Note: Descriptions are shown in the official language in which they were submitted.
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ROTATIONAL P08I~ION 8EN80R
BACXGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally related to nondestructive
inspection of tubes and more particularly to a rotary position
sensing apparatus.
2. General Background
In the commercial nuclear industry steam generators used in
energy production are generally of the straight tube or U-tube
design. Primary coolant from the nuclear reactor travels through
the tubes and transfers heat to the secondary coolant in the
steam generator. Due to the serious consequences which can
result from a tube leak, the tubes are inspected for defects on a
routine basis. To perform such inspections, it is typical to use
a pancake eddy current coil mounted in a rotating probe that is
caused to travel through each tube being inspected. This results
in a helical inspection path for the coil. During inspection,
the coil induces eddy currents in the tube wall. Variations in
these eddy currents, caused by tube defects, can be sensed by
monitoring the coil impedance. Changes in the coil impedance may
be used to indicate the presence of defects in the tube. The
quality of the inspection results is directly related to the
ability to accurately synchronize the eddy current data from the
probe head coil with the rotation of the probe head. Patents
directed to determining the rotary position of a shaft which
applicants are aware of include the following:
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U.S. Patent No. 4,746,859 discloses the use of a pair of
magnetic sensors and a ferromagnetic shaft. The sensors are
placed adjacent to one or two wheels having continuously
increasing radial dimensions with radial offsets on the outer
surfaces at common points of maximum and minimum radial
dimension. The change in the output signals indicate the change
in the length of the air gap between the sensors and wheels and
the corresponding position of the wheel and a rotor attached to
the same axle.
U.S. Patent No. 3,297,940 discloses the use of a shaped
magnetic shield on a rotating disc between an oscillating coil
and a pick up coil for use as a potentiometer that eliminates the
need for a contact slider.
U.S. Patent No. 3,786,459 discloses a position coder for a
moving shaft that generates a DC voltage having an amplitude
proportional to the magnitude of position shift of the moving
shaft with respect to a fixed point and a sign that changes
whenever a shift in direction takes place.
U.S. Patent N0. 3,562,741 discloses an electromagnetic pulse
generating system utilizing a single transducer and a disc having
teeth of various shapes that generate different signals as they
move past the transducer during rotation of the disc.
U.S. Patent No. 4,719,419 discloses a noncontact rotary
position sensor for measuring the rotary position of a shaft that
' 25 includes a magnetic member mounted for rotation with the shaft
; where the central axis of the magnetic member is eccentric from
' the axis of rotation of the shaft. A Hall effect device spaced
from the sha t measures the varying magneti= ~lux density
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produced by the magnetic member to determine the position of the
shaft.
U.S. Patent No. 4,507,638 discloses a rotatable plate shaped
to allow predetermined amounts of magnetic flux to pass from a
5drive coil to a sensing coil as the plate is rotated to determine
a selected angle of rotation of the plate.
U.S. Patent No. 4,764,767 discloses a rotor section with a
first pattern that repeats change in the circumferential
direction with a predetermined pitch and a second pattern that
10has a change of one cycle with respect to one circumference.
Comparison of signals from a detector for each pattern provides a
value of the rotational position of the rotor.
', U.S. Patent No. 4,631,510 discloses a harmonically graded
air gap reluctance-type rotating electric resolver. A rotor with
, 15a single lobe receives magnetic flux as it rotates, with the flux
~ received by the rotor being directly related to the position of
v the rotor relative to teeth on a stator.
- U.S. Patent No. 3,819,025 discloses an apparatus for
determining the angular position of print on a paper by
' 20determining the angle of rotation of a revolving axis from a
s predetermined reference position. A magnetic shield plate in the
; form of a spiral is used to control the degree of magnetic
coupling between primary and secondary windings.
, U.S. Patent No. 3,835,373 discloses a rotational position
25sensor where a half cylinder provides a varying signal by varying
the air gap between the half cylinder and a C-shaped permanent
magnet.
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Although there are a number o~ electromagnetic methods for
determining the circumferential position of a shaft, most tend to
be complicated. Devices such as that in U.S. Patent No.
4,746,859 require a pair of a magnetic sensors to be used with a
ferromagnetic shaft and circuitry for adding, subtracting,
multiplying, and dividing the signal. Magnetic sensors are
usually three-wire hall devices that are fabricated using
expensive semiconductor technology. This leaves a need for
devices capable of accurately determining the circumferential
position of a shaft that does not require multiple sensors, will
fit into small areas, and can be inexpensively fabricated in
small quantities.
SUMMARY OF THE INVENTION
The present invention addresses the above need in a
straightforward manner. What is provided is an eddy current
encoder that provides circumferential orientation feedbacX to an
acquisition computer that registers and aligns the tube
inspection data. A probe body assembly used in the examination
of tubes has a drive motor mounted therein that drives or rotates
an inspection head at one end of the probe body. A spiral shaped
encoder wheel is rotatably mounted in the probe body and attached
to the drive motor. The encoder wheel is rotated by the drive
motor simultaneously with rotation of the inspection head. An
eddy current coil is mounted in a stationary position in the
probe body and radially spaced from the encoder wheel. The eddy
current coil senses the air gap between the encoder wheel and the
eddy current coil. This information is used to synchronize the
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eddy current data obtained from the eddy current coil with the
rotation of the inspection head.
B~IEF DESCRIPTION OF THE D~AWINGS
For a further understanding of the nature and objects of the
present invention reference should be made to the following
drawings taken in c~njunction with the accompanying description
in which like parts are given like reference numerals and,
wherein:
Fig. 1 is a perspective view of the invention.
Fig. 2 is a sectional view illustrating the invention in a
probe body.
Fig. 3 is an enlarged view taken along lines 3-3 of Fig. 2.
Fig. 4 is an illustration of signals generated by the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the drawings, it is seen in Fig. 1 that the
invention is generally indicated by the numeral 10. Rotational
position sensor 10 is comprised of encoder wheel 12, eddy current
` coil 14, and eddy current instrument 16.
' 20 Encoder wheel 12 has a main body portion 18 that is
substantially cylindrical in shape. One end of main body portion
` 18 is adapted to be attached to a drive motor by being provided
with slot 20 that extends radially and axially through the end.
, The opposite end of main body portion 18 is provided with axial
threaded bore 22 for threaded mounting on an axle. Main body
portion 18 is provided with section 26 substantially along its
center that has a larger diameter than main body portion 18.
Section 26 is formed in a substantially spiral shape such that it
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has a continuously increasing radial dimension as measured from
the longitudinal axis of main body portion 18. Section 26 is
provided with radial offset 28 at the common point of maximum and
minimum radial dimension. Eddy current coil 14 ~s mounted on
probe body 24 in a stationary position relative ta encoder wheel
; 12. Eddy current coil 14 is connected with eddy current
instrument 16 by wiring 30. Electrical signals generated by eddy
current coil 14 that indicate the size of the air gap between
eddy current coil 14 and spiral section 26 of encoder wheel 12
are received, recorded, and displayed by eddy current instrument
16. As seen in Fig. 4, the signals generated by eddy current
coil 14 and displayed on screen 32 and recorded by eddy current
instrument 16 are proportional to the air gap or distance between
eddy current coil 14 and spiral section 26 of encoder wheel 12.
As an example, point 28A of the signal generated on screen 32
corresponds to the rotational position of encoder wheel 12 where
radial offset 28 is at its closest position to eddy current coil
14. It can be seen that the variable electrical signals from
eddy ~urrent coil 14 can be used to determine the rotational
position of encoder wheel 12. This also allows the rotational
position of equipment rotated simultaneously with encoder wheel
12 to be determined.
An example of such a use is illustrated in Fig. 2 and 3.
; Encoder wheel 12 is rotatably mounted inside probe body 24
between a drive motor/gear assembly 38, and a slip ring assembly
40. Axial threaded bore 22 threadably receives the threaded slip
` ring shaft axle 34. At the opposite end, slot 20 receives flats
on drive shaft 36 that extend from drive motor 38. The torque of
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the motor 38 is transmitted through the encoder wheel 12, slip
ring 40, a short flexible drive shaft 47 and an
electrical/mechanical connector 45 to probe head 44.
Electrical signals are carried by non-rotating wires 42 to
the slip ring 40. Within the slip ring, brushes transmit the
electrical signals to the inspection head through rotating wires
43 running through the flexible drive shaft 47. Inspection head
44 is used to inspect tubing walls for defects as probe body 24
is moved through the tubing while inspection head 44 is rotated.
Inspection head 44 may be provided with means 46 for inspecting
the tubing such as an eddy current coil. In a manner known in
the art, the eddy current coil is used to induce eddy currents
and sense the presence of defects in the tubing. It should be
noted that although inspection head 44 is generally indicated as
one that uses an eddy current coil to detect the presence of
defects in the tubing, any of sever~l different types of
inspection heads and means for inspecting the tubing known in the
art may be used in conjunction with rotational position sensor
10. The rotational position of means 46 relative to the
rotational position of encoder wheel 12 is recorded upon
installation of inspection head 44. This allows the signals from
means 46 on inspection head 44 to be synchronized with the
signals from eddy current coil 14 whereby the variable electrical
signals from eddy current coil 14 are representative of the
rotational angular position of inspection head 44. This provides
a means for determining the rotational position of defects in the
tubing. This can be useful in diagnosing specific problems in
equipment being examined.
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In operation, probe body 24 is lnserted into a tube o~ a
steam generator or similar type of equipment for examination of
the tube. Probe body 24 is inserted and moved through the tube
by cable 48 attached to one end of probe body 24. Cable 48
houses wiring from equipment inside probe body 24 such as eddy
current coil 14, drive motor 38, and inspection head 44 for
` monitoring and control of the equipment. While probe body 24 is
travelling through the tube, drive motor 38 is actuated. This
causes simultaneous rotation of encoder wheel 12 and inspection
head 44. The changing air gap between eddy current coil 14 and
encoder wheel 12 produces a variable electrical signal from eddy
current coil 14 that is registered and recorded by eddy current
instrument 16. A visual representation of these varying signals
may be provided in form of a display on screen 32 such as that in
Fig. 4. Signals from inspection head 44 and tube inspection
means 46 are recorded and analyzed to determine the presence of
i defects in the tube being examined. Since the rotational
position of inspection head 44 relative to encoder wheel 12 is
known, the variable electrical signal from eddy current coil 14
is representative of the rotational angular position of
inspection head 44.
Because many varying and differing embodiments may be made
; within the scope of the inventive concept herein taught and
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because many modifications may be made in the embodiment herein
detailed in accordance with the descriptive requirement of the
-~ law, it is to be understood that the details herein are to be
interpreted as illustrative and not in a limiting sense.
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