AU511762B2 - Refrigeration condensation detection - Google Patents

Description

FORM 4 COMMONWEALTH OF AUSTRALIA

PATENTS ACT 1962-69

IL C dnvention Application for a Patent

-T We ARDCO, INC.

of 12400 S. Laramie Avenue, Chicago, Illinois 6.658 United States of America

hereby apply for the grant of a Patent for an invention entitled

0O "INPUT CONTROL SYSTEM" S S*

S. 5@5S50

AP CAON ACCEED AND ANDM/ 5*

S S *6S S At-LOWED *5 0

SSS S which is described in the accompanying ci;omplete specification. S. This application is a Convention Application and is based on the S *5 S application numbe'red 719,804

for a patent or similar protection made in United States of America

.4'

5 5 1st September, 1976. 0*S

S W= address for service is: Our I 25 J PAT7E M11977 PATEKT OFZiC oFz s IC Care: SPRUSON. FERGUSON S S PATENT ATTORNEYS ESSO HOUSE, 127 KENT STREET SYDNEY. NEW SOUTH WALES. AUSTRALIA.

Dated this TWENTY-THIRD day of MAY 19 77. ARDCO, INC

By Segistered Patf=eApnl orney To: Tregistered Paten ttorney the Commissioner of Patents

FORM 8 COMMONWEALTH OF AUSTRALIA PATENTS ACT 196240

DECLARATION IN SUPPORT OF A CONVENTION APPLICATION FOR A PATENT OR PATENT OF ADDITION In support of the Convention Application made for a

patent for an invention entitled "INT CONTrOL SYSTM"

Full name and ad- 1, Thomas R. Rehberg dress of Declarant.

of 17931 Lahaigh Homewood, ll. 60430 United States of America do solemnly and sincerely declare as follows:-

(or, in the case of an application by a body corporate) 1. I am authorised by ARDCO,INC.

the applicant for the patent make this declaration on its behalf. basic The application p2. as defined by Section 14 of the Act was made in 9e Insert country and 2. The basic application as defined by Section 141 of the Act was made in *a date of basic appli- 9 cation and name of the United States of America on the foreign applicant. 909e*9 1st day of September 1976 by 99

9 9 ERNEST DAGEFORD 9* 9 9

3. 1 n "vM t.,uiua iii, th9 1.L%I1 'u IL i;t it, Ml L ittien. Jppl Xt. 99 (or where a person other than the inventor is the applicant) Full name and ad- dress of Inventor(s) 3. ERNEST DAGEFORD

.9 (f 3772 Cosley Avenue, Irvine, California 92705 998 United States of America *99

9 9. 9 2 5 MAY 1977 pATEajT OFTICE

9* is the actual inventor of the invention and the s upon which the applicant 9*9 9 is/aoentitled to make the application are as follows: 9 The said Conpany is the assignee of the actual inventor.

4. The basic application referred to in paragraph 2 of this Declaration was the first application made in a Convention country in respect of the invention the subject of the application. Declared at chicago,- Ill., this First day of April 1977 U.S.A.

Signature of Declarant

The Commissioner of Patents, SPRUSON 1, FERGUSON, SYDNEY.

S(11) AU-B1-25,512/77

(12) PATENT SPECIFICATION ABRIDGEMENT (19) AU

(21) 25,512/77 511,762 (22) 25.5.77 (23) 25.5.77 (24) 1.9.76 (31) 719304 (32) 1.9.76 (33) US (43) 30.11.78 (44) 4.9.80 2 F25D 21/04 H05B 1/02 G01N 25/68 (51) F25D 21/02 GO0N 27/12 (54) Refrigeration condensation detection (71) Ardco, Inc. (72) Dageford, E. (74) SF (56) 67,281/74 65,006/74 G05D, B60B, G01N, A47L (57) Claim 1. A dew point sensing system for controlling operation of electrical heating mea.s for a refrigerated unit to preclude formation of condensation o the exposed surfaces of said refrigerated unit, said system comprising: transducer means affixed to an exposed surface of.said refrigerated unit and having a surface temperature which varies with the temperature of said refrigerated 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, including operational amplifier means for producing said detection output having an amplitude representative of the maximum amplitude of the voltage across said transducer means; /2

33:4

(ii)AU-B1-25,512/77 2

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.

COMMONWEALTH OF AUSTRALIA Form PATENTS ACT 1952-69

C OMPLETE SPECOIFICATFION (ORIGINAL) F',R OFFICE USE

Int. Class Application Number Lodged

Zkis douno~ Complete Specification Lodged Se1t091 114 Acc.,-pted Published

Priority I__i !elated Art

S *40559 S S

ARDCO, INC. Q:

::1Addes oi Applicant 12400 S. Laramie Avenue, Chicago, Illinois 60658 United States of America OvaO

ERNEST DAGEFORD

."'Address for Service Spruson Ferguson, patent Attvrneys, Esso House, 127 Kent Street, Sydney, New South Wales, 2000 Australia.

a: o:Complete Specification for the invention entitled:

"INPUT CONTROL SYSTEM" 225MA 6/ PATENT OFFICE

The following statement is a full description of this inventicon, including the best method of performing it known to rTii/us:

Abstract of the Disclosure

An input control system having a sensing circuit, a

switching circuit dnd a source "f power isolated from the

sensing circuit and the switching circuit. The senI ag cir-

cuit includes a sensor having a variable electrical character-

istic, a detector for detecting variations in that character-

istic and for producing a representative output, a signal

producing circuit for producing a predetermined signal in re-

sponse to the detector output achieving a selected value, and

coupler responsive to the predetermined signal to produce a

coupling output. The switching circuit which is isolated from

the sensing circuit produces a switching signal in response

to the coupling signal to operate a switch for connecting an

electrical load to the power source. 04 Background of the Invention I 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.

20 Examples of prior art techniques for detecting

moisture content of the air, 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; 3,552,186; 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 2

r _r approach involves a substantial waste of energy. It has been recognized that it is only necessary to heat the exposed sur- faces being monitored periodically to keep them sufficiently warm in view of existing conditions to prevent the formation of moisture and frost.

The necessity to selectively and intermittently

control a variety of electrical loads often presents signific-

ant problems. Fcr example, co.mercial refrigerated units,

refrigerators and freezers, particularly commercial up-

right 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 V

necessary to prevent formation of condensate on these units,

not only for aesthetic reasons, but more importantly because g* condensate, moisture and/or frost, reduces.visibility

through the glass doors and, reduces sales.

To overcome this problem, a number of techniques o '"20 have been utilized for heating the exposed portions of the

refrigerated unit, the door frame, the mullion, and/or

the glass itself to preclude the formation of condensate.

A number of techniques have been developed for

intermittently heating the exposed surfact-s of refrigerated

units in an attempt to prevent the formation of condensate

and to keep 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 include presetting a heater to operate inter-

mittently, 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 surfaces 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 temperature of the exposed surfaces o:i the units, said surface temperature being partial- ly determined by the temperature within the refrigerated 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 su :fice, it requires con- r o

stant 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, therefore, 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. 9.

20 As a variation of the relative humidity sensors,

there are systems which adjust the duty cycle as a function a of the relative humidity increasing the duty 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 formation 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

4

I_ 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 the prevention of formation of condensation at minimum energy levels. The sensor being in the same cir- cuit as the load, the required safety for use in areas where the sensor is exposed to personnel is not present.

In order to properly insure against formation of

condensate on the exposed surfaces of a refrigerated unit,

any control system should utilize as input information all of

oe the factors which determine the point at which condensate

oo forms on the exposed surfaces of the unit. The factors that determine this point are the ambient temperature in the room, S

h the ambient relative humidity and the temperature of the ex- posed surfaces of the unit being monitored. Any sctisfactory system should be reliable, automatic, efficient, should effect

operation of the heaters for the minimum amount of time

necessary to prevent formation of condensate, and must be safe.

20 Summary of the Invention

Spr d In accordance with the present invention there is

r provided a control system for controlling input energy to a

load such as electric heaters connected to portions of a re-

frigerated unit or other units where ambient conditions on

opposite sides of a. thermal barrier differ, which is respon-

sive to all of the conditions which affect the formation of

condensate on the surfaces being monitored.

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 temperature of the surface, to the ambient temperature and to ambient relative humidity for initiating energizatiun of the heaters to prevent formation of condensation on the surfaces of the refrigerator unit being monitored. When the sensor is exposed, it is also necessary, for safety purposes, that the sensor be electrically isolated so that inadvertent contact between personnel and the sensor cannot result in an unsafe condition.

The energy control system of the present invention

provides a transducer or sensor suitably located to monitor

the exposed surfaces of a refrigerated unit. 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.

S.More specifically, a variable resistive element is affixed to the exposed surfaces of a refrigerated unit in a

manner that the temperature of the variable resistor exposed V'. 20 to ambient conditions varies in accordance with the temper-

ature of the exposed surfaces of the unit. Thus, moisture on the surface of the sensor can be indicative of the conditions

on the surfaces of the unit being monitored, and may be o 9* .utilized to control operation of heaters to maintain the unit

surfaces at a temperature just above that at which condensate

forms with a minimum expenditure of energy.

In accordance with the present invention, the sensor

includes a plurality of exposed spaced apart conductors em-

bedded 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 conductivity between the spaced con- ductors. A peak detector circuit is connected to the vari- able resistance transducer to produce a signal having an amplitude representative of the peak signal across the trans- ducer which, in turn, varies as a function of the resistance of the transducer.

Since the resistance of the transducer varies as a

function of the moisture formation on its surface, the

o.: :detected signal has an amplitude which varies in accordance with the monitored condition, the incipient formation of condensate. This detection signal is applied to one input

of a differential amplifier which produces an output of .9

O. selected magnitude when the difference between the detection signal and a constant reference signal reaches a preselected

magnitude.

This control output is terminated when the differ- 4

ence between the detection signal and the reference sigral

drops to a value less than the value required to initiate the

"output by a selected amount. The control output energizes a light emitting diode for producing a coupling signal.

A phototransistor is respcnsive to the light emitted by the light emitting diode to produce a switching signal in

response to those emissions which is applied to the control

electrode of an electronic switching element connected in

series between a source of energy and a load being controlled.

When monitoring a refrigerated unit, the load may be a plural-

ity of resistance heaters appropriately located to raise the

7

I 9, L.

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 temper-

ature 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 term-

inates 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 4:

heaters are deenergized until the incipient formation of con-

densate is again detected on the surface of the sensor.

4 A system in accordance with the present invention

*o 4 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 '20 necessary safety by isolating the exposed sensor to prevent 4

electrical hazards.

4Numerous other advantages and features of the pre-

sent invention will become readily apparent from the following

detailed description of the invention and of one embodiment thereof, from the claims and from the accompanying drawings

in which each and every detail shown is fully and completely

disclosed as a part of this specification in which like

numerals refer .to like parts.

Brief Description of the Drawings

FIGURE 1 is a perspective view of a refrigerated

8

c; -r I unit with which.the system of the present invention may be used;

FIGURE 2 is a perspective view of a sensor assembly

for use in the system of the present invention; and

FIGURE 3 is a circuit diagram of a system incorpor-

ating 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 thct the present disclosure is to be

considered as an exemplification of the principles of the invention and is not intended to limit the invention to the

embodiirent 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 unit 10 to provide a large area for the display

and viewing of merchandise contained intiie unit 10. Each door assembly 12 comprises a stationary mounting frame 14 and

a pair of pull doors 16, adapted to close the opening in the

stationary frame 14. Each of the doors 16 is of the type

which includes a metal frame 18 in which a transparent panel

20 is mounted so that merchandise in the refrigerated unit

will be clearly visible to customers. Tynically, the trans-

parent panel 20 is made of glass. The frame 14 of the unit,

the door frame 18, the transparent glass panel 20 and other

surfaces of the unit, mullions, are typically heated by

resistive heaters to preclude the formation of condensate

thereon.

9

i-

The input control system of the present invention

when used in conjunction with a refrigerated unit such as the

type shown in FIGURE 1 monitors the conditions at exposed sur-

faces 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 accomplish that purpose.

A system incorporating the present invention, in-

corporates a sensor assembly 25, shown in FIGURE 2. The sensor assembly 25 includes a thermally conductive support

plate 30 which is affixed to an exposed surface of the re-

frigerated unit, e.c to the mullion at 32 in FICURE 1, and is maintained in surface to surface contact therewith. The support plate 30 may be 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 in-

sure 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 "20 aluminum, is approximately one inch square and 1/32 inch

thick.

The sensor unit 35 is affixed to the surface of the

support plate 32. The sensor unit 35 comprises an electric-

ally insulative disk 36 which in the illustrated embodiment is a 1/32 inch thick epoxy glass disk. A pair of spaced con- ductors 38, 40 are formed on the surface of the disk 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 ele-

ment such as nickel plate and with a low contact resistance material such as gold.

In the illustrated embodiment, the insulated sup-

port disk 36 affixed to the support plate 30 is a 1/32 inch

thick epoxy glass disk on which is photoprinted a one-half

ounce copper pattern defining the spaced contacts 38,

The surface of the copper is electroplated with a 0.00005

inch anticorrosive layer of nickel plate which is electro-

plated 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 113 volt ac power line. S4

The system includes a sensing circuit 50 and a switching cir-

cuit 52 responsive to operation of the sensing circuit 50 for

S. operating an electronic switch 54 to connect a load 56, e.g.,

S* the resistive heaters, directly to the ac power source 48. o 0 Since tl' control system of the present invention

controls t.T -energization of the load 56 by selectively con-

necting 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 both from 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 conden-

sation on the unit being monitored.

Accordingly, both the sensing circuit 50 and the

switching circuit 52 are coupled to the power source 48

11

-r I through isolating step down transformers 58, 60, respectively, the primaries of which are connected across the ac source 48.

The secondary of the sensing circuit transformer 58 produces

a 12 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 dc 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

Soperational amplifier 68. The output of amplifier 68 is fed 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 volt-

age divider resistors 80, 82. The output of the differential amplifier is fed back to the positive input through a feed-

back resistor 84.

When the resistance of the resistive sensor drops to a selected value, as determined by the value of the input votage divider resistor 62 to the peak detector ampli-

f.er 68, the output of the peak detector will exceed the

reference voltage sufficiently to cause the differe'tial

amplifier to produce an output signal 85. This output is applied to a light emitting diode (LED) 86 which produces light emission i- response to this signal.

When the resistance of the sensor 35 rises as

moisture evaporates from the surface thereof, the differential

amplifier 78 terminates its signal when the level of the out-

put of the peak detector 68 achieves a second value lower than

the amplitude which initiated the output signal. This hysteresis characteristic minimizes continuous system oscil-

lation. Resistors 87a and 87b act as a voltage divider to

insure that the LED turns off in the absence of signal

The value selected for discontinuing the output signal 85 is

such as to deenergize the load 56, when desired, turn

off the electric hteters when they have been on sufficiently 0# 0

Slong tc insure the re::rigerated unit has reached a t~nmpr-

ature that precludes formation of condersate. .4 The switching circuit 52 includes the switching

•transformer 60, the secondary of which produces a 4.5 volt

250 mA signal rectified in a full wave rectifier 88 and

filtered by a filter capacitor 90 as is well known. The rectified output provides a source of power for a photo- V. transistor circuit including phototransistor 92 and resistors o* 93 and 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 phototransistor 92

produces a signal at its emitter in response to light emitted

by the LED 86 which signal is amplified by the amplifier cir- cuit 95. The output 100 of t-he 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 switch- ing amplifier 95 in response to emissions from the L2D 86.

The optical coupling 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 mai.. al switch 102 may be connected across the

triac 54 for the purpose of testing and .anual operation of the heaters when desired.

In operation, when condensate begins to form on the

Ssurface of the sensor 35, the resistance between the pair of spaced electrodes drops, until, in the illustrated embodiment,

the resistance achieves a level of 2 meg ohms The amplitude of the output of the peak detector 68 increases to

cause the differential amplifier 78 to produce a signal

which energizes the LED 86.

The phototransistor 92 responds to the light

emitted by the LED 86 to produce a signal amplified in the 0 switching amplifier 95 to close the triac switch 54 and U o energize the load 56.

As the surface of the refrigerated unit begins to

rise, so does the temperature of the s'ensor 35. Moisture evaporates from the surface of the sensor 35 causing an

increase in its resistance thereby reducing the m-iplitude of the output of the peak detector 68. When the resistance of the sensor increases sufficiently, the amplitude of the out-

put of the peak detector 68 drops to a value which terminates

the signal 85 produced by the differential amplifier 78 to de-

energize 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 condensate again begins to form on the surface of the

sensor 35 causing its resistance to drop to a value suffi-

ciently low to trigger the system once again.

Thus, there has been disclosed a condition respon-

sive input control system for sensing a condition to 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 from 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 a to be automatically responsive to a variety of factors which a. may affect the condition to which the system is designed to a* respond.

In the circuit shown in FIGURE 3, the following

components have been used satisfactorily:

'20 Diode 64 IN4006 I

o Bridge 88 each IN4006

'Diode 70 IN4446

Capacitor 66 220uf

Capacitor 72 luf

Capacitor 90 1000uf

Capacitor 102 0.05uf

Resistor 62 2 meg ohm 1%

Resistor 74 2 meg ohm

Resistor 76 47 k ohm

Resistor 80 100 k ohm 1%

15

I

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

Resistor 94 10 meg ohm

Resistor 98 100 ohm

Resistor 99 10 ohm 1 watt

Operational amplifiers 68, 78 each 1/2 LM1458 LED 86 and phototransistor 92 OPI5000 Triac 54 SPT225

Transistors 96 and 97 2N3569

From the foregoing, it will be observed that *S numerous variations and modifications may be effected without

S* departing from the true spirit and scope of the novel concept

S.of the invention. It is, of course, intended to cover by the appende. claims all such modifications as fall within the scope of the claims.

ft

0

0

Claims (1)

1. The claims defining the invention are as follows:

   1. 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

   of said refrigerated unit, said system comprising:

   transducer means affixed to an exposed surface of

   said refrigerated unit and having a surface temperature which

   varies with the temperature of said refrigerated 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 pro-

   ducing a detection output having a characteristic represen-

   tative of the value of the variable characteristic of said

   transducer means, including operational amplifier means for

   producing said detection output having an amplitude represen- tative of the maximum amplitude of the voltage across said

   it '-ransducer means;

   means responsive to said detection output for pro-

   ducing a predetermined signal in response to the characteristic

   of said detection output achieving a selected value;

   coupling means responsive to said predetermined sig-

   nal for producing a coupling control signal electrically

   A 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.

   2. A system as claimed in claim 1 wherein:

   said means responsive to said detection output in-

   cludes differential amplifier means for producing a said pre-

   determined signal having a constant amplitude in response to

   the amplitude of said detection output achieving said selected

   value.

   3. A system as claimed in claim 2 wherein:

   said differential amplifier means terminates said predetermined signal in response to the amplitude of said

   detection output achievino a second value lower than said

   selected value.

   4. A system as claimed in claim 3 wherein: e.:

   said coupling means includes means for producing an

   optical signal in response to said predetermined signal and

   oee• means electrically isolated from said optical signal producing

   means for producing said coupling control signal in response to said optical signal.

   A dew point sensing system for controlling operation of electrical heating means for a refrigerated unit so to preclude formation of condensation on the exposed surfaces

   of said refrigerated unit, said system comprising:

   transducer means affixed to an exposed surface of

   said refrigerated unit and having a surface temperature which

   varies with the temperature of said refrigerated unit, said

   transducer being exposed to the ambient air surrounding said

   refrigerated unit and having an electrical characteristic vary-

   AW ing as a function of condensation formed on the surface thereof;

   Sa 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 pro- ducing a detection output having a characteristic representat- ive of the value of the variable characteristic of said transducer means; means responsive to said detection output for pro- ducing a predetermined signal in response to the characteristic of said detection output achieving a selected value and for i :terminating said predetermined signal in response to said characteristic of said detection output achieving a second value different from said 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.

   6. A sensiny system as claimed in claim 5 wherein:

   said transducer means comprises a resistive trans-

   ducer the resistance of which varies as a function of

   condensation formed on the surface thereof.

   7. A sensing system as claimed in claim 6 wherein:

   said resistive trdnsducer m-eaA comprises an

   electrically insulating member mounted on a thermally conduct-

   ive member in contact with said refrigerated unit, and a plurality of spaced apart electrically conductive members

   w supported by said electrically insulating member and exposed to the ambient air surrounding said refrigerated unit.

   8. In a refrigerated unit, a dew point sensing system for controlling operation of electrical heating means

   for the refrigerated unit to preclude formation of condensation on the exposed surfaces of said refrigerated unit, said

   system comprising:

   resistive transducer means affixed to an exposed

   surface of said refrigerated unit and having a surface tempera-

   ture which varies with the temperature of said unit, said S. transducer means comprised of an electrically insulated member

   mounted on a +'.ermally conductive member in contact with said

   refrigerated unit, and a plurality of spaced apart electrically conductive members supported by said electrically insulated "member and exposed to the ambient air surrounding said

   t refrigerated unit;

   the resistance of said transducer varying as a function of condensation formed on the surface thereof;

   o. a source of alternating current; means coupled to said source for applying an ac vol-

   tage across said resistive transducer means and for electric-

   ally isolating said resistive transducer means from said

   source;

   operational amplifier means connected to said

   variable resistive transducer means for producing a detection

   output having an amplitude representative of the maximum amplituae of the voltage across said resistive transducer means;

   differential amplifier means connected to receive

   said detection output and producing a signal having a constant

   amplitude in response to the amplitude of said detection out- put achieving a selected value, and terminating said signal in response to said detection output achieving a second value lower than said selected value; light emitting diode means connected to the output of said differential amplifier for emitting light in response to said constant amplitude signal; phototransistor means electrically isolated from said light emitting diode for prod-,cing a control sigital in response to light emitted from said light emitting diode; a semiconductor switch having its main electrodes connected in series between said source of electrical energy and electrical heating means and having a control electrode connected to the output of said phototransistor means for connecting said source to said electrical heater means in response to said control signal; whereby said electrical heating means is energized.

   9. A system as claimed in claim 8 wherien:

   S. said operational amplifier produces said detection

   output having said selected value in response to a drop in the

   resistance of said transducer to a value indicative of the a incipient formation of condensation,

   whereby said electrical heating means is energized to preclude formation of condensation on the exposed surfaces

   of said refrigerated unit.

   A dew point sensing system being substantially

   as described herein with reference to the drawings.

   DATED this FOURTH day of JULY, 1980

   ARDCO, INC.

   Patent Attorneys for the Applicant SPRUSON FERGUSON

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