JPS6412328B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a displacement measuring device for measuring the displacement of an object to be measured, and more particularly to a displacement measuring device with improved synchronous detection means.

Generally, this type of displacement measuring device measures the displacement of the object to be measured by attaching a displacement detector to the movable core of a differential transformer that is excited by an AC excitation power source. 2 of differential transformer
Since the output proportional to the displacement of the secondary coil has a phase inversion of 180° depending on the position of the movable core, by synchronously detecting the output of the secondary coil, the displacement of the movable core, that is, the displacement of the object to be measured can be measured.

By the way, there are various conventional displacement measuring instruments, but
One of them is the configuration shown in FIG. This displacement measuring device includes an AC excitation power source 1, a movable core 2c having a primary coil 2a, and a displacement detector 2b.
and a pair of differentially connected secondary coils 2d, 2
The difference signal (A+B) between the outputs of the secondary coils 2d and 2e is the synchronous signal obtained from the primary coil 2a. The configuration uses synchronous detection.

However, in this displacement measuring device, there is a phase difference θ between the voltage waveforms of the primary coil 2a and each of the secondary coils 2d and 2e, and this phase difference θ changes depending on the temperature or the cable length. The reason for this is that the primary coil 2a is not a pure inductance. Now, the inductance of the primary coil 2a is Lp,
If the resistance value is Rp, the phase difference θ is as follows: θ=tan ⁻¹ Rp/ωLp (1). Since copper wire is usually used for the coil winding, Rp changes with a large temperature coefficient.
Moreover, the primary coil 2a, secondary coils 2d, 2e
When these are integrated with the AC excitation power supply 1, AC amplifier 3, etc. via a cable, the resistance and stray capacitance of the cable itself will also have an effect, and the phase will change even more.

Therefore, in order to solve the above-mentioned problems, a displacement measuring instrument having a configuration as shown in FIG. 2 has been conventionally used. This measuring device extracts a synchronizing signal from the differential connection point of two secondary coils 2d and 2e and connects it to the differential transformer 2.
It is configured to synchronously detect the difference signal (A+B).

Such a configuration is extremely advantageous in terms of implementation when connecting a cable to the secondary coil 2d. However, although it is assumed that there is no phase difference between the secondary coils 2d and 2e, in reality both coils 2d and 2e
There is a phase difference between them, causing a deterioration of the S/N ratio.
This point will be discussed below. In other words, in an ideal state where the phase difference between the two secondary coils 2d and 2e is 180°, the phase difference between the synchronization signal and the difference signal (A+B) will be 0° or 180°, and there will be an error due to the phase change. does not occur. Now, if the output of the secondary coil 2d is A and the output of the secondary coil 2e is B, as is clear from FIG. 3, the amplitudes of the outputs A and B of the secondary coils 2d and 2e are equal, The phase of output B of
When the phase shift of the output A of the secondary coil 2d from 180° when the angle is 0° is φ _A , the phase difference φ _C of the difference signal C with respect to the synchronizing signal B is φ _C = {(180° −
φ _A )/2}, and in this case, when φ _A =0, the difference signal C of (A+B) becomes zero, and the phase difference φ _C becomes 90°.

However, in practice, φ _A is rarely zero;
As shown in FIG. 3, a phase difference of φ _C is generated. Here, φ _C is φ _C =tan ⁻¹ Asinφ _A /B−A cosφ _A (2). Therefore, when the phase difference between the two secondary coils 2d and 2e is ±(180°-φ _A (φ _A ≠0)), there is a difference between the synchronizing signal B and the difference signal C: φ _C =± {(180゜−
A phase difference of φ _A )/2} is generated. At this time, φ _A
is close to 0°, and the increase and decrease of A and B are extremely small relative to their respective absolute values, so φ _C is a value near 90°, and the difference signal C at that time is expressed as an unbalanced 90° component. It is called. Also, if the amplitude of synchronization signal B changes due to a change in the core position and becomes B<A, B>A, the phase difference will similarly change from the above equation, and the imbalance will be 90 degrees with respect to synchronization signal B. The phase difference of the components is B<A,B
> There is no symmetrical relationship when A. Therefore, even if the difference signal C is synchronously detected using the synchronization signal B, the difference signal C will not become zero at the core zero position, resulting in a measurement output with a degraded S/N ratio. Note that a similar problem occurs when output A is used as the synchronization signal.

The present invention has been made to eliminate the above-mentioned drawbacks, and its purpose is to
By using the sum signal of both outputs of the secondary coil as the synchronization signal, the phase difference between the synchronization signal and the difference signal approaches 90°, improving the deterioration of the S/N ratio due to the phase difference of the unbalanced 90° component, and providing stable measurement output. The present invention provides a displacement measuring device that can be used to measure displacement.

An embodiment of the present invention will be described below with reference to FIG. In particular, the points that are different from FIG. To find the sum signal. Further, the difference signal is amplified by the AC amplifier 3, and the synchronous detection means for this amplified output is performed using the sum signal obtained by the arithmetic circuit 11. Although the sum signal is not shown, it goes without saying that the sum signal may be amplified or attenuated by an amplifier to an amplitude required as a synchronous detector driving source.

Next, the operation of the displacement measuring device configured as described above will be explained. Differential transformer 2 from AC excitation power supply 1
When an AC excitation signal is applied to the primary coil 2a, outputs A and B are obtained from the differentially connected secondary coils 2d and 2e. Both outputs A and B are input as they are to the arithmetic circuit 11, where (A+B) and (A-B) are computed to obtain a difference signal C and a sum signal D.

The difference signal C and the sum signal D will now be explained with reference to FIG. First, the difference signal C is the third
As shown in the figure, since the vector is a combination of A and B, the signal has a phase difference φ _C. On the other hand, the sum signal D is vector-wise a signal that is a combination of B and -A, and the phase difference φ _D at this time is φ _D =tan ⁻¹ Asinφ _A /B+A cosφ _A (3). Therefore, the sum signal D is used as a synchronization signal, and the difference signal C
When is the measured output, the phase difference between D and C φ _C +φ _D is
When B=A, φ _C = 180° − φ _A /2 …(4) φ _D = φ _A /2 …(5) Therefore, φ _C + φ _D = 180° − φ _A /2 + φ _A /2 = 90° ...(6) On the other hand, when B>A, the phase difference φ _C +φ _D is not 90°, but it is closer to 90° compared to the conventional one in FIG. 2, and when B<A, the difference signal C is Since it moves to the left side of the vertical axis (Y axis), the phase difference of the difference signal C is expressed as 180° - (φ _C +φ _D ), which approaches 90° as in B>A.
That is, when B>A and B<A, from φ _C (φ _C +φ _D )
is closer to 90°, and even if φ _A changes, the phase difference (φ _C +φ _D ) of difference signal C with respect to synchronizing signal D is φ _A
Since there is a mutually proportional relationship with respect to the 90° component, it is possible to improve the influence of the S/N ratio due to the phase difference of the unbalanced 90° component. Therefore, if the difference signal C amplified by the AC amplifier 3 is synchronously detected with the sum signal D, the difference signal becomes zero at the core zero position, making it possible to perform highly accurate measurement with a significantly improved S/N ratio.

Next, FIG. 6 is a diagram showing another embodiment of the present invention. This displacement measuring device uses a transformer 12 with two primary windings and one secondary winding separately from the differential transformer 2 as a calculation means.
The sum signal is taken out from the common connection of 2a and 12b as a synchronizing signal, and the secondary winding 12 of the transformer 12 is
The configuration is such that a difference signal is extracted from c. That is, this measuring instrument is configured to obtain a sum signal using an AC bridge circuit consisting of coils 2d, 2e, 12a, and 12b.

Note that the present invention can be implemented with various modifications without departing from the gist thereof.

As detailed above, according to the present invention, if the sum signal of the two coil outputs of the differential transformer is used as a synchronization signal, a synchronous detector drive source can be obtained, and the amplitudes of the two secondary coils are the same. So, two 2
When the phase difference between the secondary coils is different, the phase difference of the unbalanced 90° component generated as a difference signal with respect to the synchronous signal is 90°, so if the difference signal is synchronously detected using the sum signal, the difference signal becomes zero. . Furthermore, even when the amplitudes of the two secondary coils are different and the phase difference between the two secondary coils is different, the phase difference between the difference signal and the synchronization signal is closer to 90°, so it virtually approaches zero. . Therefore, when connecting the secondary coils with a cable, the SN ratio due to the phase difference between the coils can be significantly improved, and it is possible to provide a displacement measuring instrument that can obtain measurement outputs with high precision and stability.

[Brief explanation of the drawing]

1 and 2 are respectively block diagrams of conventional displacement measuring instruments, FIG. 3 is a vector diagram explaining synchronous detection of the measuring instrument shown in FIG. 2, and FIG. 4 is a diagram of a displacement measuring instrument according to the present invention. FIG. 5 is a vector diagram illustrating synchronous detection of the measuring instrument shown in FIG. 4, and FIG. 6 is a configuration diagram showing another example of the present invention. 1...AC excitation power supply, 2...Differential transformer, 2b
...Displacement detector, 2c...Movable core, 2d, 2e
... Secondary coil, 3 ... AC amplifier, 4 ... Synchronous detector, 11 ... Arithmetic circuit.

Claims (1)

[Claims] 1. In a displacement measuring device that measures the displacement of an object to be measured by providing a displacement detector in the movable core of a differential transformer that is excited by alternating current, the differentially connected 2 of the differential transformer is
A circuit that obtains a difference signal and a sum signal using both outputs of a secondary coil, and a synchronous detector that uses the sum signal obtained by this circuit as a synchronous signal to synchronously detect the difference signal. A displacement measuring instrument featuring: