JPS61164124A - Self-diagnosing circuit - Google Patents

Abstract

PURPOSE:To improve the accuracy of self-diagnosis, by connecting a correction circuit and drifting quantity detecting circuit to the bridge circuit of a load cell and outputting a constant voltage which is free from the influence of output voltage of the load cell. CONSTITUTION:A correction circuit 20 composed of a primary-stage amplifier 21, amplifier 22, resistances R01-R03, etc., and a drifting quantity detecting circuit 3 are connected to the output side of a bridge circuit 1. When a switch SW1 is turned off and another switch SW2 is turned on under an ordinary condition, the output of a load cell is inputted in the primary-stage amplifier 21 without receiving any influences from the correction circuit 20 and detecting circuit 3. Since a constant voltage which is free from the influence of the output voltage V0 of the load cell is outputted from the primary-stage amplifier 21 when the switch SW1 is turned on and switch SW2 is turned off at the time of a self-diagnosis, a precise and accurate self-diagnosis which is free from the influence of temperature can be performed, if the voltage is set so that it can correspond to the output of the load cell when a reference counterweight is loaded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a self-diagnosis circuit that is capable of self-diagnosing defects in amplifier circuit systems of electronic scales, combination weighing devices, and the like.

(Prior art and its problems) There are various types of failures in measuring devices such as electronic scales and combination weighing devices, but failures in the amplifier circuit system in particular cannot be determined from the outside, so they may exceed the allowable range. Even if there is a measurement error, there is a risk of using it as is without noticing it. For this reason, in the past, users would occasionally load test weights and check whether the scale was normal based on the weight displayed at that time. This is extremely troublesome, and improvements in this respect have been desired.

Therefore, the present applicant proposed the invention of a circuit capable of self-diagnosing defects in the amplifier circuit system of a measuring device in Japanese Patent Application No. 58-24751.
No. 3 and Japanese Patent Application No. 58-228961.

However, the technology disclosed in the above application can only roughly check the circuit system, and does not take into consideration checking whether the circuit system is operating with the required precision.

(Objective of the Invention) The purpose of the present invention is to solve the problems of the prior art and provide a self-diagnosis circuit that can check with necessary accuracy whether or not the amplifier circuit system is operating normally. That is.

(Summary of the Invention) The self-diagnosis circuit of the present invention includes a load cell that outputs an electric signal proportional to the load, a drift amount detection circuit that detects the amount of drift of the output terminal voltage of the load cell, and amplifies the output signal of the load cell. An operational amplifier and a switch for self-diagnosis of the circuit system are provided, and when the self-diagnosis of the circuit system is performed by switching the switch, the output voltage of the load cell is changed from the output terminal of the operational amplifier according to the detected amount of drift. A correction means is provided to output a constant voltage without any influence.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. FIG. 6 is a circuit diagram illustrating the present invention in detail. In the figure, the output terminal of the bridge l of the load cell is connected to the operational amplifier 2.
A feedback resistor Rf is connected between the negative input terminal 2b and the output terminal 2c of the operational amplifier, and the positive input terminal 2a of the operational amplifier 2 is connected to the ground. A voltage dividing resistor Rd having the same value as the feedback resistor Rf described above is connected between the two, and a differential amplifier circuit is formed by the operational amplifier 2 and the bridge 1.

The bias resistor R is connected between the switch SW and the feed resistor Rf, and when the switch SW is closed, the output current of the bridge I and the current flowing through the bias resistor R flow to the feedback resistor Rf, and the operational amplifier Acts as an amplifier.

Further, to the output side of the operational amplifier, a low-pass filter (not shown) for flicker prevention 11-, an A/D converter that converts analog signals into digital signals, and a microcomputer that performs various calculation processes are connected. .

Self-diagnosis (self-check) of such a circuit involves adding the current flowing through the bias resistor H (for example, the current corresponding to the output of the load cell when a reference weight is loaded) to the output of the load cell when no load is loaded, and calculating the current value at that time. A defect in the amplifier circuit system (circuit system from the preamplifier to the input end of the A/D converter) is determined from the output value of the A/D converter.

However, in a bridge circuit in which a temperature-sensitive resistor (Rm) is inserted on the input side as shown in the figure, the potential (V) at both output ends of the load cell changes due to a change in the resistance value due to temperature. Therefore, −h bias voltage (V
Due to the change in bias), the current flowing through the bias resistor H during self-check changes, and this appears as an error.

That is, Vex=Vbias+V -.-■ Here, -Vex is a DC voltage and is a constant value, and both Vbias and V change.

Therefore, in the present invention, the amount of drift in the potential at the output end of the load cell, which changes depending on the temperature, is detected, and the current applied to the first stage amplifier during self-check (current equivalent to the reference weight) is kept constant based on this detected amount. It has been corrected as follows.

FIG. 1 is a circuit diagram showing an example of the present invention.

As shown in the figure, a correction circuit 2° consisting of a first stage amplifier 21, an amplifier 22, a resistor Rot-RO 45, etc. and a drift amount detection circuit 3 are connected to the output side of the bridge circuit.

Now, assuming that the resistance R61''RO5 of the correction circuit can be considered according to the circuit constant of the drift amount detection circuit, this circuit operates as follows.Note that Ro1=R
Let O2=RO5.

(1) The drift amount detection circuit 3 detects the amount of drift (A − B) of the output terminal voltage of the load cell from the respective voltages input from the A terminal and the B terminal, and adds the output voltage of the load cell input from the A terminal to this. Add (A+(A-)
B)). However, while the value of this output signal can be changed depending on the circuit constant of the drift amount detection circuit, in the correction circuit 2o, the value of the input voltage (C) from the C terminal and from the drift amount detection circuit is Current addition is performed based on the input voltage (A+(A-B)) of , and as a result, the output voltage V is 5th order.

You can get u-shi.

((CVo)　10(A+　(A　B)　V.

)]...Q) (2) In the normal state, SW is off and SW2 is on, and the voltage Vo at the output end of the load cell is input to the A terminal and B terminal of the drift amount detection circuit, respectively. Ru. Therefore, from the drift detection circuit, A + (A B) = Vo + (Vo
A voltage of V0) =V0...■ is output, and on the other hand, the voltage V at the output end of the load cell is also output to the C terminal of the correction circuit. is input. As a result, the addition output from the correction circuit is expressed as follows.

((CVo) + (a+ (A B)
V.

)] = (Vo Vo )+ (Vo + (VO
-Vo)-■. )=0...■ Therefore, in the normal state, the output of the load cell is input to the first stage amplifier without being influenced by the drift amount detection circuit, correction circuit, etc.

(3) When performing a self-check, switch SW] to ON and SW2 to OFF. By this, Drift I
- The voltage of Vo is input to the A terminal of the quantity correction circuit, and the voltage of (172) V e x (any voltage value may be used), for example, is input to the B terminal. As a result, A+ (A
-B) -V. + (Vo-Vex/2)=2Vo-V
A voltage of ex/2...■ is output from the drift amount detection circuit.

On the other hand, the C terminal of the correction circuit has (R2/(R1+R2)
Since the voltage of IVex is input, the addition output from the correction circuit is -((c-vo)+ (A+ (A-B)-■.))] =-((R2/ (R1+R2))VexV o ) +
(2VoVeX/2Vo)-(R
2/(R1+R2)-(1/2))Vex...■.

In this way, when performing a self-check, the first stage amplifier can output a constant voltage of formula 0 that is not affected by the load cell's output terminal voltage V0, so this voltage can be used as the output of the load cell when a reference weight is loaded ( If the setting is made to correspond to the potential difference (potential difference at the output terminal), precise and accurate self-checks can be performed without being affected by temperature. Figure 2 shows the second embodiment of the present invention. FIG. 3 is a circuit diagram of a diagnostic circuit. In this example, resistor R8 in FIG.
1. Instead of RQ 2 + RO5, common resistor R8
and the drift amount detection circuit includes amplifiers 23 and 24.
are connected in two cascades. In addition, in each individual, the resistances surrounded by 0 marks in the figure, in this example, R, 2R, R1 + R2
+ RMs should each have good relative temperature characteristics.

FIG. 3 is a circuit diagram of a third embodiment of the present invention. In this example, the resistance on the negative input terminal side of the drift amount detection circuit is set to R/2, and the switch SW2 is connected between the switch SWl and the switch SW2 of the correction circuit. It is assumed that the resistances of the switches SWI and SW2 on the drift welfare detection circuit side can be sufficiently ignored compared to the resistance H.

FIG. 4 is a circuit diagram of a fourth embodiment of the present invention, which is a modification of the second embodiment, but the structure of the drift detection circuit is different. Note that the resistance values of the switches S1 and S2 are configured so as not to affect the circuit.

As described above, in the first to fourth embodiments, SW.

The SW2 can be operated manually or automatically. Further, either an electronic scale or a combination weighing device can be used.

FIG. 5 is a circuit diagram when the present invention is used in a combination weighing device. In this case, the self-check signal (SCK signal) output from the microcomputer etc. is sent to the self-check circuit of an empty weighing device when the weighing device starts operating or during a certain cycle when it is not filled with an object to be weighed. is input.

(Effects of the Invention) As explained above, in the present invention, it is possible to check with the necessary accuracy whether or not the amplifier circuit system is operating normally, and also to check the potential at the output end of the load cell, which changes with temperature. The amount of drift is detected and corrected using this detected amount so that the current applied to the first stage amplifier (current equivalent to the reference weight) during self-check remains constant, so it is accurate and unaffected by temperature. Accurate self-checks can be performed.

[Brief explanation of drawings]

1 to 5 are circuit diagrams of the present invention, and FIG. 6 is a circuit diagram illustrating the present invention in detail. l...bridge circuit, 2o...correction circuit, 3...
Drift a detection circuit, 21-25...Amplifier.

Claims (1)

[Claims] A load cell that outputs an electrical signal proportional to the load, a drift amount detection circuit that detects the amount of drift in the output terminal voltage of the load cell, an operational amplifier that amplifies the output signal of the load cell, and a switch for self-diagnosis of the circuit system. and a correction means for outputting a constant voltage from the output terminal of the operational amplifier, which is not affected by the output voltage of the load cell, according to the amount detected by the drift amount detection circuit when performing self-diagnosis of the circuit system by switching the switch. A self-diagnosis circuit characterized by: