GB2483337A

GB2483337A - Active damping of an acoustic transducer

Info

Publication number: GB2483337A
Application number: GB1113627.2A
Authority: GB
Inventors: Nico Bannow; Tobias Kirchner; Simon Hufnagel
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2010-08-06
Filing date: 2011-08-05
Publication date: 2012-03-07
Anticipated expiration: 2031-08-05
Also published as: CN102378094B; GB201113627D0; FR2963718B1; ITMI20111409A1; FR2963718A1; GB2483337B; DE102010039017B4; CN102378094A; DE102010039017A1

Abstract

The transducer is stimulated to emit acoustic waves by supplying it with a stimulation pulse. Next, the transducer is damped by applying a damping pulse which is at least partly opposed in phase to the oscillation movement of the transducer.Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â The frequency of the damping pulse is set during a capture step, which provides stimulation of the transducer and capture of a resulting existing oscillation frequency of the transducer. Thus the damping pulse is provided with the existing oscillation frequency, taking account of the phase positions of the driving and transducer oscillation.

Description

Description Title

Method and device for active damping of an acoustic transducer

Prior art

In the technical field of the invention, i.e. in the field of acoustic scanning of an environment, methods which include emitting pulses in an environment and receiving pulses which are reflected there in order to be able to deduce the distance to an object by evaluation are known.

The transducers which are used are acoustic transducers with a membrane which necessarily has a certain mass, and the membrane also has elasticity, so that the result is post-pulse oscillation behaviour. Piezo-electric transducers behave in the same way for their piezo-electric layer. Post-pulse oscillation after previous stimulation is undesirable for several reasons, not least because of the resulting minimum distance, since the transducer cannot be used as the receiver of the sound reflected on an object until the oscillation (as the result of a previous stimulation) has largely died away.

Damping the membrane passively by means of damping elements, in addition to its inherent damping, is therefore known, foam being used for example. However, in this way the sensitivity in both transducing directions, i.e. emitting and receiving sound pulses, is significantly reduced.

Damping the membrane actively by applying an opposite signal to the transducer in order to suppress post-pulse oscillation is known from application DE 101 36 628 A for example, overlaying this damping signal with the post-pulse oscillation resulting in an amplitude reduction. This method results in satisfactory damping only if the predetermined constant frequency of the damping signal corresponds to the resonant frequency of the transducer. A constant measuring signal during damping is also necessary.

This restricts the use of the known method to certain transducer types.

It is therefore an object of the invention to provide a mechanism for active damping of an acoustic transducer, said mechanism being suitable for many transducer types or transducers of a type with statically (between individual operating cycles) different and dynamically (during an operating cycle) changeable properties.

Disclosure of the invention

This object is achieved by the method and device according to the independent claims.

The invention makes active damping possible for almost all acoustic transducers, in particular transducers of which the resonant frequency is not precisely known in advance at production, and transducers of which the resonant frequency is changeable. The method can be used for transducers of which the resonant frequency changes during operation, in particular because of changes in the acoustic system because of dirt, aging, changeable bearing stress, changes of surrounding acoustic elements, etc. The invention makes it possible to obtain a significantly reduced post-pulse oscillation time for many transducers without using passive damping elements, which significantly reduce the range and efficiency of the transducer. The invention can also be used in combination with traditional passive damping or passive damping with reduced damping properties, in order to be able to damp even more continuously for example. In particular, it is possible to use transducers with strong production tolerances, since the invention makes individual, continuous adjustment possible. Finally, no frequency adjustment has to be carried out during production; similarly, no components (e.g. capacitors) which provide adjustment to properties which change dynamically or in the course of a lifetime, e.g. temperature compensation or compensation for aging effects, are necessary in order to (among other things) adjust the damping signal corresponding to the existing resonant frequency. By means of the invention, beats which would significantly extend the post-pulse oscillation time can be prevented by active damping intervention.

The concept on which the invention is based is, in the case of active damping, not to start from a fixed, predetermined resonant frequency (which is given by the model for example), but to capture the existing oscillation frequency, in order to provide the active damping with this frequency, which is adjusted to the existing acoustic and system-associated properties. This is achieved by observing a post-pulse oscillation, in order to be able to deduce the existing, changeable resonant frequency or oscillation frequency of the transducer or of the whole acoustic system from it. On the basis of this information, the frequency of the damping signal which is applied to the transducer for active damping can be specifically adjusted. In the case of small mismatches (e.g. different resonant and damping frequencies, phase shift), such as can occur in the prior art, extinction does not occur, but the result is oscillations of which the amplitude is greater than the damped post-pulse oscillation amplitude. The invention prevents this effect, and thus generates a significantly more robust system.

As well as observing a post-pulse oscillation process, in order to generate the existing oscillation frequency and thus the resonant frequency of the transducer, an electrical magnitude, e.g. the current which flows when a stimulating voltage is applied, can be captured while the transducer is stimulated. From the phase offset which may be present, the inherent resonant frequency of the transducer can be deduced, said resonant frequency specifying the frequency at which the damping pulse must occur.

Preferably, however, according to the invention, the frequency of the oscillation resulting from the stimulation of the transducer is captured. The transducer is therefore stimulated, and subsequently (i.e. after or at the end of the stimulation), a transducer signal is captured. Its frequency corresponds to the resonant frequency, and thus the frequency also corresponds to the frequency at which stimulation and damping take place. For stimulation, the result is two alternatives. In a first alternative, a normal stimulation pulse, which is used for acoustic scanning of the environment, is also used to generate post-pulse oscillation, so that the oscillation frequency is captured after the stimulation pulse is fed in.

In the case of known methods, the duration of the post-pulse oscillation is usually masked out, since no echo data can be extracted from it, whereas in contrast the invention provides for observing the post-pulse oscillation behaviour in order to be able to deduce the resonant frequency of the system. The existing resonant frequency, i.e. the existing oscillation frequency during the post-pulse oscillation process, corresponds to the frequency of the damping pulse to be generated. The second alternative provides that an additional measuring pulse, which can be significantly shorter than the stimulation pulse and is used only to deflect the transducer membrane, is applied, in order to be able to capture the frequency of the resulting post-pulse oscillation process. The second alternative therefore makes it possible to capture the existing oscillation frequency independently of the normal stimulation during operation of the transducer. In the case of some transducers, the resonant frequency changes depending on the amplitude. Thus the first alternative, in particular, is very suitable for capturing this dependency. Thus in the case of damping during operation, the damping frequency can be adjusted precisely to the appropriate, dynamically changing resonant frequency of the transducer.

As well as the most optimal possible damping, knowledge of the resonant frequency can also be used to stimulate the transducer as optimally as possible. For this purpose, the stimulation signal should be applied to the transducer at the same frequency and phase position, corresponding to the transducer oscillation.

The invention is particularly suitable for distance capturing sensors for motor vehicles, said sensors being based on ultrasound and the pulse-echo method. Such systems are suitable for parking assistance methods or tracking methods, for example. The method can also be used for transducers which capture a flow rate, in particular of a fuel mixture, on the basis of ultrasound, and in which ultrasound pulses are guided through a fuel stream and in particular received again by the same transducer.

The invention can be implemented by a method or a device which is described below.

The invention includes a method for active damping of an acoustic transducer, which in particular is implemented as a piezo element. Such transducers can be arranged on outsides of motor vehicles. The invention provides for stimulating the transducer by feeding an electrical stimulation pulse tc the transducer. The purpose of this step is to emit an ultrasound pulse, in order to scan the environment acoustically. According to the method, the transducer is then damped by feeding a damping pulse to the transducer. The damping pulse is fed to it during the post-pulse oscillation time, in order to damp the post-pulse oscillation. For this purpose, the damping pulse is at least partly opposed to the oscillation movement of the transducer, so that movement components cancel each other out as destructive interference. This is called active damping. The damping pulse is fed to the transducer as an electrical signal, the oscillation movement of the transducer being thus reduced. The correspondence of electrical and acoustic signals results from the reciprocal two-port network which is provided by the acoustic transducer. The cancellation can therefore take place on the electrical side, on the acoustic side of the equivalent circuit of the transducer, or between these two sides within the equivalent circuit. The damping pulse is opposed to the oscillation movement by them being phase-shifted relative to each other in such a way that the amplitude of the damping pulse at least partly cancels the amplitude of the oscillation movement, in particular the wave crests of the damping pulse and of the oscillation movement having opposite signs. This corresponds to a phase offset of essentially 180°, i.e. half a period; however, partial cancellations also result if the damping pulse is offset by an angular amount of more than 90° and less than 270° to the phase of the oscillation movement.

Because the invention works with the existing oscillation behaviour of the acoustic transducer, the method also provides a capture step, during which the transducer is again stimulated and the resulting existing oscillation frequency of the transducer is captured. As noted above, the stimulation of the transducer within the capture step can coincide with the feeding in of the stimulation pulse for the acdustic scanning of the environment, or a separate stimulation from the normal stimulation during operation of the transducer can take place, having only the purpose of generating a post-pulse oscillation, in order to be able to capture the existing oscillation frequency. Capturing the resulting existing oscillation frequency corresponds to observing the frequency of the post-pulse oscillation, where the term "resulting" can be equated to the post-pulse oscillation process as a reaction to a stimulation. The existing oscillation frequency corresponds to the frequency with which the transducer moves during the post-pulse oscillation, i.e. after stimulation. Since the resulting oscillation is not affected by stimulation or other external influences, the resonant frequency of the transducer is set up as the existing oscillation frequency.

The existing oscillation frequency can therefore be equated to the resonant frequency, or at least a simple link between these frequencies can be provided, said link including an additional correction which reproduces the dependency of the resonant frequency on further transducer properties and transducer states, in particular the existing oscillation amplitude.

Stimulation by feeding in the stimulation pulse, in order to scan the environment acoustically by a pulse-echo method, preferably also takes account of the captured resonant frequency, i.e. the captured existing oscillation frequency. The stimulation pulse is therefore preferably emitted with the resonant frequency, i.e. the captured, existing oscillation frequency.

This captured oscillation frequency, which results from the application and can also be considered as the resonant frequency, is used to generate the damping pulse. In particular, the damping pulse is provided or generated with the existing oscillation frequency. The frequency of the fundamental oscillation of the relevant signal is considered to be the captured oscillation frequency and the frequency of the damping pulse. The oscillation frequency which is captured during the post-pulse oscillation is thus the fundamental frequency of the post-pulse oscillation, and the fundamental frequency of the damping pulse corresponds to this frequency. The damping pulse thus emulates the fundamental frequency of the signal which is present at the transducer during the capture step. The captured oscillation frequency thus emulates at least the fundamental frequency of the oscillation movement which is captured after the stimulation pulse is fed in. The damping pulse is then used to damp this oscillation movement, or subsequent oscillation movements which occur after subsequent stimulation pulses.

In particular, the method provides that the stimulation is provided within the capture step by feeding the stimulation pulse to the transducer, i.e. by a stimulation step which is used to scan the environment acoustically. Also, a measuring signal, which the transducer emits after the end of the stimulation pulse, is captured, in particular its frequency. This frequency is captured as the existing oscillation frequency. The method can provide, in particular, that the whole measuring signal is not investigated, but that only the (fundamental) frequency is captured. The resulting frequency is captured as the existing oscillation frequency, with which in turn the damping pulse is provided.

An alternative embodiment provides that the stimulation is provided during the capture step by feeding a measuring pulse to the transducer. This measuring pulse is an additional measuring pulse, which is not used to scan the environment acoustically, in particular because of its short duration. In this way, the measurement of the existing oscillation frequency and the stimulation for scanning the environment acoustically can be separated in time. A frequency of a measuring signal which the transducer emits after the end of the measuring pulse is captured as the existing oscillation frequency. The damping pulse is provided with this existing oscillation frequency.

In other words, a damping pulse is generated according to the requirement that it at least has as the fundamental frequency the captured frequency of the measuring signal.

The measuring pulse which is used here is used only to generate a post-pulse oscillation of the transducer. In particular, in this way it is possible to achieve that to determine the resonant frequency, a measuring pulse with a duration of only a few half-waves is enough (e.g. fewer than ten, fewer than five or only two or one half-wave).

The measuring pulse also does not necessarily have to have a frequency which corresponds exactly to the resonant frequency, but all that is necessary is that the transducer system can be stimulated by the frequency spectrum of the measuring pulse. For this purpose, the frequency spectrum of the measuring pulse includes at least one (predefined) frequency component which is near a resonant frequency, which is to be accepted, of the transducer. The frequency spectrum can also have further frequency components, so that generating it requires no special precautions. The resonant frequency to be accepted can be predetermined, and in particular can correspond to a model-dependent standard resonant frequency. As well as at least one frequency component which can be predefined and is used to stimulate the transducer, the measuring pulse can have further frequency components, which do not necessarily cause post-pulse oscillation in the transducer. It is merely advantageous that at least one frequency component of the measuring pulse is capable of stimulating the acoustic transducer, so that a wide number of signal shapes can be used to create the measuring pulse. This simplifies the implementation of components which are used to generate the measuring pulse. As an example, a simple rectangular pulse can be used, it being possible to provide the edge steepness, pulse width and edge course within broad tolerance limits. The measuring pulse can also be provided with a frequency corresponding to a previously captured resonant frequency or oscillation frequency.

The existing oscillation frequency, which according to the invention is captured within the capture step after stimulation, corresponds to the existing resonant frequency of the transducer, if the latter is in a freely oscillating state. In particular, a freely oscillating state means that further stimulation signals are not applied to the transducer. The existing oscillation frequency is captured by determining a period duration of a signal which the transducer emits, in particular of the measuring signal which the transducer emits after the end of the stimulation pulse or after the measuring pulse is fed in. The period duration is determined, in particular, by capturing the time gap between two successive zero crossings. For this purpose, the signal which the transducer emits can be high pass filtered, e.g. by setting an offset (e.g. of an operational amplifier connected to the transducer) to zero.

Alternatively, the time gap between two successive maxima or minima of the signal which the transducer emits can be captured. Further possibilities for capturing the existing oscillation frequency, i.e. the resonant frequency, are differentiating the signal which the transducer emits with respect to time and determining the maximum of the derivative with respect to time. In particular, the period duration can be captured by means of a clock generator and a counter, which captures the number of clock pulses between two events which can be assigned uniquely with respect to time (e.g. two or more successive zero brossings for measuring the duration of a half-wave, the period duration, or the duration of a full wave or a multiple of it). The oscillation frequency and the period duration behave reciprocally to each other.

The one frequency of the measuring signal, which the transducer emits after the end of the stimulation pulse or special measuring pulse, is captured as the existing oscillation frequency, with which the stimulation pulse and damping pulse are provided.

The oscillation frequency is captured by determining a period duration of a signal which the transducer emits, by capturing the time gap between two or more successive zero crossings, in particular zero crossings of the alternating current part of the captured signal, between two or more successive relative maxima, between two or more successive relative minima, between two or more successive signal courses which are repeated in the same way or with opposite signs, or between two or more successive signal courses, which are uniquely identifiable in their temporal relationship, of the signal which the transducer emits.

The stimulation pulse and/or the measuring pulse (which is used only to generate a post-pulse oscillation) can be provided as burst signals, i.e. as a periodic signal which extends over a time window. As periodic signals, preferably rectangular signals are used, but in principle sinusoidal signals, triangular signals, sawtooth signals or similar can be used for stimulation. Preferably, the stimulation pulse has no direct component, and in particular is a bipolar rectangular pulse, which can also include a zero state. The damping pulse can be in the same form as the stimulation pulse or measuring pulse, with the difference that the frequency of the damping pulse corresponds to the existing oscillation frequency. The damping pulse is preferably provided with variable power. For this purpose, the damping pulse can have a maximum amplitude amount and/or a duty factor which is below the corresponding size of the measuring pulse or stimulation pulse. Preferably, the smaller the power or amplitude of the signal which the transducer emits after stimulation (by the stimulation pulse or measuring pulse), the smaller the amplitude and/or duty factor. In particular, the amplitude can be reduced by connecting a resistor or resistor network upstream from the transducer, in which case the resistor network can include switches which increase the total series resistance according to the amplitude of the post-pulse oscillation signal, the smaller the power of the signal which the transducer emits after stimulation is, or the smaller the energy which is still found in the transducer is. In particular, the amplitude can also be reduced by using controllable or different, switchable current sources, in which case the current reduces according to the amplitude of the post-pulse oscillation signal, the smaller the power of the signal which the transducer emits after stimulation is, or the smaller the energy which is still found in the transducer is.

The transducer is preferably driven by a ternary driver, which emits the stimulation pulses, the damping pulses and the measuring pulses. The ternary driver emits a positive amplitude, a negative amplitude and a zero amplitude, the driver being in a high resistance state during the zero amplitude. While the signal emitted by the transducer is being captured, the driver is either disconnected from the transducer or put into the high resistance state. In the high resistance state, the internal resistance of the output of the driver is many times greater than during the positive or negative amplitude.

To set the damping pulse corresponding to the oscillation movement to be suppressed, before the damping pulse is fed in phase information of the oscillation movement of the transducer is captured. The phase information can be determined from the measuring signal, in particular by capturing the zero crossings, e.g. all zero crossings leading into the positive or into the negative. The damping pulse is then fed in with a phase which is offset relative to the thus captured phase information, i.e. by an amount greater than 90°, by an amount less than 270° or essentially 180° (e.g. with a deviation of ±10%) . Thus not only the frequency, but also the phase of the damping pulse are adjusted to the measuring signal and/or to the associated oscillation movement, so that subsequent oscillation movements can be optimally compensated for and thus damped.

Whereas the existing oscillation frequency can be captured in a previous step or a previous phase, i.e. in a capture step which is already in the past, the phase information of the oscillation movement which is currently to be damped is captured, in order to be able to synchronise the damping pulse with the existing oscillation movement of the transducer (with a 180° offset) . The phase is thus preferably determined immediately before each damping pulse is fed in, whereas the oscillation frequency can be determined in previous steps. In special operating cases, the phase position of the transducer can be known, so that explicit synchronisation is not required. In particular, the phase position is not known during stimulation and following stimulation, so that the damping can begin immediately after the stimulation.

Since the oscillation frequency is constant for a certain period, the existing oscillation frequency can be captured once and stored temporarily until a new oscillation frequency is to be captured because of the expiry of a predetermined time window. Additionally, a new oscillation frequency can be captured whenever an environment sensor captures a change which is above a specified threshold value, e.g. a temperature change which is greater than a predetermined amount.

Since changes of the acoustic system and in particular of the resonant frequency of the transducer are associated with such changes of the environment, in this way the oscillation frequency, according to which the damping pulse is provided, can be updated whenever a change of the resonance behaviour of the transducer is to be expected.

Also, the capture step according to the invention can be carried out whenever it is detected that a vehicle which carries the acoustic transducer undershoots a specified speed limit, e.g. 10 or 5 km per hour. If a resulting existing oscillation frequency is thus captured, it is stored temporarily and used for future generation of the triggering pulse and damping pulse, until the oscillation frequency is updated in a new capture step. In particular when the captured oscillation frequency is older than a predetermined period, a new capture step is carried out.

In particular, updating by means of the capture step according to the invention takes place whenever the most recently captured oscillation frequency is older than a predetermined period. The period is preferably based on environmental conditions, in particular the temperature or precipitation. The capture step according to the invention can also be carried out in the case of temperature changes or if contamination or icing are detected. Alternatively or in combination with this, the capture step or updating can be carried out every 0.1, 1 or 10 minutes, in particular once a month or once a year, so that aging effects can be taken into account. As already noticed, the capture step can be carried out if the transducer or a component which is spatially near it changes the temperature so that a temperature change, because of which the oscillation frequency also changes, on the transducer can be deduced.

The inventicn is further implemented by a method of capturing the environment of a motor vehicle on which the transducer is arranged. The invention provides for carrying out an acoustic pulse-echo method, which provides sending and receiving the ultrasound pulse. According to the invention, before the ultrasound pulse is received the method according to the invention for active damping is carried out. In particular, at least once but preferably repeatedly or repeated periodically, the capture step according to the invention, in which the existing oscillation frequency (according to which the damping pulse is created) is captured, is carried out. After the ultrasound pulse is sent (by stimulating the transducer by feeding in a stimulation pulse), the damping pulse is applied to the transducer. On the basis of the captured existing oscillation frequency, or on the basis of a currently stored oscillation frequency, the damping pulse is adjusted to the existing post-pulse oscillation behaviour of the transducer. Each feeding in of the damping pulse is preferably preceded by capturing phase information, which reproduces the oscillation movement of the transducer, in order to emit the damping pulse synchronised to the existing oscillation movement, in order to achieve the maximum possible combination effect in this way.

The invention is also implemented by means of a device for active damping of an acoustic transducer, with a capture device which is connected to the transducer. The capture device is capable of capturing an existing oscillation frequency of a measuring signal from the transducer, in particular by means of a clocked counter and zero crossing detection, summit measurement or other means, in order to determine the time gap between signal courses which are repeated with the same or oppositely set signs, or in order to determine the time gap between at least two successive signal courses which are uniquely identifiable in their timing relationship. These are set up to capture the number of clock pulses between two or more zero crossings or signal courses of the same kind, in order to deduce the oscillation frequency (or a magnitude which reproduces it).

The device also has a signal generator, which is connected to the transducer and also connected to the capture device for receiving the oscillation frequency. The signal generator is capable of generating a damping pulse, the signal generator taking account of the previously received oscillation frequency when it generates the damping pulse, and providing the frequency of the damping pulse according to it. The signal generator is also capable of providing the damping pulse, with respect to phase, at least partly oppositely to the (currently captured) oscillation movement of the transducer. Because of the connection to the transducer, the signal generator is capable of applying the damping pulse to it.

According to a preferred embodiment, the device includes a memory, in which a currently captured oscillation frequency can be held, the memory also being connected to the signal generator, which can call up the existing oscillation frequency from the transducer, in order to provide the damping pulse with the oscillation frequency. The signal generator can be a binary or ternary output stage, in which case the ternary output stage in the zero state also switches to a high internal resistance.

The signal generator can also have an input for a duty factor, the duty factor of the damping pulse being provided using said input. The signal generator can also, alternatively or in combination with it, have an input for an amplitude, using which the amplitude of the damping pulse can be controlled, e.g. by means of a controllable resistor network or by means of a controllable current or voltage source. The device can have an amplitude capturing device, which captures the amplitude of a measuring signal (or its power), the duty factor and amplitude or power of the signal generator being controlled according to this amplitude capturing device through a corresponding connection, in order to provide a higher amplitude or a higher duty factor in the case of higher captured power of the measuring signal than in the case of lower power of the measuring signal. In this way, the damping strength can also be adjusted adaptively to the existing post-pulse oscillation behaviour. As well as measuring the oscillation amplitude, alternatively a so-called energy counter, which represents the existing energy in the oscillator circuit in S suitable form, can be used. The oscillation energy which is added by the stimulation is added to the energy counter at time intervals. The energy which is removed from the oscillator circuit by internal damping and active damping is correspondingly subtracted from the energy counter at time intervals.

As well as measuring the oscillation amplitude or using an energy counter, the residual energy can also be determined on the basis of the duration of preceding reference damping processes, which were received during controlled operation for acoustic scanning of the environment, or during special measurement cycles to determine the resonant frequency. By evaluating the energy counter, the required intensity of the damping pulse can be determined.

In a further embodiment, the capture device also includes a phase capture unit, in particular in the form of a zero crossing detector (which preferably can also capture the direction of the zero crossing). The phase capture unit is connected to the transducer, and is set up to capture, on the basis of the measuring signal, a phase position of the signal from the transducer (= the measuring signal) . The phase capture unit is also connected to the signal generator, in order to transmit the captured phase position to it. In this way, the signal generator can process synchronisation information in the form of the phase position, in order to provide the damping pulse precisely oppositely to the phase position of the existing oscillation movement of the transducer. The phase capture unit is set up in order to determine the phase position after a stimulation and damping pulse of the transducer has ended. The signal generator is set up to provide the damping pulse with a phase which is at least partly opposite the phase position, in particular by essentially 180°, by which means the active suppression of the post-pulse oscillation movement of the transducer can be optimised.

The device can be implemented by means of hard wired components, by means of programmable hardware and associated software, or of a combination of them. In particular, the device can be in the form of a controller, microcontroller, DSP, or hard wired, digital logic circuit (ASIC or FPGA). In particular, the power driver (output stage) can be part of an integrated circuit, which also implements most or all other components of the device, or it can be an external driver. The phase capture unit and/or the capture device to capture the measuring signal can include an A/D converter, a comparator, low pass filter, high pass filter, peak value detector or similar, in particular components of an input/output interface of an integrated circuit (e.g. a microcontroller) . The transformer can be connected to the capture device via an amplifier circuit (e.g. an operational amplifier) The damping pulse can be provided with exactly the frequency which corresponds to the oscillation frequency, or it can be provided with a frequency which is assigned to the oscillation frequency depending on the stimulation amplitude or depending on the existing amplitude in the oscillator circuit of the transducer. The last-named possibility takes account of the dependency of the resonant frequency on a signal amplitude or the power, the signal amplitude or power being assigned to a specified stimulation pulse or a specified damping pulse (i.e. a stimulation to generate an acoustic scanning pulse). The dependency of the resonant frequency on signal amplitude or power can be measured completely by the transducer. It is also possible to deduce correctly, from an oscillation frequency which was captured at lower amplitudes, according to the amplitude-dependent behaviour of the transducer, the oscillation frequency at higher amplitudes, and by measuring at high amplitudes to deduce the oscillation frequency at lower amplitudes. For this purpose, a device according to the invention can include a correction factcr or a lookup table, which reproduces a correction curve for the dependency of the resonant frequency on the amplitude.

For example, a volatile memory can be used if the correction information is determined dynamically. If the correction information is statically or rarely determined during operation, it can be held in a non-volatile read-write memory such as a flash memory. In an alternative embodiment, the dependency between resonant frequency and amplitude exists in the dependency of the resonant frequency and the amplitude or power which is provided for the damping pulse, and which can be dependent according to the amplitude or power of the measuring signal.

According to this embodiment, the existing oscillation frequency which is used for damping corresponds to the correspondingly corrected frequency. In particular, the correction is carried out within the frequency capture, for which reason the corrected frequency is observed as the result of the measurement. Since the correction includes no essential frequency shift, the frequencies essentially correspond. In the case of different amplitudes, the frequency is multiply adjusted correspondingly.

Instead of a single existing oscillation frequency, multiple averaged oscillation frequencies can be used, in which case the average value of the frequencies determines the frequency of the damping pulse. Furthermore, the existing oscillation frequency can be measured not only on the basis of two directly adjacent zero crossings or extremes, but on the basis of a predetermined plurality of zero crossings, extremes, summits, repeating signal states or uniquely identifiable signal states, in order to reduce measurement noise, in particular jitter errors. Finally, the phase position can be captured by measuring adjacent zero crossings, extremes, summits, repeating signal states or uniquely identifiable signal states, i.e. by measuring adjacent extremes, the instant of the zero crossing being produced by arithmetical averaging of the instants of the extremes. In particular, the measurement and averaging of reflected signal courses in Nth derivation, e.g. zero crossings with rising and falling signal course and upper and lower summit of the amplitude, are specially suitable for eliminating circuit-specific deviations.

According to a further aspect of the invention, a method and/or a device, which are also provided for elimination of additional running time effects in signal processing and generation in the device in the form of running time delays, are provided. According to the invention, the running time delays which occur in the signal chain -starting with the signal generator, via the amplifier circuit, to the movement of the membrane and finally in the capture device and evaluation unit -can be captured because by knowing the precise phase position of a driving pulse and comparing it with the signal course, which is measured next, of the membrane, the running time delay between signal generator and capture device can be determined by subtraction. For this purpose, the device includes a time capture device, which is connected to the signal generator which stimulates the transducer, and which is also (preferably indirectly via an amplifier) connected to the transducer, in order to capture the signals resulting from the stimulation of the transducer. The resulting time offset completes the resonant frequency, which is determined as described above, so that together a damping signal, which because of the offset, i.e. the running time delays described above, has an absolute time reference, can be generated.

The transducer can determine the running time delay dynamically during operation. It is also possible to determine the running time delay once statically before the transducer is used. In the case of dynamic determination, the running time delay can be held in volatile memory, whereas in the case of static determination, a non-volatile memory, e.g. a FLASH memory, is specially suitable.

The embodiment according to the invention is characterized in that it includes the effect of the running time delay in the damping with a phase offset of 180°. For this purpose, the antiphase damping is carried out earlier, by the amount of the running time delay, than in the case of a circuit which is assumed to be ideal, without running time delay.

In particular, it is possible to compensate for aging and temperature effects in this way, also with the result that components with greater tolerance can be used.

The deviations in the measurement of phase position and oscillation frequency compared with the actual transducer signal, which can be caused in particular by circuit or quantisation effects, can be minimised by an average value over time of the measured transducer signal being formed at instants with the same rise but opposite sign, in particular zero crossings with rising and falling signal course and in the upper and lower summits of the amplitude.

Alternatively, the transducer signal can be analysed by means of a filter, in particular a low pass filter or band pass filter, which in particular is implemented as a FIR filter, IIR filter or analogue circuit. The filter can also be implemented by means of control engineering filtering (e.g. by means of a recycled or not recycled loop, in which delay elements are connected to each other). The oscillation frequency can also be captured by means of a PLL. Additionally, a high pass filter can be used to block a voltage offset of the signal which the transducer emits.

An embodiment of the invention provides that in addition to the active damping, damping is done with a passively damping foam. The passively damping foam is in contact with an oscillating component of the transducer, in particular with a membrane of the transducer. The passively damping foam absorbs oscillation energy of the transducer. The foam has a damping capability which essentially agrees with the damping capability of foam which is used for (exclusively) passive damping in transducers. The damping capability of the foam which is used according to the invention can also be below that of foam which is used for exclusively passive damping of transducers (in particular ultrasound transducers) . In particular, the damping capability of the foam used according to the invention is similarly high as the damping capability of a foam which is used according to the prior art as pure foam damping. Alternatively, the damping capability of the foam used according to the invention is lower than the pure (i.e. purely passive) foam

damping which is used according to the prior art.

Brief description of the drawings

In the drawings, embodiments of the invention are shown, and they are explained in more detail on the basis of the

following figure descriptions.

Fig. 1 shows a diagram for a more detailed explanation of the method according to the invention; Fig. 2 shows a schematic diagram of a device according to the invention.

Embodiments of the invention Fig. 1 shows a sequence chart with three different curves, which are plotted on the same time axis leading to the right. The top curve shown in Fig. 1 shows a measuring signal 10, which is captured at the terminals of a transducer which is operated according to the invention.

The curve below it shows the amplitude of the membrane movement 20 which the membrane of the transducer executes.

The curve 20 thus shows the course of the actual transducer stimulation on the acoustic side. The bottom curve shown in Fig. 1, which is shown together with its zero line 30, is the stimulation signal 40 with which the transducer which is operated according to the invention is stimulated. Since the stimulation takes place at the same terminals as the capture of the measuring signal 10, the signal forms of the measuring signal 10 and stimulation signal 40 are similar where the stimulation signal is not zero (i.e. on the zero line 30) . If the stimulation signal 40 is on the zero line, e.g. in section 43, a driver connected to the transducer is switched to high resistance. In these time sections, the transducer is used to reproduce the membrane movement as it is shown on the electrical side of the transducer. The mostly rectangular signal course of the measuring signal 10 is because the amplifier circuit which is used goes very quickly into saturation.

Fig. 1 is divided into several time sections 41-49. In the first time section 41, the transducer is idle; before the end of time section 41, no stimulation takes place, so that the end of time section 41 can be identified as the start of an active period. In the subsequent time section 42, a measuring signal is applied to the transducer, and stimulates it. It should be recognised that in this period the membrane movement 20 begins. The stimulation takes place by means of a period of a rectangular signal, the period duration and/or the frequency of the measuring signal which is used for stimulation not corresponding to the resonant frequency. In the subsequent period 43, the resulting existing oscillation frequency is determined on the basis of the measuring signal. For this purpose, first electively the zero crossings, extremes, summits, repeating signal states or uniquely identifiable signal states, but in particular the zero crossings, of the measuring signal are captured, and if required circuit-specific offsets and jitter are eliminated by averaging. For example, by determining the time difference of two successive zero crossings, the half period duration can be determined, or by the time difference of the first and last zero crossing of three successive zero crossings, the full period duration can be determined. In order to capture the state of the transducer membrane, it is advantageous to measure the voltage at the transducer at the instant when the driver is in the high resistance state. Via a measuring resistor, it is also possible to capture the course of the current in the transducer membrane. Here too, it is advantageous but not an obligatory requirement to switch the driver to high resistance.

After the measuring pulse was applied to the transducer in section 42 and the subsequent post-pulse oscillation was used in section 43 to capture the resulting existing oscillation frequency, in the subsequent section 44 the transducer is stimulated over a relatively long period by applying a relatively long stimulation pulse. It can be seen that in this period the membrane movement 20 clearly increases, so that an acoustic scanning pulse is generated.

At the end of section 44, the stimulation pulse is ended, and this is followed by a time section 45, during which the transducer is not stimulated. Time section 45 ends with the capture of the edge 12, on the basis of which the existing phase position of the transducer is determined. Time section 45 is also used to synchronise the later damping pulse with the membrane movement in order to achieve maximally effective damping. At the start of the subsequent section 46, the active damping begins, and during it a damping pulse, which is shown on the basis of the stimulation signal 40 in time section 46, is applied to the transducer. On the basis of the membrane movement curve 20 in section 46, it can be seen that the post-pulse oscillation is strongly damped by the application of the damping pulse in section 46. Time section 46, which includes part of the damping pulse, is followed by a section 47, during which the transducer is not driven, in order to capture again the autonomous (residual) oscillation and the phase position of the transducer. Time section 47 ends with the edge 14 in the measuring signal, which reproduces the desired phase information, on the basis of which the further part of the damping pulse can be synchronised. In the subsequent time section 48, therefore, a further, second part of the damping pulse, which during the preceding repeated synchronisation section 47 was again synchronised with the actual movement of the membrane, is shown. During the second antiphase driving 48 (i.e. during the second damping pulse), if required the damping pulse is driven with a lower duty factor or a lower amplitude than in the first part, in order to take account of the already reduced amplitude of the post-pulse oscillation, so that too high damping and thus renewed stimulation are avoided.

Time sections 46 and 48 end after a predetermined duration (e.g. defined by the number of periods), whereupon the movement of the membrane is carried out by a renewed measurement on the basis of the measuring signal, in order to capture the still remaining post-pulse oscillation strength. Since at the end of time section 48 the measuring signal 10 has a low amplitude, no further part of the damping pulse is applied, since either the low amplitude of the measuring signal 10 indicates this, or a defined number of damping pulses indicates that no further essential membrane movement 20, and thus no essential post-pulse oscillation, takes place any longer.

Fig. i shows that between the step of capturing the existing oscillation frequency in section 43 and the application of the damping pulse in sections 45-48, a stimulation pulse to generate an acoustic scanning pulse during the period 44 is applied. During the period 43, the captured existing oscillation frequency is stored temporarily as the resonant frequency, and is reused both in the stimulation in section 44 and in the generation of the damping pulse in sections 46 and 48. Additionally, in Fig. 1, a method according to which an individual measuring pulse is fed to the transducer (in section 42), its only purpose being to capture the post-pulse oscillation behaviour in a subsequent time section 43, is shown.

An alternative embodiment to this provides that the stimulation pulse, which is also used to generate an ultrasound scanning pulse, is also used to stimulate the transducer for the purpose of capturing the resulting oscillation frequency. In the case of such a method, time sections corresponding to time sections 45 and 47, in which no stimulation signal is applied (in contrast to sections 46 and 48, curve 40), can be used, so that on the basis of the measuring signal 10, in sections 45 and 47 (or also in subsequent sections), the post-pulse oscillation behaviour can be read off, and in particular the resulting existing oscillation frequency can be captured. In this case, the stimulation according to section 44 is not possible with the resonant frequency, or only with a resonant frequency which was determined by a preceding stimulation pulse and is present in memory.

In Fig. 2, a schematic diagram of a device according to the invention for active damping is shown. The device includes a transducer 100, which has two terminals 102, 104. On the one hand, these are connected to a signal generator 110 of the device which stimulates the transducer. The terminals 102, 104 are also connected to the capture device 120 according to the invention, the terminals 102, 104, during a stimulation pause of the signal generator 110, providing a signal which reproduces the movement of the membrane of the transducer 100. The movement is captured by the capture device 120 through the connection to the terminals 102, 104. The capture device is equipped with a frequency measuring unit 122, which enables the capture device 120 to capture the frequency of the signal at the terminals 102, 104, in particular if the signal generator 110 has a high resistance state at the output. The capture device 120 also includes a phase capture unit 124, using which phase information, in particular a zero crossing, of the signal which is present at the terminals 102, 104 can be captured, while the signal generator 110 carries no signal to the transducer 100. The frequency and phase capture units 122, 124 preferably include zero crossing detectors, in order to capture the phase position and period length between two zero crossings. The capture units 122, 124 can use a common zero crossing detector.

The captured oscillation frequency and the captured phase position are transmitted via a connection to the signal generator 110, which provides the damping pulse, which is opposite the movement behaviour of the transducer 100, according to the frequency and phase position. The device according to the invention can also include a controller 130, which drives the signal generator 110, in order to allow stimulation pulses, damping pulses and if required measuring pulses to be generated by the signal generator at the corresponding instants. In one embodiment, which is an alternative to the embodiment of Fig. 2, the capture device 120 is not (only) connected to the signal generator 110, but to the controller 130, which processes the frequency information and phase information and drives the signal generator correspondingly.

Claims

Claims 1. Method for active damping of an acoustic transducer (100), with the following steps: stimulation (44) of the transducer by feeding a stimulation pulse to the transducer, followed by damping (46, 48) the transducer by feeding a damping pulse, which is at least partly opposed to the oscillation movement of the transducer, to the transducer, characterized in that the method also includes a capture step, which provides stimulation of the transducer and capture of a resulting existing oscillation frequency of the transducer, the damping pulse being provided with a frequency corresponding to the captured existing oscillation frequency.
2. Method according to Claim 1, wherein the stimulation of the capture step is provided by feeding a pulse to the transducer, either the actual stimulation pulse (44) or a special measuring pulse (42), which includes only few, in particular fewer than 10 or 5, or only one half-wave, the frequency spectrum of the measuring pulse preferably including a predefined frequency component which is near a model-dependent standard resonant frequency.
3. Method according to any one of the preceding claims, wherein the damping pulse is provided with a frequency which corresponds to the existing oscillation frequency, and which is also adjusted to an existing residual energy of the transducer or the existing amplitude of the oscillation movement, according to a transducer-dependent dependency between the resonant frequency of the transducer and the amplitude and residual energy of the oscillation movement.
4. Method according to any one of the preceding claims, wherein the oscillation frequency of the transducer, according to which the damping pulse is provided, is updated at latest when a change of the oscillation frequency of the transducer is to be expected because of changing properties of the transducer or has already occurred, when the last captured oscillation frequency is older than a predetermined duration (in particular in the case of temperature changes, contamination, icing, e.g. every 0.1, 1 or 10 minutes, in particular in the case of aging effects, e.g. once per month or once per year), when a vehicle which carries the acoustic transducer undershoots a specified speed limit, in particular a speed of 10 or km per hour, or when it is detected that the transducer or a component which is spatially near it changes the temperature so that a temperature change, because of which the oscillation frequency also changes, on the transducer can be deduced, or a combination of these.
5. Method according to any one of the preceding claims, wherein an additional running time delay because of the logical driving, because of the inertia of the transducer membrane, and because of evaluation of the measured signal course, is captured by the phase position of a driving pulse being compared with the signal course of the transducer measured following the damping, in order to determine the additional running time delay between signal generator and capture device by subtraction, the running time delays of the active damping being determined repeatedly during operation of the acoustic transducer for acoustic scanning of the environment or being determined once before the start of operation, and the temporal control of the feeding in of the damping pulse being compensated for by the additional running time delay, or in the case of calculation of the required phase offset, the additional running time delay being taken into account.
6. Method according to any one of the preceding claims, wherein after the feeding in of a stimulation pulse, the feeding in of the damping pulse is carried out multiple times, post-pulse oscillation being damped by one or more damping pulses, the totality of which results in complete damping of the transducer, between the successive feeding in of the damping pulses phase information of the oscillation movement of the transducer preferably being determined, and the damping pulse being emitted according to the phase information, so that at least partial compensation of post-pulse oscillation movement of the transducer is achieved by the damping pulse.
7. Method according to any one of the preceding claims, wherein during the damping, the intensity of the damping process, i.e. the amplitude of the current or voltage, is adjusted to the energy which is currently contained in the sensor, and the intensity of the damping is reduced, in particular towards the end of the damping.
8. Method according to any one of the preceding claims, wherein the transducer is driven by a ternary driver (110), which generates a positive amplitude, a negative amplitude with the amplitude amount of the positive amplitude, and a zero amplitude, the driver being switched into a high resistance state during the zero amplitude.
9. Method of capturing the environment of a motor vehicle by means of an acoustic pulse-echo method, the pulse-echo method providing sending and receiving an ultrasound pulse, and the method being carried out according to any one of the preceding claims for sending and receiving the ultrasound pulse.
10. Device for active damping of an acoustic transducer, with a capture device (120) which is connected to the transducer (100), and is set up to capture an existing oscillation frequency of a measuring signal from the transducer, the device also having a signal generator (110) which is connected to the transducer, and which is also connected to the capture device (120) to receive the oscillation frequency, and the signal generator (110) being set up to generate a damping pulse, which is at least partly opposed to the oscillation movement of the transducer, with the received oscillation frequency, and to apply it to the transducer.
11. Device according to claim 10, wherein in addition to the active damping, damping is done with a passively damping foam, which is provided on an oscillating component of the transducer, in particular on the it) I membrane, and absorbs oscillation energy of the transducer, the foam having a damping capability which essentially agrees with the damping capability of foam which is used for passive damping in transducers, or is below it.
12. A method for active damping of an acoustic transducer substantially as herein described with reference to the accompanying drawings.
13. A device for active damping of an acoustic transducer substantially as herein described with reference to the accompanying drawings.