GB2212616A

GB2212616A - Piezo-electric transducers

Info

Publication number: GB2212616A
Application number: GB8827115A
Authority: GB
Inventors: John B Davis
Original assignee: Davis
Current assignee: Davis
Priority date: 1987-11-20
Filing date: 1988-11-21
Publication date: 1989-07-26
Anticipated expiration: 2008-11-21
Also published as: GB8827115D0; GB2212616B

Abstract

In order to avoid damage to a piezo-electric crystal (3) through overloading of the crystal, the crystal is backed by a pad (10,11) of a low Youngs modulus material, and a parallel force-transmission path is provided by an adjacent member (15, 16) having a Youngs modulus which is intermediate that of the crystal and that of the pad. In an assembly for monitoring the individual loading of a series of adjacent press tools, a series of piezo-electric crystals 3 of tablet-shape are located in holes 4 in a spacer strip 2 and are sandwiched between common low-modulus strips 10, 11 clamped between elongate plates 13, 14 to pre-load the crystals, the pre-load being limited by steel bars 15, 16 which also provide the parallel force-transmission paths. Individual electrical connections to the crystals are provided by a printed circuit layer (9) and a common contact layer 5. <IMAGE>

Description

PIEZO-ELECTRIC TRANSDUCERS This invention relates to piezo-electric transducers and particularly, but not exclusively, to their use for monitoring the load on press tools.

Problems are encountered when attempts are made to monitor loads on high speed presses when the sensors are mounted on the machine structure. The measured loads are not an accurate representation of the actual load on the press tool, which is really the important load to monitor, the measured load usually being greater than the actual tool load. The errors are caused by dynamic effects within the machine structure and are generally independent of the measuring system used, both piezo-electric and foil strain gauge systems, for example, exhibiting the same effect.

Two possible methods of overcoming this problem are (a) to mount the tool or the bedplate of the press on load cells at each corner, and (b) to measure the load directly behind the tool itself. Whilst (a) can be a useful technique, a disadvantage is that the resulting tool mounting is likely to be less rigid than without the measuring system fitted.

When piezo-electric transducers were incorporated into the tool backing, as in (b), a problem was encountered that, because of the location of the piezo-electric crystal in the direct force-transmission path for the total tool load, the piezo-electric crystal was likely to bear a very substantial load.

This problem, it is realised, is partly due to the very high modulus of the piezo-electric crystal compared with that of the materials normally used for the tool backing. It is therefore quite likely that the piezo-electric crystal, or a series of crystals when these are used, would convey a very substantial part of the total tool load.

Excessive loading and/or overheating of piezo-electric crystals can destroy their piezo-electric properties.

According to the invention a piezo-electric transducer assembly for monitoring loads comprises a piezo-electric crystal which is cushioned by a pad of a material having a relatively low Youngs modulus arranged in series with the crystal in the force transmission path, the crystal/pad combination being arranged in parallel in the force-transmission path with an adjacent member having a Youngs modulus which is intermediate that of the crystal and pad, and electrical connection means associated with the crystal to enable the electrical output of the crystal to be monitored.

With this arrangement it can be arranged, by suitable choice of the thickness and modulus of the pad, that the piezo-electric crystal bears only a proportion of the total force which the entire unit of crystal, pad, and adjacent member has to bear.

The pad can, for example, be made of roughly comparable thickness to that of the crystal, in the direction of force transmission, but of a modulus which is perhaps only one tenth of that of the crystal.

An electrical contact for one face of the crystal is eferably sandwiched between the pad and the adjacent face of the crystal.

The transducer assembly may comprise only a single crystal or it may comprise a plurality of crystals for measuring the local loads at different positions of, say, a press tool.

When the transducer assembly has only one crystal the assembly may take the form of a cylinder of intermediate modulus material in which is housed a cylindrical crystal and cylindrical low modulus pad.

The cylinder can be closed at one end by an endwall integrally formed with the remainder of the cylinder, or by an endwall rigidly attached thereto, the opposite end of the cylinder being closed by a plug which is arranged to apply a pre-load to the pad-crystal combination. The pre-loading of a piezo-electrical crystal is usually desirable in a transducer.

The plug may have a threaded engagement with the cylinder body so that the pre-load can be set by screwing the plug in or out of the cylinder.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a transverse cross-section of an elongate transducer assembly; and Figure 2 is a perspective view of a locating strip mounting three piezo-electric crystals.

The illustrated transducer assembly is adapted for use with a series of press tools which are arranged in line on a common tool mounting, and where it is desirable to be able to monitor the load to which the individual tools are subjected.

It will be appreciated that if one were simply to monitor the load in the structure of the press this would provide a measure of the total load, but not of the individual tool loads.

The transducer assembly 1 comprises a locating strip 2 of material of thickness less than that of the tablet-shaped crystals 3 which are a loose fit in respective spaced-apart holes 4 in the locating strip 2. The material of the strip is not important since it only acts as a location means for the crystals 3 and does not bear any of the load. The piezo-crystals are typically of diameter 8 mm and thickness 3 mm.

A common electrical contact for the crystals is provided by an electrically conductive strip 5 of the same width as the locating strip with which it is in register, the contact strip being arranged in face contact with the lower face 6 of each of the crystals The opposite, upper faces 7 of the crystals 3 which project from the upper face 8 of the locating strip 2 are contacted by a further contact strip 9 provided with printed circuit contacts on its lower face to provide individual contacts and connections for the upper faces 7 of the respective crystals 3. The modulus of the contact strips 5,9 is not particularly important.

A pad strip 10 of low modulus material, such as cellulose acetate, is provided by a strip of the same width as the locating and contact strips and cf thickness typically 2 mm. The pad strip 10 is positioned in face contact with the rear face of the contact strip 9. A further pad strip 11 of low modulus material is positioned in face contact with the underside of contact strip 5. In a modification, not shown, one or other of the pad strips 10,11 may be omitted, it being understood that the thickness of other components are adjusted accordingly, to ensure that the piezo-electric crystals are subjected to a desired pre-load.

The entire sandwich structure of contact strip 5, crystals 3, locating strip 2, further contact strip 9 and pad strips 10,11 is received in an elongate cavity 12 defined between a pair of spaced apart parallel steel plates 13,i4, of greater transverse width than that of the strips, the plates 13,14 being spaced apart by a pair of transversely spaced apart steel bars 15,16 of oblong-rectangular transverse cross-section, the spacing of the bars 15,16 being substantially equal to the transverse width of the various strips which all fit therebetween.

The plates are connected together by a series of recessed screws 17 which pass through the respective bars 15,16, and tightening of the screws applies a pre-load to the crystals 3, this being arranged by suitable choice of the combined thickness of the sandwich structure compared with that of the bars 15,16.

The steel bars 15,16 have a modulus which is intermediate that of the crystals 3 and pad strips 10,11 and thereby provide two force transmission paths between the plates 13,14 which are in parallel with the force transmission paths through the crystals 3 themselves. All the load which the crystals 3 bear is carried by the pad strips 10,11, and so the crystals 3 are each cushioned by the pad strips 10,11 and are protected from being overloaded.

One of the metai plates 13,14 can be directly secured to one platen of the press, a series of tools being mounted in line on the oppositely facing side of the other plate, the crystals having been positioned in line with the respective tool positions so that by monitoring the outputs of the individual crystals, the individual tool loads are measured.

If desired a respective signal conditioning circuit module could be incorporated in the assembly for conditioning the outputs of the respective crystals.

In a modification, not shown, an array of crystals 3 is provided to monitor the respective loads at an array of tool positions, the locating member 8 then being in the form of a sheet with a first array of suitably positioned holes for the crystals 3, and with a second array of holes to receive steel blocks, corresponding to bars 15,16, interspaced with the crystal holes, the blocks providing the adjacent force paths parallel to the force paths through the crystals.

Claims

1. A piezo-electric transducer assembly for monitoring loads comprises a piezo-electric crystal which is cushioned by a pad of a material having a relatively low Youngs modulus arranged in series with the crystal in the force transmission path, the crystal/pad combination being arranged in parallel in the force-transmission path with an adjacent member having a Youngs modulus which is intermediate that of the crystal and pad, and electrical connection means associated with the crystal to enable the electrical output of the crystal to be monitored.

2. A transducer assembly as claimed in claim 1 in which the pad is of a modulus which is less than one fifth of that of the crystal.

3. A transducer assembly as claimed in claim 1 in which the pad is of a modulus which is less than one tenth of that of the crystal.

4. A transducer assembly as claimed in any one of the preceding claims in which an electrical contact for one face of the crystal is sandwiched between the pad and said one face of the crystal.

5. A transducer assembly as claimed in any one of the preceding claims comprising only a single crystal.

6. A transducer assembly as claimed in claim 5 in the form cf a cylinder of intermediate modulus material in which is housed a cylindrical crystal and a cylindrical low modulus pad.

7. A transducer assembly as claimed in claim 6 in which the cylinder is closed at one end by an endwall rigid with the cylinder body, the opposite end of the cylinder being closed by a plug which is arranged to apply a pre-load to the pad-crystal combination.

8. A transducer assembly as claimed in claim 7 in which the plug has a threaded engagement with the cylinder body, the arrangement being such that the pre-load can be set by screwing the plug in or out of the cylinder.

9. A transducer assembly as claimed in any of claims 1 to 4 in which a plurality of piezo-electric crystals are arranged in side-by-side, spaced-apart relationship in a plane which is transverse to the applied load direction, each of the crystals being cushioned by a respective pad of low Youngs modulus, with electrical connection means being provided for each crystal so arranged as to enable the electrical outputs of the respective crystals to be individually monitored.

10. A transducer assembly as claimed in claim 9 in which the crystals are held in said relationship by a spacer member provided with apertures in which respective crystals are located.

11. A transducer assembly as claimed in claim 10 in which the spacer member is a sheet of material having a thickness, in the applied load direction, which is less than the corresponding dimension of the crystals.

12. A transducer assembly as claimed in any one of claims 9 to 11 in which the electrical connection means comprises a printed circuit contact layer in engagement with the one face of each crystal, the printed circuit layout providing individual connections to the respective crystals.

13. A transducer assembly as claimed in any one of claims 9 to 11 in which the assembly is elongate in a direction transverse to the applied force direction, and the crystals are longitudinally spaced-apart, in said transverse direction.

14. A transducer assembly as claimed in claim 13 in which an elongate sheet of low modulus material provides a common pad for the crystals,

15. A transducer assembly as claimed in claim 14 in which the pad is located between a pair of bars which extend in the longitudinal direction of the assembly, the bars being held between a pair of plates and constituting said adjacent member, the plates being secured to the bars and providing a preloading to the sub-assembly of crystals/pad in the direction normal to the plates.

16. A transducer assembly substantially as described with reference to the accompanying drawings.

17. A transducer assembly as claimed in any one of the preceding claims attached to a press and so arranged as to monitor a tool load.