GB2208139A - A pneumatic adjusting device for ultrasonic sensors for non-destructive testing of elongate test-pieces - Google Patents

Description

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A penumatic adjusting device for sensors for non-destructive testing of elongate testpleces The invention relates to a pneumatic adjusting device for sensors for non- destructive testing of elongate test-pleces, in which a receiving part abuts with preset force against a test-piece end bears the sensors and is connected to an adjusting frame by at least one bellows cylinder filled with air at low pressure.

The "elongate test-pieces" are e.g. pipes, billets, rails, metal sheets and the like. These test-pieces can either move during the test, as normally in the testing of pipes, or can remain stationary. In the first care the adjusting device remains stationary. In the second case the adjusting device is moved relative to the test-piece, e.g. when testing elongate test-pieces where there is no roller table, or when testing a pipeline or hull or an assembled railway track or the like. - Test-pieces of this kind practically never have an ideal shape. When the adjusting device travels over their surface, the receiving part thereof must be able to compensate and absorb deviations of the test-piece from the ideal geometrical shape, When a pipe is being tested, for example, it twists and/or moves lengthwise when travelling through the non- destructive materials- ter. t ing device, so that the overall scanning track is helical or linear. During the entire scanning process, the receiving part and the sensors borne thereby must always abut the test- piece with constant force but must be able to retain the set acoustic irradiation angle in any position.

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The abutment force must be independent of deviations of the pipe from the Ideal cylindrical shape.

In the prior-art pneumatic adjusting device of the initially-mentioned kind, the bellows cylinder disposed between the receiving part and the adjusting frame is filled with air at low pressure end provides an air cushion between the receiving part and the adjusting frame. The receiving part is pressed against the teBt-plece by the pressure of the air-filled bellows cylinder. If the test-piece deviates from the Ideal geometrical shape during testing, the bellows cylinder compensates the deviations by acting as an air cushion; if the testpiece moves locally away from the receiving part, the bellows cylinder extends so that the receiving part remains in contact with the test-piece. If on the other hond the testpiece Is locally overdimensioned, the receiving part gives way in the direction towards the adjusting frame, end the bellows cylinders are compressed and thus continue to abut the test-piece.

However, the known adjusting device comprising an air cushion has the disadvantage that when the bellows cylinder extends, i.e. when the receiving part moves away from the adjusting frame, the pressure inside the bellows cylinder inevitably decreases, resulting in a decrease in the resilience -of the air cushion or bellows cylinder. As a result, the abutment force of the bearing part decreases. 1 f on the other hand the bellows cylinder. retracts because the receiving part has to give way to the test-piece, the air In the bellows cylinder is compressed, so that the pressure therein rises end the air cushion becomes stiffer. This inevitably increases the force with which the receiving part bears on the test-piece. Consequently this method cannot provide a constant bearing force. The aforementioned disadvantage Is also dependent on travel - the greater the extension or compression of the bellows cylinder, the greater are the changes in air pressure in the bellows cylinder and the greater Is the deviation of the pressure of the receiving part on the test- 1 1 1 1 1 1 z 3.

piece from the preset force when the bellows cylinder is in the zero position.

An air cushion in the form of a bellows cylinder con be replaced by a mechanical spring, e.g. c tension scroll spring, the force of which is to some extent independent of the travel. The disadvantage of mechanical springs, however, is that the shock-absorption Is much lower then for bello4s cylinders. In practice, If mechanical springs are used, It Is impossible to prevent the receiving part from vibrating during compensation movements.

The object of the invention, starting from this situation, Is to develop and improve the known pneumatic adjusting device of the InitiallYmentioned kind so that the bearing force of the receiving part on the test-piece is practically independent of the distance travelled by the receiving part relative to the adjusting frame, so that the bearing force of the receiving part on the test-piece remains constant irrespective of the motion of the receiving part (end consequently of the sensors). The aim, for example, Is for the receiving part to have a total stroke of 50 mm, i.e. 25 mm out of the zero position in either direction, during which there is practically no change In the force with which the receiving part bears ori the test-piece over the entire range of adjustment.

To this end, starting from the pneumatic adjusting device of the initially-mentioned kind, the bellows cylinder is connected by a line to a compensating unit comprising a cylinder and a piston movable therein, and the piston is preferably connected to the piston of a high-pressure unit comprising a piston having a considerably smaller area than the piston of the compensating unit, the piston cylinder chamber being connected to a pressure-regulating valve.

The inventive solution, therefore, is as follows.. air is not imprisoned in the bellows cylinder, but the Ijiterior of the bellows cylinder is 1 4.

connected to the compensating unit. The internal volume of the compensating unit varies to the same extent as the internal volume of the bellows cylinder, but the two changes in volume are of opposing value, so that the total volume remains substantially constant. Volume changes inside the compensating unit cause volume changes inside the pressure cylinder but of opposite sign. The air pressure Is always accurately kept constant by the high-pressure unit In conjunction with the piston of the compensating unit. The compensating-unit pis ton Is mechanically connected to the piston (or cylinder) of the high-pressure unit. A change in volume of the compensating unit causes a change of pressure in the high-pressure unit. When the pressure rises the control valve in front Is vented. If the pressure drops, air is introduced. The product of pressure and area remains constant in both chambers.

A slight excess pressure e.g of 0.16 bar prevails in the bellows cylinder and consequently also in the compensating unit. The excess pressure in the high-pressure unit is normally about 4 bars. The advantage of intensifying the pressure between the compensating unit and the high- pressure unit is that the pressure can be adjusted by a pressure- regultsting valve which operates at the high pressure. The resulting accuracy in pressure regulation Is much higher then would be possible at low pressure, e.g. et an excess pressure of 0.16 bar. More particularly, however, commercial pressure-regulating valves can be used, In which case the pressure Intensification is advantageously chosen so that the pressure at the high-pressure side is suitable for commercial high- precision press urereg ula, t ing valves.

An important feature of the invention, therefore, is the combination of a compensating unit and a highpressure unit. As a result of this combination the pressure Is accurately regulated in the lowpressure region, although the actual pressure-regulating elements are disposed in the high-pressure region.

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The invention enables the bellows cylinder to have a long stroke without its spring force being dependent on the length of travel. Since the low pressure in the bellows cylinder Is always constant, irrespective of the mechanical state of the cylinder, the force exerted by the cylinder Is always constant. Constancy can be maintained if there are large variations in the length of the bellows cylinder; more particularly adaptation can be made to practical requirements, i.e. to. the deviations from Ideal shape expected during typical tests. An Important advantage of the invention Is that the air cushion, i.e. the bellows cylinder, has a force which remains constant over a long travel distance.

Another critical advantage of the invention is the relatively short time taken by the adjusting device to respond to changes in the shape of the test-piece, In practice the adjusting device is designed so that it can absorb even the most abrupt changes in shape and the force with which the receiving part beers on the test-piece remains constant even in these extreme cases. The adjusting device can be quickly readjusted by giving sufficiently large dimensions to the connecting lines between the previous ly-de scribed components of the adjusting device and by using movable parts having minimum mass. The letter particularly applies to the piston of the compensating unit.

The accuracy with which the pressure in the bellows cylinder (and consequently also in the compensating unit) Is kept constant, mainly depends on the construction of the compensating unit, and the construction of the high-pressure unit and the pressure- reg ula t ing valve. The first-named two units are designed so that friction is very small and does not appreciably affect operation. The pressureregulating valve has very little hysteresis.

According to another proposed feature of the invention, the receiving part is made substantially rectangular and each corner is supported 6.

by a bellows cylinder. The bellows cylinders (four in all) are preferably connected to one another and to a compensating and high pressure unit. If four bellows cylinders are provided, the adjusting device bears more accurately and uniformly on the test piece. If the size of the test-piece is irregular, the bellows cylinders can move in ways completely Independent of one another.

For example, one bellows cylinder can become shorter whereas another expands. Since the bellows cylinders are independent of one another, each corner of the four-cornered receiving part is optimelly adjusted and supported, the overall result being a uniform distribution of bearing force.

Since the f our bellows cylinders are interconnected, a single compensating and high-pressure unit can be used. Theoretically also, each individual bellows cylinder can be associated with a separate compensating and high-pressure unit, but the expense will be considerably higher. Also, the connecting line between the four bellows cylinders has the advantage that the four cylinders can exchange air. If for example one cylinder is compressed by the test-piece and another cylinder is lengthened, air expelled from the first cylinder can flow into the second cylinder. This alters the time constant after which the adjusting device responds to a change in shape.

1 In accordance with the preceding description, there can also be an embodiment in which the low-pressure chamber is sealed on all sides. This construction, however, has disadvantages in practice, because leaks, more particularly very slow leaks, are inevitable in practice, so that when the system is shut off, there is a risk of a slow, gradual pressure drop. To avoid this danger, according to another feature of the invention, the low-pressure side is permanently connected via a low-pressure regulating valve to a compressed-air source (e.g. an air pipe at a pressure of 6 bars). This feature also improves the response speed of the adjusting device. If there are very quick changes in shape, more particularly changes of all four bellows cylinders in the same direction, air can either be blown away or re-supplied via the low-pressure regulating valve.

The low excess pressure in the low-pressure region is chosen so that the desired bearing force is reached and also the weight of the receiving part Is supported together with the additional equipment connected thereto. The additional equipment typically weighs about 50 kg. At a contact pressure of 100 N the low-pressure valve is adjusted to produce a force of 600 14. Other weights or contact pressures can be obtained by adjusting the low-pressure and highpressure regulating valves.

According to another feature of the invention, an adjusting cylinder abuts the adjusting frame and is likewise in the form of a bellows cylinder. though it can also have a different construction (e.g. a hydraulic pistonlcylinder unit or a threaded spindle). Its other end is connected to c stationary base frame. The adjusting cylinder can raise or lower the adjusting frame, so that the entire device can be set in an operatin8 position or an inoperative position.

Finally, according to another feature of the Invention, a mechanical means is proposed for guiding the receiving part in the directions of its main plane. The bellows cylinders only move transversely to the receiving part and have relatively little rigidity in the transverse direction. It Is preferable to use mechanical means for securing the receiving part, In one possible embodiment an eyelet is provided on the side of the receiving device which Is in front in the operating direction during testing.

adjusting frame engages the eyelet.

A rod connected to the Other constructions for laterally securing the receiving part are possible, e.g. a chain or a flexible part at the front end connected at its other end to an arm projecting upwards from the adjusting frame, like the shaft of a cart or the like.

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Other features and advantages of the invention will be clear from the following claims and description of an embodiment not to be regarded as limitative. In the drawings:

Fig. 1 1E3 a diagrammatic sectional sketch through the pneumatic adjusting device; Fig. 2 corresponds t.o Fig. 1, also showing an adjusting cylinder, Fig. 3 is a perspective view obliquely from above, showing the entire adjusting device, the adjusting frame being partly cut away, and Fig. 4 Is a side view, partly in section, of the adjusting device.

The drawings show a pneumatic adjusting device in an ultrasonic testing system for non-destructive testing of pipes, comprising a receiving part 20 substantially in the form of a rectangular plate. The underside of part 20 is connected to four bellowe cylinders 22 disposed on the diagonals and at equeil distances from the geometrical centre of the receiving part 20. A trough 26 disposed on top of part 20 comprises sensors 24 in the form of ultrasonic transducers. The sensors are disposed inside trough 26, which has V-8uides 28 on each ride.

Trough 26 is substantially bounded by two inclined wells. Intersecting at a line at their. lowest point which passes perpendicularly above a transverse centre of the receiving part 20 and extends in the direction of advance of the tube 30 under test (Fig. 3). A pool of water Is formed in this region by transversely extending resilient lips 32, and the tube 30 under test is partly immersed in the pool. This measuring technique, however, Is generally known and will not be described here. Arrow 34 shows the direction of motion of pipe 30 durin8 testing.

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The bottom ends of the four bellows cylinders 22 are each connected to an adjusting frame 36 which has substantially the rame dimensions as the receiving part 20 and, when cylinders 22 are in their normal position, lies in a plane parallel to the plane of the receiving part 20. In the system as Illustrated (see particularly Fig. 3), frame 35 Is disposed under part 20. A suspended arrangement is also possible in principle.

Frame 36 has a passage 38 for each of the four bellows cylinders 22, the passage being the only connection to the interior of each bellows cylinder 22. Below the passage there is a compensating line 40 which interconnects all four bellows cylinders 22. Ar. shown in Fig. 4, line 40 has a relatively large inner cross-section. It Is connected via a line 42 to a compensating unit 44. Unit 44 comprises a cylinder in the form of a cylindrical pot closed by a cover and in which a piston 48 Is axially movable. A roller diaphragm 50 provider. a real between cylinder 46 and piston 48. A s compared with a sliding real on piston 48, the diaphragm has the advantage of lower friction and a more reliable real. Piston 48 Is connected to a piston rod which at Its lower end bears a piston 52 of a hiSh-pressure unit 54. The area ratio of the two pistons 45 and 52 is about 25: 1, The difference in volume In the compensating unit 44 betwen the two end positions of piston 45 corresponds to the change in volume at the maximum stroke of the four bellows cylinders 22.

The high-pressure unit 54 is a pneumatic cylinderlpiston unit with rainimum friction between the sealing rings for piston 52 and the inner well of a cylinder 56. the low friction is obtained by flexible lips, used for sealing the piston. These flexible lips need only very slIght radial prestress.

They have a rounded sealing edge by means of which the lubricant on the moving surface of the cylinder Is uniformly distributed over the entire stroke, so that there is always a film of grease. The sealing lip has a V cross- 10.

section, both limbs or which rest in a groove in piston 52, the groove being connected by at least one bore to the high-pressure s ide. The high pressure thus radially expands the sealing ring to form a flexible lip.

Since both pistons 48 and 52 are disposed on a single piston rod, there is no need for the high-pressure unit 54 to have other guide units, more particularly a guide for the piston rod. The interior of the highpressure unit 54 is connected by a line 58 to a pressure regulating valve 60 having a hysteresis of about 0.1 bar. The pressure in line 58 is about 4.1 bars and Is somewhat higher (about a tenth of a bar higher in the embodiment under discussion) than the pressure in the low-pressure region multiplied by the pressure intensification ratio (i.e. 0.16 x 25 = 4 bars).

The pressure-regulating valve 60 maintains a constant pressure in the high-pressure range, i.e. in line 58 and inside the high-pressure unit 54. Accordingly, valve 63 is constructed so as to keep the preesure constant in optimum manner. The other side of valve 60 lea connected to a compressed-air supply line 62 connected to a compressed-air system. The pressure therein is typically 6 bars.

The compensating line 40 is also connected by a line 64 to a lowpressure regulating valve 66, the other side of which is likewise connected to the compressed air supp.ly line 62. This ensures that even if the lowpressure part leaks, the pressure cannot fall below 0.16 bar. If the pressure drops as a result of a leak or after simultaneous extension of all four bellows cylinders 22, air is supplied to the low-pressure region through line 64. Correspondingly, air is blown off if all four bellows cylinders 22 are simultaneously strongly compressed.

Chains 68, which could be replaced by other flexible tension-proof elements, ere tensioned at all four corners between the receiving 11.

part 20 and the adjusting frame 36. The chains limit the maximum travel of the four cylinders 22. Finally, an attachment rod 70 projects from frame 36 to part 20, where it engages an eyelet 72 substantially bounded by two parallel rods 74 extending transversely to the direction or arrow 34. Parts 70 - 74 prevent the receiving pert and the devices carried by it from being shifted sideways relative to frame 36. Parts 70 -74 are disposed on that ride of the receiving part 20 which extends transversely to the direction of arrow 54 and is nearest to a pipe 30 arriving for testing.

As shown in Figs. 2 and 4, an adjusting cylinder 76 Is secured to the underside of frame 36 and is likewise in the form of a bellows cylinder. The other end of cylinder 76 Is connected to a base plate 78 which also has a passage 80 for connecting the cylinder interior to a line 82. The construction Is such that the bareplate 78 and adjusting frame 36 always lie in parallel offset planer.. Lin e 8 2 contains serially connected devices 84 and 86 each comprising a nonreturn valve and a throttle valve connected in parallel. The nonreturn valves of systems 84, 86 operate in opposite directions. The throttle valves are adjustable. At their lower ends, line 82 is connected to a control valve for filling the adjusting cylinder 76 from the compressed-air supply line 62, blocking the bottom end of line 82, and opening the bottom end to the exterior. When the adjusting cylinder 76 fills, air flows through the non-return valve or the lower system 86, which is inoprative in thir. direction, and through the throttle valve of the top system 84. When the bottom end of line 82 is cornected to the exterior, air flows from cylinder 76, through the nonreturn valve of the top system 84, which Is inoperative in this direction, and through the throttle valve in the lower system 86. The two systems 64, 86 can therefore be used for independenty setting the adjusting or lowering speed of cylinder 76.

v k Since cylinder 76 operates in only one direction, tension springs (not shown) ere disposed between base plate 78 and frame 36 inside a 1 1 12.

spherical guide unit 92, and pull the two parts together. The lowest possible spacing obtainable between plate 78 and frame 36 Is limited by set screws 90, which consequently also preset the stroke of cylinder 76. Two spherical guide units 92 ensure accurat e parallel guidance between plate 78 and frame 36.

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Claims (11)

1. A pneumatic adjusting device for sensors for non-destructive testing of elongate test-pieces, in which a receiving part abuts with preset force against a test-piece and bears the sensors and is connected to an adjusting frame by at least one bellows cylinder filled with air at low pressure, connected by a line to a compensating unit comprising a cylinder and a piston movable therein, and the piston is preferably connected to the piston of a high-pressure unit comprising a piston having a considerably smaller area than the piston of the compensating unit, the piston cylinder chamber being connected to a pressure- regulating valve.

2. An adjusting device according to claim 1, wherein the receiving part is made substantially rectangular and each corner thereof is supported by a bellows cylinder, the bellows cylinders being preferably disposed along the diagonals and at equal intervals.

3. An adjusting device according to claim 1 or 2, wherein a plurality of bellows cylinders are provided and are interconnected via a compensating line.

4. An adjusting device according to any one of claims 1 to 3, wherein at least one bellows cylinder is continuously connected via a low-pressure regulating valve to a compressed-air source.

5. An adjusting device according to any one of claims 1 to 4, wherein the piston of the compensating 14 unit and the piston o-f the high-pressure unit are rigidly interconnected by a piston rod and preferably the area ration of the pistons is 25: 1.

6. An adjusting device according to any one of claims 1 to 5, wherein an adjusting cylinder is disposed at the underside of the adjusting frame and its other end is connected to a base frame, and preferably the adjusting frame and the base frame are interconnected by spherical guide units.

7. An adjusting device according to any one of claims 1 to 6, wherein a roller diaphragm is disposed in the compensating unit between the piston and cylinder.

8. An adjusting device according to any one of claims 1 to 7, wherein the pressure-regulating valve connected to the high-pressure unit is set at a pressure which is somewhat higher, preferably 0.1 bar, than the pressure in the compensating unit multiplied by the area ratio of the pistons.

9. An adjusting device according to any one of claims 1 to 8, wherein the pressure-regulating valve has minimal hysteresis, preferably less than 0. 1 bar, and minimal friction.

10. An adjusting device according to any one of claims 1 to 9, wherein the receiving part bears a trough with V-guides connected by a line extending parallel to the direction of advance of a test-piece.

11. An adjusting device substanially as described 1 V 1 herein with reference to any one or more ot figures 1 to 4.

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