An optical sensor arrangement comprises a light-emitting diode
which is connected to a light detector over a light connection such
as a fiber optics fiber. The fiber may be provided in the vicinity
of a vortex shedding flow meter and is bent by the passage of vortices
past the fiber. Bending the fiber causes attenuation of the light
signal from the light-emitting diode to the light detector. The
light-emitting diode is powered by a pulse generator which supplies
the diode with low duty cycle high-current pulses. The resulting
detected pulses are amplified, sampled and held to obtain peak values
for each pulse. The detected pulse is suppressed by a feedback circuit
portion which is activated only after each low duty cycle high current
pulse. The detected peaks are utilized to generate a square wave
which is used as a measurement of the variable, in particular the
passage of vortices in a vortex shedding flow meter.
What is claimed is:
1. An optical sensor arrangement comprising:
light-connection means connected between said light-emitting means
and said light-detecting means for varying an attenuation of light
from said light-emitting means to said light-detecting means according
to a variable to be measured;
pulse generator means connected to said lightemitting means for
applying low-duty cycle, high-current pulses to said light-emitting
means for generating lowduty cycle light pulses which are transmitted
to said light-detecting means for generating detected pulses;
sample and hold means having an input connected to said light-detecting
means for sampling said detected pulses and for holding a peak value
for each pulse at an output of said sample and hold means;
feedback means connected between said input and said output of
said sample and hold means for returning a signal to said input
immediately after each low duty cycle high current pulse of said
pulse generator means to suppress said detected pulses after each
low-duty cycle, high-current pulse; and
square wave generator means connected to said sample and hold means
for forming a square wave signal corresponding to the variable to
2. An arrangement according to claim 1 wherein said sample and
hold means includes a first amplifier connected to said light detecting
means, said first amplifier having a first high-power, high bandwidth
position and a second low-power low bandwidth position, said first
amplifier having a first position only when one of said low-duty
high-current pulses is present and said first amplifier having second
position at all other times.
3. An arrangement according to claim 2 including switching means
having an input connected to said pulse generator means and an output
connected to said sample and hold means for controlling said sample
and hold means to obtain a peak value for each detected pulse during
each low-duty cycle high-current pulse and for applying a signal
to said square wave generator means immediately after each low-duty
cycle high current pulse.
4. An arrangement according to claim 3 including connecting means
for connecting, said switching means to said feedback means for
returning the signal immediately after each low-duty cycle high
5. An arrangement according to claim 4 wherein said sample and
hold means includes a first sample and hold circuit including a
capacitor for being charged by an output of said first amplifier,
said switching means including a first switch connected to said
capacitor for grounding said capacitor to remove its charge immediately
before each low-duty cycle high current pulse of said pulse generator
means, and a second sample and hold circuit including a second amplifier
having an input connected to said first capacitor and an output
connected to a second switch, a second capacitor connected to said
second switch, said second switch being closed after each low-duty
cycle high-current pulse for charging said second capacitor.
6. An arrangement according to claim 5 wherein said feedback means
comprises a third amplifier connected to said output of said second
amplifier, a third switch connected to an output of said third amplifier,
an integrating amplifier having an input connected to said third
switch and an output connected to said input of said first amplifier,
said third switch being closed immediately after each low-duty cycle
high-current pulse of said pulse generator means.
7. A method of operating an optical sensor arrangement having a
light-emitter, a light-connection which attenuates light by a variable
amount in accordance with a variable to be measured, and a light-detector
connected to the light connection, comprising:
applying low-duty cycle high-current pulses to the light emitter
to generate light pulses;
detecting the light pulses with the light detector to generate
sampling and holding peak values for the detected pulses during
each low-duty cycle high-current pulse;
returning a suppressing signal to the light detector immediately
after the termination of each low-duty cycle high-current pulse;
generating a square wave using the peak values for the detected
pulses which corresponds to the variable to be measured.
8. A method according to claim 7 including amplifying the detected
pulses during each low-duty cycle high-current pulse using a high
power and high bandwidth amplifier and, at all other times, amplifying
the detected pulse using a low power low bandwidth amplifier.
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates, in general, to electronic sensor
circuits and, in particular, to a new and useful circuit for use
in a fiber optic vortex shedding flow meter which utilizes the bending
of a fiber optics fiber for determining the flow of a fluid.
Fiber optic vortex shedding flow meters are known which utilize
a fiber that can transmit light which is connected between a light
emitting device and a light detecting device. Bending in the fiber
due to the passage of shedded vortices from an obstruction in a
flow passage give a measurement of the flow rate.
Two-wire, 4 to 20 ma (milliamps) control transmitters are known
and utilized as a standard mechanism for transmitting signals.
The electronics for a two-wire, 4-20 ma industrial control transmitter
has only about 3.5 ma and 10 volts with which to operate. Fiber
optic systems presently require several ma for the light emitter,
often 200 ma or greater and as such are not compatible with two-wire,
4-20 ma transmitters.
SUMMARY OF THE INVENTION
An object of the present invention is to utilize a two-wire, 4-20
ma industrial control transmitter with a fiber optic vortex shedding
flow meter circuit which would normally not be compatible for such
a control transmitter.
Pulse mode or low-duty-cycle operation is necessary to utilize
a fiber optic sensor in a 4-20 ma transmitter. This invention gives
a method and circuit to achieve such low-duty-cycle operation and
the associated techniques to make it suitable for use in a two-wire,
4-20 ma vortex shedding flow meter transmitter.
A microbend sensor or other sensor with variable light attenuation
controlled by a process variable being measured, may be used. The
microbend sensor modulates the received light by only a small amount
which is on the order of 2 percent maximum. The electronics must
make this small change into a full-scale output. This is accomplished
by bucking the signal from the light detector and amplifying it.
The bucking is controlled by a feedback circuit so that the average
height of the peaks of the pulsed light signal are controlled to
a fixed level. This control has a long time-constant so that rapid
changes in the signal, the vortex shedding frequencies, are passed.
These frequencies are demodulated from the pulse signals by sample
and hold circuits and used to control the 4-20 ma output.
Power is gated to a preamp circuit to save power. The preamp uses
a programmable current opamp. High current operation is necessary
to amplify the fast pulses from the fiber optics. However, the low
current mode is adequate during the off period of the sampling.
Gating the current to the preamp in conjunction with the optic system
pulse results in a significant power savings.
Accordingly, an object of the present invention is to provide an
optical sensor arrangement which includes light-emitting means,
light-detecting means and light-connection means connected between
the light-emitting means and the light-detecting means for attenuating
light from the light-emitting means to the light-detecting means
according to a variable to be measured. A pulse generator with appropriate
isolation elements is connected to the light-emitting means for
applying low duty cycle high-current pulses to the light-emitting
means for generating a low duty cycle light pulse which is transmitted
to the light detector which, in turn, forms detected pulses. Sample
and hold means have an input connected to the light detecting means
for sampling the detected pulses and for holding a peak value for
each pulse at an output of the sample and hold means. Feedback means
are connected between the output and input of the sample and hold
means for returning a signal from the output to the input only immediately
after a pulse from the pulse generator to suppress the detected
pulse after each light pulse. Square wave means are connected to
the sample and hold means to provide a square wave corresponding
to the variable to be measured.
A further object of the invention is to provide a square wave corresponding
to the variable to be measured.
A further object of the invention is to provide a method of operating
an optical sensor comprising providing low duty cycle high current
pulses to a light-emitting device which is connected to a light-detecting
device over a light-connection which transmits light with varying
attenuation according to a variable to be measured. Detected pulses
in the light detector are sampled and held to obtain peak values
for each pulse which can be utilized to generate square waves corresponding
to the variable to be measured. Immediately after the end of each
low-duty cycle high-current pulse, a signal is fed back from the
output of the sample and hold mechanism to the input thereof for
suppressing the detected pulse from the detector.
A still further object of the invention is to provide an electronic
circuit for an optical sensor which is simple in design, rugged
in construction and economical to manufacture.
For an understanding of the principles of the invention, reference
is made to the following description of a typical embodiment thereof
as illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the Drawings:
FIG. 1A is a schematic diagram showing part of a circuit in accordance
with the invention;
FIG. 1B is the remainder of the circuit shown in FIG. 1A;
FIG. 2 is a graph showing the low duty cycle high current pulses
to be applied to a light-emitting diode (LED) in the optical sensor
arrangement of the invention;
FIG. 3 is a graph showing the output of a preamp connected to a
light detector associated with the light-emitting diode;
FIG. 4 is a graph showing the output of a sample and hold circuit
portion connected to the preamp; and
FIG. 5 is a graph with a portion shown in an enlarged scale illustrating
the detected current.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIGS. 1A and 1B are schematic diagrams of the electronics which
are suitable for a readout of a fiber optic microbend sensor as
used in a vortex shedding flowmeter, according to the invention.
Current to an LED 10 (Light-Emitting-Diode) is supplied as a series
of pulses, typically having a duty cycle of 1 to 2 percent, an amplitude
of 200 ma and a repetition rate or frequency of 500 to 500 Hz. Oscillator,
U6 typically a low-power CMOS version of a 555 timer such as 7555
is used to generate the control signal for the LED current. Transistor
Q1 and Q2 amplify the 7555's output. Transformer T1 serves to match
the drive requirements of 1.5 volts for the LED to the circuit's
higher drive voltage of typically 6 to 10 volts. This transformer
is typically a pulse transformer with a 4:1 turns ratio. Current
regulator U9 and capacitor C5 serve to isolate the high pulses of
current from creating voltage pulses on the power supply for the
rest of the transmitter circuit by limiting the peak current to
around 1 ma and storing charge in the capacitor between the LED
pulses. Then the LED current primarily comes from the charge stored
in capacitor C5. FIG. 2 shows the current waveform to LED 10.
The light pulses are transmitted to a light detector 20 by a fiber
optic cable shown schematically at 15. Varying attenuation is effected
typically by application of bending to the fiber 15 or the changing
of coupling at a discontinuity in the fiber. The light detector
20 converts the received light into an electrical signal, typically
a current. In the implementation in FIG. 1A, the detector supplies
a current to the following circuit.
A preamp U1 converts the detector current pulses into voltage pulses.
The integrated circuit used must be capable of low power operation
and have sufficient bandwidth to faithfully amplify the pulses.
Type T1C251 from Texas Instruments is a programmable CMOS opamp
which meets these requirements. In the low-current mode it meets
the power requirements. The high-current mode has the bandwidth
necessary for amplifying the pulses. The amplifier is switched into
the high power and high bandwidth mode only when the pulse is present
(controlled by the drive signal to the LED from line 12). Thus,
it is not drawing the high power during periods when such is not
necessary to the circuit's operation.
A peak-following sample and hold function is performed by the combination
of capacitor C1 diode CR1 and switch S1 (which is part of switch
circuits U5). S1 discharges the voltage of C1 at the beginning of
the light pulse. S1 is controlled by a one-shot multivibrator circuit
in U4 (MC14538 or MC14528) which is triggered by the beginning of
the pulse to the LED10 over line 12. Then C1 charges through diode
CR1 from the output of the preamp U1. C1 stops charging at the peak
of the preamp output and the diode prevents the immediate discharge
necessary to follow the downside of the pulse. FIG. 3 shows this
operation. Opamp U3 buffers the voltage on C1 allowing the following
circuitry to operate without affecting the signal on C1.
A second sample and hold is performed by switch S2 (also part of
U5), resistor R37 and capacitor C2. Switch S2 is closed after the
LED Pulse has finished. The peak of the pulse as stored on capacitor
C1 is sampled and stored on C2. The resistor R37 and capacitor C2
perform to low-pass filtering action to reduce the sampling frequency
(LED pulse frequency) component from the signal received from the
optical system. FIG. 4 shows the output of this circuit which is
on line 14. Circuit U5 may be a 4066 circuit.
Opamps U2b and U2c form a feedback control loop. The loop compares
the peaks of the pulses with signal ground and returns a current
to the preamp input over line 16 to drive the peaks back to ground.
This is necessary since the pulses are quite large, sufficient to
drive the preamp U1 into saturation.
FIG. 5 shows the signal and the typically 2 percent maximum modulation.
The effect of this circuit on the signal is shown also. U2B is an
integrator (or low pass filter) so that the adjustment effect is
slow-acting. Thus long term variations are removed and signal components
are not affected. Switch S3 controls the operation of this loop
so that it only operates immediately following the end of the pulse
to the LED by line 18. This removes any influences from decay on
capacitor C1's voltage between signal pulses.
Internal power supply is regulated by U8a and its associated components,
including Q3 a series pass field effect transistor (FET). Opamp
U2d divides the internal power supply, typically 10 volts, into
two 5-volt supplies with signal ground in the middle. This allows
for operation of amplifiers that have voltage swing and below signal
The typically low level sine wave signal on line 14 from the second
sample and hold is operated on by hysteresis comparator U8c, and
converts it to a rectangular or square wave. This rectangular or
square wave is used to trigger a one-shot multivibrator U7 (FIG.
1B) to give a fixed length, fixed amplitude pulse for each cycle
of the sine wave signal from the optical system. This is then averaged
by R19 and C9 and used to control the 4-40 ma output signal by the
circuit of opamp U8b, Q4 and their associated resistors. U7 may
be an MC14538 and is connected to hysteresis comparator by line
Two-wire 4-20 ma output is thus obtained at P2 and P1 from diodes
CR4 and CR6 respectively.
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.