GB2343751A

GB2343751A - Bi-axial stress sensor assembly

Info

Publication number: GB2343751A
Application number: GB9824492A
Authority: GB
Inventors: Michael John Meehaw; Richard Hillier
Original assignee: LOUGHBOROUGH PROJECTS Ltd
Current assignee: LOUGHBOROUGH PROJECTS Ltd
Priority date: 1998-11-10
Filing date: 1998-11-10
Publication date: 2000-05-17
Also published as: GB9824492D0

Abstract

The sensor assembly comprises a square steel plate 5 manufactured to provide two free arms at right angles to each other, where one pair of Dual Linear resistance strain gauges 1, 2 is installed on one of the right angle arms of the sensor element 5, monitoring a selected primary axis, and a second pair 3, 4 are installed on the other right angle arm, monitoring a secondary axis, are connected to a pcb or terminal strip 6 located on the remaining area of steel plate 5, to the effect that when the sensor is applied to a structural element it will monitor compressive and tensile stresses in the primary and secondary axes so that effects of the stresses monitored in the secondary axis are eliminated from the signal output of the primary axis. The server may be attached to the structural element by means of bolts fitted in holes 9,10,11 or by a suitable adhesive. Alteration of the cross-sectional area of either arm may be performed to equalise the outputs from the strain gauges (Fig.5, not shown). Full strain gauge bridge circuits may be arranged on each arm (Fig.6, not shown).

Description

The invention relates to a sensor to accurately monitor tensile and compressive forces and linear movements in structural elements.

Sensor assemblies manufactured with a strain gauge bridge circuit are frequently used to monitor structural elements forming part of a larger structure or assembly to provide data about the mechanical stresses or mechanical micromovements experienced by the complete structure or assembly. The sensors are affixed mechanically by means of bolts or adhesives to the structural element, and provide an electrical signal proportional to the stress and or length change experienced by the structural element.

However the accuracy of this measurement is affected by additional stresses caused by other extraneous forces and factors such as those caused during the mechanical operation of the complete structure or assembly. This can develop stresses in all planes of the element being monitored, thus altering the signal output derived from the primary stress.

The invention is a bi-axial sensor assembly providing a method of arranging the strain gauge bridge circuits and the mechanical components within the sensor assembly so that the effects of secondary stresses are cancelled out, leaving the primary stress to be more accurately monitored without difficulty.

The bi-axial sensor assembly is an externally powered self contained unit with a square steel plate manufactured so as to provide two arms at right angles to each other, only connected to the remainder of the steel plate by a small web of remaining material at the inner angle. Each arm has a strain gauge circuit applied. The remaining area of the steel plate is used for siting electronic components and wiring connections. The upper surface of the whole plate is covered and protected by a protective moulding or a coating of a protective polymer. The biaxial sensor assembly is applied to the structural element which is to be monitored by means of three fixing bolts, one through the free end of each arm, and the third through the meeting point of the two arms. Alternatively, where the surface of the structural element has been suitably prepared, a structural adhesive may be used to fix the sensor in the desired position.

When determining the sensor site, one arm of the sensor is designated as the primary axis, and the sensor is arranged so that the major axis of this arm lies in the same plane as the anticipated compressive and tensile stress that is to be monitored. The second arm will thus lie at right angles, to sense the anticipated compressive and tensile secondary stresses along its major axis, this secondary stress being related to that which is detrimentally affecting the signal output from the strain gauge circuits in the first arm. Assuming that the detrimental stress is equal in all directions then the strain gauge circuits on each arm can be so connected that the output signal from the circuit on the second arm is deducted from the output signal from the first arm, to leave an output signal which only relates to the stress along the major axis of the first arm. In some circumstances, it may be required to alter the signal output from either or both arms, and this can be achieved by mechanical enhancement of the stress within the arm through a change in the cross sectional area of the arm at the area where the strain gauge is applied. The change in cross sectional area can be controlled with some precision so that the signal output is reliably enhanced to any desired level. Alternatively the output signal from each arm may be enhanced by electronic means through an individual amplification circuit, the output from either arm being increased or reduced as desired by gain changes within the appropriate amplification circuit to give the desired output.

A specific embodiment of the invention will now be described by way of example with reference to accompanying diagrams in which: Figure 1 shows a plan view of the sensor element and the arrangement of the strain gauges Figure 2 shows the interconnection of the strain gauges shown in Figure 1 Figure 3 shows a plan view of the sensor and an alternative strain gauge arrangement Figure 4 shows the interconnection of the strain gauges shown in Figure 3 Figure 5 shows a plan view of alternative shapes for each arm of the sensor element Figure 6 shows a plan view of the sensor element and a further alternative strain gauge arrangement and amplification pcb Figure 7 shows the interconnection of the strain gauges shown in Figure 6 to the amplification circuit pcb Referring to Figure 1, showing the format of a bolt down bi-axial sensor where one pair of Dual Linear resistance strain gauges 1, 2 is installed on one of the right angle arms of the sensor element 5, and a second pair 3, 4 are installed on the other right angle arm. The gauges are connected either to a printed circuit board (pcb) or terminal strip 6 via separate wires or a planar flexi cable, schematically shown as 7. A cable lead 8, to provide power in and take the sensor signal out, is also connected to the pcb or terminal strip 6. The sensor element 5 has three holes 9,10,11 through it for location and fixing, and these must be positioned on the mutually perpendicular primary axis 27 and secondary axis 28 as shown, preferably with centre distance between 9 and 10 the same as that between 10 and 11. It is the function of the bolt fixings through these holes to transfer either the load or extension of the structural element along the primary axis 27 and secondary axis 28 onto the right angle arms of the sensor element 5.

Figure 2 indicates the connection of the gauges 1,2 and 3, 4 shown in Figure 1 into a bridge network. When the transducer is strained along its primary axis only, a proportional output is produced by the bridge. If an equal strain due to a load is applied to both primary and secondary axis, no output is obtained from the bridge.

Figures 3 shows an alternative configuration utilising one pair of Poisson's Ratio format gauges 12,13 on one of the right angle arms, and a second pair 14,15 on the second right angle arm of the sensor element 5. They are connected as illustrated in Figure 4. In this case the output from the gauge arrangement is not as great as that obtained from the configuration of Fig 2, when experiencing the same load conditions. Otherwise the performance is similar.

Consequently, in either of the arrangements shown in Figures 1 and 3, if the primary axis 27 of the transducer is set along the line of the loading to be monitored, then any other overall effects giving equal loadings on both primary axis 27 and secondary axis 28 will be ignored.

If the effects of loading on the primary axis 27 and secondary axis 28 are not equal then in the case of a bolt down transducer, mechanical/shape enhancement or electrical amplification can be applied to compensate as illustrated in Figures 5, 6 and 7.

It is also possible for a sensors in either of the formats shown in Figs 1 & 3 to be bonded down with an appropriate adhesive so that all of the undersurface of the sensor element 5 is in intimate contact with the structural element. The through holes 9,10,11 could be used in conjunction with bolt fixings for initial alignment and accurate positioning, the bolts being then withdrawn or left in place, but if left in place they do not provide load transfer to the sensor element 5. Figure 5 illustrates the nature of shape revisions 16 and 17 which can be used to increase the strain on either arm in order to equalise their output due to different load effects.

Figures 6 & 7 indicate the arrangement for electronic amplification of the strain gauge outputs. This arrangement provides a full strain gauge bridge on each arm of the sensor.

Gauges 18,19,20,21 are installed on the primary arm and gauges 22,23,24,25 are on the secondary arm. The bridges input into two separate amplifier circuits on a pcb 26. The different gain factors of each circuit permits the outputs to be matched when both arms are registering the undesirable loads only, so when'added'they cancel each other out.

Claims

CLAIMS 1. A sensor assembly known as the biaxial sensor assembly comprising a square steel plate manufactured to provide two free arms at right angles to each other, connected to the plate and each other by a small web of remaining material, with a strain gauge applied to each arm, with associated electronic components and wiring located on the remaining area of steel plate, which when applied to a structural element will monitor compressive and tensile stresses in primary and secondary axes so that effects of the stresses monitored in the secondary axis are eliminated from the signal output of the primary axis.
2. A sensor assembly known as the biaxial sensor assembly as claimed in Claim 1 which can be applied to a structural element in a selected position and alignment by means of fixing bolts in a three point fixing arrangement, one through the free end of each arm, and the third through the meeting point of the two arms, so as to provide a signal output due only to compressive and tensile stresses along a primary axis.
3. A sensor assembly known as the biaxial sensor assembly as claimed in Claim 1 which can be applied to a structural element in a selected position and alignment by means of a structural adhesive applied to the suitably prepared surface of a structural element so as to provide a signal output due only to compressive and tensile stresses along a primary axis.
4. A sensor assembly known as the biaxial sensor assembly as claimed in Claim 1 where the signal output from the strain gauge arrangement of either or both arms can be enhanced to ensure an equalisation of the outputs from each strain gauge by means of amendments in the cross sectional area at a section where the strain gauge is applied so as to provide a signal output due only to compressive and tensile stresses along a primary axis.
5. A sensor assembly known as the biaxial sensor assembly as claimed in Claim 1 where the signal output from the strain gauge arrangement of either or both arms can be enhanced by means of an electronic amplification circuit to provide an equalisation of the signal output from each arm so as to provide an output due only to compressive and tensile stresses along a primary axis.