DE2136096C3 - Piezoelectric transducer - Google Patents

Description

The invention relates to a piezoelectric transducer for measuring force, pressure or acceleration, with a du-.ch action of a mechanical force dielectric polarization in crystal material and thus a piezoelectric crystal arrangement generating an electric field outside the crystal, at the one directly on a surface of the crystal that intersects the direction of polarization its electrical resistance as a function of the piezoelectrically generated electrical field and thus Depending on the measured variable to be determined influenced semiconductor path is provided.

In the case of piezoelectric transducers, the measured variable generates a charge on the crystal surface, which are removed by electrodes and fed to an amplifier with a high-impedance input will. Since the charges are relatively small, it is necessary to use charge amplifiers and the Insulate the lead to the amplifier very well. It is known to be able to keep this supply line short (Austrian patent specification 251913), the piezo transducer and a transistorized charge amplifier summarized in a handy unit.

It is also known to use semiconductor elements in the form of strain gauges. These semiconductor elements serve as piezoresistive transmitters (US Pat. No. 3,023,627; 3,358,257; 3417,322 and 3,513,430), which are used like strain gauges and only have a higher sensitivity than conventional metallic strain gauges. In the case of piezoresistive elements, the resistance of the element is changed under the action of expansion or compression, whereas in the case of piezoelectric effects, under the action of a force, electrical charges appear on the corresponding surfaces of the crystal. It is known (French patent 890 391) to lay the crystal cut in such a way that maximum sensitivity is achieved.

Finally, it is also known that a crystal generates as a result of the action of a measured value δ th electrical charges to act directly on a semiconductor path, which is in the manner of a Field effect transistor is constructed. The control voltage at the gate electrode of the field effect transistor is replaced by the one directly applied to the

to semiconductor-acting charge of the artU In this way, the metrologically favorable Properties of the crystal combined with the advantages offered by field effect transistors. The object of the present invention becomes

seen in increasing the sensitivity of such transducers. Based on a piezoelectric transducer of the type mentioned, this object is achieved according to the invention in that a conical shape of the crystal bodies is provided at the location of the semiconductor paths in order to achieve a reinforced electrical field.

A particular advantage of the arrangement according to the invention is that with a certain Change in measured value means a considerably larger change

»5 tion of the resistance of the semiconductor path and thus an increase in the output signal is achieved. It is thereby possible to have a better resolution to achieve and to realize smaller measuring ranges without the downstream amplifier and / or display arrangement must be complex.

In a preferred embodiment of the invention, when the transverse piezo effect is used at least one surface of the piezocrystal has a semiconductor layer and there are semiconductor layer in this one or more resistance paths diffused, which are part of a Wheatstone Form a bridge circuit.

In other embodiments of the invention, the longitudinal piezoelectric effect is used. In further embodiments, the piezoelectric polarization is caused by a shear effect generated. Depending on the different metrological tasks and the existing one Space is one or the other type of construction cheaper.

The piezo crystals can be designed differently. They are preferably in the form of a cylinder segment executed.

Measuring transducers according to the invention are suitable for force measurements as well as for pressure and acceleration measurements. The connected circuit arrangement can be in a partial or full bridge circuit be executed. For further processing and display of the measured value, all commercially available Signal processing equipment can be used.

The state of the art and execution are shown in the drawing.

Fig. 1 shows schematically a piezoelectric force measuring device as it is readily available commercially Parts can be put together;

F i g. 2 shows a force-time diagram of a loading process;

F i g. 3 shows a voltage-time diagram of the process according to FIG. 2;

F i g. 4 shows schematically in section a field effect transistor with an insulated control electrode;

F i g. 5 shows schematically a known force measuring arrangement with the aid of the piezoelectric Transversal works;

: jg. 6 shows a crystal arrangement according to the invention

f {g, 7 shows the arrangement of Fig. 6 in section (line AA-

Oie figures erlSutem the current state 'Piezoroeßtechnik and then run over today to Prinerklarung field effect transistors with ■ FOURTH control electrode as they came on the market in recent Jab- _L. Then an example of the example of the piezoelectric field effect transducers according to the invention is shown. ( _n ρ i »1, a conventional piezoelectric transducer is shown in principle, which works according to the sversaltn piezo effect. If a force P er acts a stamp 1 on a piezoelectric Kn-» s Jl 2, z. 8th quartz, and this along When its axis is compressed, deformations arise in the quartz which result in a different shift in the electrical charges. As indicated, the totality of the positive charges 3 can be shifted somewhat to the left and the totality of the negative charges 4 can be shifted somewhat to the right In the interior of the crystal, the balance of positive and negative charges is not disturbed, but on the surface there is a layer of excess bound positive charges on the left and a layer of negative charges on the right. The electric field of these positive bound charges creates neeative free charges on the left Charges attracted from the mass, which are then located on electrode S, or d The positive charge that flows off to the ground on the right is drawn by the field of the negative bound charges from electrode 6, positive free charges from the control electrode 7 of the field effect transistor 8 to the electrode 6, or free neeative charges pushed over to the control electrode of the field effect transistor 8. These latter charges 9 act through their electric field on the channel 10 of the field effect transistor 8 and, depending on the type of field effect transistor, cause an increase or decrease in the channel resistance a current is led through channel 10 of the field effect transistor, which causes a voltage table on this transistor, which can be measured with an oscilloscope 13 or other voltage display or writing device. Every change in the channel resistance causes a voltage change on the channel itself, which is then displayed by the measuring instrument, ie indirectly the magnitude of the force P acting on the piezoelectric quartz is displayed. The previously mentioned repelled negative charges 9 can under certain circumstances partially flow away to ground via a resistor, whereby their ability on the control electrode? of the field effect transistor8 is lost. In the practical case, these derivation possibilities consist in the limited insulating capacity of the connected means of connection such as cables, sockets, etc.

Fig. 2 shows the force-time diagram of the constant punch force P, which acts on a crystal for a short time. The: corresponds to a square pulse in the diagram.
3 shows the measurement with the oscilloscope 14 * voltage-time diagram of the IngJ> g * according to FIG. 2 As a result of the drainage "ewes TeUw of the U dung through the above-mentioned insulators""J JJ voltage loss 15 which in the case of the quartz even produces a wrong signal 16

It's a much complained night! the ^ oetek Irish measuring method that such charge * losses are not completely avoided kflw «. although one will not intentionally incorporate such a resistance, especially when long pressures are to be measured, where the error is particularly pronounced. The piezocrystals are very small, and the low conductivity of the quartz is sufficient to cause such voltage losses 15 (see Fig. 3) and after removing the punch force zero point errors 16 (see Fig. 3) arise which are only very long afterwards Time to disappear. In order to immediately restore the next zero point in order to be able to continue measuring, it is necessary to connect electrode 6 (Fig. 1) to earth via an earth button (not shown) in order to dissipate the charges that are still present gnal is measured 16 were explained by the fact that the lebundeSen negative surface charges of the quartz disappear and the positive / ^ei <£ charges on the electrode 6, the see through the elec π field of bound charges held freigeben'ursprünglich before the bedding mjd polarization of Crystalline, positive and negative free charges in the isolated system consisting of electrode 6, control electrode 7 and connection of the two in equilibrium. Since now, during the 5 "output and shift of the charge within this isolated system, part of the negative charge has flowed off via resistor 14" ^ while the positive free charges are fully retained on the electrode 6 Is there an excess of positive charges after the quartz has been relieved and the charge shift is canceled in the insulated system, which can be ^measured on the insulated system from the electrode s to the control electrode? Including connecting wire and also acting on the field effect transistor 8 by means of the control electrode 7, which leads to a display of a negative force 16. The charges which have flowed off over the resistance 14 flow back over the same path and compensate for these positive excess charges as a result of this

negative force slowly ^disappears . ^d ^ _n ^ S _r . Deiungen after extraordinarily good Isolat.onsw.de ^

"Tändcn, which is the order of magnitude of 10 ohms have to reach are often made by the user piezoelectric measuring method perceived as cumbersome, 0 especially if you have long transmission kits, between the piezoelectric transducer ™ d U dunes amplifier, d. H. in our case the SransStor field. The piezoelectric system is therefore only suitable for measuring gears, after all, such but can be calibrated quasi-statically.

According to the idea of the invention; soltenjJ.e Ben advantages of piezoelectric quartz

5 6

to measure fast processes, as well as the field changes the resistance of the U-shaped half

no other transducer element reached large lcitcrs 54 located on this surface

Measurement range are retained, the disadvantages of and on this very closely rests or grown

unavoidable loss of measurement charges and is. 55 and 56 are the contact points with the HaIb-

thus restricting the measurement to dynamic 5 conductors 54, they are connected to the measuring device,

see events to be avoided. Electrode 57 is not absolutely necessary, it is jc-

Fig. 4 shows schematically the structure of a but advantageous, on the unused quartz object

the usual field effect transistor with an insulated surface

Control electrode. Such consists of a silicon surface charge through an electrode,

Utnsubstrat 40, into which a conductor channel 41 is doped and connected to the semiconductor, to be neutralized.

became, via the rcn from the source 42 to the drain 43, otherwise also a strong one on this side

a current can be carried. An electric field occurs on this channel, which is the one that causes the

An insulation layer 44 is applied. Mostly intended to influence semiconductors, weakens. So it is

it is silicon dioxide SiO ₄ , i.e. quartz, possible with this measuring transducer, static forces,

on which the control electrode 45 is applied. Will 15 which act a long time, constantly monitor

to the control electrode 45 from the outside opposite and to measure without the zero point over time

a voltage is applied to the drain 43, is lost.

The quartz layer creates an electric field that polarizes the silicon dioxide. It would be compared to the control element, as well as the display device 58, which measures the bulkhead 45 not the channel 41, but also a change in voltage. Instead of a U-shaped, made of metallic conductor, the whole thing would not have been provided with a semiconductor channel 54, a bridge as a capacitor and the electrical field arrangement can also be selected. For the application, the transistor technology would never end on the inner surfaces of the semiconductor channel, and because charges in conductors are very mobile, a number of interesting possibilities. So are the emergence of an electric field at home "5 could prevent a metal r example, on a crystal surface is a silicon Africans. In a semiconductor layer 59 by vapor deposition or with chemical, however, are generated because of the lack of free charge means. In this layer the situation would be somewhat different. The electrical known methods a P or N doped half-field is able to diffuse something into the semiconductor. It is also possible, however, and, depending on the direction of the field 30, causes a displacement or partially doped semiconductor material to be applied directly to the crystal surface in amorphous form and type of semiconductor and this enrichment of charge carriers, which either give the desired shape through etching processes give. Increasing or decreasing the channel resistance in view of the complex work processes involved. It is important that only one side of the piezocrystals with an electric field in the silicon dioxide occupies a dielectric layer of a semiconductor layer; shifted as it is treated above stall areas in this way,
already in connection with the Pie ^ ueffekt he in F i g. The transducer shown in FIG. 5 has only been clarified, and such connections also result in low sensitivity because of the piezoelectric displacement surface charges, which see the electrical effect of quartz, is quite weak. Instead of amplifying the field by a multiple, and thus quartz, piezoceramics made of barium titanate or the effect on the semiconductor by the same biocontact titinate were used, the piezo effect of which increases a factor of about. Such a dielectric is 100 times stronger than that of quartz. The additional field generated by cadmic polarization also arises, unless it was used, which at the same time is semiconductive if the dielectric polarization is not by a +5 tend and piezoelectric. The practice in the construction of primary electric field has been generated, but measuring transducers has shown that Piezokt.amik because of the z. B. by the piezo effect, and it remains its hysteresis, non-linearity, temperature-dependent equal in relation to the semiconductor whether the field by speed, AHers instability and lower insulation an electrode and external voltage plus polarization not allowed, measuring tents of high quality or only through Polarization alone was generated. The invention provides in spite of the swath. chen piezo effect to use quartz, and by

The idea of the invention consists in a piezo-conical shape of the crystal pieces

to build an electrical element, in which the piezoelek lines of force are pressed together in order to

trically generated dielectric polarization in a H electric field, which leads to a significant

adjacent semiconductor generates an electric field, increasing the sensitivity of the transducer

which leads to the same as in the principle of field efficiency explained above.

Effect transistors change the resistance of a channel. 6 shows a crystal arrangement according to the invention

This change in resistance can be achieved with electrification consisting of a number of segment-shaped

ce means are measured and is a measure of gene crystals, in which the transversal piezo effect

the force P, which acts on the element 60, is used. For a better view there is em

Fig. 5 shows a schematic diagram of a model of a known segment omitted. The force P acts on the transducer designed to measure force. One face of the segments. Piezo crystal 51 is placed on the inner sides 110 between the stamp 52 and the semiconductor arms 111 are in turn placed on the supports 53 by a force P which sets compressive or tensile stress on the end faces of the segments, which connecting points 112 which cause zo and zoelectric polarization. As a result of this polarization of the current through the semiconductor, polarization emerges from the quartz surfaces vertically. This not shown plate-shaped bottom and

Cover pieces pressed together in the axial direction, with corresponding, embedded in these plates CountercontactC, establish the electrical connection with the semiconductor element.

FIG. 7 shows a section through the quartz crystal group at point AA in FIG. Through the action of forces in the axial direction, dielectric polarization 125 arises in the outer segment areas, which itself would like to run parallel to the x-axis. By influencing the neighboring segment, these vectors 126 are displaced against the considerably reduced inner surface 127, whereby a concentration of the polarization or field vectors is achieved

will. With such an arrangement there is thus a reinforcement of the effect on the half-liter tracks electric field achievable.

The invention thus allows a substantial expansion the existing quartz piezomeasurement technique by using the field effect transistor technique with the piczo is united in a new way, but no more electrostatic charges appear as a signal, but where from the Piczoc effect only the generated dielectric polarization of the crystalline crystal and the associated impact are important. The piezo measuring technology can thus be transferred from the purely dynamic application technology to the quasi-static and static without losing its great advantages.

1 sheet of drawings

Claims (2)

Patent claims: 1. Piezoelectric measuring transducer for force, pressure or acceleration measurement, with a dielectric polarization in the crystal material due to the action of a good mechanical force and thus a piezoelectric crystal arrangement generating an electric field outside the crystal, with the direct call of a surface of the crystal that intersects the polarity direction Resistance m depending on the piezoelectrically generated electric field and thus depending on the force to be determined influenced semiconductor path is provided, characterized in that a conical shape of the crystal bodies is provided at the location of the semiconductor paths in order to achieve an increased electric field i;> .. 2. Piezoelectric transducer according to claim 1, characterized in that when used of the transverse piezo effect, at least one surface of the piezo crystal has a semiconductor layer and that one or more resistance paths diffused into this semiconductor layer which form part of a Wheatstone bridge circuit.