Power control for appliance having a glass ceramic cooking surface

An improved power control system for a household cooking appliance of the type having a glass ceramic cooking surface and at least one radiant heating unit disposed beneath the cooking surface operable at a plurality of user selectable power settings. At least one of the power settings has associated with it predetermined maximum and minimum reference temperatures defining a temperature band representative of the steady state temperature range for the glass ceramic support surface proximate the heating unit when heating normal loads at that power setting. The power control system includes an arrangement for sensing the temperature of the glass ceramic cooking surface proximate the heating unit and is operative to operate the heating unit at a power level other than the power level corresponding to the user selected power setting when the sensed glass ceramic support surface temperature is outside the predetermined reference temperature band associated with the selected power setting to more rapidly bring the temperature within the band and thereby causing the heating unit to respond quickly to changes in user selected power setting.

BACKGROUND OF THE INVENTION 
This invention relates generally to glass ceramic cooktop appliances and 
particularly to electronic power control systems for such appliances. 
Commonly assigned U.S. Pat. No. 4,740,664 to Payne et al, which is hereby 
incorporated by reference, discloses a cooktop appliance equipped with 
heating units which radiate substantially in the infrared region (1-3 
microns) in combination with a glass ceramic cooktop support surface which 
is substantially transparent to infrared radiation. Utensils placed on the 
cooktop surface are heated primarily by radiation directly from the 
heating unit rather than by conduction from the glass ceramic material. 
Though the glass ceramic is substantially transparent to the radiation, a 
portion of the energy radiating from the heating unit is absorbed by the 
glass ceramic, as is a portion of the energy reflected by the utensil 
being heated. Heat transfer from the glass ceramic is primarily by 
conduction to the utensil. 
The power control system disclosed in the aforementioned Payne et al 
patent, using glass ceramic temperature information derived from a 
temperature sensor located directly over each heating unit controls the 
output power of each heating unit to protect the glass ceramic against 
overheating caused by abnormal load conditions such as operating the unit 
with no utensil present, use of badly warped utensils, or heating an empty 
utensil. 
In that arrangement the temperature measurements are obtained by measuring 
the resistance of the bottom surface of the glass ceramic material above 
the heating units. The temperature information so obtained is sufficiently 
accurate for protecting the glass ceramic against overheating. 
Since radiation is the primary heat transfer mechanism for utensils being 
heated on such cooktops, the system responds more quickly to changes in 
user selected power settings than the conventional cooktops relying on 
conduction heating. However, the thermal inertial of the glass ceramic 
material results in a slower response than that achievable with closed 
loop automatic surface unit systems which measure utensil temperature 
directly and control the output power of the heating unit to achieve and 
maintain the user selected utensil temperature. Inherent inaccuracies in 
this temperature measurement system due to the temperature gradient 
through the glass ceramic material, the temperature gradient from the top 
of the glass ceramic material to the bottom of the potentially warped pan, 
and other sources of error, render this temperature sensor arrangement 
incompatible with such closed loop systems. Locating a sensor to directly 
sense utensil temperature would add cost and complexity to the 
manufacturing process and by protruding above the cooktop would negate at 
least to some extent the appearance and cleanability advantages of the 
smooth cooktop surface. Hence, there exists a need for a control 
arrangement which provides a faster response to changes in power setting 
than that of typical open loop control systems while retaining the 
advantages in cost, cleanability and appearance of the smooth glass 
ceramic cooktop surface. 
Therefore, it is a primary object of the present invention to provide an 
improved power control system for a glass ceramic cooktop appliance which 
reduces the time required for the system to reach steady state conditions 
in response to changes in the user selected power setting using a 
temperature sensor mounted to the bottom or inner surface of the glass 
ceramic cooktop support surface. 
SUMMARY OF THE INVENTION 
In accordance with the present invention an improved power control system 
is provided for a household cooking appliance of the type having a glass 
ceramic cooking surface for supporting loads to be heated and at least one 
radiant heating unit disposed beneath the glass ceramic cooking surface to 
heat loads supported thereon. User actuable input selection means enables 
the user to select one of a plurality of power settings for the heating 
unit. The power control system includes temperature sensing means for 
sensing the temperature of the glass ceramic cooking surface proximate the 
heating unit, and power control means responsive to the input selection 
means and to the temperature sensing means and operative to normally 
operate the heating unit at a power level corresponding to the user 
selected power setting. 
Advantageous use is made of the novel discovery that for at least some of 
the user selectable power settings, the steady state temperature of the 
glass ceramic surface will come within a predictable temperature band for 
substantially all normal loads when the heating unit is operated at the 
corresponding power level. To this end, at least one of the plurality of 
power settings has associated with it predetermined maximum and minimum 
reference temperatures defining a temperature band representative of the 
steady state temperature range for the underside of the glass ceramic 
support surface proximate the heating unit when heating normal loads at 
that power setting. The power control means is further operative to 
operate the heating unit at a power level other than the power level 
corresponding to the user selected power setting when the sensed glass 
ceramic support surface temperature is outside the predetermined reference 
temperature band associated with the selected power setting to more 
rapidly bring the temperature within the band and thereby causing the 
heating unit to respond quickly to changes in user selected power setting. 
In a preferred form of the invention the minimum reference temperature for 
each power setting represents the temperature level which the glass 
ceramic will normally at least reach under steady state conditions when 
heating a relatively dark flat bottomed utensil at the steady state power 
level for the selected power setting. The maximum reference temperature 
for that setting corresponds to the temperature which would normally not 
be exceeded by the glass ceramic material when heating a shiny aluminum 
utensil having a warped bottom surface at the corresponding power level. 
While the novel features of the invention are set forth with particularity 
in the appended claims, the invention both as to organization and content 
will be better understood and appreciated from the following detailed 
description taken in conjunction with the drawings.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT 
FIG. 1 illustrates a glass-ceramic cooktop appliance designated generally 
10. Cooktop appliance 10 has a generally planar glass-ceramic cooking 
surface 12. Circular patterns 13(a)-13(d) identify the relative lateral 
positions of each of four heating units (not shown) located directly 
underneath surface 12. A control and display panel generally designated 15 
includes a complete set of touch control keys 17 and a seven-segment 
digital LED display element 19 for each heating unit. 
The term glass-ceramic with reference to the material comprising cooktop 
surface 12 refers to a boron silicate material such as the Ceran family of 
materials. In particular in the illustrative embodiment the glass-ceramic 
material is an infrared transmissive glass-ceramic material designated 
Ceran-85 manufactured by Schott, Incorporated. 
A heating unit is disposed beneath each of the circular patterns 
13(a)-13(d). In the discussion to follow the designators 14(a)-14(d) shall 
be understood to refer to the heating unit disposed under patterns 
13(a)-13(d) respectively. Surface unit 14(a) is shown in greater detail in 
FIGS. 2 and 3. For purposes of illustration only one of the heating units 
is shown. It will be understood that heating units 14(b)-14(d) are similar 
in structure to that shown in FIGS. 2 and 3. Heating units 14(a) and 14(c) 
are 8 inches in diameter. Units 14(b) and 14(d) are 6 inches in diameter. 
Referring again to FIGS. 2 and 3, heating unit 14(a) comprises an open coil 
electrical resistance element 16 of spiral configuration, which is 
designed when fully energized to radiate primarily in the infrared (1-3 
micron) region of the electromagnetic energy spectrum. Element 16 is 
arranged in a concentric coil pattern and staked or otherwise secured to a 
support disk 18 formed of Micropore material such as is available from 
Ceramaspeed under the name Microtherm. Disk 18 is supported in a sheet 
metal support pan 20, by an insulating liner 22 formed of an aluminum 
oxide, silicon oxide composition. This insulating liner 22 includes an 
annular upwardly extending portion 22(a) which serves as an insulating 
spacer between base 18 and the glass-ceramic cooktop 12. When fully 
assembled, pan 20 is spring loaded upwardly forcing the annular portion 
22(a) of insulating liner 22 into abutting engagement with the underside 
of cooktop 12 by support means not shown. Heating units 14(a)-14(d) are 
manufactured and sold commercially by Ceramaspeed under the part name Fast 
Start Radiant Heater with Concentric Coil Pattern. 
FIG. 4 illustrates in simplified schematic form an embodiment of a system 
to be controlled in accordance with the present invention. Each of four 
heating units 14(a)-14(d) is coupled to a standard 240 volt, 60 Hz AC 
power source via power lines L1 and L2 through one of four triacs 
24(a)-24(d) respectively, the heating circuits being connected in parallel 
arrangement with each other. Triacs 24(a)-24(d) are conventional 
thyristors capable of conducting current in either direction irrespective 
of the voltage polarity across their main terminals when triggered by 
either a positive or negative voltage applied to the gate terminals. 
The power control system 26 controls the power applied to the heating units 
by controlling the rate at which gate pulses are applied to the triac gate 
terminals in accordance with power setting selections for each heating 
unit entered by user actuation of tactile touch membrane switch keyboard 
28. The columns of keys designated SU0 through SU3 provide the control 
inputs for heating units 14(a)-14(d) respectively. In the illustrative 
embodiment power pulses applied to the heating units are full cycles of 
the 240 volt, 60 Hz AC power signal; however, power signals of different 
frequencies and voltage levels such as 120 volts could be similarly used. 
A plurality of discrete power settings are provided, each having uniquely 
associated with it a particular power pulse repetition rate. In the 
illustrative embodiment nine power settings plus Off and On are selectable 
for each heating unit by user actuation of the keys in keyboard 28. Table 
I shows the pulse repetition rate associated with each power setting. 
TABLE I 
__________________________________________________________________________ 
Power 
Power 
Power Pulse 
Look Up Table 
Settings 
Level 
Repetition Rate 
Address Power Pulse Code 
__________________________________________________________________________ 
OFF 0 -- ADDR 0000 
0000 
0000 
0000 
ON 0 -- ADDR 0000 
0000 
0000 
0000 
1 1 1/64 ADDR +8 
8000 
0000 
0000 
0000 
2 2 1/32 ADDR +10 
8000 
0000 
8000 
0000 
3 3 1/16 ADDR +18 
8000 
8000 
8000 
8000 
4 4 1/8 ADDR +20 
8080 
8080 
8080 
8080 
5 5 10/64 ADDR +28 
8088 
8080 
8088 
8080 
6 6 15/64 ADDR +30 
8888 
8888 
8888 
8880 
7 7 21/64 ADDR +38 
AA88 
A888 
A888 
A888 
8 8 28/64 ADDR +40 
AA8A 
AA8A 
AA8A 
AA8A 
9 9 36/64 ADDR +48 
EAAA 
EAAA 
EAAA 
EAAA 
A 41/64 ADDR +50 
EEEA 
EAEA 
EAEA 
EAEA 
B 45/64 ADDR +58 
EEEE 
AEEE 
EAEE 
EEAE 
C 51/64 ADDR +60 
FEEE 
EEEE 
FEEE 
FEEE 
D 55/64 ADDR +68 
FEFE 
FEFE 
FEFE 
FEEE 
E 59/64 ADDR +70 
FFEF 
FEFF 
EFFE 
FFEF 
F 64/64 ADDR +78 
FFFF 
FFFF 
FFFF 
FFFF 
__________________________________________________________________________ 
The power pulse code in Table I represents 64-bit control words in 
hexadecimal format. The distribution of ON power cycles over a 64 cycle 
control period for each power setting is defined by the bit pattern of the 
associated control word. On and OFF cycles are represented by logical one 
and logical zero bits respectively. These repetition rates have been 
empirically established to provide a range of power settings for good 
cooking performance in the appliance of the illustrative embodiment. The 
bit patterns have been selected to minimize the duration of idle or OFF 
cycles for each power level. 
In FIG. 5 waveforms A-D represent the voltage applied to the heating 
element for each of power settings 1 through 4 respectively. Wave form E 
represents the power signal appearing across lines L1 and L2. Power pulses 
or ON cycles are represented by full lines. Those cycles of the power 
signal during which the triac is non-conductive are shown in phantom 
lines. As shown in Table I and FIG. 5, the pulse repetition rate for the 
first four power settings range from 1 pulse per 64 power cycles for power 
setting 1, the lowest non-Off power setting to 1 power pulse for every 8 
cycles for power level 4. 
The maximum user selectable power setting, level 9, corresponds to a 
repetition rate of 36 cycles per 64 cycles to permit the heating unit to 
be designed for steady state operation at an effective voltage which is 
lower than the 240 volt line voltage as is described in greater detail in 
commonly assigned co-pending patent application Ser. No. 000,426 filed 
Jan. 5, 1987, now pending, for Thomas R. Payne, the disclosure of which is 
hereby incorporated by reference. 
A temperature sensor for measuring the temperature of the glass ceramic 
support surface is provided in the illustrative embodiment in the form of 
four pairs of precious metal strips 30 formed on the underside of 
glass-ceramic plate 12. One pair is associated with each heating unit. 
Strips 30 serve as electrical contacts and the glass-ceramic material in 
the gap 32 between the strips is a resistance, the value of which varies 
as a function of the temperature of the glass. 
Strips 30 may be silk screened and fired onto the underside of the 
glass-ceramic cooktop 12 at a temperature of about 1300.degree. F. Strips 
30 are built up to a thickness of about 50 to 100 angstroms and extend 
from the outer edge of cooktop surface 12 nearly to the center of each of 
the circular patterns 13(a)-13(d). The strips are spaced apart a distance 
of approximately 0.3 inches. The approximate length of each strip is 3 
inches and 4 inches for the 6" and 8" heating units respectively. The 
minimum width of each strip is 0.1 inches. Such a construction gives a 
finite measurable resistance value for each strip conductor. The 
resistance of the strips is not critical provided it is small, but a value 
in the range of 1-10 ohms is preferred. Gold is used in the illustrative 
embodiment to form the strips 30; however, it will be appreciated that 
other precious metals and combinations thereof such as gold palladium 
combinations or the like could be similarly employed. The particular 
tapered pattern for strips 30 in the illustrative embodiment was selected 
somewhat arbitrarily for enhanced appearance since that portion of the 
strips which extends over the heating unit will be visible through the 
cooktop when the heating units are operating. This pattern is not 
essential for proper operation. 
An improved method for applying strips to the glass ceramic surface is 
disclosed in commonly assigned co-pending U.S. patent application Ser. No. 
091,528, filed Aug. 31, 1987, now pending, by Schulz. 
The resistance between strips 30 is a function of the distance between the 
strips, the length, glass-ceramic thickness, cooktop material as well as 
the temperature. The temperature vs. resistance characteristic of the 
glass-ceramic material comprising the temperature sensor of the 
illustrative embodiment is graphically represented in FIG. 6. At the 
maximum temperature of 1300.degree. F. (700.degree. C.) the resistance of 
the glass-ceramic is approximately 200 ohms. At room temperature the 
resistance of the glass-ceramic is in the multi-megaohm range. 
As hereinbefore briefly described, the main heat transfer mechanism in the 
cooktop of the illustrative embodiment is radiation from the heating unit 
to the utensil through the glass. The glass-ceramic is substantially 
transparent to infrared radiation; however, not totally so. Thus, a 
portion of the energy radiating from the heating unit is absorbed by the 
glass-ceramic. Similarly, a portion of the energy reflected from the 
utensil is also absorbed by the glass-ceramic. Consequently, the 
conduction from glass ceramic surface to the utensil makes a significant 
contribution to the heating of the utensil. Thus, the thermal inertia of 
the glass ceramic material slows the response of the heating system to 
changes in the user selected power setting. 
It will be recalled that an object of the present invention is to cause the 
system to respond more quickly, that is, to reach steady state heating 
conditions more quickly in response to changes in the selected power 
setting. To meet this objective, advantageous use is made of a novel and 
unexpected empirical observation that for each power level, under steady 
state conditions the temperature of the underside of the glass ceramic 
material supporting a utensil load to be heated will come within a 
corresponding relatively wide but predictable temperature band or range 
for substantially all normal utensil loads being heated on the glass 
ceramic surface. Table II shows the minimum and maximum temperatures which 
define the bands for power levels 4-7 of the illustrative embodiment. 
As used herein, the phrase "normal load" refers to a range of utensils 
likely to be heated on the cooktop. At one extreme are dark flat bottomed 
Corningware type pans and at the other extreme are warped shiny aluminum 
pans. The dark flat Corningware type of pans provide the most efficient 
heat transfer from the glass ceramic material. The phrase "Corningware 
type" refers to pans made of ceramic material, available under a variety 
of names. For a given power setting the measured glass ceramic temperature 
will be lowest for this type of pan. The warped bright metal pan provides 
poor heat transfer by conduction and also tends to reflect radiant energy 
back toward the glass ceramic and thus establishes the high temperature 
end of the band. 
TABLE II 
______________________________________ 
Steady State 
Overdrive Underdrive 
Fast Heat 
Fast Cool 
Power Level 
Level Level Threshold 
Threshold 
______________________________________ 
1 -- 0 -- 350.degree. F. 
2 -- 0 -- 350.degree. F. 
3 -- 0 -- 350.degree. F. 
4 6 2 280.degree. F. 
650.degree. F. 
5 7 3 340.degree. F. 
710.degree. F. 
6 9 3 400.degree. F. 
840.degree. F. 
7 9 4 480.degree. F. 
960.degree. F. 
8 9 5 600.degree. F. 
1050.degree. F. 
9 -- -- -- -- 
______________________________________ 
The minimum temperatures in Table II were derived from empirical testing 
using a 8" size flat dark Everware pan containing 2 liters of water; the 
maximum temperatures were derived using a 6" size shiny aluminum pan with 
a severely warped bottom containing 1/4 liter of water. 
In accordance with the present invention, the control system includes means 
responsive to the temperature sensing means operative to apply a power 
level other than the steady state power level corresponding to the user 
selected power setting, when the sensed temperature of the glass ceramic 
material is outside the corresponding steady state temperature band. When 
the sensed glass ceramic temperature is less than the minimum threshold 
temperature defining the lower limit of the temperature band for the 
selected power setting, the heating unit is overdriven, that is, a power 
level higher than the steady state power level for the selected power 
setting is applied to the unit. Similarly, when the sensed glass ceramic 
temperature is greater than the maximum threshold temperature defining 
upper limit of the temperature band for the selected power setting, the 
heating unit is underdriven, that is, a power level less than the steady 
state power level is applied to the heating unit. When the sensed glass 
ceramic temperature is within the corresponding steady state temperature 
band, the power level corresponding to the selected power setting is 
applied to the heating unit. By this arrangement, the glass temperature is 
more rapidly brought within the temperature band associated with the 
selected power level than would be the case with a conventional open loop 
control arrangement. 
In the illustrative embodiment, resistance of the glass when operating at 
the power levels 1-3 is so high that reliable measurement of usable 
temperatures requires very expensive circuitry. Thus, the overdrive or 
fast heat mode is not implemented for these power settings. The maximum 
reference temperature for power level 3 is used to implement the 
underdrive or fast cool mode for power levels 1-3. Power setting 9 is the 
maximum setting for which the unit is designed. Thus, no overdriving is 
implemented in response to selection of this power level. 
Table II also shows the power levels applied for overdriving and 
underdriving the units for each power setting. These power levels have 
been empirically selected to provide satisfactory performance in the 
cooktop of the illustrative embodiment. The objective in choosing these 
levels is to bring the temperature to within the desired limits quickly 
but without overshoot. 
It will be appreciated that the specific temperature and power level 
parameters described in Table II are intended to be illustrative only and 
not intended to be limitations on the invention. 
FIG. 7 schematically illustrates an embodiment of a power control circuit 
for the cooktop of FIG. 1 which performs the power control function in 
accordance with the present invention. In this control system power 
control is provided electronically by microprocessor 40. Microprocessor 40 
is a M68000 series microprocessor of the type commercially available from 
Motorola. Microprocessor 40 has been customized by permanently configuring 
its read only memory to implement the control scheme of the present 
invention. 
As previously described with reference to FIG. 4, keyboard 28 is a 
conventional tactile touch type entry system. The keyboard array comprises 
four columns of 11 keys each. Columns for controlling heating elements are 
designated SU0 through SU3 respectively. The keys enable a user to select 
power levels 1 through 9 in addition to On and Off for each of the four 
heating units. Keyboard 28 has one input line for each column commonly 
shared by all keys in that column and 11 output lines, one for each row of 
keys. Each particular column of keyboard 28 is scanned by periodically 
generating scan pulse sequentially at outputs P400 through P403 of 
microprocessor 40. These pulses are transmitted as they appear to the 
corresponding column input lines of keyboard 28. This voltage is 
transmitted essentially unchanged to the output lines of all the untouched 
keys. The output of an actuated key will differ, signifying actuation of 
the key in that row and column. 
In this manner each column of keyboard 28 is scanned for a new input 
periodically at a rate determined by the control program stored in the ROM 
of microprocessor 40. As will become apparent from the description of the 
control routines which follow, each column is scanned once every four 
complete power cycles of the power signal appearing on lines L1 and N. The 
output from keyboard 28 is coupled to input ports P1IO-P1IA of 
microprocessor 40 via a 410 parallel port interface circuit. 
A zero crossing signal marking zero crossings of the power signal appearing 
on lines L1 and N from the power supply is input to microprocessor 40 at 
input port P8I0 from a conventional zero crossing detector circuit 44. The 
zero crossing signal from circuit 44 is illustrated at wave form F of FIG. 
5. The pulses mark the positive going zero crossings of the power signal 
across lines L1 and N of the AC power supply. The zero crossing signals 
are used to synchronize the triggering of the triacs with zero crossings 
of the power signal and for timing purposes in the control program 
executed by microprocessor 40. 
Glass cooktop temperature information is provided to microprocessor 40 at 
input ports PAI0 through PAI3 via a standard VME 600 A-D converter circuit 
46. An analog voltage signal representative of the temperature of the 
glass-ceramic in the vicinity of each heating unit is provided via 
temperature sensor voltage bridge network 48 comprising for each heating 
unit, a 2K resistor 49 connected in parallel with 200K resistor 50, via an 
analog multiplexer circuit 51 serially connected to resistor 49, an 
isolating diode 52, and a 10 uf filter capacitor 54. The resistance of the 
glass-ceramic is represented schematically as variable resistor 56 coupled 
between the junction of resistor 50 and diode 52 and ground. The other 
side of resistor 50 is coupled to an AC supply source 57. AC supply 57 is 
used to drive the glass-ceramic sensor resistance circuitry in order to 
minimize parasitic and diffusion affects. The analog voltage signal 
applied to the input of the A--D converter from each individual sensor 
circuit is converted internally to a digitized value which is stored in 
the RAM of microprocessor 40. 
Analog multiplexer circuit 51 is connected in series with current limiting 
resistor 49 to effectively expand the temperature range of the sensing 
circuit. Multiplexer circuits 51 act as analog switches triggered by 
enable signals from output ports P404-P407 to selectively switch the 2K 
ohm resistor into the sensing circuit. If only 2K resistor 49 is employed, 
the temperature readings at the low end of the range are difficult to 
resolve. As will be hereinafter described in greater detail in the 
description of the control routines, when the sensed temperature is above 
a predetermined threshold temperature arbitrarily set at 750.degree. F., 
an enable signal is transmitted from the appropriate one of I/O ports 
P404-P407 to its associated multiplexer circuit 51 to switch in the lower 
resistor 49 before reading in the temperature for measurement purposes. 
For temperatures less than the threshold temperature, the I/O port is 
reset, effectively switching resistor 49 out of the circuit. 
Microprocessor 40 transmits triac trigger signals from I/O ports P500 
through P503 to the gate terminals of triacs 24(a)-24(d) respectively via 
a conventional 615 triac driver circuit. Triac driver circuit 64 amplifies 
the outputs from ports P500-P503 of microprocessor 40 and isolates the 
chip from the power line. Display data is transmitted from I/O ports 
P200-P20F. Display 58 is a conventional four digit display, each digit 
comprising a 7-segment LED display. Display information is coupled from 
I/O ports P200-P20F to the display segments via a conventional 410 
parallel port interface circuit 60 and a conventional segment display 
decoder driver circuit 62 in a manner well known in the art. 
It will be recalled that microprocessor 40 is customized to perform the 
control functions of this invention by permanently configuring the ROM to 
implement a predetermined set of instructions. FIGS. 8-14 are flow 
diagrams which illustrate the control routines implemented in the 
microprocessor to obtain, store and process the input data from the 
keyboard and generate control signals for triggering the triacs in a 
manner which provides the power pulse repetition rate required for the 
power setting selected and the sensed glass-ceramic temperature for each 
of the heating units. From these diagrams one of ordinary skill in the 
programming art could prepare a set of instructions for permanent storage 
in the ROM of microprocessor 40 which would enable the microprocessor to 
perform the control functions in accordance with this invention. 
The control program comprises a set of predetermined control instructions 
stored in the read only memory (ROM) of microprocessor 40. A separate file 
in the random access memory (RAM) of the microprocessor is associated with 
each of heating units 14(a)-14(d). Each file stores the control 
information for its associated heating unit which is acted upon by the 
instructions in the ROM. Execution of the control program is synchronized 
with the 60 Hz power signal such than the set of control instructions in 
the ROM is cycled through once during each cycle of the power signal. A 
file register common to all four files functioning as a four count ring 
counter is incremented once during each pass through the control program. 
The count of this file register identifies the RAM file to be operated on 
by the control instructions during the ensuing pass through the control 
program. By this arrangement the control program is executed for any one 
particular heating unit once every four cycles of the 60 Hz power signal. 
The control program is logically divided into a set of sub-routines which 
includes the Scan routine, the Keyboard Decode routine, the Rate Calc 
routine, the Rate Control routine, the Steady State routine, the TEMP 
FH/FC routine, the PSET routine, and the Power Out routine. It will be 
appreciated that other sub-routines may also be included to perform 
control functions unrelated to the present invention. 
The Scan routine (FIG. 8), which contains the file register identifying the 
RAM file to be acted upon during the ensuing pass through the control 
program, sets the scan line for the keyboard column associated with the 
heating unit which is the subject of the current pass through the routine, 
reads the input from the keyboard, and stores the user selected power 
setting selection information in temporary memory. The Keyboard Decode 
Routine validates keyboard entries and updates the control variable 
representing the power level selected by the user as appropriate to 
reflect the most recent valid user input. The Rate Calc routine reads in 
the glass-ceramic cooktop temperature information, and periodically 
calculates the rate of change of temperature. This information is used in 
the Rate Control and Steady State Control routines which perform a 
temperature limiting function by making adjustments to the power level to 
be applied to the heating unit as a function of the glass-ceramic 
temperature, the rate of change of glass-ceramic temperature and the user 
selected power setting. The TEMP FH/FC routine uses the glass ceramic 
temperature information read in by the Rate Calc routine to overdrive or 
underdrive the heating units when the sensed temperature is outside of the 
temperature band associated with the selected power setting to speed up 
the response of the appliance to power setting changes in accordance with 
the present invention. 
While the determination of what power level to be applied to the surface 
unit is determined only during the pass through the program for that 
particular heating unit, a power control decision must be made for the 
ensuing power cycle for each of the units during each pass through the 
program. The PSET routine obtains power level information from each file 
during each pass through the routine, performs a table look-up for each 
heating unit to check the appropriate bit for the power level control word 
for each surface unit, and generates a four bit trigger control word which 
identifies which heating units are to be triggered on and which are to be 
off during the next power cycle. This four bit control word is then used 
by the Power Out routine which monitors the input from the zero crossing 
circuit and triggers those triacs associated with surface units to be 
energized during the next power cycle into conduction upon detection of 
the next occurring positive going zero crossing of the power signal. Each 
of these control routines except for the Rate Control and Steady State 
Control routines will now be described in greater detail with reference to 
its flow diagram in the discussion to follow. The Rate Control and Steady 
State routines, which implement the temperature limiting function, are 
described in detail in U.S. patent application Ser. No. 000,684 
hereinabove incorporated by reference. 
SCAN Routine--FIG. 8 
The function of this routine is to address the appropriate RAM file for the 
current pass through the program, set the appropriate scan line for the 
keyboard, and read in the input information from the keyboard for the 
heating unit associated with the designated RAM file. RAM file register SU 
functions as a four count ring counter which counts from 0 to 3. Counts 0 
through 3 of the SU counter identify RAM files for surface units 
14(a)-14(d) respectively. 
Upon entering the Scan routine the register SU is incremented (Block 102) 
and Inquiry 104 determines if SU is greater than 3. If so, the counter is 
reset to 0 (Block 106). Next the address of the RAM file to be acted upon 
during this pass through the control program is set equal to SU (Block 
108). The scan line set during the previous pass through the control 
program designated R(SU-1) is reset (Block 110). The scan line associated 
with the surface unit for the current pass through the program designated 
R(SU) is set (Block 112). The data of input lines P1IA through 9 are read 
in, conveying the current input information for this RAM file from 
keyboard 28 (Block 114) and this information is stored as variable KB 
(Block 116). The program then branches (Block 118) to the Keyboard Decode 
routine of FIG. 9A. 
KEYBOARD DECODE Routine--FIGS. 9A and 9B 
The Keyboard Decode routine validates inputs from keyboard 28 and updates 
the user selected power setting variable PWD accordingly. The routine 
first determines if the new keyboard entry is a blank signifying no input, 
an OFF entry, an OR entry, or one of the power levels 1 through 9. To be 
valid when switching the heating unit from Off to another power setting, 
the On key must be actuated first followed by the desired power setting. 
The power setting must be entered within 8 seconds of actuation of the On 
key. If not, the On key must be re-actuated. 
The variable PWD represents the user selected power setting. PWD is only 
changed in response to user inputs. However, in accordance with the 
present invention the power level actually applied to the heating unit may 
be different from the level corresponding to the user selected power 
setting. The variable PLVL is introduced in this routine to represent the 
power level to be actually applied to the heating unit. PLVL is initially 
assigned the value of PWD. However, PLVL is subject to be changed in the 
control routines hereinafter described. 
A flag designated the On flag and a timer or counter designated the ONTIMER 
are used to establish the eight second period for entering a valid power 
setting after actuation of the On key. The On flag is set when the On key 
is actuated and is only reset in response to actuation of the Off key or 
timing out of ONTIMER. 
Referring to the flow diagram of FIGS. 9A nd 9B, Inquiry 120 first 
determines if KB represents a blank signifying that no key is presently 
actuated. If KB is blank, the system branches to the Decode 2 sub-routine 
(FIG. 9B). In the Decode 2 sub-routine Inquiry 122 determines if the On 
flag is set. If the On flag is not set, the power level stored in PWD is 
assigned to the variable PLVL (Block 124). If the On flag is set, Inquiry 
126 determines if the previously selected power setting presently stored 
as PWD is the Off setting. If not, the system is presently operating at 
one of power settings 1 through 9 and the program proceeds to assign the 
value of PWD to PLVL (Block 124) and branches (Block 128) to the Rate Calc 
routine (FIG. 10). If Inquiry 126 determines that PWD equals 0 
representing an Off power level, this indicates that the user has switched 
from Off to On and the ON timer is decremented (Block 130). When On timer 
equals 0 as determined at Inquiry 132 signifying that the time to enter a 
valid power level has expired, the On flag is cleared (Block 134) and 
program proceeds to Block 124 as before. 
Referring again to FIG. 9A, if KB is not a blank, Inquiry 135 determines if 
the new entry is the Off setting. If so, the On flag is cleared (Block 
136) and the variable PWD is assigned the value 0 representing the Off 
power setting (Block 138). The variable PLVL is assigned the value of PWD 
(Block 140) and the program branches (Block 142) to the Rate Calc routine 
of FIG. 10. If KB is not Off, Inquiry 144 determines if the new entry is 
the On setting. If it is, the On timer is re-initialized (Block 146). 
Inquiry 148 checks the state of the On flag. If set, the program proceeds 
to Block 140. If not set, the flag is set (Block 150) and the PWD is 
assigned the value 0 which corresponds also to the On setting (Block 152). 
The program then proceeds to Block 140 as before. 
If the answer to Inquiry 144 is No, signifying that the new entry is one of 
power levels 1 through 9, Inquiry 154 checks the state of the On flag. If 
it is not set, signifying the user has attempted to go from Off to a power 
level without first actuating the On key, the new entry is ignored and the 
program proceeds to Block 140 with PWD unchanged. If the On flag is set, 
the power setting input is valid, and variable PWD is assigned the new 
value corresponding to the new entry KB (Block 156). 
Having assigned the value of PWD representing the most recent valid user 
selected power setting to the variable PLVL the system proceeds to the 
Rate Calc routine (FIG. 10). 
RATE CALC Routine--FIG. 10 
The function of this routine is to read in the glass ceramic temperature 
data and to determine the rate of change of the glass-ceramic temperature. 
Pursuant to reading in the data, this routine generates enable signal to 
switch the lower resistance into the sensing network when the initial 
reading signifies a temperature higher than the threshold reference 
temperature which is set at 750.degree. F. Of course, the A/D readings 
will differ for the same actual sensed temperature depending upon which of 
the two resistors is in the circuit when making the reading. For example, 
for a sensed temperature of 750.degree. F., with the 200K ohm resistor 50 
in the circuit the sensor circuit voltage will measure 2.9 volts which is 
converted by the A/D circuit to an A/D reading of 253; with the 2K ohm 
resistor 49 in the circuit, for the same actual temperature the sensor 
circuit voltage will be 9.7 volts which converts to an A/D reading of 7C3. 
In the microprocessor implementation of the illustrative embodiment, a 
look-up table is employed when the 200K resistor is in the circuit to 
convert the A/D reading to the reading equivalent to that generated by the 
A/D circuit for the same temperature with the low valued resistor 49 in 
the circuit. In the previous example, the look-up table converts the A/D 
reading of 253 to 7C3. 
The rate of change information determined in this routine is used in the 
temperature limiting routines described in the aforementioned patent U.S. 
Pat. No. 4,740,664. The rate calculation is repeated every two seconds to 
provide a rapid control response. However, the rate of change is 
calculated by measuring the difference between glass-ceramic temperature 
measurements separated by eight seconds. The eight second separation 
provides a more accurate rate determination. These time intervals provide 
satisfactory results in the illustrative embodiment. 
Referring to the flow diagram of FIG. 10, first that one of the I/O ports 
P404-P407 for the particular heating unit for which the program is then 
executing, identified by index (SU+4), is reset (Block 158). Next, the 
glass-ceramic temperature input from A/D converter is then read in (Block 
159) and stored as the variable designated GLSTMP. Inquiry 160 compares 
this temperature to threshold temperature of 750.degree. F. represented by 
the variable THTMP. If the sensed temperature is higher than the threshold 
reference value, I/O port P40(SU+4) is set (Block 161) to switch low value 
resistor 49 (FIG. 7) into the circuit. The temperature input from the A/D 
converter is read in again (Block 162) with the low valued resistor in the 
circuit and stored as variable GLSTMP. If the sensed temperature is lower 
than THTMP, the value of GLSTMP entered at Block 159 using the high valued 
resistor 50 (FIG. 7) is converted via the look-up table (Block 164). The 
converted value is stored as GLSTMP and the program proceeds. 
A two second timer SLPCLK is incremented (Block 163). At two second 
intervals (Inquiry 165) the timer is reset (Block 166). 
As shown as Block 168, when the rate of change is to be updated, the 
current value of GLSTMP is stored as GLSTMPO, the previous reading is 
stored as GLSTMP1; the previous GLSTMP1 is stored as GLSTMP2; the previous 
GLSTMP2 is stored as GLSTMP3, and the previous GLSTMP3 is stored as 
GLSTMP4. By storing temperature measurements every two second in this 
fashion, the time span between the most recent temperature measurement 
GLSTMP0 and the oldest stored temperature measurement GLSTMP4 is 
approximately eight seconds. 
The rate of change of temperature, TMPSLP, is calculated as the difference 
between the most recent measurement and the oldest stored measurement 
(Block 170). This difference is proportional to the rate of change with a 
proportionality factor of 1/8. After reading in the temperature data and 
updating the rate of change calculation as appropriate, the program then 
branches (Block 172) successively to the Rate Control routine (not shown) 
and then the Steady State routine (not shown) to implement a temperature 
limiting function. From the Steady State routine the program branches to 
the TEMP FH/FC routine (FIG. 11A). 
TEMP FH/FC Routine--FIGS. 11A-11C 
The function of the TEMP FH/FC routine is to determine if the sensed 
temperature of the glass ceramic surface is within the steady state 
temperature range for the user selected power setting and to adjust the 
power level applied to the heating unit upwardly if the temperature is 
below the temperature range and downwardly if the temperature is above the 
temperature range. If the sensed temperature is within the temperature 
range, no adjustment is made and the steady state power level is applied 
to the heating unit. 
The maximum and minimum reference temperatures lited in Table II are used 
in this routine. The maximum and minimum reference values for the nth 
power setting are assigned the variable names MAXTMP (n) and MINTMP (n) 
respectively. 
Referring now to the flow diagram 11A, Inquiry 174 determines if the 
selected power setting represented by the variable PWD is 0 representing 
the OFF setting, in which case no modifications to the power setting is to 
be made and the program branches immediately (Block 175) to the PSET 
routine of FIG. 12. If one of power settings 1-9 has been selected, the 
program proceeds to Inquiry 176. 
Inquiry 176 determines if the selected power setting is one of power 
settings 1-3. If so, Inquiry 178 compares the sensed glass temperature to 
the maximum reference temperature for power setting 3, GLSTMP3. If the 
temperature is greater than GLSTMP3, a fast cool mode is initiated by 
setting PLVL to zero (Block 180); if not, no change is made to PLVL. The 
program then branches (Block 182) to the PSET routine of FIG. 12. If the 
selected power setting is higher than power setting 3, Inquiry 184 
determines if power setting 4 has been selected. A No response to Inquiry 
184 signifies that power setting 4 has been selected. If so, the program 
proceeds to Inquiry 186 which compares the glass temperature represented 
by the variable GLSTMP to the maximum temperature for power setting 4. If 
the sensed temperature is greater than the reference, the applied power 
level PLVL is reduced by two levels (Block 188) and the program branches 
(Block 190) to the PSET routine of FIG. 12. If the sensed glass 
temperature is not greater than the maximum temperature for power setting 
4, Inquiry 192 compares the temperature to the minimum temperature for 
power setting 4. If the sensed temperature is less than the minimum 
temperature, the applied power level is increased by 2 (Block 194). 
Otherwise, the program branches (Block 190) to the PSET routine. If the 
selected power setting is higher than power setting 4, the program 
proceeds to entry point FHFC2 at FIG. 11B. Inquiry 196 determines if the 
selected power setting is greater than power setting 5. A No response to 
Inquiry 196 signifies power setting 5 has been selected and the program 
proceeds to Inquiry 198, which compares the sensed temperature to the 
maximum reference temperature for power setting 5. If the sensed 
temperature exceeds the maximum temperature, the applied power level is 
reduced by 2 (Block 200). Otherwise, Inquiry 202 compares the sensed glass 
temperature to the minimum reference temperature for power setting 5. If 
the sensed temperature is less than the minimum, the power level to be 
applied is increased by 2 (Block 204); otherwise, no change is made to the 
power level to be applied and the program branches (Block 206) to the PSET 
routine of FIG. 12. 
Referring again to Inquiry 196, if Yes, the program proceeds to Inquiry 
208. A No response signifies that power setting 6 has been selected. 
Inquiries 210 and 212 compare the sensed glass temperature to the maximum 
and minimum reference temperatures for power setting 6 respectively. If 
the maximum reference is exceeded, the power level is reduced by 3 (Block 
214). If the sensed temperature is less than the minimum reference 
temperature, the power level is increased by 3 (Block 216). Otherwise, no 
change is made to the power level and the program branches (Block 217) to 
the PSET routine. 
Referring back to Inquiry 208, if the response to Inquiry 208 is a Yes 
signifying a power setting greater than 6 has been selected, the program 
proceeds to entry point FHFC3 of FIG. 11C. 
A No response to Inquiry 218 signifies power setting 7 has been selected. 
Inquires 220 and 222 compare the sensed glass temperature to the maximum 
and minimum reference temperatures for power setting 7 respectively. If 
the glass temperature exceeds the maximum reference temperature, the power 
level is decreased by 3 (Block 224) and the program branches (Block 228) 
to the PSET routine of FIG. 12. If the sensed glass temperature is less 
than the minimum reference temperature for power setting 7, the power 
level 9 representing an increase of 2 power levels is applied (Block 226). 
Otherwise, no adjustment is made to power level and the program branches 
to the PSET routine (Block 228). 
If the power setting is greater than 7, the program proceeds to Inquiry 229 
which checks for the selection of power setting 8. If the response to 
Inquiry 229 is No, signifying power setting 8 has been selected, Inquiries 
230 and 232 compare the sensed glass temperature to the maximum and 
minimum reference temperatures respectively for power setting 8. If the 
glass temperature exceeds the maximum reference temperature, the power 
level is decreased by 3 (Block 234) and the program branches (Block 236) 
to the PSET routine. If the glass temperature is less than the minimum 
reference temperature, the power level is increased by 1 to maximum power 
level 9 (Block 238), and the program branches (Block 236) to the PSET 
routine. If the sensed temperature is not less than the minimum reference 
temperature, no adjustment is made to the power level and the program 
branches (Block 236) to the PSET routine. 
PSET Routine--FIG. 12 
Having established the appropriate power level to be applied to the heating 
unit, it remains to make the triac triggering decision for the next 
occurring power signal cycle. This decision is made for each of the four 
heating units during each pass through the control program. Use is made in 
this routine of information from each of the four heating unit RAM files 
each time through the routine. It will be recalled that the power pulse 
repetition rate for each power level is defined by the bit pattern of a 
64-bit word with the logical one bit representing an On cycle and logical 
zero representing an Off cycle. The bits of the word representing the 
power level to be applied to the heating unit are tested sequentially with 
one bit being tested each pass through this routine. The state of that 
tested bit determines whether the triac for the corresponding heating unit 
will be triggered on or not in the next power signal cycle. 
This routine performs a Table Look-Up function to find the appropriate 
control word and then checks the state of the appropriate bit in that for 
each of the four surface units. The triac triggering information is then 
stored in a four-bit word designated TMPON, which is used in the Power Out 
routine (FIG. 13) to generate the appropriate triac trigger signals. 
The variable ADD represents the address in RAM of the starting location 
for the look-up table containing the 64-bit control words. The address and 
associated bit pattern in Hex representation is shown in Table I. Each of 
the 16 digits in the code as shown for each control word is the 
hexidecimal representation of four binary bits. 
The variable designated BITADD represents the location within the 64 bit 
control word of the bit to be tested with 0 and 63 corresponding to the 
location of the most significant bit and least significant bit 
respectively. 
An indexing variable n is used to iterate the table look-up loop four times 
during each pass through the routine, once for each heating unit. The 
variable PWDADD is the address of the control word representing the power 
level to be applied to the n.sup.th heating unit. As can be seen in Table 
I, the address for any particular power word is obtained by multiplying 
the value of PLVL for its associated power level, which is a number 0 
through 9, multiplied by a factor of 8 and adding this to ADD. 
Referring to FIG. 12, on entering this routine the control word TMPON is 
cleared (Block 272) and a ring counter which counts from 0 to 63 is 
incremented. Inquiry 276 determines if the counter is greater than its 
maximum count of 63. If so, it is reset to 0 (Block 278). Next BITADD is 
set equal to the count of the ring counter thereby defining the location 
within the control word for the bit to be tested for each heating unit 
(Block 280). The same bit location is tested for each of the heating 
units. 
The variable n is initialized to zero at Block 282. PWDADD for the power 
level to be applied to the n.sup.th heating unit is determined at Block 
284. The state of the bit location defined by the variable BITADD in the 
control word located at the address PWDADD is then tested (Inquiry 286). 
If the tested bit is a logical 1, the n.sup.th bit of the control word 
TMPON is set (Block 288). Otherwise, the n.sup.th bit of TMPON will remain 
0. After the index n is incremented (Block 290) the value of n is checked 
(Inquiry 292). If greater than 3, signifying that the loop comprising 
Blocks 284, 288 and 290 and Inquiries 284 and 286 has been iterated four 
times, n is reset (Block 294) and the program proceeds to the Power Out 
routine (FIG. 13). If n is not greater than 3, the program returns to 
Block 284 to test the bit for the power word for the next heating unit. 
After the appropriate state for all four bits of the variable TMPON have 
been established, the program branches (Block 296) to the Power Out 
routine (FIG. 13). 
POWER OUT Routine--FIG. 13 
The function of this routine is to trigger triacs 24(a)--24(d) to implement 
of the triac triggering decision for the next power cycle for each of the 
four heating units. The triggering of the triacs is synchronized with the 
positive going zero crossings of the power signal. 
Referring now to the routine in FIG. 13, on entering this routine the 
output latches P500-P503, which control the triacs, are reset (Block 302). 
Next the program reads in the input from the input port P8I0 representing 
the state of the zero cross detector (Block 304) and Inquiry 306 checks 
the state of this input until it switches to a logical 1 signifying the 
occurrence of a positive going zero crossing of the power signal. When 
P8I0 equals 1, the program proceeds to Inquiry 308 to sequentially check 
the four bits of the power word TMPON and set the appropriate one of 
output latches P500-P503. Index variable n is again used to sequentially 
check bits 0 through 3. It will be recalled that prior to branching from 
the PSET routine the n is reset to 0. Inquiry 308 tests the n.sup.th bit 
for a 1. If it is a 1, the output P50(n) is set (Block 310), n is 
incremented (Block 312) and Inquiry 314 checks for an n greater than 3. If 
n is less than 3, the program returns to Inquiry 308 to check the next bit 
and set the corresponding output port as appropriate. Those ones of output 
latches P500-P503 associated with bits in the variable TMPON which are in 
the logical one state are set. Those ones with output latches associated 
with zero bits in TMPON are not set. In the latter case these latches 
remain in the reset state since each of the latches is reset upon entering 
this routine. 
In this fashion each bit of the control word TMPON is tested each pass 
through the Power Out routine, and a decision to trigger or not trigger 
each triac is carried out during each pass through the control program. 
Once the loop comprising Inquiries 306 and 312 and Blocks 308 and 310 is 
iterated four times, once for each heating unit, the power control 
decision for the next power cycle has been implemented and the program 
returns to the Scan routine to execute the program for the next heating 
unit. 
In the power control arrangement herein described, it is contemplated that 
the control functions of the present invention be implemented in 
cooperation with the temperature limiting functions at least to the extent 
that the temperature limiting function overrides the fast heat/fast cool 
functions. It will be appreciated, however, that the fast heat/fast cool 
function is readily implementable in a power control system in which the 
temperature limiting function is performed in a totally different manner, 
if at all. For example, in the system of the illustrative embodiment, this 
could be achieved by simply deleting the Rate Control and Steady State 
Control routines entirely from the ROM of microprocessor 40 and retaining 
only that portion of the Rate Calc routine in ROM which reads in and 
stores the glass ceramic temperature measurement data. 
While in accordance with the Patent Statutes a specific embodiment of the 
present invention has been illustrated and described herein, it is 
realized that numerous modifications and changes will occur to those 
skilled in the art. For example, the illustrative embodiment employs 
infrared heating units. However, the invention could also be used in 
conventional conduction cooktops as well. It is therefore to be understood 
that the appended claims are intended to cover all such modifications and 
changes as fall within the true spirit and scope of the invention.