Image forming apparatus and method for measuring toner charge

1. An image forming apparatus comprising:

an intermediate transfer body;

a plurality of image formation units arranged in a direction along which the intermediate transfer body moves, each image formation unit including:

a first image formation unit including a first toner bearer and a first image bearer, the first image formation unit being configured to move first toner on the first toner bearer to the first image bearer to form a first toner image on the first image bearer, and

at least one second image formation unit disposed downstream of the first image formation unit, the second image formation unit including a second toner bearer and a second image bearer, the second image formation unit being configured to move given second toner on the second toner bearer to the second image bearer to form a second toner image on the second image bearer, and

a first transfer unit configured to cause the first toner image to be transferred onto the first image bearer onto the intermediate transfer body based on a first transfer bias current supplied to the first transfer unit, the first toner image being a predetermined pattern of defining a reference toner image;

at least one second transfer unit configured to cause the second toner image on the second image bearer to be transferred onto the intermediate transfer body based on a second transfer bias current supplied to the second transfer unit;

a first measuring unit configured to measure a total charge of the first toner included in the reference toner image based on a developing current that flows in response to moving the first toner from the first toner bearer to the first image bearer;

a second measuring unit configured to measure an adhered amount of the first toner included in the reference toner image that is transferred from the first image bearer onto the intermediate transfer body;

a third measuring unit configured to measure a first toner charge per unit mass based on the total charge and the adhered amount; and

a current controller configured to set, with respect to the second-transfer bias current, a first current value for measuring the first toner charge per unit mass and a second current value for forming an image on a print medium, the first current value being lower than a current value of the first transfer bias current, and the first current value being different from the second current value.

2. The image forming apparatus according to claim 1 , wherein the current controller is configured to set the first current value of the second transfer bias current to be lower than the second current value of the second transfer bias current.

3. The image forming apparatus according to claim 1 , further comprising a controller including:

a receiver configured to receive a switching signal; and

a switching unit configured to switch between a first operation mode of forming the image on the print medium and a second operation mode of measuring the first toner charge per unit mass, in accordance with the switching signal.

4. A method for measuring a toner charge comprising:

moving, by a first image formation unit among a plurality of image formation units, first toner on a first toner bearer to a first image bearer to form a first toner image on the first image bearer, the first toner image being a predetermined pattern of defining a reference toner image;

transferring the first toner image on the first image bearer onto an intermediate transfer body based on a first transfer bias current;

moving, by at least one second image formation unit downstream of the first image formation unit, second toner on a second toner bearer to a second image bearer to form a second toner image on the second image bearer;

transferring the second toner image on the second image bearer onto the intermediate transfer body based on a second transfer bias current;

measuring a total charge of the first toner included in the reference toner image based on a developing current that flows in response to moving the first toner from the first toner bearer to the first image bearer;

measuring an adhered amount of the first toner included in the reference toner image that is transferred from the first image bearer onto the intermediate transfer body;

measuring a first toner charge per unit mass based on the total charge and the adhered amount; and

setting, with respect to the second-transfer bias current, a first current value for measuring the first toner charge per unit mass and a second current value for forming an image on a print medium, the first current value being lower than a current value of the first transfer bias current, and the first current value being different from the second current value.