Automatic temperature and humidity regulation – Motors – Electric
Reexamination Certificate
2001-01-04
2002-01-08
Walberg, Teresa (Department: 3742)
Automatic temperature and humidity regulation
Motors
Electric
Reexamination Certificate
active
06336593
ABSTRACT:
This invention relates to an electronic digital thermostat control unit and its use in a Multipoint Temperature Controller for Refrigeration/Heating Systems and other systems such as in the automobile industry.
BACKGROUND
Electrical controls in refrigeration/heating systems basically comprise a simple thermostat, motor starting relay and an over load protector for controlling the motors. Larger models also incorporate a timer and a simple logic to control an electric heater (for the automatic defrost function). Some expensive models include one or more solenoids or motors to control blowers/air flow vanes for automatic temperature control in additional compartments of the unit.
The traditional apparatus for measuring and controlling temperatures in thermostats, consists of:
i. Gas/liquid filled capillaries in which the expansion/contraction of the gas/liquid with change in temperature is used to determine/control the temperature.
ii. Bi-metallic elements in which the deflection/deformation of a bi-metallic strip of two metals with widely different coefficients of thermal expansion, determines the temperature sensed by it.
iii. Mechanical bellows that are mechanically pushed by the expanding gas/liquid and that in turn move the mechanical contact and actuate the electrical circuit at a definite ‘set’ value.
iv. The deflecting bi-metallic strips itself performs the function of a moving mechanical switches that controls the electrical circuit.
These traditional methods/apparatus suffer from the following drawbacks:
a. Imprecise and imperfect sensing of temperatures
b. Low reliability
Analog thermostat units are also known in the art, see for instance the U.S. Pat. Nos. 3,666,973, 4,119,835, 4,137,770, 4,290,481, 4,638,850, 5,520,327, 5,528,017 and 5,592,989. However they suffer from the following drawbacks:
i) Tendency to drift with temperature and time
ii) Variation from unit to unit in behavior owing to the effect of tolerances in component values and characteristics
iii) Sensitivity to noise
Use of silicon diode for sensing temperature is also known, see for instance the U.S. Pat. No. 4,137,770 wherein a forward biased silicon diode is used in a bridge circuit for sensing the temperature. The analog thermostat described in this said US Patent is useable only for a fixed temperature and not for a variable temperature. Further, the use of the silicon diode for sensing temperature has a difficulty in its calibration over a temperature range. These limitations have not been addressed in the said US Patent.
Electronic digital thermostats are also available for use. These thermostats have been described, for instance, in U.S. Pat. Nos. 5,329,991; 5,107,918; 4,948,044; 4,799,176; 4,751,961; and 4,669,654. However, these electronic digital thermostats use expensive temperature sensors such as thermistors, thermocouples or platinum resistance thermometers. These sensors require complex and expensive interface circuits. This has made these thermostats unacceptable for use in all but the most expensive models of the refrigerators. In addition, many of the benefits of electronic thermostats, such as improved reliability of operation, are not effectively realized, when these are used with the conventional starting relay, over load protector, defrost timer, and the like. Replacing each of these elements with electronic equivalents or providing energy saving and other useful end-user functions, has so far proved to be economically viable in only the most expensive models of refrigeration units.
Conventional over load protection mechanisms are based on one of the following mechanisms:
a. Bi-metallic elements in which the deflection/deformation of a bi-metallic strip of two metals with widely different coefficients of thermal expansions determines the temperature sensed by it. The mechanical dimensions and profile of the bi-metallic strip determines the temperature at which the thermal trip action occurs to perform the over load protection function.
b. Positive temperature coefficient (PTC) resistance elements, the electrical resistance of which increases dramatically with increase in temperature beyond a certain ‘threshold’ temperature, so that the resistance element effectively reduces the current in the electrical circuit to an insignificant value.
Both these methods have drawbacks. The bi-metallic over-load protector is a mechanically moving part that experiences electrical arcing every time it breaks the electrical circuit, causing electrical interference while at the same time resulting in corrosion of the contacts.
The PTC resistance element is similarly exposed to constant heat-cool cycles that create thermal stress and reduce reliability. At the same time, the electrical and temperature characteristics of the PTC element need to be matched with the load, in order to produce the correct electrical behavior. This limits flexibility and is at best a compromise in terms of effectiveness, as exact matching of PTC characteristics to the load characteristics is rarely possible.
Similarly, the conventional methods of implementing the starting relay functions and the associated problems are:
a. The use of a conventional mechanical relay, which suffers from the standard problem of electrical arcing and reduce reliability resulting from the use of a moving mechanical contact to make/break an electrical circuit.
b. The use of a positive temperature coefficient (PTC) resisted element which suffers from the same problems that are encountered in the use of a PTC element for the over load protector function.
The conventional defrost timer in a refrigeration system is an electro-mechanical or motorized timer mechanism. Since it has constantly moving mechanical parts, and an arcing electrical contact, its reliability is quite limited. Besides the problems listed above, conventional electrical controls in refrigeration systems have proved to be unwieldy and even expensive in terms of implementing multi-zone temperature control functions, that are desirable in larger refrigeration systems. In fact, some desirable functions that result in energy saving or provide useful features for the end user, are impractical to implement using such control mechanisms.
The object of this invention is to overcome the above mentioned drawbacks and provide an electronic digital thermostat which is cost effective, operationally safe and reliable.
A further object of this invention is to provide a single, multipoint compact electronic control unit by using the said electronic digital thermostat that overcomes all the above-mentioned drawbacks and provides the advantages of the expensive electronic controls currently available, at low cost.
To achieve the said objective this invention provides an electronic digital thermostat control unit which comprises:
a linear temperature sensing element,
a constant current source to drive the said linear temperature sensing element,
the output of said linear temperature sensing element is connected to an analogue-to-digital converter to produce a digital output,
the said digital output is connected to a circuit for correcting the sensitivity and offset values of the sensor using calibration data stored in non-volatile memory,
the corrected output is connected to one input of at least one digital comparator and the other input of each digital comparator receives a digital reference value from the said non-volatile memory or from variable control means,
the output of the said comparators is filtered using digital noise filters, to eliminate spurious outputs and is stored in a control latch to set/reset the input of a control latch whenever the output of digital comparator is ‘true’, for actuating the device in the consumer/industrial product that performs the temperature correction.
The said linear temperature-sensing element is resistance temperature detector and the said temperature detector is of platinum or nickel. The said liner temperature-sensing element employed in the present invention can sense temperature upto 650° C.
The linear temperature sensing element might be a semicondu
Needle & Rosenberg P.C.
Robinson Daniel
Varma Trafag Limited
Walberg Teresa
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