Current controller for resistive heating element

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.

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.

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.