JPH0216841B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a Wheatstone bridge circuit (hereinafter simply referred to as a "bridge circuit") composed of strain gauges.
It is called. ) strain gauge and a strain gauge disconnection detection device for detecting disconnection of its input/output lead wires. Originally, in a bridge circuit consisting of strain gauges, the conditions required for detecting disconnection are not only the disconnection of the four gauges but also the unbalance of the input lead wire that excites the bridge circuit and the bridge circuit due to the structure of the bridge circuit. It is also possible to detect a disconnection in the output lead wire that takes out the voltage. However, conventionally, there has been no suitable disconnection detection means that satisfies these requirements. For this reason, for example, even if one or more of the strain gauges that make up the bridge circuit break, the abnormality will not be detected.
In many cases, this was considered to be a normal measurement. By the way, the following method can be considered as a wire breakage detection means that can satisfy the above-mentioned requirements. In other words, disconnection of the gauge and input lead wire is detected by measuring the bridge excitation current from the input lead wire side of the bridge circuit and determining whether the current value is within a certain range. Regarding breakage of the lead wire, a type of unbalanced voltage generation circuit called a burnout circuit is inserted on the output lead wire side of the bridge circuit, and when the output lead wire breaks, an excessive voltage is input to the measurement display system and the display is displayed. Disconnection is detected by overscaling. FIG. 1 shows the configuration of an example of a strain measuring device employing such a method. In Fig. 1, 1 is the gauge resistance R of four gauges G ₁ ,
This is a bridge circuit consisting of G ₂ , G ₃ , and G ₄ .
2a, 2b, 2c, 2d are the input/output terminals of the bridge circuit 1.
Connecting terminals a, b, c, d of the strain measuring device 3 which are output lead wires and have a built-in gauge disconnection detection device
are connected to each other. Input lead wires 2c, 2d
, a bridge power supply E with an excitation voltage E and an excitation current detection resistor Rs inserted in series with the bridge power supply E.
is connected. Differential amplifier 4 that amplifies the voltage generated across the resistor Rs by the bridge excitation current
The output of the first comparison/judgment circuit 5 is input to the first comparison/judgment circuit 5, and the output of the first comparison/judgment circuit 5 is input to the alarm 7 via the OR circuit 6. On the other hand, a voltmeter 8 is connected to the output lead wires 2a and 2b to measure the bridge unbalance voltage generated due to the original strain.
A display 9 is connected to display the measured value of the voltmeter 8 as an amount of strain. In addition, in order to detect disconnection of the output lead wires 2a and 2b, a voltmeter 8
It consists of resistors R ₁ , R ₂ , and R ₃ on the input side of ±V ₀
The burnout circuit 10 is supplied with a reference voltage of
Further, the output of the voltmeter 8 is input to a second comparison judgment circuit 11 for detecting excessive input voltage from the burnout circuit 10, and the output is sent to the alarm 7 via the OR circuit 6. is guided by. Next, the operation of the apparatus configured as described above will be explained. First, the detection of disconnection of the gauges _G1 to _G4 and the input lead wires 2c and 2d will be described. Under normal conditions, the bridge excitation current is =E/R+Rs≒E/R (where R≫Rs), so the voltage Es generated at Rs is Es=Rs=Rs/RE. On the other hand, when considering the combinations of disconnections that may occur in the gauges _G1 to _G4 and the input lead wires 2c and 2d, the excitation current due to the disconnection becomes the largest when any one of the gauges is disconnected. At this time, the excitation current I' is I'=E/2R+Rs≒E/2R, and the voltage E's generated at Rs is E's='・Rs=Rs/2R・E=Es/2, and the wire is disconnected. The excitation current during normal operation is less than 1/2 of the excitation current during normal operation. Therefore, the voltage generated in the resistor Rs is detected by the differential amplifier 4, and the impedance is converted, so that the output can be used advantageously in circuit design.
The magnitude is given to the first comparison judgment circuit 5 which has a threshold value of Es+E's/2=3/4・Rs/RE, which is the intermediate voltage value between the voltage Es during normal operation and the maximum voltage E's at the time of disconnection. Performs judgment and detects disconnection. The first comparison/judgment circuit 5
When the output of the differential amplifier 4 is larger than the threshold value of 3/4 Rs/RE, an affirmative signal indicating normality detection is output, and when it is smaller than the threshold value, a negative signal indicating disconnection detection is output. If a negative signal is output, OR circuit 6
The signal is inputted to the alarm device 7 via a lamp and a buzzer to issue an alarm to alert the operator. Next, detection of disconnection of the output lead wires 2a and 2b will be explained. When the output lead wires 2a and 2b are normal, if the signal source impedance R of the bridge circuit 1 is sufficiently small compared to the signal source impedance 2R ₁ + R ₃ of the burnout circuit 10, then (2R ₁
+R ₃ >>R) The voltage supplied from the burnout circuit 10 to the input of the voltmeter 8 can be approximated to zero. On the other hand, if at least one of the output lead wires 2a, 2b is disconnected, the signal source impedance of the bridge circuit 1 becomes infinite, and a maximum of 2R ₃ /2R ₂ +R ₃ · V ₀ is applied from the burnout circuit 10 to the input of the voltmeter 8. An excessive voltage higher than the measured strain voltage occurs, and the voltmeter 8
The output of is saturated and the display 9 is overscaled. (However, each constant of the voltmeter 8 is set in advance so that the output of the voltmeter 8 is saturated in the event of excessive voltage.) Check whether the output of the voltmeter 8 is saturated or not.
The second comparison/judgment circuit 11 makes a decision, and when it is not saturated, an affirmative signal indicating normality is output, and when it is saturated, a negative signal indicating a disconnection is output, which is given to the alarm 7 via the OR circuit 6 as described above. . By the way, the device configuration shown in FIG. 1 has the following drawbacks. (a) Since the excitation current changes depending on the gauge resistance and the input lead wire resistance, the threshold value of the first comparison judgment circuit 5 is set based on the gauges G ₁ to _{G 4} and the input lead wire 2.
It is necessary to reset each time according to the resistances c and 2d. (b) Resistor R ₁ configuring the burnout circuit 10
The influence of the drift voltage generated in R ₃ and the reference voltage V ₀ appears as an error in the indicated value of the originally intended amount of strain, leading to a decrease in measurement accuracy. (c) The signal source impedance of the burnout circuit 10 is made sufficiently large compared to the gauge resistance during normal operation so that the inserted voltage by the burnout circuit 10 does not occur as a bridge unbalanced voltage. Make it sufficiently small compared to the input impedance of voltmeter 8 so that an excessive input voltage is generated at the input of voltmeter 8.
There is a restriction that impedance matching must be achieved between the gauge resistor, the burnout circuit 10, and the voltmeter 8. (d) Disconnection detection means must be provided on both the input and output sides of the bridge circuit 1, making the circuit complex. The present invention has been made in view of these circumstances, and it is possible to bridge the bridge without reducing measurement accuracy or causing problems such as impedance matching, regardless of the gauge resistance or the cable resistance of the input lead wire. It is an object of the present invention to provide a strain gauge disconnection detection device that can reliably detect disconnection of a gauge and an input/output lead wire only on the output side of a circuit. That is, the feature of the present invention is that a bridge excitation voltage detection circuit is provided on the output side of the bridge circuit, and when the bridge circuit is switched and excited with two different excitation voltages, the bridge excitation voltage detection circuit detects the By determining whether each value corresponds to the excitation voltage of the bridge power supply that actually excited the bridge circuit, disconnection of the gauge and its input/output lead wires can be detected from the output side of the bridge circuit. be. First, the principle of wire breakage detection in the present invention will be explained. FIG. 2 shows a bridge circuit 1 composed of four strain gauges _G1 to _G4 . Each strain gauge G ₁ ,
When the resistance of G ₂ , G ₃ , and G ₄ is R, and an excitation voltage E is applied from terminals c and d through input lead wires 2 c and 2 d, the strain gauge G ₁ , which is proportional to the amount of strain ε,
A resistance change of -△R occurs in G ₃ and a resistance change of +△R occurs in strain gauges G ₂ and G ₄ , and an unbalanced voltage e as shown in the following equation is generated at terminals a and b via output leads 2a and 2b. can get. e=Va−Vb=Ks・ε・E …(1) (Ks is the gauge factor of the strain gauge) Figure 3 shows the equivalent unbalanced voltage when the bridge circuit in Figure 2 is viewed from terminals a and b. This shows the circuit, and the voltages Va and Vb at terminals a and b are the difference between the common-mode voltage component E/2 of the bridge excitation voltage shown by the following equations (2) and (3), and the unbalanced voltage proportional to strain. It consists of a dynamic voltage component e/2. Va=E/2+e/2...(2) Vb=E/2-e/2...(3) Figures 2 and 3 show an example of a general bridge circuit consisting of four gauges. However, 1
A bridge circuit configured with a single gauge or two gauges basically yields an unbalanced voltage proportional to strain, as in equation (1), and Va and Vb also correspond to the bridge excitation voltage as in equations (2) and (3). The bridge circuit may consist of an unbalanced voltage component, and may be composed of one gauge or two gauges. The following table shows the gauges G ₁ to _{G 4} and their input/output leads 2a to 2 in the bridge circuit 1 of Fig. 2.
This figure shows the combinations in which disconnection of wire d can occur, and the values of the voltages Va and Vb of terminals a and b and the disconnection unbalance voltage e at the time of each disconnection. (In this specification, the expression disconnection unbalanced voltage is used to distinguish it from the unbalanced voltage of the bridge circuit output, which is originally expressed by equation (1) and is generated due to distortion during normal operation.)

【table】

[Table] As shown in this table, wire breaks can be broadly classified into the following ○

Claims (1)

[Claims] 1. In a strain gauge disconnection detection device for detecting disconnection of a strain gauge and its input/output lead wire in a strain measuring device using a Wheatstone bridge circuit configured with a strain gauge, two types of excitation voltages are used. a bridge power supply capable of outputting a power supply voltage; a first switch means for switching between the two types of excitation voltage of the bridge power supply and supplying the same to the Wheatstone bridge circuit; an excitation voltage detection circuit that detects the
first and second comparison and judgment circuits which switch and input the detected voltage of the excitation voltage detection circuit by a switch means and compare it with a set value to determine a disconnection; and judgment outputs of these first and second comparison and judgment circuits. 1. A strain gauge disconnection detection device comprising: a circuit that receives a signal and outputs a disconnection detection signal when at least one of the signals is determined to be disconnected. 2. In the strain gauge disconnection detection device according to claim 1, the excitation voltage detection circuit includes a pair of buffer amplifiers and a summing amplifier that adds the outputs of the pair of buffer amplifiers to obtain a detection output. A strain gauge disconnection detection device featuring: 3. In the strain gauge disconnection detection device according to claim 1, the excitation voltage detection circuit includes a pair of buffer amplifiers and an A/D for converting the outputs of the pair of buffer amplifiers into digital values.
A strain gauge disconnection detection device comprising a converter and a digital calculator that calculates these A/D converted values and obtains a detection output. 4 Any one of claims 1 to 3
In the strain gauge disconnection detection device described in
A strain gauge disconnection detection device characterized in that the first and second comparison and determination circuits have a common configuration and are configured to switch only the comparison set value in conjunction with the second switch means. 5. A bridge power supply capable of outputting two different excitation voltages in a strain gauge disconnection detection device that detects disconnection of a strain gauge and its input/output lead wire in a strain measurement device using a Wheatstone bridge circuit configured with a strain gauge. a switch means for switching between the two types of excitation voltage of the bridge power supply and supplying the same to the Wheatstone bridge circuit; An excitation voltage detection circuit that detects and outputs one output voltage of the output terminal of the Tsuji circuit, and a first comparison judgment circuit that compares the detected output of the one output voltage of the excitation voltage detection circuit with a first set value. a storage means for storing the detected output of the excitation voltage of the excitation voltage detection circuit every time the excitation voltage supplied by the switch means is changed; an arithmetic unit that calculates the difference between the detected values of the excitation voltage; a second comparison and judgment circuit that compares the output of this arithmetic unit with a second set value; and judgment outputs of these first and second comparison and judgment circuits. 1. A strain gauge disconnection detection device comprising: a circuit that receives a signal and outputs a disconnection detection signal when at least one of the signals is determined to be disconnected. 6. The strain gauge disconnection detection device according to claim 5, characterized in that the excitation voltage detection circuit includes a pair of buffer amplifiers and a summing amplifier that adds the outputs of the pair of buffer amplifiers. Strain gauge disconnection detection device. 7 In the strain gauge disconnection detection circuit according to claim 5, the excitation voltage detection circuit includes a pair of buffer amplifiers and an A/D for converting the outputs of the pair of buffer amplifiers into digital values.
A strain gauge disconnection detection device comprising a converter and a digital calculator that calculates these A/D converted values. 8 Any one of claims 5 to 7
In the strain gauge disconnection detection device described in
A strain gauge disconnection detection device characterized in that the first and second comparison and determination circuits have a common configuration and are configured to switch only the comparison set value in conjunction with a switch means.