CA1202708A - Microprocessor control circuit responsive to sensors for electrically conductive liquids and solids - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A microprocessor is used to control the compressor and the fan of a refrigeration system, by the coolant water agitator and the carbonator water supply pump of a carbonated beverage dispensing machine. Water levels in the carbonator and carbonator water supply and the sizes of ice banks built up therein are monitored by interrogating signals applied to an array of sensors or probes in the carbonator and its water supply reservoir. The interrogating signals are under control of the microprocessor and are of short duration with low duty cycle to prevent any plating effects which could otherwise be caused by the probes. The source of the interrogating signals is a low voltage AC supply necessary for components of the dispensing system, in combination with a rectifier and smoothing capacitor together with a clipping diode and a voltage dropping resistor. The microprocessor causes the resistor to be grounded repetitively to generate a square wave voltage signal thereacross. This square wave signal is capacitively coupled to the probes to block any DC component and to apply the balanced AC interrogating signals to the probes.

Description

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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 312~;~7~

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 ~Z~2'7(~

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 -lb~

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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 27~

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 .. . . .. . . .
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, 6- ?

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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~2~Z~7~3 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 g _ ~Z~ ti~

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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~2~ 8 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 -- l]

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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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Claims (2)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A sensing circuit for responding to resistance value variations between a first point and a ground reference point at a ground potential comprising:
a resistance/capacitance network connected between said first point and a second point;
means connecting said resistance/capacitance network to said ground potential through a voltage adjusting resistor to provide a discharge path for the capacitance of said resistance capacitance network;
buffering means for connecting said first point to an input line of a microprocessor;
a D.C. voltage source connected to said second point;
means connecting an output line of said microprocess-or to said second point; and program means resident in said microprocessor for periodically causing said output line to be at said ground potential for a predetermined period of time;
whereby a balanced bipolar voltage signal is provided to said first point in response to said output line being at said ground potential for said predetermined period of time.

2. A circuit for testing the impedance of at least one electrolytic solution between a plurality of probe electrodes and a reference electrode comprising in combination a plurality of first resistors, each of said first resistors being connected between one of said probe electrodes and a first point, a second resistor connected between said first point and said reference electrode, a capacitor connected between said first point and a select-ively operable voltage source at a second point, and control means connected to said selectively operable voltage source for periodically causing said voltage source to provide a pulse of predetermined voltage and pre-determined duration to said second point.