JPH0447761B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a strain measurement circuit using a semiconductor strain gauge.

[Technical background of the invention and its problems]

A conventional example of a strain measurement circuit using a semiconductor strain gauge is shown in FIG. In the figure, 1 is a bridge circuit made up of a semiconductor strain gauge 1A, 2 is a temperature compensation voltage generation circuit, and has a transistor 2A and a resistor 2.
B and 2C, and is connected between the bridge circuit 1 and a constant voltage source. 3 is a constant voltage diode (Zena diode) that obtains a constant voltage as the power supply voltage of the bridge circuit 1; 4 is an operational amplifier 4;
This is a differential amplifier circuit using A, and its input terminal is connected to the output terminal of the bridge circuit 1.

In this strain measurement circuit, a power supply voltage is applied to a bridge circuit 1 via a temperature compensation voltage generation circuit 2, and the bridge drive voltage is determined by the function of the temperature compensation voltage generation circuit 2, that is, when the temperature rises (or decreases). Then, the voltage drop decreases (or increases) and increases or decreases depending on the temperature change due to the voltage adjustment effect. This compensates for the decrease in the gauge factor of the semiconductor strain gauge 1A as the temperature rises, and eliminates sensitivity changes due to temperature of the strain gauge. The output of the bridge circuit 1 is sent to the differential amplifier circuit 4.
It is amplified by , and becomes the output voltage e ₀ .

Temperature compensation is performed in this way to prevent sensitivity changes due to temperature of the strain gauge 1A, but if there are variations in the resistance temperature characteristics of the semiconductor strain gauge 1A, the output of the bridge circuit 1 will change even if the strain is zero. As shown by the solid line or dotted line in FIG. 2, it changes as the temperature changes. That is, a zero point temperature error will occur.

In order to compensate for this, as shown in FIG. 3, a resistor 1B is connected in series on one side of the bridge, and a resistor 1C is connected in parallel on the other side.

By attaching resistors 1B and 1C in this way, the zero point temperature error can be made zero, but when making adjustments, it is necessary to change both resistors 1B and 1C in conjunction with each other, and resistors with various resistance values can be used. Since it must be prepared for each product, process management becomes complicated.

[Purpose of the invention]

An object of the present invention is to provide a strain measurement circuit that can easily adjust zero point temperature characteristics with a simple configuration.

[Summary of the invention]

The present invention provides a strain measurement circuit in which a power supply voltage is applied to a bridge circuit having semiconductor strain gauges on at least two sides via a temperature compensation voltage generation circuit, and the output of the bridge circuit is amplified by a differential amplifier circuit using an operational amplifier. A variable resistor is connected between the two input terminals of the operational amplifier, and a reference voltage circuit is provided, and the slider of the variable resistor is connected to this reference voltage circuit via a resistor. The ratio of the drive voltage of the bridge circuit to the reference voltage is set to a constant value, and the output terminal potential of the bridge circuit is adjusted to be equal to the reference voltage using a variable resistor to achieve the required zero point temperature characteristics. be.

[Embodiments of the invention]

FIG. 4 shows an embodiment of the present invention, in which 1 is a bridge circuit composed of 1A of semiconductor strain gauges, 2
is a temperature compensation voltage generation circuit, 3 and 3' are Zener diodes, and 4 is a differential amplifier circuit using an operational amplifier 4A, and the output terminal of the bridge circuit 1 is connected to the input terminal of the operational amplifier 4A, respectively. A variable resistor 5 is connected between both input terminals.

By connecting the Zener diode 3' in series with the Zener diode 3, the Zener diode 3' constitutes a part of a constant voltage source as a power source for the measuring circuit, and also constitutes a reference voltage circuit. The reference side input terminal (non-inverting input terminal) of the operational amplifier 4A is connected to this reference voltage circuit via a resistor, and the slider of the variable resistor 5 is connected via a resistor 6. There is.

Note that the reference voltage circuit sets the reference voltage e _r so that the ratio of the driving voltage of the bridge circuit 1 to the reference voltage e _r at room temperature is 2:1.

Next, we will discuss the effect. However, regarding the generation of output voltage with respect to distortion, it is similar to the conventional example except that in the conventional example, the output voltage e ₀ is output with respect to zero potential, and in this example, it is output with respect to the reference voltage e _r . The same is true, and the zero point temperature compensation effect will be explained here.

Regarding zero point temperature compensation, the differential amplifier circuit 4 in FIG. 4 can be treated as the circuit shown in FIG. 6. Since we consider the case where the strain is zero, ea=e _b =e ₂ , e ₁
= e _r , the correspondence of e ₃ −e ₁ = e ₀ is established. Also, R/α,
R/β indicates the action of the variable resistor 5 and the resistor 6, and when α=0, the slider is connected to the operational amplifier 4.
On the inverting input terminal side of A, β=0 corresponds to when the slider is turned to the non-inverting input terminal side, and α=β corresponds to when the slider is at the center.

Each voltage in FIG. 6 is expressed by the following formula.

e ₀ = e ₃ − e ₁ = k (β − α) / 1 + k − α・(e ₂ − e ₁ )
...(1) On the other hand, the driving voltage of the bridge circuit 1 is twice the reference voltage e _r at room temperature, and since it changes depending on the temperature, it is expressed as 2e _r +γ·T=2e ₁ +γ·T. However, γ is a proportionality constant, and T is a temperature difference from room temperature.

Since e ₂ is 1/2 of the bridge drive voltage, e ₂ =
e ₁ +γ·T/2. Therefore, by substituting into equation (1), e ₀ = e ₃ − e ₁ = k(β−α)/1+k+α・γ/2・T
…(2) becomes. From this equation (2), when the strain is zero and the ambient temperature changes, the output e ₀ changes as shown in FIG. 5 in response to the values of α and β.

This appropriately selects the values of α and β for the zero point temperature error of the bridge circuit 1. This means that the zero point temperature error can be eliminated by changing the position of the slider of the variable resistor 5. Moreover, at room temperature, the output e ₀ is T=0, so α,
It is zero regardless of the value of β, and therefore adjustment is easy because it is only necessary to adjust the variable resistor 5 to make the output voltage e ₀ zero when the temperature becomes high or low.

FIG. 7 shows another embodiment of the present invention, in which the reference voltage circuit is constituted by a voltage divider formed by resistors 7A and 7B, and the constant voltage circuit is constituted by a Zener diode 3.

Even with this configuration, the potential at the point where the slider of the variable resistor 5 is connected, that is, the connection point of the resistor 6
e _e is the potential e _c of the two input terminals of the operational amplifier 4A,
The same potential as e and _d is the same as in the previous embodiment, and zero point temperature compensation is performed in the same way. However, in the case of FIG. 7, since the current i flows out from the output terminal of the bridge circuit, a load is applied to the bridge circuit.

〔Effect of the invention〕

A variable resistor 5 is connected between the two input terminals of the operational amplifier 4A in the differential amplifier circuit 4, and a reference voltage circuit is provided that generates a reference voltage that has a constant ratio with the driving voltage of the bridge circuit at room temperature. By simply connecting the slider of the variable resistor to this reference voltage circuit via a resistor, in other words, when the potentials e _a to e _d of each part in Fig. 4 are at room temperature and the distortion is zero, they all become the reference voltage. The circuit configuration is very simple because it is simply configured to be equal to e _r . Moreover, since it is only necessary to adjust the position of the slider of the variable resistor so that the output voltage becomes zero when the strain is zero, the adjustment to eliminate the zero point temperature error is easy. In addition, since the amount of compensation for zero point temperature error can be continuously changed using a variable resistor, there is no need to prepare resistors with many resistance values for each product, contributing to simplified process control. can.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a conventional example of a strain measurement circuit, Fig. 2 is a zero point temperature characteristic diagram of the same strain measurement circuit, Fig. 3 is a circuit diagram showing an example of compensation means for zero point temperature error, and Fig. 4 is a circuit diagram showing an example of compensation means for zero point temperature error. A circuit diagram showing an embodiment of the strain measurement circuit according to the present invention, FIGS. 5 and 6 are characteristic diagrams and a circuit diagram for explaining the zero point temperature compensation effect of the embodiment, and FIG. FIG. 7 is a circuit diagram showing another embodiment. 1...Bridge circuit, 1A...Semiconductor strain gauge, 2
...Temperature compensation voltage generation circuit, 3 and 5...Zena diode, 14...Differential amplifier circuit, 4A...Operation amplifier, 5...Variable resistor, 6, 7A and 7B...Resistor.

Claims (1)

[Claims] 1. A bridge circuit having semiconductor strain gauges on at least two sides, a temperature-compensated voltage generation circuit connected between the bridge circuit and a power supply, and a differential amplifier using an operational amplifier to amplify the output of the bridge circuit. In the distortion measurement circuit, a variable resistor is connected between the two input terminals of the operational amplifier, and a reference voltage circuit is provided that generates a reference voltage having a predetermined ratio to the driving voltage of the bridge circuit at room temperature. , A strain measuring circuit characterized in that a slider of the variable resistor is connected to the reference voltage circuit via a resistor.