Abstrict An instrumental error compensation circuit for a flow meter is
disclosed in which a flow rate calculated from an output signal
of the flow meter is identified with respect to a plurality of predetermined
sections of flow rates. Flow rate compensation coefficients are
calculated by linear interpolation on the basis of actually measured
data which are obtained in advance in accordance with instrumental
errors of the flow meter. Actual flow rates of a fluid are measured
with the instrumental errors of the flow meter compensated for by
the calculated coefficients.
Claims What is claimed is:
1. A circuit for compensating for an instrumental error of a flow
meter which generates a pulse signal having a frequency proportional
to a flow rate, said circuit comprising:
pulse distributor means for generating two different pulse signals
in response to the pulse signal output from the flow meter, said
two pulse signals having a predetermined frequency ratio to each
other;
flow rate calculator means for calculating an instantaneous flow
rate based on the frequency of the outer signal of the flow meter
and generating an output signal indicative of said instantaneous
flow rate;
flow rate section identifier means for identifying, in response
to the output signal of the flow rate calculator means, specific
one of predetermined flow rate sections to which the calculated
instantaneous flow rate belongs, and generating a signal indicative
of the identified section;
compensation constant memory means for storing in advance a plurality
of compensation constants necessary for calculating an instrumental
error of the flow meter for each of the sections, selecting specific
one of the compensation constants in response to the output signal
of the identifier means, and generating a signal indicative of said
compensation constant;
compensation constant calculate and write circuit means for calculating
a compensation coefficient in response to the output signal of the
compensation constant memory means by performing interpolation corresponding
to a flow rate, and generating a signal indicative of the compensation
coefficient;
compensation amount calculator means for calculating an amount
of compensation corresponding to the output signal of the compensation
constant calculate and write circuit means in synchronism with one
of the pulse signals output from the pulse distributor means, and
generating a signal indicative of said amount of compensation, said
compensation constant memory means, in response to an output of
said flow rate section identifier means, producing a compensation
value associated with the input, said compensation value being loaded
into said compensation amount calculator means such that each time
a pulse signal is delivered from said pulse distributing means to
said compensation amount calculator means, said compensation amount
calculator means calculates a compensation value based on the output
data of said compensation constant memory means;
adder means for adding up the output signals of the compensation
amount calculator means, and every time the sum reaches a predetermined
numerical value, generating an overflow pulse signal as a compensated
flow rate pulse;
output sync circuit means for sumchronizing the output pulse signal
of the adder means to said other output signal of the pulse distributor
means to generate a sync signal;
a write circuit means for entering data in said flow rate section
identifier means and data in said compensation constant memory means
corresponding to outputs of said flow rate section identifier means;
and
temporary flow rate section means for temporarily storing said
entered data such that said data is received by said compensation
constant calculate and write circuit means via said temporary flow
rate sections means, whereby said compensation constant calculate
and write circuit means performs said calculations.
2. A circuit as claimed in claim 1 in which the ratio in frequency
of said other output pulse signal of the pulse distributor means
fed to the output sync circuit means to said one output pulse signal
fed to the compensation amount calculator means is 2:1.
3. A circuit as claimed in claim 1 in which the output sync circuit
means comprises an up-down counter which accumulates the overflow
pulse signals output from the adder means and generates a logical
"1" output signal only when a count is not zero, an AND
gate to which an output signal of said up-down counter and said
other output signal of the pulse distributor means are supplied,
and a circuit for delivering an output signal of said AND gate to
a subtraction input terminal of the up-down counter.
4. A circuit as claimed in claim 1 further comprising data input
means for entering and temporarily storing instrumental error data
on the flow meter for each of actually measured flow rates, calculating
compensation values for the individual flow rate sections, and loading
the calculated values in the identifier means.
5. A circuit as claimed in claim 4 in which the data input means
comprises keys for writing data.
6. A circuit as claimed in claim 4 in which the data input means
comprises a read only memory.
Description BACKGROUND OF THE INVENTION
The present invention relates to a circuit for compensating for
instrumental errors of a flow meter.
In a flow meter, an instrumental error has to be compensated for
by using a compensation coefficient or the like so that measurement
of a flow rate of a fluid may give a true volume.
Supposing that an instrumental error is E, a displayed volume is
Vi, a volume actually flown is Vq, and a compensation coefficient
is (1+.alpha.), there holds an equation: ##EQU1## or
The instrumental error E, which is a function of the flow rate
Q, cannot be reduced beyond a certain limit by a mechanical means
but may be successfully reduced to a negligible value if use is
made of an electronic means. The electronic means for such an application
has heretofore been proposed in various forms.
One of the proposed electronic means is applied to a flow meter
of the type wherein a pickup coil or the like senses rotations of
a rotor which is rotated by a fluid flowing through the flow meter,
the rotations being converted into electric pulses. The error reducing
means is constructed to vary the pulse rate of detected pulses in
order that an error curve may undergo a parallel shift within a
certain range of flow rates Q to bring the errors closer to zero
in an average sense. However, it is difficult for such a prior art
implementation to approximate the errors to zero over the entire
range of flow rates.
In light of this, there has been proposed a system in which a plurality
of sections are defined within an operable range of a flow meter
and a ROM or like memory stores error compensation amounts matched
to the individual sections. The parallel shift of the error curve
in this system occurs over a width which differs from one section
to another. This system, however, suffers from the drawback that
because a single compensation amount is assigned to each compensation
section, the error reaches a critical value unless the range is
fractionalized into a number of sections and, therefore, without
a memory having a disproportionate capacity.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an
instrumental error compensation circuit for a flow meter which insures
a desirable accuracy of compensation even though each compensation
section may be wide, by calculating compensation amounts in different
compensation sections in correspondence to flow rates by linear
interpolation.
It is another object of the present invention to provide a generally
improved instrumental error compensation circuit for a flow meter.
A circuit for compensating for an instrumental error of a flow
meter which generates a pulse signal having a frequency proportional
to a flow rate of the present invention includes a pulse distributor
for generating two different pulse signals in response to the pulse
signal output from the flow meter. The two pulse signals have a
predetermined frequency ratio to each other. A flow rate calculator
calculates an instantaneous flow rate based on the frequency of
the output signal of the flow meter and generates an output signal
indicative of the instantaneous flow rate. A flow rate section identifier
identifies, in response to the output signal of the flow rate calculator
specific one of predetermined flow rate sections to which the calculated
instantaneous flow rate belongs, and generates a signal indicative
of the identified section. A compensation constant memory stores
in advance a plurality of compensation constants necessary for calculating
an instrumental error of the flow meter for each of the sections,
selects specific one of the compensation constants in response to
the output signal of the identifier, and generates a signal indicative
of the compensation constant. A compensation constant calculate
and write circuit calculates a compensation coefficient in response
to the output signal of the compensation constant memory by performing
interpolation corresponding to a flow rate, and generates a signal
indicative of the compensation coefficient. A compensation amount
calculator calculates an amount of compensation corresponding to
the output signal of the compensation constant calculate and write
circuit in sychronism with one of the pulse signals output from
the pulse distributor, and generates a signal indicative of the
amount of compensation. An adder means adds up the output signals
of the compensation amount calculator and, every time the sum reaches
a predetermined numerical value, generates an overflow pulse signal
as a compensated flow rate pulse. An output sync circuit synchronizes
the output pulse signal of the adder to the other output signal
of the pulse distributor to generate a sync signal.
In accordance with the present invention, an instrumental error
compensation circuit for a flow meter is disclosed in which a flow
rate obtained from one output signal of the flow meter is identified
with respect to a plurality of predetermined sections of flow rates.
Flow rate compensation coefficients are calculated by linear interpolation
on the basis of actually measured data which are obtained in advance
in accordance with instrumental errors of the flow meter. Actual
flow rates of a fluid are measured with the instrumental errors
of the flow meter compensated for by the calculated coefficients.
The above and other objects, features and advantages of the present
invention will become apparent from the following detailed description
taken with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph representing the principle of operation of an
instrumental error compensation circuit in accordance with the present
invention;
FIG. 2 is a block diagram of an instrumental error compensation
circuit embodying the present invention; and FIG. 3 is a waveform
diagram showing pulses which appear in various parts of the circuit
shown in FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
While the instrumental error compensation circuit for a flow meter
of the present invention is susceptible of numerous physical embodiments,
depending upon the environment and requirements of use, a substantial
number of the herein shown and described embodiment have been made,
tested and used, and all have performed in an eminently satisfactory
manner.
Referring to FIG. 1 of the drawings, the graph shows flow rates
Q along its abscissa and compensation coefficients (1+.alpha.) along
its ordinate. A curve E represents a flow rate to compensation coefficient
characteristic and may be represented by lines F.sub.1 F.sub.2
F.sub.3 and F.sub.4 in discrete compensation sections by approximation.
When a flow rate Q is calculated, specific one of the compensation
sections i, ii, iii and iv to which the flow rate Q belongs is identified.
Thereafter, based on data An and Bn which were computed in advance
using instrumental errors of flow rates Q, a compensation coefficient
is determined for the specific section by linear interpolation.
That is, a compensation coefficient (1+.alpha.) is calculated in
a section in which an instantaneous flow rate Q lies, using corresponding
one of the equations: ##EQU2##
The calculated coefficient (1+.alpha.) is used for compensating
for an error. The data An and Bn are obtained from actually measured
instrumental errors by a method of least squares or like method
of tolerance.
Reference will be made to FIGS. 2 and 3 for describing a practical
construction and operation of the instrumental error compensation
circuit of the present invention.
A flow meter 10 generates flow rate pulses having a frequency f
which is proportional to a flow rate. The flow rate pulses are delivered
to a flow rate calculator 12 and, via a low frequency output terminal
14a of a pulse distributor 14 to a compensation amount calculator
16. While pulses P appearing at the low frequency output terminal
14a of the pulse distributor 14 are common in frequency (f) to the
incoming flow rate pulses, pulses Q appearing at the other or high
frequency output terminal 14b have a frequency double the frequency
f, 2f.
The flow rate calculator 12 includes a pulse period measuring circuit
12a for measuring a period of the flow rate pulse signal. The circuit
12a connects to a reciprocal calculator 12b which is adapted to
obtain a reciprocal of the period of the pulse signal, i.e. a flow
rate. The flow rate is fed from the calculator 12b to a flow rate
section identifier circuit 18 which identifies specific one of the
predetermined compensation sections to which the input flow rate
belongs.
A compensation constant memory 20 which may be constituted by a
ROM or like known memory stores compensation amount data which were
calculated in advance for the individual flow rate sections on the
basis of an instrumental error of the flow meter. In response to
an output of the section identifier 18 the memory 20 produces a
compensation value associated with the input and this value is loaded
in the compensation amount calculator 16. Every time a pulse P is
delivered from the terminal 14a of the pulse distributor 14 to the
compensation amount calculator 16 the latter calculates a compensation
value (1+.alpha.) based on the output data of the memory 20 and
using corresponding one of the Eqs. (3-i) to (3-iv). The output
of the calculator 16 is fed to an adder 22. The calculator 16 may
be constituted by the circuit a shown in FIG. 1 of Japanese Utility
Model Publication No. 51-9895.
The adder 22 adds up the sequential outputs of the compensation
amount calculator 16 and, before the sum reaches "1 (one)",
delivers no pulses therefrom. Upon the increase of the sum to "1",
the adder 22 overflows to supply an output sync circuit 24 with
a pulse. Describing it more concretely, pulses P having a frequency
f proportioanl to a flow rate appear at the output terminal 14a
of the pulse distributor 14 while pulses Q whose frequency is double
the frequency of the pulses P appear at the output terminal 14b.
Each time a pulse P appears, the calculator 16 provides a compensation
value (1+.alpha.) in response to an output of the memory 20 and
using one of the Eqs. (3-i) to (3-iv). The output of the calculator
16 is fed to the adder 22. The adder 22 every time a pulse P is
produced, adds up an incoming compensation value (1+.alpha.). As
soon as the sum increases to "1", the adder 22 overflows
and produces a numerical value "1". Upon the increase
of the sum to "2", the adder 22 will produce a numerical
value "2".
The output sync circuit 24 comprises an AND gate 24a and an up-down
counter 24b. The AND gate 24a receives at one input terminal thereof
the pulses Q which are output from the terminal 14b of the pulse
distributor 14 with a frequency double the frequency of the pulses
P. The up-down counter 24b receives at its addition input terminal
the overflow pulses which are output from the adder 22. The output
of the AND gate 24b is fed back to the other or subtraction input
terminal of the up-down counter 24b.
Because a correction value (1+.alpha.) is integrated by the adder
22 for each output pulse of the flow meter 10 the pulses from the
flow meter 10 and output pulses of the adder 22 are eventually N
(1+.alpha.) pulses. While this provides compensated correct flow
rate pulses as far as the number is concerned, a problem still exists
concerning density of the pulses with respect to time. Thus, the
output sync circuit 24 serves to set up synchronization for evenness.
In detail, the outputs of the adder 22 are once accumulated in the
up-down counter 24b and, as the count reaches "1", the
counter 24b makes its output logical "1" thereby providing
an output which is synchronous with a pulse appearing at the terminal
14b of the pulse distributor 14. If the count of the counter 14b
is "0", the output thereof will be logical "0"
so that no outputs are produced by the AND gate 24a.
In FIG. 3 I shows a train of pulses output from the flow meter
10 II a train of pulses appearing at the output terminal 14b of
the pulse distributor 14 III numerical values output from the compensation
amount calculator 16 IV a train of pulses output from the adder
22 and V a train of compensated pulses output from the output sync
circuit 24.
Where .vertline..alpha..vertline.<1 the adder 22 produces overflow
pulses substantially one for each output pulse of the flow meter
10 as shown in FIG. 3. Where (1+.alpha.)>1 the output of the
adder 22 becomes "2" at the ratio of 1:.alpha. as shown
in the left half of FIG. 3 and the output sync circuit 24 delivers
one additional pulse. Further, if (1+.alpha.) <1 one overflow
pulse is lost at the same ratio and, as shown in the right half
of FIG. 3 one of the outputs of the output sync circuit 22 is lost.
Data in the section identifier 18 and those in the memory 20 corresponding
to outputs of the identifier 18 may be suitably entered through
keys associated with a write circuit 26 via a temporary flow rate
section and error storage 28 a compensation constant calculate
and write circuit 30 which may be constituted by any known device.
That is, when the keys are operated to enter actually measured error
data for the respective flow rates, the data become stored in the
temporary storage 28 whereupon the calculate and write circuit 30
calculates flow rate sections necessary to perform compensation
with a predetermined accuracy and values An and Bn for each of the
sections. The values provided by the circuit 30 are loaded in the
section identifier 18.
In summary, it will be seen that the present invention provides
an instrumental error compensation apparatus which is operable with
accuracy and with wide compensation sections due to the compensation
amounts which are calculated by interpolation for individaul flow
rates.
Various modifications will become possible for those skilled in
the art after receiving the teachings of the present disclosure
without departing from the scope thereof. For example, the write
circuit 26 with keys may be substituted for by a read only memory
(ROM) which stores necessary data and is replacable with another.
Concerning the ratio of the output frequencies at the output terminals
14a and 14b of the pulse distributor 14 the requisite is merely
to make the pulses appearing at the terminal 14b higher in frequency
than those appearing at the output terminal of the adder 22 and,
hence, the ratio of 1:2 shown and described is not restrictive.
For example, if the compensation amount (1+.alpha.) output from
the calculator 16 is preselected to be smaller than one, the output
ratio of the pulse distributor 14 may be 1:1; if the flow meter
10 oscillating pulses at a frequency proportional to a flow rate
is conditioned to employ n pulses to represent a unit amount, another
ratio may be selected in which case the output of the calculator
16 may be (1+.alpha. )/n. The adder 22 may comprise a preset counter.
The sections of flow rates may be replaced by those of pulse periods
or those of frequencies, if desired. Furthermore, the illustrated
constructions such as one of the output sync circuit 24 may be suitably
modified within the scope of the invention. |