Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
The present invention relates to sensory circuits of
the type which measure impedance (primarily resistance) between
a pair of electrodes for determining the presence of an elect-
rolytic solution, such as water, between the electrodes. The
environment which led to the creation of the present invention
was the use of such circuits in carbonated beverage dispensing
machines.
Circuits for sensing the presence or absence of an elect-
rolytic solution (a liquid) between a pair of electrodes havebeen used to sense the levels of liquid in a reservoir or tank
for a number of years. This is accomplished by judicious phy-
sical placement of at least one of the electrodes within the
tank. When the liquid falls below the level of one of the
electrodes, creating an air gap between a pair of electrodes,
essentially an open circuit will be seen by electronic apparatus
measuring the impedance between the electrodes.
It is also known to use sensor circuits of this type to
detect the presence of the solid physical state of the elect-
rolytic solution between the electrodes. By way of example,the formation of ice in a cooled reservoir or tank of water
is a condition which must often be detected. In the particular
application of carbonated beverage dispensers, it is often
desirable to make sure that a layer of ice is present within
a water container and to maintain the thickness of the layer
of ice within a range defined by predetermined maximum and
minimum thicknesses.
Thus, it is known in the art to apply a voltage between
the electrodes in such a circuit to measure the electrical
resistance provided by whatever material exists between the
electrodes. This is commonly accomplished by making the
resistance appearing between two probes one leg of a resistive
voltage divider and comparing the voltage from the di~ider to
a predetermined reference voltage to gather in~ormation about
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the physical state of material between the electrodes.
In the environment of carbonated beverage dispensing
machines, the following circumstances will be well known to
those skilled in the art. When water or carbonated water is
present between a pair o~ testing electrodes, the resistance
seen between the electrodes is relativelv low and, for practical
purposes, may be considered a short circuit when other resist-
ances within the sensor circuit are on the order of several
kilohms or more. When the water reaches a level for which
one of the electrodes becomes uncovered, an air gap exists
between the electrodes ~one of which may still be covered with
water) and the resistance measured between the electrodes in-
creases dramatically. It may be effectively considered an open
circuit.
Likewise, it is known to those s~illed in the art that the
electrical resistance of water in its solid phase (ice) is
orders of magnitude greater than water in its liquid phase.
As known to those skilled in the art, continuous, or
repetitive, application of a voltage of the same polarity be-
tween two electrodes immersed in electrolyte tends to causeelectroplating on one or both of the probes. This occurs
when the ions within the electrolytic solution (which give it
its characteristics as an electrolyte) tend to migrate toward
the electrode having a polarity opposite to the electrical charge
of the particular ion. These ions tend to either accept an
electron from the negative electrode (in the case of positive
ions) or give up an electron ak the positive electrode (in the
case of negatively charged ions), thus c~using akoms of the
compound that was an ion in solution to plate out at the elect-
rodes.
It is well known that as plating continues, a layer ofa particular substance (or a pluraliky oE substances) forms
aro~nd the electrodes tending to increase the resistance thus
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leading to erratic results as the apparatus continues to
attempt to operate as a sensor. Since electrical resistance is
the primary quantity ~eing meagured between electrodes in sensor
circuits of this type J i.t is certainly possible to design a
circuit where the polarit~ of the applied voltage between
electrodes is periodically reversed. However, such an approach
would require the addition of multiplexing and/or swi-tching
components, thus increasing the complexity and cost of such a
circuit.
Thus, there is a need in the art for an inexpensive circuit
which may be used in a probe sensor cixcuik for an elect~olytic
solution, which will minimize plating of ions onto the probe
electrodes.
BRIEF SUMMARY OF THE INVENTION
The invention in one broad aspect pertains to a circuit
for testing the impedance of at least one electrolytic solution
between a plurality of probe electrodes and a reference elect-
rode comprising in combination a plurality of first re~istors,
each of the first resistors being connected between one of the
probe electrodes and a first point. A second resistor is con-
nected between the first point and the reference electrode and
a capacitor is connected between the first point and a select-
ively operable voltage source at a second point~ Control means
is connected to the selectively operable voltage source for
periodically causing the voltage souxce to provide a pulse of
predetermined voltage and predetermined duration to the second
point.
~ nother aspect of the invention pertains to a sensing
circuit for responding to resistance value variations between
a flrst point and a ground reference point at a ground potential
including a resistance/capacitance network connected between the
first point and a second point, and means connecting the resist-
ance~capacitance network to the grollnd potential through a
voltage adjusting .resistor to provide a discharge path Eor -the
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capacitance of the resistance capacitance network. Buffering
means connect the ~irst point to an input line of a micro-
processor and a D.C. voltage source is connected to the second
point. Means connect an output line of the microprocessor to
the second point, and program means resident in the micro-
processor periodically cause the output line to be at the ground
potential for a predetermined period of time, whereby a balanced
bipolar voltage signal is provided to the first point in re-
sponse to the output line being at the ground potential ~or
the predetermined period of time.
More particularly, the invention as disclosed in the
environment of carbonated beverage dispensing machines employs
a microprocessor as the principal control element of the system.
The sensors or probes for sensing ice bank sizes which are too
small and/or water levels which are too low rely upon changes in
electrical contact with the water to create abrupt and large
changes in the resistance values of the sensor circuits. The in-
terrogating signals for these sensors are balanced AC signals to
prevent electropla~ing ef~ects. These signals are also of very
short duration and with low duty cycle so that the probss are
electrically quiescent most of the time. The source of
the interrogating signals is derived from the low voltage
AC which is employed in the dispenser system, e.g. to
power dispensing solenoids, but is converted to the balanced
AC interrogating signals under control of the microprocessor.
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BRIEF DESCRIPTION OF THE DN~JING FIGURES
Figure 1 is a block diagram illustrating the
general arrange~,ent of the invention in association with
components of a carbonated beverage dispensing machine;
Figure ~ is a circuit diagram of the invention;
and
Figures 3-6 are logic flow diagrams illustrative
of microprocessor control.
DETAILED DESCRIPTION OF T~iE INVENTION
Figure 1 is a general view which includes certain
entities employed in a carbonated beverage dispensing system.
~s shown, a carbonator 1 is employed which, although not shown
in detail, essentially comprises a tank 2 pressurized with
C2 and containing a quantity of water which is carbonated by
the CO2 under pressure. This tank is provided with a grounding
conductor 3 and it is also provided, interiorally thereof, with
a portion of the evaporator coil of a refrigeration system.
The other portion of this evaporator coil is located in the
water supply tank or reservoir 4. The compressor 5 of the
refrigeration systen~ is operated to build up ice banks around
the aforesaid portions of the evaporator coil.
The carbonator 1 is also provided with a conductor
6 1eading to a probe or sensor which senses the size of the ice
bank in the carbonator and the conductor 7 leads to a water
level sensor withln the carbonator.
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Th,e conductors 8 and 9 lead respectively to the water
- level and ice bank siæe sensors in the supply tank 4, and con-
ductor 10 is the ground conductor. The water level sensors
normally contact the water in the respective tanks 2 and 4
and therefore establish a minimal resistance path to the
respective ground conductors 3 and 10. When the water
level in the tank 2 fallc below the water level sensor~ a
maximum resistance value (infinite) is established between
the conductors 7 and 3 whereas for the same condition in the
tank 4, the maximum resistance value is established between
the conductors 8 and 10. However, when an ice bank attains
the desired size such that the corresponding ice bank size
sensor becomes encased with the ice bank, the resistance
value sensed becomes maximum. Thus, for the ice bank sensors,
the normal condition is a maximum resistance path between the
conductors 6 and 3 and ~etween the conductors 9 and 10.
The conductor~ 7, 6, 3, 8, 9 and 10 are connected
as shown to the respective conductors 11, 12, 13,'14, 15
and 16 from the electronic control circuitry 17 of this
invention. As will appear later, interrogating pulses
are repetitively appl'ied to the conductoxs 11, 12, 14 and
15 so that the circuitry 17 can sense whether the aforesaid
normal conditions prevail (no control needed) or whether
one or both of the conductors 7 and 8 have a high resistance
to ground (water level control needed) and whether one or
both of the conductors 6 and 9 have a low resistance to
ground (ice bank size cont~ol needed~. The control circuitry
17 also performs an indicator function by flashing the LED 18
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when the water level in the tank 4 is low. For this
purpose, the circuitry 17 provides a pulsed signal across
the conductors 19 and 20.
In addition to the refrigerant compressor 5, other
components of the beverage dispenser are the agitator motor
21 for stirring the water in the tank 4, the fan 22 which is
energized in conjunction with the compressor 5 in order to
cool the condenser coil of the refrigerant system, and the
pump 23 which functions to transfer water from the supply
tank 4 to the carbonator 2.
The line voltage source is indicated at 24 and 25, the
line 24 being connected to the compressor 5, pump 23 and fan
22 as shown and the line 25 being selectively switchable by
the circuitry 17 to one or both of the output lines 26 and
27, The line voltage is also connected to a step-down transformer
(not shown) to provide a 24VAC source, one side of which is
grounded (see Figure 2). The outp~t line 28 completes the 24vAC
circuit to the agitator 21 when the controI so demands.
The logic performed by the control circuit is shown
in Figures 3-6. The logic flow of Figure 3 controls the
compressor 5 and the fan 22. As shown, if the ice bank
sizes of both tanks 2 and 4 become too small so as to expose
the respective sensors to contact with the water in these
tanks, this condition will be sensed as described herein-
after and the compressor (and fanjwill be switched on.
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The logic flow then illustrates a minimum "on" time of
five minutesO ~his is to assure maximum efficiellcy of
the refrigerant system. If, within this time, neither
ic bank increases in size sufficiently to "bury" its sensor,
the compressor continues to run until one (but not necessarily
both) of the sensors indicates that the corresponding ice
bank has attained the proper size (l.e., the sensor is
"buried"). The refrigexa~ion system is then switched off~
To pro~ect the compre s~r, a minimum "off" time of five
minutes is scheduled.
Figure 4 illustra~es the logic flow for the LED 18.
As shown, if the r~servoir tank 4 has its supply depleted
to the point a~--which the eorresponding low level sensor
no longer contacts the water, this condition is sensed and
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the LED is alternately nergized and deenergized 0.5 second
periods until the water supply in the tank 4 has been replenished
by personnel.
In Figure 5, it is shown that the pump 23 is
energized at any ~ime that the level sensor in the
carbonator 2 indicates too low a level o~ carbonated water,
provided there is sufficient water present in the water
supply reservoir 4. If both of these conditions prevail,
the pump 23 is switched on for a minimum of te~ seconds~
After this time, unless the water supply in $he reservoir -
4 ~alls too low in which case ~he pump is switched o~f
as shown, the pump continues to run until the carbonator
level sensor is again contacted by water and, as shown,
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the twenty second delay has timed out. This delay is
to prevent "hunting" by assuring that the water level in
the carbonator has risen to a predetermined height above the
tip of the low level sensor, If, during this twenty second
delay time the reservoir level falls too low, the pump is
immediately switched off as shown.
Figure 6 illustrates the agitator control. As
noted, the agitator 21 stirs the water in the supply tank 4.
If the pump 23 is running, the agitator is energized continuously
and is switched off only after a twenty second period subsequent
to deenergization of the pump 23. So long as the pump 23 is
not energized, the agitator is cycled twenty second "on" and
eighty second "off", as shown.
All of the above logic has been programmed into the
microprocessor for and by the manufacturer. The microprocessor
is indicated at 30 in Figure 2 and is an Intel type 3020H,
specially programmed as noted above. This chip is connected
with an external crystal circuit 31 to provide a 3.58M Hz
clock. The microprocessor is interfaced with the sensors through
the buffer/inverter 32 and which isa conventional IC, type CD40~9
as shown. The microprocessor is interfaced by means of the
output driver 33 with the control switches 34, 35 and 36
and with the sensor circuits also, as presently described.
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The driver 33 is conventional, preferably being a type
ULN 2003 as shown. The output line 20 pulses the LED 18
whereas the output lines 40,41 and 42 control the
respective switches 36, 35 and 34 through the optical coupling
circuits 37, 38 and 39, as shown.
The switch 34 connects the 24VAC supply line 43
to the conductor 28 to energize ~he agitator 21, the
switch 35 completes the cirçuit between the conductor
27 and 44 to energize the pump 23, and the switch 36 com-
pletes the circuit between the conductors 26 and 44 to
energize the compressor 5 and the fan 22.
The driver output lines 20, 42, 41 and 40 are
controllea, respectively, by the logic output lines 45,
46, 47 and 48 from the microprocessor. The remaining output
line 49 from the microprocessor strobes the output line 50
of the driver 33 twice per second to sround the line 50 for
about thre~ milliseconds per strobe. This produces a rect-
angular wave at the junction 51 with the resistor 52 having
an amplitude of six volts peak-to-peak. The voltage source
is taken from the 24VAC output at 53,54 of the step-down
transformer, after rectification by the diode 55, smoothing
by the capacitor 56 and clipping by the six volt Zener diode 57.
The resisto~ 52 and 58 provide current limiting functions.
The signal at the junction 51 is capacitively coupled
by the capacitor S9 to the sensor input lines 11, 12, 8 and
9 through the voltage adjusting resistor network 60.
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The signal coupled by the capacitor 59 to khe sensors is
a balanced AC signal and eliminates any electroplating action
by the sensors not only because of the balanced AC na-ture
of the interrogation signals but also because of their low
duty cycle.
A regulated five volt supply is provided by the type
7805 regulator 51, as shown.
The signals at the buffer output lines 62, 63, 64
and 65, by means of the logic described in conjunction with
Figures 3-6, provide the respective inputs to the driver 33
at the respective lines 45, 46, 47 and 48.
As previously noted, approximately twice every
second, line 50 and thereore point 51 is taken to ground
for approximately three milliseconds. Point 51 normally
exhibits a potential of approximately plus six volts.
Resistor 52 is current limiting and therefore, when
pin 14 of driver 33 goes low, point 51 will be pulled to ground.
Upon a subse~uent release of pin 14, point 51 returns to its
normal plus six volts.
Through the action of coupling capacitor 59 and
voltage adjusting resistor 60, a symmetrical A.C. signal is
provided to the probe lines. Most of the time, point 51 is at
a plus six volt potential. Since the lefthandmost one of voltage
adjusting resistor 60 is connected to ground, capacitor 59 will
charge to six volts during the long in~erpul~e periods. When
point 51 is suddenly taken to ground, the voltage across
capacitor 59 will not change instantaneously, and therefore the
junction between capacitor 5~ and resistor 60 is immediately
brought to minus six volts~
Although the applicant has used the words sensor or
probe in describing the invention, it is the resistance between
one of the probe leads and ground that is being measured, the
resistance being determined by whe-ther water, ice, or air
separates the particular probe lead and a second lead at
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ground potential. This resistance is in series with a
particular one of resistors 60 connected to the probe lead.
The series combi.nation of the two is in parallel with the
lefthandmost one of the voltage adjusting resistors 60.
During the three millisecond low going pulse,
capacitor 59 will begin to discharge through the lefthand-
most one of resistors 60 and pin 14 of driver 33. Since the
resulting signal at the probe lead is symmetric, it is apparent
that the time constant associated with the discharge of
capacitor 59 is such that the capacitor becbmes substantially
completely discharged during the three millisecond low going
pulse at point 51.
Because the rectangular wave at point 51 is capacitiv-
ely coupled through capacitor 59 to the probe leads, the shape
of the voltage waveform at the probe leads will be a spike
imrnediately down to minus six volts with an exponential portion
returning to zero as capacitor 59 discharges. Since resistor
52 is current limiting, and point 51 is being held at ground,
the six volts normally present at the cathode of zener 57 is
all being dropped across resistor 52 while pin 14 of driver
circuit 33 is held low.
Upon termination of the three millisecond pulse,
point 51 is returned to its plus six volt state. Since
capacitor 59 has discharged, the junction between capacitor
59 and resistors 60 is immediately elevated to plus six volts
since the voltage across the capacitor will not change
i.nstantaneously. This voltage decays exponentially as
capacitor 59 charges toward its quiscent value of a six volts
across the capacitor.
Accordingly, the capacitive coupling of the short
rectangular pulses at point 51 to the probe leads results in
the symrnetric signal.
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One of the voltage adjusting resistors 60 and the
resistance between the probe lead and ground, form a voltage
divider which is tapped at the inputs to one of the bufEers
inside buffer chip 32. The output signal from the buffers
are read by processor 30 to determine if the high resistance
or low resistance condition is present between the probe lead
and ground.
Accordingly, the present invention is designed to be
used for detecting the presence or absence of a substance
between a probe lead and a lead connected to ground potential
by directly determining a resistance variation of the medium.
Furthermore, the present invention is designed to be used in
an environment in which the probe lead and the grounded lead
will be immersed in an electrolytic solution. It i9 well known
that forcing D.C. current through an electrolytic solution be-
tween two terminals (such as the probe lead and the grounded ]ead)
will cause plating of atoms at the two terminals, wherein each
of the atoms is represented by ions in the electrolytic solution.
The plating of substances out of solution onto the leads has
the well known effect of drastically altering the resistance
between the probe material and the solution. Thus, plated
probe leads of this type quickly become useless for measurin~
resistance. Furthermore, it is known that problems of
plating can be avoided by providing bipolar symmetric signals
between probe leads immersed in such an electrolyte.
The invention allows a symmetric bipolar waveform
to be applied between the probe lead and a ground lead without
the use of double-ended balanced amplifiers which can provide
both positive and negative voltage swings with respect to
ground. It can be seen from inspection of Fig. 2 that
applicant's circuit is "single-ended", in -that all of the
processor circuitry uses a single positive power supply and a
ground lead. The use oE a resistance capacitance network, as
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descri.bed, does provide an arrangement wherein the unipolar
rectangular wave is capac.itively coupled to the probe leads
with a resulting symmetric A.C. siynal. This prevents the
problem of electroplating and provides an accurate scheme
for determining whether a high resistance condition (from
ice or the absence of the electrolytic solution) is present
between the probe terminal and the ground terminal.
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