GB1587127A - Method and means for determining flow values from signals derived by ultrasonic observation - Google Patents

Description

(54) METHOD AND MEANS FOR DETERMINING FLOW VALUES FROM SIGNALS DERIVED BY ULTRASONIC OBSERVATION (71) We, BRITISH AEROSPACE, a Corporation established under the Aircraft and Shipbuilding Industries Act 1977, of Brooklands Road, Weybridge, Surrey, KT13 0SJ do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a method and means for determining the flow of a liquid in a duct and is concerned with displaying the peak velocity of fluid flow.

In ultrasonic observation of the flow of liquids containing scattering centres, using the Doppler effect, a signal is obtained which carries information in the form of a spectral power distribution. This will be referred to as the Doppler signal. The frequencies of such signals correspond linearly to fluid velocities, and the problem of determining the maximum instantaneous velocity is the same as the problem of determining the maximum or minimum instantaneous frequency.

According to the present invention, the Doppler signal is heterodyned with a variable frequency signal in such manner that the modified spectral density distribution has a substantially fixed upper and lower frequency.

The carrying of the invention into practice will now be discussed in more detail with reference to the accompanying drawings, in which: Figures 1, 2, 3 and 5 are explanatory plots of power spectral density against frequency, Figure 4 is a block diagram of an embodiment of apparatus for performing the invention, and Figure 6 is a block diagram of a modification of the apparatus of Figure 4.

The following description relates to finding the minimum frequency, since in a certain bi-directional blood flowmeter this corresponds to peak positive velocity (V). The extension of the method to maximum frequencies is obvious.

Consider Figure 1. The power density spectrum indicated there has the following features f, corresponds to V, the peak velocity f2 corresponds to VO, the minimum velocity The co-ordinate P represents power spectral density.

The basic concept according to this invention, as already stated, is that the whole of the Doppler Signal shall be heterodyned with a variable frequency (fh) signal in such a way that the modified spectral density distribution has a fixed upper frequency. In such a case the variations in frequency fh follow the variations in ft, and the signal causing the variations in fh carries the same information.

Consider Figure 2. This shows two power spectra. The upper frequency spectrum is identical in form with the spectrum in Figure 1, and the lower is its mirror image with respect to the frequency fh. They represent the signal obtained by modulation of the original signal with a signal of frequency fh.

Now consider Figure 3. The upper sideband has been filtered out, and the frequency f is encompassed by a narrow band filter of bandwidth Af and centred on ff. The presence or absence of output signals from such a filter may be used to drive fh up or down, so as to ensure that f off at all times. When this is achieved, fh = f + fl = ff + fl (1) i.e. fh is linearly related to fl, and therefore to V.

Now consider Figure 4, the block diagram of an embodiment of apparatus for carrying the invention into effect.

The variable frequency fh is produced by a voltage controlled oscillator (VCO), and applied with the input signal to a modulator (MOD) from which emerge the signals of Figure 2. A wide band filter (WBF) suppresses the upper sideband. Its outut now encounters the narrow band filter (NBF) of centre frequency ff, referred to in Figure 3. The presence or absence of a singla in the frequency interval Af corresponds to the presence or absence of an output from this filter, and this is determined by a preset threshold (THRES). Signals from the narrow band filter are rectified and smoothed and applied to the threshold circuit, an output from which indicates a certain minimum signal level in the band Af. Such a signal drives the trigger circuit (TRIG) output to a 1 state; absence of such a signal drives it to a 0 state, or vice-versa.

This binary output signal is applied to an integrator (INT), the output signal from which rises or falls accordingly, and in doing so causes fh, the output frequency from the VCO, to change up or down. By arranging that presence of a signal in Af reduces fh, we have a negative feedback loop maintaining the validity of equation 1. The signal driving the VCO is now linearly related to V.

The function of the threshold circuit is to exclude a pre-determined noise level (e.g. of thermal origin) from the decisions of the trigger circuit.

The system described above deals adequately with signals which have a wide spectral distribution. However, there are cases, e.g. in the case of flow of fluid in which all elements have nearly the same velocity, when the spectral distribution is narrow (see Figure 5). In such a case, it is possible for the frequency ff to be below fO, with the result that the system described would behave as if ff were above f (at f'f) since the trigger circuit output would be 0, i.e. it would behave as if to decrease the reported value of V, and this process would continue unchecked.

In order to avoid this contingency, the device is best used in conjunction with a circuit for determining average velocity V, such as described in our patent application 44830/75.

Since the peak instantaneous velocity can never be less than the average velocity, the system of Figure 4 can be modified (Figure 6) appropriately. The computed values of V and V are compared by a comparator 11, and if V exceeds V, a binary 1 is supplied to the logic circuit 12.

The truth table for this unit is A B C C 0 0 1 0 0 1 0 1 C=A + B 1 0 1 0 1 1 ~ 1 0 C or C may now be used to drive the integrator appropriately. For example, if a 1 in C is required to increase the reported value of V, the condition A = 0, B = 1 is the only one capable of diminishing this value.

WHAT WE CLAIM IS: 1. A method of determining the maximum instantaneous velocity of a liquid flowing in a duct, employing ultrasonic observation to obtain a Doppler signal in the form of a spectral power distribution, characterised in that the Doppler signal is heterodyned with a variable frequency signal in such manner that the modified spectral density distribution has a substantially fixed upper or lower frequency.

2. Apparatus for performing the method according to claim 1, comprising means for generating a signal of controllably variable frequency, a modulator receiving the Doppler signal and said variable frequency signal as inputs, filter means receiving the modulator output and passing only a narrow frequency band substantially at one frequency limit of a sideband. and means responsive to the output of said filter means to derive a feedback signal for controlling the frequency of said variable frequency signal to keep said narrow frequency band substantially at said frequency limit.

3. Apparatus according to claim 2, wherein the filter means comprises a wide band filter to remove one sideband, and a narrow band filter passing only said narrow frequency band which is substantially at one frequency limit of the other sideband.

4. Apparatus according to claim 2 or claim 3, wherein said means responsive to the output of the filter means comprises a threshold device followed by a trigger and an integrator.

5. Apparatus according to claim 4, wherein the filter means output is rectified before

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. fh = f + fl = ff + fl (1) i.e. fh is linearly related to fl, and therefore to V. Now consider Figure 4, the block diagram of an embodiment of apparatus for carrying the invention into effect. The variable frequency fh is produced by a voltage controlled oscillator (VCO), and applied with the input signal to a modulator (MOD) from which emerge the signals of Figure 2. A wide band filter (WBF) suppresses the upper sideband. Its outut now encounters the narrow band filter (NBF) of centre frequency ff, referred to in Figure 3. The presence or absence of a singla in the frequency interval Af corresponds to the presence or absence of an output from this filter, and this is determined by a preset threshold (THRES). Signals from the narrow band filter are rectified and smoothed and applied to the threshold circuit, an output from which indicates a certain minimum signal level in the band Af. Such a signal drives the trigger circuit (TRIG) output to a 1 state; absence of such a signal drives it to a 0 state, or vice-versa. This binary output signal is applied to an integrator (INT), the output signal from which rises or falls accordingly, and in doing so causes fh, the output frequency from the VCO, to change up or down. By arranging that presence of a signal in Af reduces fh, we have a negative feedback loop maintaining the validity of equation 1. The signal driving the VCO is now linearly related to V. The function of the threshold circuit is to exclude a pre-determined noise level (e.g. of thermal origin) from the decisions of the trigger circuit. The system described above deals adequately with signals which have a wide spectral distribution. However, there are cases, e.g. in the case of flow of fluid in which all elements have nearly the same velocity, when the spectral distribution is narrow (see Figure 5). In such a case, it is possible for the frequency ff to be below fO, with the result that the system described would behave as if ff were above f (at f'f) since the trigger circuit output would be 0, i.e. it would behave as if to decrease the reported value of V, and this process would continue unchecked. In order to avoid this contingency, the device is best used in conjunction with a circuit for determining average velocity V, such as described in our patent application 44830/75. Since the peak instantaneous velocity can never be less than the average velocity, the system of Figure 4 can be modified (Figure 6) appropriately. The computed values of V and V are compared by a comparator 11, and if V exceeds V, a binary 1 is supplied to the logic circuit 12. The truth table for this unit is A B C C 0 0 1 0 0 1 0 1 C=A + B 1 0 1 0 1 1 ~ 1 0 C or C may now be used to drive the integrator appropriately. For example, if a 1 in C is required to increase the reported value of V, the condition A = 0, B = 1 is the only one capable of diminishing this value. WHAT WE CLAIM IS:

1. A method of determining the maximum instantaneous velocity of a liquid flowing in a duct, employing ultrasonic observation to obtain a Doppler signal in the form of a spectral power distribution, characterised in that the Doppler signal is heterodyned with a variable frequency signal in such manner that the modified spectral density distribution has a substantially fixed upper or lower frequency.

2. Apparatus for performing the method according to claim 1, comprising means for generating a signal of controllably variable frequency, a modulator receiving the Doppler signal and said variable frequency signal as inputs, filter means receiving the modulator output and passing only a narrow frequency band substantially at one frequency limit of a sideband. and means responsive to the output of said filter means to derive a feedback signal for controlling the frequency of said variable frequency signal to keep said narrow frequency band substantially at said frequency limit.

3. Apparatus according to claim 2, wherein the filter means comprises a wide band filter to remove one sideband, and a narrow band filter passing only said narrow frequency band which is substantially at one frequency limit of the other sideband.

4. Apparatus according to claim 2 or claim 3, wherein said means responsive to the output of the filter means comprises a threshold device followed by a trigger and an integrator.

5. Apparatus according to claim 4, wherein the filter means output is rectified before

being applied to said threshold device.

6. Apparatus according to claim 4 or claim 5, wherein the variable frequency generating means comprises a voltage-controlled oscillator responsive to the output of the integrator.

7. Apparatus according to claim 4 or claim 5 or claim 6, wherein the trigger output is applied to a logic circuit the output of which is in turn applied to the integrator input, and the logic circuit receives a second input from a comparator which compares the integrator output signal, representing the minimum or maximum Doppler signal frequency, and a signal representing the average Doppler signal frequency.

8. Apparatus for performing the method of claim 1, and substantially as described with reference to Figure 4 or Figure 6 of the accompanying drawings.