Patent ID: 8340543

Claim:
A temperature control method for use in a fixing device that fixes a toner image on a recording sheet by passing the recording sheet through a fixing nip, the fixing device including: a fixing member disposed to indirectly press against a pressure roller to form the fixing nip therebetween; and a heater to heat the fixing member to a target temperature, the method comprising: detecting a temperature of the fixing member with a temperature detector; controlling operation of the heater by changing a duty thereof according to the detected temperature, wherein controlling the heater operation includes calculating a calculation equation to obtain each heater duty; and executing a heater duty control to change a heater duty for a current control cycle discontinuously from that for a previous control cycle when the current control cycle precedes entry of the recording sheet into the fixing nip by a given period of time, wherein the executing of the heater duty control causes the heater duty for the current control cycle to exceed a value obtained from the calculation equation when the current control cycle precedes entry of the recording sheet into the fixing nip by the given period of time, wherein the calculation equation comprises a proportion-integral-derivative (PID) algorithm given by: D n =D n−1 +Kp *( T n−1 −T n )+ Ki *( T−T n )+ Kd *(2* T n−1 −T n −T n−2 ) where D n is the heater duty for a n-th control cycle, D n−1 is the heater duty for a (n−1)th control cycle, T is the target temperature, T n is a temperature of the fixing member detected for the n-th current control cycle, T n−1 is a temperature of the fixing member detected for the (n−1)th control cycle, T n−2 is a temperature of the fixing member detected for a (n−2)th control cycle, Kp is a proportional gain, Ki is an integral gain, and Kd is a differential gain, and the heater duty control obtains the heater duty for the current control cycle by substituting a given corrective value greater than the actual heater duty for the previous control cycle for D n−1 in the PID algorithm.