GB2134267A - Load transducer - Google Patents

Abstract

A load transducer comprises a strain-gauged bobbin 11 located between and insulated from two metal diaphragm members 14, 15. Bosses 16, 17 project from the diaphragm members 14, 15 into opposite ends of a bore which extends centrally of the bobbin 11. The opposed end faces of the bosses are separated by compressible insulant material 22 that permits the bosses to move towards each other when a load is applied across the diaphragm members along the axis of the bobbin bore. The load transducer is particularly suited for embodiment in hand portable apparatus for measuring and indicating the force exerted between the electrodes of resistance welding equipment as it does not require that the electrical current supplied to the electrodes be switched off whilst measurements are being made. <IMAGE>

Description

SPECIFICATION Load transducer THIS INVENTION relates to load transducers and is particularly concerned with a load transducer which is suitable for embodiment in apparatus for measuring the force exerted between the electrodes of resistance welding equipment.

Resistance welding techniques are frequently employed for joining sheet metal components, in particular sheet steel components, on mass production assembly lines and, when there is no requirement for a sealed joint, spot welding is normally used. In setting up the equipment and in controlling the quality of the welds produced during a production run, it is necessary from time to time to measure and/or to check a number of parameters. One of these parameters is the force exerted between the tips of the electrodes, this force usually being generated by a pneumatic or hydraulic ram operating a scissor action mechanism.

Using presently available equipment and techniques, measurement of the force exerted between spot welding electrodes takes an inordinately long time in comparison with rates of production on mass production lines.

For example, one method employs a Bourdontube pressure gauge and requires that the current to the equipment be switched off before the force measurement is made. Also, the Bourdon-tube pressure gauge requires an open electrode gap dimension of at least 20mm so that the open electrode gap dimension, for example 12mm, used in sheet material spot welding operations has to be increased for the measurement to be made and then reset.

It is an object of the present invention to provide a load transducer which can be used to measure the force exerted between the electrodes of resistance welding equipment with the current switched on and which requires minimal alteration of the open gap dimension between the electrodes to enable the exerted force to be checked.

It is another object of the invention to provide a load transducer which can be embodied in hand portable apparatus for measuring and indicating the force exerted between the electrodes of spot welding equipment.

Accordingly, in its broadest aspect, the present invention provides a load transducer comprising a strain-gauged bobbin located between and insulated from two metal diaphragm members having opposed bosses projecting one from each diaphragm member into opposite ends of a bore extending centrally of the bobbin, and compressible insulant material separating opposed end faces of the bosses.

In use of a load transducer in accordance with the present invention loads are applied to the external surfaces of the diaphragm members along the axis of the central bore of the bobbin and the compressible insulant material permits the opposed end faces of the bosses to move towards each other when load is applied.

In a preferred form of load transducer in accordance with the present invention the bobbin is of cylindrical form having a central bore and the external circumferential surface is recessed to locate strain gauges which are bonded to the bobbin surface at the bottom of the recess and at equal spacings around the circumference thereof. Preferably four strain gauges are bonded to the bobbin and wired as a bridge.

The metal diaphragm members are preferably insulated from the bobbin by a substantially rigid material such as, for example, a phenol-formaldehyde laminate material; however, a less rigid but substantially incompressible material, such as hard rubber may be used.

The compressible insulant material is preferably a flexible elastomer sealant material such as, for example, polychloroprene.

The metal diaphragm members may be held in relationship to each other and to the bobbin by being fixed to a substantially rigid insulating member which is sandwiched between the diaphragm members so as to extend around the circumference of the bobbin. The fixing may be by means of screws which locate with the insulating member.

In a load transducer in accordance with the invention intended for embodying in hand portable apparatus for measuring and indicating the force exerted between the electrodes of resistance welding equipment, the insulating member may project outwardly of the peripheral side edges of the diaphragm members and be adapted, as for example by the provision of bolt holes, for attachment to a body portion of the hand portable apparatus.

The invention also provides hand portable transducer apparatus for insertion between the electrodes of resistance welding equipment, the apparatus including a body member, a load transducer comprising a strain-gauged bobbin located between and insulated from two metal diaphragm members having opposed bosses projecting one from each diaphragm member into opposite ends of a bore extending centrally of the bobbin, a compressible insulant material in said bobbin bore separating opposed end faces of the bosses, a substantially rigid insulating member sandwiched between and fixed to said diaphragm members and projecting outwardly of the peripheral side edges of the diaphragm members to attachment with said body member, and means for processing and displaying signals output by said load transducer in terms of force exerted on said diaphragms by said resistance welding electrodes.

Apparatus adapted for use with spot welding equipment preferably incorporates a pair of movable jaws having opposed mating faces with an indent portion in the mating face of each jaw for locating with and gripping one of the electrodes so as to accurately align the load transducer with the electrodes.

For operator convenience in using the apparatus it is preferable to provide the body member with a handle. Such a handle may conveniently provide means for actuating a mechanism to move the jaws in a synchronous manner to grip the said one electrode so that it is accurately aligned with the axis of the bore of the bobbin.

In one embodiment of the invention the handle comprises a fixed member and a finger grip member movable with respect to the fixed member and connected with a suitable linkage mechanism for actuation of the movable jaws.

The means for processing and displaying signals output by the load transducer may comprise a DC amplifier which amplifies the output of the load transducer and feeds it to a moving coil analogue meter for display.

However, in a preferred embodiment the load transducer signal output processing and display means comprises a microprocessor with a programming board controlling the output display on an alpha-numeric display.

In another embodiment the load transducer signal output processing and display means comprises a microprocessor controlling the operation of an analogue-to-digital converter that changes an amplified DC signal output by the load transducer to binary form, this then being changed to denary by decoder drivers which drive an LCD display.

The apparatus is preferably self-powered by a battery of power cells which may be rechargeable and the apparatus may incorporate a battery charging circuit for this purpose.

The invention will now be further described by way of example and with reference to the accompanying drawings in which: Figure 1 is a cross-section through a load transducer and the body portion of apparatus to which it is attached in one embodiment of the invention, the cross-section being taken on line I-I of Fig. 2; Figure 2 is a view in plan of apparatus in accordance with one embodiment of the invention for measuring and displaying the force exerted between the electrodes of a spot welding machine; Figure 3 is a block diagram illustrating load transducer signal output processing and display means in one embodiment of the invention; Figure 4 is a block diagram illustrating load transducer signal output processing and display means in another embodiment of the invention; and Figure 5 is a block diagram similar to Figs.

3 and 4 and relating to a further embodiment of the invention.

Referring first to Fig. 1, a load transducer 10 in accordance with one embodiment of the present invention comprises a metal bobbin 11 preferably manufactured from mild steel.

The bobbin 11 is of cylindrical form with a central bore extending between its end faces.

The external circumferential surface of bobbin 11 is recessed and four electrical resistance strain gauges 12, only two being visible in Fig. 1, are bonded to the bobbin surface at the bottom of the recess so as to be equispaced around the circumference of the bobbin and are wired in a wheatstone bridge circuit. After bonding and wiring of the strain gauges the recess is filled with a protective insulating sealant 1 3 such as a cold setting silicon rubber. The bobbin 11 is located between two mild steel diaphragm members 14 and 1 5 having bosses 1 6 and 17, respectively, projecting into the central bore of bobbin at opposite ends thereof.The end faces of the bobbin 11 are insulated from the diaphragm members 14 and 1 5 by substantially incompressible insulating washers 18 and 19, respectively, manufactured from a phenol-formaldehyde laminate material or any other suitable insulating material. Similarly, the circumferential surfaces of the bosses 1 6 and 1 7 are insulated from the circumferential surface of the bobbin bore by substantially incompressible rings 20 and 21, respectively. The opposed end faces of the bosses 1 6 and 1 7 are separated by a compressible insulating material 22 such as polychloroprene.

A rigid insulating member 23 is sandwiched between the diaphragm members 14 and 1 5 so as to extend around the circumference of the bobbin 11 and the diaphragm members are secured to the insulating member by screws 24. The insulating member 23 is sealed with respect to the diaphragm members 14 and 1 5 by sealing rings 25 and 26, respectively, and projects outwardly of the peripheral side edges of the diaphragm members to form a flange 27.

In incorporating the load transducer 10 in apparatus 30, as illustrated in Fig. 2, for measuring and displaying the force exerted between the electrodes of a spot welding machine, the load transducer is secured to spaced projecting portions 31 and 32 of a body member 33 of the apparatus 30 by bolting the flange 27 of the insulating member 23 to the projecting portions 31 and 32 with screws 34.

The apparatus 30 incorporates a pair of movable jaws 35 and 36 positioned above one of the diaphragm members 14 and the load transducer 10. Each of the jaws 35 and 36 has an indent 37 which locates with an electrode (not shown) of a spot welding machine when, in use of the apparatus, the jaws are brought together. By this means the electrodes (not shown) are accurately aligned with the axis of the bore of the bobbin member 11 and their force is exerted along that axis. The opposed surfaces of the jaws 35 and 36 diverge with respect to each other from the indents 37 to their outer ends so as to assist in location of the jaws with respect to an electrode.

The jaws 35 and 36 are moved by a squeeze action handle 38 which also provides for ease of operator handling and use of the apparatus. The handle 38 comprises a fixed member 39 secured to the body member 33 and a pivoted member 40 having finger grip indents 41. The pivoted member 40 is connected through a suitable mechanism (not shown) with the jaws 35 and 36 whereby movement of the pivoted member 40 under the squeezing action of an operator's hand is translated into synchronous movement of the jaws to grip the electrode and accurately align it with the axis of the bore of the bobbin member 11.

The apparatus 30 is provided with means for supplying electrical power to the load transducer 10, means for processing signals output by the load transducer, and means for displaying the force measured by the load transducer, all of which are housed in the body member 33 of the apparatus.

Referring to Fig. 3, the load transducer 10 of apparatus in one embodiment of the invention is supplied with electrical power from a battery of nickel-cadmium cells 50 which are re-chargeable from an external power source through a battery charging circuit 51. The voltage of the cells is suitably adjusted by a voltage supply circuit 52. Signals output by the load transducer 10 are amplified by a DC amplifier 53 before being fed to an analogueto-digital converter 54. A control circuit 55 comprising a suitably programmed microprocessor controls the operation of the analogueto-digital converter 54 and feeds in a calibration signal to convert the voltage level into pounds force.The amplified DC signal is converted to binary form and fed to decoder drivers 56 which change the binary form to denary and drive an LCD display 57 for read out of the load transducer output in terms of pounds force.

In the embodiment of Fig. 4 the load transducer 10 is again provided with electrical power from a battery source 60 which is rechargeable through a charging circuit 61 and a voltage supply circuit 62 is included for suitable adjustment of the voltage supplied by the battery source 60. Signals output by the load transducer 10 are amplified by a DC amplifier 63 and fed to an alpha-numeric display arrangement 64 controlled by a control circuit 65 comprising a microprocessor with a programming board. The microprocessor linearises the calibration of the load cell so that a moving line display of the force exerted on load transducer is provided and also provides for a numerical read out of the measured force. The microprocessor may also be programmed to display a warning when the power output from the battery power source is approaching a low level.

The embodiment of Fig. 5 again provides a battery power source 70, charging circuit 71 and voltage supply circuit 72. Signals output by the load transducer 10 are amplified by a DC amplifier 73 and fed to a moving-coil meter 74 for display.

In the embodiments of Figs. 3 and 4 the microprocessor may be programmed so that the apparatus ignores the initial impact of the electrodes on the load transducer by blanking out, substantially, the first 40 milli-seconds of load. The subsequent peak force may then be stored and displayed until the apparatus is manually reset. Switching means (not shown) may be incorporated for enabling one of two or more ranges of force to be selected for display purposes, the particular range to be selected being dependent upon the force expected to be exerted by the particular spot welding equipment which is being set up or checked.

One transducer apparatus that we have designed having a construction similar to that hereinbefore described with reference to and as shown in Fig. 2 of the accompanying drawings, embodies a load transducer having a 12mm thickness dimension and being of similar construction to the load transducer herein before described with reference to and as shown in Fig. 1 of the accompanying drawings. This apparatus meets a particular requirement for checking the force exerted between the electrodes of a spot welding gun having an open electrode gap dimension slightly in excess of 12mm, with the electrical current switched on and no alteration of the open gap dimension being necessary to carry out the force measurement.

Claims (20)

1. A load transducer comprising a straingauged bobbin located between and insulated from two metal diaphragm members having opposed bosses projecting one from each diaphragm member into opposite ends of a bore extending centrally of the bobbin, and compressible insulant material separating opposed end faces of the bosses.

2. A load transducer as claimed in Claim 1, wherein the bobbin is of cylindrical form and strain gauges are bonded to a recessed portion of the external circumferential surface of the bobbin.

3. A load transducer as claimed in Claim 1 or Claim 2, wherein the metal diaphragm members are insulated from the bobbin by a substantially rigid insulant material.

4. A load transducer as claimed in Claim 3, wherein the substantially rigid insulant material comprises laminated phenol-formaldehyde.

5. A load transducer as claimed in Claim 1 or Claim 2, wherein the metal diaphragm members are insulated from the bobbin by a substantially incompressible material.

6. A load transducer as claimed in Claim 5, wherein the substantially incompressible material comprises hard rubber.

7. A load transducer as claimed in any preceding claim, wherein the said compressible insulant material separating the bosses in said bobbin bore comprises a flexible elastomer sealant material.

8. A load transducer as claimed in any preceding claim, wherein the metal diaphragm members are held in relationship to each other and to the bobbin by being fixed to a substantially rigid insulating member sandwiched between the diaphragm members so as to extend around the bobbin.

9. Hand portable transducer apparatus for insertion between the electrodes of resistance welding equipment, the apparatus including a body member, a load transducer comprising a strain gauged bobbin located between and insulated from two metal diaphragms, opposed bosses projecting one from each diaphragm member into opposite ends of a bore extending centrally of the bobbin, a compressible insulant material in said bobbin bore separating opposed end faces of the bosses, a substantially rigid insulating material sandwiched between and fixed to said diaphragm members and projecting outwardly of the peripheral side edges of the diaphragm members to attachment with said body member, and means for processing and displaying signals output by said load transducer in terms of force exerted on said diaphragms by said resistance welding electrodes.

10. Transducer apparatus as claimed in Claim 9 further comprising a pair of movable jaws having opposed mating faces and an indent portion in the mating face of each jaw for locating with and gripping one of the electrodes so as to accurately align the load transducer with the electrodes.

11. Transducer apparatus as claimed in Claim 9 or Claim 10, having a handle on the body member.

1 2. Transducer apparatus as claimed in Claim 11, wherein the handle provides means for actuating a mechanism to move the jaws in a synchronous manner to grip the said one electrode.

13. Transducer apparatus as claimed in Claim 12, wherein the handle comprises a fixed member and a finger grip member movable with respect to the fixed member and connected with suitable linkage mechanism for actuation of the movable jaws.

14. Transducer apparatus as claimed in any one of Claims 9 to 13, wherein the means for processing and displaying signals output by the load transducer comprise a DC amplifier which amplifies the output of the load transducer and feeds it to a moving coil analogue meter for display.

1 5. Transducer apparatus as claimed in any one of Claims 9 to 13, wherein the means for processing and displaying signals output by the load transducer comprise a microprocessor having a programming board controlling the output display on an alphanumeric display.

16. Transducer apparatus as claimed in any one of Claims 9 to 13, wherein the means for processing and displaying signals output by the load transducer comprise a microprocessor controlling the operation of an analogue-to-digital converter that changes an amplified DC signal output by the load transducer to binary form which is then changed to denary by decoder drivers which drive an LCD display.

1 7. A load transducer substantially as hereinbefore described with reference to and as shown in Fig. 1 of the accompanying drawings.

1 8. Apparatus for measuring and displaying the force exerted between the electrodes of resistance welding equipment substantially as hereinbefore described with reference to and as shown in Fig. 2 of the accompanying drawings embodying a load transducer as claimed in Claim 1 7.

19. Apparatus as claimed in Claim 18, including means for processing and displaying signals output by the load transducer substantially as hereinbefore described with reference to and as shown in any one of Figs. 3, 4 or 5 of the accompanying drawings.

20. Any new or improved features, combinations and arrangements described, shown and mentioned or any of them together or separately.