Liquid sensor systems for liquid-employing apparatus and sensors for use in such systems

A liquid sensor system particularly suitable for commercial washing machines for drinking glasses and soft drink dispensers employs a capacitive detector providing a tortuous path through which the liquid passes and constituting part of the dielectric of the resultant capacitor. The pumps supplying the liquid through the capacitor to the dispensing means are controlled in accordance with the capacitance, as measured by a circuit. A preferred measuring circuit comprises a pulse generator; one series of pulses split from the generator output is delayed by a fixed period and fed to a flip-flop, while the second series also from the generator output are delayed by a period dependent upon the sensor capacitance and are also fed to the flip-flop, the output of which is dependent upon which series of pulses is in advance of the other. The circuit also provides for automatic adjustment of its sensitivity upon start-up or reset to adjust for differences in the sensors and the liquids.

FIELD OF THE INVENTION 
The invention is concerned with liquid sensor systems for liquid-employing 
apparatus, such as commercial washing machines for drinking glasses and 
drink vending machines, and with sensors for the liquids dispensed in such 
apparatus. 
REVIEW OF THE PRIOR ART 
One kind of apparatus to which the present invention is particularly 
applicable is a commercial washing machine for drinking glasses, such as 
is employed in restaurants, hotels, saloons, etc., to wash the glasses 
automatically or under control of an operator. An example of apparatus of 
this type is described in our U.S. Pat. No. 4,334,547, issued June 15, 
1982, the disclosure of which is incorporated herein by this reference. In 
this machine the glasses are carried by a horizontal moving perforated 
belt through successive stations in which they are pre-rinsed, washed with 
a detergent solution, and hot rinsed. Typically the wash water is 
discarded after each wash and the hot rinse water is used for the next 
wash operation. In the interests of good hygiene, in many jurisdictions, 
the operation of such machines is now the subject of health regulations 
which set minimum standards for the washing and rinsing operations, 
including the nature and concentration of the substances added to the wash 
and rinse waters. In practice, such a machine must include some method of 
detecting when any of the added chemicals, which are almost always liquids 
because of their ease of dispensing is exhausted and indicate this fact to 
the operator, usually by means of an "out of chemical" light and a buzzer. 
It may also be arranged that the machine is shut down when this occurs. It 
is desirable, if not essential, for the machine not to operate when an 
operator substitutes plain water for the exhausted chemical solution in 
order to try to keep the machine running, or attempts to economize by 
using a more diluted solution than is proper. 
Another machine to which the invention is applicable is a drink vendor of 
the kind which contains an assortment of liquid concentrates (syrups), a 
cylinder of carbon dioxide, and header tanks of hot and cold water, and 
prepares each drink upon demand. Such a machine must include a detector 
for each concentrate which will stop it from dispensing the respective 
drink when the concentrate is exhausted. A prior example of such a machine 
is disclosed in our U.S. Pat. No. 3,537,616, issued Nov. 3, 1970, in which 
a coil of tubing through which the syrup must pass to the dispensing 
nozzle is weighed and a relay is operated when the mechanical weighing 
mechanism detects that the coil is empty. 
DEFINITION OF THE INVENTION 
It is an object of the invention to provide a new liquid sensing system for 
liquid-employing machines that is flexible in the nature of the liquids 
that can be employed with it. 
It is another object to provide a new form of capacitive detector for use 
in such systems. 
In accordance with the present invention there is provided a liquid sensor 
system for liquid-employing apparatus comprising: 
a capacitive liquid sensor adapted to have the liquid to be detected as 
part of the dielectric therein, 
motor and pump means receiving liquid to be detected from a source thereof 
and pumping it through the capacitive detector, 
a capacitance measuring circuit connected to the said capacitive detector 
and producing a first electric output when the detector is empty of liquid 
and a second output when it is full of liquid, 
and motor control means operative upon receipt of said first electric 
output of the capacitance measuring circuit to stop operation of the motor 
and pump means. 
Preferably, the said capacitance measuring circuit comprises: 
an oscillator producing a pulse output, 
pulse delay means receiving the oscillator pulse output and producing a 
fixed delay comparison pulse output, 
an RC circuit including the said capacitive liquid sensor as at least part 
of the capacitance thereof receiving the oscillator pulse output and 
producing a variable delay pulse output with a delay in dependence upon 
the sensor capacitance, 
comparison means receiving the said fixed delay comparison pulse output and 
said variable delay pulse output and producing either said first electric 
output or said second electric output as the result of its comparison 
thereof. 
Also in accordance with the invention there is provided a capacitive liquid 
sensor for liquid-employing apparatus comprising a pair of spaced flat 
parallel conductive plates, and a liquid-carrying structure of 
non-conductive material between the plates establishing a tortuous 
elongated path between them for liquid which thereby constitutes part of 
the dielectric of the resultant capacitor. 
Preferably, the said liquid-carrying structure comprises a pair of flat 
parallel tight spirals of tubing of non-conductive material having 
disposed between them a flat planar member to prevent the turns of one 
coil entering between the turns of the other, each of the coils being in 
physical contact with a respective one of the parallel conductive plates.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a typical glass washing machine 10 of the type with 
which the invention can be employed, consisting of a frame mounting for 
longitudinal movement an endless perforated conveyor 12 on which the 
glasses are placed upside down. Upon activation of the conveyor, whether 
automatically or manually, the glasses are carried beneath a hood 14 in 
which they are spray-washed and then spray-rinsed by respective solutions 
delivered from nozzles 16 and 18 respectively. The wash and rinse liquids 
drain into respective tanks 20 and 22 from which they are pumped to the 
respective nozzles and to which they again drain. After each cycle of use 
the wash liquid is dumped to a drain, while the rinse liquid is 
transferred to the wash tank and used for washing. The pumps, pipes and 
valves employed for the rinsing, washing and recyling operations do not 
constitute part of the present invention and do not require specific 
illustration herein. The concentrated liquid chemical solutions to be 
added to the wash and rinse waters may be stored in respective storage 
containers (not shown) in the base of the machine or, more usually, are 
stored in larger containers external to the machine (also not shown) and 
are fed to a dispensing unit 24 of the invention via inlet pipes 26, 28 
and 30, being fed by the unit 28 to the respective nozzles 16 and 18 via 
outlet pipes 32, 34 and 36. The dispensing unit illustrated is able to 
operate with three different liquids, but only two are employed in the 
washing machine illustrated. 
The unit 24 comprises a metal container which is lockable by a lock 38, so 
that once appropriate adjustments have been set by the serviceman they 
cannot be changed by the machine operator. Liquid entering the unit 24 
from one of the storage containers, first passes to a respective 
capacitive detector 40 of the three detectors that are provided in this 
particular embodiment. As seen more clearly in FIG. 3, each detector 
consists of two flat approximately square metal plates 42 between which 
are tightly sandwiched two parallel tightly wound spiral coils 44a and 
44b, formed from plastic tubing and wound on a central core 46 so that 
their turns contact one another, the two coils having a flat planar member 
48 constituted by a thin sheet of plastic interposed between them to 
facilitate their winding by preventing the turns of one coil from slipping 
randomly into the recessed between the turns of the other coil; this also 
ensures a more uniform value of capacitance for each detector. The liquid 
is drawn through the elongated tortuous path that is thereby established 
in the detector by a respective peristaltic-type pump 50, three of which 
are provided in this embodiment, although only one is seen in full in FIG. 
2, one for each detector, each consisting of a single loop 52 of flexible 
plastic tubing, the inner surface of which is engaged by a rotor 54 having 
four axial equally circumferentially spaced bars 56 mounted thereon 
parallel to its axis of rotation. As each rotor is rotated by its 
respective motor 58 the bars press successively sufficiently tightly 
against the respective loop 52 to flatten it and force its liquid contents 
in the direction of rotation, the liquid exiting through outlet pipe 32. 
The number of detectors, pumps and motors in a particular dispensing unit 
will of course depend upon the number of liquids to be handled in the 
particular apparatus. A glass washing apparatus usually will not require 
more than three (pre-rinse sanitizer, wash detergent and final rinse), but 
a drink dispenser will usually require many more than three to give an 
adequate selection of flavours. 
In a particular embodiment each pair of detector coils 44a and 44b consists 
of about 120 cm (48 in.) of flexible transparent PVC pipe of 0.6 mm (0.25 
in.) external diameter and 0.3 mm (0.125 in.) internal diameter wound 
tightly about a central core 46 of about 10 cm diameter, the plates 42 
measuring about 10 cm by 10 cm (4 in. by 4 in.). The resultant capacitor 
when empty or full of a low dielectric liquid such as plain tap water has 
a capacitance between 30 and 50 picofarads; the specific example had a 
capacitance empty of 46 pF. The liquids normally used in glass washing 
machines will produce an increase in capacitance in such a detector of 
about 5-10 pF, when the coils are full. 
An alternative form of detector is illustrated by FIG. 4 and it will be 
seen that a plastic moulded body 60 is formed to provide, by means of 
internal baffles and partitions, in as compact a space as possible, the 
necessary elongated tortuous path 62 to ensure that a sufficient change of 
capacitance is obtained with the liquids employed, as compared with plain 
tap water. As explained above, it is desirable for the detector to detect 
the use of plain water as well as complete emptying thereof and, for the 
purpose of the subsequent description, reference to the detector being 
"empty" may also be taken as a reference to its being full of a low 
dielectric constant liquid such as tap water. 
The operation of the electrical circuit portion of the machine will now be 
described with reference to FIGS. 5 and 6. The power supply (+5 volts 
regulated) required for its operation is not illustrated and will be 
apparent to those skilled in the art. An integrated circuit element 64 and 
the associated resistor and capacitor constitute a pulse oscillator 
producing an output voltage of about 1 KHz of approximately square wave 
pulse characteristic. A practical frequency range for the oscillator is 50 
Hz to 10 KHz, the sensitivity of the circuit decreasing with increase in 
frequency. A single oscillator suffices to supply all of the detectors 
that may be provided in a single machine, and the circuit for each 
detector is fed from the oscillator via a respectivce NAND logic buffer 
66a, 66b or 66c. Each buffer output is fed through a respective time delay 
means consisting of two series-connected NAND gates 68 and 70 to one input 
of a flip-flop circuit 72; the two gates together delay the pulse output 
of the buffer by a constant period of about 300-600 nanoseconds, as 
compared to the pulse length of about 500 microseconds. The buffer output 
is also fed to an RC circuit consisting of a variable 
sensitivity-adjusting resistor 74 and the capacitive sensor 40, and the 
resulting time delay applied to this train of pulses will depend upon the 
RC value, the value of C depending upon the dielectric constant of the 
liquid, if any, present in the coils 44a and 44b. The pulse train 
appearing at junction 76 is fed to a NAND gate 78 that reshapes the pulses 
and ensures that they are at proper voltage level to be fed to the 
flip-flop 72. 
If the capacitor detector is empty, or contains a low dielectric constant 
liquid, the pulses from shaper 78 arrive with their leading edges ahead of 
the corresponding pulses from delay means 68, 70, maintaining the 
flip-flop 72 with a low output and consequent low input to an inverter 
amplifier 80; the resulting high output from amplifier 80 extinguishes an 
LED indicator or annunciator 82 to show the operator that the 
corresponding liquid is exhausted, or is not suitable. Most of the 
detergent liquids used in a glass or dish washing machine are such as 
produce an increase in capacitance of the detector of about 5-10 
picofarads; with such a liquid present, the RC value is increased by a 
corresponding amount and the delay produced at junction 76 is increased to 
the extent that the leading edges of the pulses therefrom now arrive at 
the flip-flop 72 just behind the leading edges of the corresponding pulses 
from gate 70, maintaining the flip-flop with a high output and inverter 
amplifier 80 with a low output, whereupon the LED display 82 is lit. A 
negative feedback resistor 83 is provided to stabilize the response of the 
flip-flop 72. 
The outputs of all of the flip-flops 72 of the three detectors are fed to a 
common NAND gate 84 and if any of them becomes low the output of the gate 
goes from low to high; the consequent high input to an inverter amplifier 
86 causes its output to open the contacts 87 of a relay 88 that controls 
supply of operating current to the pump motors 58. Thus, the contacts 87 
are connected via an on/off switch 112 to the axial A.C. power supply 114. 
In one position of these contacts, power is supplied to a pump control 116 
will operate the pumps, the type of control employed depending upon 
whether the pumps are of A.C. or D.C. type; the control may also operate 
the machine heaters. Upon "opening" of the contacts to stop the motors, 
another pair of the contacts is closed which cause an alarm buzzer 118 to 
sound and also cause a neon "out of chemical" indicator 120 to light. With 
this "empty" condition present the pump motors cannot operate, unless the 
relay is over-ridden by the operator by means of priming switches (not 
shown) that are operated so as to fill the system with liquid at start-up 
or re-start. 
With the small changes in capacitance of the detector involved between its 
"full" and "empty" states some means must be provided for zeroing each 
detector circuit, and also for adjusting its sensitivity to suit the 
magnitude of the dielectric property of the solution detected. Thus, a 
highly ionized liquid, such as an iodine-based sanitizing liquid, will 
produce a much larger change in capacitance than the typical syrup used 
for soft drink dispensers. To calibrate the respective circuit the 
operator presses push button switch 90 resulting in the production of a 
pulse of about 50 milliseconds duration by the series-connected RC circuit 
92, the output of which is fed to an NAND gate element 94 that is opeative 
to prevent multi-pulse production by contact bounce and to ensure the 
production of a pulse of the required level to actuate the remainder of 
the circuit. The pulse output from gate 94 is fed to the reset terminal 
RST of a counter 96 and resets it to zero if it is not at zero. The pulse 
also goes to one input of an NOR logic gate 98, causing it to produce an 
output pulse of about 200 milliseconds duration that, when fed to NAND 
logic element 100 connected to operate as an oscillator of about 1 KHz 
frequency, switches that oscillator on, causing it to feed its output 
pulses to the clock input terminal CLK of the counter 96. In this 
embodiment the counter 96 has eight couputs Q.sub.1 through Q.sub.8 to 
which a resistance chain 102 is connected, the output of the chain being 
an analog voltage whose value depends upon the count of the oscillator 
giving a digital/analog conversion. The analog voltage is fed to an input 
of an inverter 104, the output of which is connected to the junction 76 
and biases the input to the gate 78. A change in the bias voltage at 
junction 76 is equivalent in effect to a change in the value of the 
sensitivity adjusting resistor to change the RC value and the consequent 
time delay. 
Immediately when the counter 40 is set to zero the bias voltage at 76 is at 
its maximum and the circuit is at minimum sensitivity. When the counter 
oscillator 100 is switched on by operation of switch 90, the counter 96 
counts progressively upward and an increasingly positive voltage is 
generated by the chain 102, resulting in the delivery of an increasingly 
negative bias voltage to the junction 76, progressively increasing the 
sensitivity. The output from the flip-flop 72 is also affected and is fed 
as another input to the NOR gate 98; when the output signal from the 
flip-flop suddenly changes from low to high and the indicator lights the 
pulses from the gate 98 cease and the oscillator 100 stops, stopping the 
counter at the output to which it has counted, the circuit thus being set 
automatically and held at the maximum sensitivity that is required for 
effective operation, it being understood that too great sensitivity could 
result in unstable operation. Similarly, when the output signal from the 
flip-flop 72 goes from high to low because the capacitor 42 is "empty", 
the element 98 is switched off and cannot operate in the absence of an 
enabling pulse, e.g. from the switch 90. This low signal is however also 
fed to a monostable oscillator consituted by NOR gate 106 which generates 
an output pulse that is fed to the reset input RST of the counter, 
returning it to zero and minimum sensitivity so that the circuit will no 
longer operate at all. An operator may attempt to re-start the apparatus 
with a sensor "empty" by pushing reset button 90, whereupon the counter 96 
may count up to the most sensitive condition and stay there, giving the 
possibility that the apparatus will operate; this is prevented by the 
final counting stage also being connected so that it outputs to the reset 
pulse generator 106, which will reset the counter to zero and, as 
described above, cause the circuit to maintain minimum sensitivity. 
It will be understood that practical operation of the circuit will usually 
require adjustment by the manufacturer between the manual adjustment 
provided by resistor 74 and the automatic sensitivity adjustment provided 
by the circuit. An empty sensor will stop the automatic adjustment at the 
same minimum sensitivity value as a tap water filled sensor, which will in 
turn stop at a lower sensitivity than with the usual chemical materials. 
For factory calibration of the circuit it is convenient to use a "standard 
chemical" solution, which is a solution prepated so as to have a 
dielectric constant slightly higher than tap water, so as to give a set 
point standard. Upon factory calibration of the circuit, and attempting to 
reset the circuit with the standard chemical it will not adjust 
automatically, then the sensitivity is decreased manually until operation 
is obtained, which will ensure automatic operation with any proper washing 
chemicals present. 
Voltage reference in each sensor circuit is provided by a voltage follower 
operation amplifier 108 having an input fed from a voltage divider 110 
supplied with the input voltage, the output of which amplifier is fed to 
the inverter 104 in the sensor circuit to increase the bias voltage and 
reduce the sensitivity as the input voltage increases. Temperature 
compensation is also be provided, if required, by a thermostat (not shown) 
in the ground leg of the resistors 110 of the voltage divider. 
It will be seen that I have provided a new capacitive liquid sensor for 
liquid-employing machines that is simple in structure and an operating 
circuit therefor that is simple, reliable and flexible in operation, 
without the possibility of circumvention by an operator.