WO1997023005A1

WO1997023005A1 - Resonant coupled transducer drive

Info

Publication number: WO1997023005A1
Application number: PCT/US1996/019426
Authority: WO
Inventors: Carl Rutschow; Richard A. Arseneau; Russell C. Watts
Original assignee: Robert Bosch Corporation
Priority date: 1995-12-04
Filing date: 1996-12-04
Publication date: 1997-06-26
Also published as: AU1281697A

Abstract

A resonant acoustic coupling circuit (10) having a tank circuit (34) with a capacitor (22) and an inductor (24) in operational conjunction with a transducer capacitance (32), the transducer capacitance (32) being the inherent capacitance of a transducer (28). A first switch (16) introduces energy to begin oscillation, a second switch (18) tickles the tank circuit (34) to maintain oscillation. A third switch puts the tank circuit (34) in a hold mode for lengthening the period of the tank circuit (34) for matching the electrical resonant frequency of the tank circuit (34) to the mechanical resonant frequency of the transducer (28), and further for stopping oscillation such that a high static voltage remains on the transducer (28) such that the transducer (28) is in a receive mode.

Description

RESONANT COUPLED TRANSDUCER DRIVE

TECHNICAL FIELD

The present invention relates generally to the field of electronic circuitry, and more particularly to an improved method for providing an increased drive voltage to an acoustic transducer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S.

Application No. 08/58709 filed on 4 December, 1995, and having the same title as this present invention. This application is also related in by subject matter to issued U.S. Patent No. 5,482,314 entitled AUTOMOTIVE OCCUPANT SENSOR SYSTEM AND

METHOD OF OPERATION BY SENSOR FUSION which was issued to

Corrado et al. on 9 January, 1996, and to the PCT

application PCT/US 95/04780 claiming priority thereof, in that the present application describes and claims a component suitable for use in the system described and claimed therein. Finally, the present application is related by subject matter to Provisional U.S. Patent Applications BLOWER CUT OFF SYSTEM FOR KOZI Serial No. (to be filed) and KEEP OUT ZONE INCURSION FAST SENSING MADE FOR AIRBAG DEPLOYMENT SYSTEMS Serial No.

60/028, 844.

BACKGROUND ART

In the past, methods for converting electrical power from one voltage level to another generally all involved the use of a transformer. Indeed, in the early days of electronics, transformers provided the only viable alternative for this purpose, so much so that fairly extreme measures were

sometimes taken to enable the use of a transformer. For example, costly and unreliable multivibrators were provided so that lower direct current ("DC") voltages, as from an

automobile battery, could be "stepped up" through a

transformer to provide the higher voltage levels necessary to operate early tube type radios.

However, as advancements in electronics have occurred, it has become increasingly important to recognize that

transformers are generally quite bulky, heavy, and expensive components as compared to most other electronic component devices Furthermore, transformers are generally quite inefficient, and thus produce even more bulk in a finished appliance, given that space for heat dissipation must be provided Therefore, while there remains, and probably will continue to remain, a place for the transformer m specific electronic design applications, it is accepted that avoidance of the use of transformers is frequently a superior design alternative in the creation of modern sophisticated electronic devices.

Probably most notable among the 'transformer avoidance' (or, at least, 'transformer minimization') ideas has been the switching power supply and variations thereof. Switching power supplies have become the default choice for providing DC power for most new electronic appliance designed to be powered from alternating 'house' current. Similarly, 'flying

capacitor' circuits such as voltage doublers and voltage multipliers are today frequently employed for stepping up DC voltages

While alternatives have been developed to eliminate transformers in various coupling circuits, these have suffered from problems such as lack of linearity and, therefore, transformers remain the designers' first choice in many such applications. Means for electrically resonant coupling an input transducer to an amplifier circuit have existed for some limited applications (such as the 'pick-ups of electric guitars) Also, means for acoustically and/or mechanically resonant coupling the output of transducers are known (such as tuned speaker enclosures).

A specific problem is posed by those transducers which require a high alternating current ("AC") drive voltage riding on a direct current ("DC") bias voltage. For example some transducers, such as those used for ultrasonic acoustic ranging, require a high voltage sine wave drive when used as transmitters. A DC bias must also be provided such that the, typically approximately 200 volts peak-to-peak, signal does not drop below zero volts. When such a transducer is used as a receiver, it is still required to provide the high DC bias voltage in order to achieve optimal sensitivity. To the inventor's knowledge all prior means for providing this sort of signal/bias combination has involved the use of a

transformer to develop the required high AC voltage, which high AC voltage is then rectified (as with a diode rectifier and capacitor filter) to separately generate the required DC bias.

Prior to the present invention, it has been necessary to provide some form of transformer coupling to generate the required high voltage signals, and also to provide a diode and capacitor (or even more elaborate rectifying and filtering means) to produce the required DC offset voltage. To the inventor's knowledge, no prior art method has existed for coupling a transducer requiring both high level signal and bias voltages, which method did not include the use of a transformer. All methods for coupling an acoustic transducer to a signal source either used a transformer and/or did not provide both a high level drive signal and an appropriate DC bias.

DISCLOSURE OF INVENTION

Accordingly, it is an object of the present invention to provide a means for increasing the voltage from a signal source to be applied to a transducer.

It is another object of the present invention to provide a signal coupling means which is light in weight and small in physical dimension.

It is still another object of the present invention to provide a signal coupling means which will provide a high voltage alternating drive signal to a transducer, along with a DC bias voltage.

It is yet another object of the present invention to provide a signal coupling means which is inexpensive and easy to produce in large quantities.

It is still another object of the present invention to provide a signal coupling means which is rugged and reliable in operation.

It is yet another object of the present invention to provide a signal coupling means for an acoustic transducer which will assist in stabilizing the output from the acoustic transducer.

It is still another object of the present invention to provide a high voltage DC bias source which does not require the use of a transformer.

Briefly, the preferred embodiment of the present

invention is an acoustic transducer driver circuit which, using the inherent resonance in the transducer along with an inductor and an additional capacitor as a resonant circuit provides a high voltage alternating, generally sine wave shaped, signal to the transducer such that the signal, once initiated, does not drop below zero volts reference. The cycle is periodically disturbed, to put the transducer in a receive mode, generally at the signal peak such that a high static voltage remains on the transducer during the receive mode. A tickle switch is provided for maintaining energy in the resonant circuit during operation thereof. The resonant circuit is electrically tuned to the frequency of the signal being applied to the transducer such that reflected voltages from the capacitor are effectively added to initial signal voltages.

An advantage of the present invention is that there are no bulky transformer connections such as are prone to loosen or break when subjected to repeated vibration or other stress. A further advantage of the present invention is that it is efficient, and thus does not give off a substantial amount of heat nor use a significant amount of power to perform its function.

Still another advantage of the present invention is that it is small and light weight.

Yet another advantage of the present invention is that it is inexpensive to manufacture.

Still another advantage of the present invention is that it is reliable in operation.

Yet another advantage of the present invention is that DC bias voltages and AC drive signals are coupled to a transducer using a minimum of components.

Still another advantage of the present invention is that a coupling circuit for driving a transducer requiring both signal and bias voltages can be constructed into an integrated circuit package.

Yet another advantage of the present invention is that the coupling circuit can be electrically tuned, thereby avoiding the need for trimming inductor and capacitor values.

These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as

illustrated in the several figures of the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is basic schematic of an acoustic resonant

coupling circuit according to the present invention;

Fig. 2 is a timing diagram illustrative of the functioning of the acoustic resonant coupling circuit of Fig. 1; and

Fig. 3 is a schematic of an equally preferred alternate embodiment of an acoustic resonant coupling circuit according to the present invention. BEST MODE FOR CARRYING OUT INVENTION

The best presently known mode for carrying out the invention is a resonant acoustic transducer coupling circuit. The predominant expected usage of the inventive resonant acoustic transducer coupling circuit is in the production of sensor modules using an acoustic signal, where size, weight, cost and accuracy of signal are each important factors.

An embodiment of the inventive resonant acoustic

transducer coupling circuit is shown in schematic form in Fig. 1, and is designated therein by the general reference

character 10. The resonant acoustic transducer coupling circuit 10 has a first diode 12, a second diode 14, a first switch 16, a second switch 18, a third switch 20, a capacitor 22, an inductor 24, and a DC supply voltage 26 having a value V⁺. In the best presently known embodiment 10 of the present invention V⁺=12 VDC . The inventive resonant acoustic transducer coupling circuit is depicted connected to a

transducer 28. A ground potential 30 is indicated, where present, in the generalized schematic of Fig. 1. A transducer capacitance 32 is indicated in the schematic of Fig. 1. One skilled in the art will recognize that the transducer

capacitance 32 does not represent a distinct physical

component of the resonant acoustic transducer coupling circuit 10. Rather, the transducer capacitance 32 is the inherent capacitance of the transducer 28. The inductor 24, the capacitor 22 and the transducer capacitance 32 form a tank series resonant circuit ("tank circuit") 34.

In the best presently known embodiment 10 of the present invention, the switches 16, 18 and 20 are MOS insulated gate field effect ("IGFET") transistors. Biasing and timing circuit portions for providing operating power to, and for turning off and on the switches 16, 18 and 20, are entirely conventional and are eliminated from the simplified view of

Fig. 1 for the sake of clarity. The transducer 28 is a

Polaroid 9000 Series'" Environmental Transducer having a transducer capacitance 32 value of 600 pF. Therefore, the value of the capacitor 22 is chosen to also be 600 ρ F . The value of the inductor 24 is 35 mH.

According to the best presently known embodiment 10 of the present invention, the first capacitor 22 is selected to be approximately equal in value to the transducer capacitance 32 of the transducer 28. The inductor 24 is chosen such that the tank series resonant frequency of the inductor 25, the capacitor 22 and the transducer capacitance 32 is slightly above the mechanical resonant frequency of the transducer 28.

Fig. 2 is a timing diagram illustrating a typical operating timing pattern of the resonant acoustic transducer coupling circuit 10 of Fig. 1 depicting a resultant signal 35 as seen at the transducer 28. As is illustrated in the timing diagram of Fig. 2 , transmission from the transducer 28 is initiated at an initiate operation time 36 when the first switch '"S1") 16 is closed, the second switch ("S2") 18 and the third switch ("S3") 20 being open at the initiate

operation time juncture. Closing the first switch 16 causes current to flow in the coil 25, this current having a terminal value (I_term) of I_term=V⁺ ( t/L) , where t is the time the first switch 16 is closed and L is the inductance value of the inductor 24. When the desired current level (I_term) is attained through the inductor 24, the first switch 16 is opened at a begin cycling time 38. At this point in time, energy stored in tne inductor 24 is described by the term ½LI_term ² . The stored energy in the inductor 24 then begins to resonantly charge the transducer capacitance 32 to a voltage dependant upon the amount of energy then stored in the

inductor 24 according to the relationship ½LI _term ² = (½) CV_{C (peak)} ² , which may be rearranged as:

Where C is the capacitance value of the transducer capacitance 32 and V_C(peak) is the peak voltage developed across the

transducer capacitance 32.

When the voltage V_c across the transducer capacitance 32 reaches its resonant peak at a first peak time 40, the current through the inductor 24 becomes zero and then reverses as the transducer capacitance 32 begins discharging. The first diode 12 blocks the reverse current path of the current from the inductor 24, such that the reverse current from the inductor 24 charges the capacitor 22. This resonant charging continues until the voltage on the transducer capacitance 32 is near zero and the voltage on the capacitor 22 has reached its peak. Assuming that the capacitance value of the transducer

capacitance 32 generally equals that of the capacitor 22, the peak voltage on the capacitor 22 will then be approximately V_c+V⁺ (given that the transducer capacitance 32 was

previously charged to V⁺. As can be appreciated in light of the above discussion, given the above described resonant action, the voltage across the transducer 28 will thereby oscillate between V_c and approximately V⁺ without ever going below zero volts, which is the desired condition as discussed previously herein. In the example of the best presently known embodiment 10 of the present invention, V_c is approximately 200 volts.

In order to provide energy to the tank circuit 34 to replace that lost due to parasitic tank and transducer

resistance and radiation energy loss, and thereby to maintain the drive signal amplitude, the second switch 18 acts as a "tickle" switch. To inject energy into the resonating tank circuit 34, the third switch 20 is opened at a prepare for tickle time 42 and the second switch 18 is closed at a start tickle time 44. The third switch 20 remains open and the second switch 18 remains closed during all or part of the half cycle when the capacitor 22 is discharging through the

inductor 24 into the transducer capacitance 32. To end the tickle, the second switch 18 is opened at an end tickle time 46 and, thereafter, the third switch 20 is closed at an allow further cycling time 48.

The time during which the second switch 18 is closed while the third switch 20 is open, both during the the time during which the capacitor 22 is discharging through the inductor 24 into the transducer capacitance 32, is the tickle 50. The tickle 50 raises the voltage on the ground side of the capacitor 22 which has the same effect as would increasing the discharge voltage across the inductor by V⁺ volts. This increases the current in the inductor 24 (and, consequently, the energy stored in the inductor 24) according to the

equation I=V⁺ t₂/L where t₂ is the time that the second switch 18 is closed The time t₂ can be adjusted such that the energy injected into the tank circuit 34 just replaces energy lost through attrition.

In order to place the transducer 28 in the receive mode, the oscillation must be stopped with a high DC voltage left remaining across the transducer capacitance 32. This is accomplished by opening the third switch 20 at any point in that half cycle when the capacitor 22 is discharging into the transducer capacitance 32 through the inductor 24, with both the first switch 16 and the second switch 18 being left open, as is indicated occurring at a start hold time 52. The tank circuit 34 will then finish the half cycle through the second diode 14 until the capacitor 22 is discharged and the

transducer capacitance 32 is at its maximum voltage.

Thereafter, while the switches 16, 18 and 20 all remain open, the second diode 14 blocks the return cycle and the

oscillation stops with the transducer capacitance 32 remaining at its maximum voltage This DC bias on the transducer 28 puts the transducer 28 in its high sensitivity "microphone" mode, any acoustic signal impinging upon the transducer 28 can be detected by a receiver electronics 54. The receiver

electronics 54 is entirely conventional in nature and is not shown in detail herein. The period during which the

oscillation is thus stopped with high voltage across the transducer capacitance 32 is a hold time 56 The hold time 56 can also be used to slightly adjust (lower) the frequency of oscillation of the tank circuit 34 so that the electrical oscillation period matches the mechanical resonant frequency of the transducer 28. This is desirable, since any

transαucer, such as the transducer 28, generally is most efficient when the electrical oscillation frequency is matched to its mechanical resonant frequency. This objective is achieved by selecting a component value for the inductor 24 such that, when used as described and shown herein with the capacitor 22 and the transducer capacitance 32 causes the tank circuit 34 to have an electrical resonant frequency slightly higher than the mechanical resonant frequency of the

transducer 28. The oscillation period of the tank circuit 22 can then be periodically increased, as required, by

momentarily placing the inventive resonant acoustic transducer coupling circuit 10 in the hold mode (as demonstrated by the hold time 56) thereby matching the electrical resonant

frequency of the tank circuit 22 to the mechanical resonant frequency of the transducer 28. In the best presently known embodiment 10 of the present invention, this adjustment is made during each cycle of oscillation of the tank circuit 34, although it is conceivable that the desired result could be obtained by making such adjustment at intervals of less than every cycle.

One skilled in the art will recognize that the above discussed method for matching the electrical resonant

frequency of the tank circuit 34 to the mechanical resonant frequency of the transducer will produce some distortion in the electrical signal provided to the transducer 28. However, the inventor has found that the high Q of the transducer 28 (which characteristic is generally typical of all such

mechanical transducers) sufficiently suppresses such

distortion such that it has no significant effect on the operation of the resonant acoustic transducer coupling

circuit, the transducer 28, or the receiver electronics 54.

There are two primary ways to restart the oscillation of the tank circuit 34. The first is to restart the oscillation as has previously been disclosed herein, by closing the first switch 16. This discharges the transducer capacitance 32 and allows current to build up in the inductor 24. The second preferred method for restarting operation is to reclose the third switch 20. The tank circuit 38 will then commence oscillation upon the energy stored in the transducer

capacitance 32. As previously discussed herein, the second switch 18 can then be used to tickle the tank circuit 34 in order to restore the resonant acoustic transducer coupling circuit 10 to the desired energy level.

The resonant acoustic transducer coupling circuit 10 has been simulated using Simulation Program with Integrated

Circuit Emphasis ("SPICE™"). In the SPICE simulation, an ideal lossless circuit was assumed, this being a not

unreasonable simulation mode given that circuit resistances are relatively negligible given the components previously discussed herein, the intended purposes of the resonant acoustic transducer coupling circuit, and the relatively short operating cycles intended. The timing diagram of Fig. 2 is derived from the SPICE simulation. As can be seen in the view of Fig. 2, an inductor current trace ("I_c") 58 shows that current begins building up in the inductor 24 when the first switch 16 is closed and that reverse current begins to flow in the inductor generally simultaneously with the opening of the first switch 16.

One skilled in the art will recognize that the timing and control of the switches 16, 18 and 20 of the best presently known embodiment 10 of the present invention are accomplished under microprocessor control. Such microprocessor control will in most applications, be derived from a processor (not shown) which is generally available for the operation and control of a larger object detection or location circuit, or the like, of which the resonant acoustic transducer coupling circuit 10 will form a component part thereof.

Fig. 3 is a basic schematic diagram of an equally

preferred alternate embodiment 10a of the present inventive resonant acoustic transducer coupling circuit. One skilled in the art will recognize that the equally preferred alternate embodiment 10a of the invention is essentially a simplified version of the first preferred embodiment 10. The chief advantage of the alternate preferred embodiment 10a as

compared to the first disclosed embodiment 10 is that the alternate preferred embodiment 10a provides comparable performance at a lower cost in that, for example, fewer switches and diodes are required. The second diode 14, the first switch 16, the capacitor 22, the inductor 24, the transducer 28 (and associated transducer capacitance 32), the ground potential 30 and the receiver electronics 54 are retained in the equally preferred alternate embodiment 10a and these have essentially the same functionality therein as has been previously described in relation to the first described embodiment 10 of the present invention.

In the equally preferred alternate embodiment 10a of the invention, an alternate DC supply voltage 26a provides

regulated positive 48 volts DC to the alternate resonant acoustic transducer coupling circuit 10a. Also, as can be seen in the view of Fig. 3, a modified tank circuit 34a has a second capacitor 58 added thereto. A current limiting

resistor 60 limits current from the alternate DC supply voltage 26a in the equally preferred alternate embodiment 10a of the invention.

As can be appreciated by one skilled in the art, the alternative resonant acoustic transducer coupling circuit 10a lacks components for rapidly causing the decay and/or hold of resonance of the alternative tank circuit 34a. Similarly, the alternative resonant acoustic transducer coupling circuit 10a lacks components for "tickling" the alternative tank circuit 10a as has been previously described herein in relation to the first preferred embodiment 10 of the present invention.

In operation, the equally preferred alternate embodiment 10a of the present invention functions much like the operation previously described herein in relation to the first preferred embodiment 10. Oscillation of the tank alternative tank circuit 34a is induced by the introduction of a pulse width modulated signal across the first switch 16, which excites the alternate resonant circuit 34a, producing a large voltage swing between the capacitor 22 and the inductor 24. The oscillating signal is clamped to the ground potential 30 by the diode 14. This clamping stores charge on the transducer capacitance 32 and the capacitor 22, producing an offset bias that is approximately one half of the peak to peak signal swing. The capacitor 22 is chosen to maintain bias in the presence of leakage current through the diode 14 and other parasitic paths during the receive portion of the operation. The current limiting resistor 60 value is chosen to limit the current through the first switch 16 to a value tolerable thereby The second (coupling) capacitor 58 is introduced to provide additional wave shaping to monimize low frequencies in the output excitation pulse presented to the transducer 28.

After oscillation of the alternative tank circuit 34a ceases (due to parasitic losses) the alternate resonant acoustic transducer coupling circuit 10a is then in condition to pass signal from the transducer 28 to the receive

electronics 54.

One skilled in the art will appreciate the fact that the requisite DC voltage can be introduced into the tank circuit 34 (Fig. 1) or the alternative tank circuit 34a (Fig. 2) in any of several places in the circuitry. Similarly, it will be recognized that the functionality described herein can be accomplished using any of a multitude of different component configurations.

Various other modifications may be made to the invention without altering its value or scope. For example, the best presently known embodiment 10 of the present invention has been described herein as being used with the specific

transducer 28 called forth in this disclosure. Where another transducer might be adapted for use with the present

invention, component values and circuit timings will be adjusted, accordingly, with only a minimal amount of

experimentation required for such adaption Likewise, because the best presently known embodiment 10 of the present

invention is intended for automotive use, it is convenient to use a 12 volt power source, but this is by no means a

restriction of the present inventive resonant acoustic

transducer coupling circuit.

All of the above are only some of the examples of available embodiments of the present invention. Those skilled in the art will readily observe that numerous other

modifications and alterations may be made without departing from the spirit and scope of the invention. Accordingly, the above disclosure is not intended as limiting and the appended claims are to be interpreted as encompassing the entire scope of the invention.

INDUSTRIAL APPLICABILITY

The inventive resonant acoustic transducer coupling circuit is intended to be widely used wherever it is desirable to provide an AC drive signal riding on a DC bias voltage. The predominant current usages are for providing signal and bias to acoustic transducers used for distance measurement, and the like, particularly the Polaroid 9000 Series™

Environmental Transducer.

The primary intended use for the inventive resonant acoustic transducer coupling circuit 10 is in an automobile, for operation in cooperation with a transducer, receiver and microprocessor to comprise a position locating sonar type apparatus which will determine if a person is seated in a seat of the automobile and, if so, whether the person is facing forward, what size the person is, and other information necessary for making a decision about whether an air bag should be deployed when other sensors determine that an accident condition exists, such as for example as is

described in U.S. Patent No. 5,482,314 entitled U.S. Patent No. AUTOMOTIVE OCCUPANT SENSOR SYSTEM AND METHOD OF OPERATION BY SENSOR FUSION, issued to Corrado et al.

It should be noted that the resonant acoustic transducer coupling circuit 10 will also act, somewhat, as a filter such that Electro-Mechanical Interference (EMI) will be reduced. This will be of particular importance in automotive

applications. Similarly, any tendency of the transducer 28 to make an audible "click" when it is initially excited will be reduced by the filtering action of the tank circuit 34. The resonant acoustic transducer coupling circuit 10 of the present invention may be utilized in any application wherein conventional circuits for driving mechanical acoustic transducers, of the type here considered, are used. It is further anticipated that the present invention will find application for driving devices other than transducers, wherever it is necessary to provide a high voltage alternating current signal riding on a direct current biasing voltage. Primary areas of improvement are in the reliability, cost savings, economy and savings in size and weight.

Since the resonant acoustic transducer coupling circuit 10 of the present invention may be readily produced and integrated into existing applications wherein an acoustic transducer is used, it is expected that it will be acceptable in the industry as a substitute for conventional transducer driving circuits. For these and other reasons, it is expected that the utility and industrial applicability of the invention will be both significant in scope and long-lasting in

duration.

Claims

WHAT IS CLAIMED:

1. A transducer driver circuit for driving an acoustic transducer, comprising:

a resonant tank circuit including a capacitor and an inductor, the capacitor and the inductor acting in conjunction with the inherent capacitance of the transducer to produce series resonant oscillation resulting in a signal to the transducer;

a first switch for initially charging the inductor such that current begins to build through the inductor when the fist switch is closed; and

a second switch for introducing additional energy to the tank circuit.

2. The transducer driver of claim 1, wherein:

the first switch and the second switch are insulated gate field effect transducers.

3. The transducer driver circuit of claim 1, and further including:

a third switch for interrupting oscillation of the tank circuit such that the transducer is left with a static charge thereon, the static charge generally equaling the peak value of the signal.

4. The transducer driver of claim 3, wherein:

the third switch is an insulated gate field effect transducer.

5. The transducer driver circuit of claim 1, wherein:

the resonant tank circuit is an electromechanical

interference filter.

6. The transducer driver circuit of claim 1, wherein:

said acoustic transducer is an electrostatic transducer.

7. A driver circuit for coupling a mechanical transducer to electronic circuitry, wherein:

said transducer has an inherent reactance which forms a resonant circuit in combination with an inductor and a

capacitor such that the resonant circuit will resonate to provide an output signal through said transducer, the output signal being generally a sine signal which has a minimum value greater than zero volts.

8. The driver circuit of claim 7, wherein:

said resonant circuit is electrically tuned to a

frequency of the output signal, such that voltages reflected from the capacitor are added to the input signal.

9. The driver circuit of claim 7, and further including: a first switch for disrupting the operation of the driver circuit at the peak value of the output signal such that a static voltage remains at the transducer.

10. The driver circuit of claim 7, and further including: a second switch for maintaining energy in the resonant circuit by raising the voltage on a ground side of the

capacitor during a part of a cycle of the resonant circuit.

11. The driver circuit of claim 10, wherein:

the first switch and the second switch are insulated gate field effect transistors.

12. The driver circuit of claim 7, wherein:

the resonant circuit is a resonant tank circuit.

13. The driver circuit of claim 7, wherein.

the inherent reactance of said transducer is a

capacitance .

14. The driver circuit of claim 7, wherein:

the transducer is an acoustic transducer.

15 The driver circuit of claim 7, wherein:

the transducer is an electrostatic transducer

16. The driver circuit of claim 7, wherein:

the transducer is a piezo electric transducer.

17. A transducer driver circuit for driving an acoustic transducer for an automotive occupancy sensor, the driver circuit comprising:

an inductor and a capacitor in cooperative combination with the inherent reactance of the acoustic transducer such that a resonant circuit including the acoustic transducer is formed such that oscillations in the resonant circuit excite the acoustic transducer.

18. The transducer driver circuit of claim 17, and further including :

an oscillation stopping means for stopping oscillation of the resonant circuit such that a high DC voltage remains on the acoustic transducer.

19. The transducer driver circuit of claim 17, and further including:

tickler means for maintaining a signal level during oscillation of the resonant circuit.

20. The transducer driver circuit of claim 17, and further including:

an initial excitation means for introducing energy into the resonant circuit such that oscillation of the resonant circuit is initiated.