GB2141050A - Plain paper copier - Google Patents

Abstract

In a plain paper copier in which toner can be added automatically to the developer at different rates, the toner density is monitored by measuring the light reflected from a toner-covered patch 17 on the photoreceptor 12. The toner density is compared with parameters which vary according to whether the copier has its bias selected to 'copy normal', 'copy lighter' or 'copy darker', and the rate of toner addition adjusted accordingly. <IMAGE>

Description

SPECIFICATION Toner Density Control This invention relates to controlling the density of black lines or areas in copies made from an original by a plain paper copier using an electrostatographic process.

In such a process, a charged particulate material (usually called 'toner') presenting an optical contrast with a sheet of paper or like support material is deposited on an electrostatic latent image formed on a photoreceptor, usually in the form of a drum or belt. The real image, in the form of a pattern of toner replicating the pattern of data on the original, is then transferred to the support material and subjected to heat to cause the tone particles to become fused to the support to form a permanent image, in the sense in which any image on a sheet of paper is permanent.

The toner is usually applied by contacting the photoreceptor with a developer, which is a mixture of relatively-large particles of a carrier material and relatively-small particles of toner.

The two types of particles are usually charged triboelectrically in opposite senses, with the polarity of the toner being opposite to that of the latent image, so that when the developer is brought close to, or in contact with, the latent image the toner particles are extracted preferentially from the developer in order to form the real image.

In order to accommodate originals having coloured backgrounds, or being of low contrast, it is usual for copiers to be provided with 'copy lighter' and 'copy darker' manually-operable selectors. When 'copy lighter' has been selected, the density of the toner or both the data and the background is reduced, which can result in higher copy contrast when the use of coloured paper would normally give rise to a significant deposit of toner on the otherwise-blank background.

Similarly when 'copy darker' is selected, the density of toner is increased, which can increase preferentially the data images compared with the background, so as again to increase the copy contrast.

When monitoring toner density changes during copying, in order to measure degradation of the copy images, and to signal that fresh toner needs to be added, it is known to use interdocument patches that have toner deposited on them at normal bias irrespective of the bias used in depositing toner on the adjacent latent copy images. This enables toner depletion of the developer to be detected and corrected as the machine is running. This is possible only with single-roll developers, which permit the bias on the patch area to be different from that on the image area. However, this is not possible when a twin-roll developer is used, because while the patch area is under one roll the image area is under the other, necessitating the same bias being applied to both areas.

The present invention aims at detecting and responding to small changes in the density of successive toner deposits, irrespective of the bigger changes in density brought about by unscheduled selection of the 'copy lighter' and 'copy darker' modes, in order to initiate replenishment of the toner.

Accordingly the present invention provides a copier which is as claimed in the appended claims.

The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a plain paper copier of the present invention; Figure 2 is a graph of voltage against density to illustrate how equatchanges n sensor voltages give rise to unequal changes in one density; Figure 3 is a diagrammatic view of the means forming part of Figure 1 for deriving the respective control voltages; Figure 4 is an algorithm of a 'low-toner' strategy for the copier of Fig. 1, and Figure 5 is an algorithm of an 'out-of-toner' strategy to which the copier may resort at need.

The copier of the present invention includes a hopper 2 into which packs of fresh toner can be emptied either at random intervals or in response to an 'out-of-toner' signal displayed by the copier.

Toner from the hopper passes through a control valve in the form of a foam-covered- roller rotated by a motor 5 under the control of signals passing along a line 6 from a microprocessor 8.

When the motor is stationary, the roller prevents any toner from falling into developer tank 10.

When the motor 5 is pulsed or otherwise driven at a low speed, the roller 4 rotates slowly in order to replenish the developer with toner at a so-called 'normal' rate. When the motor 5 is energised continuously or otherwise, it rotates the roller 4 at a higher speed so as to introduce toner into tank 4 at a maximum rate. The toner dropping into tank 10 is mixed uniformly with the toner-depieted developer contained therein, and at the same time acquires triboelectrically a charge of the appropriate size and polarity as dictated by the polarity of the electrostatic latent image produced on a photoreceptive drum 12.The developer in tank 10 passes to an applicator 14 designed to apply developer uniformly to the surface of drum i2, to enable the toner particles to adhere preferentially to the latent image, with the carrier particles falling off or otherwise being removed from the surface of the drum. The process by which the image is produced on the photoreceptor 12 is well known to workers in the electrostatographic art, and so will not be described herein in any further detail.

In order to ensure that the image density produced by the copier is as faithful to that on the original as possible, or is of an enhanced contrast, the copier of the present invention monitors the density of toner laid down on the photoreceptor each time a copy is made. To do this, a small charged area 1 7 (to be called 'patch' below) is produced on the drum in the inter-document space provided between adjacent image areas.

The manner in which this patch is produced on the drum is well known in the art, and so will not be further described, but it is sufficient to note here that the patch has a charge produced on it of the same polarity as the charged parts of the latent image and with a value corresponding to the selected copy density which is prevailing at the time in question. This means that the patch has produced on it during the development process a uniformly-thin film of toner of which the thickness is a function of the bias applied to the patch area during the development process.

Incident on the toner-covered patch are timed pulses of light from a sensor 16, the light after reflection from the coated patch being caused to fall on a suitable photodetector 1 9 to produce an output signal V5 which varies with variations in the density of toner on the path. The intensity of the light emitted from the lightemitting diode (LED) 1 6 is controlled by the output from an automatic gain control device 18, which in turn receives its input from a microprocessor 8.

As shown more fully in Figure 3, the output from the photodetector 1 9 is fed to microprocessor 8 through a logic board acting as an amplifier 30 producing a gain factor G, the output of which passes through either a switch 32 and adjustable attenuator 20, or switch 22, to an analog-to-digital convertor 36. The output of convertor 36 is applied in both cases to an inlet of microprocessor 8, for further processing as described below.

When the copier is first being set up or calibrated, no toner is deposited on the patch, so that the LED 1 6 bounces light off an uncoated photoreceptor, known as a 'bare drum". This causes the photodetector 1 9 to emit a voltage signal V5. This is amplified to G.V5 in the logic board 30 and from there it is passed through the closed switch 32 to attenuator 20, which applies an attenuation factory, so that the input signal to convertor 36 is Vs.G.y. The switches 32 and 22 are under the control of microprocessor 8. The microprocessor is such that signals input to it must lie within a specified range before they are acceptable.In a microprocessor used in the copier of the present invention, the input signal must be greater than 1.054 V and less than 1.94 V. in order to achieve an input signal lying within this range, and allowing for the different reflectivities of photoreceptor 12, and for contamination and aging of the LED and photodetector, the microprocessor sends selected ones of various trains of pulses to the automatic gain control (AGC) device 1 8 when the copier is first switched on.

When the density of the toner or patch 1 7 is being monitored, the switch 22 is closed so that the output of the amplifier 30 passes directly to convertor 36, and from there to the microprocessor, bypassing attentuator 20. Under these conditions, the variations of this signal are produced by variation in the density of the toner, which will be called below variations in the 'patch signal'. These variations in the patch signal in turn result in variations in the input signal to microprocessor 8, which variations are automatically monitored during the copying process. When the density of toner or patch 1 7 is excessive, the resultant additional attenuation of the light signal reduces the input signal to microprocessor 8, leading to the production of a 'dark patch' signal.The converse, i.e. an excessive amount of light giving rise to a high input signal, produces a 'light patch' signal.

When a dark path signal is produced, a signal passes along line 6 to motor 5 to cause the flow of toner to the developer tank 10 to be stopped until a light path signal is produced.

It has been found that the attenuation y introduced by attenuator 34 varies exponentially with input voltage. In other words, Vout=Vin.eK.

This is shown in Figure 2, wherein the x-axis represents the output voltage (V) of the attenuator, and the y-axis represents the equivalent density of the patch. When the copier is being set up or calibrated initially, the value of attenuator 20 is adjusted to make Vpd2=yVdyd 1 where Vp=the output voltage from convertor 36 after scanning a toner-covered patch at normal density; Vd=the equivalent voltage after scanning a bare drum, and d=the offset voltage generated when there is no LED current.

This set-up value for normal density is shown as Vnr giving rise to a density signal DnX The subtraction of offset voltages from the bare drum and the patch voltages has two major advantages: 1) The general density control is improved.

Consider the algorithm Sp+ofsset > y(SD+offset), where Sp and stare true signal values. If the LED output moves the true signal by a factor K we obtain KSp (KSD+offset).

Thus it can be seen that the value of K does not cancel, and the toner concentration is altered to balance the inequality.

2) The automatic density control set-up is improved, because only one adjustment should be necessary to make Vpd2=y(VDd 1) When the 'copy lighter' or 'copy darker' selector is operated, Vn is altered by a constant amount dV. Thus the copy lighter signal Vn+dV gives rise to a reduced density signal DndD2, and the 'copy darker' signal VndV to density signal Dn+dD1. It will be noted that dDa; dD1, because of the non-linearity of the curve.

In order to compensate for aging of the photodiodes 16 and 19, the LED 16 can be supplied with a drive current which is altered in fixed steps (of 55%) under the control of the microprocessor 8 to ensure that the output voltage VAID of the photodetector circuit lies within the limits acceptable to the microprocessor. The different drive currents are achieved by using a pulsiform current so that the effective level of the current is dictated by the mark: space ratio. At the highest level, the current is on continuously. It in this state VAID is still too low, the copier may still be operated, but a signal is generated to indicate that it is faulty in that the copy densities will not be as selected.

Regarding a drive current as being at a level L, the relationship between the drive currents is that L,=1.55. L,--l.

When the copier is being set up, it is important to ensure that the photodetector 1 9 is not saturated i.e. that its output is able to increase further when it detects a lighter patch. This is achieved by comparing its output voltage (y.Vd) with the minimum threshold voltage for a bare drum to which the microprocessor can respond, i.e. (min Vd < 1.094V).

When the copier is first switched on, a midlevel drive signal (L5) is applied to LED 16. If the above output voltage is above the minimum threshold, it might be so far above if that the photodetector 1 9 is saturated. This is counteracted by reducing the drive signal to L1 and remeasuring. If this time the output voltage is, as expected, below the threshold, then the drive signal is stepped up to level L2, and the process repeated until the threshold is reached or exceeded. The change in successive drive signal levels is such as to ensure that the output voltage cannot reach its saturated value in one step from below the threshold value.This also ensures that for most of the time the system sets up for a given sensitivity and hence the density equivalents of the (voltage) correction factors in the out-of-toner strategy are maintained.

If when the copier is switched on the threshold is not reached, then the drive signal is stepped on to L6, and so on.

When the copier is in its 'copy normal' mode, so as to apply a normal bias to the developer, the respective patch voltage (Vp) minus the offset voltage (d2) is compared with the bare drum voltage at set up (y.Vd) minus the offset value after passing through the attenuator. The occurrence of a 'light patch' is indicated when basically Vpoffset > yVdyoffset.

Bearing in mind the non-linearity of the voltage v. density curve, giving rise to unequal density variations from equal voltage variations, this inequality in practice would become Vpd2 < Y.(Vdd1) (1) When the inequality is reached, a signal is generated causing fresh toner to be added to the developer at a nominal rate, as by pulsing motor 5. When more than say eight such successive 'light patch' signals are generated, the copier is set to add toner at a maximum rate, as by energising motor 5 continuously. If the number of successive 'light patch' signals continues to increase, when it has reached a specified count, of say 98, the copier enters an out-of-toner strategy, to be described below.

If the addition of toner is effective to increase the copy density so that the inequality is not satisfied, this gives rise to a 'dark patch' count.

When say three successive 'dark patches' have been counted, the toner dispense motor 5 is deenergised, and the 'light patch' counter is reset to O.

Until three successive dark patches are counted, the system maintains its then-current dispense rate.

When the copier is in its 'copy lighter' mode, the change in relative densities of the lighter patches compared with the bare drum signal is used to initiate the replenish toner strategy by the modified inequality 3(Vpd2) > 5(yVdd1)- (2) Similarly, when the copier is in its 'copy darker' mode, the governing inequality is 4(Vpd2) > 3(y.Vdd,). (3) In the general case, the inequality becomes m(V,-d2) > n(yV,-dl) wherein m and n are integral values chosen empirically after evaluation of the copy quality and desired control density achieved by the combination of a particular photoreceptor, toner, etc.

The theory behind this implementation of the present invention is as follows.

For a typical system yl,e-K.dU wherein K=a constant, and du=an arbitrary amount of density units.

If 'copy lighter' or 'copy darker' is selected, the preferred attenuation for control changes to 1,2=eK.(du+d) wherein d=change in density from switching from normal to lighter or darker.

If we divide these, we arrive at y2/y 1K.d This is independent of both density (to the first order), and of light level from the LED. Thus obtaining the ratio of attenuation factors in the two modes compensates for the absolute change in densities in switching between the two modes, and allows the copier to respond to incremental changes in density.

Whereas it would be possible for the microprocessor to be programmed to divide the respective voltages to decide on whether or not toner is to be added, or at what rate, in practice it is easier to cause the microprocessor to add the respective voltages the necessary number of times (m or n) and to compare the resultant sums to see if the inequalities (2) or (3) are reached.

The general control alogrithm is shown in Fig.

4, and the out-of-toner strategy in Fig. 5. These should be fully explanatory when considered in conjunction with the following description.

The 'out-of-toner' routine is entered if the maximum dispense rate has been used for over 90 consecutive copies generating 'light patch' signals.

There is an interruptible tone-up cycle at the end of the first three jobs in the out-of-toner routine. This allows the processor to recover if toner concentration has been reduced by copying very dark originals.

During the interruptible tone-up cycle, the processor's main drive and toner dispense motors run. The lamp is turned 'on', but the high voltage power supply is turned 'off'. The operator can reselect features and copy numbers and start another job. If the 'start' button is not pressed, or the 'instant start' does not operate, the tone-up cycle stops after 50s.

If three interruptible tone-up cycles have taken place with a single adequate patch signal having been generated, no more interruptible tone-up cycles are performed. The machine then stays in this part of the out-of-toner strategy, with continuous dispense rate, until either it sees one patch of adequate density (in which case it leaves the out-of-toner strategy) or until the density of the patch has dropped by 0.5 density units from the set (basic control) point. A different threshold voltage is used, dependent on the bias voltage selected.

This threshold does not have to be very accurate.

Hence, a plain (constant) threshold is used rather than a proportional one to simplify the machine programming code. If the density does drop by 0.5 du, the message 'Add Toner' is displayed after the current job is completed. The operation cannot then reselect features or alter copy number. If 'start' is pressed, the machine goes through a non-interruptible tone-up cycle for 90s.

At the start and end of this tone-up cycle a solenoid-driven patch generator is operated, and the two patch voltages are stored and then compared with each other. The 'copy normal' mode bias (developer bias) is used throughout the non-interruptible tone-up cycle.

If a 'correct' patch density is detected, the outof-toner strategy is left and the main control strategy is joined at the light patch count corresponding to the beginning of the maximum toner dispense rate. The 'Add Toner' message can be cancelled only by obtaining a positive increase in density from the stored patch value above a given noise threshold after a non-interruptible tone-up cycle, or by exceeding a second voltage (density) threshold. This second threshold corresponds to a patch voltage which is equivalent to the voltage at which 'Add Toner' is displayed, minus the noise threshold voltage. This gives an optimum safeguard without the two thresholds conflicting.

If a second non-interruptible tone-up cycle takes place, the previously-stored start and end patch value are replaced by the new patch values.

If toner is added and the machine does not leave the out-of-toner strategy the first time (perhaps because of too much system noise), a second non-interruptible tone-up cycle may still not give a density increase which exceeds the noise threshold, but it will almost certainly give a voltage which exceeds the second voltage (density) threshold. In practice, this second threshold corresponds to about 0.3 du below the automatic density control set point.

The message 'Please Wait-Adjusting Copy Quality' is displayed during each non-interruptible tone-up cycle. If the above exit conditions are met, the main control strategy is rejoined at light path count=9 (see above).

Claims (11)

1. A plain paper copier including a toner hopper for delivering toner to a developer at at least three different rates, of which one is zero; a photoreceptor; means for charging a patch in the interdocument area of the photoreceptor, and for depositing toner on it, each time a copy is to be made; means for monitoring the intensity of light reflected from the toner-covered patch, and means for selectively biasing the developer under manual control whereby toner is applied to the electrostatic image of an original document and the patch at preset rates to give rise to toner images of different densities, and for comparing the density of toner deposited against parameters which differ according to the selected bias, and for controlling the admission of fresh toner in accordance with the results of the comparison.

2. A copier as claimed in claim 1, in which the hopper has its outlet closed by a rotary cylinder which, when stationary, prevents toner from being delivered to the developer, and which can be rotated at two different rates corresponding to desired rates of toner delivery.

3. A copier as claimed in claim 1 or 2, in which light from a light-emitting diode is caused to fall on the patch before being reflected onto a photodetector.

4. A copier as claimed in claim 3, in which the photodetector supplies its output to a microprocessor through an amplifier in series with a switch and attenuator, bypassed by a switch, and from there to an analog-to-digital converter connected to the microprocessor.

5. A copier as claimed in claim 4, in which the microprocessor sends selected trains of pulses to an automatic gain control device when the copier is first switched on, the trains being applied in a controlled sequence until the input signal to the microprocessor lies within chosen limits.

6. A copier as claimed in claim 3, or claim 3 and any claim dependent therefrom, in which the light-emitting diode is supplied with a drive current alterable in steps under the control of the microprocessor adjacent steps being in a fixed ratio relative to each other.

7. A copier as claimed in claim 6, in which the ratio is 55%, and in which at the highest step the current is on continuously.

8. A copier as claimed in any preceding claim, in which it automatically enters an 'out-of-toner' routine after more than a preset number of consecutive 'light patch' signals have been generated.

9. A coper as claimed in claim 8, in which an interruptible 'replenish toner' cycle is automatically started at the end of each of a small number of copy runs selected after the copier has entered the 'out-of-toner' routine.

10. A copier as claimed in any preceding claim, in which ,as and when the density of the patch drops to 0.5 density units, the copier goes through a non-interruptible replenish toner cycle for a chosen time after the 'start' button is actuated.

11. A plain paper copier adapted to go through the toner replenishment steps shown in Fig. 5 of the accompanying drawings.