Note: Descriptions are shown in the official language in which they were submitted.
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LTRASONIC PROBE
DESCRIPTION
This invention relates generally to ultrasonic
probes, and is concerned with wheel probes which incor-
porate a rolling element to traverse a material under
test.
Conventionally, when using wheel probes to detect
flaws in workpieces over which the probe travels, one
uses a coupling fluid on the surface of the workpiece
to provide the necessary acoustic coupling between the
workpiece and the tyre, i.e. the rolling element, of the
probe. However, with certain materials it is not
possible to use an oily coupling medium on the surface
of the material. This is the case for example when
testing for faults in friction materials, such as for
brake linings, where one is testing for delaminations,
unresinated areas and faulty bonding of the friction
material to its backing. A further problem which arises
from the use of a liquid coupling medium between the
probe and test object is the difficulty of maintaining
the thickness of the liquid layer constant. During
testing it is necessary to maintain the liquid layer at
a constant thickness and as thin as possible in order
to obtain reasonably consistent results.
It is an object of the present invention to
provide a wheel probe which does not require the use of
a coupling medium between the surface of the rolling
element and the material under test.
In accordance with the present invention there
is provided an ultrasonic probe arranged to traverse a
material under test with rolling contact therebetween~
the probe comprising a rotatable rolling element which
is filled with an acoustic coupling medium, and trans-
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ducer means within the rolling element, the rolling ele-
ment being arranged to elongate axially along its axis of
rotation when subject to external pressure arising from
the rolling element having its rolling surEace pressed
against a material under test to bring the internal surface
of the rolling element into acoustically coupled lubrica-
ted bearing contact with a face of the transducer means.
The rolling element is preferably a tyre with the
transducer means mounted within the contour of the tyre
so that there is an acoustically good interface between
the transducer means and the internal surface of the
tyre as the probe traverses the material under test.
There is thus provided what may be referred to as
a "bellows" probe, in which the contour of the rolling sur-
face is designed to change when the wheel is pressed into
contact with a material under test, this change in shape,
in a preferred embodiment, both causing an axial elonga-
tion of the wheel and also causing the internal surfaceof the tyre to contact a face of the transducer means.
Preferably, the rolling element is filled with a
thin oil at a pressure of about 2 psi.
Preferably, the transducer means is spring-urged
towards the internal surface of the rolling element to
provide a loading of said face of the transducer means
against the rolling element.
One embodiment of probe in accordance with the
invention will now be described by way of example and
with reference to the accompanying drawing, in which:
Fig. 1 is a side elevation of the probe; and,
Fig. 2 is a front elevation of the probe.
As shown in the drawings, the wheel probe comprises
a centre axle 10 which carries a housing 12 for a trans-
ducer 14 which includes a piezoelectric crystal. The
emitting/receiving face of the crystal is positioned at
the underside of the transducer 14. The transducer
housing 12 is spring-loaded in a downward direction away
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from the axle lO with a suitable constant spring force.
One end of the axle lO is journalled in a downwardly
extending leg of a connection box 16 into which passes
a coaxial cable 18 for taking signals to and from the
probe transducer.
The rolling element of the probe is a tyre 20
which is hollow and comprises a membrane which at rest
has a circumferential surface which is curved convexly.
The tyre 20 may be a moulded element of rubber or
equivalent material which is resilient and deformable.
One side of the tyre is fitted with a sealing disc 22,
and axially outwardly of this disc there is a non-
friction bearing 24 which permits rotation around the
centre axle lO. The other side of the tyre 20 like-
wise is fitted with a sealing disc 26 which has fittedinto it a non-friction bearing seal 28. This non-
friction bearing seal is designed so that it will still
provide a seal between the membrane of the tyre and the
axle even when the bearing seal moves axially along the
length of the axle lO.
When the wheel probe is assembled, the hollow
interior of the tyre membrane is filled with an acoustic
coupling medium, such as a very thin oil, to a pressure
of about 2 psi. When the wheel probe is pressed into
contact with a material under test,or when a load is
otherwise applied to the underside of the tyre, the
membrane deforms under this pressure so that the surface
of the tyre which is in contact with the test material
forms a flat region beneath the transducer, as indicated
by the broken line in the drawing. At the same time,
the bearing seal 28 is displaced axially outwards along
the axle lO due to the resilience and deformability of
the tyre material. As a result, the face of the trans-
ducer 14 which carries the piezoelectric crystal comes
into contact with the internal surface of the membrane.
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The pressure of the oil within the tyre, taken in con-
junction with the bias force on the transducer, is a
critical factor. If the pressure is too high, one does
not achieve the proper displacement, i.e~ there is an
excessive and possibly irregular thickness of oil between
transducer and tyre, and side reflections can arise. If
the pressure is too low, then there will be insufficient
acoustic coupling and also the generation of mechanical
noise. The spring loading of the transducer housing
maintains a constant loading of the crystal face against
the tyre membrane on the one hand, while on the other
hand the couplant which fills the tyre acts as a load
bearing between the crystal and the internal surface of
the memhrane to maintain a constant acoustic coupling
between these two surfaces. Because of the spring
pressure and the couplant pressure there is a constantly
maintained film of coupling fluid between the crystal
and the membrane, thus ensuring a constant path length for
the ultrasonic waves while the wheel probe is in motion.
This constant path length acts as a near field stand-off,
thus eliminating any reflection echoes from the internal
surface of the membrane. The membrane thickness then acts
as the transducer stand-off. One thus achieves a constant
thickness stand-off in good acoustic contact with
the crystal face by virtue of the film of couplant between
the membrane and the transducer. The deformation of the
tyre whereby the membrane surface is brought into contact
under load with the transducer ensures that one does not
have a varying thickness of fluid between the internal
surface of the membrane and the crystal face.
In one particular embodiment of wheel probe as
described above, with a tyre of 25 mm diameter and a
tyre membrane having a thickness of 0.635 mm, good
results have been achieved when the transducer housing
is spring loaded with a fixed loading of 8 o~nces and
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the oil pressure within the tyre is 2 psi.
Although in the example illustrated in the drawing
the axial displacement of the tyre, i.e. the bellows
effect, is to one side of the tyre only, one could arrange
for the tyre to deform in both axial directions by the use
of appropriate bearing seals. Again, the acoustic
couplant within the tyre and the pressure under which it
is loaded will vary depending upon the particular applic-
ation and mode of use of the probe. Furthermore,
although the probe is shown as having only a single trans-
ducer within the tyre, the invention is to be understood
as extending also to the use of more than one transducer,
for example arranged side-by-side.
The rolling element probe of the present invention
can be used at relatively high speeds of rotation on a
variety of surfaces. It is particularly suited to the
detection of laminations within various types of material
without the use of any coupling liquid between the probe
and the material under test.
