Note: Descriptions are shown in the official language in which they were submitted.
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Back~ound of the Invention
The present invention relates to energy input control
systems, and in particular, to a condensate sensing and control
system for preventing formation of condensation on a unit being
monitored.
Examples of prior art techniques for detecting moisture
content of the air, e.g., dew point, or relative humidity, and/
or for controlling formation of condensate on surfaces being
monitored are disclosed in U.S. Patents Nos. 2,435,895; 2,687,035;
2,720,107; 2,733,594; 2,733,607; 2,904,995; 2,975,638; 3,142,986;
3,293,901; 3,161,056; 3,166,928; 3,195,344; 3,195,345; 3,287,974; -
3,416,356; 3,422,677; 3,460,352; 3,552,1~6; 3,599,862; 3,696~360;
3,859,502; and British patent No. 900,194. Continuously heating "
such components is not desirable because the heated surfaces may
appear warm to the touch, and because that approach involves a
substantial waste of energy. It has been recogni~ed that it is
only necessary to heat the exposed surfaces being monitored
periodically to keep ~hem ~ufficiently ~arm in view of existing
conditions to prevent the formation of moisture and frost.
The necessity to selectively and intermittently con-
trol a variety of electrical loads o~ten presents significant
problems. For example, commercial refrigerated units, e.g.,
refrigerators and freezers, particularly commercial upright units
located in retail stores, are typically enclosed with glass
doors with the products contained therein visible to the consumer.
Typically, metal framed glass doors are used in these
units. From the retailers point of view, it is necessary to
prevent formation of condensate on these units, not only for
aesthetic reasons, but more importantly because condensate,
e.g., moisture and/or frost, reduces visibility through the
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glass doors and, reduces sales.
To overcome this problem, a n~mber of techniques have
been utilized for heating the exposed portions of the refrigerated
unit, e.g., the door frame, the mullion, and/or the glass itself
to preclude the formation of condensate.
A number of techniques have been developed for inter-
mittently heating the exposed surfaces of refrigerated units in
an attempt to prevent the formation of condensate and to }ceep
the surface temperatures of the glass, the door frame~ the
outer frame, and the mullions just above that point at which
formation of condensate commences. Some of these techniques in-
clude presetting a heater to operate intermittently, but according
to a fixed cycle. Another approach is to sense the relative
humidity in the room in which the unit is disposed and to turn on
the heaters when the relative humidity exceeds a preselected value.
However, formation of condensate on the qurfaces of refrigerated
units is a function not only of the relative humidity in the
room, but also of the temperature in the room and of the temper-
ature of the exposed surfaces of the units, said surface temper-
ature being partially determined by the temperature within therefrigerated units. Sensing relative humidity alone does not
provide sufficient information to minimize energy utilization
while simultaneously precluding formation of moisture.
Another approach is to adjust the duty cycle of the
heater manually. While this may suffice, it requires constant
monitoring by store personnel since the formation of frost can
vary as a function of the number of times the doors are opened
and as a function of changes in ambient conditions. It is
common, thexe~ore, for such systems to be set at a level to
insure prevention of frost on the unit under the worst conditions,
resulting in wasted energy.
As a variation of the relative humidity sensors, there
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are systems which ad~ust the duty cycle as a function of the
relative humidity-increasing the du~y cycle of the heaters as
relative humidity increases. Again, since the point at which
condensate forms is a function of more than the relative humidity
in the ambient atmosphere, such systems are often adjusted to
operate with a longer duty cycle than is necessary in order to
preclude ~ormation of condensate.
One of the patents identified above, U.S. patent
No. 3,696,360, discloses an alarm for warning of impending
condensation on an element being monitored. While the system
disclosed in this system is designed to be responsive to the
various conditions which affect formation of condensation,
it is believed the circuit disclosed, which includes a sensor
and a load would not provide the sensitivity or accuracy required
to insure pre~ention formation of condensation at minimum energy
levels. The sensor being in the same circuit as the load, the
required safety for use in areas where the sensor is expos~d
to personnel is not present.
In order to~properly insure against formation of
condensate on the exposed su~faces of a refrigerated unit, any
control system should utilize as input information all o~ the
factors which determine the point at which condensate forms on
the exposed surfaces of the unit. The factors that determine
this point are the ambient temperature in the room, the ambient
relative humidity and the temperature of the exposed surfaces of
the unit belng monitored. Any satisfactory system should ba
reliable, automatic, efficient, should effect operatlon of
the heaters for the minimum amount of time necessary to prevent
formation of condensate, and must be safe.
Summary of tha Invention
In accordance with the present invention there is
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provided a control system for controlling input energy to a
load such as electric heaters connected to portions of a
refrigerated unit, or other units where ambient conditions
on opposite sides of a thermal barrier differ, which is
responsive to all of the conditions which affect the formation
of condensate on the surfaces being monitorecl.
A system in accordance with the present invention
incorporates a sensor affixed to an exposed surface of the
refrigerated unit, the sensor being responsive to the tempera-
ture of the surface, to the ambient temperature and to ambientrelative humidity for initiating energization of the heaters to
prevent formation of condensation on the surfaces of the refriger-
ator unit being monitored. When the sensor is exposed, it is
also necessary, for safety purposes, that the sensor be elec-
trically isolated so that inadvertent contact between personnel
and the sensor cannot result in an unsafe condition.
The energy control system of the present invention pro-
vides a transducer or sensor suitably located to monitor the ex-
posed surfaces of a refrigerated unitO The sensor is electrically
isolated from the power circuit connected to electric heaters,
and accurately and reliably detects the point at which condensate
forms on the unit and controls operation of heaters to prevent
formation of condensation on the exposed surfaces being monitored.
More specifically r a variable resistive el~ment is
affixed to the exposed surfaces of a refrigerated unit in a
manner that the temperature of the variable resistor exposed
to ambient conditions varies in accordance with the temperature
of the exposed surfaces of the unit. Thus, moisture on the sur-
face of the sensor can be indicative of the conditions on the
surfaces of the unit being monitored, and may be utilized to
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control operation of heaters to maintain the unit surfaces at
a temperature just above that at which condensa~e forms with a
minimum expenditure of energy.
In accordance with the present invention, the sensor
includes a plurality of exposed spaced apart conductors embedded
in an electrically insulated body which, in turn, is mounted on
a thermally conductive member suitably affixed to or mounted on a
surface of the unit. A signal is applied across the resistive
element, the resistance of which varies in accordance with the
amount of moisture on its surface, moisture altering the con-
ductivity between the spaced conductors. A peak detector circuit
ls connected to the variable resistance transducer to produce a
signal having an amplitude representative of the peak signal
across the transducer which, in turn, varies as a function of
the resistance of the transducer.
Since the resistance of the transduGer varies as a
function of the moisture formation on its surface, the detected
signal has an amplitude which varies in accordanca with the
mo~itored condition, i.e., the incipient formation of condensate.
This detection ~ignal is applied to one input of a differential
amplifier which produces an output of selected magnitude when
the dif~erence between the detection signal and a constant
reference signal reaches a preselected magnitude.
This control output is terminated when the dif~erence
between the detection signal and the reference signal drops to
a value less than the value required to initiate the output by
a selected amoun~. The control output energizes a light lemitting
diode for producing a coupling signal.
A photo transistor is responsive to thle light emittea
by the light emitting diode to produce a switching signal in
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response to those emissions which is applied to the control
electrode of an electronic switching element connectèd in series
between a source of energy and a load being controlled. When
monitoring a refrigerated unit, the load may be a plurality of
resistance heaters appropriately located to raise the temperature
of the exposed surfaces to preclude formation of condensate on
those surfaces.
When the temperature of the exposed surfaces rises, in
response to energization of the heaters, above the temperature
at which moisture forms, the temperature of the sensor also rises
causing moisture to evaporate from its surface. The resulting
increase in the resistance of the sensor terminates the control
output.
Emissions from the light emitting diode terminate, the
switching signal from the phototransistor terminates and the
signal applied to the control electrode of the switching element
is thus ended. The switching element opens and the heaters
one deenergized until the incipient formation of condensate is
again detected on the surface of the sensor.
A system in accordance with the present invention
provides efficient, accurate and reliable monitoring of the ~ ;
condensate formation or other conditions to be monitored, utilizes
the minimum amount of energy necessary to maintain the desired
condition, and at the same time provides the necessary safety
b~ isolating the exposed sensor to prevent electrical hazards.
Thus in accordance with a broad aspect of the invention,
there is provided a dew point sensing system for controlling
operation of electrical heating means for a refrigerated unit to
preclude formation of condensation on the exposed surfaces o said
refrigerated unit comprising:
transducer means affixed to an exposed surface of said re-
frigerated unit and having a surface temperature which varies
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with the temperature of said ref~igerated unit, said
transducer being exposed to the ambient air surrounding
said refrigerated unit and having an electrical .
characteristic varying as a function of condensation formed
on the surface thereof;
a source of electrical power;
means coupled to said source for applying a voltage
across said transducer means and for electrically isolating
said transducer means from said source; ~
means connected to said transducer means for :
producing a detection output having a characteristic
representative of the value of the variable characteristic
of said transducer means;
means responsive to said detection output for
producing a predetermined signal in response to the
characteristic of said detection output achieving a selected
value,
coupling means responsive to said predetermined
: signal for producing a coupling control signal electrically
isolated from said predetermined signal; and
switching circuit means responsive to said isolated
coupling control signal for connecting said source to said
electrical heating means;
whereby said electrical heating means is energized. : r
Numerous other advantages and features of the present
invention ~ill become readily apparent from the following
detailed description of the invention and of one embodiment
thereof, fram the claims and from the accompanying drawing
in which each and every detail shown is fully and completely
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1~13369~
~, .
disclosed as a part of this specification in which like numerals
refer to like parts.
Brief Description of the Drawln~
FIGURE 1 is a perspective view of la refrigerated unit
with which the system of the presen~ invention may be used;
FIGURE 2 is a perspective view of a sensor ass~ly
for use in the system of the present invention; and
FIGURE 3 is a circuit diagram of a system incorporating ;~
the present invention.
Description of the Preferred Embodiment
While this invention is susceptible of embodiment
in many different forms, there is shown in the drawings and
will herein be described in detail one specific embodiment, with
the understanding that the present disclo~ure is to be
considered as an exemplification of the principles o~ the invention
and is not intended to limit the invention to the embodiment
illustrated. The scope of the invention will be pointed out
in the appended claims.
FIGURE 1 illustrates the front of refrigerated unit 10
incorporating a pair of door assemblies 12 mounted side
by side in the uni~ 10 ~o provid~ a large area ~or ~he display ~-
and viewing of marchandise contained in the unit 10. Each
door assembly 12 comprises a stationary mounti~g frame
14 and a pair of pull doors 16, adapted to close the opening
in the stationary frame 1~. Each o~ the doors 16 is
of the type which includes a metal frame 18 in which a transparent
panel 20 is mounted ~o that merchandise in the refrigerated unit
will be clearly visible to customers. Typically, the transparent
panel 20 is made of glass. The frame 14 of the unit, the door
frame 18, the transpare~ glass panel 20 and other surfaces of
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the unit, e.g., mullions, are typical}y heat~ed by resistive
heaters to preclude the formation of condensate thereon.
The input control system of the prlesent invention
when used in conjunction with a refrigerated unit such as the
type shown in FIGURE 1 monitors the conditions at exposed surfaces
of the unit and controls operation of electric heaters to
preclude formation of condensate on such surfaces while utilizing
the minimum amount of energy required to accomplish that purpose.
A system incorporating the present invention, incor-
porates a sensor assembly 25, shown in FIGURE 2. Thesensor assembly 25 includes a thermally conductive support plate
30 which is affixed to an exposed surface of the refrigerated
unit, e.g., to the mullion at 32 in FIGURE 1, and is maintained
in 6urface to surface contact therewith. The support plate 30
may ~e affixed to the mullion 32 by metallic fasteners such as
screws (not shown) which pass through the apertures 34 in the
support plate 32 into the mullion to insure maximum thermal
conductivity between the plate 32 and that portion of the
refrigerated unit 10 to which it is affixed. In one embodiment,
the support plate is made of aluminumr is approximately one inch
square and 1/32 inch thick.
~ he sensor unit 35 is affixed to the sur~ace of the
support plate 32. The sensor unit 35 comprises an electrically
insulative disk 36 which in the illustrated embodiment is a
1/32 inch thick epoxy glass disc. A pair of spaced conductors
38, 40 are formed on the surface of the disc 36 which, in the
illustrated embodiment, include interleaved generally circular
conductive fingers 38a, 40a spaced apart from each other~and
electroplated with an anticorrosive conductive element such as
nickel plate and with a low contact resistance material such
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as gold.
In the illustrated embodiment, the insulated support
disc 36 affixed t~ the support plate 30 is a 1/32 inch thick
epoxy glass disc on which is photoprinted a one-half ounce
copper pattern defining the spaced contacts 38, 40. The
surface of the copper is electroplated with a 0.00005 :inch
anti-corrosive layer of nickel plate which is electroplated
with a 0.00003 inch thick low contact resistance layer of
gold.
The sensor 35 forms part of the input control system ;.
shown in FIGURE 3. The system of FIGURE 3 includes a source 48
of ac potential, typically a 110 volt ac power line. The system
includes a sensing circuit 50 and a switching circuit 52 responsive
to operation of the sensing circuit 50 ~or operating an electronic -r
switch 54 to connect a load 56, e.g., the resistive heaters,
directly to the ac power source 48.
Since the control system of the present invention
controls the energization of the load 56 by selectively connecting
it directly to a 110 volt source 48 and since the sensor 35
which forms a part of the control system is located on exposed
surfaces of a refrigerated unit which is being monitored, an
: electrical shock hazard could exist unless the system including
the sensor 35 is isolated bo~h f~om the load 56 and from the
source 48. Isolation is further beneficial in that the energizing
and deenergizing of the load does not affect the performance of
the system in sensing incipient formation of condensation and
precluding formation of condensation on the unit being monitored.
Accordingly, both the sensing circuit 50 and the
switching circuit 52 are coupled to the power source 48 through
isolating step down transformers 58, 60, respectively, the primaries
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of which are connected across the ac source 48. The
secondary of ~he sensing circuit transformer 58 produces a
twelve volt 25 mA output which is applied across a voltage
divider consisting of the resistive sensor 35 and a second
resistor 62 connected in series across the secondary of the
sensing circuit transformer 58. This secondary voltage is also
applied across a rectifier 64 and filter capacitor 66 to produce
a d.c. control voltage and reference voltage. The junction
between the resistive sensor 35 and the voltage divider resistor
62 is connected to the plus input of an operational amplifier
68.
The output of amplifier 68 is ed back to the
negative input of amplifier 68 through rectifier 70. The
operational amplifier 68 acts as a peak detector and produces a
dc output which is integrated by capacitor 72 and resistor 74
and is applied through an input resistor 76 to the positive
terminal of a second operational amplifier 78 which acts as a
differential amplifier. The other input to the differential
amplifier 78 is connected to the junction of a pair of voltage
divider resistors 80, 82. The output of the differential amplifier
is fed back to the positive input through a feedback resistor
84.
~ hen the resistance of the xesistive sensor 35 drops
to a selected value, as determined by the value of the input
voltage divider resistor 62 to the peak detector amplifier 68,
the output of the peak detector will exceed the reference
voltage sufficiently to cause the differential amplifiex to
produce an output signal 85. This output i5 applied to a
light ~emitting diode (~ED) 86 which produces light emission in
response to this signal.
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When the resistance of the sensor 35 rises as moisture
e~aporates from the surface thereof, the differential amplifier
78 terminates its signal when the level of the ou~put of the
peak detector 68 achieves a second value lower than the amplitude
which initiated the output signal. This hys1teresis characteristic
minimizes continuous system oscillation. Resistors 87a and
87b act as a voltage divider to insure that the LED turns off
in the absence of signal 85. The value selected for discontinuing
the output signal 85 is such as to deenergize the load 56 , when
desired, i.e., turn off the electric heaters when they have been
on sufficiently long to insure the refrigerated unit has reached
a temperature that precludes formation of condensate.
The switching circuit 52 includes the switching
transformer 60, the secondary of which produces a 4.5 volt250mA
signal rectified in a full wave receifier 88 and filtered by a
filter capacitor 90 as is well known. The rectified output
provides a source of power ~or a phototransistor circuit including
phototransistor 92 and resistors 93and 94 and for an amplifier
circuit 95 which includes a pair of transistors 96, 97 and
resistors 98, 99 connected to the output of the phototransistor
92. The phototransis~or 92 produces a signal at its emitter in
response to light emitted by the LED 86 which signal is amplified by
the amplifier circuit 95. The output 100 of the amplifier circuit
95 is applied to a gate electrode of the electronic switch 54, a
triac. A capacitor 101 is connected across the gate electrode
to minimize transients.
The main electrodes of the triac 54 are connected in
series with the power source 48 and the load 56. The triac 54
closes in response to the output 100 of the switching amplifier
95 in response to emissions from the LED 86. The optical coupling
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between the sensing circuit 50 and the switching circuit 52
isolates the sensor 35 from the load 56 to positively insure
safety and insure that the sensor may in no way be connected
across the 110 volt line.
A manual switch 102 may be connected across the triac
54 for the purpose of testing and manual operation of the
heaters when desired.
In operation, when condensate begins to form on the
surface of the sensor 35, the resistance between the pair of
spaced electrodes drops, until, in the illustrated embodiment,
the resistance achieves a leyel of 2 meg-ohms ~ 5%. The amplitude
of the output of the peak detector 68 increases to cause the
dif~erential amplifier 78 to produce a signal 85 which energizes
the LED 86.
The phototransistor 92 responds to the light emitted
by the LED 86 to produce a signal amplified in the switching
amplifier 95 to close the triac switch 54 and energize the load
56.
As the surface of the refrigerated unit begins to rise,
so does the temperature of the sensor 35. Moisture evaporates
from the surface of the sensor 35 causing an increase in its
resistance thereby reducing the amplitude of the output of the
peak detector 68. When the resistance of the sensor increases
sufficiently, the amplitude of the output of the peak detector
68 drops to a value which terminates the signal 85 produced by
the differential amplifier 78 to deenergize the LED 86, thereby
terminating the output of the phototransistor 92 and causing
the triac switch 54 to open. The load 56 is deenergized.
Formation of condensate has been precluded. The load remains
deenergized until such time as the condensa e again begins to
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form on the surface of the sensor 35 causing its resistance to
drop to a value sufficiently low to trigger the system once again.
Thus there has been disclosed a condition responsive
input control system for sensing a condition ~o be monitored,
for providing a switching signal to control a load related to
that condition in which the sensor, the sensing circuit and the
switching circuit are all isolated from the load and ~rom any
power source required to operate the load. The system in
accordance with the present invention is safe, accurate, reliable,
simple and self-contained, and is adapted to be automatically
responsive to a variety of factors which may effect the condition
to which the system is designed to respond.
In the circuit shown in FIGURE 3, the following
components have been used satisfactorily:
Diode 64 - IN4006
Bridge 88 - each IN4006
Diode 70 - IN4446
Capacitor 66 220uf 25v ~ !
Capacitor 72 luf 25v
Capacitor 90 lOOOuf lOv
Capacitor 102 0.05uf lOv
Resistor 62 2 meg ohm 1
Resistor 74 2 meg ohm
Resistor 76 47 k ohm
Resistor 80 100 k ohm 1%
Resistor 82 100 k ohm 1
Resistor 84 2 meg ohm
Resistor 87a 15 k ohm
Resistor 87b 2 k ohm
Resistor 93 270 ohm
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Resistor 94 l0 mey ohm
Resistor 98 l00 ohm
Resistor 99 10 ohm l watt
Operational Amplifiers 68, 78 - each l/2 LM1458
~ED 86 and phototransistor 92 - OPX5000
Triac 54 - SPT225
Transistors 96 and 97 - 2N3569
From the foregoing, it will be observed that numerous
variations and modifications may be effected without departing
from the true spirit and scope of the novel concept of the
invention. It is, of course, intended to cover by the appended
claims all such modifications as fall within the scope of the
claims.
