Apparatus for Sensing the Concentration of Toner in a Developer Mix

This invention relates to an apparatus and method for sensing and controlling the concentration of toner in a developer mix used in a xerographic or the like copier to develop electrostatic images.

It is well known in the art that the proper concentration of toner in a developer mix comprising both toner and carrier particles used to develop latent electrostatic images produced on a photo-conductive or the like medium in a xerographic-type copier is important to the quality of the copies provided. Many different types of devices are known in the art for monitoring and controlling the toner concentration of a developer mix in a copier. One well known device uses Nesa glass over which a sample of developer mix is passed. The Nesa glass is provided with a pattern which is charged and developed by the developer mix. Light passing through the Nesa glass subsequent to development indicates the density of the toner and therefore the concentration thereof in the mix. The latter is used to control the replenishment of toner to the mix in the developer apparatus of the copier.

Other devices are also known in the art for determining the toner concentration of a developer mix. These devices employ circuitry to measure the resistance or inductance of the mix and thereby determine its toner concentration.

While the above-described devices work satisfactorily for the most part to provide an indication of the concentration of toner in a developer mix comprising both toner and carrier particles, they have certain drawbacks which make them less desirable. In the case of the first-described device, sample development with the mix is required to make the determination of toner concentration. Thus, after the glass is developed, it must be cleared of mix for the next sampling. Also, light sources and coated Nesa glass material is needed which can be expensive. In the case of the resistance or inductance measuring devices, relatively complex circuitry is required to perform the operation.

In an electrostatic copier developer of the type that employs a triboelectric mix of toner and carrier which is recirculated over a path and to which toner is to be added periodically, the present invention provides for a novel apparatus for sensing the concentration of toner. This apparatus includes a surface of material triboelectrically dissimilar from that of the mix. The surface is positioned in the flow path so that the mix flows over it. The surface is electrically isolated from the system except for an impedance coupled between it and the system ground. The flow of the dissimilar triboelectric mix over the surface generates a current from it through the impedance which current is related to the toner concentration.

The invention, together with further advantages and features thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements.

FIG. 1 and FIG. 2 are side sectional views of developer assemblies of a xerographic-type copier incorporating a toner concentration detector according to the invention;

FIG. 3 is a block diagram of a circuit for controlling the replenishment of toner to the developer mix of the developer asssemblies of FIGS. 1 and 2 in accordance with the detection of a voltage produced triboelectrically by the interaction of the developer mix and a triboelectrically dissimilar surface according to the invention.

Referring now to the drawing in greater detail, there is illustrated in FIG. 1 a developer assembly 10 of a xerographic or the like copying machine which includes a toner concentration detector arrangement 11 according to the invention.

The developer assembly comprises a plurality of magnetic brush rolls 12, 14, 16 and 18 mounted about the periphery of a photoconductive drum 20 upon the surface of which latent electrostatic images are produced in a conventional manner and developed by the developer assembly 10. The developer rolls of the assembly are of the conventional type including an outer rotatably mounted insulating cylinder 13 (See roll 12) surrounding a magnet assembly 15 which produces a predetermined magnetic field at the periphery of the cylinder for picking up and transporting developer mix.

The four developer rolls of the developer assembly are mounted on a suitable frame (not shown). A housing 22 surrounding the rolls defines the developer mix sump 24 in which developer mix including toner and carrier particles is supplied to the rolls for developing the images on the drum 20. It should be noted that the toner and carrier particles comprising the developer mix are of the conventional type, the carrier being iron, ferrite, or the like particles.

A cross-mixer device 23 is provided adjacent the uppermost developer roll 18 for keeping toner and carrier intermixed as it is recirculated back to the sump 24.

In operation, developer mix in sump 24 is picked up magnetically by a fifth, "pickup" roll 26. The mix is transported magnetically to roll 12 whereat a magnetic "brush" of developer mix is formed on the surface thereof. From there the mix is transported to roll 14 whereat another brush is formed and so on to rolls 16 and 18. The brushes rub against the surface of the drum and toner is attracted to the electrostatic image thereon for development of the image.

The developer mix is carried about the surface of roll 18 as illustrated by the arrow and deposited into the cross-mixer 23. The developer mix passes through the cross-mixer, flowing over the surface thereof and from there is deposited again in sump 24.

The cross-mixer assembly is isolated electrically from the developer housing by means of an insulating layer and as such is electrically ungrounded. The cross-mixer assembly is formed of a conductive metal which is triboelectrically dissimilar from the developer mix employed in the developer assembly. It is, however, only required that the material be triboelectrically dissimilar from the developer mix. Being metal and conductive is not necessary to the operation of the apparatus according to the invention. The movement of the developer mix over the surface of the cross-mixer produces a making and breaking of contact therebetween to generate a current flow in the cross-mixer. The current is fed via conductor 28 connected to the cross-mix assembly, through a resistor 30 to ground. By ground is meant system or chasis ground, that is, the potential of the housing 22, which may be, but need not be earth ground. The flow of current through resistor 30 produces a voltage thereacross which is detectable by a suitable voltmeter such as 32, coupled as shown in FIGS. 1 and 2. The reading of the voltmeter is thus indicative of the direct current generated. Of course, this direct current is proportional to the d.c. voltage that is generated by the flow across the surface. The voltage and current through the resistance 30 generated by the interaction of the triboelectrically dissimilar developer mix and cross-mixer surface material, it can be shown, varies with the concentration of toner in the mix.

It should be noted that while in a preferred embodiment of the invention the developer mix is shown flowing over the surface of a cross-mixer of a developer assembly, the cross-mixer is used for convenience only. Any surface triboelectrically dissimilar from the developer mix and isolated electrically from ground can be employed for the purpose of tribovoltage generation.

The toner concentration detector according to the invention can be used with other types of developers as well. Referring now to FIG. 2 of the drawing, there is illustrated therein a cascade developer assembly 34 of a well-known type including a toner concentration detector arrangement 36 according to the invention. In the developer assembly developer mix is carried up by conveyor 38 which is driven by a motor 40 or other suitable drive means. The developer mix is released onto chute 42 at the top of the conveyor run, wherefrom it cascades down over the surface of the photoconductive drum 20 of the xerographic copier. Excess developer mix is recirculated into the developer mix sump 46 formed at the lower end of the outer housing 50 enclosing the assembly. The toner component of the developer mix which is used in developing a latent image on the surface of drum 20 is stored in a toner dispenser 44, and is released into the developer mix sump 46 as required. The release of the toner is controlled by gate 48.

In the developer assembly 34, chute 42 is isolated electrically from ground and is formed of a material dissimilar tribolectrically from the developer mix used in the assembly. As such, as the mix is carried by gravity over the chute 42, a current is generated to produce a triboelectric voltage detectable at volt meter 32. As in the case of the arrangement of FIG. 1, a current flows via conductor 28 and resistor 30 to ground. The volt meter 32 connected across the resistor detects the voltage produced by the current flow therethrough. This voltage or tribovoltage varies with the concentration of the toner in the developer mix.

The voltage output detected by the action of the developer mix flowing over the triboelectrically dissimilar surface can be used to control replenishment of toner to the mix as the toner concentration decreases. The latter is illustrated in FIG. 3 of the drawing wherein an automatic density control unit 52, which may include standard voltage sensing circuitry well known to one skilled in the art, is employed to control the feeding of toner from a toner hopper such as 44, shown in FIG. 2 of the drawing into the developer sump 46 when the tribovoltage value reaches a predetermined value. In this manner, a proper toner concentration in the developer mix can always be maintained.

While a particular embodiment of the invention has been shown and described, it should be understood that the invention is not limited thereto since many modifications may be made. It is therefore contemplated to cover by the present application any and all such modifications as fall within the true spirit and scope of the appended claims.