Constant current closed loop controller for rotating system

Temperature control apparatus for a rotatable heating system comprising:

• (a) a rotatable heating member having at least one heating element;
• (b) means rotating with said rotatable member and coupled to a stationary controller and a power supply via a plurality of respective control and power slip rings for resistively sensing the temperature of said heating element, including means for comparing the sensed temperature to a reference signal of a selected magnitude and producing a current control signal proportional to the sensed temperature within a predetermined range about the magnitude of said reference signal and wherein the magnitude of the current control signal varies inversely to said temperature;
• (c) stationary means responsively coupled to said current control signal via said control slip rings for comparing said magnitude varying current control signal to a separately selected set-point control signal, the magnitude of which set-point control signal corresponds to a desired temperature, and including means for producing a third control signal so long as the magnitude of said current control signal is less than that of said set-point signal;
• (d) means coupled to said power slip rings for applying power to said heating element in response to said third control signal and in synchronization with the zero-crossings of an AC power supply signal;
• and wherein
• (e) the magnitude of the reference signal at said rotating means is selected to be sufficiently large so as not to be affected by noise at said control slip rings.