DD247083A1 - EVALUATION ASSEMBLY FOR PIEZOELECTRIC KOERPERS - Google Patents

Abstract

To evaluate a piezoelectric body with respect to its piezoelectric material coefficients, this body is arranged parallel to the direction of a force acting on it serially between two punches and generated by the force on the body a first measurement. The aim and object of the invention is to provide in routine operation using the direct piezoelectric effect, a simple, insensitive to wide variations in force variations and reproducible results delivering evaluation arrangement. For this purpose, an etalon is provided between one of the two punches and a third punch for producing a second measuring quantity which is functionally related to the force. A device is used to compare electrical signals derived from both measured quantities. figure

Description

With the arrangement according to the invention, it is possible, regardless of fluctuations in the force as standard to assess the various objects to be measured and weed out those that are below a certain, previously definable Schweliwertes. It is accessible to automation.

embodiment

The invention will be explained in more detail with reference to the schematic drawing. In a column 18, a first punch 1, an etalon 2, a second punch 3, a piezoceramic body 4 and 5 are arranged serially. The contact surfaces 6, 7, 8, 9 between the punches 1, 3, 5 and the etalon 2 or the body 4 are designed so that a complete contact is ensured for a reliable contact. The stamp 1,3,5 are therefore made of conductive material. From the contact surfaces 6, 7, 8, 9, electric lines 10, 11, 12, 13 go to a comparator 14, wherein a capacitor 15, 16 is respectively provided between the lines 10, 11 and 12, 13. In the comparator 14, the charges applied to the line pairs 10, 11 and 12, 13, respectively, for deriving voltage signals are compared with one another and the difference between the voltages applied to the capacitors 15, 16 is measured with a voltmeter 17 connected to the comparator 14.

If a force F adf acts on the pillar 18 and thus on the etalon 2 and the body 4 via the punches 1.5, charges Q2 and Q4 are induced in the latter, which are dependent on the force F and the piezoelectric charge constant d. Since the etalon 2 in the present example is quartz and the body 4 is a piezoceramic, their piezoelectric modules are different from each other. The integration of the charges Q _{2 of} the etalon 2 and Q _{4 of} the body 4 takes place on the suitably dimensionierfen capacitors 15,16, whose inherent capacitances are substantially greater than those of the etalon 2 and the body 4. The voltage applied to the capacitors 15,16 electrical voltages are equal in amount if the ratio of the capacitances C ₁₅ : Ci _{6 of} the measuring capacitors 15,16 gleiph the ratio of the piezoelectric modules d ₂ , d ₄ of Etalon 2 and Körper4. By an appropriate choice of the capacitances C ₁₅ and C ₁₆ , this condition can be met. Since for the piezoelectrically generated charge of the etalon 2 and the body 4, the relationship

Q ₂ = d ₂ F 'I

Q ₄ = d ₄ · F '. Il

and the electrical voltages U ₂ and Un due to the integration of the piezoelectrically generated charges Q ₂ and Q ₄ on the capacitors 15,16

d ₂   1- UL C ₁₅ . C 15 au Cl ₄

H. η. , ρ

^ ₁₆ U ₁ S

are, results for the piezo module (the piezoelectric charge constant) of the body. 4

d ₄ Ci6 "j" _ j ' d ₂ · C _{1 6}

- = --oderd ₄ = -, V

d ₂ Ci ₅ Ci ₅

when U ₂ = U ₄ . It follows that changes in the force F within finite limits and with sufficient functional agreement of the dependence of the piezoelectric effect of the etalon 2 and the body 4 on the force F have no influence on the evaluation of the body 4. If the piezo module and the orientation change from body to body in the routine examination, it is possible for the capacitors 15, 16 and etalon 2 to be unchanged, for the voltmeter 17 to be in the form of an occurring voltage or to deviate from a predefined voltage for the fitness or unsuitability of the body 4 demonstrated. The comparison of the voltages is made in the comparator 14; the voltmeter 17 indicates the voltage difference.

Instead of the two electrical lines 11 and 12, it is also possible to use only one line, which is mounted centrally on the existing of conductive material stamp 3. The Quarzetalon 2 can also be replaced by a piezoelectric film of polyvinylidene fluoride or a piezoresistive material. When using a piezoresistive material, the capacitor 15 would be omitted. The force F can be exercised for example by means of an eccentric.

Claims (3)

Invention claim: An arrangement for evaluating the direct piezoelectric effect of a piezoelectric body, which is arranged between two punches and with these substantially parallel to an acting force, wherein on the body a first measured quantity is generated, which is in a functional relationship to the force characterized in that between one of the two punches and a third punch an etalon for producing a functionally related second measurand is also arranged serially and that means are provided for comparing electrical signals derived from the measurands. 2. Arrangement according to item 1, characterized in that the etalon aus'Quarz consists. 3. Arrangement according to item 2, characterized in that in the electrical connections des.Etalons or the body with the device in each case a capacitor is arranged and that the ratio of the capacitances of the capacitors substantially equal to the ratio of the piezoelectric charge coefficients of etalon and body is. For this 1 page drawing Field of application of the invention The invention relates to an arrangement for evaluating the direct piezoelectric effect of piezoelectric vibration, preferably piezoceramic body for use in force gauges, igniters, etc. In the evaluation arrangement, the piezoelectric body between two punches and with these arranged substantially parallel to an acting force serially, wherein The body generates a functionally dependent on the electric charge and is converted into an electrical signal. Characteristic of the known technical solutions As is known, piezoelectric, in particular piezoceramic, transducers are evaluated for the magnitude of the piezoelectric effect by measuring dynamic quantities, such as the resonant frequency, in accordance with the equivalent electromechanical circuit diagram. Thus, the electromechanical coupling factor can be determined exactly. The measurements are based on the reciprocal piezoelectric effect and allow the calculation, corresponding material coefficients. An immediate assessment of the direct piezoelectric effect is not possible by measuring dynamic quantities. In certain applications, however, it is of paramount importance to determine the charge constant by, for example, by repeated lifting of a weight, piezoelectrically generated charges are absolutely measured by charge integrations. The total generated charge can be calculated as the product of the observed voltage and the sum of the shunt and static capacitance of the piezoelectric body. Apart from the necessity of carrying out calculations, the reproducibility of the loading or unloading of the piezoelectric bodies under routine conditions is not guaranteed. Object of the invention The indicated deficiency should be remedied by the invention and a simple way of assessing the quality of piezoelectric bodies or transducers as charge sources to be created. Explanation of the essence of the invention The invention is therefore based on the object to enable a particularly suitable for routine operation arrangement for evaluating the direct piezoelectric effect of a piezoelectric body or transducer. The application of a Relativmeßverfahrens should result in a simple arrangement that is usable without calculations and reducible for quality control. According to the invention, this object is achieved in that between one of said two punches and another punch an etalon for generating a functionally connected with the force acting on the body quantity is also arranged in series and that means for comparing the charge corresponding electrical Signal is provided with dem'der measured variable corresponding electrical signal. The functional relationship may be the same for the body and the etalon. Advantageously, the etalon is made of quartz, which has a high reproducibility and temperature independence of the piezoelectric values. Finally, it is advantageous for the use of the arrangement according to the invention if in each case a capacitor is arranged in the electrical connections of the etalon or of the body with the device. The ratio of the capacitances of the capacitors is chosen to be substantially equal to that of the piezoelectric charge coefficients of the etalon and of the body. The inherent capacitance of the capacitors is significantly higher than that of the piezoelectric charge sources.