GB2066466A - Ultrasonic Flow Measurement - Google Patents

Abstract

An ultrasonic measurement system for measuring the velocity or suspended solids concentration (for example) of a flowing fluid, comprises ultrasonic transmitting and receiving transducers 1, 2 on opposite sides of a flow pipe, an oscillator 4 driving the transmitting transducer and a phase locked loop 7-11 controlling the oscillator such that an integral number of half wave lengths exist in the ultrasound wave between the transducers. The system exhibits good stability. In the phase locked loop, the transmitted signal is passed through a 90 DEG phase shifter 8 to a phase comparator 7 for comparison with the receiving transducer output signal, so that the loop goes into lock when the transmitted and received signals are 0 or 180 DEG out of phase. <IMAGE>

Description

SPECIFICATION Ultrasonic Flow Measurement This invention relates to ultrasonic measurements of parameters of flowing fluids, for example flow velocity and suspended solids concentration.

The measurement of flow rate and suspended solids concentration using ultrasonic techniques has been under investigation since the mid 1 970's (see Ph.D. theses at the University of Bradford by J. Coulthard, 1973, C.N. Wormald, 1973, K.H. Ong, 1975, J.S. Battye, 1976, A.S.

Trivedi, 1 978 and K.G. Leach 1978). Such work has been largely successful but the stability of the measurement system has been poor.

In the simplest system, usable for ultrasonic flow measurement in slurries and sludges, an open-loop technique is used. Transmitting and receiving transducers are set up on diametrically opposite sides of the flow pipe and an oscillator drives the transmitting transducer at the resonant frequency of the "transmitter-flow mediumreceiver" system. A standing wave of ultrasonic energy develops across the pipe between the two transducers. Owing to turbulence the standing wave becomes amplitude modulated. The signal issuing from the receiver is fed to a demodulator to extract the flow information contained in the amplitude modulated carrier. A random flow noise signal is recovered by the demodulator and passed to an averaging or integrating voltmeter to give an indication of the suspended solids concentration.Alternatively, the demodulated output may be fed to an auto-correlator to display the mean square power of this signal. If a twochannel system is used, with two pairs of transducers spaced along the pipe and provided with respective demodulators, the outputs of the two demodulators may be fed to a real time cross-correlation to obtain an indication of the flow velocity within the pipe.

The major problem with an open loop system is to maintain the system within the required operating regime, once it has been set up initially.

One cause of instability can be directly attributed to the oscillator stability, i.e. frequency drift. As the oscillator frequency changes, the acoustic pathlength between the transmitter and the receiver will vary. The effect of this will be as if the receiver is moving along the standing wave pattern, and the acoustic pressure due to standing wave modulation will alter in amplitude and phase. When this situation occurs, the amplitude and phase of the total received signal will vary.

Variation in amplitude will produce erroneous results in the suspended solids concentration measurement, and the phase change may effect the correlability of the two signals in the twochannel system and result in a skewed or inverted correlation peak. This problem may be minimised using a high stability crystal oscillator, but as this cannot be adjusted over a wide range of frequencies, it is not always possible to adjust the operation frequencies of both channels to obtain good correlation.

Another cause of instability is due to other parametric variations, such as temperature, flow velocity and rheology of the flowing medium.

Chanyes in these alter the acoustic pathlength between the transmitter and the receiver, thus affecting the amplitude and phase of the received signal as discussed above. Although the crystal oscillator was used and improved the performance of the system, the long term stability remained poor for the second reason. Thus some form of feed back system was considered necessary to achieve optimal operating conditions.

In the recent work by J.S. Battye, a discrete component phase locking system was devised to control the oscillator frequency by controlling the phase between the transmitted and received signals. He postulated that the "skewness" of the cross-correlation peak was due to the nonlinearity of the demodulator and used a closed loop system, maintaining the demodulator at a set cross-over point. Further work carried out by us and by K.G. Leach and A.S. Trivedi has shown that the demodulator stability did not cause problems, instead the acoustic path length variation caused by the instability of the oscillator frequency and variation in standing wave pattern were the main causes.

In accordance with this invention, there is provided an ultrasonic measurement system for measuring a parameter of a flowing fluid, comprising ultrasonic transmitting and receiving tranducers and an oscillator for driving the ultrasonic transmitting transducer to transmit ultrasound across the flowing fluid to the receiving transducer, and a phase lock loop controlling the oscillator such that an integral number of half wave lengths exist in the ultrasound wave between the transducers.

An embodiment of this invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a block diagram of an ultrasonic measurement system in accordance with this invention; and Figure 2 is a circuit diagram of the system.

Referring to Figure 1, transmitting and receiving transducers 1, 2 are set up on diametrically opposite sides of a flow pipe 3. A voltage controlled oscillator 4 drives the transmitter through an R.F. power amplifier 5. The output of the receiver is connected through an R.F. amplifier 6 to an input of a phase comparator 7 in the phase lock loop. The other input of the phase comparator is provided from the amplifier 5 through a 900 phase shifter 8. The output of the phase comparator is taken through a low pass filter 9, a d.c. amplifier 10 and a limiter 11 to the control input of the oscillator 4.The elements 4, 7, 9, 10 and 11 making up the phase lock loop are provided by an integrated circuit type NE 562 B. The output of receiver amplifier 6 is taken to a signal processor for the measurement, comprising a high pass filter 12, R.F. amplifier 13, demodulator 14, low pass filter 1 5 and audio amplifier 16, the output of which may be taken to an averaging voltmeter or to a correlator.

In this system the phase of the transmitted signal is 900 phase shifted and compared with the phase of the received signal using the phasecomparator. In a conventional phase locked loop, the loop goes into lock when the transmitted and received signals are 900 out of phase. The additional 900 phase shifter 8 extends the locking capability of the loop such that it goes into lock when the transmitted and received signals are 0 or 1 800 out of phase. If the phase of the received signal changes with respect to the transmitted signal, then the output voltage of the comparator will be in the appropriate direction so as to drive the oscillator to correct for the error. Thus the feed back nature of the phase lock loop causes the oscillator to synchronise (or lock) with the incoming signal.Once in lock, the oscillator frequency is identical to the input signal except for a finite phase difference. Hence, any phase variation due to path length variation can be corrected for. In this circuit an integrated circuit form of phase lock loop (NE562B) is used which is specificaily designed for accepting the output of the oscillator, and is therefore not connected internally to the phase comparator as in other phase lock loop circuits.

Turbulent flow in the pipe modulates the ultrasound in amplitude and frequency. Amplitude modulation is removed by passing the received signal through the limiter 11, but frequency modulation cannot be removed. However, lowpass filter 9 of 1 Hz cut-off frequency was designed to ensure efficient locking of the PLL for any slight changes in flow medium parameters as well as avoiding loss of flow noise information.

Flow noise signals generally have a bandwidth of about 300 Hz. When the loop is in lock and when small changes occur in the medium some variations would occur at the lower end of the frequency spectrum, thereby causing the loop to unlock. The low-pass filter incorporated will not permit the transients to pass but will pass signals with longer time constants. If the frequency of the error voltage from the phase comparator falls outside the band edge of the filter, no information is transmitted around the loop and oscillator remains at its free running frequency, but if the error voltage frequency falls within the band of the low-pass filter, the locking operation will occur.

Figure 2 shows the circuit diagram of the PLL, closed loop system. The frequency of the oscillator is determined by whichever of the two capacitors shown is connected between terminals (5) and (6) of the integrated circuit NE562B. To increase the system bandwidth two resistors of 10 kO are connected to ground across the capacitor. The low-pass filter cut-off frequency, of 1 Hz is defined by the 13.3 ,uF capacitor and 56 ohm resistor between terminals (13) and (14). In this NE562B PLL, a buffer ampiifier is connected to the oscillator to provide differential square wave output at terminals (3) and (4). These outputs are emitter follower and have no internal load resistance, therefore the external 12 kQ resistors are connected as shown.It is essential that the resistance from each pin to aground be equal in order to maintain output waveform symmetrical and to achieve a minimum frequency drift. The phase comparator inputs, terminals (2) and (12) are biassed by connecting 1 kQ resistor from each pin to the 8V bias available at pin (1).

Noise threshold in a PLL is always difficult to estimate, as it is a statistical quantity. Noise will show up in input signals as amplitude and phase modulation. However, amplitude modulation is eliminated by a limiting amplifier, before the phase detector, which is incorporated in the NE 562B. As the input signal to noise ratio increases, the phase jitter of input signal increases, and the possibility of unlocking due to instantaneous phase excursions will increase. It is impossible to acquire lock if the signal to noise ratio is less than 6 dB. If the modulation and transient phase errors are present, a better signal to noise ratio is required to achieve a constant phase difference of the transmitted and received signals. Thus the parameter fluctuations are compensated by changing the transmitter frequency.When the phase difference between the transmitted and received signals is 0 or 1 800, the equivalent acoustic path length is equal to nA/2, where n is an integer and A is the wavelength of the ultrasound. In this case the system should be acoustically matched. However, in practice due to the acoustic impedence mismatch between the medium and the transducer, a standing wave may exist between the transmitter and receiver.

Although a standing wave exists in the acoustic system, due to the fact that the acoustic pathlength is an integral number of halfwavelengths, the receiver will be situated in a position such that the acoustic pressure of the standing wave is a minimum. Hence, the signal received due to a standing wave modulation is minimised.

The phase locked loop locks its operating frequency when the phase relationship is satisfied. In this particular system, when the loop is locked the acoustic pathlength between the transmitter and receiver is equal to no/2, where n is an integer. Hence, by either adding or subtracting an integral number of half-a-wave between the transmitter and receiver, the phase condition would be still satisfied. Addition or subtraction of extra waves in the acoustic system can be achieved by shanging the operating frequency. If the operating frequency is 1,:: 1 06Hz and the distance between the transmitter and receiver is 28 mm, then in order to introduce an additional half-a-wave, a frequency shift of 26.3 kHz is required. For a pipe where the maximum velocity obtainable is 3 mS-l. the maximum frequency change which could occur due to any parametric change in the flowing medium is 2kHz.

Since the change in locking frequency (26.3kHz) is much greater than the frequency changes (2kHz) which can occur in the flow rhedium, the phase locked loop will track the disturbance without adding extra waves, such that the operating frequency changes by the corresponding amount keeping the loop in lock.

However, if the change in pathlength is greater than 0.75 mm, the phase locked loop will add an extra halfwave in the system instead of trying to keep within the original number of waves. In doing so, the operating frequency will change by 26.3 kHz and the loop goes into lock at a different position. When this happens, it would appear as if the receiver has moved to a new position along the standing wave, but the acoustic pressure would still remain constant at the minimum value.

For large random variations in the system, it is possible for the phase locked loop to drive the oscillator to operate at a frequency outside the bandwidth of the transducers. If this occurs, then the received signal drops to zero and the phase locked loop loses contro. At this point the PLL drives the oscillator back to its free running frequency. In so doing the operating frequency again falls within the transmission band of the transducer and the phase locked loop will lock at any frequency where the phase conditions are satisfied.

The performance of the PLL system was checked by studying the control of the loop during temperature changes in the medium. The fluid temperature was steadily increased over a period of time and the resulting changes in operating frequency noted. Temperature was increased slowly to ensure a uniform temperature profile in the acoustic field. A linear relationship was found between the temperature and change in frequency, but the loop remained in lock. The change in frequency was about 10 kHz, which is less than that required (26.3 kHz) to introduce half-a-wave, for a typical example of an environmental temperature change that may occur in a real situation. The time stability of the system in lock was observed over a period of 24 hours and the change in frequency encountered was less than +5 kHz and the loop remained in perfect lock while tracking the environmental changes.With this closed loop system, when single phase liquids (e.g. water) were used as test samples, no amplitude modulation was observed over a range of velocities from 0.2set to 2ms-'.

But when small quantities of second phase (e.g., sand) was introduced into the flowing stream, noticeable amplitude modulation was observed.

Hence, with this system it is possible to make the sensors insensitive to single phase liquid turbulent eddies. Using this system, a velocity calibration curve obtained for different solid/liquid suspensions was formed and a reproducibility of ~2% of the reading and an overall accuracy of +1% was achieved. A calibration curve was also obtained for suspended solids concentration in a constant velocity system.

The closed loop control system was found to entirely eliminate the difficulties encountered in the past and resulted in good system controllability. Another attractive feature is that it made the acoustic system highly sensitive to solids and relatively insensitive to liquid phase.

Hence, a solution is provided for the longstanding problem of non-availability of an instrument for on-line monitoring of the flow parameters in hostile environments.

Claims (7)

Claims

1. An ultrasonic measurement system for measuring a parameter of a flowing fluid, comprising ultrasonic transmitting and receiving transducers and an oscillator for driving the ultrasonic transmitting transducer to transmit ultrasound across the flowing fluid to the receiving transducer, and a phase lock loop controlling the oscillator such that an integral number of half wave lengths exist in the ultrasound wave between the transducers.

2. An ultrasonic measurement system as claimed in claim 1, in which a drive signal applied to the transmitting transducer and an output signal of the receiving transducer are both applied (one through a 900 phase shifter) to a phase comparator of the phase lock loop.

3. An ultrasonic measurement system as claimed in claim 1 or 2, in which a control signal for the oscillator is connected to the oscillator through a low pass filter and a limiter respectively to remove any amplitude and frequency modulation caused by turbulence in the fluid flow.

4. An ultrasonic measurement system as claimed in any preceding claim, in which the receiving transducer output is connected to a measurement channel incorporating a A.M.

demodulator.

5. An ultrasonic measurement system as claimed in any preceding claim; arranged to measure velocity of the flowing fluid.

6. An ultrasonic measurement system as claimed in any one of claims 1 to 4, arranged to measure suspended solids concentration in the flowing fluid.

7. An ultrasonic measurement system substantially as herein described with reference to the accompanying drawings.