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Claims  |
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What is claimed is:
1. Apparatus for controlling the flow of heating current through a
resistive element in order to heat the resistive element from ambient
temperature to a desired temperature, said apparatus comprising:
heating current supply means for supplying the resistive element with
heating current;
measuring means for measuring the resistance of the resistive element;
detection means connected to said measuring means for detecting changes in
the resistance measured by said measuring means in order to determine when
the resistive element is at ambient temperature, said detection means
including detection circuit means for generating a first logic signal
indicative of ambient temperature conditions in the resistive element;
computing means connected to said measuring means and said detection means
for computing the predicted resistance of the resistive element at the
desired temperature, said computing means including a first computing
circuit means responsive to said first logic signal for receiving the
value of said resistance measured by said measuring means when the
resistive element is at ambient temperature, said computing means also
including a second computing circuit means for computing said predicted
resistance as a function of the desired temperature and said value of said
resistance received by said first computing circuit means; and
comparison means connected to both said measuring means and said computing
means for comparing said predicted resistance with said resistance
measured by said measuring means, said comparison means including a
control signal means for generating a control signal as a function of the
difference between said predicted resistance and said resistance measured
by said measuring means, said comparison means also including a control
circuit means connected to said control signal means for conducting
heating current through the resistive element in response to said control
signal.
2. Apparatus as set forth in claim 1, further comprising a timing means for
generating timing signals which define a succession of operating cycles
for said operating apparatus, said measuring means functioning during a
first operating cycle and a second operating cycle in said succession of
operating cycles to measure the resistance of the resistive element and
said detection means functioning during said first operating cycle and
said second operating cycle to compare the resistance measured by said
measuring means during said first operating cycle with the resistance
measured by said measuring means during said second operating cycle.
3. Apparatus as set forth in claim 2, wherein said detection circuit means
includes a sample-and-hold means for holding a value representing said
resistance measured by said measuring means during said first operating
cycle, said detection circuit means also including a a detection
comparison means for comparing said value held by said sample-and-hold
means with a value representing said resistance measured by said measuring
means during said second operating cycle, said detection comparison means
functioning to generate said first logic signal when the difference
between said value of said resistance measured by said measuring means
during said first operating cycle and said value of said resistance
measured by said measuring means during said second operating cycle is
within a predetermined range.
4. Apparatus as set forth in claim 1, wherein said first computing circuit
means includes a memory means for storing said value of said resistance
measured by said measuring means when the resistive element is at an
ambient temperature.
5. Apparatus as set forth in claim 4, further including a switch means
connected to supply said second computing circuit means with a value
representing the desired temperature, said second computing circuit means
having a means for multiplying said value of said resistance stored by
said memory means with a temperature factor derived from said value of the
desired temperature.
6. Apparatus as set forth in claim 5, wherein said temperature factor is
equal to (1+.alpha..DELTA.T), where .alpha. is a temperature coefficient
having a value dependent upon both the ambient temperature and the
composition of the resistive element and .DELTA.T represents the
difference between the desired temperature and the ambient temperature.
7. Apparatus as set forth in claim 5, wherein said second computing circuit
means includes a first digital-to-analog converter connected to receive
from said switch means said value representing the desired temperature, an
output amplifier stage connected to said first digital-to-analog converter
and adjusted to supply an output representing said temperature factor and
a second digital-to-analog converter connected to receive both said value
of said resistance stored by said memory means and said output from said
output amplifier stage.
8. Apparatus for controlling the flow of heating current through a
resistive element in order to heat the resistive element to a desired
temperature, said apparatus comprising:
heating current supply means for periodically supplying heating current to
the resistive element;
measuring current supply means for periodically supplying measuring current
to the resistive element;
logic means connected to said heating current supply means and said
measuring current supply means for respectively enabling said heating
current supply means and said measuring current supply means to supply the
resistive element with heating current and measuring current in
alternating fashion;
measuring means for measuring the voltage drop across the resistive element
as measuring current flows therethrough and for responsively generating a
measured resistance signal indicative of the actual resistance of the
resistive element;
computing means for computing the predicted resistance of the resistive
element at the desired temperature and for generating a computed
resistance signal indicative of said predicted resistance, said computing
means including a memory means for storing an ambient resistance value
representing the resistance of the resistive element at ambient
temperature, said computing means also including a computing circuit means
connected to said memory means for generating said computed resistance
signal as a function of the desired temperature and said ambient
resistance value; and
comparison means connected to both said measuring means and said computing
means for comparing said computed resistance signal with said measured
resistance signal, said comparison means including a control signal means
for generating a control signal as a function of the difference between
said predicted resistance and said actual resistance of the resistive
element, said comparison means also including a control circuit means
connected to said control signal means for conducting heating current
through the resistive element in response to said control signal.
9. Apparatus for controlling the flow of heating current through a
resistive element in order to heat the resistive element to a desired
temperature, said apparatus comprising:
timing means for generating timing signals which define operating cycles
for said apparatus, each said operating cycle including first and second
intervals of predetermined duration;
measuring current supply means connected to receive said timing signals for
supplying the resistive element with a measuring current during said first
interval of each said operating cycle;
heating current supply means connected to receive said timing signals for
supplying the resistive element with heating current during said second
interval of each said operating cycle;
measuring means for measuring the voltage drop across the resistive element
as measuring current flows therethrough and for responsively generating a
measured resistance signal indicative of the actual resistance of the
resistive element;
computing means for computing the predicted resistance of the resistive
element at the desired temperature and for generating a computed
resistance signal indicative of said predicated resistance, said computing
means including a memory means for storing an ambient resistance value
representing the resistance of the resistive element at ambient
temperature, said computing means also including a computing circuit means
connected to said memory means for generating said computed resistance
signal as a function of the desired temperature and said ambient resistive
value; and
comparison means connected to both said measuring means and said computing
means for comparing said computed resistance signal with said measured
resistance signal, said comparison means including a control signal means
for generating a control signal as a function of the difference between
said predicted resistance and said actual resistance of the resistive
element, said comparison means also including a control circuit means
connected to said control signal means for conducting heating current
through the resistive element during said second interval of each said
operating cycle in response to said control signal.
10. Apparatus as set forth in claims 8 or 9, wherein said measuring means
includes an amplifier means connected across the resistive element to
generate said measured resistance signal in the form of an amplifier
output.
11. Apparatus as set forth in claim 10, wherein said amplifier means
includes an amplifier having a gain which is adjusted such that the value
of said amplifier output equals the value of the actual resistance of the
resistive element.
12. Apparatus for controlling the flow of heating current through a
resistive element having a plurality of resistive segments in order to
heat each of said plurality of resistive segments to a desired
temperature, said apparatus comprising:
heating current supply means for supplying the resistive element with
heating current;
measuring means for respectively measuring the resistances of each of the
plurality of resistive segments;
detection means connected to said measuring means for determining when the
resistive segments are at ambient temperature;
computing means for respectively computing the predicted resistances of
each of the resistive segments at the desired temperature, said computing
means including a first computing circuit means which receives the value
of the resistance measured by said measuring means for each resistive
segment when each resistive segment is at ambient temperature as
determined by said detection means, said computing means also including a
second computing circuit means which respectively computes said predicted
resistance for each resistive segment as a function of the desired
temperature and said value of said resistance of each resistive segment as
measured by said measuring means at ambient temperature; and
comparison means connected to both said measuring means and said computing
means for respectively comparing each of said predicted resistances
computed by said computing means with a corresponding one of said
resistances measured by said measuring means, said comparison means
including a control signal means for generating a plurality of control
signals as a function of the respective differences between each of said
predicted resistances computed by said computing means and each of said
resistances measured by said measuring means, said comparison means also
including a control cirucit means connected to said control signal means
for selecting individual resistive segments and for conducting heating
current through the individual resistive segments so selected in response
to said plurality of control signals.
13. Apparatus as set forth in claim 12, wherein said comparison means
further includes a multiplexing means connected to said computing means
for synchronizing the operation of said comparison means such that said
predicted resistances are sequentially supplied to said comparison means
and uniquely associated with a corresponding one of said resistances
measured by said measuring means.
14. Apparatus as set forth in claim 13, wherein said comparison means
includes storage means for respectively storing said predicted resistances
supplied from said computing means through said multiplexer means in order
to enable simultaneous comparisons between each of said predicted
resistances and each of said corresponding resistances measured by said
measuring means.
15. Apparatus as set forth in claim 12, further comprising timing means for
synchronizing the operation of said measuring means, said detection means
and said computing means such that said predicted resistances for the
resistive segments are computed in sequential fashion. |
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Claims |
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Description |
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TECHNICAL FIELD
The present invention is directed to a controller which heats a resistive
heating element to a desired temperature and more particularly to a
controller which regulates the flow of current through the resistive
heating element in accordance with the comparison between the measured
resistance of the resistive heating element and the predicted resistance
of the heating element at the desired temperature.
BACKGROUND ART
Resistive heating elements provide a simple and economically practical
means for generating heat in a wide variety of situations. For example,
devices as diverse as those employed in the home appliance field, the
industrial equipment field and the medical/surgical instrument field all
utilize resistive heating elements to achieve or maintain desired
temperatures under varying conditions. In some applications, such as home
heating systems, relatively gross temperature control over the resistive
heating element is sufficient to satisfy temperature demands. Other
applications, such as those involving the use of hemostatic scalpel blades
during surgical operations, require precise control over resistive heating
element temperatures if the resistive heating element is to be
successfully operated. It is with this latter category of applications
that the controller of the present invention is concerned.
DISCLOSURE OF THE INVENTION
It is therefore an object of the present invention to provide a controller
for a resistive heating element.
It is another object of the present invention to provide a controller for a
resistive heating element wherein the amount of current flowing through
the resistive heating element is precisely regulated to govern the
temperature of the resistive heating element.
It is yet another object of the present invention to provide a controller
for heating a resistive heating element to a desired temperature wherein
the controller predicts the resistance of the heating element at the
desired temperature.
It is a further object of the present invention to provide a controller for
heating a resistive heating element to a desired temperature wherein a
sensing current is coupled across the resistive heating element and used
to determine the heating element resistance at ambient or room
temperature, the predicted resistance of the heating element at the
desired temperature thereafter being determined by multiplying the ambient
resistance with a parameter which varies as a function of the desired
temperature.
It is still a further object of the present invention to provide a
controller for heating a resistance element to a desired temperature
wherein the controller measures the actual resistance of the resistive
heating element and compares the actual resistance so measured with a
predicted value of the heating element resistance at the desired
temperature in order to derive a control signal having a value which
varies as a function of the difference between the measured resistance and
the predicted resistance, the control signal thereafter being used to
regulate the flow of current through the resistive heating element.
These and other objects of the present invention are achieved by a
controller which measures the ambient resistance R.sub.amb of the heating
element at ambient temperature and uses the measured value of the ambient
resistance to compute the predicted resistance R.sub.hot of the heating
element at the desired temperature. After the predicted resistance
R.sub.hot has been computed, heating current is passed through the
resistive heating element to generate heat while the actual resistance of
the heating element is periodically measured. The actual resistance
measurements R.sub.m are compared with the value of the predicted
resistance R.sub.hot and the difference between the two serves to generate
a control signal for regulating the flow of heating current. When R.sub.m
and R.sub.hot equal one another, the heating element is assumed to have
reached the desired temperature. Heating current is then shunted past the
resistive heating element until cooling of the heating element or an
increase in the desired temperature setting once again causes the value of
the measured resistance to differ from the value of the predicted
resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
The various features, objects and advantages of the present invention will
become more apparent from the following Brief Description of the Drawings
and Best Mode for Carrying Out the Invention, wherein:
FIG. 1 is a schematic illustration depicting the fundamental operations of
a current controller constructed in accordance with the present invention;
FIG. 2 is a circuit diagram of the analog resistance measuring and heating
current control circuitry employed in conjunction with a hardware version
of the controller of FIG. 1;
FIG. 3 is a circuit diagram of a logic board for use in governing the
operation of the circuitry disclosed in FIG. 2;
FIGS. 4A-4M graphically represent various timing and address pulses
generated by the logic board of FIG. 3.
FIGS. 5A and 5B are circuit diagrams which illustrate the parameter
input/output board employed in conjunction with the controller of FIG. 2;
and
FIGS. 6A and 6B are circuit diagrams respectively illustrating the sensing
current generator and heating current supply.
BEST MODE FOR CARRYING OUT THE INVENTION
The simple electrical relationship which exists between the resistance of a
resistive heating element and the amount of heat given off by the element
in response to current flow therethrough makes resistive heating elements
particularly attractive from a control standpoint. Basically, the amount
of heat given off by any electrical conductor in response to current flow
is a function of the resistance of the conductor. Resistance in turn
varies as the temperature T of the conductor changes. This relationship
between resistance and temperature can be expressed as follows:
R(T)=R.sub.amb X(1+.varies..DELTA.T), (1)
where R.sub.amb is the resistance of the conductor at ambient temperature
T.sub.amb,.varies. is a temperature coefficient having a value dependent
upon both the ambient temperature and the composition of the conductor and
.DELTA.T=T-T.sub.amb. Using Equation (1), the value of the resistance
R.sub.hot presented to the flow of current through a given conductor can
be predicted for any desired conductor temperature T.sub.hot. Precise
temperature control over the heat generated in the conductor then becomes
a simple matter of regulating current flow until the measured value of the
conductor resistance R.sub.m equals the predicted value of the conductor
resistance R.sub.hot at the desired temperature, i.e., until
R.sub.m =R.sub.hot (2)
A controller which regulates current flow through an electrical conductor
such as a resistive heating element for the purpose of generating a
desired amount of heat is schematically illustrated in FIG. 1. Resistive
heating element 2 is alternately connected to receive heating current
I.sub.H from a heating current supply 4 and sensing current I.sub.s from a
sensing current generator 6. A resistance measuring means 8 is connected
across resistive heating element 2 to provide on-going measurements of the
heating element resistance while a resistance computing means 10 computes
the predicted resistance R.sub.hot of the resistive heating element at the
desired temperature T. The values of R.sub.m and R.sub.hot are compared in
a comparison means 12 and used to generate a control signal for adjusting
the flow of heating current I.sub.H from heating current supply 4. The
operation of heating current supply 4, sensing current generator 6,
resistance measuring means 8, resistance computing means 10 and comparison
means 12 is coordinated by timing pulses supplied from controller logic
15.
A preferred operating sequence for the controller of FIG. 1 will now be
described. When controller operation is initiated, sensing current
generator 6 directs a flow of sensing current I.sub.s through resistive
heating element 2 while heating current supply 4 is deactivated. Sensing
current I.sub.s produces a voltage drop across the resistive heating
element, which voltage drop furnishes a measure of the resistance R.sub.m
of the heating element. If the change in measured resistance R.sub.m over
time is sufficiently small, resistive heating element 2 is assumed to be
at ambient temperature, and the value of the measured resistance thus
represents the ambient resistance R.sub.amb of the heating element. This
value is stored in resistance computing means 10 along with a desired
temperature setting enabling the resistance computing means to compute the
predicted resistance R.sub.hot of the resistive heating element at the
desired temperature in accordance with Equation (1).
The desired temperature setting may be entered into the resistance
computing means in the form of a temperature index TI, where TI is related
to the desired temperature T by the equation:
T=2TI+100 (3)
After the value of R.sub.amb has been measured and stored, heating current
I.sub.H from heating current supply 4 is intermittently passed through
resistive heating element 2 to generate heat as a function of power
dissipation in the heating element. During the intervals between
intermittent heating current flow, the sensing current I.sub.s from
sensing current generator 6 is supplied to the resistive heating element
and resistance measuring means 8 continues to measure the actual heating
element resistance R.sub.m. The value of R.sub.m and the value of the
predicted resistance R.sub.hot from the resistance computing means 10 are
compared with one another in comparison means 12, with the difference
between the two values serving as an indication of the extent to which
flow of heating current I.sub.H from heating current supply 4 ought to be
adjusted. That is, where R.sub.m and R.sub.hot differ, resistive heating
element 2 is not yet at the desired temperature and intermittent flow of
heating current I.sub.H from heating current source 4 is maintained. On
the other hand, if the value of the measured resistance and the value of
the predicted resistance at the desired temperature are equal, an
assumption is made that resistive heating element 2 has reached the
desired temperature and comparison means 12 generates a control signal to
interrupt the intermittent flow of current from heating current source 4.
This interruption in heating current flow will continue until the
resistive heating element 2 cools to a point sufficient to lower the value
of the measured resistance R.sub.m or until the desired temperature
setting (and hence the value of the predicted resistance R.sub.hot) is
increased, whereupon the difference between R.sub.m and R.sub.hot again
causes comparison means 6 to signal the need for additional heating
current flow through the resistive heating element.
It is to be understood that the controller scheme of FIG. 1 may be
implemented in either software or hardware form. One suitable
configuration for a hardware-implemented controller constructed in
accordance with the present invention is illustrated in FIGS. 2 through 5.
The FIG. 2-5 controller operates in cycles such that each cycle coincides
with any one of several operating states depending upon whether the
controller is in a calibrating mode or a heating mode. More particularly,
as soon as power to the controller is switched on, the calibrating mode is
initiated and the controller enters a first state wherein certain limit
checks, discussed in greater detail hereinbelow, are performed on the
resistive heating element 2 to ensure that the controller and the
resistive heating element are properly connected and that the resistive
heating element is not defective. Once the initial limit checks are
completed, the controller cycles through a series of calibration states
ST.sub.1 through ST.sub.5 during which states the ambient resistance
R.sub.amb of the resistive heating element 2 is measured and stored.
Following successful completion of the calibrating states, the controller
advances to an ST.sub.6 or heating state to initiate the heating mode, and
current is thereafter cyclically supplied to the resistive heating element
for the purpose of generating heat in accordance with periodic comparisons
between the measured resistance R.sub.m of the resistive heating element
and the computed resistance R.sub.hot at the desired heating element
temperature.
Turning now to FIG. 2, the resistance measuring and current supply
circuitry of the controller can be seen. Resistive heating element 2 may,
if desired, comprise a set of three resistive heating segments 20, 22 and
24 connected in series to receive either heating current I.sub.H from
heating current supply 26 or sensing current I.sub.s from sensing current
generator 28. Appropriate circuit modifications, of course, will render
the controller of the present invention useable with resistive heating
elements having greater or lesser numbers of resistive heating segments. A
series of instrumentation amplifiers 30, 32 and 34 are respectively
connected across resistive heating segments 20, 22 and 24 to provide a
measure of the resistance of each resistive heating segment. When heating
current I.sub.H is flowing through resistive heating segments 20-24, the
instrumentation amplifier inputs are grounded through digital switches 36,
38, 40 and 42. When sensing current I.sub.s is supplied to the resistive
heating segments 20-24 by sensing current generator 28, however, the
digital switches 36-42 are opened in response to a switch open signal INS
from the controller logic (not shown in FIG. 2). The respective voltages
generated as the sensing current flows through the resistive heating
segments 20-24 then appear at the instrumentation amplifier inputs. The
gain of each instrumentation amplifier 30-34 is adjusted such that the
value of the instrumentation amplifier output equals the value of the
resistance of the corresponding resistive heating segment. For example,
where a sensing current of 0.2 amps is utilized to obtain resistance
measurements, the gain of each instrumentation amplifier is set to five,
resulting in an amplifier output of one volt per ohm of measured
resistance. Upon completion of each resistance measuring sequence or
interval, digital switches 36.varies.42 are again closed in response to
the INS signal from the controller logic and the instrumentation amplifier
inputs are shorted to ground. The presence of resistors 43, 44, 46 and 48
serve to limit the flow of current through each digital switch.
The output from each instrumentation amplifier 30, 32 and 34, representing
the measured resistance R.sub.m of each resistive heating segment 20, 22
and 24, is supplied to a corresponding comparator 50, 52 and 54 for
comparison with the predicted value of the resistive heating segment
resistance R.sub.hot at the desired resistive heating segment temperature.
The instrumentation amplifier outputs are also multiplexed through
multiplexer 56 and supplied to an A-to-D converter 58. An amplifier 60
connected between multiplexer 56 and A-to-D converter 58 furnishes the
necessary scaling. Multiplexer 56 is controlled by address bits A.sub.0
and A.sub.1 supplied from the controller logic, which address bits shift
at the outset of each controller operating cycle to assume one of four
binary states 00, 01, 10, and 11. Each resistive heating segment 20-24 is
associated with a unique address, say address 00 for resistive heating
segment 20, address 01 for resistive heating segment 22, and address 10
for resistive heating segment 24. It can also be seen that one of the
addresses, e.g., 11 in the present example, is a fictitious address when
used in connection with the 3-segment heating element of the FIG. 2
embodiment. During the calibration mode of controller operation, each of
the instrumentation amplifier outputs multiplexed to A-to-D converter 58
is monitored, as described in further detail below, to ensure that the
corresponding resistive heating segment is at room or ambient temperature.
If ambient temperatures exist, the instrumentation amplifier output, which
now represents the ambient resistance R.sub.amb of the corresponding
resistive heating segment, is digitized by A-to-D converter 58 in response
to a data write pulse DW from the controller logic. The digitized value of
R.sub.amb is then written into random access memories 62, 64 in response
to a memory write MWR pulse supplied from the controller logic. It can be
seen upon examination of FIG. 2 that both of the random access memories 62
and 64 are addressed by the same address bits A.sub.0, A.sub.1 controlling
multiplexer 56. Hence, a one-to-one relationship between the measured
value of the ambient resistance for each resistive heating segment and the
storage location of the measured value is preserved.
When the controller is operating in the heating mode, the values of
R.sub.amb for each resistive heating segment 20, 22 and 24 are
sequentially recalled from random access memories 62 and 64 in response to
successive memory read pulses MRD supplied from the controller logic. The
sequentially recalled values of R.sub.amb are received by a D-to-A
multiplying converter 66 having a reference voltage set to represent the
value of a predetermined constant multiplied by the value of the
temperature perature factor (1+.alpha..DELTA.T). The magnitude of the
predetermined constant is chosen to offset the scaling factor associated
with amplifier 60 at the input to A-to-D converter 58. The temperature
factor (1+.alpha..DELTA.T) is derived by providing a D-to-A converter 68
with a digital representation TI.sub.0 -TI.sub.4 of the Temperature Index
defined by Equation (3) and thereafter adjusting gain and offset in the
converter output amplifier stage 70 to produce a voltage representative of
the temperature factor. The output of D-to-A converter 66 accordingly
represents the value of a given ambient resistance R.sub.amb multiplied by
the temperature factor, which value in turn represents the predicted
resistance R.sub.hot for a given resistive heating segment at the desired
temperature. The D-to-A output is converted to a voltage by amplifier 72
and multiplexed through multiplexer 56 to one of the comparators 50, 52 or
54 in accordance with the address A.sub.0, A.sub.1 entered into
multiplexer 56. The outputs of comparators 50, 52 and 54, respectively
designated HT.sub.1, HT.sub.2 and HT.sub.3, provide a ready indication of
the difference between the predicted resistance R.sub.hot and the measured
resistance R.sub.m for each resistive heating segment 20, 22, and 24, and
may thus serve as control signals for increasing or decreasing the amount
of heating current required by each resistive heating segment in order to
reach or maintain the desired heating segment temperature.
It should here be noted that the use of address A.sub.0, A.sub.1 for both
multiplexer control and random access memory control ensures that the
value of a predicted resistance based on the ambient resistance of a given
resistive heating segment will be supplied only to that comparator
associated with the given resistive heating segment. It should also be
noted that simultaneous comparisons between the measured resistance
R.sub.m and the predicted resistance R.sub.hot for each resistive heating
segment 20, 22 and 24 occur during each operating cycle of the controller
heating mode, even though the value of the predicted resistance for a
given resistive heating segment is supplied to the comparator associated
with that resistive heating segment only once for every four operating
cycles. During the remaining three operating cycles, capacitors 74, 76 and
78 respectively connected to the predicted resistance inputs of
comparators 50, 52 and 54 operate to store the values of the predicted
resistances, enabling simultaneous comparisons between the predicted
resistances and the measured resistances for each operating cycle.
The regulation of heating current flow I.sub.H through resistive heating
segments 20, 22 and 24 is accomplished via control circuit 80. Control
circuit 80 includes a D flip-flop 84 having data inputs connected to
receive the control signals HT.sub.1, HT.sub.2, and HT.sub.3 from
comparators 50, 52 and 54. The Q outputs of flip-flop 84 are respectively
connected through a series of NAND gates 86, 88 and 90 to supply single
inputs to a series of triple-input NAND gates 92, 94 and 96. The remaining
inputs to NAND gates 86-90 are connected to receive timing pulses Q.sub.1
from the controller logic. As will be seen in connection with FIGS. 3 and
4, Q.sub.1 is high for all but a short interval at the outset of each
controller operating cycle. The remaining inputs to triple-input NAND
gates 92-96 are supplied by the output of NAND gate 98 and an S.sub.6
signal from the controller logic. NAND gates 92, 94 and 96 respectively
output signals designated FET.sub.1, FET.sub.2 and FET.sub.3, which
signals act to shunt heating current I.sub.H from the resistive heating
segments in a manner to be described. Finally, NAND gate 98 is connected
to receive the aforementioned Q.sub.1 timing pulse as well as an ISON
timing pulse from the controller logic.
Flip-flop 84 is clocked once during each operating cycle by timing pulse
Q.sub.3 from the controller logic. As long as the HT.sub.1, HT.sub.2 and
HT.sub.3 outputs from comparators 50, 52 and 54 indicate no demand for
heat from the resistive heating segments 20, 22 or 24 associated
therewith, the Q outputs from flip-flop 84 remain low to drive the outputs
of NAND gates 86, 88 and 90 high. If the resistance measuring interval of
the operating cycle has been completed, the controller logic generates a
low ISON signal and the output of NAND gate 98 switches high. Assuming
that the controller is in its heating mode, the S.sub.6 signal from the
controller logic will also be high and the triple-input NAND gates 92, 94
and 96 will output low signals. If, on the other hand, the HT.sub.1,
HT.sub.2 or HT.sub.3 output from a given comparator 50, 52 or 54 indicates
a demand for heating current in the associated resistive heating segment,
the corresponding Q output from flip-flop 84 will clock high to drive the
output of the associated NAND gate 86, 88 or 90 low, whereupon the
FET.sub.1, FET.sub.2 or FET.sub.3 output of the associated triple-input
NAND gate 92, 94 or 96 will switch high.
Control circuit 80 also includes a series of power transistors 100, 102 and
104 respectively connected in shunt configuration across resistive heating
segments 20, 22 and 24. Each power transistor is activated by biasing the
gate thereof positive with respect to the source thereof. Such biasing is
in turn accomplished by conducting a suitable current from a constant
current source 106 through a resistor 108, 110 or 112 connected between
the transistor source and gate. A series of transistors 114, 116 and 118
are connected between constant current source 106 and resistors 108, 110
and 112. The gates of each transistor 114, 116 and 118 are respectively
connected to receive the control signals FET.sub.1, FET.sub.2 and
FET.sub.3 from NAND gates 92, 94 and 96. When any of the HT.sub.1,
HT.sub.2 and HT.sub.3 outputs from comparators 50, 52 and 54 is low,
indicating no demand for heating current in a particular resistive heating
segment, the resulting low FET.sub.1, FET.sub.2 or FET.sub.3 signal will
turn off the appropriate transistor 114, 116 or 118. Current from constant
current source 106 thus continues to flow through the corresponding
resistor 108, 110 or 112, activating the associated power transistor 100,
102 or 104 and shunting current around the particular resistive heating
segment. If, however, the comparator associated with a given resistive
heating segment has signaled a demand for heating current, the associated
control signal FET.sub.1, FET.sub.2 or FET.sub.3 will switch high to turn
on the appropriate transistor 114, 116 or 118 to draw off current from
constant current source 106. Current flow through the associated resistor
108, 110 or 112 is thereafter stopped, deactivating the associated power
transistor 100, 102 or 104. With the associated power transistor in an off
condition, heating current flows through the given resistive heating
segment to generate heat. Diodes 120, 122 and 124 provide transient
protection for the power transistors 100, 102 and 104, while diodes 126,
128 and 130 prevent current from flowing back into constant current source
106 when transistors 114, 116 and 118 are on. If desired, constant current
source 106 may be constructed using a series of transistors 132, 134 and
136 supplied by 60 volts unregulated power. A zener diode 138 biased by
resistor 140 provides the desired reference for gating transistors
132-136.
When control circuit 80 is examined closely, several constraints on the
controller operation are apparent. First, no heating current can flow
through resistive heating segments 20, 22 and 24 during the calibrating
mode of the controller. This is because the S.sub.6 signal from the
controller logic is low for all controller operating cycles which occur
during the calibrating mode. NAND gates 92-96 in power transistor control
circuitry 80 are accordingly disabled to produce high FET.sub.1, FET.sub.2
and FET.sub.3 control signals and power transistors 100, 102 and 104
remain in an off condition to permit flow of sensing current I.sub.s
through the resistive heating segments. When the controller is in a
heating mode, the ISON signal supplied by controller logic to NAND gate 98
is driven high to signal the beginning of a resistance measuring interval
in the operating cycle. Unless the Q.sub.1 timing pulse also input to NAND
gate 98 is low, which only occurs for a brief interval at the outset of
each operating cycle, the output of NAND gate 98 will switch low to drive
the outputs from NAND gates 92-96 high, again turning power transistors
100-104 off to permit the flow of sensing current through the resistive
heating segments. Of course, during the brief interval at the outset of
each operating cycle defined by a low Q.sub.1 control pulse, the output
from NAND gates 86-92 will all switch high to drive triple-input NAND
gates 92-96 low, turning power transistors 100-104 on. This latter
procedure serves to dissipate any energy inductively stored in the
resistive heating segments prior to performing the resistance measurement.
Finally, during the period between the end of the resistance measuring
interval and the end of each operating cycle of the controller heating
mode, power transistors 100-104 will be deactivated as previously
described whenever the resistive heating segments associated therewith
require heating.
The controller logic of the present invention is illustrated in FIG. 3. All
of the logic timing is derived from a master clock 142 which, in one
embodiment of the controller logic, comprises a 320 KHz clock.
Accordingly, clock L 142 outputs a series of 3.125 .mu.second pulses, as
indicated in FIG. 4A. The 3.125 .mu.second pulses clock octal counter 144
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