GB2058349A

GB2058349A - Method and Apparatus for Sensing the Distance of a Surface

Info

Publication number: GB2058349A
Application number: GB8025811A
Authority: GB
Original assignee: National Research Development Corp UK; National Research Development Corp of India
Current assignee: National Research Development Corp UK; National Research Development Corp of India
Priority date: 1979-08-22
Filing date: 1980-08-07
Publication date: 1981-04-08

Abstract

The distance of an irregular surface is sensed by illuminating it with a beam of acoustic radiation which has a variable polar pattern, sensing acoustic radiation reflected by the surface, comparing the generated and sensed radiation by a signal averaging technique such as cross correlation, and determining the transit time of the radiation. The acoustic radiation of variable polar pattern is generated by modulating an electromagnetic carrier wave to provide a pseudo random series of pulses, and applying the modulated carrier wave to an acoustic generator as an exciting signal, the generator polar pattern depending on the frequency spectrum of the signal.

Description

SPECIFICATION Method and Apparatus for Sensing the Distance of a Surface The position of a surface is often sensed by measuring the transit time of ultrasonic radiation reflected from the surface. Difficulties occur when the surface is not flat; e.g. when there is wave motion or surface turbulence in a liquid, or when a solid surface is irregular as in a rock bunker; in such circumstances, it is difficult to determine a true average surface level by conventional techniques. Another difficulty arises when the surface is surrounded by the walls of a vessel which may reflect pulses to the receiver and cause incorrect readings.

We have now discovered the basic cause of the incorrect readings and have developed apparatus which overcomes the problem. We have also developed a novel method of generating ultrasonic radiation in the required form.

In this specification, the term polar pattern refers to the locus of one end of a vector, the other end being centred on an acoustic transmitter, the length of the vector in any direction being proportional to the sound pressure emitted by the transducer in that direction. The term "acoustic radiation" refers to longitudinal vibrations at audible or ultrasonic frequencies.

According to the invention, apparatus for generating an acoustic radiation comprises generating means for generating an electromagnetic carrier wave; modulating means for modulating the carrier wave to provide a series of pulses of the carrier wave of different duration; and an acoustic generator to which the modulated radiation is applied as an exciting signal whereby a beam of acoustic radiation of varying polar pattern is emitted.

Preferably the modulation is such that a pseudo random binary sequence of pulses is produced. In one arrangement the carrier wave is amplitude modulated at 100% modulation depth so that acoustic radiation at one fundamental frequency is generated intermittently as a pseudo random sequence. In another arrangement the carrier wave is frequency modulated so that acoustic radiation is generated continuously at two alternate fundamental frequencies, the periods during which each frequency is generated each being a pseudo random sequence.

Also according to the invention, apparatus for sensing the distance of a surface comprises apparatus for generating acoustic radiation having a variable polar pattern; sensing means for sensing acoustic radiation reflected by the surface; and comparison means for comparing the generated and sensed radiation by a signal averaging technique and deriving information related to the transit time of the radiation between the generating and sensing means. The signal averaging technique may be, for example, cross correlation.

Preferably the sensing means has an output in response to received radiation which also varies in accordance with the polar pattern of the radiation.

Further according to the invention, a method of sensing the distance of a surface comprises generating a beam of acoustic radiation which has a variable polar pattern; sensing acoustic radiation reflected by the surface; comparing the generated and sensed radiation by a signal averaging technique; and deriving information related to the transit time of the radiation.

The invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates ultrasonic echo signals from material having a rough surface and contained in a vessel; Figure 2 illustrates two pulses of carrier waves of different pulse lengths; Figure 3 illustrates the frequency spectra of the two pulses; Figure 4 illustrates the polar diagrams of ultrasonic radiation at three frequencies excited by the two pulses; Figure 5 illustrates the unnormalised resultant polar diagrams; Figure 6 illustrates the normalised resultant polar diagrams; Figure 7 illustrates the time response of an ultrasonic receiver; Figure 8 illustrates three exciting pulses and Figure 9 illustrates the corresponding receiver outputs; Figure 10 illustrates the unnormalised polar diagrams of the receiver for the three pulses;; Figure 11 illustrates in block diagram form a level sensor according to the invention operating by amplitude modulation; and Figure 12 illustrates a sensor operating by frequency modulation.

In Figure 1, a vessel 10 contains large solid objects 12, such as rocks in a rock bunker. The general level of rocks is sensed by a system comprising an ultrasonic transmitter 14, an ultrasonic receiver 16, and suitable electronic circuitry 1 8 for sensing pulse transit time.

Ultrasonic radiation emitted by the transmitter 14 is indicated by the dotted lines. Radiation following path (a) (b) is a directly reflected beam and can be regarded as indicating the true level of the surface; however, if the beam (a) strikes a high point as illustrated, an incorrect value of surface height results. Radiation following path (a) (c) (d), i.e. reflected by the surface and by the wall of vessel 10, also gives an incorrect value as does radiation reflected directly by the vessel wall, path (e) (f).

To obtain a true reading of surface height, it is essential to reject spurious beams such as those reflected by the vessel walls, and also to ensure that the direct beam does not sense merely a local high point or low point. With prior art untrasonic level detectors, which used short pulses of radiation, it was not possible to eliminate these errors and such level meters were suitable for use only on flat surfaces, usually stationary froth-free liquids.

We have now discovered that the spurious signals described above are accepted as genuine signals when the ultrasonic radiation is generated at a fixed polar pattern. According to our invention therefore, radiation at a varying polar pattern is generated. We have further discovered that the polar pattern is dependent on the pulse length of the excitation carrier wave applied to the ultrasonic transmitter. We believe that this was not previously known; earlier studies were limited to very short ultrasonic pulses, and the aim was to maintain a narrow, constant beam width with minimum variation in pulse length.

Consider now an ultrasonic crystal excited by a carrier wave of frequency fe and suppose the carrier wave is applied as two pulses of three and six carrier cycles respectively, durations Ti and T2, as illustrated in Figure 2. The frequency spectra of the two pulses will be as illustrated in Figure 3; the spectral amplitude is greatest at the carrier frequency fct but there are also contributions at higher and lower frequencies. Contributions decrease to zero at frequencies T and 1 fe+7 T then increase to secondary maxima at still lower and higher frequencies.

The beam width of the ultrasonic transmitting crystal depends on the ratio of its diameter to the operating frequency; let the bandwidth be 2fw and consider the output of the crystal at the three frequencies f,--f,; f,; f,+f,; assuming the response of the crystal to be equal at all frequencies within the applied frequency spectrum.

It is known that the divergence of the output beam of ultrasonic radiation is smaller at high frequency than it is at low frequencies. For a crystal of constant diameter, the beam angle is inversely proportional to the frequency of oscillation. The polar patterns of the output beams at each of the three frequencies are shown in Figure 4. The carrier frequency is present in all three patterns, in the form of a narrow beam in a clearly defined general direction. At the lowest frequency, the polar pattern has little directional property and the beam angle is much greater.

Referring again to the frequency spectra in Figure 3, the contribution of the carrier frequency fc is proportionately greater in the long pulse than in the short pulse, and the spectrum of the long pulse is narrower than that of the short pulse. The relative sizes of the polar patterns at the three frequencies considered reflect the different proportionate contributions.

Other frequencies present in the exciting frequency spectrum also contribute, and the resultant unnormalised polar patterns corresponding to the two pulse lengths are shown in Figure 5. The total radiated pressure is greater in the longer pulse because it contains more energy; the pattern sizes reflect the difference.

However, the amplitude of the spectrum of the long pulse decreases more quickly with frequency than that of the short pulse, so that the ratio A(fc) A(fc+fas) where A is amplitude at a particular frequency, is larger for the short pulse than it is for the long pulse. The normalised polar diagrams are illustrated in Figure 6; that for the short pulse is more directional than that for the long pulse.

Consider now the effect of varying the length of successive exciting pulses applied to transmitter 14 in Figure 1. When a short pulse is applied a narrow beam is emitted; and when a long pulse is applied, a wide beam is emitted.

Thus the beam irradiates different areas of the surface with each pulse, with the great practical advantage that the effect of local high or low spots can be averaged out. If a pseudo random binary sequence of pulses is applied to the transmitter, a randomly scanning output beam is generated.

We have shown that changing the length of the exciting pulse changes the polar pattern and the beam angle of the emitted ultrasonic radiation. We have also discovered that the angle of the beam of radiation which can be sensed by an ultrasonic receiving crystal is also dependent on the pulse length. A typical response curve of a receiver is shown in Figure 7 in which pressure amplitude is plotted against time. The output increases from zero and after a period Ts reaches a steady state. If the receiver is excited with a burst of carrier frequency for a period less than Ts, the output of the receiver will not reach its steady state level. Figure 8 shows two exciting pulses of duration iess than Ts and one pulse longer than T9 and Figure 9 illustrates the corresponding transducer output signals. Clearly, output increases with the pulse length. If the detectable pressure level is a fraction of the steady level indicated by the line Pd in Figure 10, then the angle of the beam which can be sensed by the transducer will also depend on the pulse length.

Longer pulses produce greater acoustic pressures and therefore wider beams can be detected. The detectable beam angle is directly proportional to pulse width.

This discovery can be used to reinforce the change in the polar pattern of the output beam with varying exciting pulse length: in both effects, a short exciting pulse corresponds to a narrow transmitted or sensed beam and a long exciting pulse corresponds to a wide transmitted or sensed beam.

An ultrasonic level sensor according to the invention is illustrated in Figure 11. An ultrasonic transmitting crystal 1 4 is connected through an impedance matching circuit 20 and amplifier 22 to a modulator 24. The modulator is supplied with carrier wave frequency by a carrier wave generator 26. The lengths of the pulses of carrier waves are controlled by a pseudo random binary sequency (PRBS) generating circuit comprising a series of shift registers 28 supplied by a clock pulse generator 30: the registers are connected in a ring through an "EXOR" gate 32 and there is feedback from the third and the last shift register.

The PRBS generator is also connected to a cross correlating circuit 34.

The ultrasonic receiving crystal 1 6 is connected through an impedance matching circuit 36 and amplifier 38 to a demodulator 40, and through an automatic gain control amplifier 42 to the cross correlator 34.

In operation, the PRBS signals are supplied to the modulator 24 and the carrier wave is amplitude modulated with 100% modulation index to give a series of pulses of random width which are applied to the transmitter 14 which produces a corresponding series of acoustic pulses. The PRBS signals also form one input to the cross correlator, the signal derived from the receiver 16 providing the second input. The two signals are cross correlated by conventional techniques; the cross correlation function of the two signals Rxy (T) is given byt

where T is the transit time of the pulses between the transmitter and the receiver, t is the time and x and y are the amplitudes of the transmitted and received signals.The function has a peak value when t=T; the time at which this peak occurs is measured and from a knowledge of the velocity of sound the distance of the reflecting surface from the transmitter 14 and receiver 1 6 can be determined. Since a randomly varying area of surface is viewed as the polar patterns of the transmitter and receiver vary with pulse length, the effect of the local high or low spots is eliminated, forming only a low amplitude side peak in the correlation function and being rejected in favour of the true, high amplitude correlation peak.

The transit time may be displayed on an optical display device 44 either as a time or as a distance, or the correlation function may be displayed, or the calculated value may be supplied to automatic control equipment.

The carrier and clock frequencies will be chosen in accordance with the type of surface which is to be sensed and its distance. Typically a PRBS clock frequency of 1.45 kHz is used for a smooth surface such as stationary water at a distance of up to 50 metres, while 750 Hz would be suitable for rough water, or a solid surface having variations in height of five or six centimetres at a distance of up to five metres. For large rocks, even lower frequencies could be used.

Each pulse is arranged to be a whole number of wavelengths of the carrier frequency, where the whole number is usually between one and 250.

As distance increases, if frequency is kept constant the risk of spurious reflections increases.

Typically the angle of the output beam will vary between 80 and 360. The maximum and minimum angles are chosen, by selecting signal bandwidth, depending on the distance to be measured and the roughness of the surface and therefore the size of the area to be scanned. As would be expected, a wide signal bandwidth reduces the transmitted energy and the signal to noise ratio is poor. If bandwidth is narrow, the signal to noise ratio improves but the accuracy of measurement is degraded.

While the illustrated system uses separate transmitting and receiving crystals a combined transmitter/receiver may also be used with suitable electronic circuitry.

The apparatus illustrated schematically in Figure 11 provides a 100% amplitude modulated signal. In Figure 12, a slightly modified system is shown, identical parts being given the same reference numerals, which operates by frequency modulation. The PRBS generating circuit is connected through a modulator 46 to a frequency generator 48 capable of operating at one of two frequencies f and f+8f; the difference Af is sufficiently small for the ultrasonic transmitting crystal 14 to be capable of operating at either frequency. The modulator 46 switches the generator 48 from one frequency to the other in accordance with the PRBS signal; at each frequency, a PRBS of pulses is produced and the polar pattern of the output beam varies accordingly.Since the change in frequency Sf is small the variation of the polar pattern is smaller than in the amplitude modulation system, but advantages are that the crystal 14 is continuously excited so that transmission efficiency is increased and lower peak power signals can be used and also higher PRBS frequencies can be used (because they are not limited by the ringing characteristics of the transducer) which increases the accuracy and reduces measurement time.

In the Figure 1 2 arrangement, the demodulator 40 is arranged so that for a reflected echo signal at frequency f a binary "0" is produced and for a signal at frequency f+6f a binary "1" is produced.

The demodulated binary signal is then crosscorrelated with the output from the PRBS generator to derive the transit time.

In either an amplitude modulated or a frequency modulated system, as an alternative to the cross correlation circuit, other signal averaging systems could be used, such as that conventionally used in a progressively adding sonar system.

A level sensor according to the invention can be used to sense the surface of either a solid or a liquid; the liquid may have a smooth or a turbulent surface, and the sensor may be placed either above the liquid or below the surface at the bottom of the containing vessel.

Claims

1. Apparatus for generating acoustic radiation comprises generating means for generating an electromagnetic carrier wave; modulating means for modulating the carrier wave to provide a series of pulses of carrier wave radiation of different durations; and an acoustic generator to which the modulated carrier wave is applied as an exciting signal, whereby acoustic radiation of varying polar pattern is generated.

2. Apparatus according to Claim 1 in which the modulating means is arranged to provide a pseudo random binary sequence of pulses.

3. Apparatus according to Claim 1 or Claim 2 in which the carrier wave is amplitude modulated at 100% modulation depth so that acoustic radiation at one fundamental frequency is generated intermittently as a pseudo random sequence.

4. Apparatus according to Claim 1 or Claim 2 in which the carrier wave is frequency modulated so that acoustic radiation is generated continuously at two alternate fundamental frequencies, the periods during which each frequency is generated each being a pseudo random sequence.

5. Apparatus according to Claim 4 in which the acoustic generator is a piezoelectric crystal and the two frequencies are such that the crystal can generate an acoustic wave at either frequency.

6. A method of generating acoustic radiation comprises generating an electromagnetic carrier wave; modulating the carrier wave to provide a series of pulses of carrier wave radiation of different duration; and applying the series of pulses to an acoustic generator whereby acoustic radiation of varying polar pattern is generated.

7. Apparatus for sensing the distance of a surface comprises apparatus for generating acoustic radiation of varying polar pattern; sensing means for sensing acoustic radiation reflected by the surface; and comparison means for comparing the generated and sensed radiation by a signal averaging technique and deriving information related to the transit time of the radiation between the generating and sensing means.

8. Apparatus according to Claim 7 in which the signal averaging technique is cross correlation.

9. Apparatus according to Claim 7 or Claim 8 in which the sensing means has an output in response to the sensed radiation which varies in accordance with the polar pattern of the radiation.

10. Apparatus for sensing the distance of a surface comprises an electromagnetic carrier wave generator; a pseudo random binary sequence generating circuit arranged to amplitude modulate the carrier wave to provide a series of pulses of carrier wave of different durations; an ultrasonic transmitting device to which the series of pulses is applied as an intermittent exciting signal; an ultrasonic receiving device; and a cross-correlating circuit to which the pseudo random binary sequence generating circuit and the ultrasonic receiving device are connected.

11. Apparatus for sensing the distance of a surface comprises an electromagnetic carrier wave generator; a pseudo random binary sequence generating circuit arranged to switch the generator between operation at a first frequency and operation at a second frequency so as to provide at each frequency a series of pulses of the respective carrier waves of different durations; an ultrasonic transmitting device to which the two series of pulses are applied as a continuous exciting signal; an ultrasonic receiving device; and a cross-correlating circuit to which the pseudo random binary sequence generating circuit and the ultrasonic receiving device are connected.

12. A method of sensing the distance of a surface comprises generating acoustic radiation of varying polar pattern and directing it towards the surface; sensing acoustic radiation reflected by the surface; and comparing the generated and sensed radiation by a signal averaging technique and deriving information related to the transit time of the radiation between the generating and the sensing means.

1 3. Apparatus for sensing the distance of a surface substantially as hereinbefore described with reference to Figure 11 or to Figure 12 of the accompanying drawings.