In this model, a non-contact toner density (TD) sensor, which we call µ sensor, is used for the toner density control.
The TD sensor is attached on the lower side of the development unit. Unlike HST sensor, the board of TD sensor is exposed. So there is a cover around the sensor to protect the sensor and to maintain a good contact condition of the sensor and development unit.
The TD sensor measures the permeability of the developer without contacting from outside of the case, and converts the measured value to the toner density.
According to the toner density measured by this sensor, the proper amount of toner is supplied to the developer.
A counter corresponding to the frequency is used as the unit of TD sensor output. Thus, unlike HST sensor which directly detects Vt, the TD sensor output is converted into Vt for the toner supply control.
In the TD sensor, there is an ID chip storing the machine identification information, the running distance information of Development unit and PCU, and other information used by the image density control.