JPH08159884A - Load cell type weighing equipment employing single electric power source and correcting method based on temperature characteristic - Google Patents

Abstract

PURPOSE: To provide a highly precise weighing equipment at a low cost by installing a third voltage different from an electric power source voltage and a standard voltage and dividing the voltage obtained by amplification of the output of a strain gauge attached to a load cell and the third voltage with a temperature sensing resistor and a normal resistor. CONSTITUTION: A third voltage VP different from an electric power source voltage VDD and a standard voltage VSS is installed and the output VX obtained by dividing the voltage V1 obtained by amplifying the output of a strain gauge attached to a load cell and the third voltage VP with a temperature sensing resistor R3 and a normal resistor R4 is connected with an A/D converter. The output VX can be expressed as VX=R4 /(R4 +R3 )×(V1 +VP)+VP in the case where the temperature sensing resistor R3 is set in this way. If VP is so set as to satisfy V1 =VP, VX becomes equal to VP and is constant independently of the R3 value. That is, it is indicated that an indicated value does not fluctuate even if temperature alters in a no load state by setting VP=V1 ,0 while the amplified output V1 in no load state being set to be V1 ,0 .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a load cell type weighing scale, and more particularly to correction of a temperature characteristic of a load cell type weighing scale using a single power source so that a measured value is not influenced by an environmental temperature change.

[0002]

2. Description of the Related Art Generally, a strain gauge itself and a strain element that gives strain to the strain gauge in proportion to a load value have temperature characteristics in terms of material, and it is very difficult to eliminate the influence of the temperature of the output of a single load cell. Is. For this reason, various methods for correcting the temperature characteristic of the load cell using a temperature sensitive resistor having a temperature characteristic opposite to the output temperature characteristic of the load cell alone have been proposed and implemented.

In a load cell type weighing scale using a single power source, which is relatively low in cost compared to the conventional one, a circuit typified by FIG. 3 is used, in which a temperature opposite to the temperature characteristic of the load cell is provided between the power source voltage V _DD and the strain gauge bridge. A method of inserting a temperature-sensitive resistor having characteristics and controlling the input voltage has been generally adopted.

However, in this method, when V _SS = 0V in FIG. 3 and R _G1 = R _G2 = R _G3 = R _G4 , the output of the bridge circuit is output.
V _{G +} is expressed as V _{G +} = 1/2 * R _G / (R _G + R ₃ ) * V _DD (1), where R _G is the combined resistance of the bridge. Here, the temperature characteristic of the load cell is 600ppm /
When set to ℃, R ₃ is set so that the bridge circuit has a temperature characteristic of about -600ppm / ℃. However, at that time, the +
The voltage V _{G + at the} input terminal also fluctuates at −600 ppm / ° C., and as a result, the amplifier output V ₁ drops by the voltage drop of V _{G +} .

That is, the fluctuation voltage ΔV ₁ of V ₁ per 1 ° C. is
When V _{G +} of the voltage drop and _{ΔV G + ΔV 1 = ΔV G} + = 1/2 * R G / (R G + R 3) * (- 600ppm) * 10 -6 ···
It is represented by (2). Here, if R _G = 350Ω, R ₃ = 72Ω, and the temperature characteristics of R ₃ are 3,500ppm / ° C and V _DD = 5V, the temperature correction is almost -600ppm, so from equation (2), ΔV ₁ = 1/2 * 3
50 / (350 + 72) * (-600 ^-6 ) * 5 = -1.24 mV, and if the fluctuation of V ₁ under no load and under weighing load is 0.6 V, it changes about 1/500 when 1 degree changes. It will be. This means that if the environmental temperature changes by 1 ° C, the zero point shifts by 2 in a weighing scale whose scale is 1 / 1,000 of the weighing scale, and in the scale of 1 / 3,000 it shifts by 6 scales. It shows that it will end.

In order to make up for such a drawback with a single power source, as shown in FIG.
As shown in, a method of connecting a temperature-sensitive resistor for temperature correction to positive and negative power supplies is used. R ₃ = R ₃ 'R _G1 = R _G2 = R _G3 = R
_{With G4} , the gauge output is 0V, which means that the gauge output does not change even if the temperature changes. However, A / D
In the converter, especially in the A / D converter used in the load cell type weighing machine, there are few things that can convert the negative voltage, and the output V ₀ of the balance in the unloaded state is adjusted to a positive value. There is a need. The fact that V ₀ has a constant value means that the output value V ₀ fluctuates as the temperature fluctuates as described above.
It is not an essential solution, and the temperature-sensitive resistance is generally expensive, and using two of them is not desirable because it causes a cost increase. On the other hand, it is known that the no-load instruction value fluctuates due to changes in the state of the load cell due to changes in temperature, changes in the offset voltage of the amplifier, and the like.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a load cell type weighing machine using a single power source, which corrects the temperature characteristics without any change in the indicated value at the time of no load, and is low cost and highly accurate. It is to provide a load cell type weighing scale.

[0008]

[Means for Solving the Problems] A third voltage V _P different from a power supply voltage V _DD and a reference voltage V _SS is provided, and a voltage V obtained by amplifying an output of a strain gauge attached to a load cell with an amplifier.
₁ and the third voltage V _P , the temperature sensitive resistance R ₃ and the normal resistance R
Connect the output V _X divided in ₄ to the A / D converter.

[0009]

By setting the value of the third voltage V _{P that} can be easily set to a value equal to V ₁ , ₀ , which is the output voltage obtained by amplifying the output of the load cell under no load, the temperature change under no load condition Even if there is, the indicated value will not change.

[0010]

1 and 2 are block diagrams showing details of essential parts of an embodiment of the present invention, and FIGS. 5 and 6 are circuit diagrams used for explaining the embodiment of the present invention.

If a temperature-sensitive resistor R ₃ is arranged as shown in FIG. 5, V _X = R ₄ / (R ₄ + R ₃ ) * (V ₁ -V _P ) + V _P (3) To be done. Here, if V _P is set so that V ₁ = V _P , V _X = V _P , which is a constant value regardless of the value of R ₃ . This means that if the unloaded amplifier output V ₁ is V ₁ , ₀ , then V _P
= V ₁ , ₀ indicates that the indicated value does not change even if the temperature changes in the no-load state.

Obtaining an arbitrary V _P is easy by V _DD , V _SS and R ₁ , R ₂ as shown in FIG. 5, and a variable resistor is used for R ₁ to adjust V _P. It is also easy to enable. The temperature correction of the load cell is performed by the amplifier output V _{1 of} formula (3).
Value of R ₄ , the value of R ₄ and the temperature-sensitive resistance R ₃ so that R ₄ / (R ₄ + R ₃ ) has temperature characteristics that cancel the temperature characteristics of the load cell.
You can set the value of.

Another embodiment, which is a simpler diagram shown in FIG.
It is possible to obtain the same result with an equivalent circuit to the circuit shown in 5.
come. That is, Fig. 6 is based on Kirchhoff's law (V_DD-V_X) / R₁+ (V₁-V_X) / R₃ = V_X/ R₂ (4) is established. Therefore R₂ R₃(V_DD-V_X) + R₁ R₂(V₁-V_X) = R₁ R₃ V_X Transform this equation to V_X(R₁ R₂+ R₂ R₃+ R₃ R₁) = R₂ R₃ V_DD+ R₁ R₂ V₁ ... (5) is obtained. Equation (5) is V_XSolving for V_X = (R₁ R₂ V₁+ R₂ R₃ V_DD) / (R₁ R₂+ R₂ R₃+ R₃ R₁) = R₁ R₂/ (R₁ R₂+ R₂ R₃+ R₃ R₁) * (V₁+ R₃/ R₁* V_DD) = R₁ R₂/ (R₁ R₂+ R₂ R₃+ R₃ R₁) * [V₁+ {R₂/ (R₁+ R₂) -R₂/ (R₁+ R₂) + R₃/ R₁} * V_{D D} ] = R₁ R₂/ (R₁ R₂+ R₂ R₃+ R₃ R₁) * (V₁-R₂/ (R₁+ R₂) * V_DD+ (R₁ R₂+ R₂ R₃+ R₃ R₁ ) / R₁(R₁+ R₂) * V_DD} = R₁ R₂/ (R₁ R₂+ R₂ R₃+ R₃ R₁) * (V₁-R₂/ (R₁+ R₂) * V_DD} + R₂/ (R₁+ R₂) * V_DD = (R₁ R₂/ (R₁+ R₂)} / {R₁ R₂/ (R₁+ R₂) + R₃} * {V₁-R₂/ (R₁+ R₂) * V_DD} + R₂/ (R₁+ R₂) * V_DD (6) Where R_Four = R₁ R₂/ (R₁+ R₂), V_X = R_Four/ (R_Four+ R₃) * (V₁-R₂/ (R₁+ R₂) * V_DD} + R₂/ (R₁+ R₂) * V_DD ... (7) is obtained. Therefore, R₂/ (R₁+ R₂) = V₁/ V_DDTo be R₁, R₂To
If you put it, from formula (7), V_X= R₂/ (R₁+ R₂) * V_DD And V_X
Is R₃Becomes constant regardless of the value of
Output V_XHas no temperature effect. one
One, V₁Coefficient of R_Four/ (R_Four+ R₃) Indicates the load cell temperature characteristics
R to kill₃Select the to correct the load cell temperature.
come. Here, setting VP = R2 / (R1 + R2) * VDD, the formula (7)
Matches the expression (3).

The output V ₁ of the load cell at no load is
Even when there is a change in a constant value due to a temperature change due to the offset voltage of the amplifier, the characteristics of the load cell, etc., it can be canceled. That is, the variation of V ₁ of the time of temperature variation in Delta] t ° C. [Delta] V _1, the variation of R ₃ to the variation of [Delta] R _3, V _X and [Delta] V _X (7) from equation _{ΔV X = R 4 / (R} 4 + R ₃ + ΔR ₃ ) * (V ₁ + ΔV ₁ -R ₂ / (R ₁ + R ₂ ) * V _DD } -R ₄ / (R ₄ + R ₃ ) * {V ₁ -R ₂ / (R ₁ + R ₂ ) * V _DD } = R ₄ / (R ₄ + R ₃ + ΔR ₃ ) * ΔV ₁ + (R ₄ / (R ₄ + R ₃ + ΔR ₃ ) -R ₄ / (R ₄ + R ₃ )} {V ₁ -R ₂ / (R ₁ + R ₂ ) * V _DD } = R ₄ / (R ₄ + R ₃ + ΔR ₃ ) * ΔV ₁ + (-ΔR ₃ R ₄ ) / (R ₄ + R ₃ + ΔR ₃ ) (R ₄ + R ₃ ) * {V ₁ -R ₂ / (R ₁ + R ₂ ) * V _DD } = R ₄ / (R ₄ + R ₃ + ΔR ₃ ) * [ΔV ₁ -ΔR ₃ / (R ₄ + R ₃ ) * {V ₁ -R ₂ / (R ₁ + R ₂ ) * V _DD }] where ΔV _X = if the value in [] on the right side is 0 Since it is 0, ΔV ₁ -ΔR ₃ / (R ₄ + R ₃ ) * {V ₁ -R ₂ / (R ₁ + R ₂ ) * V _DD } = 0, R ₂ / (R ₁ + R ₂ ) * V _DD -V ₁ = -ΔV ₁ * (R ₄ + R ₃ ) / ΔR ₃ ∴ R ₂ / (R ₁ + R ₂ ) = 1 / V _DD * {V ₁ -ΔV ₁ * (R ₄ + The objective can be achieved by selecting R ₁ and R ₂ that satisfy R ₃ ) / ΔR ₃ }. However, since R ₄ = R ₁ R ₂ / (R ₁ + R ₂ ), (R ₁ + R ₂ ) R ₄ = R ₁ R ₂ ∴ R ₁ R ₂ -R ₁ R ₄ = R ₂ R ₄ ∴ R ₁ = R ₂ R ₄ / (R ₂ -R ₄ ) must be satisfied. Also in FIG. 5, when there is a constant change due to temperature fluctuation, it is clear that there is a value that can cancel the change. Also, in the case of a circuit having positive and negative power supplies (FIG. 7), it is apparent that the same problem can be solved by selecting V _p so that the output voltage V ₀ = V _p when there is no load.

[0015]

According to the present invention, the temperature characteristic of a load cell type weighing machine using a single power source is corrected to be the temperature characteristic with no change in the indicated value when there is no load, and the output V of the load cell when there is no load. ₁ can be offset even when there is a constant change due to temperature fluctuations due to the offset voltage of the amplifier and the characteristics of the load cell.

[Brief description of drawings]

FIG. 1 is a block diagram showing details of essential parts of an embodiment of the present invention.

FIG. 2 is a block diagram showing details of essential parts of another embodiment of the present invention.

FIG. 3 is a circuit diagram used for explaining a conventional embodiment.

FIG. 4 Conventional example for positive and negative power supplies

FIG. 5 is a circuit diagram used to describe an embodiment of the present invention.

FIG. 6 is a circuit diagram used to explain another embodiment of the present invention.

FIG. 7: Embodiment of the present invention in positive and negative power supplies

[Explanation of symbols]

R _1, R _2, R ₄ normal resistor R _3, R ₃ 'temperature sensing resistor R _G1, R _G2, is R _G3, R _G4 Sutorengedji V ₁ amplified load cell output V _P 3 was voltage V _X Temperature correction , Load cell output input to A / D converter V _SS Reference voltage V _DD Power supply voltage V _{G +} Amplifier input terminal voltage

Claims (2)

[Claims] 1. A voltage V ₁ obtained by amplifying the output of a strain gauge attached to a load cell with an amplifier, and a third voltage V _P different from the power supply voltages V _DD and V _SS and a temperature-sensitive resistor R ₃ , Resistance of R ₄
A method for correcting temperature characteristics of a load cell type weighing scale using a single power source, characterized in that a voltage V _X divided by and is connected to an A / D converter. 2. A voltage V ₁ obtained by amplifying the output of a strain gauge attached to a load cell with an amplifier is passed through a temperature sensitive resistor R ₃ to R ₁
* R ₂ / (R ₁ + R ₂ ) is the same value as R _{4 according} to claim ₁
A method for correcting the temperature characteristics of a load cell type weighing scale using a single power supply, characterized in that the output V _X obtained by dividing V _DD and V _SS by R ₂ and R ₂ is connected to an A / D converter.