Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
1998-12-18
2001-07-03
Shah, Kamini (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C702S049000, C702S050000, C702S100000, C073S861110, C073S861160, C073S861000, C073S861780, C073S861356
Reexamination Certificate
active
06256590
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electromagnetic flowmeters used to monitor fluid flow.
2. Description of Prior Art
Electromagnetic flowmeters operate, as is well known, by passing a current through a coil to generate a magnetic field and detecting an electro-motive force, measured in volts, induced in a fluid passing through the field. The output voltage, E, is proportional to the product of the field, B, and flow velocity, v, and thus:
E=k·B·v
. (equation
1)
The constant k will depend on factors such as the sensitivity of the meter and the spacing of the electrodes, but can be considered constant for a given apparatus. It is generally assumed that the magnetic field strength in the coil is linearly proportional to the current, I, flowing in the coil so that:
B=m·I
(equation
2)
The value of m is normally considered a constant approximating to:
m
≈
N
μ
1
where
μ
=
permeability
of
free
space
l
a
=
the
length
of
the
air
gap
,
and
N
=
the
number
of
turns
of
the
coil
.
Thus, the output signal is proportional to the product of current and velocity, and can be expressed by substitution for B from equation (2) into equation (1) as:
E=km·I·v.
(equation
3)
Measurement is normally carried out at a fixed current, provided by a constant current source, but, in order to take into account small fluctuations in the current, the output current is usually measured, and the velocity is determined from the ratio of output voltage to input current from equation (3) rewritten as:
v=
(1
/km
)·(
E/I
) (equation
4)
The constant (1/km) thus relates the flow velocity to the ratio of measured voltage to applied coil current, which we will hereafter refer to as R, giving v=R·E/I.
The above equation represents a reasonably good approximation for most purposes, at constant current. However, the inventor has appreciated that, in fact, the field strength is not directly proportional to the input current. Thus, errors can arise, which can be significant if the flowmeter is highly accurate (the inventor has found the problem particularly acute in the development of meters of accuracy of the order of 0.1%), hitherto necessitating the use of high stability accurately calibrated constant current sources, which can add to manufacturing time and cost, and may present particular problems in battery powered equipment. In addition, pursuant to the invention, to optimise power consumption, it has been proposed to make measurements at one of a plurality of currents or at a current which may be continuously varied.
SUMMARY OF THE INVENTION
In general terms, the invention provides that, at least for circumstances in which the non-proportionality of field strength to input current could be significant, in place of the constant calibration factor, m in equation 2 or any derived constant such as R a variable factor, based on the measured value of current, is utilised. An alternative way of considering the invention is to consider the induced field to be equivalent to a function M(I), where M is not directly proportional to I. M is normally a non linear function, but may in one embodiment simply be equal to I minus an offset; in any event the output signal will be a non-linear function of (
1
/I).
According to a first aspect of the invention, there is provided an electromagnetic flowmeter signal processing apparatus for use with an electromagnetic flowmeter including a coil past which a fluid can flow, the coil being arranged so as to generate a magnetic field of strength dependent upon the magnitude of current supplied to the coil thereby to induce in said fluid a voltage indicative of a parameter of the fluid's flow, the apparatus comprising means for measuring the current supplied to the coil, means for measuring the voltage induced in the fluid and means for producing an output signal representative of the flow parameter and generally proportional to the ratio of the measured voltage and the measured current, characterised in that the producing means is arranged to calculate or adjust the output signal in dependence on the measured current value to compensate for non-linear dependence of the field, and thus the induced voltage, on applied current.
In another aspect, the invention comprises a method of obtaining a measure of a parameter representative of flow rate in an electromagnetic flowmeter, the method comprising dividing the voltage induced across the fluid by a function of the current flowing through a magnetic field-generating coil of the flowmeter, wherein the function is not directly proportional to the current. The function is preferably selected to be a better approximation to the actual field generated by the coil than a linear approximation, over at least a range of current values.
By flowmeter signal processing apparatus, in this specification, we mean apparatus capable of processing signals derived from a flowmeter; the signal processing apparatus will typically be integrated into a flowmeter assembly including the field generating coils, voltage sensing electrodes and means for supplying current to the coils and other components, but may be provided separately for connection to a flowmeter skeleton comprising only coils and sensing electrodes.
In one embodiment of the invention, the producing means is arranged to multiply the ratio of output voltage to input current by a calibration factor; the calibration factor being adjusted as a function of current amplitude. The calibration factor need not, however, be continuous, and may instead be constituted by a plurality of discrete values which are sequentially selected as the input current amplitude varies.
The calibration factor may be evaluated either from a look-up table or by calculation. In the event that calculation is used, one simple algorithm that can be used is to subtract a relatively small offset value from the measured current.
Substituting into equation 4, we have
v=R·E/
(
I−I
0
) (equation
5),
Electromagnetic flowmeters typically employ a ratiometric design; that is, the flow is generally determined from the ratio of the output EMF, E to the input current, the ratio being multiplied by a constant to give the flow rate. Re-eriting equation (5) in terms of the ratio E/I gives:
v=R
(
I/
(
I−I
0
))·
E/I
(equation
6)
This is equivalent to employing a calibration factor which varies as a function of I/(I−I
0
), or as a function of 1/(1−p), where p is equal to I
0
/I. In each case, I
0
is empirically determined.
This correction is relatively simple to calculate, and gives significantly better results than an uncorrected value of current for reasonably large values of I. As I reduces to a comparable value to the offset current I
0
, the value for v obtained becomes too large, and becomes undefined at I=I
0
. The value of I
0
may be altered for different ranges of current.
Exponential correction
A better correction is found by considering B to be a function, M(I), as follows:
B=I·K·
(1
−e
−I/K
2
) Equation 7
Substituting into equation 1 gives
v=R·E/
(
K
1
(1−e
−I/K2
)) Equation 8
or
v=R·[I/
(
K
1
(1
−e
−I/K
2
))]·
E/I
Equation 9
in which the term [I/(K
1
(1−e−
I/K2
))] gives a correction factor to be applied to the velocity derived in the conventional manner.
Polynomial Correction
As an alternative to the exponential correction factor mentioned above, empirical compensation using a (preferably at least third order) polynomial may be employed; this is found to be still better in many cases.
The flow velocity can be computed
ABB Instrumentation Limited
Larson & Taylor PLC
Shah Kamini
Vo Hien
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