Abstrict An ultrasonic flow meter (10) has a pair of transducers (36 38).
A first (36) of the pair of transducers is coupled to a first divider
(18) having a first predetermined divisor. A second (38) of the
pair of transducers is coupled to a second divider (24) having a
second predetermined divisor. The second predetermined divisor is
not equal to the first predetermined divisor. An input (16) of the
first divider (18) is coupled to a first oscillator (12). An input
(22) of the second divider (24) is coupled to a second oscillator
(14). A decoder circuit (50) is coupled to an output (15) of the
first oscillator (12) and an output (20) of the second oscillator
(14). The decoder circuit (50) determines a difference frequency.
Claims What is claimed is:
1. An ultrasonic flow meter comprising:
a pair of transducers;
a first divider having a first predetermined divisor coupled to
a first of the pair of transducers;
a second divider having a second predetermined divisor coupled
to a second of the pair of transducers, the second predetermined
divisor not being equal to the first predetermined divisor;
a first oscillator coupled to an input of the first divider;
a second oscillator coupled to an input of the second divider;
a decoder circuit coupled to an output of the first oscillator
and an output of the second oscillator, the decoder circuit determining
a difference frequency.
2. The ultrasonic flow meter of claim 1 wherein the decoder circuit
mixes the difference frequency with an output of the first divider
to form a flow data signal.
3. The ultrasonic flow meter of claim 2 wherein the flow data
signal has a flow frequency that is an integer multiple of a beat
frequency.
4. The ultrasonic flow meter of claim 1 further including a path
length divider having a variable divisor coupled between the first
of the pair of transducers and the first divider.
5. The ultrasonic flow meter of claim 1 wherein the first predetermined
divisor is equal to the second predetermined divisor plus one.
6. The ultrasonic flow meter of claim 1 wherein the first oscillator
has a first output frequency and the second oscillator has a second
output frequency and the first output frequency is never equal to
the second output frequency for a flow rate less than 80 feet per
second (24 meters per second).
7. The ultrasonic flow meter of claim 1 wherein the first oscillator
is a voltage controlled oscillator.
8. The ultrasonic flow meter of claim 1 wherein the first predetermined
divisor is less than eighteen.
9. An ultrasonic flow meter comprising:
a first voltage controlled oscillator;
a second voltage controlled oscillator;
a first divider having a first dividing factor coupled to the first
voltage controlled oscillator;
a second divider having a second dividing factor that is different
than the first dividing factor, the second divider coupled to the
second voltage controlled oscillator;
a first switch having a first terminal coupled to an output of
the first divider and a second terminal coupled to an output of
the second divider;
a path length multiplier connected to a third terminal of the first
switch;
a second switch having a forth terminal coupled to the output of
the path length multiplier;
a first transducer coupled to a fifth terminal of the second switch;
and
a second transducer coupled to a sixth terminal of the second switch.
10. The ultrasonic flow meter of claim 9 further including a decoding
circuit coupled to the first terminal and to the second terminal.
11. The ultrasonic flow meter of claim 10 wherein the decoding
circuit has a flow rate signal as an output.
12. The ultrasonic flow meter of claim 11 wherein the flow rate
signal has a flow frequency that is an integer multiple of a flow
velocity frequency.
13. The ultrasonic flow meter of claim 9 wherein the first voltage
controlled oscillator has a first frequency and the second voltage
controlled oscillator has a second frequency and the first frequency
is not equal to the second frequency for a flow rate up to 80 feet
per second (24 m/s).
14. A method of determining a flow rate comprising the steps of:
(a) adjusting an upstream oscillator signal so that an upstream
signal has an upstream wavelength equal to an upstream path length
between a pair of transducers;
(b) adjusting a downstream oscillator signal so that a downstream
signal has a downstream wavelength equal to a downstream path length
between the pair of transducers;
(c) multiplying the upstream oscillator signal by the down stream
oscillator signal to form a difference signal;
(d) multiplying the difference signal by the downstream signal
divided by a downstream multiplier to form a flow data signal having
a flow frequency;
(e) determining the flow rate by multiplying the flow frequency
by a predetermined factor.
15. The method of claim 14 wherein step (a) further includes the
steps of:
(a1) dividing the upstream oscillator signal by an upstream factor
to form an intermediate upstream signal; and
(a2) dividing the intermediate upstream signal by a path length
divider to form the upstream signal.
16. The method of claim 15 wherein step (b) further includes the
steps of:
(b1) dividing the downstream oscillator signal by a downstream
factor to form an intermediate downstream signal; and
(b2) dividing the intermediate downstream signal by the path length
divider to form the downstream signal.
17. The method of claim 16 wherein the downstream factor is selected
to not be equal to the upstream factor.
18. The method of claim 16 wherein the downstream factor is selected
to be equal to the upstream factor plus one.
19. The method of claim 16 wherein step (a1) further includes
selecting the upstream factor and the downstream factor so that
the upstream oscillator signal and the downstream oscillator signal
will never have the same frequency for the flow rate of zero to
a maximum flow rate design.
Description FIELD OF THE INVENTION
The present invention relates generally to the field of flow meters
and more specifically to an ultrasonic flow meter and method.
BACKGROUND OF THE INVENTION
Ultrasonic flow meters have many advantages over other methods
of determining flow rates. Ultrasonic flow meters can continuously
measure the flow rate, while other methods generally measure average
flow rates. In addition, ultrasonic flow meters are obstructionless
and work with non-conductive materials.
Ultrasonic flow meters have a pair of transducers that are placed
on either side of the flow path of a fluid flowing through a pipe.
The transducers are pointed at each other and the line between them
has a component in the direction of the fluid flow. The principle
used to detect flow rates is that the wavelength of an ultrasonic
packet will lengthen in the upstream path and shorten in the downstream
path. The amount by which the wavelength changes is directly proportional
to the flow rate. Ultrasonic flow meters have an upstream and a
downstream voltage controlled oscillator. The upstream oscillator's
frequency is adjusted until the upstream wavelength is equal to
a path length between the two transducers. Similarly, the downstream
oscillator's frequency is adjusted until the downstream wavelength
is equal to the path length between the two transducers. The output
of the upstream oscillator is then mixed with the output of the
downstream oscillator to form a beat signal. The beat signal has
a beat frequency that is proportional to two times the flow rate.
Unfortunately, voltage controlled oscillators tend to lock in frequency
when operated at similar frequencies. When the flow rate is zero
the upstream oscillator has the same frequency as the downstream
frequency. As a result it is common for the oscillators in ultrasonic
flow meters to track each other and give false results.
One solution has been to shield the oscillators so that they do
not see each other electrically. Unfortunately, it is expensive
and difficult to shield the oscillators from each others EMI (Electro-Magnetic
Interference).
Thus there exists a need for an ultrasonic flow meter that is accurate
and overcomes the frequency lock in phenomenon associated with voltage
controlled oscillators.
SUMMARY OF THE INVENTION
An ultrasonic flow meter that overcomes these and other problems
has a pair of transducers. A first of the pair of transducers is
coupled to a first divider having a first predetermined divisor.
A second of the pair of transducers is coupled to a second divider
having a second predetermined divisor. The second predetermined
divisor is not equal to the first predetermined divisor. An input
of the first divider is coupled to a first oscillator. An input
of the second divider is coupled to a second oscillator. A decoder
circuit is coupled to an output of the first oscillator and an output
of the second oscillator. The decoder circuit determines a difference
frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a pipe section used with an radial
ultrasonic flow meter;
FIG. 2 is a block diagram of an ultrasonic flow meter;
FIG. 3 is a timing diagram associated with the ultrasonic flow
meter of FIG. 2;
FIG. 4 is a schematic diagram of a decoder circuit used in the
ultrasonic flow meter of FIG. 2;
FIG. 5 is a schematic diagram of the ultrasonic flow meter of FIG.
2; and
FIG. 6 is a schematic diagram of an alternative embodiment of the
ultrasonic flow meter of FIG. 2.
DETAILED DESCRIPTION OF THE DRAWINGS
The invention uses a pair of unequal dividers to insure that the
two voltage controlled oscillators do not operate at the same frequency.
FIG. 1 is a schematic diagram of a pipe section for use with a radial
ultrasonic flow meter. The pipe 6 has a pair of cylindrical arms
7 that are open to the flowing fluid. A transducer 8 is inserted
into each of the cylindrical arms 7. The sonic path between the
transducers 8 has a component in the direction of fluid flow. The
transducers 8 are connected to an ultrasonic flow meter by a pair
of transmission lines 9. The invention works with both radial ultrasonic
flow meters, as shown if FIG. 1 and axial ultrasonic flow meters.
FIG. 2 is a block diagram of an ultrasonic flow meter 10. The flow
meter 10 has a first voltage controlled oscillator (downstream oscillator,
first oscillator) 12 and a second voltage controlled oscillator
14 (upstream oscillator, second oscillator). The downstream oscillator
12 has an output 15 coupled to an input 16 of a downstream divider
(first divider) 18. The upstream oscillator has an output 20 coupled
to an input 22 of an upstream divider (second divider, second dividing
factor) 24. An output 26 of the downstream divider 18 and an output
28 of the upstream divider 24 are coupled to a switch 30. The switch
30 alternately connects the upstream oscillator 14 and the downstream
oscillator 12 to a path length multiplier 32. The path length multiplier
32 has a variable divisor. The variable divisor is adjusted so that
a wavelength of the oscillator signal at the output 34 of the path
length multiplier 32 is approximately equal to a path length (L)
between a pair of transducers 36 38. The output 34 of the path
length multiplier 32 is connected to a switch 36. The switch 36
alternately connects the output 34 of the path length multiplier
32 to a first 36 of the pair of transducers or to a second 38 of
the pair of transducers. The transducer that is not connected to
the output 34 of the path length multiplier 32 is connected to a
phase comparator 39. The phase comparator 39 determines whether
the oscillator's wavelength is longer or shorter than the path length
L. A switch 40 connects the output 42 of the phase comparator 39
to the appropriate oscillator controller 46 48. Once the oscillators
are locked onto the correct frequency a decoding circuit 50 samples
the oscillator signals at various points and converts the oscillator
signals into flow data signal 52. A processor 54 controls all the
various switches 30 36 40 and sets the path length multiplier's
variable divider value. The processor 54 is also in communication
with the oscillator controllers 46 48. The processor 54 converts
the flow data signal into a flow rate.
FIG. 3 shows a timing diagram that explains how the flow rate meter
10 adjusts the oscillator signal's frequency. The output of one
of the oscillators is coupled by the switches 30 36 to the appropriate
transducer 36 38. If we assume that the fluid is flowing from transducer
36 to transducer 38 then the downstream oscillator 12 is coupled
to the transducer 36 and the transducer 38 is coupled to the phase
comparator 39. In this situation the output 42 of the phase comparator
39 is coupled to the controller 46. The downstream oscillator signal
15 after being divided down by the downstream divisor (downstream
dividing factor, first dividing factor) 18 and the path length multiplier
32 is shown as the first trace 80 of FIG. 3. The output of the transmitting
transducer 36 is shown as the second trace 82. The transmitting
transducer 36 sends out a very short pulse 84 when it detects an
initial rising pulse 86 from the oscillator. The receiving transducer
38 detects the transmitted sonic pulse approximately one cycle after
the pulse was transmitted. The received pulse 88 is compared to
the next rising edge 89 of the trace 80 in the phase comparator
39. When the phase comparator 39 detects that the received pulse
88 arrived before the next rising edge 89 it sends the early message
to the controller 46. The controller 46 increases the frequency
(shortens the wavelength) of the oscillator, in response to an early
message from the phase comparator. This causes the received signal
88 to eventually align with the second rising edge 89. In this way
the oscillator signal's frequency is adjusted so that the wavelength
at the transducers is equal to the path length L. This process is
alternately performed for the upstream oscillator 14 and the downstream
oscillator 12 by setting the switches 30 36 40.
FIG. 4 shows a schematic diagram of the decoding circuit 50. The
decoding circuit (decoder circuit) 50 mixes the downstream oscillator
signal (first frequency) 15 with the upstream oscillator signal
(second frequency) 20 in a mixer 100. The output (difference signal)
102 of the mixer 100 is mixed in a second mixer 104 with the output
26 of the downstream divider 18 to form the flow data signal 52.
Mathematically, it can be shown that if the downstream divisor is
chosen to be one less than the upstream divisor then the frequency
of the flow data rate signal 52 is proportional to the flow rate.
In addition, the frequency of the flow data rate signal 52 is an
integer multiple of the beat frequency (flow velocity frequency)
that would be obtained by mixing the upstream signal at point 34
with the downstream signal at point 34 as is done in the prior
art. Because the flow rate signal's frequency is an integer multiple
of the beat signal used in most prior art ultrasonic flow meters,
the resolution is an integer multiple greater than obtained with
prior art methods.
FIG. 5 is schematic diagram of the ultrasonic flow rate meter 10
of FIG. 2. If f.sub.2 is defined as the frequency of the downstream
signal at the transducer, when the downstream wavelength is equal
to the path length (L), then f.sub.2 =(C+V)/L. Where C is the speed
of sound in the flowing fluid, V is the flow rate between the transducers
and L is the sonic path length between the transducers. f.sub.4
is the frequency of the intermediate downstream signal and is equal
to N.sub.1 times f.sub.2. Where N.sub.1 is the path length divider
(multiplier). Similarly, f.sub.3 which is the frequency of the
intermediate upstream signal, is equal to the N.sub.1 times f.sub.1.
Where f.sub.1 is the frequency of the upstream signal, when the
upstream wavelength is equal to the path length (L) and equals (C-V)/L.
The frequency of the downstream oscillator signal 12 is equal to
N.sub.1 *(N.sub.2 -1)*f.sub.2. The frequency of the upstream oscillator
signal 20 is equal to N.sub.1 *(N.sub.2)*f.sub.1. By selecting the
downstream multiplier 18 to be one less than the upstream multiplier
24 the flow data signal frequency, f.sub.d, is equal to N.sub.1
*N.sub.2 *V/L. Where V is the flow rate (if the path between the
two transducers is parallel to the flow direction) and L is the
acoustic path length between the transducers. Thus the flow rate
is found by multiplying the flow frequency by a predetermined factor.
When the downstream divisor (downstream factor) is not chosen to
be one less than the upstream divisor (upstream factor) the equation
for the flow data signal's frequency (flow frequency) is a function
of the speed of sound. This is undesirable because the speed sound
will fluctuate with temperature and pressure and the speed of sound
may not by known for the particular fluid.
As can be seen from FIG. 5 the downstream oscillator signal's
frequency (first output frequency) will not be equal to the upstream
oscillator signal's frequency (second output frequency) when the
flow rate is zero. When the flow rate is zero f.sub.1 =f.sub.2 and
therefore f.sub.3 =f.sub.4. As a result, the upstream oscillator
signal's frequency will be f.sub.3 *N.sub.2 and the downstream oscillator
signal's frequency will be f.sub.3 *(N.sub.2 -1). Because the two
oscillators do not have the same output frequency they will not
frequency lock to each other as happens in the prior art. The other
situation where the two oscillators can have the same frequency
is at high flow rates (maximum flow rate design). The higher the
flow rate the lower the upstream oscillator signal's frequency and
the higher the downstream signal's frequency. The lower the divider
N.sub.2 is chosen to be, the farther apart the oscillator's frequencies
will be with no flow. As a result if N.sub.2 is selected below about
18 then there are almost no practical situations in which the two
oscillator's frequencies would be the same. In the preferred embodiment,
N.sub.2 is selected to be 16.
FIG. 6 shows an alternative embodiment of the invention. In this
embodiment the flow data signal is created by mixing the intermediate
upstream signal 28 and the intermediate downstream signal 26. As
a result the flow data signal's frequency f.sub.d is N.sub.1 times
beat signal frequency (i.e., the signal generated by mixing the
upstream signal f.sub.1 and the downstream signal f.sub.2). Where
the beat frequency is 2V/L. The expression does not include the
speed of sound and has N.sub.1 times the resolution of the beat
signal. In addition, the upstream oscillator's frequency and the
downstream oscillator's frequency are spaced apart when there is
no flow in the pipe.
Thus there has been described a ultrasonic flow meter that does
not result in the two voltage controlled oscillators becoming locked
in frequency. In addition, the ultrasonic flow meter described herein
has a significantly higher resolution than prior art ultrasonic
flow meters. The invention described herein applies to both axial
and radial ultrasonic flow meters. While the invention has been
described in conjunction with specific embodiments thereof, it is
evident that many alterations, modifications, and variations will
be apparent to those skilled in the art in light of the foregoing
description. Accordingly, it is intended to embrace all such alterations,
modifications, and variations in the appended claims. |