Current controller for resistive heating element

Claim 2

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.

Claim 3

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.

Claim 4

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.

Claim 5

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.

Claim 6

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.

Claim 7

Apparatus as set forth in claim 5, wherein said temperature factor is equal to (1+αΔT), where α is a temperature coefficient having a value dependent upon both the ambient temperature and the composition of the resistive element and ΔT represents the difference between the desired temperature and the ambient temperature.

Claim 8

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.

Claim 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: 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.

Claim 10

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.

Claim 11

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.

Claim 12

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.

Claim 13

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.

Claim 14

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.

Claim 15

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.

Claim 16

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.

Claim 17

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.

Claim 18

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.

Claim 19

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.