Oximetrix, Inc.

Robert J Strehlow

Paul S Tamura

A controller for regulating current flow through a resistive heating element in order to adjust the temperature thereof is disclosed. The controller includes a means for periodically measuring the resistance of the heating element. During controller calibration, the rate of change of the measured heating element resistance is ascertained in order to determine whether the heating element is at ambient temperature. Computations are performed on the value of the measured heating element resistance associated with ambient temperature conditions, enabling the controller to predict the value of the heating element resistance at a desired temperature. Current is then conducted through the heating element to generate heat, while periodic measurements of the heating element resistance are performed. The resistance values obtained during these latter resistance measurements are compared with the predicted resistance of the heating element associated with the desired temperature and the difference between the predicted resistance and each measured resistance is used to generate a control signal which regulates the flow of heating current through the heating element until desired temperature is reached.

Current controller for resistive heating element

The controller of the present invention regulates the flow of current through a resistive heating element by computing the predicted resistance R.sub.hot of the heating element at the desired temperature and then adjusting the voltage drop across and current flow through the resistive heating element until the relationship V=IR.sub.hot is satisfied. In an analog embodiment of the present invention, measurement of the voltage drop is obtained from a voltage amplifying circuit connected across the resistive heating element, while a measure of the current flow through the resistive heating element is obtained from a current amplifying circuit connected across a current measuring resistor in series with the resistive heating element. The output of the current amplifying circuit is multiplied by first and second variable gain amplifier circuits to produce a signal output representing the product of the current flow and the predicted resistance at the desired temperature. The signal from the voltage amplifying circuit and the product-representative signal from the two variable gain amplifier circuits are balanced in a control amplifier to produce a control signal having a value which varies as a function of the difference between the voltage-representative signal and the product-representative signal. The control signal is thereafter utilized to adjust the voltage drop across the resistive heating element until the current through the resistive heating element multiplied by the predicted resistance at the desired temperature, i.e., the product-representative signal, equals the voltage drop across the resistive heating element.

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Mountain View, CA, United States of America

Robert J Strehlow