Image forming apparatus

1. An image forming apparatus comprising:

a plurality of image forming units each of which includes

an image carrier having a photosensitive layer formed on a surface thereof, and

a developing device including a developer carrier that is disposed to face the image carrier and carries a developer containing toner, and configured to form a toner image by adhering the toner to an electrostatic latent image formed on the image carrier, and forms an image by superimposing the toner image of a same color;

a developing voltage power supply that applies, to the developer carrier, a developing voltage acquired by superimposing an AC voltage on a DC voltage;

a density detection device that detects a density of the toner image formed by the developing device;

a current detection unit that detects a DC component of developing current that flows when the developing voltage is applied to the developer carrier; and

a control unit that controls the image forming units and the developing voltage power supply, wherein

the plurality of image forming units use the developer containing the toner of a same color and a same type, and substantially same development conditions are set to evenly divide an image density among the image forming units,

the control unit calculates a toner charge amount for each of the developing devices, based on a DC component of the developing current detected by the current detection unit when a reference image is formed on the image carrier by each of the developing devices at a time when an image is not formed, and a density of the reference image detected by the density detection device, and detects whether there is an anomaly in each of the developing devices, based on the calculated toner charge amount, and

when an anomaly is detected in any of the developing devices, the control unit inhibits use of the image forming unit including the developing device, and resets the development conditions to evenly divide the image density among the usable image forming units.

2. The image forming apparatus according to claim 1 , wherein

the control unit inhibits use of the image forming unit including the developing device, when the toner charge amount Q within the developing device is equal to or lower than a lower limit value Q1 or is equal to or higher than an upper limit value Q2.

3. The image forming apparatus according to claim 1 , wherein

the control unit inhibits use of the image forming unit including the developing device, when the toner charge amount Q within the developing device is deviated from an average value of the toner charge amounts within all the developing devices by a certain value or more.

4. The image forming apparatus according to claim 1 , wherein

the control unit performs a recovery operation of recovering the toner charge amount of the developing device in which an anomaly is detected.

5. The image forming apparatus according to claim 4 , wherein

the control unit performs, as the recovery operation, a forcible ejection operation of ejecting the toner within the developing device onto the image carrier.

6. The image forming apparatus according to claim 4 , wherein

the control unit resets the development conditions to evenly divide the image density among all the usable image forming units including the developing device, when the toner charge amount of the developing device in which an anomaly is detected is recovered by the recovery operation.

7. The image forming apparatus according to claim 4 , further comprising a notification device that notifies a state of the image forming unit, wherein

the control unit causes the notification unit to notify to urge replacement of the developing device, when the toner charge amount of the developing device is not recovered after the recovery operation is performed.

8. The image forming apparatus according to claim 1 , further comprising three or more of the image forming units, wherein

the control unit stops an image forming operation, when the number of usable image forming units is one or less.