Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
2003-10-08
2004-08-24
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207120, C340S870360
Reexamination Certificate
active
06781366
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a circuit for a displacement detector incorporating with a sensor coil forming a differential transformer.
FIG. 28
shows the circuit structure of a prior art contact displacement detector with a sensor
201
incorporating a differential transformer
202
and a sensitivity-adjusting resistor
203
. The differential transformer
202
has a mobile core (not shown) and two coils
204
and
205
disposed around this mobile core. These two coils
204
and
205
are connected in series and driven by an AC voltage provided as a driving signal from an oscillator
206
through an amplifier
207
. Output signals are taken out from a junction point in between.
This displacement detector is a transducer of the half-bridge type. The inductance of the two coils
204
and
205
driven by an AC voltage is' a function of the position of the mobile core. The inductive voltages generated in the two coils
204
and
205
are equal to each other if the mobile core is at the center of the two coils
204
and
205
. If the mobile core is displaced from this center position, the inductive voltage of one of the coils
204
or
205
increases and that of the other coil
204
or
205
decreases. A contact member (not shown) for contacting the target object of measurement is attached to this mobile core and the sensor is adapted to detect the displacement of this contact member.
The output signal from the junction at the center of the two coils
204
and
205
is an AC output of which the amplitude changes according to the displacement of the mobile core. After being amplified by an amplifier
208
, this output AC signal is subjected to a full-wave rectification process by an AC-DC converter
209
and inputted to the non-inversion input terminal of a differential amplifier
210
. Another AC voltage applied from amplifier
207
to the differential transformer
202
is inputted to the inversion input terminal of this differential amplifier
210
through amplifier
211
and AC-DC converter
212
to serve as a standard signal. The differential amplifier
210
amplifies the standard signal and the output signal from the differential transformer
202
differentially and outputs a signal corresponding to the displacement of the mobile core.
According to this illustrated example, not only is the sensor
201
itself provided with a sensitivity-adjusting resistor
203
, but the amplifier
208
for amplifying the output signal from the differential transformer
202
is provided with a gain-switching resistor
213
such that the gain of the amplifier
208
can be changed, depending on the kind of the sensor
201
, that is, such that the same circuit can be used with sensors of different kinds with different ranges of measurement (or strokes).
According to this example, furthermore, a pull-down resistor
214
is connected to the output signal line of the differential transformer
202
and there is also provided a comparator
215
for comparing the output from the AC-DC converter
209
with a threshold value to provide a detection output. If there is a breakage in the sensor cable connected to the sensor
201
, or when the wire for transmitting a signal for driving the sensor
201
is broken (as indicated by A
1
) or the sensor signal output line A
2
is broken, for example, the AC voltage signal outputted from the sensor
201
is not communicated and becomes zero by the pull-down resistor
214
such that the breakage can be detected by the comparator
215
. If the breakage is only in the grounding line, as shown by A
3
, the sensor driving signal is not divided by the coils
204
and
205
and hence the sensor driving signal is directly outputted. This, too, can be detected by the comparator
215
.
For carrying out measurements with a high level of accuracy with such a prior art sensor, very small signals from the differential transformer must be taken out at a high level of stability and with a high S/N ratio. Moreover, the output from the amplifier
207
to become a standard signal must also be stable. For this purpose, an oscillator and an amplifier such as an operational amplifier with high accuracy and stability are required. For obtaining a high S/N ratio and stability, a dedicated IC incorporating an operational circuit for temperature compensation, etc. must be used, and this affects the production cost adversely.
Since different sensors have different sensitivities, furthermore, the gain of the amplifier
208
is adjusted by means of the gain-switching resistor
213
. Thus, if a sensor with low sensitivity is used, the S/N ratio becomes lowered as the gain is increased. Although it is desirable to use processing systems having similar processing characteristics for the standard signal and the output signal from the differential transformer
202
, the processing system for the output signals from the differential transformer
202
is different from that for the standard signal, being adapted to switch to change the gain. Thus, it is difficult to make the temperature characteristics of the components uniform and to place the components in a thermally well balanced manner.
Moreover, since the breakage of the sensor cable is detected on the basis of the output AC signal, if the inductance of the differential transformer
202
is increased in order to improve the sensitivity of the sensor
201
, the output AC signal from the differential transformer becomes unstable due to the capacitive coupling between the signal lines at both ends of the coil
204
or
205
when there is a breakage in the sensor cable and the breakage may not be detected dependably. It may be attempted therefore to reduce the resistance of the pull-down resistor
214
in order to reduce the effect of the capacitive coupling but if the resistance of the pull-down resistor
214
is reduced, the linearity characteristic of the differential transformer
202
becomes adversely affected. A similar result is obtained even if a pull-up resistor is used instead of the pull-down resistor.
Another problem of prior art displacement sensors of this kind relates to their structure. If the diameter of a sensor is reduced from &phgr;8 to &phgr;6, for example, the sensor can be attached to a target object (such as a machine) more intimately and the target object can be made more compact. Since the weight of the mobile parts of the sensor must be reduced accordingly, the load to the sensor can be reduced and hence the sensor becomes usable for the measurement of an object which could not be measured because of its large load. When the diameter of a sensor is reduced from &phgr;8 to &phgr;6, however, it is not sufficient to merely reduce its linear dimensions to three quarters (0.6/0.8) of the original and to reduce the cross-sectional area by a factor of (0.6/0.8)
2
=0.56. It cannot be ignored that stoppers for the rotation of a mobile component for driving the core member, for example, must retain their original function and capability. Moreover, the difficulty in assembly because of reduced size of components must be considered and the need for water-proofing between the mobile components for the core member becomes more important.
FIG. 23
shows the structure of an example of prior art displacement sensor, having a linear bush
81
and the bobbin assembly of a differential transformer
95
inside a housing
80
. A mobile member
101
having a mobile shaft
91
and a core member
89
connected to this mobile shaft
91
is movable longitudinally inside this housing
80
through a linear bush
94
. The core material
89
is inserted into the bobbin assembly of the differential transformer
95
to form the differential transformer
95
. The mobile member
101
is biased by means of a spring member (not shown) such as a parallel coil spring with invariable coil diameter so as to, protrude the tip of the mobile shaft
91
out of the housing
80
and a contact member
93
is formed at the protruding portion of the mobile shaft
91
. The linear bush
94
is of a structure having inserted inside an ou
Hiramatsu Naruaki
Itou Tadashi
Kuramoto Satoshi
Nakamura Kimihiko
Beyer Weaver & Thomas LLP
OMRON Corporation
Patidar Jay
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