Developing device having inductance sensor

1. A developing device comprising:

a developer bearing member which bears developer including toner and carrier to develop an electrostatic latent image formed on an image bearing member;

a developing container which accommodates the developer;

a conveying screw which conveys the developer accommodated in the developing container; and

an inductance sensor which includes a detecting portion which detects magnetic permeability of the developer accommodated in the developing container,

wherein in a state that a predetermined magnetic material is disposed at a position in contact with the detection portion, an output value when the detection unit detects magnetic permeability of the predetermined magnetic material is A, and

in a state that the predetermined magnetic material is disposed 1 mm away from the detection portion in a vertical direction, an output value when the detection unit detects magnetic permeability of the predetermined magnetic material is B, with B/A≥0.1 being satisfied,

wherein the conveying screw includes:

a rotary shaft;

a blade which is spirally formed on an outer circumference of the rotary shaft; and

a rib formed as to protrude outwardly from the outer circumference of the rotary shaft,

wherein the rib is disposed opposite the detection portion with respect to a conveying direction of the conveying screw, and

wherein the rib is provided with a magnet, with magnetic flux density of the magnet being in the range of 20 mT to 60 mT.

2. The developing device according to claim 1 ,

wherein when the rib rotates synchronously with the rotation of the conveying screw, a ratio of occupancy of a magnetic brush of the developer borne on the magnet to an area where the inductance sensor has a detection sensitivity is 70% or more,

wherein the detection sensitivity of the inductance sensor is a ratio of the output value when the detection portion detects the magnetic permeability of the predetermined magnetic material in a state that the predetermined magnetic material is disposed at a position X, with X>0, mm away from the detection unit in the vertical direction, to the output value when the detection portion detects the magnetic permeability of the predetermined magnetic material in a state that the predetermined magnetic material is disposed at a position in contact with the detection portion.

3. The developing device according to claim 1 ,

wherein a length of the magnet in a rotation axis direction of the conveying screw is 40% or more of a length of an area in the rotation axis direction of the conveying screw in which area the inductance sensor has a detection sensitivity of 10% or more,

wherein the detection sensitivity of the inductance sensor is a ratio of the output value when the detection portion detects the magnetic permeability of the predetermined magnetic material in a state that the predetermined magnetic material is disposed at a position X, with X>0, mm away from the detection unit in the vertical direction, to the output value when the detection portion detects the magnetic permeability of the predetermined magnetic material in a state that the predetermined magnetic material is disposed at a position in contact with the detection portion.

4. The developing device according to claim 1 ,

wherein the inductance sensor further includes an output portion which outputs a pulse signal in accordance with the magnetic permeability detected by the detection portion.

5. The developing device according to claim 1 ,

wherein the inductance sensor further includes a circuit board, and

wherein the detection portion is an area on the circuit board, on which an area a pattern of a coil is formed.