GB2170605A - Piezo-electric movement transducers - Google Patents

Abstract

A transducer for providing an electrical signal indicative of the movement of a component, such as a fuel injection valve, comprises a piezo-electric crystal 31 on one side of which is located a permanent magnet 30 and adjacent the other side of which a magnetic member (not shown in Fig. 2) is movable so as to vary the mechanical stress applied to the crystal upon movement of the member. The magnet 30 forms an electrical connection 33,34 with one face of the crystal 31 and an electrical connection is effected to the opposite face of the crystal by a suitable member, conveniently a non- magnetic metallic end closure 28 of a metallic sleeve 27 in which the magnet 30 and crystal 31 are located. <IMAGE>

Description

SPECIFICATION Transducers This invention relates to a transducer for providing an electrical signal indicative of the movement of a component in particular, but not exclusively, the movement of the valve member of a fuel injection nozzle of an internal combustion engine.

Various forms of transducer have been proposed for providing an electrical signal indicative of the movement of a component. The proposals range from a simple switch in which the component forms one or is connected to one element of the switch, to more complex proposals such for example as capacitive or inductive transducers. The component itself providing it is formed from a suitable material, can form the moving element of the transducer. Such forms of transducer have their disadvantages, in the case of the capacitive ana inductive transducers, special electrical circuits have to be provided to enable the required output signal to be obtained.

It is known to utilize in a fuel injection nozzle, a transducer which incorporates a piezoelectric crystal, the force applied to which is varied when the valve member of the nozzle is lifted from or returns to its seating. A known example has a piezo-eiectric crystal which forms an abutment surface for a spring which biases the valve member to the closed position. The crystal is positioned at the end of the spring remote from the valve member and an output signal is obtained from the crystal whenever there is a change in the force applied to it.However, with the arrangement described the spring, which is positioned between the valve member and the crystal has an influence on the response of the crystal to movement of the valve member and while the signal obtained from the crystal can provide a reasonably good indication of the movement of the valve member from the seating it does not in addition, provide a reliable indication of when the valve member returns into contact with the seating. It is impractical to position the crystal between the spring and the valve member because of the difficuity of providing the electrical connection. Moreover, it will be understood that such an arrangement wouid not be practical where for instance it is required to sense the passage of a tooth on a rotating member.

The object of the present invention is to provide a transducer for the purpose specified in a simple and convenient form.

According to the invention, a transducer for providing an indication of the movement of a component comprises a piezo-electric crystal one face of which is held against movement, a magnetized member and a magnetic member, said members being positioned on opposite sides of the crystal with one of said members engaging a face of said crystal opposite to said one face whereby the attraction force between said members results in a compressive stress being applied to said crystal, movement of said component being arranged to vary the reluctance of the magnetic circuit between said members whereby the attraction force between the members, and therefore the compressive stress applied to the crystal is varied, variation in the compressive stress causing a variation in the voltage developed by said crystal.

One example of a transducer in accordance with the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a sectional side elevation of part of a fuel injection nozzle incorporating the transducer, and Figure 2 is a view to an enlarged scale of part of the nozzle seen in Figure 1.

Referring to Figure 1 of the drawings the fuel injection nozzle is of the so-called inwardly opening type and includes a nozzle body 10 which is secured by means of a cap nut (not shown), to a tubular holder 11, there being located between the holder 11 and the body 10 a distance member 12. Formed in the nozzle body is a stepped blind bore 13 having an enlargement 14 intermediate its ends which is connected by way of an inlet passage 15 formed in the body, the distance member and the holder, to a fuel inlet 16.

At the blind end of the bore there is formed a seating 17 which surrounds an outlet orifice 18 and slidable within the bore 13 is a valve member 19. The portion of the valve member lying between the enlargement 14 and the seating is of reduced diameter and the end of the valve member is shaped for co-operation with the seating. In addition, the valve member in the example has a projection which extends through the outlet orifice 18, the purpose of the projection being to shape the resulting fuel spray.

At the end of the valve member remote from the seating, is formed a pin 20 which extends with clearance through a hole formed in the distance member 12 and the pin mounts a spring abutment 21, the abutment being engaged by one end of a coiled compression spring 22 the other end of which engages a shim 23 which engages a step defined in the tubular holder.

In operation, when fuel under pressure is supplied to the inlet the pressure in the enlargement 14 acts upon the valve member to lift the valve member against the action of the spring 22, away from the seating 17 and as soon as the valve member is clear of the seating fuel can flow through the outlet orifice 18 in known manner. The exten-t of movement of the valve member 19 is limited by its engagement with a surface 24 defined by the distance member 12 and the force which is ex erted by the spring 22 is determined in use by the thickness of the shim 23. When the supply of fuel to the inlet ceases, the valve member will return to the closed position in which it is shown in Figure 1.

In order to control the timing of delivery of fuel to the engine associated with the nozzle, it is desirable to know the instants of the start of delivery of fuel and the cessation of delivery of fuel and the best indication of when delivery of fuel starts is obtained by sensing the lifting of the valve member 19 from the seating 17. To provide an indication of the cessation of fuel delivery, the movement of the valve member into contact with the seating must be sensed. For this purpose there is mounted within the injection nozzle a probe 25 which extends centrally along the axis of the nozzle and is housed within the spring 22. The construction of the probe will be described with reference to Figure 2.

Referring to Figure 2 the probe comprises a tubular body 26 which is adapted to be secured in the holder 11 of the nozzle and which has a lower portion of reduced diameter about which is located a metallic sleeve 27.

The lower end of the sleeve is closed by a metallic non-magnetic closure 28 which may be formed integrally with the sleeve. A space is provided between the closure and the lower end of the tubular body 26 and located against the lower end of the tubular body is an electrically insulating bush 29. Adjacent the bush there is located a magnetized member in the form of a cylindrical permanent magnet 30 which is poiarized along an axis perpendicular to its plane end faces. Between the magnet 30 and the closure 28 there is disposed a piezo-eiectric crystal 31 the one face of which bears against the closure 28 and the opposite face of which is engaged by the adjacent end face of the magnet 30. The end faces- of the crystal make electrical connection with the end closure and the magnet.

The magnet and the crystal are surrounded by insulating material 32 and a plate engages the end face of the magnet remote from the crystal the plate being connected to a terminal 33 which is connected by means of an insulated wire 34 to a terminal accessible from the exterior of the nozzle. The spring abutment 21 is formed from magnetic material and forms the other member of the transducer and when the various components are assembled together, a magnetic circuit will exist between the abutment 21 and the magnet 30. As a result there will be an attraction force between the abutment 21 and the magnet, this resulting in compressive stress being applied to the piezo-electric crystal 31. As the valve member lifts from its seating, the reluctance of the aforesaid magnetic circuit will be reduced and as a result an increased compressive stress will be applied to the crystal.

The crystal during its manufacture is polarized so that variation in the force applied to said two end faces will result in the development of a voltage between the two faces. As the compressive stress increases, one of the faces will become increasingly positive with respect to the other and as the stress is reduced the difference in the voltage will become less. The amplitude of the signal is therefore representative of valve movement, a rising or falling voltage indicating the direction of movement.

The crystal 31 can be of annular form and the magnet 30 provided with an integral projection which extends through the aperture in the crystal. In this manner the initial reluctance of the magnetic circuit will be reduced.

In the use of the nozzle, the magnet and crystal will be subject to vibration in the plane of the magnetic field which will give rise to spurious signals. These can be obviated by providing a mass equivalent to that of the magnet and a similar piezo-electric crystal positioned on the nozzle holder with the crystals being connected in inverse parallel relationship so as to effect cancellation of the spurious signals.

When the transducer is used in some other situation for example the detection of the rotation of a wheel, a tooth or a plurality of teeth can be formed on the wheel which is arranged to rotate in a plane normal to the magnetic axis of the magnet with the tooth or teeth being formed from magnetic material.

The magnet or a plurality of magnets may be mounted on the component and the position of the magnet occupied by a magnetic member.

The example described illustrates an inwardly opening injector having a projection in the outlet. The transducer can be used on any inwardly opening injector. Moreover, it is convenient to have an impedance transforming circuit positioned on or adjacent to the holder 11 of the injector to provide a low impedance output to minimise the risk of electrical noise appearing in the signal.

Claims (6)

1. A transducer for providing an indication of the movement of a component, comprising a piezo-electric crystal one face of whrch is held against movement, a magnetized member and a magnetic member, said members being positioned on opposite sides of the crystal with one of said members engaging a face of said crystal opposite to said one face whereby the attraction force between said members results in a compressive stress being applied to said crystal, movement of said component being arranged to vary the reluctance of the magnetic circuit between said members whereby the attraction force between the memoers, and therefore the compressive stress applied to the crystal is varied, variation in the compressive stress causing a variation in the voltage developed by said crystal.

2. A transducer according to Claim 1 in which one of said members is defined by the component.

3. A transducer according to Claim 1 or Claim 2 for use in a fuel injection nozzle for an internal combustion engine, said magnetized member being located in facial contact with one face of the crystal, the member and crystal being mounted in electrically insulated relationship within a sleeve, said sleeve having an end closure against which bears the face of the crystal opposite to said one face, the transducer further including a plate engaging the magnetized member, the injection nozzle including a spring abutment movable towards and away from said end closure to vary the mechanical stress applied to the crystal, the sleeve and said plate forming the electrical connections to the crystal.

4. A transducer according to Claim 3 in which said crystal is of annular form and the magnetized member is provided with an integral projection extending through the aperture in the crystal.

5. A transducer according to Claim 3 in which said sleeve is mounted about a body and including an insulating bush located between the end of the body and said plate, said bush and said body defining a passage for an electrical connection to said plate.

6. A transducer for providing an indication of the movement of a component comprising the combination and arrangement of parts substantially as hereinbefore described with reference to the accompanying drawings.