This invention relates generally to toner image creation and more particularly to developability control which enables a wider usable A.sub.t (i.e. a toner material's effectiveness in charging with a given carrier) range. 
The invention can be utilized in the art of xerography.  In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoreceptor.  The photoreceptor comprises a charge retentive surface.  The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images.  The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation. 
A common type of developer comprises carrier granules having toner particles adhering triboelectrically thereto.  The two-component mixture is brought into contact with the photoconductive surface, where the toner particles are attracted from the carrier granules to the latent image.  This forms a toner powder image on the photoconductive surface which is subsequently transferred to a copy sheet.  The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques. 
In most two component development processes, the developer/toner materials are at the mercy of the development housing design: auger configuration, sump geometry, magnetic brush roll size, magnetics design, roll and auger velocity, etc.  to cause tribocharging of the toner against the carrier.  The auger speeds and roll velocities are usually adjusted to provide adequate flow balancing of the developer within the housing and developability that provides ample operating latitude, respectively.  Once these speeds are determined, the level of tribocharging of the toner against the carrier is fixed.  Hence, to modify the tribocharging, the formulation of the materials are adjusted to provide the desired tribocharging performance. 
In conventional two-component xerographic development, the ability of a toner material to charge with a given carrier material is quantified as follows: EQU A.sub.t =Tribo*(TC+C.sub.0)
where Tribo is the average charge to mass ratio of toner, TC is the toner concentration in percent by weight, and C.sub.0 is a constant.  A.sub.t is a critical specification parameter for toner and developer; it tends to vary from batch to batch, with developer age, and with operating relative humidity.  The variation with humidity is a special problem with many color toners, since this variation tends to be much larger than with comparable black toners.  In general, the higher the A.sub.t, the better the material charging. 
Modification of the developer A.sub.t by changing the material's formulation is a long process whereby the materials must be subjected to a significant amount of both bench and lengthy and expensive full process experiments before they can be qualified for satisfactory use in a product.  Also, when a product like the 4850 printer, for example, has more than one color toner, it is desirable to maintain commonality in the carrier formulation so as to minimize cost. 
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention.  In addition to possibly having some relevance to the patentability thereof, these references, together with the detailed description to follow hereinafter, may provide a better understanding and appreciation of the present invention. 
In copending U.S. patent application Ser.  No.  08/359,357, there is disclosed a developer control for enabling the use of developer and toner materials with widely varying A.sub.t in high quality xerographic copying and printing.  Pixel count data is combined with toner test patch reflectance data during a brief toner rundown to determine the rate of change of density per unit change in toner concentration.  During toner rundown, dispensing of toner is suspended for a period of time for effecting toner concentration reduction by approximately 0.25%.  The change in Toner Concentration (TC) is estimated using pixel counting.  Additionally, toner test patches are created and the reflectance thereof is measured for determining the change in toner density.  The estimated TC change and the change in toner density are processed using linear regression to find the average change in density sensor output for the estimated change in TC which is referred to as the rundown slope. 
The rundown slope is then compared to a target value.  If it exceeds the target value by more than .epsilon. ( a noise factor), the dispense setpoint is reduced by one unit.  If the rundown slope is less the target value by more than .epsilon.,  the dispense point is increased by one unit.  The noise factor, .epsilon.  is attributable to errors in pixel count or reflectance sensor drift. 
According to the foregoing, the nominal control line and control band in TC-Tribo space is altered to produce a much wider usable A.sub.t range. 
U.S. Pat.  Nos.  5,153,642 and 4,639,115 relate to devices disposed in a developer housing for removing contaminants from a developer mixture. 
The '642 patent discloses an apparatus in which an electrostatic latent image recorded on a photoconductive member is developed with developer material stored in a developer housing.  The developer material advances along a path of travel to a development zone closely adjacent to the latent image.  A cleaner, positioned in the path of the developer material and spaced from the photoconductive member, cleans contaminants from the developer material without impeding the flow thereof.  The cleaner has a multiplicity of fibers disposed in the path of travel of the developer material. 
The '115 patent discloses an apparatus for purifying toner prior to its use in developing latent electrostatic images.  An electrically biased roll supported in the developer housing contiguous at least one of the development rolls serves to attract paper debris from the toner contained in the toner carried by the developer roll.  The roll is fabricated from a suitable insulating material and electrically biased in a manner suitable for attracting the paper debris contained in the toner.  The roll is rotated and a scraper blade is provided for removing the debris therefrom.  The debris so removed is allowed to fall into a toner catch tray which can be provided with an auger for moving it out of the tray to thereby increase the capacity of the system for debris removal. 
While the aforementioned patents are not directed to developer A.sub.t control they deal with functions carried out in a developer housing which may be relevant to patentability of the present invention. 
While increasing the A.sub.t latitude is known as disclosed in the aforementioned patent application, it will be appreciated that being able to vary the developer A.sub.t directly in order to compensate for changes in ambient conditions such as humidity is a desirable goal.  Also, the capability of compensating for developer materials that do not meet an At target value is desirable.