GB2033584A

GB2033584A - Accelerometers

Info

Publication number: GB2033584A
Application number: GB7843235A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-11-04
Filing date: 1978-11-04
Publication date: 1980-05-21
Also published as: GB2033584B

Abstract

A piezoelectric accelerometer comprising a piezoelectric element- mass-spring stack (16, 18, 24) uses the spring (24) as part of the electrical path from one side of the piezoelectric element (16) to an output terminal (38). The spring (24) is the only flexible component in this path, thereby avoiding the use of wires and enabling the component parts to be assembled by simple screw connections. The mass (18) is preferably of a high density tungsten alloy, thereby reducing the axial length of the mass. <IMAGE>

Description

SPECIFICATION Accelerometers This invention relates to accelerometers and is concerned with such devices which incorporate a piezoelectric element as a transducer to convert a stress exerted on the element, e.g. as a result of shock or vibration, into an electrical output signal.

In one known type of piezoelectric accelerometer the active element consists of a number of piezoelectric discs on which rests a relatively heavy mass. The mass is pre-loaded by a stiff spring and the whole assembly is sealed in a metal housing with a thick base. When the accelerometer is subjected to vibration, the mass exerts a variable force on the discs which, due to the piezoelectric effect, develop an electrical output proportional to the force and therefore to the acceleration of the mass. According to one known form of construction the piezoelectric element is mounted under compression from the pre-load spring with the piezoelectric element-massspring system mounted on a cylindrical centre post attached to the base of the accelerometer.The electrical output from the piezoelectric element is obtained by way of a wire which is connected at one end to the piezoelectric element and which is led out through a passageway in the base of the accelerometer to an output terminal.

This known arrangement has a number of disadvantages. Firstly, the use of a wire as the electrical connection between the piezoelectric element and the output terminal creates a number of problems. It is a difficult assembly task to connect the wire satisfactorily and to lead it through the passageway in the base. Moreover, the fact that the wire lead is connected to the piezoelectric element at one side of the element can lead to distortion of the electrical output. Furthermore, this design of accelerometer has only a moderate sensitivity, e.g. the ratio of the electrical output to the acceleration of the vibration causing the output.

According to another known form of piezoelectric accelerometer, the piezoelectric element is again mounted under compression within a housing. A seismic mass rests upon the piezoelectric element and is subjected to the force of a spring. In this arrangement, instead of using a flexible wire for the electrical output, a terminal screw is set into the centre of the seismic mass so that the electrical output from the piezoelectric element passes though the seismic mass directly to the terminal screw and thence to an external connecting wire which can be soldered to a tag held by the screw.

This known arrangement also has a number of disadvantages. Firstly, because the terminal screw is directly coupled to the seismic mass by being screwed into it, any external forces acting on the terminal screw, for example due to the mass of the external connecting wire, also have an effect on the seismic mass and can give rise to spurious outputs.

Secondly, in order to obtain the necessary sensitivity with this arrangement the axial length of the seismic mass has to be longer than is desirable, for example it has an axial length substantially equal to its diameter. When the accelerometer is subjected to a lateral acceleration, the seismic mass tends to twist off the piezoelectric element and when the mass settles back on the element it can chip the piezoelectric material or short-circuit against the body of the housing.

It is an object of the present invention to provide an improved piezoelectric accelerometer which is simple to assemble, is reliable in use, can be manufactured at low cost and has moderately high sensitivity.

The piezoelectric accelerometer of the present invention can be used for intruder detection systems, for the monitoring of machine vibration, and for general purpose measuring applications where vibration or shock needs to be indicated and/or measured.

In accordance with the present invention there is provided an accelerometer comprising a piezoelectric element mounted on a base, a mass in contact with a face of the piezoelectric element, spring means biasing the mass towards the piezoelectric element, and a rigid contact link extending from the spring means to an output terminal to enable an electrical output to be taken from said face of the piezoelectric element via the spring means and the contact link to said output terminal, said spring means constituting the only flexible component in the path from the piezoelectric element to the output terminal.

By including the spring means in the electrical path to the output terminal from the non-earthed side of the piezoelectric element one eliminates the main problem associated with the second of the aforementioned known accelerometers, namely that of the terminal screw being directly connected to the seismic mass without any flexibility in the conductor path. The accelerometer of the present invention also overcomes the assembly difficulties inherent in the design of the first of the aforementioned known accelerometers which use a fine contact wire between the piezoelectric element and the output terminal. According to a preferred embodiment of the present invention the accelerometer can be assembled entirely by the screwing of component parts together in an axial stack, thereby reducing assembly costs and improving reliability.

Preferably, the rigid contact link comprises an elongate element extending from the output terminal to an electrically conductive thrust plate which abuts the spring means, preferably a pre-load spring washer.

According to a preferred embodiment, the piezoelectric element, the mass and the spring means are stacked axially in contact with each other, and at least a part of the rigid contact link is positioned off the central axis of the stack.

According to an important preferred feature of the present invention, the mass is made from a high density tungsten alloy. This means that one can use a mass having a much shorter axial length than is the case with the conventional stainless steel mass, thereby reducing the moment of inertia about its centre of gravity. This increases the resistance of the mass to lateral shock and reduces the overall length of the accelerometer, without impairing its sensitivity.

In order that the invention may be fully understood, one preferred embodiment of accelerometer in accordance with the invention will now be described by way of example and with reference to the accompanying drawing which is a longitudinal sectional view through the accelerometer.

Referring to the drawing it will be seen that the accelerometer comprises a cylindrical cup-shaped housing 10 which is internally screw-threaded around the lip as indicated at 12. The base of the housing 10 is provided with an internal recess 14. A piezoelectric element 16 is seated in the recess 14. A seismic mass 18 which is substantially cylindrical in shape is seated on the piezoelectric element 16. The bottom of the mass 18 is provided with a recess 20 within which the element 16 is seated, and the upper face of the mass 18 is also provided with a shallow recess in order to define a small peripheral lip 22.

The housing 10 includes a screw-threaded fixing stud 21 to enable the device to be mounted for use.

The seismic mass 18, in comparison with the conventional use of stainless steel, is preferably made from a high density tungsten alloy, preferably having a composition of 90 to 95 percent tungsten.

The axial length of the mass 18 can thus be approximately half the diameter, thus reducing the moment of inertia about its centre of gravity by about a factor of 4 as compared with the use of conventional materials. This substantially increases the resistance ofthe accelerometer to lateral shock.

It also has a further advantage which will be referred to later.

In assembling the piezoelectric element 16 and the seismic mass 18 within the housing 10, these parts may be simply seated one on the other or a suitable adhesive can be used to ensure that they are in firm abutting engagement.

A pre-load spring washer 24 is positioned above the seismic mass 18 and is located thereon by the peripheral lip 22. The washer is preferably arranged in an upwardly convex attitude. Mounted above and in contact with the spring 24 is a screw-in assembly which fits into the upper end of the cup-shaped housing 10. The assembly comprises a metal thrust plate 26, an electrically insulating washer 28 and a metal plug 30 which has a peripheral screw thread.

Extending up through the assembly of thrust plate 26, washer 28 and plug 30 is a contact screw 32. This screw 32 is electrically insulated from the metal plug 30 by being encased in a sheath or sleeve 34 of suitable electrically insulating material. On top of the plug 30 is placed a further electrically insulating washer 36, and a solder terminal 38 is secured in place on the contact screw 32 by 3 nut 40.

At a diametrically opposed position from the contact screw 32 a second screw 42 is screwed down into the plug 30 and retains a solder tag 44 which serves as an earth terminal. This terminal is connected by way of the plug 30 and the body of the housing 10 to the lower side of the piezoelectric element 16.

A cylindrical cup-shaped terminal shield 46 is fitted on to the housing 10 and is secured in place by a centre screw 48 which tightens down into the plug 30. Connecting wires (not shown) extend from the tags 38 and 44 out through a hole (not shown) in the shield 46 to external indicating and/or measuring circuitry.

The electrical signal path from the upper surface of the piezoelectric element 16 is therefore by way of the seismic mass 18, the spring 24, the thrust plate 26 and the screw 32 to the terminal 38. The only component in this path which has flexibility is the spring 24 which provides a flexible interface between the thrust plate and seismic mass.

It was mentioned above that by using a high density tungsten alloy for the mass 18 the axial length of the mass 18 could be kept small. This means also that the overall length of the accelerometer is kept relatively small and that the axial length of the wall of the housing 10 can be kept to a minimum. This has the added advantage that when the accelerometer is subjected to a vibration or shock there is relatively little "stretching" of the housing wall and the effect of the seismic mass 18 will produce an accurate response to the shock or vibration.

It will also be appreciated that the assembly of the accelerometer can be performed by simple screwing together of the component parts, thus simplifying the assembly operation, reducing cost and improving reliability.

In an alternative arrangement, the construction of the accelerometer can be simplified by replacing the screw-in plug 30 and associated parts with a fixed closure plate secured across the top of the cupshaped housing and by having a rigid contact link rivetted to the closure plate and extending down into contact with the spring. Such an arrangement would further reduce the cost of manufacture but would not provide accessibility to the inside of the accelerometer as is possible with the preferred construction shown in the drawing.

Claims

1. An accelerometer comprising a piezoelectric element mounted on a base, a mass in contact with a face of the piezoelectric element, spring means biasing the mass towards the piezoelectric element, and a rigid contact link extending from said spring means to an output terminal to enable an electrical output to be taken from said face of the piezoelectric element via the spring means and the contact link to said output terminal, said spring means constituting the only flexible component in the path from the piezoelectric element to the output terminal.

2. An accelerometer as claimed in claim 1, in which the piezoelectric element, the mass and the spring means are arranged in an axial stack within a housing.

3. An accelerometer as claimed in claim 2, in which the rigid contact link comprises an elongate element extending from the output terminal to an electrically conductive thrust plate which abuts the spring means.

4. An accelerometer as claimed in any preceding claim, in which the spring means is a pre-load spring washer.

5. An accelerometer as claimed in any preceding claim, in which the rigid contact link is at least partly positioned off-centre from the central axis of the mass.

6. An accelerometer as claimed in any preceding claim, in which said mass is of a high density tungsten alloy.

7. An accelerometer as claimed in claim 6, in which the alloy includes from 90 to 95 percent tungsten.

8. An accelerometer as claimed in any preceding claim, in which the axial length of the mass is approximately half its maximum transverse dimension.

9. An accelerometer as claimed in any preceding claim, in which the piezoelectric element has one face seated within a recess in a housing and an opposite face seated within a recess in the mass.

10. An accelerometer as claimed in any of claims 1 to 8, in which the rigid contact link is screwed into a housing which encloses the piezoelectric element, the mass and the spring means, the link being screwed into the housing until it abuts the spring means.

11. An accelerometer as claimed in any preceding claim, which includes a two-part housing, one portion thereof enclosing the piezoelectric element, the mass and the spring means, and the other portion thereof enclosing the output terminal.

12. An accelerometer as claimed in any preceding claim, in which the contact link comprises a screw having one end in electrical contact with said spring means and the other end in electrical contact with said output terminal, the screw extending through and being insulated from a plate which forms part of an earthed conductor path from the opposite face of the piezoelectric element to an earth terminal carried by said plate.

13. A piezoelectric accelerometer substantially as hereinbefore described with reference to the accompanying drawing.