JPS58212207A - Zero point correcting device of voltage amplifying circuit - Google Patents

Abstract

PURPOSE:To attain zero correction for a short time, by comparing a voltage amplifying output of a measuring point with the 2nd reference point potential, storing a comparison output in a capacitor while a zero correcting switch is depressed and feeding it back to the other input of a measuring point voltage amplifying circuit. CONSTITUTION:In an electronic scale using a load cell Rx for example, the cell Rx is connected in series with a resistor R1 between power supplies +V and -V and a center point B is connected to an inverting input terminal 1 of an operational amplifier OP1 through an R7. Further, a potential of a connecting point E between resistors R4, R5 connected to the power supplies +V, -V is given to a non-inverting input of the amplifier OP1, and a feedback resistor R6 is provided to the amplifier OP1. An output C of the amplifier OP1 and a potential at a point F are compared at an amplifier OP2, and the output charges up a capacitor C1 via a switching element S and a resistor R2. The potential of the capacitor C1 is fed back to the non-inverting input of the amplifier OP1 via a voltage follower OP3. The zero correction is performed for a short time by operating the element S in this way and the weight is measured with the output C.

Description

DETAILED DESCRIPTION OF THE INVENTION 27-' The present invention relates to a device for correcting zero point drift in an amplification circuit using a load cell or an amplification circuit such as a thermometer.

In cases where a load cell is used, the output of the load cell is very small, so the degree of amplification of the amplification circuit is usually as high as 100 times or more. In such a case, if there is a drift at the zero point, the drift is also amplified, and the output of the amplification circuit is therefore saturated, making it impossible to measure. To prevent this, conventional methods have been used to select circuit elements that cause drift, such as resistors, operational amplifiers, and operating temperatures, to reduce variations in these components, or to select individual components that have minimal drift. The above problem has been addressed by constructing a circuit using expensive components.

In order to eliminate these drawbacks, the present invention automatically performs zero correction in the amplifier circuit during zero adjustment, and continues the zero correction only during a short measurement time, thereby solving the above problems at low cost. Provide a method to remove it.

3. A voltage comparison circuit is provided to compare the output of the voltage amplification circuit that amplifies the voltage at the second measurement point with the potential of the second reference point, and the output voltage of this voltage comparison circuit is adjusted to zero when the zero correction switch is pressed. This voltage is stored in a capacitor during the voltage amplification circuit, and this voltage is constantly fed back to the other input of the voltage amplification circuit, thereby performing zero correction for a short period of time.

FIG. 1 shows an example in which the present invention is implemented in an electronic scale using a load cell Ibc.

In particular, the present invention is suitable for simple applications such as a microwave oven equipped with such a scale, where measurement as a scale can be completed in a short time and operations such as tare subtraction are necessary. .

First, the load cell Rx is connected to the power supply +V in series with the fixed resistor R1.
The midpoint (point B) is connected to the inverting side input terminal 1 of the operational amplifier oP1 through a resistor R7. In addition, the connection point E of resistors R4 and R5 connected between 1 power supply +V and -■:1 is resistor R
8 to the non-inverting side input 2 of the operational amplifier OP1. Further, a feedback resistor R6 is connected between the inverting input 1 and the output 3 of OPl.

In the above state, the voltage at point B is inverted and amplified using the voltage at point E as a reference and outputted to output 3. Now, as described above, this part is a voltage amplification circuit that amplifies the measurement point (point B) potential with respect to the reference point potential (point E), and its output is basically A. Amplifies the difference between the point potential and the B point potential.

Although the inversion amplification circuit is explained in FIG. 1, it may be configured as a differential amplification circuit.

Now, if you want to perform tare subtraction etc. as is, Rx
Since the resistance value of is different from that when there is no load, the voltage amplification circuit has already outputted an output corresponding to a certain weight to the zero point. This is the same even when there is no load and when the balance of each resistance is lost. However, the control circuit (not shown, the part that uses a microcomputer etc. to calculate weight, display, etc.) that will be connected after the zero point assumes that the second reference point potential (point F) is zero weight. In order to overcome this drawback, when the voltage appears at the 0 point as shown above and the weight becomes zero, the control circuit calculates the voltage at this point. It is possible to convert the weight to zero, but in order to do so, the range of variation must be calculated by adding the following three factors: (required measured weight) + (tare weight) + (variation due to zero point drift) A measurement system with a dynamic range of is required. in general,
Increasing the dynamic range reduces precision, making the circuit unusable with inexpensive methods.

Therefore, the present invention provides a means to improve accuracy by limiting the dynamic range to only the range of necessary measurement weights, and the purpose can be achieved by simply adding the circuit described below to the above-mentioned circuit. .

First, it is obvious that it is better to make the voltage at the output point C equal to the second reference point potential F point during tare subtraction or zero point correction. It is well known that this can be done by changing the voltage of The present invention utilizes this principle6.-2.

However, its feature is that it automatically performs zero correction.

First, the operational amplifier OP2 compares the 0 point output and the F point potential.
A voltage comparator circuit using a voltage comparator, whose output point G is connected via a switching element S to a series circuit of a resistor R2 and a capacitor C1 between the power supply -2. Further, the potential at the connection point between the resistor R2 and the capacitor C1 is transferred to the reference side input point A of the voltage amplification circuit through the resistor R3 via a voltage follower P3 provided for impedance conversion.
It has been returned to the point.

Now, the switching element S is a switch that is closed when a switch such as a tare subtraction switch is pressed.At this time, if the potential at point C is higher than the second reference point potential at point F, the output of the voltage comparison circuit becomes low. Therefore, since the switching element S is closed, the voltage across the capacitor C1 is lowered through the resistor R2.

Therefore, the potential at the reference side input point A of the voltage amplifier also becomes low through the voltage follower P3.

Therefore, the output of the voltage amplifier becomes low, and finally 7.
〜−・ Matches point F.

At this time, if switching element sl is opened, the voltage at that point in time (that is, when the output of the voltage amplification circuit becomes equal to point F) is held in capacitor C, and the voltage is applied to point A via the voltage follower. Because it continues to be
The output at this point becomes zero weight. From now on, if you apply a load to the scale, the corresponding output will be at the 0 point, so you can measure weight.

Note that as the charge stored in the capacitor C1 is discharged as desired, the zero point moves, but there is no problem as long as the measurement is completed within the allowable error range.

As described above, the present invention stores all the voltage necessary for correction in the capacitor only during zero correction such as tare subtraction, and even after the correction is completed, zero correction is carried out by feeding back the voltage stored in the capacitor to the voltage amplification circuit. Easy to do 11...: This is a simple method.

FIG. 2 shows another embodiment of the present invention using a one-chip microcomputer with a built-in analog-to-digital converter.

1o is a microcomputer. Inside it,
It incorporates many circuits, including an analog-to-digital converter shown at 11, memory, a control section, and an output buffer 12, and operates according to programmed procedures. Using this function, some functions of the circuit shown in Figure 1 can be implemented in a microcomputer.
It can be processed internally.

In Figure 2, Rx, R1, R4, R5, R71
Re, R6, Rs, OPl, OF2, R
2. CI is the same as in FIG. Now, the output of the voltage amplifier is connected to the input 13 of the analog-to-digital conversion circuit of the microcomputer, and the voltage is read by the analog-to-digital conversion circuit.

Trl and Tr2 are turned on or off under the control from the microcomputer, and inside the microcomputer 10, the voltage at the 0 point and the voltage at the second reference point F are changed according to the program. ', and depending on the size, the transistor Tr1
．． Turn Tr2 on or off. Therefore, the part composed of these programs is the voltage comparator circuit OP in Figure 1.
Corresponds to 2.

In the figure, the part that functions as this is marked 13.

By using a microcomputer in this manner, several functions can be housed inside the microcomputer, but the operation is exactly the same as that described in FIG. 1.

As explained above, according to the present invention, the following effects can be expected.

(1) Since a switching element that is closed when performing zero point correction is used and voltage is stored in the capacitor so that the voltage during this closed period becomes zero, arbitrary zero point correction can be performed.

(2) Automatic zero correction can be easily performed.

10,... FIG. 2 is a circuit diagram showing another embodiment of the present invention.

oPl...Operation amplifier forming a voltage amplification circuit, OF2...Operation amplifier forming a voltage comparison circuit, S...Switching element.

Claims (1)

[Scope of Claims] The measuring point potential is connected to either the inverting or non-inverting of an amplifier circuit having an inverting or non-inverting input for amplifying the measuring point potential (point B) with respect to the reference point potential (point E). and another inversion circuit with the reference point potential connected to the other input. A voltage comparator circuit in which another reference point potential (point F) is connected to one input of another amplifier circuit having a non-inverting input, and the output (point 0) of the voltage amplifier circuit is connected to the other input. , A series circuit of a switching element S, a resistor (R2), and a capacitor C1 is connected to the output of this voltage comparison circuit, and the voltage at the connection point (point D) of this capacitor C1 and resistor R2 is expressed via a voltage follower. A zero point correction device for a voltage amplification circuit that feeds back the reference point potential of the voltage amplification circuit to a connected input.