Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2002-05-20
2004-06-22
Deb, Anjan K. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S207180, C324S607000
Reexamination Certificate
active
06753686
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for detecting failure of a differential transformer used with an electric micrometer, etc.
This invention also relates to a method and apparatus for signal processing of a differential transformer for obtaining a measurement signal from the differential transformer. Particularly, the present invention relates to a method and apparatus for signal processing of a differential transformer suitably when small displacement measuring instruments such as electric micrometers are incorporated inline for use.
2. Description of the Related Art
As a displacement gauge for measuring displacement of the object to be measured, an electric micrometer is used for inline multipoint measurement as illustrated in FIG.
11
.
In
FIG. 11
, numeral
10
denotes an electric micrometer containing a differential transformer, numeral
12
denotes a measurement unit, numeral
12
A denotes offset and gain adjustment controls. Further, numeral
14
denotes a unit case in a built-in manner, numeral
14
A denotes a selector, numeral
14
B denotes a check meter, and numeral
16
denotes a personal computer (PC).
As shown in detail in
FIG. 12
, in the electric micrometer
10
, displacement of a spindle
22
forming a part of a probe
20
is transmitted to a core
26
. Sensor coils
28
and
30
are placed symmetrically with respect to the mechanical center position of the core
26
(called neutral point) and connected in series so as to form a differential transformer. The electric micrometer
10
uses change in the impedance of the sensor coils
28
and
30
depending on the position of the core
26
to detect displacement of the spindle
22
. That is, the electric micrometer
10
detects displacement of the spindle
22
based on change depending on the position of the core
26
in the difference between voltages E
1
and E
2
(E
1
-E
2
) as shown in
FIG. 13
occurring across each of the sensor coils
28
and
30
when a voltage is applied to the sensor coils
28
and
30
from an external oscillator
32
.
Specifically, as shown in
FIG. 14
, a drive signal shaped like a sine wave generated in the oscillator
32
is applied to the sensor coils
28
and
30
through a transformer
40
, for example. An amplifier
46
amplifies output at the neutral point of the sensor coils
28
and
30
and output of a variable resistor (called control)
42
for zero adjustment (offset correction). Output of the amplifier
46
is input through a span adjustment (gain correction) control
48
and a capacitor
50
for offset removal to a synchronous rectifier
52
, which then half-wave or full-wave rectifiers the input in synchronization with the drive signal output by the oscillator
32
. Then, output of the synchronous rectifier
52
is smoothed through a filter
54
and the analog signal output through the filter
54
is converted into a digital signal by an analog-digital (A/D) converter
56
. Then, the digital signal is displayed in digital form on a display
58
implemented as a liquid crystal display (LCD), for example. In
FIG. 14
, numeral
44
denotes a resistor. However, in the electric micrometer using such a differential transformer, the sensor signal becomes zero at the mechanical neutral point of the probe
20
(core
26
) and thus it is difficult to discriminate between a failure caused by a broken wire, etc., and the normal signal from the neutral point; this is a problem.
Further, to conduct high-accuracy measurement, the frequency stability and oscillation stability of the oscillator
32
need to be high, the amplification stability of the amplifier
46
needs to be high, and offset needs to be low.
However, the parts accuracy of the coil and capacitor used with the oscillator
32
for generating the sine wave to drive the sensor coils
20
and
30
are insufficient. Further, it becomes necessary to adjust the oscillation frequency and amplitude with trimmer control at the assembling time and the calibration time, and the frequency and voltage easily change due to temperature change. On the other hand, high-accuracy parts are expensive and moreover involve a problem of being still hard to raise stability.
Offset correction to set the origin and adjust the zero point as shown in
FIG. 15
is made with the control
42
. Gain correction to adjust the measurement span using a master work, etc., as shown in
FIG. 16
is made with the control
48
. However, the two controls
42
and
48
affect each other and thus adjustments are hard to make. Since the controls are used, the effects of temperature change and secular change are easily received. Further, since the control attachment space is required, there is a problem of being hard to miniaturize, etc.
Particularly, to conduct multipoint measurements with the electric micrometers incorporated inline, it is substantially almost impossible to make manual offset correction and manual gain correction for each probe.
Further, in the synchronous detection system using the synchronous rectifier
52
, to deal with the various types of probes and lengths of signal cables, a phase shift occurs in an output signal to the probe and an input signal from the probe.
FIG. 17A
shows phase change depending on the probe type and
FIG. 17B
shows phase change caused by the cable length difference. Therefore, due to the phase shift occurring in the output signal to the probe and the input signal from the probe, synchronous rectification cannot well be conducted and the waveform may get out of shape as shown in the upper stage of FIG.
17
C.
When the A/D converter
56
converts an analog signal into a digital signal, unless a filter having a large time constant is inserted, it is hard to suppress display flicker as shown in
FIGS. 18 and 19
and improvement in the response speed is inhibited. This can lead to a fatal problem to incorporate the electric micrometers inline for use for automatic control in addition to simple display.
SUMMARY OF THE INVENTION
The invention is intended for solving the above-described problems in the related art.
It is a first object of the invention to provide a method and apparatus for failure of a differential transformer which can discriminate between the break state in an input signal line or a drive signal line and the normal state in the differential transformer.
It is a second object of the invention to provide a method and apparatus for signal processing of a differential transformer which can be incorporated inline for use for automatic control like a linear encoder by reducing the effects of temperature change and secular change, eliminating the need for adjustment at the assembling time, and facilitating calibration.
It is a third object of the invention to provide a method and apparatus for signal processing of a differential transformer which can deal with various types of probes, cable extension, etc.
It is a fourth object of the invention to provide a method and apparatus for signal processing of a differential transformer which can improve the response speed and suppress display flicker, etc., even with a filter having a small time constant.
In order to achieve the first object of the invention, there is provided a method for detecting failure of a differential transformer comprising: applying an offset signal for failure detection, that cannot be removed if an input signal line of the differential transformer is broken, from an offset application circuit to an output signal of the differential transformer, the offset application circuit having an output impedance higher than an impedance of the differential transformer.
Further, in order to achieve the first object of the invention, there is provided an apparatus for detecting failure of a differential transformer comprising: an offset application circuit for applying an offset signal for failure detection, that cannot be removed if an input signal line of the differential transformer is broken, to an output signal of the differential transformer, the offset application circuit
Deb Anjan K.
Mitutoyo Corporation
Oliff & Berridg,e PLC
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