A sensor circuit method of operating a vortex shedding flow meter
utilizes A.C. single cycle bursts to intermittently drive a sensor
which changes its state with the vortex shedding frequency. The
bursts are spaced in time by an amount larger than the burst duration.
What is claimed is:
1. A vortex shedding flow meter circuit comprising:
a sensor which varies its state with a vortex shedding frequency;
a drive circuit connected to an input of said sensor having means
for applying A.C. single cycle bursts which are spaced in time by
an amount greater than the duration of each burst to alternating
opposite ends of said sensor for driving said sensor to change its
state with the vortex shedding frequency;
wherein said sensor comprises a bridge having at least one element
which changes its state with the vortex shedding frequency and balancing
elements, said drive circuit comprising a bridge drive transformer
having an output coil connected to said bridge and an input coil,
said drive circuit being effective to apply a positive one half
cycle of each burst to one end of said input coil and a negative
one half cycle of each burst to an opposite end of said input coil.
2. A circuit according to claim 1 wherein said drive circuit comprises
a first transistor connected to one end of said input coil and a
second transistor connected to an opposite end of said input coil,
each of said transistors having bases respectively connectable to
determined positive and negative half cycles of each burst.
3. A circuit according to claim 2 wherein said output circuit
comprises an isolating transformer having an input coil circuit
connected to said bridge and an output coil, a sample and hold circuit
connected to said output coil and a differential amplifier connected
to said sample and hold circuit.
4. A method of operating a vortex shedding sensor circuit having
a drivable sensor member which produces an output that varies at
a vortex shedding frequency comprising:
driving the sensor member only during single cycle A.C. bursts
which are spaced in time by an amount greater than a duration of
each burst, wherein a frequency of said A.C. single cycle bursts
is chosen to be at least five times a maximum of the vortex shedding
BACKGROUND OF THE INVENTION
a. Field of the Invention
The present invention relates in general to circuitry for vortex
shedding flow meters and, in particular, to a new and useful circuit
and method of driving the sensor connected to a vortex shedding
flow meter to reduce energy consumption and heating of the sensor.
b. Description of the Prior Art
Vortex shedding flow meters are utilized to measure the volumetric
flow of fluids by presenting a non-streamlined obstruction termed
a bluff body in the flow path. As the flowing fluid separates around
the obstruction, vortices are produced in the wake of the obstruction
which alternate from one side of the obstruction to the other. The
number of these vortices per unit time is directly proportional
to the volumetric flow rate. The number of vortices thus can be
counted to provide a measure of the flow rate.
It is known to utilize a continuously operating bridge circuit
to sense the passing of these vortices. The bridge circuit usually
has one resistor or other element which has a varying parameter
such as resistance with the passage of each vortex. Circuit elements
are connected to the bridge to count the thus sensed passage of
vortices and to provide a signal which is proportional to the flow
rate of the fluid to be measured.
Transformers are utilized to drive one side of the bridge and also
to tap the signal from the other side of the bridge. When large
driving voltages are utilized, the sensor bridge has a tendency
to heat and lose accuracy due to drift and also impairs useful life.
Moreover, many transmitter applications do not have this high power
available on a continuous basis.
Thus an accurate vortex shedding frequency sensor was required
which was drift free, over extended operating times.
SUMMARY OF THE INVENTION
The present invention solves the prior art problems as well as
others by providing an improved method and circuitry for driving
and sensing the signal from a sensing bridge utilized in a vortex
shedding flow meter.
Rather than continuously driving the bridge, the present invention
utilizes an A.C. signal to drive the bridge sensor in discrete single
cycle bursts which are widely spaced. The sensor is thus activated
to sense the passing of vortices only during discrete time intervals,
providing a low average power to the sensor so that it remains in
a stable state having increased product life.
Accordingly, an object of the present invention is to provide a
method of operating a vortex shedding flow meter sensor circuit
having a drivable sensor member with an output which varies at a
vortex shedding frequency comprising, driving the sensor member
in A.C. bursts, said bursts being equally spaced in time and being
faster by a factor of approximately ten times the vortex shedding
Since the sensor is driven at a high peak power, the sensitivity
is improved, as it is directly related to the peak driving power.
This achieves a main object of the invention.
Another object of the invention is to provide a vortex shedding
flow meter circuit comprising, a sensor which varies its state with
a vortex shedding frequency, a drive circuit connected to said sensor
having means for applying A.C. single cycle bursts spaced in time
by an amount greater than the duration of each burst to the sensor
for producing a signal corresponding to the vortex shedding frequency.
Another object is to provide isolation between the sensor and the
output circuit to prevent grounding interactions.
Another object of the invention is to provide a sensor circuit
for a vortex shedding flow meter which is simple in design, rugged
in construction and economical to manufacture.
The various features of novelty which characterized the invention
are pointed out in the claims annexed to and forming a part of this
disclosure. For a better understanding of the invention, its operating
advantages and specific objects attained by its uses, reference
is made to the accompanying drawings and descriptive matter in which
a preferred embodiment of the invention is illustrated.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a schematic representation of a circuit utilized in accordance
with the invention;
FIG. 2 shows a typical wave form which is applied to the bridge
in the circuit of FIG. 1;
FIG. 3 shows the driving wave form applied along input 24 of the
FIG. 1 circuit to the transistor Q.sub.1 ;
FIG. 4 shows the driving wave form applied along input 26 of the
FIG. 1 circuit to the transistor Q.sub.2 ;
FIG. 5 shows the vortex shedding frequency 1/T sensed by resistor
R.sub.1 of the bridge in the circuit of FIG. 1;
FIG. 6 shows the activation of the bridge of the FIG. 1 circuit
during each vortex shedding period T, by the switches S.sub.1 and
FIG. 7 shows the input from resistor R1 of the bridge to the coil
28 of transformer T.sub.2 in the FIG. 1 circuit;
FIG. 8 shows the signal from the transformer T.sub.2 as stored
in capacitors C.sub.1 and C.sub.2 of the FIG. 1 circuit;
FIG. 9 shows the output signal of the differential amplifier measuring
the difference between signals C.sub.1 and C.sub.2 in the FIG. 1
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the drawings, FIG. 1 shows a typical circuit which
can be utilized to practice the invention. A bridge generally designated
10 comprises four resistors R.sub.1 through R.sub.4. The bridge
is utilized as a strain gauge for sensing the passage of vortices
past a bluff body of a vortex shedding flow meter (not shown). One
of the resistors R.sub.1 of the bridge 10 acts as an active component
of the bridge and reacts to the passage of a vortex by changing
resistance. Another resistor, for example R.sub.4 acts as a dummy
resistor and cooperates with the two balancing resistors R.sub.2
and R.sub.3 in a known fashion.
The bridge drive 10 has a transformer T.sub.1 with a winding 12
connected across the north and south terminals 14 and 16 respectively
of the bridge 10. The other coil 20 of the drive transformer T.sub.1
has a center tap 21 with opposite ends 23 25 respectively, connected
to collectors of a pair of switching transistors Q.sub.1 and Q.sub.2.
The center tap 21 has a voltage V+connected thereto which induces
a current which alternately flows either through switching transistors
Q.sub.1 or Q.sub.2 to a common point 22. The emitters of the transistors
Q.sub.1 and Q.sub.2 are connected to the circuit common point 22
and allow current flow thereto whenever the bases of transistors
Q.sub.1 and Q.sub.2 are excited by first and second drive signals
applied thereto from terminals 24 and 26 and switches S.sub.1 and
S.sub.2. As is shown in FIGS. 3 and 4 respectively these drive
signals are produced by the alternate actuation of the switches
S.sub.1 and S.sub.2 and are combined to form the upper and lower
half cycles of the FIG. 2 wave form by inverting the FIG. 4 wave
form through the operation of the transformer T.sub.1 in which the
currents from Q.sub.1 and Q.sub.2 flow in opposite directions from
tap 21 through coils 23 and 25 of the primary of 20. An RCA CD4066B
assembly may be used for the switches S.sub.1 and S.sub.2.
In operation, the FIGS. 3 and 4 wave form are applied along terminals
24 and 26 to alternately drive the transistors Q.sub.1 and Q.sub.2.
The A.C. signals designated 40 and 42 comprise greatly spaced single
cycle bursts 44 and 46. A typical duty cycle of ten percent is provided
to the bridge drive voltage applied along terminals 24 26 where
the duration of the pulse 44 or 46 is one tenth of the duration
between pulses. The frequency of the bridge drive is selected to
be at least five and preferably ten times that of the maximum vortex
shedding frequency to be measured. The sensor in the form of bridge
10 is thus gated ON in single cycle and widely spaced bursts. This
permits an increased drive level of approximately ten to one while
permitting operation at low total power modes as dictated by a known
4-20 mA two-wire transmitter application.
The combined A.C. pulse as shown in FIG. 2 is formed by applying
the stepped functions as shown in FIG. 3 along input terminal 24
and a stepped input of the type shown in FIG. 4 along input terminal
26. These stepped inputs activate the respective switching transistors
Q.sub.1 Q.sub.2. Upon activation of the switching transistor Q.sub.1
by the FIG. 3 pulse, the current from voltage V+ flows from the
center tap 21 of the transformer T.sub.1 primary coil 20 through
the switching transistor Q.sub.1 to the common 22. It will be understood
that the common point 22 could also be at a negative D.C. voltage
potential. This results in the formation of the top positive portion
of the FIG. 2 pulse. The pulse may be somewhat rounded by the action
of the transformer T.sub.1. Similarly, when the FIG. 4 pulse activates
the switching transistor Q.sub.2 the flow of current is from the
center tap 21 of the transformer T.sub.1 primary coil 20 through
Q.sub.2 to the common point 22 which is 180.degree. out-of-phase
with the Q.sub.1 directed flow resulting in the inversion of the
FIG. 4 pulse to thus form the bottom half of the FIG. 2 square wave.
Again, this wave in actuality may be somewhat rounded by the transformation
through transformer T.sub.1. The FIG. 2 wave form formed at coil
12 causes the intermittent activation of the sensing bridge 10 only
during time the pulses P resulting in a more stable and accurate
sensing of the vortex shedding frequency by the active resistor
R.sub.1 of the bridge.
Thus it is seen that the invention permits typically ten times
higher bridge drives without introducing any additional heating
problem over that which would be the case utilizing known lower
but continuous drive levels.
The FIG. 2 square wave is shown as a perfect square wave, whereas,
as mentioned, some rounding of the wave occurs due to the transformer
T.sub.1. However, it will be recognized that the use of a wide band
transformer T.sub.1 operating below saturation provides a wave form
of a pattern approximately as shown in FIG. 2. The important thing
to keep in mind is that the circuit is measuring and dealing with
frequencies and minor distortion of the wave form is thus acceptable.
During the activation of the bridge 10 the output of the Wheatstone
bridge will be a frequency as sensed by the variations in resistance
of the active resistor R.sub.1 due to the vortex shedding pressure
applied to the active resistor R.sub.1. This frequency as actually
sensed by the resistor R.sub.1 may be seen in FIG. 5. The signal
to the primary coil 28 of transformer T.sub.2 however, will be
a frequency-related signal only corresponding to the vortex shedding
frequency, since the bridge 10 will only be active during the FIG.
2 induced burst as are shown in FIG. 6. Note that there are approximately
10 bursts per period T of the vortex shedding frequency of FIG.
5. Thus the actual signal supplied to the coil 28 of transformer
T.sub.2 will be of the form shown as a solid line in FIG. 7. The
amplitude, KA, of the signal of FIG. 7 is the product of the vortex
shedding signal of FIG. 5 the bridge actuation signal of FIG. 6
and a scale factor determined by the resistors R.sub.1 R.sub.2
and R.sub.4. The factor K lumps this proportionality. This signal
shown as a solid line in FIG. 7 will be substantially duplicated
on the secondary windings 30 of the transformer T.sub.2 where it
will be alternately applied to capacitors C.sub.1 and C.sub.2 of
the sample and hold circuit 32 through the alternate actuation of
the switches S.sub.1 and S.sub.2. Thus a wave form for C.sub.1 and
C.sub.2 will be provided as is shown in FIG. 8. The difference amplifier
34 provides an output signal therefrom indicative of the difference
between the levels of capacitors C.sub.1 and C.sub.2 which due to
the negative nature of C.sub.2 provides a signal having twice the
amplitude A of either C.sub.1 or C.sub.2.
This is accomplished by having the east and west terminals of bridge
10 connected across the coil 28 of the isolating signal transformer
T.sub.2. The FIG. 7 signal received from the bridge 10 is supplied
by the coil 30 of the transformer T.sub.2 to the sample and hold
circuit 32. The switches S.sub.1 and S.sub.2 are connected to two
inputs of the differential amplifier 34. The switches S.sub.1 and
S.sub.2 operate in conjunction with capacitors C.sub.1 and C.sub.2
provide fullwave rectification of the output signal from transformer
T as shown in FIG. 8. The differential amplifier 34 provides a circuit-common
reference signal and has an output which shows the difference between
the signals on capacitor C.sub.1 and capacitor C.sub.2. The wave
form at the output of amplifier 34 is then a sampled representation
of the vortex frequency as shown in FIG. 9 with samples taken at
intervals determined by the FIG. 2 peaks.
While a specific embodiment of the invention has been shown and
described in detail to illustrate the application of the principles
of the invention, it will be understood that the invention may be
embodied otherwise without departing from such principles.