having a strain generating portion and a load receiving portion on the upper surface of an elastic body as shown in FIG. On the other hand, if it is the load receiving part of the load cell shown in FIG. 8, it is possible to mount the bellows. Therefore, for the load cell shown in FIG. 8, as shown in FIG. 9, the upper and lower lids a and b of the hermetic chamber have sufficient rigidity, and the bellows d and e are respectively provided between the load receiving portion c and the load cell. Assume that it is installed.

  In this case, the direction in which the load is applied to the load receiving portion c of the load cell is not always the vertical direction, but is inclined as indicated by a virtual line in the figure with the center point O of the strain generating portion as the rotation center. . Here, if the bellows d and e are not in a symmetrical position, the upper and lower bellows d and e are distorted into different shapes as indicated by the symbols d ′ and e ′ due to the inclination of the load receiving portion c, and the inside and outside of the hermetic chamber When a difference in atmospheric pressure occurs, different forces act on the load receiving portion c from the upper and lower bellows d and e. As a result, there is a problem that forces acting on the load receiving portion c from the upper and lower bellows d and e do not cancel each other and appear as load fluctuation errors.

  Further, in such a system using a bellows, a metal bellows must be used in order to maintain a breathable cutoff for a long period of time, and there is also a problem that an increase in cost is inevitable including a mounting cost. .

  The present invention has been made in view of the above-described problems, and detects a pressure difference between the inside and outside of the load cell body by a differential pressure detection means without using a special bellows, and corrects a load signal based on the pressure difference. Thus, it is an object of the present invention to provide a load cell and a weight measuring device capable of accurately correcting a load signal fluctuating due to a force applied to a strain generating part due to a pressure difference.

In order to achieve the above object, a load cell according to the first invention is provided.
Some or all of the strain generating portion formed on the load cell body is disposed in a closed space that is closed to the atmosphere Rutotomoni, a strain gauge for generating a load signal in response to the distortion of the strain generating portion, said closure A differential pressure detecting means for detecting a differential pressure between a pressure in the space and a pressure in the atmosphere; and a load signal correcting means for correcting a variation in the load signal based on the differential pressure detected by the differential pressure detecting means. In the load cell provided ,
A load signal output from the strain gauge and a differential pressure signal output from the differential pressure detection means are configured to detect each independently,
The load signal correcting means determines a function indicating a relationship of a tare load fluctuation amount in the load signal with respect to a change in the differential pressure signal when adjusting the load cell, and according to the input differential pressure signal when using the load cell. The load signal is corrected based on the tare load fluctuation amount calculated using the function .

In the first aspect of the invention, the load signal correcting means has the tare load fluctuation amount calculated from the relationship between the differential pressure detected by the differential pressure detecting means and the temperature of the enclosed space or the temperature of the atmosphere. Is preferably corrected ( second invention).

According to a third aspect of the present invention, there is provided a weight measuring device comprising the load cell according to the first aspect of the present invention and a control device connected to the load cell, wherein the control device comprises the differential pressure detecting means. The load signal correcting means corrects the variation of the load signal based on the differential pressure detected by the above.

Here, the load signal correction means corrects said difference detected by the pressure detecting means differential pressure, the load signal with the tare weight fluctuation amount is calculated from the relationship between temperature or temperature of the atmosphere of the closed space ( 4th invention) is preferable.

  According to the present invention, since it is configured to correct the variation of the load signal based on the differential pressure between the pressure in the enclosed space and the atmospheric pressure, the pressure difference between the inside and outside of the load cell body can be obtained without using a special bellows. As a result, it is possible to accurately correct the force applied to the strain generating portion and the fluctuation of the load signal. Therefore, the fluctuation of the load signal based on the pressure difference can be corrected by a simple and low-cost device.

  Next, specific embodiments of a load cell and a weight measuring device using the same according to the present invention will be described with reference to the drawings.

[First Embodiment]
FIG. 1 shows a longitudinal sectional view of a load cell according to the first embodiment of the present invention, and FIG. 2 shows a circuit configuration diagram of an electric circuit unit attached to the load cell of this embodiment. .

The load cell 1 of the present embodiment includes a load cell main body (distortion body) 2 made of a disc-shaped metal elastic body as a whole having an airtight chamber 3 which is a closed space closed to the atmosphere. A load introducing portion 4 is formed so as to protrude from the center portion on the upper surface side. On the outer peripheral side of the load introduction part 4 and between the outer surface and the inner surface of the load cell body 2, there is provided a thin-walled disc-shaped or annular strain-generating portion 5, and the stress of the strain-generating portion 5 Strain gauges G ₁ , G ₂ , G ₃ , and G ₄ are affixed to the maximum or minimum portion of.

The strain gauges G _{1 to} G ₄ form a strain gauge bridge circuit, and its terminals are connected to terminals of a circuit board 7 arranged in the hermetic chamber 3 by lead wires 6. Further, the terminal of the circuit board 7 is connected to a conduction pin 9 of an airtight terminal 8 that closes an opening on one side of the load cell body 2, and the conduction pin 9 is connected to a core wire of the wiring cable 10. Further, the load cell main body 2 and the wiring cable 10 are fixed by a connection fitting 11. Here, the connection fitting 11 is configured such that one end is inserted into an opening formed in the load cell main body 2 and fixed with screws, and the wiring cable 10 is inserted and fixed at the other end.

  A differential pressure sensor (differential pressure detecting means) 12 is disposed on the side wall of the hermetic chamber 3 of the load cell main body 2, and a metal outer frame of the differential pressure sensor 12 is fixed to the side wall by welding. A metal lid 13 is fixed to the lower end side, which is the opening side of the load cell main body 2, by welding. Here, the diaphragm constituting the differential pressure sensor 12 is made of metal, and a glass material is used for the gas sealing plate in which the conduction pin 9 of the airtight terminal 8 is embedded, and the airtight chamber 3 is completely blocked from ventilation. It is configured to be.

  Further, a gas injection hole 2 'is provided in the side wall of the load cell main body 2. Nitrogen gas is injected from the gas injection hole 2' to be replaced with air, and welding is performed when the inside of the hermetic chamber 3 is maintained at about 1 atm. Thus, the injection part is sealed. Since the nitrogen gas does not contain water vapor, the phenomenon that the inside is dewed by the rise and fall of the ambient temperature around the load cell does not occur, and the decrease in the insulation resistance of the strain gauge and the wiring connection solder portion is prevented.

In the airtight chamber 3, there are an amplification circuit for amplifying the load signal output from the strain gauges G ₁ , G ₂ , G ₃ , G ₄ and the differential pressure signal output from the pressure sensitive gauge of the differential pressure sensor 12, and An electric circuit unit 14 (see FIG. 2) includes an A / D conversion circuit that digitizes the above signal, an arithmetic circuit that calculates a load signal and corrects the differential pressure, and an output circuit that outputs the digital load signal corrected for the differential pressure to the outside. Built in.

In FIG. 2, the load signals from the strain gauges (SG) G _{1 to} G ₄ are amplified by the amplifier A 1, digitized by the A / D converter A / D 1, and the diaphragm of the differential pressure sensor 12. The differential pressure signal from the gauge (PS) 15 formed above is amplified by the amplifier A2, and then digitized by the A / D converter A / D3. A temperature signal from a temperature sensor (TS) 16 (which incorporates an amplifier) that outputs a gas temperature signal in the hermetic chamber 3 is digitized by an A / D converter A / D2. These signals are taken into the central processing unit CPU via the input / output circuit I / O. The input / output circuit I / O is controlled by a command from the central processing unit CPU based on a program stored in the memory M.

In FIG. 2, reference numeral 17 indicates power supply to the electric circuit unit 14, the strain gauges (SG) G _{1 to} G ₄ , the gauge (PS) 15, and the temperature sensor (TS) 16. A power supply line that is indicated by reference numeral 18 is a bidirectional data bus line that inputs and outputs digital load signals and various signals (alarm signals, setting signals, temperature signals used during adjustment, differential pressure signals). is there. The memory M includes a ROM that stores an arithmetic processing program, a RAM that stores arithmetic data while power is being supplied, a non-volatile RAM that can store data even when power is not supplied, and the like. Yes.

  Next, an outline of a load signal correction method based on the pressure difference between the inside and outside of the hermetic chamber 3 will be described.

(1) When the temperature characteristics of the differential pressure sensor can be ignored in terms of the accuracy of the weight measurement value First, the differential pressure sensor 12 is placed in the atmosphere (differential pressure = 0), and the output of the differential pressure sensor 12 is measured. To do. The output value at this time is the offset output voltage Voff, and Voff = V0.
Next, as shown in FIG. 1, the differential pressure sensor 12 is attached to the load cell main body 2, and the load cell 1 is placed in the atmospheric pressure adjustment chamber, and the atmospheric pressure in the atmospheric pressure adjustment chamber is changed in advance in the adjustment mode.
First, the atmospheric pressure is adjusted so that the output of the differential pressure sensor 12 becomes V0. If the pressure in the pressure adjusting chamber at this time is P0, this P0 is equal to the pressure in the airtight chamber 3.

  Next, the load signal output and the output signal of the differential pressure sensor 12 are measured while changing the pressure in the pressure adjusting chamber from P0 to P1, P2, P3,. Assume that the load signal output changes from W0 to W1, W2, W3,..., And the output of the differential pressure sensor 12 changes from the offset output value V0 to V1, V2, V3,. These data are measured and recorded as shown in Table 1. The offset output value V0 is stored in advance in the load cell measurement circuit.

From the data in Table 1, a relational expression of the load output signal change amount wx with respect to the signal output vx of the differential pressure sensor is determined by a method such as the least square method from FIG. 3A to obtain the following expression.
wx = f (vx) (1)

  In FIG. 3 (a), P1, P2,...> P0 atmospheric pressures P1, P2,... Are given, but P1 ′, P2 ′,. .., V1 ', V2' ... <V0 differential pressure signal is output, and v1 '= V1'-V0 <0, v2' = V2'-V0 <0,. Minus signal is output, and the load change due to this differential pressure also becomes a negative value as w1 ′ = W1′−W0 <0, w2 ′ = W2′−W0 <0,. With these data, wx ′ = f ′ (vx) can be determined in the region of vx <0, as shown in FIG.

  After the relational expression of the load change amount wx with respect to the differential pressure signal output vx is obtained as described above, the following processing is performed when the load sensor 1 is in operation.

  First, when the load cell 1 is turned on during operation, the load signal Wx of the load cell 1 is read as the initial load W0 and the output Vx of the differential pressure sensor 12 is read by the initial processing program. Assume that this output Vx is Vxi.

The value of the initial load W0 is expressed by the following equation using a tare load Wi that is previously loaded on the load cell and a load change wxi due to the current differential pressure.
W0 = Wi + wxi (2)
Therefore, the tare load Wi is obtained by the following equation.
Wi = W0−wxi (3)
Here, wxi is obtained by the output vxi = Vxi−V0 of the differential pressure sensor 12 and the equation wxi = f (vxi) obtained by substituting vxi and wxi into the above equation (1), so that the tare load Wi can be obtained. it can. The obtained Wi is stored in the memory as the initial tare load by the true tare. That is, the tare load is composed of the initial tare load Wi due to the true tare and the load fluctuation component wxi due to the differential pressure.

When the operation operation is started, the load variation component wxi is sequentially changed by the differential pressure change during the operation operation in the initial load W0 including the initial tare load Wi and the load variation component wxi due to the differential pressure.
In the normal processing program that is operated following the initial processing program during operation, the load signal Wx and the output signal Vx of the differential pressure sensor 12 are periodically detected by sampling. When the output signal of the differential pressure sensor 12 is Vx, the differential pressure output signal vx is obtained by the following equation based on the stored offset output value V0 of the differential pressure sensor 12.
vx = Vx−V0 (4)
Then, the load output signal change amount wx with respect to the differential pressure output signal vx is obtained by the equation (1). At this point, wxi is replaced with wx.

According to the calculation of the weight measurement value in the conventional load cell that does not consider the influence of the differential pressure of the gas inside and outside the hermetic chamber, the load Wn of the object to be weighed loaded on the load cell is K, the span coefficient as the load measuring device, and the zero point When the zero point weight value stored at the time of adjustment is Wz, it is expressed by the following equation.
Wn = K · (Wx−Wi−Wz) (5)
On the other hand, in the load cell 1 of the present embodiment, the load change wx due to the differential pressure is added to the load signal Wx, so that it is expressed by the following equation, and the differential pressure component included in the load signal Wx is corrected. It becomes possible to do.
Wn = K · {Wx− (Wi + wx) −Wz} (6)

(2) When it is necessary to consider the temperature characteristics of the differential pressure sensor 12 First, the differential pressure sensor 12 is placed in the atmosphere (the state where the differential pressure = 0), and the ambient temperature is set to θ0, θ1, θ2, θ3,. The temperature change characteristic of the offset output voltage Voff when it is changed is measured, and the temperature characteristic function f0 (θ) as shown in FIG.
Voff = f0 (θ) (7)

Further, if there is a span temperature drift in the output signal of the differential pressure sensor 12, the output signal varies depending on the atmospheric temperature even if the differential pressure is the same. That is, even if the load signal output change and the differential pressure are the same, the magnitude of the differential pressure sensor output signal varies depending on the temperature.
In this load cell, the magnitude of the differential pressure changes to the magnitude of the change in the load signal. Therefore, if the span of the differential pressure sensor 12 changes in temperature, the relationship between the change in the output of the load signal and the magnitude of the output signal of the differential pressure sensor. Depends on the temperature.

  Therefore, the load cell with a differential pressure sensor in the present embodiment is adjusted by setting different temperature conditions in the gas adjustment chamber. For example, θ0 = 5 ° C., θ1 = 20 ° C., θ2 = 35 ° C. is selected as the reference temperature test condition, and data is measured in the same manner as in FIG. 3 at each gas temperature, and as shown in FIG. Functions f0 (vx, θ0), f1 (vx, θ1), and f2 (vx, θ2) corresponding to the curves m0, m1, and m2 in the region of vx ≧ 0 are determined. Similarly, functions f0 ′ (vx, θ0), f1 ′ (vx, θ1), and f2 ′ (vx, θ2) corresponding to the curves m0 ′, m1 ′, m2 ′ in the region of vx <0 are determined.

  Now, assuming that these functions are expressed by a quadratic expression, the above three functions f0 (vx, θ0) are used to determine the function fx (vx, θx) in the case of an arbitrary temperature θx during operation. ), F1 (vx, θ1), f2 (vx, θ2), the amount of change w20, w21, w22 of the load signal in the differential pressure values (each excluding the offset value, which is assumed to be v2). Is stored at the time of the above test.

Then, when the temperature measurement value θx is θ2 <θx <θ1, the function fx (by the coordinates (v2, w2x) is obtained from w2x obtained by dividing the load change amount in v2 by a ratio determined between θ1 and θ2. vx, θx) is obtained. That is, w2x is determined by the following equation.
w2x = w22 + {(w21-w22) / (θ2-θ1)} · (θ2-θx)
(8)

The following processing is performed during the operation of the load sensor 1.
First, during operation, the load cell 1 is turned on and the initial load W0 is measured. At the same time, the output signal θxi of the temperature sensor 15 and the output signal Vx = Vxi of the differential pressure sensor 12 arranged in the hermetic chamber 3 are measured. .

Next, the output offset value Voff of the differential pressure sensor at the temperature θxi is obtained as Voff = f0 (θxi) from the equation (7), and is substituted into the equation (4) to obtain the differential pressure signal vxi by the following equation.
vxi = Vxi−Voff = Vxi−f0 (θxi) (9)

The load fluctuation component wxi included in the initial load signal W0 is derived after the function fxi (vx, θxi) at the temperature θxi is determined. That is, after deriving the function fxi (vx, θxi) using any of w20, w21, and w22 when vx = v2 as described above, wxi is obtained by the following equation.
wxi = fxi (vxi, θxi) (10)

Thus, when wxi is obtained, the initial tare load Wi is obtained by the following equation and Wi is stored.
Wi = W0−wxi (11)
Then, the load Wn of the object to be weighed is expressed by the following formula: Wn = K · {Wx− (Wi + wxi) −Wz} (12)
The load fluctuation component wxi with respect to the initial differential pressure / temperature fluctuation is replaced with wx calculated based on the differential pressure and temperature during operation, and is followed.

  More specifically, based on the above concept, the load signal is corrected as follows.

(1) At the time of manufacture / adjustment of the load cell At the time of manufacture / adjustment of the load cell 1, the differential pressure sensor 12 is first placed in a state of zero differential pressure in the atmosphere, and the output signal of the differential pressure sensor 12 is collected. Then, the differential pressure sensor 12 is placed in the temperature test tank, the offset voltage value of the differential pressure sensor 12 is measured while changing the temperature of the temperature test tank, and stored in the measuring instrument together with the temperature value. Thereby, in the measuring instrument, the equation (7), which is a relational expression representing the temperature characteristics of the differential pressure sensor 12 and the offset voltage, is determined.

  When the differential pressure sensor 12 is attached to the load cell main body 2 in a predetermined load cell 1, the measuring instrument is connected to the load cell. Then, the function of the expression (7) is sent through the data bus line 18 and stored in the memory M of the arithmetic circuit in the electric circuit unit 14. In addition, the characteristic data about the differential pressure sensor 12 may be sent to the load cell, and the equation (7) may be determined by the load cell arithmetic circuit.

  Next, the load cell 1 is placed in a gas adjustment chamber in which a temperature test is possible, and the load signal and temperature of the load cell 1 and the output signal of the differential pressure sensor 12 are monitored by a measuring instrument. Next, the gas adjustment chamber is adjusted to a predetermined pressure as described above, and the temperature of the pressure adjustment chamber is set to a predetermined temperature in the airtight chamber 3 of the load cell 1 (actually filled with nitrogen gas). The temperature data, the differential pressure signal, and the load signal are read into the weight measuring device while adjusting so that the typical load-differential pressure, temperature function f0 (vx, θ0), f1 (vx, θ1), f2 ( vx, θ2) is determined. Then, the determined representative function is stored in the memory M. These functions may be determined by an arithmetic circuit in the load cell hermetic chamber.

(2) When using the load cell When power is supplied to the load cell 1, the load signal W0, the temperature signal θxi, and the differential pressure signal Vxi are read by the initial program when the power is supplied. Then, the value of the offset signal Voff = f0 (θxi) of the differential pressure sensor 12 is calculated from the temperature signal θxi and the stored f0 (θ) in the equation (7). Further, the temperature-corrected differential pressure signal vxi is calculated from the stored equation (9).

  Next, a function fxi (vx, θxi) is determined from a stored representative function through a calculation procedure represented by the equation (8) from vxi and θxi, and a load fluctuation component wxi due to a differential pressure at the time of power supply is expressed as wxi. = Fxi (vxi, θxi).

When wxi is obtained in this way, the initial tare load Wi is obtained from equation (10), Wi is stored, and the load Wn of the object to be weighed is determined as in equation (12). After this, the temperature and differential pressure are detected periodically and continuously with respect to the continuation of the operation operation, the load fluctuation component wx at each time point is calculated, wxi is updated, and the load signal is
Wn = K · {Wx− (Wi + wxi) −Wz}
Asking.

[Second Embodiment]
In the present embodiment, the variation of the load signal due to the differential pressure change based on the magnitude of the load load is corrected separately from the differential pressure change due to the temperature change and the load signal variation due to the differential pressure change due to the atmospheric pressure change. This is related to the example. In other words, even if the external air pressure and the air temperature are constant, the zero point drift is corrected depending on the magnitude of the load.

  For example, when a load is applied to the load cell 1 shown in FIG. 1 and the strain generating portion 5 is bent slightly downward, the metal lid 13 is also bent accordingly, but the airtight chamber 3 is slightly changed according to the magnitude of the load. If the volume of the airtight chamber 3 changes, a change in the atmospheric pressure of the hermetic chamber 3 occurs to generate a reaction force against the load. If this reaction force cannot be ignored, the following correction is executed.

As a correction measure, the following preparatory work is performed when adjusting the load cell.
First, test samples (for example, weights) ws1, ws2, ws3, and ws4 having known weight values are prepared.
The load cell capacity is Wf, and the test sample weight values are ws1 = (1/4) · Wf, ws2 = (1/2) · Wf, ws3 = (3/4) · Wf, ws4 = Wf, respectively. . It is assumed that the load cell for performing this test has been subjected to span adjustment in a state where the hermetic chamber is electrically connected to the outside.

  In the first embodiment, in the test (see Table 1) for applying the atmospheric pressure P0, each sample load load is added to measure the differential pressure sensor output and the load signal output. As a result, the data shown in Table 2 is obtained.

For example, in test category 1 of Table 2, when the test sample ws1 is loaded on the load cell and the measured values of the load output signal Ws1 and the load sensor output Vs1 are obtained, the load signal output change is Ws1-W0. A difference from the weight value ws1 of the test sample is regarded as a reaction force (zero change amount) due to an increase in pressure in the hermetic chamber. This zero point change amount is expressed by the following equation.
−Δws1z = (Ws1−W0) −ws1
Further, the amount of change in the differential pressure due to the pressure change in the hermetic chamber due to the load load is expressed by the following equation.
Δvs1 = Vs1−Vs0
Hereinafter, other test samples can be measured in the same manner, and the zero point change amount and the differential pressure change amount for each sample load can be derived.

  Next, from the data of the zero point change amount and the differential pressure change amount according to the load loads ws1, ws2, ws3, and ws4, using a method such as a least square method, as shown in FIG. Functions fw (Wn) and fz (Wn) with respect to the load Wn of the pressure change amount are obtained.

When using the load cell (when performing normal load measurement work), first, the pressure change is temporarily removed in order to remove the differential pressure change due to the load from the atmospheric pressure change or the atmospheric pressure change due to expansion in the airtight chamber. The approximate load load measurement value Wn ′ for the load cell is calculated without considering the load cell (or the load load measurement value that compensates for changes in atmospheric pressure and hermetic chamber air pressure is ignored as Wn ′ ignoring the influence of the load load), Substituting Wn ′ into the above functions fw (Wn) and fz (Wn), leads to a zero-point variation −Δws1z ′ and a differential pressure variation Δvs1 ′.
Then, in the above equation (4), the differential pressure vx is expressed by the following equation: vx = (Vx−Δvs1 ′) − V0
As described above, the influence of the differential pressure change due to the load is removed.

Similarly, in the above-described equation (6), the load Wn of the object to be measured is expressed by the following equation: Wn = K · {Wx− (Wi + wx−Δws1z ′) − Wz}
As described above, the influence of the zero point change due to the magnitude of the load is removed.

  Further, the equations (9) and (12) when the temperature changes are also handled by the same calculation operation. Further, when considering the temperature as a function for strict compensation, in the first embodiment, “(2) Considering the temperature characteristics of the differential pressure sensor 12 under a plurality of types of gas temperature environments. The functions fw (Wn, θx) and fz (Wn, θx) may be obtained by a method similar to the method described in the “when necessary” column (see paragraph 0042).

  In each of the above embodiments, the example applied to the load cell as shown in FIG. 1 has been described. However, the present invention is a so-called compression type load cell in which the elastic body as shown in FIG. The present invention can also be applied to a load cell in which an airtight chamber is formed around a strain generating portion by sealing the strain generating portion provided in a part of the columnar elastic body with a metal case.

  Further, in each of the above embodiments, nitrogen gas is filled in an airtight chamber which is a closed space. However, instead of this nitrogen gas, normal air may be filled, or a gel such as silicon resin may be used. You may make it fill with a gaseous substance.

  In each of the above embodiments, the differential pressure sensor is used. However, a pressure sensor in which a room having a reference pressure is built in the sensor to measure the external pressure of the sensor is attached to the inside and outside of the airtight chamber. The difference between the output signals of the pressure sensors may be obtained. In this case, the installation position of the pressure sensor attached to the outside of the load cell may be on the load cell, or may be in the control device away from the load cell.

  In the load cell of each embodiment, a control device having the function of the electric circuit unit 14 is placed outside the load cell, and a differential pressure sensor signal and a temperature sensor signal are sent to the control device together with a load signal. The pressure difference correction of the load signal may be performed by the arithmetic circuit provided in FIG.

  In addition, the temperature sensor is preferably provided in the closed space (airtight chamber) of the load cell. However, it can be considered that the temperature difference between the inside and outside of the airtight chamber is small, and the temperature sensor is provided outside the airtight chamber to detect the atmospheric temperature. .

  The electric circuit unit 14 in each embodiment corresponds to the “load signal correction unit” in the present invention.

The longitudinal cross-sectional view of the load cell which concerns on the 1st Embodiment of this invention 1 is a circuit configuration diagram of an electric circuit unit mounted on a load cell according to a first embodiment. Graph showing the relationship between the load output signal change amount and the differential pressure signal output Graph showing the relationship of offset output voltage to ambient temperature Graph showing the relationship between the load output signal change amount and the differential pressure signal output The graph which shows the relationship between the differential pressure | voltage change amount with respect to the load load in 2nd Embodiment, and a zero point change amount. The figure which shows an example of the conventional load cell The figure which shows the other example of the conventional load cell Diagram explaining problems of conventional load cell with bellows

Explanation of symbols

1 Load cell 2 Load cell body (distortion body)
2 'gas injection hole 3 inner space (airtight chamber)
4 Load introduction part 5 Strain generation part 6 Lead wire 7 Circuit board 8 Airtight terminal 9 Conductive pin 10 Wiring cable 11 Connection fitting 12 Differential pressure sensor (Differential pressure detection means)
13 Metal lid 14 Electric circuit unit (load signal correction means)
15 Temperature sensor (PS)
16 gauge (TS)
17 power supply line 18 data bus lines A1, A2 amplifier A / D1~A / D3 analog-CPU Central Processing Unit I / O output circuit _G 1 ~G ₄ strain gauge M memory

Claims (4)

Some or all of the strain generating portion formed on the load cell body is disposed in a closed space that is closed to the atmosphere Rutotomoni, a strain gauge for generating a load signal in response to the distortion of the strain generating portion, said closure A differential pressure detecting means for detecting a differential pressure between a pressure in the space and a pressure in the atmosphere; and a load signal correcting means for correcting a variation in the load signal based on the differential pressure detected by the differential pressure detecting means. In the load cell provided ,
A load signal output from the strain gauge and a differential pressure signal output from the differential pressure detection means are configured to detect each independently,
The load signal correcting means determines a function indicating a relationship of a tare load fluctuation amount in the load signal with respect to a change in the differential pressure signal when adjusting the load cell, and according to the input differential pressure signal when using the load cell. A load cell configured to correct a load signal based on a tare load fluctuation amount calculated using a function . The load signal correction means, characterized in that the correction and the difference detected by the pressure detecting means differential pressure, the load signal with the tare weight fluctuation amount is calculated from the relationship between temperature or temperature of the atmosphere of the closed space The load cell according to claim 1 .   A weight measuring device comprising the load cell according to claim 1 and a control device connected to the load cell, wherein the control device is based on the differential pressure detected by the differential pressure detecting means. A weight measuring device comprising load signal correcting means for correcting fluctuation of the load signal. The load signal correction means, characterized in that the correction and the difference detected by the pressure detecting means differential pressure, the load signal with the tare weight fluctuation amount is calculated from the relationship between temperature or temperature of the atmosphere of the closed space The weight measuring device according to claim 3 .

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