A METHOD AND A SYSTEM FOR TESTING CAPACITIVE ACOUSTIC TRANSDUCERS

Abstract: A method and a system for testing one or more transducers, preferably acoustic transducers, from a central control unit. The transducers are individually connected to the inlet of a preamplifier (4) having a relatively high input resistance (6). A test conduit extends from the central control unit to each transducer. The test conduit communicates with the joint between the transducer and the inlet of the preamplifier (4) through a relatively small capacity (10). In this manner a method is obtained for testing the transducer, said testing being highly sensitive to the condition of the transducer and relatively insensitive to external fields.

Description: Title: A Method and a System for Testing Capacitive. Acoustic Transducers Technical Field The invention relates to a method of testing one or more capacitive transducers, preferably acoustic transducers, from a central control unit, where the transducers are individually connected to an inlet of a preamplifier having a relatively high input resistance, and where a .test conduit furthermore extends from the central con- trol unit to the transducers, the testing of each trans¬ ducer being performed by means of a test signal trans¬ mitted through the test conduit and a capacitor inserted in said test conduit to the joint between the transducer and the inlet of the preamplifier. Background Art It is known to test one or more capacitive measuring microphones associated with preamplifiers and connecting wires, where each microphone is connected to the inlet of a preamplifier having a relatively high input re- sistance. The microphones may be placed far away from a central control unit performing the testing optinally automatically. A test signal is transmitted through an individual test conduit from the central control unit to each microphone. By this known method, the test conduit is connected to the chassis terminal of the microphone. The test signal is transmitted through the capacity of the microphone. Mo rn preamplifiers ensure that the response to the tes ignal is almost independent of the capacity of the mi._ophone, and accordingly it is not possible to detect changes therein although said capa¬ city has become extremely high or optionally shortcir- cuits . Elder preamplifiers with a relatively high input capaci- ty provide a slight sensitivity to changes in the capa¬ city of the transducer. This sensitivity is, however, very small, typically of a magnitude smaller than the one achievable by the method according to the invention. In addition, external electromagnetic fields presents a technical problem, especially in connection with large measuring systems. The measuring system associated with the known central control unit is sensitive to external fields due to the separate chassis terminals of the mi— crophone and the preamplifier. US—PS No. 4,648,078 discloses a system for testing a plurality of acoustic transducers, a test signal being transmitted through the capacity between the screen and the inner conductor of a screened cable to the joint be- tween the individual transducer and an amplifier. This system is, however, only able to indicate whether the amplifier and the cable to a central control unit is in order. The system is unable to indicate whether the ca¬ pacity of the transducer has changed. Description of the Invention The object of the invention is to provide a method of the above type, which is highly sensitive to the condi¬ tion of each transducer simultaneously with being rela¬ tively insensitive to external electromagnetic fields. A method of the above type is according to the invention characterised by a relatively small capacity with a very high equivalent parallel resistance (leakage resistance) being inserted in the test conduit, and by the frequency characteristics measured by means of the test conduit being compared with previously determined character¬ istics so as to indicate errors, if any, in the transdu¬ cer. The high equivalent parallel resistance (the leak¬ age resistance) of the capacitor renders it possible to perform a testing even at the lowest operational fre- quencies in such a manner that it is possible to find the frequency characteristics and thereby to indicate errors, if any, in the transducer. Moreover according to the invention the equivalent pa— rallel resistance (leakage resistance) of the capacitor in the test conduit may be about 10 7 M Ω. The capacitor in the test conduit may for instance be provided by means of a substrate with a soldering layer -on one side and a conducting path on the opposite side. The resulting capacitor is provided with a very high parallel resistance (parasite resistance) . As a result it is also possible to perform tests at the lowest oper¬ ational frequencies. The invention relates furthermore to a system for car- rying out the method of testing one or more transducers, preferably capacitive transducers, from a central con¬ trol unit, where the transducers are individually con¬ nected to an inlet of a preamplifier having a relatively high input resistance, and where a test conduit from the central control unit is connected to each of the trans¬ ducers. The system is characterised by the test conduit being connected to the joint between the transducer and the inlet of the preamplifier through a relatively small capacitor with a very high equivalent parallel resist- ance (leakage resistance) , whereby a particularly advan¬ tageous system is obtained for the control of the trans¬ ducers . In addition according to the invention, the test conduit may communicate with a change-over switch in the central control unit, said change-over switch being connected either to a chassis or to a test AC voltage. Moreover according to the invention, the capacitor in the test conduit may be an adjustable screw in a cap— sule, said adjustable screw being electrically insulated from the remaining portion of the capsule. Finally according to the invention, the capacitor in the test conduit may be an adjustable screw in an adapter in form of a separate unit between the capsule and the pre¬ amplifier. Such a unit can be delivered separately to¬ gether with the necessary supply lines. Brief Description of the Drawings The invention is described in greater detail below with reference to the accompanying drawings , in which Fig. 1 illustrates a known capacitor microphone with an associated preamplifier circuit, Fig. 2 illustrates a corresponding equivalent circuit, Fig. 3 illustrates a capacitor microphone circuit with a test conduit according to the invention, Fig. 4 illustrates a corresponding equivalent circuit, Fig. 5 illustrates a portion of the test conduit accord¬ ing to the invention, Fig. 6 illustrates an adapter insertable between the preamplifier and the capsule and in which an adjustable metal screw is provided, said screw being electrically insulated from the remaining portion of the capsule, and Fig. 7 illustrates the entire microphone unit. Best Mode for Carrying Out the Invention The invention relates to a method of testing transdu¬ cers, such as microphones, which can be arranged far from a central control unit. At airports, a demand exists for instance for a possibility of continuously measuring and registering the aircraft noise. The most noisy aircrafts are sometimes penalized by fewer landing permits or optionally fined. Accordingly, it is neces¬ sary to use a comprehensive measuring system involving a high number of microphone units for the registration of the noise. In order to ensure that all the microphones of such a measuring system are in order, it is necessary at regular intervals to test said microphones. The reli¬ ability of the accuracy of the measuring system is of decisive importance because an exceeding of the allowed noise limits can have very serious consequences. Fig. 1 illustrates a known microphone unit comprising a capacitor microphone 2 and a preamplifier 4. The value of the capacitor 5 corresponds to the input capacity of the preamplifier 4 plus the capacitor of the supply line. The input resistance 6 corresponds to the input resistance of the preamplifier 4. The capacitor 5 and the resistance 6 are typically of a value of 0.2 pF and 5 • 10 10 Ω. The capacity of the microphone 2 is approxi¬ mately 20 pF. A test conduit 8 is connected to one input terminal of the microphone 2, whereby a. test signal of for instance 100 mV can be transmitted at one or more discrete frequencies through the test conduit 8 and a change—over switch 1 in the central control unit. Fig. 2 illustrates the corresponding equivalent circuit in which the change-over switch 1 is arranged in the position shown in Fig. 1. It appears, that a shortcir- cuiting of the capacitor microphone 2 (C m - OO ) results in nothing but a minor change of the output signal V out of the preamplifier 4 unlike the system according to the invention. The system according to the invention is so sensitive that even minor changes of 1% can be detected. Minor changes can for instance be caused by the parts of the microphone 2 being warped relative to one another, or by the membrane in the microphone being slackened to such a degree that it is pulled towards a backplate electrode of an electrical polarisation voltage between the membrane and the backplate electrode. The change caused by a shortcircuiting (C m - ∞ ) in the known syst¬ em is so insignificant that it can hardly be measured, cf. the formula Vou./ in - [C m /(C m +C i )]A where C m is the capacity of the microphone, C j is the input capacity of the preamplifier, and A is the amplification of the preamplifier. In connection with tube amplifiers, the input capacity is typically 3 pF, whereas the amplification is typical¬ ly 0.95. A typical 1/2" microphone of 20 pF results in the following voltage ratios. V ou./ v in - 20/(20+3) -0.95 - 0.83 A broken membrane or a drop of water in the microphone can cause substantial measuring errors by shortcircuit¬ ing the capacity of the microphone. Such serious errors can be detected by the central unit because the voltage ratio changes from 0.83 to 0.95. The system is, however, rather insensitive to changes in the capacity and insig— nificant changes cannot be detected. The system according to the invention involves such a proportional relationship that a change in capacity of for instance 5% results in a change in the output signal V out of 5%. The known capacitor microphone requires an insulation between the capsule and the remaining portion of the microphone including the preamplifier 4 as these por¬ tions must be mutually insulated during the transmission of the test signal. The change-over switch 1 in the cen¬ tral control unit connects the capacitor microphone 2 either to a test voltage of about 100 mV or to the chas¬ sis. The connecting to the chassis through the change¬ over switch 1 in the central control unit is thereby performed through a relatively long conducting path im— -plying that irrelevant signals can be picked up. The test conduit 8' includes a relatively small capaci¬ tor 10 connected to the joint between the capacitor mi¬ crophone 2 and the input of the preamplifier 4. The ca- pacitor 10 is of a magnitude of about 0.1 pF, i.e. of the same magnitude as the capacity 5. As a result a highly significant change is obtained in the output sig¬ nal V out of tie preamplifier 4 in case the capacitor mi¬ crophone 2 shortcircuits , and even small changes in the capacity of about 1% can be detected. The parallel re¬ sistance (the leakage resistance) of the capacitor 10 must, however, be of the magnitude 10 7 M Ω in order to allow the leakage current to be about 1-00 times smaller than the capacity current of the capacitor 10, as an unacceptable frequency nonlinearity is otherwise found in the lower portion of the frequency range. As a conse¬ quence of the latter, a parallel resistance (leakage resistance) must apply which is of the magnitude 10 7 M Ω in order to test a typical capacitor microphone system at a low cut-off frequency of for instance 20 Hz. Such a small capacitor 10 with a high leakage resistance is according to the invention provided by means of a substrate 14, cf. Fig. 5, provided with a soldering lay¬ er 12 on one s " =≥ and a conducting path (not shown) on the opposite side. The leakage currents are thereby very small with the result that the capacity is very low and can be trimmed by cutting off portions of the soldering layer 12 by means of a laser beam or other abrasive means. The substrate can for instance be formed by the circuit card provided with the preamplifier 4. As a result, a defect capacitor microphone 2 causes a remarkable change in the output signal from the pre¬ amplifier 4 when a test signal of the magnitude 10 V is transmitted to the test conduit 8' . The output voltage is V ou t " V in • C c /(C m + Cj + C c ) - A or V ou t " in/(C m + Ci + C c ) • A or V ou. " in/( C m + Cj) - A as C c « C m + Ci where C c is the capacity, through which the test signal is transmitted, A is the amplification, and Qi n is the input charge . When it is assumed that V in - 10 V ; C c - 0 . 1 pF ; C m - 20 pF ; C j - 0 . 2 pF ; and A - 0 . 995 , the result is V out /V in - °-^ • 0 . 995 - 4. 9 • 10 "3 corresponding 20 + 0.2 + 0.1 to V out - 49 mV at a test signal of 10 V. If C m is changed to 00 corresponding to a shortcir- cuiting, V out is almost o. A testing at several different frequencies can reveal whether the frequency charac¬ teristic has been changed relative to earlier measured characteristics, and the source of error can be evaluat— ed. It is, however, very important that the leakage re¬ sistance is very high, in practise of the magnitude 10 7 M Ω. The stability of the capacity must typically be so good that neither the temperature nor the time results in changes exceeding 1%. The capacity is typically 100 times smaller than the capacity of the microphone, i.e. 0.1 pF or lower. The relatively small capacitor in the test conduit 8' can alternatively be provided in form of an adjustable screw 16 in the microphone capsule 20 or in an adapter 19 between the capsule 20 and the preamplifier, said screw 16 being electrically insulated from the remaining portion of the capsule or the adapter. The adjustable screw 16 possesses a low capacity relative to a perma¬ nently supported conduit 17 providing the connection between the capacitor microphone and the preamplifier, cf. Fig. 6. The test conduit 8' is connected to the screw 16. The only important feature is that the capaci¬ ty is very stable. The exact value of the capacity need not necessarily be known provided it is stable. Fig. 7 illustrates the entire microphone unit comprising the encapsulated preamplifier 18 , the adapter 19 , and the microphone capsule 20. Based on the frequency characteristics of the ratio of the output voltage V out to the input voltage V jn it is furthermore possible to detect errors, if any, and to decide whether the capacitor microphone 2 should be re¬ placed. The latter is very advantageous in practise in connection with a large number of microphones far away from a central control unit. The method according to the invention can also be used in connection with other capacitive transducers, such as piezoelectric hydrophones and accelerometers , and it can for instance be used for controlling whether an acce- lerometer has been correctly mounted. The system according to the invention is furthermore rather insensitive to external electromagnetic fields, such as radar fields adjacent an airport, because the conductor path from the chassis terminal of the mi- crophone to the chassis terminal of the preamplifier is very short. In other words, a very efficient chassis connection is obtained. Other sources of errors beyond water drops in the mi¬ crophone and shortcircuiting of the capacitor microphone " are for instance an incorrect electrostatic charge. An incorrect electrostatic charge may apply to the membrane of the capacitor microphone. An incorrect charge results in a changed capacity of the capacitor microphone. Even small errors in the microphone or small changes in the tightening or tensioning of the membrane can be detected by means of the system according to the invention. The important factor is that the measuring accuracy is known, and furthermore that said accuracy does not change gradually. Only then it is possible to determine the fines relating to an exceeding of the allowed noise level. The conductor from the transducer and each preamplifier to the central unit are also tested.

Claims: Claims . 1. A method of testing one or more capacitive transdu¬ cers, preferably acoustic transducers, from a central control unit, where the transducers are individually connected to an inlet of a preamplifier (4) having a relatively high imput resistance (6), and where a test conduit furthermore extends from the central control unit to the transducers, the testing of each transducer being performed by means of a test signal transmitted 'through the test conduit (8') and a capacitor inserted in said test conduit to the joint between the transducer and the inlet of the preamplifier (4), c h a r a c ¬ t e r i s e d by a relatively small capacity with a very high equivalent parallel resistance (leakage re- sistance) being inserted in the test conduit (8') , and by the frequency characteristics measured by means of the test conduit (8') being compared with previously determined characteristics so as to indicate errors, if any, in the transducer. 2. A method as claimed in claim 1, c h a r a c t e r ¬ i s e d by the test conduit (8') in the central control unit communicating with a change—over switch (1) con¬ nected to either a chassis or a test AC voltage. 3. A method as claimed in claim 1 or 2 , c h a r a c - t e r i s e d by the capacitor (10) in the test conduit (8') having a value of approximately 0.1 pF. 4. A method as claimed in claim 3, c h a r a c t e r ¬ i s e d by the equivalent parallel resistance (leakage resistance) of the capacitor (10) in the test conduit (8') being about 10 7 M Ω. 5. A method as claimed in claim 3 or 4, c h a r a c ¬ t e r i s e d by the capacitor (10) in the test conduit (8') being provided by means of a substrate (14) with a soldering layer (12) on one side and a conducting path on the opposite side (Fig. 5). 6. A method as claimed in claim 5, c h a r a c t e r ¬ i s e d by the soldering layer (12) being trimmed by means of a laser beam or other abrasive means until the desired capacity of the capacitor (10) has been ob¬ tained. 7. A system for carrying out the method as claimed in 'one or more of the preceding claims 1 to 6 of testing one or more transducers, preferably capacitive trans¬ ducers, from a central control unit, where the transdu¬ cers are individually connected to an inlet of a pre¬ amplifier (4) having a relatively high imput resistance (6) , and where a test conduit from the central control unit is connected to each of the transducers, c h a r ¬ a c t e r i s e d by the test conduit (8') being con¬ nected to the joint between the transducer and the inlet of the preamplifier (4) through a relatively small capa¬ citor (10) with a very high equivalent parallel re— sistance (leakage resistance) . 8. A system as claimed in claim 7, c h a r a c t e r ¬ i s e d by the test conduit (8') communicating with a change—over switch (1) in the central control unit, said change—over switch being connected either to a chassis or to a test AC voltage. 9. A system as claimed in claim 7 or 8, c h a r a c ¬ t e r i s e d by the capacitor (10) in the test conduit (8') having a value of approximately 0.1 pF. 10. A system as claimed in claims 7 to 9, c h a r a c — t e r i s e d by the equivalent parallel resistance of the capacitor (10) being of the magnitude 10 7 M Ω. 11. A system as claimed in claims 7 to 10, c h a r a c— t e r i s e d by the capacitor in the test conduit (8') being an adjustable screw in a capsule (20), said ad¬ justable screw being electrically insulated from the remaining portion of the capsule. 12. A system as claimed in claims 7 to 10, c h a r c¬ t e r i s e d by the capacitor in the test conduit (8') being an adjustable screw (16) in an adapter (19) in form of a separate unit between the capsule (20) and the preamplifier (18). AMENDED CLAIMS [received by the International Bureau on 8 September 1992 (08.09.92); original claims 2 and 8 cancelled; original claims 1 and 6 amended; other claims unchanged but renumbered accordingly (3 pages)] 1. A method of testing one or more capacitive transdu¬ cers, preferably acoustic transducers, from a central control unit, where each transducer is connected to an inlet of a preamplifier (4) having a relatively high input resistance (6), and where a test conduit further¬ more extends from the central control unit to the trans¬ ducer, the testing of each transducer being performed by means of a test signal transmitted through the test con- duit (8') and a relatively small capacitor inserted in said test conduit to the joint between the transducer and the inlet of the preamplifier (4) , c h a r a c ¬ t e r i s e d by a relatively small capacity inserted in said test conduit 8' having a very high equivalent parallel resistance (leakage resistance) , and by the frequency characteristics being measured by means of the test conduit (8 f ) and being compared with previously determined characteristics so as to indicate errors, if any, in the transducer. 2. A method as claimed in claim 1, c h a r a c t e r ¬ i s e d by the capacitor (10) in the test conduit (8') having a value of approximately 0.1 pF. 3. A method as claimed in claim 2, c h a r a c t e r ¬ i s e d by the equivalent parallel resistance (leakage resistance) of the capacitor (10) in the test conduit (8') being about 10 7 M Ω. 4. A method as claimed in claim 2 or 3, c h a r a c ¬ t e r i s e d by the capacitor (10) in the test conduit (8') being provided by means of a substrate (14) with a soldering layer (12) on one side and a conducting path on the opposite side (Fig. 5). 5. A method as claimed in claim 4, c h a r a c t e r ¬ i s e d by the soldering layer (12) being trimmed by means of a laser beam or other abrasive means until the desired capacity of the capacitor (10) has been ob¬ tained. 6. A system for carrying out the method as claimed in one or more of the preceding claims 1 to 5 of testing one or more transducers, preferably capacitive trans¬ ducers, from a central control unit, where the transdu¬ cers are individually connected to an inlet of a pre¬ amplifier (4) having a relatively high input resistance .(6) , and where a test conduit from the central control unit is connected to each of the transducers , wherein the test conduit (8') is connected to the joint between the transducer and the inlet of the preamplifier (4) through a relatively small capacitor (10), c h a r a c - t e r i s e d by the relatively small capacitor in the test conduit 8' having a very high equivalent parallel resistance (leakage resistance). 7. A system as claimed in claim 6, c h a r a c t e r ¬ i s d by the capacitor (10) in the test conduit (8') having a value of approximately 0.1 pF. 8. A system as claimed in claim claim 6 or 7 , c h a r a c t e r i s e d by the equivalent parallel resistance of the capacitor (10) being of the magnitude 10 7 M Ω. 9. A system as claimed in claims 6 to 8, c h a r a c¬ t e r i s e d by the capacitor in the test conduit (8') being an adjustable screw in a capsule (20) , said ad¬ justable screw b ing electrically insulated from t e remaining portion of the capsule. 10. A system as claimed in claims 6 to 8, c h a r a c¬ t e r i s e d by the capacitor in the test conduit (8') being an adjustable screw (16) in an adapter (19) in form of a separate unit between the capsule (20) and the preamplifier (18) .

Inventors: FREDERIKSEN ERLING, FREDERIKSEN, ERLING
Applicants: BRUEEL & KJAER AS, A/S BRUEEL & KJAER
Application year: 1992
Main classification: H04R29
IPC codes: G01R27, G01R31, H04R19