The feedback type surface electric potential sensor provides the world's most accurate detection features, to lead trends for higher picture quality of color phtocopiers, digital monochrome photocopiers and next-generation color laser printers in its size class (which is the world's smallest) at a new price that responds to market needs.
It has its own highly stable output circuit to send the surface electric potential of the examined photosensitive drum back to the standard electric potential of the probe component so that electric field disrupters between the drum and the detection electrode will be completely removed. With the variable range of detection outputs (0 to 4.5V) limited to ±0.05V, it provides a detection response speed of 11 msec. (typ.) to achieve compatibility with high-speed machines. In addition, the range of detection distance supported has been extended to between 1.5 and 3.5mm. Within this range, the output voltage is highly stable with little fluctuation even if there is a change in ambient temperature.
Necessity of surface electric potential detection
Electrostatic photocopiers generally take the following steps for photocopying :
- 1) Electrification : electrifies the surface of the photosensitive drum evenly with the corona discharge of the electrifier.
- 2) Exposure : makes the part to be exposed conductive to release the electric charge on the surface.
- 3) Development : attaches toner electrified to latent images to produce visible images.
- 4) Transfer : transfers toner on the photosensitive drum to paper.
- 5) Settlement : melts the toner transferred on paper to fix it.
- 6) Cleaning : removes toner left on the photoreceptor after transfer to bring it back to the initial state.
It is the optimization of the quantity of the toner to be attached to the photosensitive drum that is, of course, important for retaining the best picture quality.
This is possible only when the electrification status of the photosensitive drum is kept at the preset optimum value at the point of development in step (3) in the process above, and it requires a function to correctly detect the electric potential on the surface of the drum after the cleaning in step (6) and to control the strength of electrification (corona discharge) in step (1).
The overview of the functions and controls are shown in the model above. The surface electric potential sensor transmits the surface electric potential detected to the CPU as voltage signals via processing circuit (a). Receiving the signals, the CPU controls high voltage generator (b) to adjust the strength of corona discharge generated by electrifier (c). This means that the series of detection and control processes prevents environmental changes and age deterioration of the electrification properties of the drum surface from impacting the optimum level of electrification of the photosensitive drum that provides quality pictures. Detection accuracy and stability in output of the surface potential sensor are decisive to the performance and reliability of this control system, and the response speed needs to further improve for quicker photocopying.
Structure of surface electric potential sensors
The probe component which detects the surface potential of the photosensitive drum, consists of a chopper (tuning fork shaped vibration board) equipped with a electric flux line shield wing and a piezoelectric element for operation, a detection electrode to detect the surface potential and a pre-amplifier (detection circuit) for impedance conversion.
The small-size actuator control mechanism, central to sensing, is enclosed in a compact metallic shield case that shields external noise, and a window open on the upper part of the detection electrode surface is provided in the photocopier such that it faces the surface of the photosensitive drum.
Surface potential detection mechanism
Next, we provide an explanation on the relationship between the actuator component and the detection circuit in the probe using the model below, in which the surface of the photo-sensitive drum is negatively charged.
The sensing electrode in the probe that faces the surface of the photosensitive drum at a distance of several millimeters has its surface induced to carry a positive charge equivalent to the negative charge (of the area restricted by the window and the electric flux line shield wing) on the surface of the photosensitive drum. This is because of the electrostatic combination through the air.
Loaded with operation voltage, the piezoelectric element adhered to the chopper base is expanded or contracted in a vertical direction in this illustration so that the electric flux line shield wing at the end will periodically open and close by the bending stress on the chopper.