JPS6144241B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a strain-to-electrical signal conversion device using a semiconductor strain gauge, and more particularly to a strain-to-electrical signal conversion device that compensates for temperature dependence.

[Conventional technology]

It is well known that various strain gauges are used to measure mass, stress, fluid pressure, and the like.

In recent years, there has been an increase in the use of highly sensitive semiconductor strain gauges that utilize the piezoresistance effect of semiconductors.

Semiconductor strain gauges have the advantage of a high rate of change in resistance with respect to the amount of strain, that is, a high gauge factor, but have the disadvantage that the resistance value of the gauge and the temperature dependence of the gauge rod are large.

To explain the temperature dependence in detail, the resistance value R of the semiconductor strain gauge is given by the following equation.

R=Ro(1+αT) {1+sγ(1+βT)} (1) In equation (1), Ro is the resistance value in the unstrained state at a given temperature, α is the temperature coefficient of the resistance value, T is the temperature of the strain gauge, and s is The amount of strain, γ is the gauge rod, and β is the temperature coefficient of the gauge factor. The polarity and value of a gauge rod vary depending on the crystal orientation of the semiconductor single crystal, the angle between the current and stress within the gauge, and other factors.

Expanding equation (1), we get the following equation.

R=Ro(1+αT)
+Ro(1+αT)(1+βT)S・γ (2) =Ro(1+αT)+Ro
{1+(α+β)T}s.γ (2a) The right-variant second term in equation 2 is the change in gauge resistance due to strain. On the other hand, α is + in the case of silicon single crystal depending on the impurity concentration in the crystal of the semiconductor strain gauge.
It has a value of 3000ppm to 600ppm/C°, and γ is regardless of the impurity concentration in the case of silicon single crystal.
It is about 2000ppm/C°. As is clear from the second term of equation (2a), the temperature dependence of the change in gauge resistance can be reduced by appropriately selecting the impurity concentration of the crystal and canceling out α and β. Therefore, a strain-to-electrical signal conversion bridge using a semiconductor strain gauge can be driven by a so-called constant current source in order to obtain only the change in resistance as a signal, regardless of the resistance value. many. In order to further reduce temperature dependence in a highly accurate strain-to-electrical signal converter, the drive current may be changed in accordance with the temperature to compensate. FIG. 1 shows a conventional example of a strain-to-electrical signal converter that compensates for such temperature dependence.

In the figure, there are semiconductor strain gauges 1 to 4,
Semiconductor strain gauge 1
It is assumed that the resistance values of the semiconductor strain gauge 2 and the semiconductor strain gauge 4 change in opposite ways with respect to the semiconductor strain gauge 3 and the semiconductor strain gauge 3. The temperature-sensitive resistors 5 and 6 are used for the purpose of compensating for the temperature dependence of the output of the device shown in FIG. 1, which corresponds to the reference strain amount. The amount of compensation is determined by the values of temperature-sensitive resistors 5 and 6, their temperature coefficients, and variable resistors 7 to 10.
Determined by the value of Since the reference strain amount is usually selected at a point corresponding to the zero strain amount, such temperature-dependent compensation will hereinafter be referred to as zero-point temperature compensation. The semiconductor strain gauges 1 and 2, the temperature-sensitive resistor 5, and the variable resistors 7 and 9 constitute one leg of the strain amount-electrical signal conversion bridge, and the semiconductor strain gauges 4,
3. The temperature sensitive resistor 6 and the variable resistors 8 and 10 form the other legs. The current flowing through the two legs is added at the connection point 15 of the semiconductor gauges 2 and 3 and flows through the resistor 11 and the temperature-sensitive resistor 12. The voltage drop in the circuit caused by the resistor 11 and the temperature-sensitive resistor 12 is compared with the voltage of the reference voltage circuit 13 by the amplifier 14, and the potential at the connection point 17 between the amplifier 14 and the semiconductor strain gauge 1 is adjusted so that the two match. be. Therefore, resistance 1
1. The temperature sensitive resistor 12, the reference voltage circuit 13 and the amplifier 14 are used to ensure that the sum of the currents flowing through the two legs of the bridge is a predetermined value at each temperature. The amplifier 16 is a means for amplifying the potential difference at the midpoint of the two legs, that is, at each sliding terminal of the variable resistors 7 and 8, inputting the output to the semiconductor gauge 4 side, and negative feedback to one end of one leg. can be,
This keeps the potential at the midpoint of each leg equal.

If the given strain is applied to semiconductor strain gauges 1 and 3, for example
If the resistance value of the semiconductor strain gauges 1 and 2 is increased and the resistance values of the semiconductor strain gauges 2 and 4 are decreased, the current in the legs including the semiconductor strain gauges 1 and 2 increases, and the connection between the semiconductor strain gauges 1 and the amplifier 14 increases. It is clear that by increasing the potential at point 17, the current in the leg containing semiconductor strain gauges 4 and 3 is reduced, and the potential at connection point 18 between semiconductor strain gauge 4 and amplifier 16 is reduced.

The potential difference between these connection points 17 and 18 is amplified by a differential amplifier circuit 19.
The output signal voltage Eout can be obtained. Note that 20 in the figure is a circuit for setting an output voltage corresponding to a predetermined amount of distortion.

[Problem that the invention seeks to solve]

As is clear from the above description, the device shown in FIG. 1 has the advantage that it can be easily implemented because it is comprised of only a standard operational amplifier and passive elements.

However, in the conventional example described above, two relatively expensive temperature sensing elements 5, 6 and two variable resistors 9, 10 are required in order to compensate for the temperature coefficient of the zero point, which varies in polarity and size depending on the device. There was no consideration given to this point. In addition, amplifier error factors such as input equivalent offset voltages of the amplifier 16 and the differential amplifier circuit 19, or temperature changes in these voltages had similar effects. Normally, the offset voltage of an operational amplifier made with a semiconductor integrated circuit is
The amount of change for a temperature change of 100° C. is about 0.3 to 1 mV, and when the temperature changes of the amplifier 16 and the differential amplifier circuit 19 are added, there is a drawback that the burden of zero point temperature compensation increases accordingly.

Additionally, the voltage applied to each leg varies in opposite polarity and with the same value of temperature coefficient as the temperature coefficient of the gauge rod. That is, for example, the temperature changes by about 20% with respect to a temperature change of 100° C., and this value corresponds to about 4 to 5 times the potential difference between the connection points 17 and 18 corresponding to the maximum amount of strain. On the other hand, it is difficult to increase the common-mode voltage rejection ratio of the operational amplifier circuit 19. For example, even if the setting accuracy of each resistor that determines the differential amplification factor is 0.25%, the common-mode voltage rejection ratio is only 40 dB.

For example, assume that the change in voltage applied to each leg due to temperature is 20% of the voltage, and the change due to strain is 5%.
and the common mode voltage rejection ratio of the differential amplifier circuit 19 is
When it is 40 dB, this means that the change in the zero point of the leg voltage due to temperature change becomes as large as 4% at the output end of the differential amplifier circuit 19.

As is clear from the above description, the conventional example described above does not take into consideration the fact that not only the zero point temperature compensation circuit is expensive, but also that the means for amplifying the bridge output signal tends to increase the temperature change at the zero point.

An object of the present invention is to provide a strain-to-electrical signal converter equipped with an inexpensive zero-point temperature compensation circuit.

[Means for solving problems]

In order to achieve such an objective, the present invention
2 including a strain gauge between the midpoint of the leg and a predetermined potential point
A distortion amount-electrical signal conversion bridge consisting of two legs,
A strain amount-to-electrical signal conversion device comprising means for setting the sum of currents of two legs to a predetermined value, means for equalizing midpoint potentials of the two legs, and means for extracting an output from the midpoint potential, Means for generating a reference potential equal to the midpoint potential of the leg when the strain gauge is at the reference temperature state, and the difference between the current flowing between the reference potential generating means and the midpoint of the leg as the temperature of the strain gauge. and means for making the temperature substantially proportional to the difference between the temperature and the reference temperature.

Further, a strain amount-to-electrical signal conversion bridge consisting of two legs including a strain gauge between the midpoint of the legs and a predetermined potential point, means for setting the sum of currents of the two legs to a predetermined value, and In a strain-electrical signal conversion position that includes means for equalizing the midpoint potentials and means for extracting an output from the midpoint potentials, the means for equalizing the midpoint potentials is configured to equalize the midpoint potentials of the two legs. It is comprised of a difference amplifying means, a means for converting a voltage proportional to the difference between the output voltage of the amplifying means and a reference potential into a current, and feeding back this current negatively to the amplifying means, and an output of the amplifying means. It is the output voltage of the voltage.

〔Example〕

The details of the present invention will be explained below using Examples.

In FIG. 2, a resistor 11, a temperature sensing element 12, a reference voltage circuit 13, and an amplifier 14 are arranged so that the sum of the currents of the bridge including semiconductor strain gauges 1 to 4 becomes a predetermined value, as in the conventional example of FIG. It is a means.

Further, when strain is applied to the semiconductor gauges 1 to 4, a voltage difference is generated at the midpoint of each leg, that is, at the connection points 38 and 39 with the input terminal of the amplifier 30. This requires explanation because of the same effect as in the conventional example. probably won't.

The potential difference between the middle point of each leg, that is, the connection point 38 and 39 with the input terminal of the amplifier 30 is amplified by the amplifier 30, divided by the resistors 31 and 32, and then sent to the current source circuit 35.
given to. The current source circuit 35 includes resistors 31 and 3.
Two currents having a current difference proportional to the potential difference between the connection point 33 with the semiconductor strain gauge 1 and the connection point 34 in the figure are applied to the resistors 36 and 37, respectively, and the connection point 46 between the resistor 36 and the semiconductor strain gauge 1, the resistance 37 and the semiconductor A potential difference is generated between the strain gauge 4 and the connection point 47, and this potential difference is generated between the connection point 38 of the semiconductor strain gauges 1 and 2 and the semiconductor strain gauges 3 and 4.
The potentials at the connection point 39 are made equal. Further, the potential of the connection point 34 can be adjusted by a variable resistor 41. That is, the amplifier 30, the resistors 31 and 32, the current source circuit 35, the resistors 36 and 3
It can be understood that 7 constitutes a feedback amplifier circuit. Therefore, if the resistors 31 and 32, the current source circuit 35, and the resistors 36 and 37 are determined so that the potential difference change at the connection points 46 and 47 is attenuated with respect to the output voltage change of the amplifier 30, the connection It will be understood that an output obtained by amplifying the potential difference between points 46 and 47 by the reciprocal of the attenuation factor is obtained at the output terminal of the amplifier 30. In addition, the current source circuit 35 and the resistor 3
6 and 37 have the connection points 33 and 34 as input ends,
Although the connection points 46 and 47 can be regarded as a voltage amplification circuit with a voltage amplification factor smaller than 1 as output terminals, the values of the resistors 36 and 37 can be selected to be 1 kΩ or less, respectively, and the current source circuit 35 is configured. It is clear that the common mode rejection ratio of the low amplification factor voltage amplifier can be set to a value sufficiently larger than 60 dB since a transistor whose collector dynamic resistance is sufficiently larger than 1 MΩ can be easily selected. Therefore, the effect that a change in the average potential of the connection points 46 and 47 due to temperature has on the output terminal of the amplifier 30 can be made sufficiently smaller than -60 dB, that is, one thousandth.

Further, the potential difference applied to the connection points 33 and 34, which are the input terminals of the current source circuit 35, is equal to the amount of distortion -
Point 46 corresponding to the output end of the electrical signal conversion bridge
It is clear that the influence of the input offset voltage of the current source circuit 35 and its temperature change can be made sufficiently smaller than that of the amplifier 30, and vice versa. This means that the input offset voltage of the current source circuit 35 and its tolerance for temperature change may be larger than that of the amplifier 30.

In the bridge circuit including the semiconductor strain gauges 1 to 4, the sum of the currents flowing through the two legs is a predetermined value depending on the temperature, and the connection point is the midpoint of each leg, as in the conventional example shown in FIG. It is clear from the above description that the potentials at points 38 and 39 are maintained approximately equal.

Next, consider temperature changes in the potentials of the connection points 38 and 39. In the embodiment shown in FIG. 2, the bridge is driven with a constant current, and the output voltage of the bridge corresponds to the change in resistance value due to strain in the semiconductor strain gauges 1 to 4, that is, the second term on the right side of the second equation. multiplied by the value of the drive current, that is, [R ₀
(1+αT)(1+βT)s.γ]Equal to I. Therefore, in order to eliminate the temperature dependent term, the current value I
The temperature dependence of the bridge output voltage can be compensated by formulating as shown in the following equation.

I=I ₀ /(1+αT)(1+βT) (3) Here, I ₀ is a constant current value. It is clear that when such a current is applied to the bridge (connection point 15 in the figure), the voltage drop V _G across the semiconductor strain gauges 2 and 3 is expressed by the following equation.

V _G = R ₀ (1+αT)×1 (4) = R ₀ I ₀ /1+βT = R ₀ I ₀ (1−βT) (4)' On the other hand, according to the inventors' measurements, the temperature coefficient of resistance α , the temperature coefficient β-α of the resistance change, and the temperature coefficient of the gauge factor vary depending on the temperature as shown in FIG. 3 in the case of a semiconductor strain gauge with a specific impurity concentration. Here, β is approximately constant regardless of temperature. Therefore, as is clear from equation (4),
The potential at the connection points 38 and 39 changes with a temperature coefficient of approximately -β.

Therefore, a reference potential point 43 equal to the potential of the connection points 38 and 39 at a predetermined temperature is provided by the correction circuit 42, and this reference potential point 43 and the connection point 3 are
If either one of 8 or 39 is connected with a variable resistor 44, the potential difference between the connection point 38 or 39 and the reference potential point 43 caused by a temperature change in the potential of the connection point 38 or 39 is proportional to the potential difference. It is clear that a potential difference proportional to the resistance value is generated. Further, the polarity of the potential difference is determined depending on which of the connection points 38 and 39 the variable resistor 44 is connected to. Moreover, since the potential difference between the connection points 38 and 39 is extremely small, the variable resistor 44 does not matter where the variable resistor 44 is connected.
The currents flowing through the two electrodes are approximately equal, so although the polarities are different, approximately equivalent compensation can be achieved.

As is clear from the above explanation, the switching means 4 selects and connects one of the reference potential point 43, the resistor 44, and the connection point 38 and the middle point 39 that provide the reference potential.
5 implements a circuit 42 that compensates for the temperature dependence of the zero point.

The above-mentioned zero-point temperature compensation circuit is not only inexpensive because it does not require a temperature sensing element, but also the amount of correction can be easily adjusted because the only element that requires adjustment is the variable resistor 44, which makes such a device industrialized. It is highly effective.

Furthermore, only one amplifier 30 is affected by amplifier error factors such as input offset voltage, and the return circuit formed by the current source circuit 35 and resistors 36 and 37 is similar to the conventional example shown in FIG. Since a high common-mode voltage rejection ratio can be achieved without using high-precision elements like the differential amplifier shown in No. 19, it is easy to integrate into a semiconductor integrated circuit, and it is small and inexpensive.

FIG. 4 shows another embodiment, in which instead of providing the switch 45 described above, resistors 51 and 5 of different values are used.
2. That is, resistors 51 and 5
It is clear that different values of 2 will result in slight differences in the currents flowing through the semiconductor strain gauges 1 and 4. Therefore, the potential difference occurring between the connection points 46 and 47 can be canceled out.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to provide an inexpensive strain-to-electrical signal conversion device that can guarantee zero point temperature.

[Brief explanation of the drawing]

Fig. 1 is an electric circuit diagram of a conventional strain-electric signal converter, Fig. 2 is an electric circuit diagram of a strain-electric signal converter showing zero implementation of the present invention, and Fig. 3 is a diagram showing temperature and each coefficient. FIG. 4 is a main circuit diagram showing another example of the present invention. 1, 2, 3, 4... Semiconductor strain gauge, 13... Reference voltage circuit, 14, 16... Amplifier, 35... Current source circuit, 50... Drive current, 42... Zero point compensation circuit, 44
...variable resistance.

Claims (1)

[Claims] 1. Four resistors are connected in sequence to form a closed circuit, and two adjacent resistors are connected to one leg and the connecting point of each resistor is the center. A bridge in which each resistance between a point and another connection point is a strain gauge, or all of the resistances are strain gauges;
means for detecting the sum of currents flowing through each leg of the bridge and comparing it with a reference value so that the same value of current as the reference value is divided and flowing through each leg; means for detecting a potential difference at midpoints, outputting a voltage according to the difference, and setting the potential difference at midpoints to 0 based on the output value; means for extracting an output from at least one of the midpoints; means for generating a reference potential equal to the midpoint potential of the leg when the gauge is at a reference temperature state, and a current flowing between the reference potential generating means and the midpoint of the leg at a temperature of the strain gauge and the reference 1. A strain-to-electrical signal conversion device characterized by comprising means for making the difference approximately proportional to the temperature. 2. The means for setting the midpoint potential difference to 0 includes means for amplifying the difference between the midpoint potentials of the two legs, and converting a voltage proportional to the difference between the output voltage of the amplifying means and a reference potential into a current; 2. The distortion amount-to-electrical signal conversion device according to claim 1, further comprising means for negatively feeding back this current to the amplifying means, and the output voltage is the output voltage of the amplifying means.