Data processing: generic control systems or specific application – Specific application – apparatus or process – Specific application of temperature responsive control system
Reexamination Certificate
1999-08-17
2002-04-30
Grant, William (Department: 2121)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Specific application of temperature responsive control system
C700S300000, C700S211000, C700S207000, C219S497000, C374S149000
Reexamination Certificate
active
06381518
ABSTRACT:
FIELD OF THE INVENTION
The instant invention relates to oven temperature control, and more particularly to electronic controllers and temperature sensors for controlling same.
BACKGROUND OF THE INVENTION
The maintenance of a consistent cooking temperature has long been a problem which has plagued appliance engineers, gourmet chiefs, and homemakers alike. The variation of temperature within the oven cavity from the center to the walls, as well as the variation of temperature over time during a cooking cycle may result in inconsistent cooking behavior. As an example, cake which should be cooked at 350° F. for 20 to 25 minutes may be nearly burned one time at 18 minutes, and may still be wet after 28 minutes another time. This problem is a result of many factors including the size of the oven cavity, the placement of the temperature sensor within the cavity, the type of heat source (gas versus electric), the amount of insulation used in the construction of the oven, convection currents within the cavity, as well as the sensor characteristics themselves.
Recognizing that some of these factors are beyond the appliance engineer's control, efforts were made to design a mechanism of temperature sensing and burner control which would minimize temperature variation within the cavity due to controller induced changes. In the past, oven temperature controllers utilized electromechanical controls which included at least one hot thermostat within the cavity which controlled the relays or solenoids which supplied the fuel (gas or electric) to adjust oven temperature. This control was a simple ON/OFF type control which operated the burners to maintain the sensed temperature within a hysteresis band defined, in large part, by the hysteresis of the temperature sensing element itself. Such a temperature control band is illustrated in FIG.
6
.
As may be seen with reference to
FIG. 6
, the set point line
10
indicates the desired temperature as set by the user. However, because the temperature sensor and control included hysteresis, the oven temperature control would actually not turn the burners off until the temperature had risen beyond the set point by a given hysteresis amount as illustrated by the line
12
. Once the burner control had turned off the burners, the interior temperature within the oven begin to fall. Unfortunately, due once again to hysteresis of the temperature sensing and control circuitry, the interior temperature would be allowed to fall beyond the set point
10
to a point along the line
14
. Once the temperature had fallen below line
14
, the burners would again be turned on and the temperature would begin to rise. This temperature rise would continue until line
12
was reached, and the cycle would continue. The temperature hysteresis band of these early oven temperature controls was typically as wide as 20° F., and was fairly constant for all temperature settings.
As electronic controls were introduced to appliance design, the operating characteristics of the electromechanical temperature control, including the hysteresis band, were emulated within the electronic controller. As with their electromechanical counterparts, a linear hysteresis band of approximately 20 F. was used throughout the set point band defined by line
10
of FIG.
6
. Unfortunately, utilizing a linear hysteresis band results in a large percent error at lower cooking temperatures, e.g. a 20 F. band at the 170° F. setting equates to a percent effort of +/−6%, while at the setting of 550° F. it equates to only a +/−2% error.
Recognizing this large disparity in the percentage error resulting from emulating the electromechanical sensors of the past, the next generation of electronic oven temperature controllers utilized a stepped turn on hysteresis limit
16
as illustrated in FIG.
7
. This stepped lower limit
16
allowed for the percent error allowed over the entire cooking cycle to be lowered to a more acceptable level. These next generation electronic controllers utilized three (3) to four (4) discrete lower limits as illustrated by line segments,
16
A,
16
B, and
16
C, resulting in three to four discrete hysteresis bands. Typically, these bands were set to 5° F., 10° F., and 15° F. for a three zone implementation, and to 5° F., 10° F., 15° F., and 20° F. for a four zone implementation. These discrete hysteresis zones greatly improved the cooking performance of the ovens in which these controllers were installed, especially when cooking delicate foods such as pastries, etc.
However, the non-linear nature of this lower hysteresis limit has also resulted in cooking control problems. Specifically, since a discontinuity exists between different cooking zones (e.g. defined by the upper hysteresis limit
12
and the first segment
16
A, the upper limit
12
and the second segment
16
B, and the upper limit
12
and the third segment
16
C), inconsistent cooking performance was observed when the oven was set at a temperature near the end point of two zones. This inconsistent cooking performance is a result of the controller oscillating between the two adjacent control zones of lower limit
16
. Attempts to stabilize this problem through software coding have met with limited success due to the limited code space available and the cost restraints imposed by the highly competitive appliance industry. As a result, this problem remains.
In addition to this problem, these next generation electronic controllers also suffer from a similar problem relating to initial turn on of the oven. When the oven is first turned on and a temperature is set by the user, the cavity temperature begins to climb. It is known in the oven art that the oven temperature will continue to climb once the burners are turned off during this initial pre-heat phase as illustrated by temperature curve
20
of FIG.
9
. Because of this effect, the controller utilizes a separate preheat turn off limit as illustrated in
FIG. 8
as line
18
P
or
18
NP
. The position of this preheat turn off limit
18
P
or
18
NP
in relation to the normal control hysteresis limits
12
,
16
shown in
FIG. 7
varies depending on many factors, including whether the oven is a pyro type (see line
18
P
) or a non-pyro type (see line
18
NP
).
Because a pyro type oven includes a self-cleaning cycle which raises the interior temperature to approximately 900° F., it includes much more insulation than a non-pyro type oven which does not include a self cleaning cycle. Because of this increased insulation, the pre-heat turn off limit
18
P
is typically lower than the pre-heat turn off limit
18
NP
in a non-pyro oven, and may be below the steady state burner turn on limit
16
of FIG.
7
. In a non-pyro type oven, the pre-heat turn off limit
18
NP
may actually be above the steady state turn off limit
12
shown in
FIG. 7
to allow for the increased need to heat the walls of the oven (which contain relatively little insulation compared to a pyro type oven). In any event, the pre-heat turn off limit
18
P
or
18
NP
is set to minimize temperature overshoot and maximize temperature settling time within the steady state temperature control band
12
,
16
of FIG.
7
.
However, the non-linear nature of this pre-heat turn off limit
18
P
or
18
NP
has also resulted in cooking control problems. Specifically, since a discontinuity exists between different cooking zones (e.g. in
FIG. 7
defined by the upper hysteresis limit
12
and the first segment
16
A, the upper limit
12
and the second segment
16
B, and the upper limit
12
and the third segment
16
C), the pre-heat limit
18
P
or
18
NP
was also discontinuous. These discontinuities also resulted in inconsistent pre-heating performance when the oven was first turned on and set at a temperature near the end point of two zones. As with the above, this inconsistent pre-heating performance is a result of the controller oscillating between the two adjacent pre-heat zones of lower limit
18
P
or
18
NP
. Attempts to stabilize this problem through software coding have met with limi
Alvord Robert J.
Courtney John L.
Huffington Jeffrey M.
Vitkus Kimble
Grant William
Hartman Jr. Ronald D
Morris Jules Jay
Ranco Incorporated of Delaware
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