1. Field of the Invention
The present invention relates to an electrostatic recording control method and apparatus for an electrophotographic printer or copier, and more particularly to an electrostatic recording control method and apparatus capable of making color printing by combination of normal development and reversal development such as a tri-level development system.
2. Description of the Related Art
There has been known a method of making two-color printing by one rotation of a photosensitive body in which normal development and reversal development are combined to form an electrostatic latent image on a photosensitive body and the electrostatic latent image thus formed is developed by two color developers charged with opposite polarities to form a two-color image. A "tri-level system" is well known in which a latent image with three potential levels on a photosensitive body is formed and the latent image thus formed is developed by two color developers charged with opposite polarities to form a two color image (see U.S. Pat. No. 4,078,929).
Now referring to FIGS. 17A and 17B, such a tri-level system will be explained. Irradiation of a photosensitive body with optical energy with three levels of 0, L.sub.W and L.sub.L provides latent image potentials of V.sub.H1, V.sub.W1 and V.sub.L1 on the surface (FIG. 17A. In the plus-charging of the photosensitive body, an area having a latent image potential of V.sub.H1 attracts minus toners, resulting in normal development, another area having a latent image potential of V.sub.L1 attracts plus toners, resulting in reversal development and a still another area having an intermediate potential of V.sub.W1 becomes a non-developed white area.
For the above reason, a development bias voltage V.sub.B1 for the normal development area must be larger than V.sub.W1 and smaller than V.sub.H1 and a development bias voltage V.sub.B2 must be smaller than V.sub.W1 and larger than V.sub.L1.
When the photosensitive body is used for a long time, as shown in FIG. 17B, the charging potential V.sub.H decreases whereas V.sub.W and V.sub.L produced by exposure to light increase. Since the increasing coefficient of V.sub.L is larger than that of V.sub.W, after time t.sub.1, V.sub.B2 &lt;V.sub.L and after time t.sub.1, V.sub.B1 &lt;V.sub.W. This makes it impossible to perform reversal development and subjects the white area which should not be developed to normal development. As a result, the image quality will be greatly deteriorated.
U.S. Pat. No. 5,208,632 describes in the prior art a method for correcting reading errors among plural surface potential sensors owing to impairing by charged particles such as toners.
In controlling the latent image potential of a photosensitive body, the measurement precision of a surface potential sensor is critical. If it is not assured, suitable control cannot be done. Generally, use of the surface potential sensor for a long time leads to invasion of toners or paper particles in the sensor, thus providing a measurement error or malfunction.
Further, where plural surface potential sensors, which provide different characteristic changes according to their different characteristics and locations, are used, the values measured by these plural potential cannot be accurately compared with one another, which will reduce reliability of control of latent image potential.
Since the surface potential sensor itself is expensive, the electrostatic recording apparatus is also expensive. The maintenance cost such as periodic calibration of the surface potential sensor cannot be neglected.
The main cause of malfunction of the surface potential sensor is invasion of toners or the like into a sensor probe as described above. In recent years, a negative-feedback type surface potential sensor has been used to correct the distance between an object to be measured and it. In this surface potential sensor, a voltage having the same polarity as the potential of the object to be measured is applied to the metallic box for a sensor probe. The applied voltage, which differs between an absolute display type and a relative display type, ranges from ten to thousand volts.
A digital-type electrostatic recording apparatus generally uses reversal development as a development manner. Therefore, where an OPC with minus charging is used, since the polarity of a toner is minus and that of the voltage applied to the box for a surface potential sensor is also minus, the toners and sensors repel each other so that the toners are hard to be applied to the sensor.
The color printing electrostatic recording apparatus on the basis of both normal development and reversal development uses two kinds of plus and minus toners. The plus toners are electrically attracted to the metallic box of the sensor and likely to be contaminated. The latent image potential in the photosensitive body cannot be suitably controlled, which will reduce the reliability of operation.