JPH0321047B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a displacement measuring device using a differential transformer.

In devices that use a differential transformer, such as length measuring machines and weighing machines, the core of the differential transformer is displaced in a linear direction in proportion to the length or weight to be measured, and this amount of displacement is converted into an electrical signal. to measure length, weight, etc. However, since such measuring devices require extremely high measurement accuracy, changes in the characteristics of the differential transformer and other electronic circuits due to changes in temperature, etc., have caused large measurement errors.

As a countermeasure against this problem, conventional methods have been used, for example, as shown in Figure 1, to insert a thermistor 2 in series on the primary coil side of the differential transformer 1 to offset changes in sensitivity due to temperature of the differential transformer 1. . However, with this method, it is difficult to match the characteristics of the differential transformer 1 and the thermistor 2, so sufficient temperature compensation cannot be achieved, and the oscillation circuit that applies the oscillation signal to the primary side of the differential transformer 1 4 and the downstream electronic circuit 3 for detecting the amount of displacement that performs processing such as amplification, detection, or A/D conversion on the output signal.
Measurement accuracy could not be maintained sufficiently. (The reference numeral 5 in the figure is a display that displays the amount of displacement based on the output signal of the electronic circuit 3.) Conventionally, as shown in FIG. 2, a differential transformer 1
The output is taken out to a separate system from the electronic circuit 3 for subsequent stage displacement detection such as amplification, detection, and A/D conversion, and is detected by the detection circuit 6 to convert it into a DC voltage, which is then converted to a preset reference voltage. E is compared with the comparison circuit 7, and the control signal is sent to the oscillation circuit 4 so that the two match.
A method of controlling the oscillation current by outputting it to the oscillator is also used. However, this method cannot compensate for changes in the sensitivity of the electronic circuit 3 for detecting the amount of displacement, and therefore measurement accuracy cannot be maintained sufficiently.

The present invention is a displacement measuring device that improves the above-mentioned drawbacks and enables sufficient sensitivity compensation. is intended to provide.

Hereinafter, one embodiment of the present invention will be described based on the drawings.

FIG. 3 is a block diagram showing an embodiment of the displacement measuring device of the present invention.

In the figure, 11 is an oscillation circuit that outputs a predetermined frequency and an oscillation signal, and 12 is a differential transformer whose core 12a is displaced in proportion to the linear displacement to be measured. 1
The secondary coil 12b is excited, and the core 12a is connected to the two secondary coils 12c and 12d connected with opposite polarity.
output voltages e ₁ and e ₂ corresponding to the displacement of , respectively. 13 is the secondary coil 12 of the differential transformer 12
The difference voltage e ₁ −e ₂ between the output voltages e 1 and _{e 2} of c and 12d is 2
The average voltage (e ₁ +e ₂ )/2 of e ₁ and e ₂ is taken out from the intermediate tap of the primary coil 13a of the input transformer 13, which is an input transformer taken out to the secondary coil 13b. Reference numeral 14 denotes a first changeover switch, which switches between output terminal A from the secondary coil 13b of the input transformer 13 and output terminal B from the intermediate tap of the primary coil 13a. 15 is an AC amplifier that amplifies the output signal from terminal A or B via the changeover switch 14; 16 is a detector that detects the output signal of the AC amplifier 15 and converts it into a DC signal; and 17 is a detector that amplifies the output signal of the detector 16. Amplifying DC amplifier, 1
8 is an A/D converter that converts the output signal of the DC amplifier 17 into a digital signal. A second changeover switch 19 distributes the output signal of the A/D converter 18 to terminals A and B in conjunction with the first changeover switch 14. A displacement output circuit 20 is connected to terminal A and outputs the output signal of the A/D converter 18 as a displacement signal. 21 is connected to terminal RO,
This is a D/A converter that converts the output signal of the A/D converter 18 into an analog signal. 22 is a comparison circuit that compares the output signal of the D/A converter 21 with a preset reference voltage E and outputs a control signal so that both are equal; 23 is an oscillation circuit that stores the control signal of the comparison circuit 22; This is a control signal storage circuit that outputs the control signal to the oscillation circuit 11 to control the amplitude of the oscillation signal of the oscillation circuit 11.

Next, the operation of the above embodiment will be explained.

The primary coil 12b of the differential transformer 12 is excited by the oscillation signal from the oscillation circuit 11, and the output voltages e ₁ and e ₂ of the secondary coils 12c and 12d are applied to the core 12.
It changes according to the displacement of a. Input transformer 13
An output voltage e ₁ −e ₂ is generated in the secondary coil 13b of
The middle tap of the primary coil 13a has (e ₁ +e ₂ )/
2 occurs.

The output signal obtained from terminal A or B via the first changeover switch 14 is amplified by an AC amplifier 15, detected by a detector 16, and converted into a DC signal.
The signal is amplified by a DC amplifier 17 and converted into a digital signal by an A/D converter 18. When the first and second changeover switches 14 and 19 are connected to the A side, the AC signal of the difference voltage e ₁ -e ₂ is amplified and detected as described above, and A/D converted. It is output from the displacement output circuit 20 as a displacement signal. 1st, 2nd
When the changeover switches 14 and 19 are connected to the low side, the AC signal with the average voltage (e ₁ +e ₂ )/2 passes through the same circuits 15 to 18 to the D/A converter 21.
It is converted into an analog signal by the comparison circuit 22, and compared with the reference voltage E by the comparison circuit 22, and a control signal is outputted so that the two become equal. This control signal is stored in the control signal storage circuit 23 and output to the oscillation circuit 11 to control the amplitude of the oscillation signal and control the oscillation voltage e ₀ .

As is well known, the secondary coil 1 of the differential transformer 12
The output voltages e ₁ and e ₂ of the differential transformers 2c and 12d change approximately linearly in response to the displacement of the core 12a of the differential transformer 12, as shown in FIG. That is, core 12
When a is displaced upward from the zero point (to the + side of the horizontal axis), 2
The output voltage e ₁ of the secondary coil 12c increases, and the output voltage e ₂ of the secondary coil 12d decreases. When the core 12a is displaced downward, e ₁ decreases and e ₂ increases. Therefore, the differential voltage e ₁ −e ₂ increases approximately in proportion to the upward and downward displacement from the zero point, but the average voltage (e ₁ +
e ₂ )/2 remains approximately constant regardless of the displacement of the core 12.

However, if the output voltages e ₁ and e ₂ of the differential transformer 12 fluctuate due to temperature changes, the differential voltage
e ₁ −e ₂ and the average voltage (e ₁ +e ₂ )/2 also vary.
As shown by the chain line in Fig. 4, e ₁ and e ₂ increase in proportion to the displacement of the core, so the fluctuation width a of e ₁ and e ₂ also increases in proportion to the displacement of the core, and therefore Although the variation width b of the differential voltage e ₁ −e ₂ also increases in proportion to the change in the core, the variation width c of the average voltage (e ₁ +e ₂ )/2 remains almost constant regardless of the core displacement. Therefore, even if changes in temperature occur, the average voltage (e ₁ + e ₂ )/2
If the fluctuation width c of is controlled in the direction of zero, the differential voltage e ₁
−e ₂ fluctuation is also eliminated, and a displacement signal with less error can be obtained. In addition to the differential transformer 12, the oscillation circuit 11, the input transformer 13, the AC amplifier 15,
Detector 16, DC amplifier 17, A/D converter 18
etc. also cause sensitivity changes due to temperature changes, etc., but the average voltage signal (e ₁ + e ₂ )/2 and the difference voltage signal e ₁
-e Since it passes through the same circuit as ₂ , it causes the same ratio of fluctuations in the average voltage and differential voltage.

Therefore, if the average voltage signal passed through the second changeover switch 19 and the D/A converter 21 is compared with the reference voltage E set in advance by the comparator 22, and a control signal is output so that the two match, the 1. Fluctuations in the differential voltage signal when the second changeover switches 14 and 19 are switched to the A side are corrected and output to the displacement output circuit 20.

Therefore, by controlling the amplitude of the oscillation circuit 11 by switching the first and second changeover switches 14 and 19 to the low side after each measurement or after a certain number of measurements, errors caused by temperature changes, etc. can be significantly reduced. can be done.

Further, as shown in FIG. 5, when the displacement of the measurement target is extremely large and the displacement of the core 12a is large, all of e ₁ , e ₂ , e ₁ −e ₂ , and (e ₁ +e ₂ )/2 Linearity deteriorates above a certain displacement. Therefore, even in such a case, if a control signal is outputted from the comparator circuit 22 so that the curved portion of (e ₁ +e ₂ )/2 shown by d in FIG. 5 becomes a straight line portion d', the differential voltage Since the curved portion f of the signal e ₁ -e ₂ can be made to be a straight line portion f', it is also possible to compensate for errors due to nonlinearity of measurement.

In the above embodiment, the control signal is output to the oscillation circuit 11, but the same result can be obtained by performing closed loop control so that the output voltage from the second changeover switch 19 is constant. 15, the detector 16, the DC amplifier 17, or the A/D converter 18.

As explained above, the displacement measuring device of the present invention determines the control amount for measurement value compensation based on the output obtained from the same output terminal as the output of the final stage of the electronic circuit for displacement output, and determines the control amount for compensation of the measured value. Since the following measurements are controlled using the stored control amount, the linearity of the temperature characteristic of the entire circuit for outputting displacement can be compensated for.

[Brief explanation of the drawing]

1 and 2 are block diagrams showing a conventional displacement measuring device with a compensation circuit, FIG. 3 is a block diagram showing an embodiment of the present invention, and FIGS. 4 and 5 are diagrams showing the core of a differential transformer. FIG. 3 is a diagram showing output characteristics with respect to displacement. 11...Oscillation circuit, 12...Differential transformer, 1
4...First changeover switch, 17...DC amplifier, 19...Second changeover switch, 20...Displacement output circuit, 22...Comparison circuit, 23...Control signal storage circuit.

Claims (1)

[Claims] 1. An oscillation circuit that generates an oscillation signal of a predetermined frequency; a primary coil is excited by the oscillation signal of the oscillation circuit, and the output voltages of the two secondary coils correspond to the displacement of the core. a differential voltage output circuit that outputs a differential voltage signal between the output voltages of the two secondary coils; an amplification and detection circuit that amplifies and detects the differential voltage signal; and a displacement output circuit that receives an output signal of the amplification detection circuit and outputs it as a displacement signal of the core, the sum voltage outputting a sum voltage signal of the output voltages of the two secondary coils. an output circuit, and a first circuit that switches the input to the amplification and detection circuit to either the difference voltage output circuit or the sum voltage output circuit.
a changeover switch, and an output signal of the amplification and detection circuit when the output of the sum voltage output circuit is input to the amplification and detection circuit by the first changeover switch is compared with a preset reference voltage. , a control circuit that outputs a control signal to the oscillation circuit, and a second switch that switches the output of the amplification and detection circuit to either the displacement output circuit or the control circuit in conjunction with the first switch. and a control signal storage circuit for storing a control signal of the control circuit, and an output signal of the amplification and detection circuit when the output of the sum voltage signal output circuit is inputted via the first changeover switch. After receiving the control signal from the control circuit through the second changeover switch and storing the control signal of the control circuit at this time in the control signal storage circuit, the first changeover switch is switched to the differential voltage signal output circuit. At the same time, the second changeover switch is switched to the displacement output circuit side, and the oscillation circuit is controlled by the control signal stored in the control signal storage circuit in subsequent displacement measurements. measuring device.