JPH056642B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a bridge circuit constituted by a resistor including a strain gauge, an AC power source that excites the bridge circuit, and a phase detector that detects the phase of the output of the bridge circuit. The present invention relates to a strain measuring device equipped with.

Conventionally, this type of device has been widely used because the amplifier is cheaper and the performance is more stable than a system that uses a DC power source as a bridge power source. However, if there is stray capacitance in the bridge circuit and unbalance occurs, an output voltage will appear in the bridge circuit even when there is no strain, and this will cause errors in strain measurements that should normally detect only changes in resistance. There was an inconvenience.

Therefore, a capacitance balancing circuit is installed in the bridge circuit and adjusted to balance the capacitance (capacitance) before measurement, so that the output of the bridge circuit due to unbalanced capacitance is reduced to zero. If the imbalance changes, the output will change and an error will occur.

It is an object of the present invention to provide a strain measuring device free from such inconveniences.

If the resistance of the bridge circuit is balanced and the stray capacitance is unbalanced, and for example, in the bridge circuit shown in the first diagram, there is stray capacitance only between terminals B and C, then the AC power supply voltage V of the bridge circuit as shown in the second diagram _B
A phase difference of slightly less than 90 degrees occurs between the output voltage V 01 and the output voltage V ₀₁ . Also, if stray capacitance exists only between terminals A and B, AC power supply voltage V _B and output voltage V ₀₂
A phase difference slightly larger than 90 degrees occurs between the two. Therefore, when phase detection is performed on this output voltage with a voltage having the same phase as the AC power supply voltage V _B , voltages -V _R and +V _R corresponding to the phase shift from 90 degrees appear as outputs.

As shown in FIG. 2, if the output voltages V ₀₁ , V ₀₂
The phase detection signal (V′ _B ) with a phase difference of 90 degrees from this
If phase detection is performed with , the output after detection will be zero.

The present invention focuses on the above relationship, and is based on the unbalance of stray capacitance that exists in a bridge circuit composed of resistors including strain gauges. A means for generating a phase detection signal having a phase difference of Features.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a block diagram of one embodiment of the invention. In the figure, 1 is a bridge circuit constituted by a resistor including a strain gauge;
AC power supply 2 was connected to the diagonal points of , and AC amplifiers 3 were connected to the other diagonal points.

The resistance and stray capacitance on each side of the bridge circuit 1 can take various values, but from the viewpoint of bridge unbalance, only the resistance R' changes as shown in the figure.
The bridge circuit is expressed as a capacitor added between terminals A and B or between terminals B and C. The output of the AC amplifier 3 is sent to a phase detector 4 and a smoothing circuit 5.
and connected to the output terminal 7 via the output amplifier 6. Reference numeral 8 denotes a phase management unit as means for creating a phase detection signal to be supplied to the phase detector 4;
A phase detection signal having a phase difference of approximately 90 degrees from the output signal of the bridge circuit 1 when no distortion is generated based on the imbalance of stray capacitance in the bridge circuit 1 is created.

The phase management unit 8 includes a phase detector that uses the output of a phase shifter 9 that delays the voltage of the AC power source 2 by 90 degrees as a phase detection signal, that is, a polarity discriminator 10 for the output signal of the bridge circuit 1, and the discriminator 10. A polarity inversion and level adjustment circuit 11 that inverts the output signal of the AC amplifier 3 as it is or inverts the polarity and adjusts the level according to the polarity determined by the circuit 10; It consists of a phase adder 12 that vector-adds the output signal and the power supply voltage.

A polarity discriminator 10 consisting of a phase detector detects the phase of the output voltage of the bridge circuit 1 using a phase detection signal obtained by delaying the phase of the power supply voltage _VB by 90 degrees using a phase shifter 9, and converts it into a positive or negative polarity voltage. , it is determined whether stray capacitance exists between the terminals AB or BC of the bridge circuit 1.

The polarity inversion and level adjustment circuit 11 consists of a polarity inversion circuit and an attenuator. When stray capacitance exists between the terminals BC and the output voltage vector of the bridge circuit 1 is in the first or second quadrant shown in the second diagram, the polarity reversing circuit is activated by the positive signal of the polarity discriminator 10 to detect the polarity. The polarity of the output voltage is inverted and output, and when a stray capacitance exists between terminals AB and the vector of the output voltage of the bridge circuit 1 is in the third or fourth quadrant shown in the second diagram, the negative polarity of the polarity discriminator 10 is output. The attenuator operates to attenuate the output voltage to a predetermined level while being inactivated by the signal and outputting the output voltage as it is.
The attenuation ratio of this attenuator is approximately 90% compared to the output signal when the phase detection signal after being added to the power supply voltage in the next phase adder 12 is balanced and the capacitance is relatively large and unbalanced. The size was set so that the phase difference was about degrees.

Its operation will be explained next.

Now set the voltage of AC power supply 2 to 2V and bridge circuit 1
If there is a stray capacitance of 5000PF between terminals _A and _B of The angle between the voltage V ₀₂ and the imaginary phase direction is α ₁ .

Output voltage of polarity inversion and level adjustment circuit 11
V _CO is, for example, twice the output voltage _{V 02 of} the bridge circuit 1, as shown in _FIG. is shifted in phase by an angle α ₂ equal to α ₁ with respect to the power supply voltage V _B , resulting in a phase difference of 90 degrees from the output voltage V ₀₂ .

Thus, even if a voltage based on capacitance unbalance is output from the bridge circuit 1 in the absence of distortion, no output voltage is generated at the output terminal 7 through the phase detector 4, smoothing circuit 5, and output amplifier 6.

When strain is applied to the strain gauge of the bridge circuit 1, the bridge circuit 1 outputs a voltage _VOS , which is the output voltage _V02 plus the output voltage _VS corresponding to the strain. This output passes through the AC amplifier 3 and the circuit 11 to become a voltage _VCS as shown in the third diagram, which is then converted to the voltage of the AC power supply 2 by the phase adder 12.
V _B and outputs the phase detection signal V _PS . Third
In the diagram, the power supply voltage V _B is shown to be extremely small compared to the output voltage V ₀₂ of the bridge circuit 1, but in reality _, the output voltage V ₀₂ ,
Since V _OS is an extremely small value (maximum 10 mV), even if an output voltage value that is different in phase and magnitude from the bridge circuit 1 when distortion occurs is generated, the phase of the phase detection signal V _PS is almost the same as the phase of the phase detection signal V _PO when there is no distortion.
In this way, even if the output voltage V _OS of the bridge circuit 1 is detected by the phase detector 4 using the phase detection signal V _PS when distortion occurs, the output voltage V _R2 due to unbalanced capacitance of the bridge circuit 1 when no distortion occurs is removed. be able to.

Next, the stray capacitance between terminals B and C is, for example, 5000PF.
Suppose it exists. At that time, the phase relationship between the output voltage V ₀₁ of the bridge circuit 1 without distortion and the voltage V _B of the AC power supply 2 is as shown in the third diagram. This output voltage V ₀₁ is passed through the AC amplifier 3 to the polarity discriminator 1.
0, a positive signal is output from the discriminator 10, which activates the polarity inversion circuit of the circuit 11 to convert the polarity of the output signal of the AC amplifier 3, and lowers the level with the attenuator. A voltage having the same magnitude and phase as V _CO shown in the third diagram is input to the adder 12, and a phase detection signal V _PO having the same phase as V _PO is output.

In this way, even if there is stray capacitance between terminals B and C, the phase detection signal will have a phase difference of approximately 90 degrees with respect to the output voltage V ₀₁ at the time of no distortion, so this signal will cause the output voltage V If ₀₁ is phase detected,
The output after phase detection becomes zero. Even when distortion occurs, the output voltage V _R1 due to the unbalanced capacitance of the bridge circuit 1 when there is no distortion can be removed for the same reason as described above.

As described above, the present invention includes means for creating a phase detection signal having a phase difference of about 90 degrees from the output signal of the bridge circuit when there is no distortion based on the unbalance of stray capacitance existing in the bridge circuit, Since the output of the bridge circuit is detected by the phase detector using the phase detection signal of the generating means, even if there is unbalance of stray capacitance in the bridge circuit or the magnitude of the stray capacitance changes, This has the effect that no output is generated due to this, and therefore no error occurs.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of one embodiment of the present invention, and FIG.
The figure shows the phase relationship between the power supply voltage V _B and the output voltages V ₀₁ and V ₀₂ due to the unbalance of the stray capacitance of the bridge circuit, and FIG. 3 is a vector diagram for explaining the operation. 1... Bridge circuit, 2... AC power supply, 3...
AC amplifier, 4... Phase detector, 5... Smoothing circuit, 8... Phase management unit, 9... Phase shifter, 10...
Polarity discriminator, 11... Polarity inversion and level adjustment circuit, 12... Phase adder.

Claims (1)

[Claims] 1. A bridge circuit configured with a resistor including a strain gauge, an AC power supply that excites the bridge circuit, and a phase detector that detects the phase of the output of the bridge circuit. the output signal of the bridge circuit based on the unbalanced stray capacitance of the bridge circuit; A distortion measuring device characterized in that the distortion is detected by a phase detector.