In a coriolis flow meter, two drivers are provided and a measure
of mass flow rate derived from the adjustment of the input signals
to each driver required to achieve a desired phase shift in the
output. This may give higher accuracy compared to conventional meters
in which the phase shift is simply measured, and may also enable
determination of other characteristics of the fluid.
What is claimed is:
1. Apparatus for obtaining a measure of mass flow rate in a fluid
excitation means for applying vibration to a portion of the conduit;
sensor means for detecting a measure of phase difference in the
vibration at first and second mutually spaced apart points in the
flow conduit; and wherein
the excitation applied by said excitation means is adjustable to
compensate for variation in said phase difference caused by flow
of fluid through the conduit and wherein adjustment of said excitation
is selected to alter said phase difference.
2. Apparatus according to claim 1 further comprising control means
for adjusting the excitation provided by the excitation means.
3. Apparatus according to claim 2 including means for determining
a measure of the mass flow rate through the conduit based on a measure
of the adjustment provided by said control means.
4. Apparatus according to claim 3 wherein the control means is
arranged substantially to maintain a predetermined phase difference,
said measure of the mass flow rate being based essentially on a
measure of said adjustment.
5. Apparatus according to claim 3 wherein the control means is
arranged to permit variation of the measured phase difference within
a predetermined range, said measure of the mass flow rate being
based on the measure of said adjustment and a measure of said phase
6. Apparatus according to claim 3 further comprising means for
deriving a measure of the accuracy of the apparatus based on the
measure of said adjustment and said measure of phase difference.
7. Apparatus according to claim 1 wherein the excitation means
comprises two spaced apart exciter elements.
8. Apparatus according to claim 7 including control means for
adjusting the excitation provided by the excitation means, wherein
the control means is arranged to drive each exciter element independently.
9. Apparatus according to claim 1 wherein the sensor means comprises
two sensor elements, each providing a signal representation of phase
at one of said spaced apart points.
10. Apparatus according to claim 9 including cross-correlation
means arranged to determine a measure of said phase difference based
on the results of cross-correlation of signals from each of the
11. Apparatus according to claim 10 arranged to apply an effective
phase shift to at least one of the sensor signals to increase the
sensitivity of the cross-correlation to changes in the phase difference.
12. Apparatus according to claim 11 wherein the effective phase
shift is selected so that the phase difference between the signals
to be cross-correlated is approximately 90.degree. (modulo 180.degree.).
13. A method according to claim 11 further comprising determining
a measure of the accuracy of the meter based on the measured phase
difference and the measure of the amount of the amount of said adjusting.
14. Apparatus according to claim 10 wherein the sensor signals
are stored as discrete samples and an offset corresponding to the
effective phase shift is determined based on a value for the number
of samples in a signal period.
15. A method of obtaining a measure of mass flow rate in a fluid
conduit comprising applying vibration to a portion of the conduit;
detecting a measure of phase difference in the vibration at first
and second mutually spaced apart points in the flow conduit;
adjusting the excitation applied to the conduit to compensate for
variation in said phase difference caused by flow of fluid through
the conduit; and
obtaining a measure of the mass flow rate based on at least a measure
of the amount of adjustment and wherein adjustment of said excitation
is selected to alter said phase difference.
16. A method according to claim 15 wherein said measured phase
difference is maintained substantially constant and the determination
is based essentially on said measure of said adjusting.
17. A method according to claim 15 wherein the determination of
the measure of the flow rate is based on the measure of the amount
of said adjusting and the measured phase difference.
18. A method of verifying the accuracy of a mass flow meter which
includes means for applying vibration to a portion of a conduit
and means for detecting a phase difference in vibration at first
and second mutually spaced apart locations in the conduit, the method
comprising adjusting the excitation applied to the conduit and comparing
a measured value of said phase difference with a predicted value
of phase difference and wherein adjustment of said excitation is
selected to alter said phase difference.
Coriolis flow meters have been used for many years to obtain accurate
mass flow measurements. The principle behind such flow meters is
that a transducer is employed to apply vibration to a conduit containing
the fluid whose mass flow rate is to be measured and the vibration
in the conduit is measured by two spaced apart sensors, typically
either side of the source of vibration. Throughout this specification,
the term fluid is intended to encompass both homogeneous fluids
such as liquids or gasses and non-homogeneous fluids such as slurries,
suspensions or particulate media. In the absence of a fluid flow,
the phase of vibration at each sensor location will be approximately
the same. However, when a fluid flows through the conduit, there
will tend to be a lag in the phase of the upstream sensor and a
lead in the phase of the downstream sensor. From the phase difference,
a measure of the mass flow rate can be obtained.
Examples of flow meters operating of this principle can be found
in U.S. Pat. No. 4422338 U.S. Pat. No. 5423221 U.S. Pat. No.
4856346 U.S. Pat. No. 5394758 U.S. Pat. No. 4192184 and U.S.
re-issue Pat. No. 31450 the disclosures of each of which are herein
incorporated by reference.
The inventor has appreciated that a problem with conventional mass-flow
meters is the need to measure the small phase differences accurately;
typically the phase differences induced are only of the order of
a few degrees.
The inventor has proposed that the excitation applied to a flow
conduit be adjusted to modify the measured phase difference, for
example to achieve or maintain a desired phase difference between
sensors (preferably a null phase difference) and the mass flow rate
derived from the adjustment applied. In this way, it may be easier
to obtain accurate measurement as detection of a particular phase
difference at a single point may be more accurately achieved than
accurate measurement over a range of possible phase differences.
The technique can be employed to extend the range or increase the
accuracy of measurement, by effectively reducing the range over
which a phase difference must be measured.
Accordingly, in one aspect, the invention provides apparatus for
obtaining a measure of mass flow rate in a fluid conduit comprising:
excitation means for applying vibration to a portion of the conduit;
sensor means for detecting a measure of phase difference in the
vibration at spaced apart points in the flow conduit; wherein the
excitation applied by said excitation means is adjustable to compensate
for variation in said phase difference caused by flow of fluid through
The apparatus preferably includes control means for adjusting the
excitation provided by the excitation means and preferably further
includes means for determining a measure of the mass flow rate through
the conduit based on a measure of the adjustment provided by said
Preferably the excitation means comprises two (or more) spaced
apart transducers, preferably electromagnetic transducers, and the
adjusting means comprises means for adjusting the relative phase
and/or amplitude, preferably at least the relative phase, of excitation
signals supplied to the transducers.
Preferably the apparatus is arranged so that the control means
maintains a substantially constant phase difference, preferably
a null phase difference at the sensor locations; this enables an
accurate measure of mass flow to be derived directly from the adjustment
applied to the adjustment means.
Alternatively, the phase difference may be measured, and the mass
flow derived from both a measure of the adjustment applied to the
excitation means and the measured phase difference. For example,
the excitation may be adjusted in discrete steps and a correction
factor may be determined from the measured phase difference.
Another benefit of the invention is that the calibration of the
meter can be checked by comparing the measured phase difference
with a stored or predicted phase difference for a given adjustment
applied to said excitation. For example, a measure of mass flow
rate may be determined based on the measured phase difference with
no compensating adjustment, and compared to a measure of mass flow
rate obtained by adjusting the excitation to produce a substantially
null phase difference; if the measures do not agree, this suggests
a fault or calibration shift in the meter. Thus, the apparatus may
include means for verifying accuracy of the meter based on said
measure of adjustment and a measure of the phase difference, preferably
based on a stored or calculated relationship between the two.
The inventor has appreciated that the measured phase shift may
be a non-linear function of the applied phase shift, depending on
further physical properties of the fluid, particularly viscosity.
Thus a further benefit of the invention is that it may be possible
to obtain a measure of a further property of the fluid, for example
viscosity, based on the variation of measured phase shift with applied
phase shift. The method preferably includes obtaining a measure
of phase shift for a plurality of values of applied excitation and
deriving a property of the fluid from the measured variation of
phase shift with applied excitation.
The invention also extends to corresponding methods of operation.
An embodiment of the invention will now be described, by way of
example, with reference to the accompanying drawings in which:
FIG. 1 is a schematic diagram of a flowmeter according to the invention;
FIG. 2 is a schematic block diagram of the control arrangement
used for the apparatus of FIG. 1.
Referring to FIG. 1 a flow meter comprises a conduit 10 provided
with an electromagnetic exciter 12 and having spaced apart sensors
14a, 14b located respectively on upstream and downstream sides of
the exciter. The conduit is coupled to a rigid housing or support
tube 16 which ensures that the portion of conduit which is vibratable
is well defined. Coupling flanges 18 will usually be provided for
joining to adjacent pipework.
The basic physical arrangement described above is purely exemplary;
a design based on any suitable known arrangement may be employed.
An important difference from conventional arrangements, however,
is that the exciter 12 of this embodiment comprises two separate
spaced apart exciter elements 12a, 12b. Each of these elements is
separately controllable. Thus, by adjusting the relative amplitude
or phase (preferably at least the phase) of the signals fed to each
exciter element, the phase detected by the sensors may be positively
adjusted. This may be employed to compensate for phase shift caused
by mass flow, as will be described further below with reference
to the control arrangement schematically illustrated in FIG. 2.
In this embodiment, control of the apparatus is effected primarily
by means of a digital signal processor 20. This drives the exciter
elements 12a, 12b by means of respective output drivers 22a, 22b
and receives inputs from the sensors 14a, 14b by means of respective
input amplifiers 24a, 24b.
As schematically illustrated, the digital signal processor implements
a number of functions, as will be described further below.
Firstly, it applies positive feedback from the sensor elements
to the exciter elements to excite and maintain resonant oscillation
of the conduit. This is essentially a simple amplification function,
preferably ignoring any small phase differences in the inputs from
the sensors, but simply taking an average of the two inputs and
preferably providing a basic output drive signal of approximately
the right phase frequency and amplitude to drive the exciter elements
to maintain resonant oscillation. The excitation is preferably arranged
to excite a preferred mode of vibration or preferred combination
of modes of vibration; this may include frequency or phase selection.
Advantageously a digital signal processor is employed, enabling
selection of a desired proportion of modes, and switching between
modes; in combination with adjustment of applied phase, this may
facilitate determination of a measure of one or more further physical
Fine adjustment of the phase, and optionally amplitude, of the
signals fed to each individual exciter element is preferably performed
subsequently in a separate output generator functional element which
receives the basic output signal and information concerning the
individual phase adjustments to be made.
In one arrangement, the apparatus is arranged to adjust the excitation
to obtain a null phase difference between the sensors; this has
the benefit that the actual phase measurement need not be calibrated.
Higher accuracy may be obtained because a null point may be more
accurately determined than the absolute value of a phase difference.
In an alternative arrangement, the apparatus is arranged to derive
a measure of mass flow from either or both of the applied phase
shift and the measured phase shift. This has the benefit that rapidly
changing flow rates can be accomodated (where the flow rate is determined
from the applied phase shift alone, there will be delays before
the apparatus tracks the flow rate and the measured phase shift
becomes zero). Even though this requires measurement of the phase
difference, accuracy may be improved because measurement may be
required over a smaller range.
In a further alternative arrangement, the "null" point
is altered, for example modulated about a zero value, or cycled
through a plurality of discrete values. In this way, a measure of
variation of measured phase shift with applied phase shift can be
obtained, and this may provide information concerning other physical
properties of the fluid. For example, the ratio of the output phase
to the applied adjustment may provide a useful measure of physical
properties of the fluid, such as viscosity.
As an alternative to positive feedback to generate the oscillation,
the signal processor may include an element arranged to produce
an oscillating signal at a determined frequency. Resonant oscillation
is, however, preferred, as the conduit behaviour may be more easily
controlled and predictable. In addition, it is possible to measure
the resonant frequency and derive therefrom a measure of the mass
and hence density of the fluid in the conduit. A further possibility
is for the resonant circuit to be incorporated in a phase-locked
The digital signal processor also includes means for obtaining
a measure of the phase difference in the signals from the sensors
14a and 14b. In this embodiment, this is achieved by cross correlation.
Assuming we have two sinusoidal signals A and B differing in phase
by predetermined angle .alpha. these can be written as follows (neglecting
any absolute phase offset):
Then the cross product A X B can be written as ##EQU1## A X B=ab/2
cos .alpha. (4)
Thus, the phase difference .alpha. may be determined by integrating
the product of the two signals. In practice, of course, this integration
will be performed numerically on a discrete series of samples, preferably
16 bit samples. For example with a resonant frequency of the order
of 100 hz and a sampling frequency of the order of 40 kHz, there
will be approximately 400 samples per cycle. The integration may
be performed over a number of cycles to increase accuracy. The coefficients
a and b may be determined by cross-correlating each signal with
It has been appreciated by the inventor that the phase measured
using cross correlation is most sensitive to small changes in the
phase when the phase difference is approximately 90.degree.(+/-n.times.180.degree.).
At this point, cos .alpha., which is proportional to the value of
the integral should theoretically be zero and has maximum gradient,
so any small change will be readily noticeable. Thus, to improve
accuracy, it is preferred that the apparatus is configured so that
the phase difference measured by cross correlation is approximately
90.degree.. This can be achieved by inserting a phase delay in the
input from at least one of the sensors, or in a preferred implementation
simply by performing the cross correlation using effectively shifted
sample data. Alternatively, the sensors may be configured to provide
an in-built phase offset in their outputs. Since the resonance frequency
can be determined and the sampling frequency is known, it is possible
to determine the number of samples per cycle, N. Then by assuming
the signals are in phase in the unperturbed condition of the conduit,
effectively shifting the data returned from one sensor by N/4 samples,
the value of the integral will be approximately zero at a null phase
difference. This technique is applicable to other applications of
cross correlation to measure phase difference.
Note that the data need not be physically moved to effect a shift;
in a preferred implementation, the data is simply read out from
memory starting at an offset point from the actual start of sampling.
For example, the first N/4 samples from one sensor may be ignored,
based on the measured phase difference, the phase of output signals
sent to the output drivers is adjusted to tend to return the measured
phase to a desired value. The measured phase difference and a measure
of the applied adjustment (for example a value for the phase difference
between the output signals) are fed to a calculator, which also
receives calibration data. From this, a value for the mass flow
rate in the conduit can be determined. In addition, by monitoring
the change in measured phase with changes in the drive signals,
faults or drifts in the calibration of the meter may be detected.
Meter accuracy verification may be performed in a number of ways,
and may be performed either continuously or intermittently.
As mentioned briefly above, a value for the density of the fluid
in the conduit may also be calculated from a measure of the resonant
frequency of the pipe and further calibration data.
In the above described embodiment, a digital signal processor has
been used to implement the majority of the functions of the apparatus.
This provides a convenient, flexible and readily adaptable arrangement.
However, it will be appreciated that each of the above functions
may be performed using analogue hardware, or a combination of analogue
hardware and digital signal processing.
For example, it is relatively straightforward to implement positive
feedback using an analogue amplifier circuit. Indeed, this function
can readily be separated from the digital signal processor and a
conventional analogue feedback circuit arranged to drive a single
exciter element may be provided, with the digital signal processor
serving to make fine adjustments to the phases of the signals supplied
to each of the individual exciter elements 12a, 12b.
Analogue phase-difference detection circuitry is also known, and
may be substituted for the cross-correlation phase difference detection
The meter accuracy verification function need not be provided,
and this can lead to simpler apparatus in which it is only necessary
to detect a null (or other predetermined) phase difference, rather
than to obtain a measure of the phase difference.
As an alternative to two separate sensor elements, a single composite
sensor providing a direct output of phase difference may be employed.
This can readily be interfaced to simpler processing circuitry.
As an example of a composite sensor, a sensor having two detector
coils mechanically coupled to two respective sensing points on the
conduit, but having windings connected in series but in opposite
sense, so that the net output is equal to the difference between
the sensor readings; this will be substantially null with no phase
shift. As a further example, a mechanical coupling may be provided
between the two sensing points, and a single sensor may be arranged
to measure deformation of the mechanical coupling.
In place of separate driver coils 12a, 12b, a single exciter element
may be employed, with the phase adjustment effected by, for example,
altering the physical coupling of the driver element to the conduit.
Indeed, any arrangement in which the phase difference between the
sensor signals can be controlled may be employed; the arrangement
of two spaced apart coils is however particularly convenient.
The apparatus is preferably calibrated empirically, by passing
a known mass flow rate through the conduit and noting the phase
adjustment and/or measured phase difference, preferably for a number
of different mass flow rates.
Each feature described may be independently provided, unless otherwise
stated. The appended abstract is herein incorporated by reference.