GB2209218A - An ultrasonic fluid flow meter with anti-fraud means - Google Patents

Abstract

In a flow-meter in which the velocity of flow along a path 2 is sensed by determining the difference in the time of flight of ultra-sonic signals from transducers 4, 5 travelling upstream and downstream the flow path, means is provided for detecting if an external transmitter of ultra-sonic signals has been operated in the vicinity of the flow-meter. The flow-meter may, in conjunction with means for sensing the density of fluid, form part of a domestic gas meter, and the detecting means will give an indication if the meter has been tampered with. The detection may be by means of the transducers 4, 5 themselves or by providing a frequency offset in the forward and reverse transmissions for zero flow rate. A transmit and receive circuit is disclosed. <IMAGE>

Description

Flow-meter for sensing the velocity of flow of a fluid This invention relates to flow-meters for sensing the velocity of flow of a fluid.

Flow-meters are known in which velocity of flow along a flow path is determined by a pair of transducers obliquely arranged across the flow path. Each transducer can transmit and receive signals to and from the other, and the difference in the "time of flight of the ultrasonic signals upstream and downstream gives a measure of the velocity of flow.

This invention provides a flow-meter for sensing the velocity of flow of a fluid, comprising means defining a flow path for the fluid, transducer means for transmitting and receiving ultrasonic signals along the flow path in both directions, and means for detecting if an external transmitter of ultrasonic signals has been operated in the vicinity of the flow-meter.

Such detecting means provides an indication if any attempt has been made to interfere with the meter.

The transducer means themselves may be arranged to detect if an external transmitter is operated.

Alternatively, in an arrangement where the difference between the time of flight upstream and downstream is sensed by varying the frequency of the transmitted signal to obtain a constant phase difference, means may be provided for producing a frequency offset so that there is a finite difference frequency even if gas flow rate is zero: otherwise it might be possible for a powerful ultrasonic transmitter to produce a false reading of the same frequency for upstream and downstream flow even when gas flow is taking place.

The flow-meter may, in conjunction with a pressure sensor which gives a measure of the density of flow, form part of a domestic gas meter, and the security of such an appliance is of course important.

A flow-meter for a gas constructed in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a plan view of the flow-meter showing the shape of the flow path; Fig. 2 is a block diagram of an electric circuit for the velocity sensing transducers; Referring to figure 1, the flow-meter is formed in a metal block 1. A gas flow path 2 is defined by a slot 3 of rectangular cross-section which is machined into the top surface of the block and closed by a cover (not shown) which is bolted to the block.Transducers 4,5 capable of emitting as well as receiving ultra-sonic vibrations are mounted in passages 6,7 which communicate with the gas flow path, and signals derived from these transducers give a measure of the velocity of gas flow along the path 2 as well as a measure of the velocity of the acoustic waves in the gas.

The flow path 2 includes a region 2a along which the ultra-sonic waves travel axially. In the interests of keeping the overall length of the block 1 to a minimum, the passages 6,7 housing the velocity measuring transducers 4,5 are orientated at right angles to the region 2a, and the ultra-sonic waves are reflected by sur-faces inclined at '450 between the region 2a and the passages 6,7.

The reflecting surfaces are formed by the walls of the regions 2b, 2c, the axes of which are inclined at 450 to the axis of the region 2a. The inclined regions 2b, 2c thus provide the necessary deflection of the ultra-sonic waves, but also do not provide any substantial impedence to gas flow. Short regions 2d,2e parallel to the region 2a form the inlet and outlet respectively of the flow meter.

The transducers 4,5, which are of identical form, emit and receive ultra-sonic waves in a cone-shaped region extending from the transducers. To prevent there being different possible path lengths between the transducers, a guide 10 for axial ultra-sonic waves only is positioned in the region 2a to suppress off-axis modes. The guide consists of an insert which has closely packed

exagonal channels extending throughout its length. The region 2a is lined with acoustic absorbing foam pads 11;12 at each end of the guide to suppress multiple reflections.

The transducers are supported in the passages 6,7 in plugs 13,14 of silicone rubber which block off the passages 6,7 and at the same time isolate the transducers from vibration.

The transducers 4,5 sense the velocity of gas flowing along the path 2 because a signal transmitted by one transducer and received by the other takes less time when it travels in the direction of gas flow than when it travels against the direction of gas flow, since the ultra-sonic signals are of course propogated in the gas itself.

The time of flight between transmitter and receiver can be conveniently measured in frequency form by coupling the transmitter and receiver together in a "sing-around" system, where the oscillation frequency stabilises at two values which maintain a fixed phase shift between transmitted and received signals. Thus, the respective times of propogation from transducer 4 to transducer 5 and vice-versa are measured by varying for each direction of propogation the driving frequency until exactly the same number of wavelengths, not necessary an intregal number, exists between the transmitting transducer and the receiving transducer, and this is done by producing a predetermined phase difference, for example, zero, between the transmitted signal and the received signal.The difference between these two frequencies is proportional to the velocity of gas along the flow path and the sum of these two frequencies is proportional to the velocity of the ultra-sonic waves in the gas.

Thus, if one assumes that the component of path along the flow path axis between the transmitter and the receiver is L and the acoustic velocity in the gas is C, with a mean flow velocity V averaged over the flow path diameter, the time taken for sound at frequency fl to propogate between the transducers in the flow direction is t1, where t = L 1 C + V If the sing-around frequency is fl the phase shift along the acoustic path is proportional to the product fl tl which is maintained at a fixed value N by the singaround system.

Thus, f1 = N = N (C + V) tl L If the transducers are now interchanged so that sound propogates against flow, sing-around frequency f2 is given by f2= N(C - V) 7 The difference in frequencies f1 - f2 is therefore 2N =fl f2 = 2N V 1 =l - 2= L The sum of the frequencies is fo = fl + f2 = 2NC 7 The difference in frequency therefore gives a measure of the flow velocity and the sum gives a measure of the acoustic velocity in the gas. The difference frequency therefore provides a measure of volume flow rate which is independent of the

of the gas.

The differencing technique allows a wide range of flow velocities to be covered without making impossible demands on the stability of operating conditions to achieve the necessary accuracy at low velocity at the low velocity end of the range.

Referring to figure 2, electronic analogue gate cross-over switch 15 switches over every t a second between states connecting contacts 15a, 15b and 16a, 16b together. In the first position, the transducer 5 is driven and the transducer 4 receives. The signal is fed to a phase detector 17 via an operational amplifier 18 which amplifies the signal until it is comparable with the signal fed to the transducer 5 which is directly fed back to the phase detector. The output of the phase detector 17 controls a voltage controlled oscillator 19 which varies the frequency of its output until there is a predetermined phase difference, for example, 00, between the two inputs to the phase detector.The same procedure happens when the contacts 16a,16b are connected together and a d}fEc-rcnec frequency corresponding to the fixed phase difference for propogation with the gas flow is locked onto.

The frequency corresponding to zero flow is chosen in relation to the distance between the transmitter and receiver to ensure that the phase differences introduced when alternating between the two states are sufficiently small that the phase locked loop locks onto the same

number of wavelengths between transmitter and receiver kin each case and cannot lock onto a larger or smaller number of wavelengths.

In order to subtract one frequency from the other, a counter 20 counts up from zero during one 2 second period and then, during the second 1 second period, counts down from the number reached. The difference between the two counts is proportional to the difference between the frequencies. To obtain a more accurate count, the output frequency of the voltage controlled oscillator is multiplied by a factor of 16 for feeding to the counter, to achieve sufficient velocity resolution at low flow rates, and then divided by a factor of 16 for feeding to the transducers and the phase detector.

As an example of suitable dimensions and operating frequencies, the length of the section 2a may be approximately 80mm and the cross-section of the passage may be approximately 20mm by 20mm. The transducers 4,5 may each consist of an 8mm diameter piezo-electric ceramic disc with a small aluminium cone attached to the centre to act as a radiator. A few volts applied to the transducers at the resonance frequency generates sound pressure levels of 100dB a few centimetres from the transmitting aperture.

When used as receivers, the transducers generate output voltage at the order of 0.1 to 1 volt when placed a few centimetres away from the transmitter. The transducers have resonance bandwidths centred on 40kHz of approximately 2kHz. With this example, four complete wavelengths can be accommodated between transmitter and receiver in each direction of propogation of the ultrasonic waves. A flow rate of approximately 3 metres/second would on this basis produce a difference frequency of approximately 800Hz.

Variations are of course possible. Thus, the section 2a could be from 60mm to 100mm long, the flow path diameter could be from lOmm to 40mm, the centre frequency of the transducer 4,5 could be from 30kHz to 100kHz, and the response curve against frequency for amplitude and phase should be as flat as possible.

Equally, the regions 2b,2c could lie at different angles to region 2a, passages 6,7 being appropriately reorientated to receive the reflected signals. If desired, the passages 2b,2c could be at right angles to passage 2a, and the transducers could be recessed with the walls of 2b,2c, with the transducers aligned along the axis of 2a.

Alternatively, the transducers could be both arranged in the section 2a, the line joining them obliquely crossing the section 2a. Any other variation of time of flight ultrasonic velocity sensing is of course possible.

As an alternative to piezo-electric materials, multiple layer PVDF could be used, or bulk ceramic transducers using acoustic matching layers could be used.

It will be apparent that the difference frequency will be zero for zero gas flow down the flow path.

However, in principle, the same effect could be achieved by placing a powerful ultra-sonic source against the meter. This could lock the operating fequencies of the device to the external source frequency. The difference frequency output in the two directions would then be zero and the meter would read zero irrespective of actual flow rate.

To prevent a possibility of such fraud, a small frequency offset may be provided between the forward and reverse states so that a non-zero difference frequency is generated at zero flow. For example, a phase shift device 32 operated during one set of alternate measurement periods only would provide such a frequency offset. An indication of this, indicating tampering with the equipment, could be provided within the meter.

Alternatively, the transducers 4 and 5 could be arranged to detect any externally applied ultra-sonic source, for example, the alternate operation of the transducers described above could be interrupted periodically to sense for such signals.

The velocity flow could, in conjunction with pressure sensing means which gives a measure of the density, form part of a mass flow-meter, in particular a gas meter, an indication that the meter had been tampered with would be particularly advantageous.

Co-pending application no.

is directed to a mass flow-meter which may include the anti-fraud device described herein, no.

is directed to a velocity sensor shown in Figures 1 and 2, and no.

is directed to a pressure sensor.

Claims (4)

1. A flow meter for sensing the velocity of flow of a fluid, comprising means defining a flow path for the fluid, transducer means for transmitting and receiving ultra-sonic signals along the flow path in both directions, and means for detecting if an external transmitter of ultra-sonic signals has been operated in the vicinity of the flow meter.

2. A flow meter as claimed in claim 1, in which the transducer means detects if an external transmitter is operated.

3. A flow meter as claimed in claim 1, in which a pair of transducers is arranged so that the signal transmitted by each will be received by the other after travelling in opposite directions along the flow path, each transducer being arranged to transmit alternately, means for varying the frequency of the transmitted signals so that the phase relation between the transmitted signal and the received signal remains the same with the same number of wavelengths between transmitter and receiver, means for obtaining a measure of the difference between the frequencies corresponding to flow along and counter to the flow path to obtain a measure of the flow velocity, and means to produce a frequency offset so that there is a finite difference frequency even if the gas flow rate is zero.

4. A flow meter as claimed in claim 3, in which a phase detector detects a phase relation between the transmitted and received signals, and means providing a phase shift is provided for connection to the phase detector for one direction of transmission between the transducers only.