In this model, a non-contact toner density (TD) sensor, which we also call a mu () sensor, is used for toner density control.
The TD sensor is attached on the lower side of the development unit. Unlike a HST sensor, the board of the TD sensor is exposed. So there is a cover around the sensor to protect it and to maintain a good contact between the sensor and development unit.
The TD sensor measures the permeability of the developer without contacting it, from the 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 a HST sensor which directly detects Vt, the TD sensor output is converted into Vt for toner supply control.
In the TD sensor, there is an ID chip storing the machine identification information, the running distance information of the development unit and the PCU, and other information used by image density control.