Image forming apparatus

1. An image forming apparatus comprising:

a rotatable photosensitive member;

a charging member configured to electrically charge a surface of the photosensitive member at a charging portion;

an exposing device exposing the surface of the photosensitive member electrically charged to light at an exposing position and configured to form an electrostatic latent image on the surface of the photosensitive member;

a developing device supplying toner charged with a normal polarity to the electrostatic latent image of the surface of the photosensitive member at a developing position and configured to form a toner image on the surface of the photosensitive member;

a transfer member configured to transfer the toner image on the surface of the photosensitive member to a transferred material at a transfer position;

a brush contacting the surface of the photosensitive member at a brush contacting position downstream of the transfer position and upstream of the charging position with respect to a rotational direction of the photosensitive member;

a voltage applying portion configured to apply a brush voltage to the brush; and

a control portion configured to control a surface potential of the photosensitive member at the brush contacting position,

wherein the toner remaining on the surface of the photosensitive member after transferring is collected by the developing device, and

wherein when a value of subtracting a value of the surface potential of the photosensitive member in the brush contacting position from a value of the brush voltage is defined as a contacting position potential difference, the control portion controls the surface potential of the photosensitive member in the brush contacting position so that the contacting position potential difference is changed from a first potential difference to a second potential difference in a predetermined direction which is either one of an increasing direction or a decreasing direction, and then the contacting position potential difference is changed from the second potential difference to a third potential difference in the predetermined direction.

2. An image forming apparatus according to claim 1 , wherein the control portion controls the surface potential of the photosensitive member in the brush contacting position so that

(i) the contacting position potential difference is changed from the first potential difference to the third potential difference via the second potential difference in the predetermined direction,

(ii) the contacting position potential difference is changed from a fourth potential difference to a fifth potential difference in an opposite direction which is either one of the increasing direction or the decreasing direction and opposite to the predetermined direction, and then

(iii) the contacting position potential difference is changed from the fifth potential difference to a sixth potential difference in the opposite direction.

3. An image forming apparatus according to claim 1 , wherein the control portion controls the surface potential of the photosensitive member in the brush contacting position so that the contacting position potential difference is changed stepwise.

4. An image forming apparatus according to claim 1 , wherein the control portion controls the surface potential of the photosensitive member in the brush contacting position so that the contacting position potential difference is changed continuously.

5. An image forming apparatus according to claim 1 , wherein the control portion controls the surface potential of the photosensitive member in the brush contacting position so that a sign of the contacting position potential difference becomes both of positive and negative.

6. An image forming apparatus according to claim 1 , wherein the control portion controls the surface potential of the photosensitive member in the brush contacting position so as to control at least one of a voltage applied to the charging member, an exposure to the photosensitive member by the exposing device, a voltage applied to the transfer member, and an exposure to the photosensitive member by a pre-exposing device provided downstream of the transfer position with respect to the rotational direction of the photosensitive member and upstream of the brush contacting position so as to expose the surface of the photosensitive member.

7. An image forming apparatus according to claim 1 , further comprising a transfer voltage applying portion configured to apply a transfer voltage to the transfer member,

wherein the control portion controls the surface potential of the photosensitive member in the brush contacting position by controlling the transfer voltage applying portion.

8. An image forming apparatus according to claim 1 , wherein the control portion controls the surface potential of the photosensitive member in the brush contacting position so that the contacting position potential difference is changed in a direction for increasing an absolute value of the contacting position potential difference during a period when a sign of the contacting position potential difference becomes a sign opposite polarity to the normal polarity.

9. An image forming apparatus according to claim 1 , wherein the brush removes and collects a paper powder from the surface potential of the photosensitive member.

10. An image forming apparatus according to claim 1 , wherein the charging member electrically charges the surface of the photosensitive member by contacting the surface of the photosensitive member.

11. An image forming apparatus according to claim 1 , wherein the brush is provided with base materials contacting the surface of the photosensitive member, and

wherein a density of the base materials of the brush is 120 kF/inch 2 or more.