GB2593477A

GB2593477A - Pulse generator

Info

Publication number: GB2593477A
Application number: GB2004228.9A
Authority: GB
Inventors: Dixon Steven; Feeney Andrew; Kang Lei; Somerset William
Original assignee: University of Warwick
Current assignee: University of Warwick
Priority date: 2020-03-24
Filing date: 2020-03-24
Publication date: 2021-09-29
Anticipated expiration: 2040-03-24
Also published as: GB202004228D0; GB2593477B

Abstract

The pulse generator 1 is configured to output a drive signal 4 causing a transmitting transducer 7 to emit a pulsed sound wave 8 having a first time duration tP. When received by a receiving transducer 11 the pulse generator is configured to to generate an electrical response signal 12 having a second time duration tR. shorter than the first time duration (figs 2 to 5). This approach is said to avoid the problems of mechanical reverberation in the transmitting transducer 7. The drive signal 4 may comprise multiple pulses that may be in anti-phase. The acoustic pulses may be reflected from or transmitted through the object under inspection (figs 8 to 10).

Description

Pulse generator

Field

The present invention relates to a pulse generator for outputting a drive signal for a 5 transducer to generate a pulsed sound wave and to a system including the pulse generator and at least one transducer including the transducer.

The present invention also relates to a method of synthesising a transmit waveform for the drive signal for the pulse generator.

Background

Transducers that use a piezoelectric element as an active component which generates or detects sound are used in a range of applications from non-destructive testing to flow measurement and proximity sensing. Herein, the term "sound" is used to cover /5 pressure waves from infrasound through to ultrasound.

A piezoelectric element has many resonant modes that could be, for example, through-thickness or radial. For example, for a transducer that operates at 1MHz, a disc-, rectangular-or square-shaped block of a suitable piezoelectric material, such as lead zirconium titanate (PZT-5H), is used which has a thickness of approximately 2 mm, corresponding to a 1MHz through-thickness resonant mode for the material. The thickness of the material can define the resonant frequency of piezoelectric element, but it could also vibrate in a different direction, such as a radial mode of vibration for a disc.

Piezoelectric elements tend to be driven at frequencies close to or around a particular mechanical resonance or, if driven by a wideband or temporally-sharp signal, will tend to vibrate at one or more resonant modes of vibration. it is the operation around resonance that tends to provide good transduction efficiency as either an ultrasonic generator or detector.

One problem with a transducer being resonant is that it can vibrate for relatively long periods of time. Steps can be taken to increase damping of the piezoelectric element, using arrangements such as mechanical damping from an energy absorbing "backing layer" attached to the piezoelectric element, by electrical damping through a circuit that is electrically connected to the electrodes of the element, or by a combination of the two approaches. Even then, the transducer may still produce long-lived ultrasonic vibrations that could make it difficult to differentiate between signals of different origin, that are closely-spaced in time, since these signals could interfere with each other. In simple circuits used for proximity sensing with a fluid ultrasonic transducer, it can also be useful to limit the temporal extent of the electrical response to a received ultrasonic waveform, to make a particular measurement more accurate as it restricts the period of time over which the signal amplitude is significant.

Approaches to address this problem include driving a transducer with two distinct /0 electrical pulses: after the transducer has been driven by a first pulse, a second pulse is used to try to damp the vibration of the transducer. The second pulse can be of smaller amplitude, with a phase that can be considered inverted with respect to that of the first pulse that initially excited the transducer.

/5 Reference is made to: G. L. Miller, R. A. Boie, &M. J. Sibilia: "Active Damping of Ultrasonic Transducers for Robotic Applications" Proceedings. 1984 IEEE International Conference on Robotics and Automation, page 379ff. (1984).

Canhui Cai and Paul P. L. Regtien: "Accurate Digital Time-of-flight Measurement Using Self-Interference", IEEE Transactions on Instrumentation and Measurement, volume 42, page 990ff. (1993).

Xinxin Liu et al.: "Reducing ring-down time f pMUTs with phase shift of driving waveform", Sensors and Actuators A, volume 281, pages 100 -107 (2018).

ICatsuhiro Sasaki, et al.: "Air-coupled ultrasonic time-of-flight measurement system using amplitude-modulated and phase inverted driving signal for accurate distance measurements", IEICE Electronics Express, volume 6, page 1516ff. (2009) -3 -

Summary

The invention is based on shortening the electrical signal produced by the transducer that detects a received pulsed sound wave (for example, a received pulsed ultrasonic wave) that has returned from or passed through a target, as opposed to aiming to 5 shorten the duration of the mechanical vibrations of the transmitting transducer.

According to a first aspect of the present invention there is provided a pulse generator configured to output a drive signal for causing a transmitting transducer to generate a pulsed sound wave (for example, a pulsed ultrasonic wave), having a first duration io which, when received by a receiving transducer, causes the receiving transducer to generate an electrical response signal having a second duration, wherein the second duration is shorter than the first duration.

The pulsed sound wave may be a pulsed ultrasonic wave and the transducers may be ultrasonic transducers.

The drive signal may comprise at least two pulses including a first pulse having a start and an end and a second pulse and wherein the second pulse starts after the start of the first pulse. The second electrical pulse may start after the end of the first electrical pulse. However, the second pulse may start before the end of the first pulse. The first and second pulses may be out of phase. The first and second electrical pulses may be in anti-phase. The second electrical pulse may be longer than the first pulse and/or has more cycles. The second electrical pulse may be the same or shorter (but have greater amplitude) than the first pulse. The drive signal may comprise at least three pulses.

or The drive signal may comprise a first pulse and a second pulse having respective starts and ends, and a third pulse, and wherein the third pulse starts after the start of the second pulse. The third pulse may start after the end of the second pulse. The second and third pulses may be out of phase. The second and third pulses may be in anti-phase. The third pulse may be shorter than the second pulse and/or may have fewer cycles. The drive signal may comprise at least four pulses.

According to a second aspect of the present invention there is provided a system comprising at least one transducer including a first transducer, the pulse generator of the first aspect of the invention configured to drive the first transducer such that the first transducer is the transmitting ultrasonic transducer and an amplifier arranged to receive a signal from the receiving transducer. -4 -

The first transducer is preferably an ultrasonic transducer.

The first transducer may also be the receiving transducer. The at least one transducer 5 may include includes a second, different transducer such that the second transducer is the receiving transducer.

The system may comprise a controller and a driver, wherein the pulse generator comprises the controller and driver.

The system may further comprise a reflector arranged such that the transmitting directs the pulsed sound wave at the reflector and the reflector directs the pulsed sound wave to the receiving transducer.

According to a third aspect of the present invention is provided a method comprising providing a transmit waveform for generating a drive signal for a transmitting transducer, determining a response of a receiving transducer, determining whether the response meets a predefined requirement, upon a positive determination, storing the transmit waveform and upon a negative determination, modifying the transmit waveform and determining a new response of the receive transducer. The drive signal is for causing the transmitting transducer to generate a pulsed sound wave (for example, a pulsed ultrasonic wave) having a first duration which, when received by the receiving transducer, causes the receiving transducer to generate an electrical response signal having a second duration, wherein the second duration is shorter than the first or duration.

According to a fourth aspect of the present invention is provided a computer program comprising instructions for performing the method.

According to a fifth aspect of the present invention is provided a computer program product comprising a computer readable medium (which may be non-transitory) storing the computer program. -5 -

Brief Description of the Drawings

Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates a pulse, a pulse sequence and first and second pulses where the second pulse is in anti-phase with the first pulse; Figures 2a, 2b and 2c illustrate, in a first experiment, an electrical signal sent to a transmitter, a far-field pressure generated by the transmitter and received by a receiver, and an amplified electrical signal generated by the receiver, respectively, Figures 3a, 3b and 3c illustrate, in a second experiment, an electrical signal sent to a jo transmitter, a far-field pressure generated by the transmitter and received by a receiver, and an amplified electrical signal generated by the receiver, respectively, Figures 4a, 4b and 4c illustrate, in a third experiment, an electrical signal sent to a transmitter, a far-field pressure generated by the transmitter and received by a receiver, and an amplified electrical signal generated by the receiver, respectively, Figures 5a to 5c illustrate first, second and third far-field pressures generated by a transmitter excited with a first, first and second, and first, second and third waveforms, respectively; Figures 5d to 5f illustrate first, second and third electrical responses generated by a receiver in response to the first, second and third far-field pressures, respectively; Figure 6 is a schematic block diagram of a system for generating and detecting ultrasonic pulses; Figure 7 is a schematic block diagram of a first transducer configuration employing separate transmit and receive transducers; Figure 8 is a schematic block diagram of a second transducer configuration employing or separate transmit and receive transducers and a signal-reflecting target; Figure 9 is a schematic block diagram of a third transducer configuration employing a transducer which is used to generate and detect ultrasonic pulses, and a signal-reflecting target; Figure ro is a schematic block diagram of a fourth transducer configuration employing a transducer which is used to generate and detect ultrasonic pulses, which is in contact with a sample; Figure 11 is a schematic block diagram of a transmit pulse waveform-synthesising system based on experimentation; Figure 12 is a schematic block diagram of a second pulse waveform-synthesising system 35 based on simulation; and -6 -Figure 13 is a process flow diagram of a method of synthesising a transmit pulse waveform.

Detailed Description of Certain Embodiments

Introduction

The present invention concerns the problem that a transducer, in particular an ultrasonic transducer, can vibrate long after excitation. Long-lived pulses at a receiving transducer can make it difficult to detect signals that arrive closely-spaced in time, since these signals could interfere with each other.

The present invention is based on the approach of generating a pressure wave with a transmitting transducer which has the effect of shortening the electrical response of a detecting transducer, instead of trying to make the duration of a pressure wave generated as short as possible. In particular, the transmitting transducer is excited by an electrical waveform that is not necessarily designed to generate a shorter pressure wave, but rather to produce a pressure wave which attempts to minimise the time duration of the electrical response of the detecting transducer. The generating transducer and the detecting transducer could also be the same transceiver transducer.

To illustrate the differences in these two approaches, a set of experiments will be described. In each experiment, a pair of transmitting and receiving flexural ultrasonic transducers (FUTs) are used.

The electrical signal that is used to excite the transmitting transducer will herein be or described as comprising electrical pulses which are modulated, sinusoidal signals with no discontinuities in phase. The resulting far-field pressure produced by the transmitting transducer when excited by one or more of these electrical pulses will be referred to as the pressure wave, pressure pulse or similar. The amplified electrical signal produced by the receiving transducer when excited by the pressure waveform outputted by the transmitting transducer will be referred to as the electrical response of the receiving transducer.

Referring to Figure 1, examples of an electrical pulse, electrical pulse sequence and first and second pulses, where the first and second pulses are in anti-phase are shown. -7 -

Damping of a transmitting transducer Referring to Figures 2a, 2b and 2c, a first comparative example of damping of a transmitting narrowband transducer is shown.

In the first comparative example, minimisation of the duration of output pressure is the only considered factor. This is achieved by supplying a first, driving electrical pulse (labelled "la") followed by a second, damping electrical pulse (labelled "lb") to the transmitting transducer, where the second pulse is applied in anti-phase to the extrapolated waveform of the first.

A first trace represents the system after excitation by the first electrical pulse only, and a second trace shows the response to the initial electrical pulse and the active damping electrical pulse. Temporal duration or duration is an arbitrarily quantified measure, but here it is defined as the difference in time at which the envelope of each trace crosses an amplitude threshold equal to 2% of the envelope's peak. The duration decrease is the percentage decrease in duration when using the damping sequence when compared to using a single pulse alone.

As shown in Figure 2a, the first electrical pulse consists of five cycles of a sine wave having total duration of (124.6±0.1) Rs. The second electrical pulse consists of five cycles of a sine wave also having a total duration of (124.6±0.1) ps, starting (12.4±0.1) ris after the end of the first pulse. The second pulse has a smaller amplitude than the first pulse.

As shown in Figure 2b, the far-field pressure response generated byjust the first electrical pulse is longer than the response generated by using the first and second electrical pulses. The pressure pulse generated by just the first electrical pulse has a duration of (1900.5±0.1) ps. The pressure pulse resulting from both the first and second electrical pulses has a duration of (282.5±0.1) s.

As shown in Figure 2c, the outputted amplified electrical response generated by the receiving transducer arising from just the first electrical pulse is longer than that resulting from the first and second electrical pulses. The electrical response caused by a single pulse has a duration of (2321.9±0.1) ps. The electrical response caused by the first and second pulses has a duration of (836.0±0.1) ps. It is clear that the both electrical responses exhibit significant temporal duration or ring-down. -8 -

Referring to Figures 3a, 3b and 3c, a second comparative example of damping of a transmitting narrowband transducer is shown.

The approach used in the second comparative example is similar to that used in the first comparative example, but follows the approach used by Katsuhiro Sasaki, etal.. ibid. In the second comparative example, an electrical pulse sequence is employed in which the second pulse has fewer cycles, but a larger amplitude than the initial driving pulse.

The first electrical pulse (labelled "2a") consists of five cycles of a sine wave having duration of (124.7±0.1) hts. The second pulse (labelled "2b") consists of three cycles of a sine wave having duration of (74.8±0.1) us starting (12.4±0.1) us after the end of the first pulse. In this case, the second pulse has a larger amplitude.

As shown in Figure 3h, the far-field pressure pulsed waveform generated by the transmitting narrowband transducer when driven byjust the first electrical pulse is longer than the pressure waveform generated by using the first and second pulses.

As shown in Figure 3c, the outputted amplified electrical response generated by the receiving transducer arising from just the first electrical pulse is longer than that arising from the first and second electrical pulses. The electrical response resulting from the single pulse has a duration of about (2331.2±0.1) us. The electrical response caused by first and second pulses has a duration of (1133.8±0.1) us. Again, it is clear or that the both electrical responses exhibit significant ringdown.

Damping of the receiving transducer Referring to Figures 4a, 41) and 4c, a first example of damping of a receiving transducer is shown.

In this case, minimisation of the duration of the signal outputted by the receiving transducer is achieved by considering the system as a whole and not just simply minimising the duration of the transmitter's outputted pressure only.

Referring to Figure 4a, a damping sequence of electrical pulses is shown, which involves the transmitting transducer being excited by three electrical pulses namely, -9 -one excitation pulse (labelled "3a") and two damping pulses (labelled "3b" and "3c"). As will be explained in more detail hereinafter, there may be only one damping pulse and/or the pulses need not be temporally separate.

The receiver is damped by mechanical damping and/or electrical damping (through the electrical components to which the receiver is attached). The technique does not distinguish between these mechanisms. The sequence of pressure waves that are incident on the receiver actively damp the response of the receiver after it has been initially excited by the first incident pressure waves.

The outputted far-field pressure from the transmitter is used to damp the resonant vibrations in the receiver. In this example, damping is achieved by creating two distinct pulses of pressure which, when incident on a receiving narrowband transducer, excite and subsequently damp the resonant ringing.

As shown in Figure 4a, the first electrical pulse consists of five cycles of a sine wave having a duration of (124.7±0.1) tI5. The second electrical pulse consists of six cycles of a sine wave also having a duration of (149.6±0.1) Its starting (11.8±0.1) Rs after the end of the first pulse. The second pulse has a larger amplitude than the first. The third electrical pulse consists of three cycles of a sine wave having a duration of (74.8±0.1) us starting (11.7+0.1) Rs after the end of the second pulse. The third pulse has a smaller amplitude than the second pulse.

As shown in Figure 4b, the far-field pressure pulse generated by just the first pulse is or longer than the response generated using the three pulses. It is noted that the pressure wave generated using the three pulses has a duration of (417.8±0.1) is. Thus, the pressure response is longer than that generated using just two pulses (which is (282.5±0.1) Rs in Figure 3b).

Even though the pressure pulse is longer than that in the first comparative example, the electrical response of the receiver is shorter.

As shown in Figure 4c, for the first electrical pulse only, the electrical response has a duration of (2322.7±0.1) Rs. For the three electrical pulses, the electrical response has a duration of (446.3±0.1) gs. In the first comparative example, the electrical response -10 -has a duration of (836.0±0.1) ris. It is clear that the electrical response resulting from the three pulses exhibits shorter ringdown.

Figures 5a to 5f illustrate the effect of each of the three electrical pulses.

Figures 5a, 5b and 5c are plots of far field pressure generated by the transmitting transducer when excited by the first electrical pulse, for the first and second pulses and for all three pulses, respectively. Figures 5d, 5e and 5f are plots of electrical response generated by the receiving transducer resulting from the transmitter being excited by jo the first pulse, the first and second pulses and all three pulses, respectively. Thus, the plots show the effect of adding one additional electrical pulse at a time.

After the system is initially driven by the first electrical pulse, a second pulse is used, the effect of which is to optimally damp the vibrations of the receiver. This second electrical pulse is higher in amplitude and cycle number than the initial electrical pulse, with the effect of bringing the oscillations of the transmitter to a halt and immediately start driving it in anti-phase with the vibrations of the receiver. This produces an outputted pressure wave that damps the ringing of the receiver quickly, but then continues to drive it in the opposite direction as it responds to the long resonant ringing of the transmitter. The vibrations of the transmitter (in anti-phase with the receiver) are then damped with a final electrical damping pulse to avoid continuing to drive the receiver with its resonant ringing in response to the second electrical pulse.

This is just one example of a set of electrical pulses which can be used to damp the receiver. An arrangement of one, two, three or more damping pulses can be used. The number, duration, amplitude and separation of the pulses can be varied and can be found experimentally or by simulation.

System Referring to Figure 6, a system 1 for generating and detecting an ultrasonic or acoustic pressure wave (herein also referred to as a "sound wave" or "pressure wave") is shown.

The system 1 uses a transmit waveform 3 (which maybe stored or generated on-the-fly) to generate a time-varying drive signal 4 (or "excitation signal") which includes two or more sections 5, 6 (or "features"), for example, in the form of pulses each comprising one or more cycles of a give wave shape (e.g., a sine wave) at a respective given frequency. The time-varying drive signal 4 is supplied to a transmit ultrasonic transducer 7 which generates a pressure wave 8. The pressure wave 8 is received by a receive transducer 11 which generates an electrical response 12. In some cases, the same transducer is used for the generation and detection of the ultrasonic pressure wave 2. In other cases, different transducers are used.

The transmit transducer 7 is driven by an electronic circuit 15 (or "driver") which receives the transmit waveform 3 and generates an excitation signal 4. The receive transducer n converts incident sound waves 8 into the response signal 12 which is fed jo into an amplifier 16 and outputs an amplified signal 17.

The system 1 may include a controller 18 which may take the form of microcontroller which controls the driver 15 and processes the amplified signal 17. The controller 18 includes (or is provided with) suitable input/output circuits (not shown) including, for example, an analogue-to-digital converter for digitising the amplified signal 17. The controller 18 may be controlled or communicate with a computer system 19. in some arrangements, the controller 18 may be omitted and the computer system 19 may provide the functions of the controller 18.

The transmit waveform 3, which may be stored in memory zo in the controller 18 or computer system 19, is used to control the characteristics of the transmitted pressure waves 2, such as signal envelope, carrier signal phase (which may vary within a pulse waveform) and duration.

or Rather than trying to make the duration of the generated ultrasonic pressure wave 8 as short as possible, the aim instead is to generate an ultrasonic wave 8 which temporally shortens the electrical response of the detecting transducer 11. In particular, the generating transducer 7 is activated by excitation signal 4 that is not necessarily designed to make the generating transducer 7 vibrate for a shorter time (e.g., the shortest time possible), but rather to produce a pressure wave 8 which makes the detecting transducer n vibrate for a shorter time (e.g., the shortest time possible).

Referring also to Figure 7, a first configuration 211 (herein referred to an "experimental set-up") comprising two 40 kHz ultrasonic transducers 7, 11 is shown.

-12 -In the experimental set-up, the generating transducer 7 transmits pressure waves 8 in the form of ultrasonic pressure pulses towards the receiving transducer n which propagating through a fluid in the form of air. In this set-up, a controller 18 is not used. Instead, the driver 15 is controlled directly by the computer system 20 which generates suitable waveforms 4 for the driver 15. Likewise, the amplifier 16 feeds directly to the computer system 20.

Alternative configurations Referring again to Figure 7, in the experimental set-up 211, the generating transducer 7 jo transmits pressure waves 8 towards the receiving transducer 11 propagating through air without any (intended) reflection.

Figures 8 to 10 show second, third and fourth transducer configurations 212, 213" 214 which involve reflection of the ultrasonic waves 8. The system i (Figure 2) hereinbefore described may employ the second, third or fourth transducer configurations 212, 214, 214.

Referring to Figure 8, in the second configuration, the transducers 7, ii are orientated towards a sound-reflecting target 26. The sound waves 8 propagate through a fluid (i.e., a gas or a liquid).

Referring to Figure 9, in the third configuration, the transducers 7, ii (Figure 8) are replaced by a single transducer 7, ii orientated towards the sound-reflecting target 26.

Referring to Figure 10, in the fourth configuration, the single transducer 7, 11 is in contact with a region 28 of fluid or solid. The transducer 7, 11 launches soundwaves into the fluid or solid via a first interface 28 and waves are reflected from an inner rear interface 29.

System for synthesising transmit waveform(s) As explained hereinbefore, adding one or more damping feature, generally a temporally delayed or overlapping pulse which is non-coherent, to an excitation feature in a transmit waveform to produce a modified transmit waveform can shorten the mechanical and the electrical response of the detecting transducer.

A suitable modified transmit waveform can be found experimentally or by modelling.

-13 -Referring to Figure 11, a first waveform-synthesising system 31 is shown.

The system 31 includes a transmit and receive transducers 32,33 and corresponding 5 driver and amplifier 34, 35 having similar or the same responses to the transducers 7, ii, driver 15 and amplifier 16 which will be used. In some cases, the system 31 uses the transducers 7, ii, driver 15 and amplifier 16 which will be later used.

The system 31 includes a computer system 36 which includes at least one processor 38 jo and memory 37 which holds waveform-finding software 39, a desired response 40 (e.g., a maximum duration of response tx and/or a maximum duration of decay) and transmit waveform(s) 40.

Referring to Figure 12, a second waveform-synthesising system 31' is shown.

The second system 31' is similar to the first system 31 but differs in that instead of using transmit and receive transducers 32,33 and corresponding driver and amplifier 34, 35, a model 42 of these components are used.

The processor 37 executes the waveform-finding software 39 and carries out a process of finding a waveform.

Referring also to Figure 13, the processor 37 retrieve an initial transmit waveform 41 and perform a real or virtual measurement of the response of the receive transducer or (step Si & S2). The processor 37 determines whether the response meets the requirements, e.g., where the duration of the response is shorter than the maximum duration of response tx (step S3). If not, the processor 37 modifies the waveform (step 84) and repeats the process. Modification may include incrementally changing the parameters in a nested fashion, e.g., phase, amplitude, number of cycles etc., adding additional damping pulses. Once a suitable waveform has been found, it is stored (step S5).

Modifications It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of pulse generators for -14 -ultrasonic transducers and component parts thereof and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment.

Generation and detection of pulses is described using flexural ultrasonic transducers (the type of transducer typically used in car parking sensors). However, other types of ultrasonic transducer can be used Although claims have been formulated in this application to particular combinations of /o features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.

/5 The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

-15 -Claims 1. A pulse generator configured to output a drive signal for causing a transmitting transducer to generate an a pulsed sound wave having a first duration which, when received by a receiving transducer, causes the receiving transducer to generate an electrical response signal having a second duration, wherein the second duration is shorter than the first duration.
2. The pulse generator of claim 1, wherein the drive signal comprises at least two ro pulses including a first pulse having a start and an end and a second pulse and wherein the second pulse starts after the start of the first pulse.
3. The pulse generator of claim 2, wherein the second electrical pulse starts after the end of the first electrical pulse.
4. The pulse generator of claim 2 or 3, wherein the first and second pulses are out of phase.
5. The pulse generator of claim 4, wherein the first and second electrical pulses are in anti-phase.
6. The pulse generator of any one of claims 2 to 5, wherein the second electrical pulse is longer than the first pulse and/or has more cycles.or 7. The pulse generator of any one of claims 2 to 6, wherein the drive signal comprises at least three pulses.8. The pulse generator of claim 7, wherein the drive signal comprises a first pulse and a second pulse having respective starts and ends, and a third pulse, and wherein the third pulse starts after the start of the second pulse.9. The pulse generator of claim 8, wherein the third pulse starts after the end of the second pulse.to. The pulse generator of claim 8 or 9, wherein the second and third pulses are out of phase.-16 -The pulse generator of claim 10, wherein the second and third pulses are in anti-phase.12. The pulse generator of any one of claims 8 to 11, wherein the third pulse is shorter than the second pulse and/or has fewer cycles.13. The pulse generator of any one of claims 2 to 12, wherein the drive signal comprises at least four pulses.14. A system comprising: at least one transducer including a first transducer; the pulse generator of any one of claims ito 13 configured to drive the first transducer such that the first transducer is the transmitting transducer; and an amplifier arranged to receive a signal from the receiving transducer.15. The system of claim 14, wherein the first transducer is also the receiving transducer.16. The system of claim 14, wherein the at least one transducer includes a second, different transducer such that the second ultrasonic transducer is the receiving transducer.17. The system of claim 14, comprising: a controller; and a driver; wherein the pulse generator comprises the controller and driver.18. The system of claim 17, further comprising: a reflector arranged such that the transmitting ultrasonic directs the pulsed sound wave at the reflector and the reflector directs the pulsed sound wave to the receiving transducer.19. A method, comprising: providing a transmit waveform for generating a drive signal for a transmitting transducer; -17 -determining a response of a receiving transducer; determining whether the response meets a predefined requirement; upon a positive determination, storing the transmit waveform; and upon a negative determination, modifying the transmit waveform and determining a new response of the receive transducer; wherein the drive signal is for causing the transmitting ultrasonic transducer to generate a pulsed sound wave having a first duration which, when received by the receiving transducer, causes the receiving transducer to generate an electrical response signal having a second duration, wherein the second duration is shorter than the first /o duration.zo. A computer program which, when executed by a computer, performs the method of claim 19.