Image forming apparatus and image forming method

1. An image forming apparatus, comprising:

an image forming station including a plurality of photoconductive drums and adapted to form toner images on surfaces of the respective photoconductive drums; and

a drive control unit for rotating and driving the plurality of photoconductive drums;

wherein:

the image forming station includes a reference pulse output section for generating a reference pulse having a cycle which is equal to a rotation cycle of the photoconductive drums or an integral multiple of the rotation cycle of the photoconductive drums and outputting the reference pulse to the drive control unit; and

the drive control unit includes:

a phase peak detector for detecting phase peaks of surface speeds of the respective photoconductive drums,

a phase difference detector for detecting phase differences between an edge of the reference pulse and the phase peaks, and

a phase synchronization controller for synchronizing phases of the surface speeds of the respective photoconductive drums by controlling rotation speeds of the respective photoconductive drums according to the phase differences;

and wherein

if the phase peak of the photoconductive drum to be synchronized is ahead of the edge of the reference pulse, the phase synchronization controller controls the phase of the photoconductive drum to be synchronized by decelerating the rotation speed of the photoconductive drum at a constant deceleration rate until the phase difference is halved, then accelerating the rotation speed at a constant acceleration rate of the same magnitude as the constant deceleration rate up to an initial speed and, thereafter, setting the rotation speed at a constant speed.

2. An image forming apparatus comprising:

an image forming station including a plurality of photoconductive drums and adapted to form toner images on surfaces of the respective photoconductive drums; and

a drive control unit for rotating and driving the plurality of photoconductive drums;

wherein:

the image forming station includes a reference pulse output section for generating a reference pulse having a cycle which is equal to a rotation cycle of the photoconductive drums or an integral multiple of the rotation cycle of the photoconductive drums and outputting the reference pulse to the drive control unit; and

the drive control unit includes:

a phase peak detector for detecting phase peaks of surface speeds of the respective photoconductive drums,

a phase difference detector for detecting phase differences between an edge of the reference pulse and the phase peaks, and

a phase synchronization controller for synchronizing phases of the surface speeds of the respective photoconductive drums by controlling rotation speeds of the respective photoconductive drums according to the phase differences; and wherein,

if the phase peak of the photoconductive drum to be synchronized lags behind the edge of the reference pulse, the phase synchronization controller controls the phase of the photoconductive drum to be synchronized by accelerating the rotation speed of the photoconductive drum at a constant acceleration rate until the phase difference is halved, then decelerating the rotation speed at a constant deceleration rate of the same magnitude as the constant acceleration rate up to an initial speed and, thereafter, setting the rotation speed at a constant speed.

3. An image forming apparatus according to claim 1 , wherein:

the phase peak detector detects the phase peak of the surface speed of one of the plurality of photoconductive drums;

the phase difference detector detects the phase difference between the edge of the reference pulse and the phase peak of the one photoconductive drum; and

the phase synchronization controller synchronizes the phase of the one photoconductive drum and those of the other photoconductive drums by controlling the rotation speeds of the other photoconductive drums according to the phase differences.

4. An image forming apparatus according to claim 1 , wherein the drive control unit further includes:

an oscillation circuit for generating clock signals;

a clock frequency meter for measuring the frequency of the clock signals based on a count number of the clock signals in one cycle of the reference pulse; and

a clock correction controller for correcting the frequency of the clock signals based on the frequency measured by the clock frequency meter.

5. An image forming apparatus according to claim 4 , wherein the reference pulse output section outputs a reference pulse having a cycle equal to the rotation cycle of the photoconductive drums when only a control by the phase synchronization controller is performed in the drive control unit.

6. An image forming apparatus according to claim 4 , wherein the reference pulse output section outputs a reference pulse having a cycle which is an integral multiple of the rotation cycle of the photoconductive drums when only a control by the clock correction controller is performed or the control by the phase synchronization controller and the control by the clock correction controller are simultaneously performed in the drive control unit.

7. An image forming method for forming toner images on surfaces of a plurality of photoconductive drums by rotating and driving the photoconductive drums, comprising the steps of:

generating a reference pulse having a cycle which is equal to a rotation cycle of the photoconductive drums or an integral multiple of a rotation cycle of the photoconductive drums and outputting the reference pulse to a drive control unit for driving and rotating the photoconductive drums;

detecting phase peaks of surface speeds of the respective photoconductive drums;

detecting phase differences between an edge of the output reference pulse and the phase peaks;

synchronizing phases of the surface speeds of the respective photoconductive drums by controlling rotation speeds of the respective photoconductive drums according to the phase differences; and

if the phase peak of the photoconductive drum to be synchronized is ahead of the edge of the reference pulse, the phase synchronization controller controls the phase of the photoconductive drum to be synchronized by decelerating the rotation speed of the photoconductive drum at a constant deceleration rate until the phase difference is halved, then accelerating the rotation speed at a constant acceleration rate of the same magnitude as the constant deceleration rate up to an initial speed and, thereafter, setting the rotation speed at a constant speed.

8. An image forming apparatus according to claim 2 , wherein:

the phase peak detector detects the phase peak of the surface speed of one of the plurality of photoconductive drums;

the phase difference detector detects the phase difference between the edge of the reference pulse and the phase peak of the one photoconductive drum; and

the phase synchronization controller synchronizes the phase of the one photoconductive drum and those of the other photoconductive drums by controlling the rotation speeds of the other photoconductive drums according to the phase differences.

9. An image forming apparatus according to claim 2 , wherein the drive control unit further includes:

an oscillation circuit for generating clock signals;

a clock frequency meter for measuring the frequency of the clock signals based on a count number of the clock signals in one cycle of the reference pulse; and

a clock correction controller for correcting the frequency of the clock signals based on the frequency measured by the clock frequency meter.

10. An image forming apparatus according to claim 9 , wherein the reference pulse output section outputs a reference pulse having a cycle equal to the rotation cycle of the photoconductive drums when only a control by the phase synchronization controller is performed in the drive control unit.

11. An image forming apparatus according to claim 9 , wherein the reference pulse output section outputs a reference pulse having a cycle which is an integral multiple of the rotation cycle of the photoconductive drums when only a control by the clock correction controller is performed or the control by the phase synchronization controller and the control by the clock correction controller are simultaneously performed in the drive control unit.