Electric heating – Heating devices – With heating unit structure
Reexamination Certificate
2002-03-21
2004-05-11
Fuqua, Shawntina (Department: 3742)
Electric heating
Heating devices
With heating unit structure
C219S528000, C219S549000, C219S212000, C219S529000, C219S211000, C338S062000, C338S296000
Reexamination Certificate
active
06734404
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to electrically resistive heating elements and more particularly to heating elements configured such that emissions of magnetic flux from the heating elements are reduced.
BACKGROUND OF THE INVENTION
Most electronic devices experience changes in operating characteristics based on their operating temperature. For most applications, these variations are slight and can either be ignored or compensated for through calibration. However, there are instances in which environmental temperature regulation is required to ensure proper operation of an electronic device. For example, in many space applications where temperatures are extremely cold, environmental temperature regulation is required. At these extreme temperatures, electronic components may have operating characteristics that are quite different from their operating characteristics at room temperature causing them to malfunction or provide erroneous readings. Further, temperature regulation is also typically required for components that are particularly sensitive to variations in temperature. One example of such a device is a precision fiber optic gyro (FOG). Precision FOGs are particularly sensitive to changes in temperature. In precision applications, changes in the operating temperature of only a few millidegrees can affect the performance of the gyros significantly.
In many applications, strip heaters are used in temperature control systems for providing heat to electronic devices. Strip heaters include a resistive element that generates heat when a current is applied thereto. The heating element is either an elongated wire or trace of resistive material deposited on a substrate. The heating element is typically arranged in a pattern over a defined area to thereby provide uniform heat over the defined area. When current is applied to the heating element, heat is emitted from the strip heater.
While strip heaters are considered an inexpensive and efficient means of providing heat to electronic devices for environmental temperature control, there are some drawbacks to these devices. Specifically, as known in the art, when a current is applied to an elongated wire or trace, a magnetic field or flux is emitted from the heating element. This magnetic flux is problematic for several reasons. In terms of public safety, studies have linked high magnetic flux emissions as contributing to increased risk of cancer and other health problems. In addition, in terms of electronic device design, magnetic flux emissions, even at substantially lower levels, are known to negatively effect the performance of electronics and some types of fiber optics. Stray magnetic flux can also introduce output changes, drift, or noise into electronic components, which can corrupt data signals in an electronic device.
The magnetic flux emitted from a wire having infinite length is defined by the equation:
B
=(&mgr;
0
i
)/(2
&pgr;d
)
where:
&lgr;
0
=4&pgr;×10
−7
henries per meter;
i=amperes (AC and/or DC); and
d=distance from wire in meters.
As mentioned above, many conventional strip heaters employ elongated heating elements that are formed into patterns. These elongated heating elements can produce significant levels of magnetic flux. As seen from the equation above, the amount of flux emitted is inversely proportional to the distance d from the heating element. In most cases, the heating element is placed as close as possible to the item to be heated, thereby intensifying the amount of magnetic flux to which the element to be heated is subjected. Thus, although a strip heater element will serve to raise or regulate the operating temperature of the electronic device to a desired level, the magnetic flux emitted by the strip heater can adversely affect the electronic device's operation.
Considerable effort, costs, and research is expended in electronic device design applications to shield devices from and eliminate sources of magnetic flux that may disrupt operation of the electronic device. For example,
FIGS. 1A and 1B
illustrate one conventional strip heater
10
having somewhat reduced magnetic flux emissions. As illustrated, the heater
10
includes a continuous heating element
12
that is folded in half to form two portions,
14
and
16
. To reduce magnetic flux emissions, the two portions
14
and
16
are twisted about each other. When current is applied, as shown by the current arrows, current flows through the first portion
14
of the heating element
12
in one direction and through the second portion
16
in an opposite direction. In this configuration, because the current is the same magnitude through both portions,
14
and
16
, magnetic flux emitted from the first portion
14
of the heating element is substantially cancelled by the magnetic flux emitted from the second portion
16
.
Importantly, the amount of magnetic flux emission cancellation is related to the proximity of the first
14
and second
16
portions of the heating element to each other. In other words, the tighter the heating elements are twisted about each other, and the finer the elements are in terms of average diameter, the better the magnetic flux cancellation. Separations and air gaps between the first and second portions of the heating element, however, and the use of loosely wound, poorly-anchored, large diameter (0.010 inches or more) wires reduce the level of magnetic flux cancellation. As such, it is important to eliminate separations and air gaps between the first and second portions of the heating element for maximum magnetic flux cancellation.
Current heating element designs, however, do not properly address these problems. Specifically, for the most part, twisted heating element type strip heaters have been employed in heating blankets and similar applications. In these applications, heating elements are typically placed in the blanket material in a loose fashion. In this instance, the heating element is free to flex with the movements of the blanket. The flexing of a heating blanket also flexes the heating element allowing for separations and/or air gaps to form between the first
14
and second
16
portions of the heating element
12
. For example,
FIG. 1B
illustrates a conventional heating element
12
in a flexed state. As can be seen, because the first and second portions are not fixed with respect to each other, air gaps
18
form in the flexed heating element, which thereby increase the amount of net magnetic flux emissions, (i.e., non-cancelled), by the heating element. While generally safe for human use, such heating blankets and similar devices can typically emit flux at levels tens or hundreds of times higher than many sensitive electronic devices can tolerate.
Some strip heating elements are substrate based, which means they are formed by depositing resistive traces on a substrate as opposed to wires. Advances have also been made to these substrate-based strip heaters to reduce magnetic flux emissions. In these systems, it is difficult to manufacture the heating element so that it has two portions twisted about each other. For this reason, conventional low flux substrate-based strip heater systems can be designed such that the resistance traces overlay each other on the opposed sides of the substrate.
FIG. 2
illustrate a typical substrate-based strip heater system. Specifically, the strip heater system
20
includes a substrate
22
and first and second electrically resistive traces,
24
and
26
. Importantly, the first and second traces are located on opposed sides of the substrate
22
. The two traces each have opposed ends, (
24
a
and
24
b
) and (
26
a
and
26
b
), and bodies
24
and
26
. Importantly, the bodies of each of the traces overlay each other in a corresponding pattern. Further, the opposed ends
24
b
and
26
b
are connected to each other by a via
28
extending through the substrate
22
to create a continuous heating element between the first ends
24
a
and
26
a
. In these systems, similar to the heating e
Alston & Bird LLP
Fuqua Shawntina
The Boeing Company
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