Heated roller fusing systems for fixing toner images to a substrate generally heat at least one roller to an appropriate surface temperature to help fix a toner image to the substrate. Too low a temperature gives poor fusing quality and too high a temperature will distort the copy substrate and reduce the life of the fusing roller. Thus, substantial development has been devoted to control of the surface temperature of fusing rollers.
U.S. Pat. No. 4,046,990 shows a commercially used control system in which the temperature of the core is sensed and controlled according to the temperature projected to be on the surface of the roller. Since the surface temperature varies according to whether the fuser is in an "idle" condition or a "run" condition the set point for the core is varied according to whichever condition it is in.
More common modern control devices involve sensing the surface temperature itself. For example, the amount of energy applied to the core is controlled according to the difference between the temperature sensed on the surface and an appropriate set point. After an elongated period of idle, a relatively cool core is difficult to bring immediately up to a high enough temperature to raise the surface temperature to compensate for losses to substrates carrying images being fused. This phenomena is commonly called temperature "droop".
A more sophisticated algorithm raises the surface temperature set point when the start copy button is pressed, thereby immediately creating an error signal and increasing the power to the source. This approach reduces droop, but if a core is still warm from a previous long run there will be a substantial temperature "overshoot", characterized by a period in which the fuser temperature is hotter than desirable.