DE3918407C2 - Piezoelectric acceleration sensor, in particular for triggering occupant protection devices in motor vehicles - Google Patents

Description

The invention relates to a piezoelectric Accelerometer, especially for triggering Occupant protection devices in motor vehicles, according to the preamble of claim 1.

Such an acceleration sensor is known from DE 33 34 603 A1. A connector of a piezoelectric accelerometer is connected to an operational amplifier via a resistor, which is fed back via a second resistor, so that the Output signal of the accelerometer is integrated. If the integrated signal reaches a certain threshold value, then triggered the protective device, possibly other circuit parts are provided to additional in the event of an impact of the motor vehicle to consider critical parameters on an obstacle.

The first resistor has the most commonly used piezoelectric accelerometers have a resistance value that typically on the order of one gigaohm to ten gigaohms lies. Since the gain V of the operational amplifier is essentially different from that Ratio of the resistance values of the second and the first resistor depends and - neglected other circuit parts - too

V = 1 + R2 / R1

can be specified, the second resistor R2, d. H. the Integration resistance has an even greater resistance value than the first Have resistor R1 if reasonable gain is desired becomes. However, resistors with such high resistance values are relative  large, so that a compact design of the acceleration sensor is prevented. With today's and rising in the future Safety standards for motor vehicles are however Occupant protection device such as air cushions, belt tensioners etc. with such acceleration sensors used more and more, so that Acceleration sensors are to be regarded as mass articles. A compact design and economical production combined with high Functional reliability should therefore be aimed for.

DE 23 34 788 B2 describes a piezo transducer Amplifier arrangement known in which the output signal of a Piezo crystal an input of an operational amplifier is supplied, the output via a capacitor and a for this purpose switches connected in parallel to the input is fed back. So this operational amplifier acts as Integrator, this circuit arrangement usually as "Miller integrator" is referred to. Such integrators have the advantage of a high input resistance and one great reinforcement. Such an amplifier arrangement is preferably used in connection with reed contacts, whereby the switched feedback essentially as Zeroing device is used.

From the book by U. Tietze and Ch. Schenk, "Semiconductor Circuit Technology ", 5th edition, Springer-Verlag, 1980, pp. 198-199, especially Figure 11.9, is an integrator with a switched feedback and an input switch known, which enables defined initial conditions set and interrupt any integration, where at the same time the output voltage to the value in Switching instant remains. However, with this integrator again in the feedback as well as in the feed line to Input integrator resistors required that are compact Construction in special conditions, as explained above, exclude.

The invention has for its object an acceleration sensor to modify the type in question so that resistors with high Resistance values are avoided and the acceleration sensor as very small unit can be manufactured.

This object is according to the invention by the in the characterizing part of claim 1 specified features solved.

The first resistor is accordingly a circuit of a first Capacitor and a first switch replaced so that the charge of the Accelerometer recorded and clocked to the Operational amplifier is passed. Likewise, the second resistance, d. H. the integration resistance by a Parallel connection from a second capacitor and a second Switch replaced. This second switch is used during the Integration phase kept open and after its end to reset the Operational amplifier closed.

The capacitors used require capacitance values of a few Picofarad, so that the acceleration sensor realized with it is essential can be built smaller than before. Typically match Capacitance values of 10 picofarads Resistance values of about 15 gigaohms with conventional construction with resistors.  

The output signal of the operational amplifier is also preferred clocked with the help of a third switch on a third capacitor managed and saved there for the individual integration phases.

The operational amplifier and the switches are preferably in one Integrated semiconductor chip, on which also the piezoelectric Accelerometer is attached. The entire arrangement Semiconductor chip and accelerometer is preferred on the Base of a small housing, e.g. B. in the form of a conventional Fixed transistor housing. The thickness of the base becomes advantageous chosen much larger than that of the semiconductor chip, once around to increase the shock resistance of the acceleration sensor, on the other hand, to set the resonance frequency of the acceleration sensor very high, so that disturbances from externally induced mechanical vibrations reduced or practically excluded.

Added to this is the fact that it is implemented using a semiconductor chip the acceleration sensor is programmable, e.g. B. from the outside supplied data, and thus on the special application can be optimally adjusted.

The acceleration sensor can be mass-produced with existing manufacturing techniques take place.

Further embodiments of the invention emerge from the subclaims forth.

The invention is closer in exemplary embodiments with reference to the drawing explained. In this represents

Fig. 1 is a cross-sectional view of an acceleration sensor according to the invention;

Fig. 2 shows a section along II-II in Fig. 1;

Fig. 3 is a circuit diagram for an acceleration sensor according to a first embodiment of the invention, and

Fig. 4 is a circuit diagram for an acceleration sensor according to a second embodiment of the invention.

In Figs. 1 and 2, an acceleration sensor 1 is shown, comprising a housing 2 having a base 3 and a cover 4, and a semiconductor chip 5 and a piezoelectric accelerometer. 6 The semiconductor chip 5 is soldered or glued to the base 3 , the accelerometer is also soldered or glued to the top of the semiconductor chip 5 . Four contact pins 7 protrude through the base 3 of the housing 2 , the ends of which protrude into the housing 2 are connected to connecting contacts 9 of the semiconductor chip 5 via connecting wires 8 . The centrally arranged acceleration sensor 5 is on the semiconductor chip 5, ambient, an evaluation circuit 10 is integrated as shown in FIGS. 3 and 4 respectively. The connection contacts of the accelerometer 6 are connected via connection wires 11 to corresponding circuit points 12 and 13 of the evaluation circuit 10 ; see. Fig. 3 and 4.

In FIG. 3, the wiring of the acceleration sensor is shown. A capacitor C is provided in parallel with the acceleration sensor 6 and is defined by the parasitic capacitance of the acceleration sensor and, if appropriate, an additionally provided capacity. The circuit points 12 and 13 , which are connected to the two electrodes of the accelerometer via the connecting wires 11 , form the input points of the evaluation circuit 10 . The circuit point 13 , which is connected to one of the contact pins 7 , is connected to an externally supplied reference voltage U-Ref and is connected to the positive input of an operational amplifier 14 . This receives its supply voltage via a further contact pin 7 .

The connection point 12 is connected to a switch contact 15 of a first switch S1, the second switch contact 16 of which is connected to the negative input of the operational amplifier 14 . The base of the switch S1 is connected to a first capacitor C1, the second contact of which is connected to the reference voltage U-Ref.

The operational amplifier 14 serves as an integrator. Its output is fed back via a second capacitor C2 to the negative input of operational amplifier 14 . A second switch S2 is connected in parallel with this second capacitor C2.

The output of the operational amplifier is led via a third switch S3 to the starting point of the evaluation circuit 10 , at which the output voltage is taken off via one of the contact pins 7 . A contact of a third capacitor C3, whose other contact is connected to ground, is located at the third switch S3 and the starting point of the evaluation circuit.

The evaluation circuit 10 also has a clock generator 17 which delivers switching signals T1, T2 and T3 to the individual switches S1, S2 and S3. The clock 17 can be programmed from the outside via a contact pin 7 . This contactor can also perform additional internal circuit functions. It is of course possible to arrange this clock generator externally, so that its clock signals are routed to the switches via one or more contact pins.

The otherwise necessary series resistor between the piezoelectric accelerometer and the operational amplifier is replaced by the first switch S1 and the capacitor C1, whereas the second switch S2 and the second capacitor C2 replace the otherwise usual integration resistor. The gain of the operational amplifier 14 is determined by the ratio of the capacitance values of the second and the first capacitor.

The function of the circuit described is as follows:  

In a first time interval, the switch S1 is placed on the switch contact 15 with the aid of the clock generator 17 , so that the charge generated in the piezoelectric accelerometer 6 is stored in the first capacitor C1. The second switch C2 is closed, so that the second capacitor C2 is discharged and the operational amplifier 14 is thus reset. The third switch S3 is kept open.

The first switch S1 is then placed on the switch contact 16 ; the second switch S2 is opened and the third switch S3 is kept open. This begins the integration while the first capacitor C1 is being discharged. When this process is complete, the first switch S1 is switched back to the switch contact 15 and the third switch S3 is closed, so that the integration value determined by the operational amplifier 14 is stored by the third capacitor C3 and taken off as the output voltage U.

In a last interval, the third switch S3 is opened again and the second switch S2 is closed, so that the operational amplifier 14 is reset while discharging the second capacitor C2.

This cycle is then repeated accordingly.

The reference voltage U-Ref serves as a compensation voltage with which u. a. offset drifts can also be compensated for.

The circuit shown in FIG. 4 has the same components as that according to FIG. 3, so that a detailed description is unnecessary. Only a fourth switch S4 and a fourth capacitor C4 are added, the switching point 12 being connected to the switching contact 15 of the first switch. The fourth switch S4 switches the fourth capacitor C4 in parallel to the accelerometer 6 in its first switching position, while the capacitor C4 is discharged in the second switching position. The fourth switch S4 and the fourth capacitor C4 simulate a resistor parallel to the acceleration sensor 6 , with which the lower limit frequency of the entire circuit can otherwise be determined. This limit frequency can be set by a certain switching frequency of the switch S4. The switch S4 can be actuated independently of the first switch S1 by clock signals T4 from the clock generator 17 . A synchronous switching with the first switch is also possible. The capacitance value of the fourth capacitor C4 is typically a few femtofarads.

Claims (10)

1.Piezoelectric acceleration sensor, in particular for triggering occupant protection devices in motor vehicles, with a piezoelectric acceleration sensor and an evaluation circuit having at least one integrator, the acceleration sensor being connected via a resistor to the input of the integrator, characterized in that the resistor is connected to the acceleration sensor ( 6 ) first capacitor (C1) connected in parallel and a first switch (S1) is formed, with which the capacitor (C1) can be connected to the accelerometer ( 6 ) or the integrator ( 14 ), that the integrator is an operational amplifier ( 14 ), the output of which is fed back to the input via a parallel circuit comprising a second capacitor (C2) and a second switch (S2), and that a clock circuit ( 17 ) is provided which mutually connects the first and second switches (S1, S2) so controls that then we nn the first switch is connected to the accelerometer ( 6 ), the second switch is closed, and when the first switch is connected to the integrator, the second switch is open. 2. Acceleration sensor according to claim 1, characterized in that the one terminal ( 13 ) of the accelerometer ( 6 ) and the associated terminal of the first capacitor (C1) are connected to a first input (positive input) of the operational amplifier ( 14 ) that the other connection of the accelerometer ( 6 ) is connected to a first switch contact ( 15 ) of the first switch (S1), that a second switch contact ( 16 ) of the first switch (S1) is connected to the other input (negative input) of the operational amplifier ( 14 ) is connected, and that the feedback takes place via the parallel connection of the second capacitor (C2) and the second switch (S2) to the negative input of the operational amplifier ( 14 ). 3. Acceleration sensor according to claim 1 or 2, characterized in that the output of the operational amplifier ( 14 ) is connected to a third switch (S3), the output terminal of which forms the output (U) of the evaluation circuit ( 10 ), and in parallel with this output a third capacitor (C3) is connected. 4. Acceleration sensor according to claim 3, characterized in that the one clock circuit ( 17 ) for the three switches (S1, S2, S3) actuates them in successive time intervals as follows:

• a) connecting the first capacitor (C1) via the first switch (S1) to the accelerometer ( 6 ) with the second and third switches (S2 and S3) closed;
• b) connecting the first capacitor (C1) via the first switch (S1) to the operational amplifier ( 14 ) and opening the second switch (S2) and keeping the third switch (S3) open;
• c) connecting the first capacitor (C1) via the first switch to the accelerometer ( 6 ) and closing the third switch (S3) and keeping the second switch (S2) open;
• d) opening the third switch (S3) and closing the second switch (S2),

after which steps b) to d) are repeated cyclically. 5. Acceleration sensor according to one of the preceding claims, characterized in that a fourth capacitor (C4) and a fourth switch (S4) are provided between the acceleration sensor ( 6 ) and the first capacitor (C1), the fourth switch (S4) being clocked the fourth capacitor (C4) in parallel with the accelerometer ( 6 ) or short-circuits the fourth capacitor (C4). 6. Acceleration sensor according to claim 5, characterized in that the fourth switch (S4) is independent of the first switch (S1) can be actuated. 7. Acceleration sensor according to claim 5, characterized in that the fourth switch (S4), when the first switch (S1) connects the first capacitor (C1) to the accelerometer ( 6 ), the fourth capacitor (C4) in parallel with the accelerometer ( 6 ) switches and then, when the first switch (S1) connects the first capacitor (C1) to the integrator ( 14 ), short-circuits the fourth capacitor (C4). 8. Acceleration sensor according to one of the preceding claims, characterized in that the evaluation circuit ( 10 ) is integrated in a semiconductor chip ( 5 ), that the acceleration sensor ( 6 ) on the surface of this chip is mechanically connected to this, and that the arrangement of semiconductor chip ( 5 ) and accelerometer ( 6 ) on an external connection contacts ( 7 ) having a base ( 3 ) of a housing ( 2 ) which has a cap ( 4 ) covering the semiconductor chip ( 5 ) and the accelerometer ( 6 ). 9. Acceleration sensor according to claim 8, characterized in that the base ( 3 ) has a thickness which exceeds that of the semiconductor chip ( 5 ).