GB2163265A

GB2163265A - Extensometer and load measuring system

Info

Publication number: GB2163265A
Application number: GB08515611A
Authority: GB
Inventors: Graham Tock
Original assignee: DUNN ENGINEERING Ltd
Current assignee: DUNN ENGINEERING Ltd
Priority date: 1984-08-18
Filing date: 1985-06-20
Publication date: 1986-02-19
Also published as: GB8421064D0; GB8515611D0

Abstract

The extensometer comprises two parts displaceable towards and away from one another, a clamping means (21) for each part to clamp the part against an elongate member (7) and a transducer (9) attached to one of said parts to sense relative displacement, each part comprising a first abutment (15) to abut on the elongate member with each clamping means (21), when in place, constituting a second abutment on the elongate member diametrically opposed to the first abutment member and each clamping means being such that it does not impose any substantial force on the first abutment tending to displace it from the diametrical plane in which it lies. A load-measuring system may comprise an electronic unit to process signals from a plurality of such extensometers and provide a read-out of load. <IMAGE>

Description

SPECIFICATION Extensometer and load measuring system employing same The present invention relates to an extensometer for use in a load measuring system and to such a system employing the extensometer.

A particular application of load measurement systems is the determination of the weight of suspended structures such as suspended boilers. It is the practice in power stations, for instance, to mount a boiler in this manner; the boiler being suspended by sling rods anchored at their upper ends on a suitable support structure located above the boiler. When the boiler is to be weighed, the weight of the boiler is taken by an hydraulic jack to release the tension in the sling rods and the rods are released from their anchorages on the support structure; the operating parameters of the jack while under load then being used to determine boiler weight.

It is also known to use an extensometer on a rod to deduce boiler weight while the rods are being loaded with the weight of the boiler.

The existing systems of weighing a suspended boiler are not, however, satisfactory in that they are laborious in use and do not readily provide for an accurate determination of boiler weight.

It is an object of the present invention to provide an extensometer to permit a more effecacious measurement of the weight of a suspended structure by measuring the suspension extension as it is loaded by the structure.

It is also an object to provide a suspended load measurement system using an extensometer according to the present invention.

There is provided by the present invention an extensometer comprising two parts displaceable towards and away from one another along a given axis, a clamping means for each part to clamp the part against an elongate member passing parallel to said axis, and a transducer attached to one of said parts to sense relative displacement along said axis of said two parts; each part comprising a first abutment to abut on the elongate member with each clamping means, when in place, constituting a second abutment on the elongate member diametrically opposed to the first abutment member, and each clamping means being such that in clamping the respective part to the elongate member, it does not impose any substantial force on the first abutment tending to displace it from the diametrical plane in which it lies.

Preferably, each clamping means comprises a chain anchored to the respective part so that it may be passed round the elongate member and having means for securing the chain at a remote end thereof to the respective part so that the chain can be tightened around the elongate member.

It is also preferred that a third abutment be provided carried by each part to lie displaced from the first abutment along said axis but in the same diametrical plane. The first abutment is preferably constituted by a knife edge and the third by a roller. In an embodiment of the invention, the roller is mounted so as to be adjustable radially of said axis in relation to the knife edge. In the embodiment, the two parts are connected by one or more springs so as normally to lie a predetermined distance apart in the null position of the or each spring and the or each spring is housed in a protective tube carried at one end by one of the parts and, in a slightly extended condition of the or each spring, contacting at the other end, the other of the parts.In the embodiment, the or each tube can be axially adjusted in position to be removed from contact with the other part to permit use of the extensometer.

The transducer is preferably a linear differential transformer; and in the embodiment it is fixed to the rear face of a protective plate carried by said one part.

An advantage of the extensometer in the present invention is that it is capable of being constituted as a very sensitive instrument for load measurement while nevertheless being robust and simple to install and use.

In a system according to the present invention, the signal from a transducer is processed by an electronic unit to provide a read out of load. Being given the use of a transducer providing an electrical analogue signal, the signal is preferably digitised and the digital signal processed. In an embodiment of a system according to the present invention, the signal is processed by a software program fed into an E.P.R.O.M. which takes into account the modulus of elasticity of the elongate member concerned and its diameter. Usually, a load determination of a boiler will require an extensometer for each sling rod; and accordingly the embodiment provides a plurality of channels of up to 12, so that up to that number of extensometers may be used and the signals from them processed simultaneously.The unit may provide for alternative moduli of elasticity so that the system may be used with sling rods of different composition, and for display of load measurement. It may also provide for data and command inputs by keyboard, and be equipped with a printer to print out load measurements effected in each channel.

The unit preferably has provision for zeroing any signal from a transducer so that tolerances in the transducers of different extensometers giving rise to a signal in the null or unextended position thereof, are eliminated.

In use of a system according to the present invention, an extensometer is attached to each elongate member supporting the load when the load is otherwise supported and the elon gate members are not under tension. Their anchorages are then secured and the load smoothly transferred to the elongate members. The readings of the transducers are sampled several times during this operation and the load is determined when the transducers show no further elongation of the elongate members. The read out from the processor then shows the load on each elongate member in addition to the computed weight supported by all the elongate members: if the weight distribution thus revealed is considered unacceptable, adjustments can then be made to the appropriate ones to achieve the required weight distribution.

In a further embodiment of the present invention, provision is made for taking load measurements on bent rods since, in practice, rods may be in such a condition. In this embodiment, the extensometers are used in pairs diametrically opposed on each side of a rod and, if need be, with one axially displaced by an amount in relation to the other to leave the clamping means of each extensometer unobstructed by the other extensometer. Assuming the transducers used in each extensometer is one providing an electrical output signal, one in each pair may comprise connections to receive the electrical lead of the other so that both can be connected to the processing unit by one cable rather than by separate cables.

The processor unit is adapted to accept pairs of inputs with each pair associated with a pair of displays so that the load on the rod as measured by each extensometer in a pair can be displayed. The processor software makes provision for processing signals of eeither polarity so that both tensile and compressive loads can be measured and indicated on the displays by a suitable change of polarity sign. The processor unit may also provide a mode facility so that the loads shown in a pair of displays can be averaged and the average displayed in one of the displays of the pair.

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a front elevation of an extensometer according to the present invention; Figure 2 is a section on the line 'X-X' of Figure 1; Figure 3 is a section on the line 'Y-Y' of Figure 1; Figure 4 is a plan view of the device of Figure 1; Figure 5 is a plan view of an electronic processor for use with an extensometer in a first embodiment of a system according to the present invention as shown in Figures 1 to 4 and Figure 6 is similar to Figure 2 but shows an extensometer pair for use in a second embodiment of a system according to the present invention.

Turning now to Figures 1 to 4, the extensometer comprises two parts 1, 3 displaceable towards and away from one another along the axis indicated by line 'X-X' in Figure 1, an identical clamping means for each part for clamping each part on an elongate member 7 such as a sling rod, whose extension under load is to be measured, and a transducer 9 attached to part 1 by a protective plate 11 carried by the part 1 with the transducer being secured to the rear face 1 3 of the plate.

Each part comprises a first abutment 15, and each clamping means (see Figure 4) constitutes a second abutment, as at 17, on the elongate member with the two abutments being diametrically opposed.

Each part comprises a transversely elongate body 1 9 (see especially Figure 1) and each clamping means comprises a chain 21 anchored to the body adjacent the right hand extremity thereof as seen in Figure 1 so that the chain can be passed round the elongate member as shown in Figure 4 and secured at a remote end to the body at the left hand extremity thereof as seen in Figure 1.For this purpose, the body at the right hand end is formed with a notch 23 so that a link 25 of the chain can be passed into the notch, with the two pins 27 connecting the link with the contiguous links of the chain, lying outside the notch, that is, one pin facing the front face 29 of the body and the other pin facing the rear 31 of the body, but so that the pins themselves cannot enter the notch, whereby when the chain is tensioned round the elongate member, the pin of the trailing contiguous link 33 holds the link firmly in place in the notch to anchor the respective end portion of the chain; and, adjacent the left hand extremity, the body is bored to receive therethrough a screw-threaded shank 35 made integral with the chain at the respective end thereof and receiving a knurled nut 37 thereon so that, by screwing up the nut on the shank, the chain can be tensioned around the elongate member.

Each abutment 1 5 is in the form of a knife edge; and because the chain can move on the elongate member when being tightened up, no substantial forces are imposed on the knife edge tending to deflect it circumferentiallv of its position, that is, from the diametrical plane containing the point of contact of the knife edge with the elongate member and abutment point 1 7 of the chain.

Each part comprises a further abutment in the form of a roller 43 mounted on a block 45 supported on the body of the part; the position of the roller radially of axis 'X-X' (Figure 1) being adjustable in relation to the knife edge, by loosening a fastening screw 47 received in a blind bore in the body and received through a slot 49 in the block. The roller assists in positioning and stably mounting the extensometer on the elongate member and, being a roller, is not affected by elongation of the elongate member. The roller position adjustment is effected by a screw 50 received through a screw threaded bore 51 in the plate 11 and bearing at its remote end on the block. The roller position once it is set during assembly and the block fastened in place, is not intended generally to be adjusted in use of the extensometer.

The two parts of the extensometer are connected by a pair of tension springs (one of which 53 is seen in Figure 3) housed in respective protective tubes 55, 57 carried in bores in the body of part 1 with the springs in slightly extended condition holding the body of part 3 against the bottom ends of the tubes as seen in Figure 4. In order that the springs in use of the extensometer may assume their null or unextended condition, the tubes are slidably displaceable in the bores of body of part 1 so that they can be displaced upwards to clear the part 3; the tubes being held in place by screwed collars 59 carried on the top portions of the tubes.

The transducer in this instance is a linear voltage differential transformer and, in use of the extensometer, is connected by leads 61 to a processor unit 63 (Figure 5).

The unit comprises a 1 2 channel analogue to digital converter to take up to 1 2 extensometers, having visual displays W1 to W1 2 respectively for the channels and input sockets S1 to 12 respectively by which each channel can be operated. The unit also comprises a microprocessor software driven via an E.P.R.O.M, a stabilised power supply, a keyboard KB for data and command input, a display DL for total load measured, a modulus of elasticity switch MS to select between that for carbon steel or the modulus for the "Duraheat" range of special steels. The unit provides a power supply via socket SKP to a printer P and a socket PI for power input; the latter being provided with a switch PIS.

The keyboard comprises keys for the digits O to 9 and a key "X" for decimal point insertion, and command keys "P", "T", "H", "E'' and "C" respectively to effect print out of a result, zeroing of transducer signals, holding of displayed values, entering of data and correction of entered data, with the first four of the command keys having light indicators LI 1 to 4 respectively to indicate activation. The keyboard numerical keys are used to input diameters; and an entered diameter is displayed on display DL while data are being processed and pending display at the end of processing of the total load.

Because of the circuitry now available and the sensitivity of the transducers, the unit can work to an accuracy of .000625 cm.

In carrying out a load determination, say, of a load suspended by sling rods, the rods are relieved of tension by suspending the boiler with e.g. a power operated block and tackle arrangement, and an extensometer is fixed to each sling rod by the chain clamps in the manner as described above. The anchorages of the sling rods are secured and the load is then smoothly transferred to the sling rods by suitably releasing the block and tackle suspension, until the rods again carry the full load.

In this operation, the transducers of all the extensometers are repeatedly interrogated, and processing to produce a load measurement is not initiated until each transducer provides a stable output indicating that the rods have ceased to extend and are carrying full load.

The print out produced by the processor unit shows not merely total load but the load on each sling rod as well. Assuming the weight distribution is acceptable as shown by those individual readings, the print out then constitutes a test certificate.

The leads of the transducers may comprise loading resistors such as resistor 65 (Figure 2) to limit the magnitude of any output signal from a transducer in the null position so as to avoid the possibility of incompatability of different extensometers with the processor unit.

In the second embodiment of the present invention described with reference to Figure 6, the extensometers are used in pairs. Referring to Figure 6, the extensometers are used in pairs. Referring to Figure 6, the two extensometers, generally indicated at "A" and "B", are clamped, diametrically opposed to the elongate member 7 with extensometer "B" slightly axially displaced from extensometer "A" so that neither obstructs the clamping means of the other. Extensometer "A" comprises a connection box 67 having a socket 69 for a terminal plug 71 of the leads 61 B from the transducer 9B of extensometer "B", the socket providing connecting wires 73 to terminals in a double socket 75 providing terminals for the leads of the transducers of both extensometers and receiving a terminal plug 77 of a cable 79 connecting with the processor unit.

The processor unit itself in this embodiment (not shown) is modified to provide 6 double input sockets, and the visual displays are connected in pairs to a respective input socket so that the transducer readings respectively of the two extensometers of the pair thereof concerned can be displayed. The processing unit is programmed to process signals from a transducer of either polarity and, in the instance of a bent rod, may display in a pair of displays, a tensile load for one of the transducers of a pair of extensometers and a compressive load for the transducer of the other one of the pair by an appropriate polarity sign, e.g. " + ve" for extension and "- ve" for compression.

The processor unit in this embodiment dis penses with the "H" key and instead provides a mode key to activate hardware to average the loads shown in the display pairs by summing each two loads and dividing by 2, one display in each pair then being used to display the average.

The printer in this embodiment is adapted to print out individual transducer loads i.e. up to 12, or the calculated average loads i.e. up to 6 depending on the position of the mode key.

Claims

1. An extensometer comprising two parts displaceable towards and away from one another along a given axis, a clamping means for each part to clamp the part against an elongate member passing parallel to said axis, and a transducer attached to one of said parts to sense relative displacement along said axis of said two parts; each part comprising a first abutment to abut on the elongate member with each clamping means, when in place, constituting a second abutment on the elongate member diametrically opposed to the first abutment member, and each clamping means being such that in clamping the respective part to the elongate member, it does not impose any substantial force on the first abutment tending to displace it from the diametrical plane in which it lies.

2. An extensometer according to Claim 1, wherein each clamping means comprises a chain anchored to the respective part so that it may be passed round the elongate member and having means for securing the chain at a remote end thereof to the respective part so that the chain can be tightened around the elongate member.

3. An extensometer according to Claim 1 or 2, wherein a third abutment is provided carried by each part to lie displaced from the first abutment along said axis but in the same diametrical plane.

4. An extensometer according to Claim 3, wherein the first abutment is constituted by a knife edge and the third by a roller.

5. An extensometer according to Claim 4, wherein the roller is mounted so as to be adjustable radially of said axis in relation to the knife edge.

6. An extensometer according to any of the preceding Claims, wherein the two parts are connected by one or more springs so as normally to lie a predetermined distance apart in the null position of the or each spring and the or each spring is housed in a protective tube carried at one end by one of the parts and, in a slightly extended condition of the or each spring, contacting at the other end, the other of the parts.

7. An extensometer according to Claim 6, wherein the or each tube can be axially adjusted in position to be removed from contact with the other part to permit use of the extensometer.

8. An extensometer according to any of the preceding Claims, wherein the transducer is one providing an electrical analogue signal.

9. An extensometer according to Claim 8, wherein the transducer is a linear differential transformer.

10. An extensometer substantially as herein before described with reference to the accompanying drawings.

11. A load measuring system employing an extensometer as claimed in any of the preceding Claims, further comprising an electronic unit to process the signal from a transducer and provide a read out of load.

1 2. A system according to Claim 11 employing an extensometer according to Claims 9 or 10, wherein the signal is digitised, and the unit is such as to process the digital signal.

13. A system according to Claim 11 or 12, wherein the signal is processed by a software program fed into an E.P.R.O.M. which takes into account the modulus of elasticity of the elongate member concerned and its diameter.

14. A system according to Claim 11, 12 or 13, wherein the unit comprises a plurality of channels so that a corresponding number of extensometers may be used and the signals from them processed simultaneously.

1 5. A system according to any of Claims 11 to 14, wherein the unit provides for the use of alternative moduli of elasticity so that the system may be used with elongate members of different composition.

1 6. A system according to any of Claims 11 to 15, wherein the unit provides for display of load measurement.

1 7. A system according to any of Claims 11 to 16, wherein the unit comprises a keyboard for data and command inputs.

1 8. A system according to any of Claims 11 to 17, wherein the unit comprises a printer to print out load measurements.

19. A system according to any of Claims 11 to 18, wherein the unit has provision for zeroing any signal from a transducer so that tolerances in the transducers of different extensometers giving rise to a signal in the null or unextended position thereof, can be eliminated.

20. A system for load measurement according to Claim 14 or any of Claims 1 5 to 1 9 as dependent thereon, wherein a pair of the extensometers are employed for each elongate member the load on which is to be measured, to be attached diametrically opposed to the elongate member, and wherein the processor unit is adapted to accept the inputs from the pair of extensometers to display load measured thereby on a respective one of a pair of displays.

21. A system according to Claim 20, wherein the processor comprises software providing for the processing of signals of either polarity so that both tensile and compressive forces can be measured and displayed by a change of polarity sign.

22. A system according to Claim 20 or 21, wherein the processor unit comprises a mode facility so that loads shown in a pair of displays can be averaged and the average displayed in one of the displays.

23. An extensometer substantially as described with reference to the accompanying drawings.

24. A system for load measurement substantially as described with reference to Figures 1 to 5 of the accompanying drawings.

25. A system for load measurement substantially as described with reference to Figure 6 of the accompanying drawings.