GB2226416A

GB2226416A - Capacitance-type deformation sensor

Info

Publication number: GB2226416A
Application number: GB8928510A
Authority: GB
Inventors: Gian Maria Timossi; Acqua Roberto Dell
Original assignee: Marelli Autronica SpA
Current assignee: Marelli Europe SpA
Priority date: 1988-12-23
Filing date: 1989-12-18
Publication date: 1990-06-27
Anticipated expiration: 2009-12-18
Also published as: IT1224034B; IT8868156A0; DE8914777U1; FR2641072B3; FR2641072A3; ES1012681U; GB2226416B; GB8928510D0; ES1012681Y

Abstract

A capacitance-type deformation sensor e.g. for measuring the load on a vehicle axle comprises a first tubular body 2 in which there is fixed a first metal plate 5 and in which there is slidably mounted a second tubular body 7 to which a second metal plate 8, facing the first plate, is fixed. The abovementioned tubular bodies are provided with anchoring attachments 15 for connection to two points of a structure 17 the relative displacement of which is to be determined. Electronic circuitry 11 including e.g. a variable frequency oscillator and a frequency-voltage converter is mounted on a board 10 within the inner body 7. <IMAGE>

Description

This invention relates to a deformation sensor intended particularly for measuring deformation of a structure.

In particular the invention relates to a sensor of the type including a capacitor with a first and a second plate facing each other and connected to two points of the structure the relative displacement of which in a direction perpendicular to said plates is to be determined by means of a corresponding variation in capacitance of the capacitor.

It is an object of the present invention to provide a deformation sensor of the type defined above, which is simple, robust and reliable.

According to the present invention there is provided a deformation sensor for measuring the deformations of a structure wherein a first plate is fixed inside a first tubular body with an internal cavity and wherein a second tubular body is mounted in said first body so as to be capable of sliding in a guided manner with respect to the first body in a direction perpendicular to the plane in which said first plate lies, a second plate being fixed to said second tubular-body, the plates facing each other and forming a capacitor, said first body and second body being provided with respective anchoring attachments for connection to the two points of the structure the relative displacement of which is to be determined by means of a corresponding variation in the capacity of the capacitor formed by said plates.

In order to enable the invention to be more readily understood, reference will now be made to the accompanying drawings, which illustrate diagrammatically and by way of example an embodiment thereof, and in which: Figure 1 is a longitudinal section through a deformation sensor according to the present invention, Figure 2 is a cross-section along the line II-II of Figure 1, and Figure 3 is a block diagram of an electronic circuit associated with the sensor.

Referring now to the drawings, there is shown a deformation sensor 1 intended in particular for measuring the load on the axle of an industrial vehicle, by determining the deformation of the axle under the effect of the load. The sensor 1 comprises a first tubular body 2 having an internal cavity 3 closed at one of its ends by an end wall 4. Inside the cavity 3 there is arranged a first metal plate 5 fixed to the end wall 4 with the interposition of a layer 6 of electrically insulating material. Inside the body 2 there is slidably mounted a second tubular body 7, at one end of which there is mounted a second metal plate 8 which faces and is at a distance from the plate 5. The plate 8 is fixed to the body 7 by means of a layer of electrically insulating material 9.The body 7 is also internally hollow and in its interior there is mounted a support board 10 on which there are arranged the components of an electronic circuit 11 which will be described below.

In the Figures the radial play between the body 2 and the body 7 has been exaggerated to facilitate comprehension: in reality this play is of the order of a tenth of a millimetre. Outside the bodies 2 and 7 there is arranged a protective bellows 13 made of elastomeric material, the ends of which are fixed by means of annular clamps 14 to the body 2 and to the body 7, respectively.

The first body 2 and the second body 7 are provided with respective end anchoring attachments 15 for fixing the sensor 1, for example by means of welding, to two points 16 of a structure 17 the deformation of which, causing the relative displacement of said points, is to be determined. The flexural deformations of the structure 17 in the directions indicated by the arrows cause, in fact, a relative displacement of the bodies 2 and 7 in a direction substantially perpendicular to the plane in which the plates 5 and 8 lie.

The capacitance of the capacitor defined by the plates 5 and 8 varies correspondingly as a function of the deformation of the structure 17.

In order to avoid temperature variations causing deformations, of varying degree, of the structure 17 and of the bodies 2 and 7, which could be interpreted as a variation in the load on the structure 17, the bodies 2 and 7 preferably consist of the same material as the structure 17, for example soft iron.

The The plates 5 and 8 are connected by means of conductors 18 to an electronic circuit 11 carried by the board 10 and designed to supply a voltage signal indicating the capacitance of the capacitor. Figure 3 illustrates the block diagram of a suitable circuit for this purpose. The circuit 11 comprises an oscillator 20, the oscillation frequency of which depends upon the capacitance of the capacitor formed by the plates 5 and 8 connected to it. The output of the oscillator 20 is connected to a monostable circuit 21 followed by lowpass filters 22 which together perform a frequency/ voltage conversion. The output terminal 23 of the monostable circuit 21, which passes out through a hole 24 in the body 7 (Figures 1 and 2) has (with respect to earth) a voltage V indicating the capacitance of the capacitor formed by the plates 5 and 8 and therefore the degree of deformation of the structure 17.

By way of an alternative to the embodiment illustrated in Figure 3, the electronic circuit 11 could comprise a fixed-frequency oscillator followed by a monostable device, the time constant of which is a variable depending on the variation in the capacitance of the capacitor formed by the deformation sensor. In this case also, a low-pass filter is connected to the output of the monostable device.

Claims

CLAIMS:

1. A deformation sensor for measuring the deformations of a structure wherein a first plate is fixed inside a first tubular body with an internal cavity and wherein a second tubular body is mounted in said first body so as to be capable of sliding in a guided manner with respect to the first body in a direction perpendicular to the plane in which said first plate lies, a second plate being fixed to said second tubular-body, the plates facing each other and forming a capacitor, said first body and second body being provided with respective anchoring attachments for connection to the two points of the structure the relative displacement of which is to be determined by means of a corresponding variation in the capacity of the capacitor formed by said plates.

2. A sensor according to Claim 1, wherein said first body has one end closed by a wall and the first plate consists of a conductive plate fixed to said wall.

3. A sensor according to Claim 1 or 2, wherein the second plate consists of a conductive plate fixed to the end of the second body which extends inside the first body.

4. A sensor according to any one of the preceding claims, wherein the first body and second body each consisting of a material having substantially the same thermal expansion characteristics as the structure the deformation of which is to be determined.

5. A sensor according to any one of the preceding claims, wherein an electronic circuit is associated with the capacitor and comprises an oscillator the frequency of which depends upon the capacitance of said capacitor and a frequency/voltage converter connected to the output of the oscillator.

6. A sensor according to any one of Claims 1 to 4, wherein an electronic circuit is associated with the capacitor and comprises a fixed-frequency oscillator connected to the input of a monostable device the time constant of which is a function of the capacitance of said capacitor, a low-pass filter being connected to the output of said monostable device.

7. A sensor according to Claim 5, wherein the components of the electronic circuit are carried by a support arranged in a cavity formed inside the second body.

8. A deformation sensor for measuring the deformations structure, substantially as hereinbefore described with reference to the accompanying drawings.