JPS60105924A - Adjustment of zero point of load cell balance - Google Patents

Abstract

PURPOSE:To prevent the change of a zero point even if a span is adjusted, by inserting a resistor for correcting bridge balance into the bridge circuit of a load cell to set the bridge balance to a predetermined value. CONSTITUTION:A load cell 2 consists of a four strain gauges 3 subjected to bridge connection and an adjusting resistor 4 inserted into one arm of said connection as a resistor for correcting bridge balance. A reference voltage generator 6 and a test voltage generator 7 are connected to a power source 1 in common and a zero point setting voltage generator 8 is connected to the reference voltage generator 6. The output voltage of the load cell 2 is applied to a microcomputer 15 through an analogue switch 5, a load cell signal amplifier 9 and an A/D converter 10. The zero point adjustment of the load cell 2 is performed by the adjusting resistor 4 and the change of the zero point is prevented even if the span adjustment of the balance is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to zero point adjustment of a load cell scale in which an analog value from a load cell part that changes the resistance value according to the load is converted into a digital value as a measured value. It is about the method.

Technical Background and Problems Generally, this type of load cell scale outputs the load as a digital value, but the digital value from the zero point to the rated load point is called the span, and in conventional models, this When the span is adjusted, it changes to the zero point, making adjustment in the initial state extremely difficult.

OBJECTS OF THE INVENTION An object of the present invention is to provide a method for adjusting the zero point of a load cell scale in which the zero point does not change even if the span is adjusted.

Summary of the Invention The present invention changes the span by changing the reference voltage, and at the same time changes the zero point, but by inserting a bridge balance correction resistor into the bridge circuit of the load cell, the span can be adjusted to a predetermined value. Once set, the relationship between the load cell load and the output digital value can be changed as a slope change with the zero point as the center.This allows the configuration to be configured so that the zero point will not shift even if the span is adjusted. It's Shitachino.

Embodiment of the Invention First, a load cell 2 to which a power source 1 of voltage VE is connected is provided, and this load cell 2 has four strain gauges 3 connected in a bridge manner and one bridge balance correction device R7.1 inserted on one side. It consists of an adjustment resistor 4 as a resistor. This load cell 2 is connected to an analog switch 5 consisting of three switch sections Sl, Sl, and S3. That is, the + side is connected to the switch S1, and the - side is connected to the grounded switch S2. and,
The output voltage of the load cell 2 is VL.

Further, a reference voltage generator 6 is connected to the power source 1 as a common power source.
and a test voltage generator 7 are connected. The reference voltage generator 6 has three elements labeled 1, 2.3 and Rr.
A variable resistor R5 and a resistor R6 are connected in series on the output side, and an output Vr is taken from the midpoint of these connections. The test voltage generator 7 has three elements labeled 4 and 5.6 and four resistors labeled R3 and R4, and a variable resistor R8 and a resistor R1o are connected in series. An output voltage of VA is taken from the midpoint of these connections, and this output is connected to the switch S3 of the analog switch 5.

Further, a zero point setting voltage generator 8 is connected to the reference voltage generator 6 . This zero point setting voltage generator 8 includes an element indicated as 7, a variable resistor R7, and a resistor R8, and generates an output voltage Vz.

The analog switch 5 is then connected to a load cell signal amplifier 9. This load cell signal amplifier 9
has an amplifier indicated as 8, the analog switch 5 is connected to the + side of this amplifier, and the feedback resistor network for determining the gain is connected to the - side. This feedback resistor network consists of resistors R11°R12 and an amplifier with a gain of 1 times denoted by 9, and the output side of the zero point setting voltage generator 8 is connected to the + side of this amplifier.

Therefore, the output voltage of the load cell signal amplifier 9 is V
o, and this load cell signal amplifier 9 is connected to an A/D converter 1o. This A/D co',yt<-
The case 10 is of a double integration type and includes an integrator II having a comparator function, a counter 112, and a counter 213, and these counters 112 and 213 have /c
A clock oscillator 14 is connected which generates pulses with a frequency of . Further, a microcomputer system 15 is connected to the output side of this A/D converter 10,
This microcomputer system 15 includes a display section 16.
, operation keys 17 are connected. Further, an analog switch controller 18 is connected to this microcomputer system 15, and this analog switch controller 18 is connected to the switches S1, . S
1 and S3.

In such a configuration, the operation of each part will be explained next. First, the output voltage VA of the test voltage generator 7 is expressed by the following equation.

The output voltage Vr of the reference voltage generator 6 is expressed by the following equation.

The output voltage Vz of the zero point setting voltage generator 8 is expressed by the following equation.

・・・・・・・・・・・・・・・・・・・・・・・・
(3) The output Vt of the load cell 2 is expressed by the following equation.

x VL =-・K-VE +Vu+・・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・・・・
...(4) Fn However, Fn: Rated load of the load cell F-x: Load cell load K: Sensitivity of the load cell ■■; Output of the bridge unbalanced load cell signal amplifier 9 of the load cell ■0 is the state of the analog switch 5 Accordingly, the following equations (5), (6), and (7) are shown.

(When only switch S1 is ON) (Switch S2
(when only the switch S3 is in the QN state) (when only the switch S3 is in the QN state) Next, when the output of the integrator 11 of the A/D converter 10 is Voz, and the integration constants are Cz and Rr,
The output vox is expressed by the following equation.

In equation (8), when V ox =O, the comparator issues a signal to stop the operation of the integrator 11. The time at this time is t3.

In addition, t1 to t2 is a period TS set to a constant value,
If t 2 to t 3 is a period TR, the following equation is obtained in order to satisfy the requirement that V ox =0.

■0 Tk-1・Ts ・・・・・・・・・－・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・・・・
(10)r Here, since Ts is a constant value, TR can be taken as a time proportional to the input voltage 0 given to the A/D converter 10. If the clock pulses fc of the clock oscillator 14 are counted within this period of TR, a digital value can be obtained. Now, fc -TR=N*, fc -Ts
=Ns.

Equation (10) can be determined from the relationship between the pulse numbers NR and Ns as shown in the following equation.

0 NR=-・Ns ・・・・・・・・・・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・・・・
...(11)r Substituting equations (2) and (5) into equation (11), we get, and into equation (12), equation (3) showing Vz and equation (4) showing VL.
) and substituting the expression.

Ri Rts +R6 RI R5+ Re Here, Vu/Vr= represents the degree of unbalance of the bridge constituting the load cell 2, and has a constant value as a unique value of the load cell 2. Therefore, equation (14) is the voltage 1 of the drive power source 1 of the load cell? , it shows that it is unrelated to , and even if VE fluctuates, NR and Rxz/Rxx=
G3. VIJ/Vg=k.

Rz/R1=Gt, Re/(R5+Ra)=rt.

When Ra/(R7+Ra)=r2,
Equation (14) becomes as follows.

Equation (16) is illustrated in FIG. 2. here,
If the digital output at zero point P is NR(Z), then the digital focus at the weighing point (rated load point of the scale) is NR<Z.
+P), it becomes. Here, let the rated load of the scale be F (F>).

F (P) = F cz + p> −F (F). Therefore, 5PAN” NR(Z+F) −
Next to NR.

Here, in order to adjust the 5PAN of the scale, r1=Ra/(R6+R8), it is sufficient to adjust the variable resistor R5 of the reference voltage generator 6 to change rl.

Also, in equation (16), the zero point satisfies the condition that (Fx/Fn) ・K+=0, so NR=-G3-
rl・Ns, and in order to adjust the zero point of the scale, R2=Ra
/ (R7+R8), therefore, it is sufficient to adjust the variable resistor R7 of the zero point setting voltage generator 8 to change rl.

Note that F (Z) is the load of the scale pan and the structure that supports it.

Next, in equation (16) and what is shown in Figure 2, A/
When the digital output of the D converter 10 is obtained with both positive and negative polarities, a zero point can be set. This can be done by determining rl in equation (17) so that N R (Z) knee NR. Even when the zero point is determined in this way, when rl is changed to adjust 5PAN, rl is not included in the terms of G3, rl, and Ns that determine the zero point, so the movement of the zero point is as follows. There is an advantage that the 5PAN can be easily adjusted using only the value of the formula, and with a very small amount.

First, let the movement of the zero point be ΔNR.

In formula (20), (F(2)/Fn) ・K +
By adjusting the bridge balance of the load cell 2 so that k = O, ΔN R (Z) = 0, which has the advantage of eliminating the movement of the zero point due to the adjustment of 5PAN. Therefore, in FIG. 2, it is possible to set an inclined straight line centered on point P, and even if 5PAN is adjusted, point P (zero point) will not change. By using this circuit system in this way, when the digital output of the A/D converter 10 can be obtained with bipolar polarity, the S PAN of the scale
By obtaining a digital output range of both polarities, the resolution of the scale can be increased without making 5PAN adjustment difficult.

Therefore, let us assume that the digital output when formulas (2) and (7) are substituted into formula (■1) is NR(A)? So, (
Substituting equation (1) into equation 21) yields the following.

However, R4/R3=Gz l RIO/(R90 Rto
)==r3.

Since equation (23) is independent of VE, it is possible to obtain a stable NR against fluctuations in VE.

Furthermore, equation (23) can be expressed as the following equation.

In this equation (24), -63・r2・Ns is the zero point, so to test the analog section, (24
) may be adjusted so that the first term of the equation (19) becomes equal to the equation (19).

Next, if the digital output when substituting equations (2) and (6) into equation (11) is NR(0), then =-G3・
r2・Ns ・・・・・・・・・・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・・・・
(27), and equation (27) is (F
(Z)/F n) - When K+ = O, it matches that of NR.

In load cell 2, (F (Z)/F n)・K
If the bridge balance of the load cell 2 is adjusted so that the ratio is 20 to 10, then Np and <o>=NR. To do this, a regulating resistor 4, RZ 1, is used.

Normally, the operation of the scale is such that the microcomputer system 15 instructs the analog switch 5 to turn on the switch S1, and the signal from the load cell 2 is input to the load cell signal amplifier 9. 5 points of the scale at this time
AN is expressed by equation (19).

To test the operation of the analog section, it is sufficient to test whether this 5PAN is a specified value. Therefore, in order to test the operation of the analog section, the first switch S2 is turned ONL, and NR(0) expressed by equation (27) is stored in the microcomputer system 15. Next, switch S3 is turned on and NR(A
) is stored in the microcomputer system.

Therefore, in order to calculate 5PAN, the microcomputer system calculates according to the formula: S PAN = NR<^)-NR...-song, between songs, between songs, between songs, between songs, and between songs (2g). Then, it is determined whether the result satisfies the specified value or not. If the result does not satisfy the specified value, an error is displayed and the scale is set to an unusable state.

Such operations are automatically performed at appropriate intervals during use.

Effects of the Invention The present invention inserts a bridge balance correction resistor into the bridge circuit of the load cell and sets this resistance value to a predetermined value, thereby adjusting the relationship between the load cell load and the digital value of the output to a zero point. It can be changed as a central slope change, so that the zero point will not shift even if the span is adjusted.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention, and FIG. 1 is a block diagram, and FIG. 2 is a graph showing the relationship between load and digital value. 2... Load cell, 4... Adjustment resistor (resistance for bridge balance correction) Applicant Tokyo Electric Co., Ltd.

Claims (1)

[Claims] In the device in which the output of the load cell is amplified by an analog section and then converted to a digital signal by an A/D converter to obtain weight data, a bridge balance correction resistor is connected to the bridge circuit of the load cell, and a resistor is connected to the bridge circuit of the load cell to apply a voltage to the load cell. The load cell output generated by the weight of the plate and the structure supporting it is adjusted to zero by the bridge balance correction resistor, so that the zero point does not change when the span of the scale is adjusted. Features: Zero point adjustment method for load cell scales.