SPECIFICATIONElectric transducer unit for sensing mechanicalvibrations or other pressure changesThe invention relates to an electric transducer unitfor sensing mechanical vibrations or other pressurechanges, comprising a membrane for sensing pressure changes and a piezoelectric transducer coupledto the membrane. Such an electric transducer unitcan be used in piezoelectric transducers.
In recent years the application of piezoelectrictransducers comprising membranes for sensingmechanical vibrations or other pressure changes hasbecome a conventional technique, and the mechanical connection between the membrane and thecrystal of the piezoelectric transducer can be solvedby many known ways. The basic principle of thefunction of such a transducer lies in that in responseto the deformation of the piezoelectric crystal occurring under the effect of pressure changes applied onthe membrane an electrical voltage is generated thatis proportional to this deformation.
In one of the known piezoelectric transducers ofthis kind the crystal is mounted on the inner surfaceof the membrane e.g. by means of sticking orsoldering. Such a construction is described in detailin Hungarian patent 160 565. In this patent theproblems connected with the different known solutions are also summarised. The transducer described in this patent is advantageous because thewhole deformation acting on the membrane isdirectly transferred to the crystal without any coupling member therebetween, and thus the electricsensitivity and the electrical transmission propertiesof such a transducer are more favourable than thatof the prior constructions.
The adhesive or soldered connection between thecrystal and the membrane transfers all kinds offorces acting on the membrane to the crystal. Thisphenomenon, which is advantageous as far aselectrical properties are concerned, becomes disadvantageous if external force effects substantiallyexceeding the operational limit values act on themembrane. Such force effects can occur if thetransducer is accidentally dropped or knockedagainst a hard object. The crystal of the piezoelectrictransducer is a fragile and thin plate which can easilybe split or broken by such forces.
In addition to the dangers outlined above, it is verydifficult to achieve the required quality control of thesoldered connection and the danger of a possible disconnection can be avoided only by using a verycareful and disciplined manufacturing technology.
An object of the present invention is to provide anelectric transducer unit that is at least equal to otherknown constructions as regards its electric properties, but which is more advantageous as regardsits mechanical stability, its protection offered for thecrystal against external force effects, its life and itsfeasibility for being manufactured. Moreover, thetransducer unit has to meet all the additionalspecifications which are characteristic to the construction made according to the prior art referred tohereinabove, including good shielding against external noise fields.
It has now been found that the direct coupling between the membrane and the piezoelectric transducer can be achieved by incorporating the piezoelectric transducer into the membrane itself. To carry out this idea it has been necessary to break with the conventional design of membranes, i.e.
with membranes in form of thin, often metallic, plates.
According to the present invention there is provided an electric transducer unit for sensing mechanical vibrations or other pressure changes, comprising a membrane for sensing said pressure changes and a piezoelectric transducer mechanically coupled to said membrane and comprising a pair of electrodes, in which said membrane comprises a membrane body of an elastic plastics material and said piezoelectric transducer is located in said membrane body, at least one of said electrodes being lead out through said membrane body.
The use of the membrane body completely pro techs the crystal of the piezoelectric transducer against any external mechanical impact, but at the same time the electric properties remain unchanged due to the direct coupling between the membrane body and the crystal.
An additional advantage occurs due to the greater mechanical strength of the membrane body, because in prior constructions high external forces could destroy not only the crystal but the membrane plate as well. Because the membrane used in the present invention is thicker than usual and elastic it will better resist these forces.
In a preferred embodiment of the invention an electrial conductive coating is provided on the outer surface of the membrane body.
Another embodiment which facilitates the manufacturing of the membrane body is made of two parts, the piezoelectric transducer abutting the inner wall of the outer part. This embodiment, in which the membrane body is made of two parts, is especially advantageous if the outer part of the membrane body is built integrally with a casing surrounding the outside of the transducer, and the inner part is placed in the inside of the casing. One of the electrode terminals and the shielding coating can be omitted when the outer part of the membrane body is made of an electrically conductive plastics material.
The membrane body can be manufactured of epoxide resin or of polystyrene or of polypropylene or of other similar elastic plastics materials.
In the accompanying drawings:Figure 1 is a vertical sectional view, not to scale, of a first embodiment of the transducer unit according to the invention;Figure 2 is a sectional view similar to Figure 1 but showing a shielding coating on the outer surface thereof;Figure 3 is a vertical sectional view of another embodiment according to the invention;Figure 4 is a vertical sectional view of a further embodiment according to the invention;Figure 5 is a vertical sectional view of an embodiment according to the invention comprising a trans ducer unit similar to that shown in Figure 2;Figure 6 is a vertical sectional view of the embodiment of Figure 1 illustrated to scale.
Figure 1 shows the simplest embodiment of the transducer unit in a vertical sectional view. The transducer unit comprises two constructional parts, namely a membrane body 1 and a piezoelectric transducer 2 placed inside the membrane body 1.
The piezoelectric transducer 2 is a square or rectangular prism with output electrode such as terminals 3 and 4 connected respectively to the opposite surfaces of the transducer 2 by means, for example, of a soldered connection.
The membrane body 1 is preferably circular in shape and has a thickness greater than that of the piezoelectric transducer 2. A condition for proper operation is that the membrane body 1 should transmit the mechanical effects (principally the bending and shear stresses caused by membranelike oscillation) to the piezoelectric transducer 2 located in it. The membrane body 1 should therefore be made of an elastic plastics material. Actually locating the piezoelectric transducer 2 can be achieved in many ways. However, a most convenient way is to mould the membrane body 1 around the transducer, so that forces which could destroy the crystal appear during the casting and the subsequent solidification steps.
In the embodiment shown in Figure 1 the transducer unit according to the invention can be used instead of the membrane of any kind of conventional transducer.
The embodiment shown in Figure 2 differs from the one shown in Figure 1 in that on the upper surface of the membrane body 1 a coating of electrically conductive material is made for completely shielding the housing of the transducer (Figure 5) against external electric noise and disturbing fields.
The coating 5 can be made by any known plastic metallisation process or by the application of a conductive paint.
The membrane body 1 shown in Figure 3 consists of two separate component parts, namely an upper portion la, and a lower portion 1b made of different materials. The upper portion la is made of an electrically conductive flexible plastics material so that there is no need for the coating 5, and the upper electrode surface of the piezoelectric transducer 2 is coupled directly to the electrically conductive upper portion 1a of the membrane body 1, whereby the terminal 3 can be dispensed with. An output lead for the upper crystal surface is provided by a wire 6 connected to or moulded or pressed in the upper portion 1 a. The lower portion 1b of the membrane body 1 is made of a non-conductive elastic plastics material and it is connected by moulding to the upper portion 1a. The lower electrode surface of the crystal has an output terminal 4.
Figure 4 shows an improved embodiment compared to that shown in Figure 3 in which the electrically conductive upper portion 1a of the membrane body 1 is designed to form also the side wall 7 of the transducer housing. The bottom of the housing is closed by a cover plate 10 and a perse known amplifier 8 for the transducer can be placed in a shielded chamber 9 so formed. The transducer output is lead out by a low impedance wire 11 from the output of the amplifier 8. Inside the housing the terminal 4 can be coupled directly to the input of thg amplifier 8. The body of the housing with a shielded chamber 9 serves as the other lead out terminal 3.
Figure 5 shows an embodiment which is similar to, that shown in Figure 4, but here the side wall 7 of the housing is separated from the membrane body 1.
The housing does not contain any amplifier in this case. The transducer unit is designed as that shown in Figure 2. The shielding coating 5 of the membrane body 1 is connected electrically to the metallic side wall 7 of the housing.
In Figure 6 the embodiment of Figure 1 is shown to scale. The membrane body 1 preferably has a diameter about 25 mm and a thickness about 1.5 mm. The crystal of the piezoelectric transducer 2 has a diameter of 16 mm and a thickness of 0.4 mm. Of course, sizes substantially differing from these exemplary values can also be used.
The membrane body can be made, for example, of epoxide resin, of polystyrene, of polypropylene or of other easily processable plastics materials.
As far as operation is concerned it is of no importance whether the transducer unit according to the invention is located completely or only partly in the membrane body 1. In a rugged design for measuring greater pressure changes the lower electrode surface of the piezoelectric transducer 2 may extend out of the bottom of the membrane body 1.
The measurements carried out with the transducer unit of the invention have proved that this unit is equivalent to conventional transducers as regards its sensitivity and electrical transmission.
However, when stability, reliability and life are considered the transducer according to the invention is substantially more favourable than conventional ones because the sensitive crystal is protected against any kind of external mechanical impacts on the membrane body 1. If the transducer according to the invention is accidentally dropped or its membrane surface is mechanically damaged, this cannot cause the splitting or breaking of the crystal of the transducer.