Controller for a fusing device of an electrophotographic printing machine

An apparatus in which images on sheets are fused during a copy run. Heat is applied to at least the images on successive sheets advanced, in seriatim, through a fusing device permanently affixing the images to the sheet. The temperature of the fusing device is detected and a signal indicative thereof transmitted. A controller compares the time derivative of the signal received from the detector at initialization of the copy run to a first constant and energize the heater of the fusing device when the first constant is less than the time derivative of the signal. During the copy run, after the time derivative of the signal is less than the first constant, the controller compares the signal received from the detector to a constant and generates an error signal indicative of the difference therebetween to control the heater of the fusing device. After the copy run, the controller compares the time derivative of the signal received from the detector to still another constant and de-energizes the heater of the fusing device when this latter constant is less than the time derivative of the signal. Once the surface temperature of the fusing device returns to the stand-by temperature at the end of the copy run, the controller returns to comparing the signal received from the detector to a constant and generating an error signal indicative of the difference therebetween which regulates the heater thereof.