WO1996008700A1

WO1996008700A1 - Method for measuring the vibrations of a test object

Info

Publication number: WO1996008700A1
Application number: PCT/SE1995/001018
Authority: WO
Inventors: Hans Wilhelm Persson
Original assignee: Hans Wilhelm Persson
Priority date: 1994-09-12
Filing date: 1995-09-11
Publication date: 1996-03-21
Also published as: SE9403028D0; SE502506C2; SE9403028L; AU3536695A

Abstract

In a method for measuring the vibrations of a test object, an ultrasonic measuring signal having a frequency that is much higher than the vibration frequency of the test object (3) is transmitted from an ultrasonic transmitter (2) towards the vibrating test object. The ultrasonic signal reflected from the test object is detected by an ultrasonic receiver (4) and is frequency-demodulated, and the frequency-demodulated signal is frequency-analysed with a view to determining the vibration frequency and/or the maximum vibration velocity of the test object. Before the vibration measurement, the absence of signals interfering with the ultrasonic signal reflected from the test object is ensured by transmitting from the ultrasonic transmitter (2) an ultrasonic burst signal having the same frequency as the ultrasonic measuring signal. Any signals received by the ultrasonic receiver (4) outside a predetermined time interval for the reception of the ultrasonic burst signal reflected against the test object (3) are essentially eliminated by altering the direction and/or distance of the ultrasonic transmitter and/or the ultrasonic receiver in relation to the test object.

Description

METHOD FOR MEASURING THE VIBRATIONS OF A TEST OBJECT

This invention relates to a method for measuring the vibrations of a test object, wherein an ultrasonic mea¬ suring signal having a frequency that is much higher than the vibration frequency of the test object is transmitted from an ultrasonic transmitter towards the vibrating test object, the ultrasonic signal reflected from the test object is detected by an ultrasonic receiver and is fre¬ quency-demodulated, and the frequency-demodulated signal is frequency-analysed with a view to determining the vibration frequency and/or the maximum vibration velocity of the test object.

This prior-art measuring method is advantageous by being contact-free, which is an absolute necessity when measuring the vibrations of light-weight objects. This method further enables a high measuring accuracy, pro¬ vided that the vibration frequency of the test object is much lower than the frequency of the measuring signal. Also, a high ultrasonic frequency enables narrow ultra¬ sonic beams and, hence, small and well-defined measuring points on the test object.

Despite the advantages of the above measuring method, use of this method has not spread to the extent that might have been expected, probably since the repro- ducibility of the measurement results has been completely unsatisfactory.

The object of the present invention is, therefore, to make the above method for vibration measurement bet¬ ter suited for use by ensuring reproducible measurement results. According to the invention, this object is achieved by a method which is characterised in that, before the vibration measurement, the absence of signals interfering with the ultrasonic signal reflected from the test object is ensured by transmitting from the ultrasonic transmit- ter an ultrasonic burst signal having the same frequency as the ultrasonic measuring signal and by substantially eliminating, through an alteration of the direction and/ or distance of the ultrasonic transmitter and/or the ultrasonic receiver in relation to the test object, any signals received by the ultrasonic receiver outside a predetermined time interval for the reception of the ultrasonic burst signal reflected against the test object.

The invention is thus based on the insight that the poor reproducibility of the measurement results, which may vary, say, something like ± 30%, is among other things due to repeated reflections of the ultrasound between the ultrasonic transmitter and the ultrasonic receiver or reflections against objects located essen- tially somewhere between the transmitter/receiver and the test object.

According to the invention, the absence of inter¬ fering signals can be ensured before each measurement of the vibrations of the test object. This is of special importance when the conditions round the test object may change between successive measurements.

The measurement results are especially adversely affected when the ultrasonic measuring signal can bounce back and forth between the ultrasonic transmitter and the ultrasonic receiver. According to the invention, this is conveniently prevented by so directing the ultrasonic transmitter and the ultrasonic receiver that the direc¬ tion of the ultrasonic receiver does not coincide with the direction of the mirror reflection of the ultrasonic measuring signal in the test object.

The invention will now be described in more detail with reference to the accompanying drawings, in which

Fig. 1 is a block diagram showing the arrangement of the equipment used for performing a vibration measurement according to the invention,

Fig. 2 shows the frequency spectrum of an output signal from the equipment in Fig. 1, and Figs 3 and 4 show a reflected measuring signal in, respectively, the presence and the absence of an inter¬ fering signal.

Fig. 1 illustrates the principle for the measurement method according to the invention. A signal generator 1 operates an ultrasonic transmitter 2, which transmits ultrasound in the direction of a vibrating measuring point, for instance on the vibrating surface of a test object 3. When reflected against the vibrating surface, the transmitted ultrasonic signal is frequency-modulated as a result of the Doppler effect. By an ultrasonic receiver 4, the reflected ultrasonic signal is detected and converted to an electric signal. After suitable amplification in an amplifier 5, the signal is demodu- lated in a detector 6 in the form of an FM demodulator. The demodulated signal from the FM demodulator 6 is proportional to the vibration velocity of the measuring point. However, the signal is also dependent on a few other parameters related to the ultrasonic signal. The transmitted ultrasonic signal u_c may be rendered as follows

u_c = U_csin(ω_ct) (1)

wherein U_c is the peak value of the ultrasonic signal, and ω_c = 2πf_c, i.e. the angular frequency. Likewise, the vibration velocity v_x of the measuring point is rendered as follows

v₁ = V₁cos(ω₁t) (2)

wherein V₁ is the peak value of the vibration velocity, and ω₁/2π = f_: is the vibration frequency. If the propa¬ gation velocity of the ultrasound is designated c and the angle of incidence of the ultrasound is designated α, the resulting Doppler shift Δf upon reflection can be render¬ ed as Δf = 2f_c V₁cos(α)/c (3)

if V <<c On the basis of the equation (1), the received, Doppler-shifted ultrasonic signal u_Cl is

u_Cl = KU_csin[2π(f_c+Δf)t] (4)

wherein K depends on the reflection coefficient, the scattering, and the reception qualities of the ultrasonic detector. The equation (4) can be rewritten with the aid of the equations (2) and (3), resulting in

u_Cl = KU_csin[2π(f_c+2f_cV₁cos(ω₁t)cos(α)/c)t] (5)

i.e. a frequency-modulated signal.

The FM demodulation results in a signal v_xβ corre¬ sponding to Δf in the expression given above, i.e.

v_xβ = 2f_cV₁cos(ω₁t)cos(α)/c (6)

In other words, the output signal from the measuring system is a function of the transmitted ultrasonic fre¬ quency f_c, the ultrasonic velocity c, the angle of inci- dence α of the ultrasound, the maximum vibration velocity V_: of the measuring point, and the vibration frequency f . The signal can be analysed with the aid of a frequency analyser, providing information on the vibration fre¬ quency f ₁ and an amplitude V_xβ (see Fig. 2) . V_xβ can be rendered as follows

V_xβ = 2f_cV₁cos(α)/c (7)

from which the maximum vibration velocity V₁ at the mea- suring point can be calculated, since all the parameters of the equation are known, except V_x. This gives

V. = V₁βc/2f_ccos(α) (8) The instantaneous vibration velocity Vi at the mea¬ suring point being now known as regards both amplitude and frequency, also the position x_x and the acceleration a₁ of the measuring point can be calculated, since

S v^t (9) and

With the vibration velocity

we have ^x _ι - (Vi/αjiJsinCci ) + constant (12) and a₁ = -(D^sinfc^t) (13)

i.e. division by ω_: and multiplication by -<o_{l r} respec¬ tively.

A high ultrasonic frequency is required to achieve good measuring accuracy as regards the frequency and the amplitude. This is so because the equation (3) presup¬ poses that the vibration frequency is much lower than the ultrasonic frequency.

From experience, we know, however, that the size of the amplitude V often varies.

The presence of interfering signals can be estab¬ lished by setting the signal generator 1 in burst mode in accordance with the invention, i.e. having the signal generator transmit the ultrasonic measuring signal during a short period which, at the most, is of the same order as the propagation time of the measuring signal from the ultrasonic transmitter to the ultrasonic receiver via the test object, and which preferably is but about a third of this propagation time, and analysing the signal received by the ultrasonic receiver.

For exemplifying purposes, Fig. 3 shows, as a func¬ tion of time, an ultrasonic measuring signal reflected against a vibrating test object and received by the ultrasonic receiver 4. The signal is shown before demo¬ dulation. A signal portion A represents the transmitted ultrasonic burst signal, as reflected once against the vibrating test object, whereas a signal portion B repre¬ sents the same measuring signal reflected twice against the vibrating test object after bouncing back from the ultrasonic transmitter. The presence of such interfering signals as the signal portion B represents normally results in an error in the amplitude v_xβ of the output signal from the detector 6.

In Fig. 3, the duration t₂-t_x of the burst period is about a third of the propagation time t_!-t₀ of the ultra¬ sonic signal from the transmitter 2 to the test object 3 and back to the receiver 4. If the burst period were to be prolonged, it might become difficult to discern the presence of interfering signals.

The presence of reflections from objects located essentially somewhere between the transmitter/receiver and the test object may result in interfering signals which, like the signal portion B, come after the signal portion A in time or come before the signal portion A in time. There may also occur such interfering signals as result in the signal portion A having a duration exceed- ing a predeterminable time interval.

In order to ensure correct and reproducible measure¬ ment results, such signals as may interfere with the ultrasonic signal reflected from the test object should, according to the invention, be essentially eliminated. According to the invention, this is achieved by the sig¬ nal generator having a measuring mode in which a conti¬ nuous ultrasonic signal is transmitted from the ultra¬ sonic transmitter 2, and a burst mode in which an ultra¬ sonic burst signal having the same frequency as the ultrasonic measuring signal is transmitted from the ultrasonic transmitter 2 towards the test object 3. It is detected whether the ultrasonic receiver 4 receives signals outside the predetermined time interval (for instance the interval t_!-t₂ in Fig. 3) for the reception of the ultrasonic burst signal reflected against the test object 3. If so, this or these interfering signals are eliminated by altering the direction and/or distance of the ultrasonic transmitter 2 and/or the ultrasonic receiver 4 in relation to the test object 3. This proce¬ dure is repeated until the signal received by the ultra¬ sonic receiver 4 essentially presents only the signal portion A according to Fig. 4 and this signal portion A further falls within the predetermined time interval for the reception of the ultrasonic burst signal reflected once against the test object.

Those skilled in the art easily realise how the ana- lysis of the ultrasonic burst signal received by the ultrasonic receiver 4 can be performed in order to meet the criteria laid down.

Depending on the stability of the measurement ar¬ rangement and the measurement conditions, the absence of interfering signals may have to be checked more or less often. In order to guarantee correct measurement results at all times, the absence of interfering signals can be ensured before each measurement of the vibrations of the test object. A particularly common interfering signal has proved to be the signal represented by the signal portion B in Fig. 3, i.e. an ultrasonic measuring signal bouncing several times between the ultrasonic transmitter 2 and the ultrasonic receiver 4. This signal may, for instance, be easily disposed of by not directing the ultrasonic transmitter 2 and the ultrasonic receiver 4 at the same angle to the normal to the surface of the test object 3 at the measuring point. Alternatively, the ultrasonic transmitter 2 and the ultrasonic receiver 4 can be offset in relation to each other in the plane of the test object 3 at the measuring point. Thus, the direction of the ultrasonic receiver 4 should not coincide with the direc- tion of the mirror reflection of the ultrasonic measuring signal in the test object 3.

Other signals interfering with the signal received by the ultrasonic receiver 4 can be disposed of by chang- ing the spacings of the test object 3 and, respectively, the ultrasonic transmitter 2 and the ultrasonic receiver 4. In particular, the amplitude of any interfering sig¬ nals can be reduced by increasing these spacings.

Also the frequency of the ultrasonic measuring sig- nal is of importance to the presence of interfering sig¬ nals. A high frequency of the ultrasonic measuring signal is advantageous already for the reason that it enables well-defined measuring points and the measurement of fairly high vibration frequencies. In addition, it is usually the case that the higher the frequency of an ultrasonic signal, the more rapid the attenuation of the signal. Conveniently, use is therefore made of ultrasonic measuring signals having a frequency of approximately 200 kHz or above, i.e. even about 1 MHz and above.

Claims

1. A method for measuring the vibrations of a test object, wherein an ultrasonic measuring signal having a frequency that is much higher than the vibration fre¬ quency of the test object (3) is transmitted from an ultrasonic transmitter (2) towards the vibrating test object, the ultrasonic signal reflected from the test object is detected by an ultrasonic receiver (4) and is frequency-demodulated, and the frequency-demodulated signal is frequency-analysed with a view to determining the vibration frequency and/or the maximum vibration velocity of the test object, c h a r a c t e r i s e d in that, before the vibration measurement, the absence of signals interfering with the ultrasonic signal reflected from the test object is ensured by transmitting from the ultrasonic transmitter (2) an ultrasonic burst signal having the same frequency as the ultrasonic measuring signal and by substantially eliminating, through an alte¬ ration of the direction and/or distance of the ultrasonic transmitter and/or the ultrasonic receiver in relation to the test object, any signals received by the ultrasonic receiver (4) outside a predetermined time interval for the reception of the ultrasonic burst signal reflected against the test object (3).

2. A method as set forth in claim 1, c h a r a c ¬ t e r i s e d in that the absence of interfering signals is ensured before each measurement of the vibrations of the test object (3).

3. A method as set forth in claim 1 or 2, c h a r ¬ a c t e r i s e d in that the ultrasonic transmitter (2) and the ultrasonic receiver (4) are so directed that the direction of the ultrasonic receiver does not coincide with the direction of the mirror reflection of the ultra¬ sonic measuring signal in the test object.

4. A method as set forth in any one of claims 1-3, c h a r a c t e r i s e d in that use is made of an ultrasonic measuring signal having a frequency exceeding approximately 200 kHz.