GB2127590A - Toner density control - Google Patents

Abstract

In an electrostatographic copier, the addition of fresh toner from hopper 2 to the developer at 10 is controlled at 4, 5 in accordance with the intensity of light reflected from a toner-covered patch 17 on the photoreceptor. As the number of successive 'light patch' (low toner) signals increases the copier switches from a low toner replenishment rate to a higher one, to long interruptible tone-up cycles, and to long non- interruptible (copying prevented) cycles. The copier leaves these modes when a 'dark patch' signal is produced. <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 presenting an optical contrast with a sheet of paper or like support material (usually called 'toner') 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 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 to the photoreceptor by contacting it 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. Thus with the passage of time, the developer would become depleted of toner, leading to the production of copies noticeably lighter than the original, if steps were not taken to replenish the developer with fresh toner.

Such fresh toner has in the past been added manually, either regularly or in response to a signal generated by the copier. With multiple-user copiers, there is a tendency for particular users at any time, when the out-of-toner signal is generated or displayed, to leave it to the next user to add toner. This can happen with several successive users, leading to copy quality degradation.

The present invention aims at removing the addition of toner to the developer from the control of the operator, so as not to allow the copy quality to be degraded excessively, by varying the rate of addition of fresh toner to the developer in accordance with usage, so that the proportion of toner in the developer is always at or near optimum, except for those times when the toner hopper is itself empty.

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 schematic view of an electrical sub-circuit by means of which the energy supplied to a light source is adjusted automatically to produce a desired output; Figure 3 is a diagram illustrating the means by which the sub-circuit of Figure 2 is made to operate; Figure 4 is a graph of voltage against time indicating the output of the Figure 2 circuit; Figure 5 is a graph of voltage against time showing how the 'patch' density is monitored each time a copy is made; Figure 6 is an algorithm of a 'low-toner' strategy for the machine of Figure 1, and Figure 7 is an algorithm for the respective 'outof-toner' strategy.

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 a 'replenish toner' signal generated by the copier.

Toner from the hopper passes either intermittently or continuously through a control valve 4 in the form of a porous roller rotated by a motor 5 under the control of signals passing along a line 6 from a microprocessor 8. The toner entrained by the roller drops into a developer tank in which the toner is mixed uniformly with the toner-depleted developer, 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 12, to enable the toner particles to adhere to the electrostatic latent image.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, 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 17 (to be called a 'patch' below) is produced on the drum immediately preceding the area dedicated to the copy being made at any one time. The manner in which this patch is produced on the drum will be described in further detail below, but at this stage it is sufficient to note that the patch has a charge produced on it of the same polarity as the charged parts of the latent image and with a known value.

This means that the patch has produced on it during the development process a uniformly-thin film of toner. Incident on the toner-covered patch are timed pulses of light from a sensor 16, the light after reflection from the coated patch being adapted to fall on a suitable photodetector to produce an output signal which varies with variations in the density of toner on the patch. The intensity of the light emitted from sensor 1 6 is controlled by the output from an automatic gain control device 1 8 as described below, which in turn receives its input from microprocessor 8.

The output from the photodetector in sensor 1 6 is fed to microprocessor 8 alternatively through an adjustable attenuator 20 or directly through a switch 22 under the control of the microprocessor through line 24.

The means by which the output of the light source in sensor 1 6 is adjusted is indicated more clearly in Figure 2, in which a resistor R1 is connected between a positive rail 30 and an earth return 32, through the emitter and collector of a transistor 01. The transistor is adapted to have trains of negative-going square-wave pulses (as shown in Figure 3) applied to its base by microprocessor 8. Connected between resistor R1 and transistor 01 is a resistor R2 leading to the base of a second transistor Q2, with the connection being designed to lead transient pulses to earth through a capacitor C. Connected in series with the emitter/collector path of transistor Q2 is a light-emitting diode (LED) D1 and a conventional diode D2, together with a third resistor R3.The functioning of this particular circuit is as follows: When a negative-going pulse arrives at the base of transistor Qi , the transistor is enabled and starts to draw current from rail 30 through resistor R1. This leads to the voltage of point A starting to fall, which fall is transmitted through resistor R2 to the base of transistor 02, which in turn is enabled when the voltage of its base becomes sufficiently negative. The base of transistor Q2 tends to be continuously charged through resistors R1 and R2, so that the circuit functions as a type of integrator. When the transistor Q2 is enabled, the diode D1 emits a quantity of light which is related to the nature of the pulse train applied to the base of transistor 01.

Some of the different alternative pulse trains are shown in Figure 3. What distinguishes the trains from each other is the effective mark:space ratio. That pulse train which produces the lowest amount of light from diode D1 is referred to as level 1, and uses a pulse (mark) of 2ms separated from the next pulse by an interval of 30ms. The second level of light produced by diode D1 comes from the second pulse train having a mark:space ratio of 2:28 or 1:14. As can be seen from Figure 3, the mark:space ratios reduce until the level 1 5 signal produces a ratio of 1:1, and level 1 6 a ratio of 2:1.However, the maximum amount of light is produced by diode D1 when there are no intervals, as indicated in Figure 3 by the application of a continuous negative voltage (level 1 7) to the base of transistor Q1 . The particular pulse train applied to Q1 at any one time is under the control of microprocessor 8.

When the copier is first being set up or calibrated, no toner is deposited on the patch, so that the sensor 1 6 bounces light off an uncoated photoreceptor, known as a "bare drum". For initial set up, the switch 22 is held open by the microprocessor 8 so that the signal from the photodetector in sensor 1 6 passes through attenuator 20 before reaching the microprocessor. The microprocessor is such that signals input to it must lie within a specified range before they are acceptable. In one microprocessor used in a copier of the present invention, the input signal must be greater than 4.0V and less than 5.2V.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 photoreceptor, the microprocessor sends selected ones of the trains of pulses shown in Figure 3 to the automatic gain control device 1 8 when the machine is first switched on. As shown in Figure 4, when the light signal is first being established, a level 8 signal is fed to transistor Q1. If this is found to give an output from diode D1 which is too low, then the signal start stepping up in levels at two second intervals until either an output signal is produced within the acceptance range or the device reaches level 1 7.

When the machine is first switched on, and at all times when the resultant input signal to the microprocessor is outside the above-specified limits, a 'bar' visible to the operator is illuminated.

The application of a level 1 7 pulse (continuouslyon) to transistor Q1 resulting in the output signal still being too low permits the user to operate the machine, although the bar is kept illuminated to show that the copier is faulty.

In the present case, and as shown in Figure 4, the output signal produced by a level 8 stream of pulses produces an excessive output signal despite attenuation. The level 8 signal is therefore removed and replaced after two seconds by a level 1 signal, giving rise to an output which is too low. The same happens when the level 2 signal is applied, but it is assumed for the purposes of illustration that when a level 3 signal is applied to transistor Q1 the attenuated signal fed to microprocessor 8 is above the 4.0V threshold.

When this happens, the microprocessor continues to supply level 3 signals, i.e. those with a 1:13 mark:space ratio, to device 1 8. This continues until during a later standby period it is found that the output signal produced by a level 3 pulse train has now fallen below the threshold, in which case it is replaced by a level 4 signal and so on until the output signal is within the necessary range.

When the density of the toner on patch 17 is being monitored, the switch 22 is closed so that the output of the photodetector passes directly to the microprocessor 8, by passing attenuator 20.

Under these conditions, the variable attenuation of this signal is produced by variations in the density of 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 on 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 patch signal is produced, a signal passes along line 6 to motor 5 to cause the flow oftonerto the developer tank 10to be stopped until a light patch signal is produced. The detection or generation of a light patch signal causes the algorithm shown in Figure 6 to be entered. However, before this is discussed in further detail, the process illustrated in Figure 5 will be described to provide additional explanation.

The graph of Figure 5 represents the output signal of the sensor 16 as supplied to micro processor 8. When the machine is switched on at time tO, the output signal climbs to its threshold value as dictated by operation of the automatic gain control device 18, as already described, light being reflected from the uncoated drum. When the threshold has been reached, at time t1, the switch 22 is closed to bypass attenuator 20. This state continues until the 'start' button is depressed as a result of which a copy cycle is initiated. The passage of the resultant black patch 1 7 in front of the sensor 1 6 leads to the reduction in output voltage as shown in time t2. During this time, three readings of the density of the black patch are taken at 4ms intervals, and are averaged for each patch.

The voltage peak after the black patch reading arises from reflection of the light from the bare drum between the black patch and the leading edge of the copy area. However, the sensor reflects light off alternating incremental areas of bare drum and toner as the copy area passes below the sensor, indicated by the ripple waveform between times t3 and t4. The latter time marks the end of the first copy, after which the signal rises to the bare drum value before falling when it reads the next black patch. The cycle is then repeated.

The algorithm in Figure 6 will now be described and explained in greater detail The black patch is generated normally by causing a grey area in advance of the registration edge on the copier platen to have light incident on it to produce the latent image of the black patch on the drum. The 'greyness' is chosen carefully to produce a known charge in the black patch latent image. However, when the copier is in its 'reduction mode', i.e. set up to produce opticallyreduced copies, the grey area just described lies outside the field of the copier optics. Under these circumstances, the motor 5 is pulsed or stepped to replenish toner at a preset (nominal) rate shown in Figure 6 as the Force Normal Toner Dispense.This rate of dispensation may be about 0.02 g/s, produced by appropriate pulsed operation of valve 4 which rate has been found is the toner-depletion rate when taking copies of originals having the usual proportion of black (data) space to white (blank) space. The dimensions of the hopper valve etc. are chosen so that when the motor 5 is operated continuously, toner enters tank 10 at the rate of about 0.05 g/s, which corresponds to the maximum rate that is envisaged would be required to replace the toner under all normal conditions of use and with originals having large black areas, but obviously not when there is a fault.

Obviously the normal rate of toner addition may be adjusted or chosen, as by altering the nature or size of the roller, or the speed of the motor, so that it is in excess of the normal rate of toner depletion, leading to control normally alternating between the zero and normal rates of addition.

Most of the steps in the Figure 6 algorithm are self-evident, and will not be discussed herein except as necessary.

The effect of the reduction mode having been discussed, and implicitly the control of the output from the LED Do, the next point to be discussed is interrogating the machine to see if it is in its 'CVT' mode, by which is meant that form of copying in which an original document is fed across the platen by a 'constant velocity transport'. The implication in this is that the copier is doing 1:1 copying from large sizes of paper, in which case the grey patch in front of the registration edge becomes obscured by the paper and is no longer useful. Thus, in this mode, it is necessary for a solenoid to be energised to introduce a black patch generator (BPG) into the optical field in advance of the leading edge of the continuouslyfed original. In either case, the density value of the resultant black patch is monitored, and a counter incremented by the production of a 'light patch' signal.

If a 'dark patch' signal is produced instead, this leads to incrementation of a counter for such signals. When the number of successive dark patch signals reaches three, this is a sign that there is excess toner in the developer, and so motor 5 is deenergised to stop the addition of any further toner until light patch signals are again generated.

When the number of successive light patch signals is less than a predetermined number, for example eight, toner is added at the above 'normal' rate. However, when the number of successive light patches exceeds eight, the rate of addition of toner is altered to its maximum i.e.

motor 5 is driven continuously. Normally this alternation between stopping the flow of toner and feeding it at its normal and maximum rates is sufficient to allow for the variable rates at which toner is extracted from the copier on the copy sheets. However, in the case of exceptionallyheavy demand, the maximum rate of replenishment during copying may be insufficient to replenish the developer, and so when more than a preset number of light patch signals have been produced, e.g. 98, the microprocessor 8 causes the copier to move into its out-of-toner strategy, as shown in Figure 7.

After the relatively-large number of successive 'light patch' signals the copier is arranged to go through a maximum of three interruptible tone-up cycles. Each of these cycles lasts for about 50 seconds after the completion of a job. During each cycle the 'Adjusting copy quality. You may still make copies' signal is displayed to the operator, indicating that it is possible for the operator to interrupt these tone-up cycles by pressing the 'start print' button, which causes the machine to go through a copy cycle at the end of which, if a 'light patch' signal is still generated, and there have less than four such tone-up cycles, the copier starts on the next interruptible cycle.

If three such cycles have passed without a single 'patch normal' or 'dark patch' signal having been generated, the machine continues to dispense toner at the maximum rate for each copy, until either a single 'dark patch' signal is produced or the patch signal drops by 0.5 density units compared with the starting value. In the latter case, once the 'start print' button has been pressed, the machine goes through another tone-up cycle (this time of 90s), but which this time is not able to be interrupted.

In this cycle, the CVT mode patch is produced on the drum both before and after the copy area, and the resultant patch signals are compared with each other. If there is an increase of density the copier reverts to the beginning of the 'out-oftoner' strategy and extinguishes the 'add toner' signal, so that the user has an immediate 'reward' for the trouble taken in adding toner. If there is no such increase, the copier remains in its noninterruptible tone-up cycle mode until the reduction in density becomes less than 0.3 density units, whereupon it reverts to the beginning of the 'out-of-toner' strategy.

The present invention is based on the foliowing relationships, which are based on the fact that the microprocessor 8 works in discrete voltage levels.

The photodetector in sensor 1 6 transduces the incoming light into a voltage directly proportional to the light intensity. Let the voltage produced by reflectance from the bare drum be Vd, and let the attenuation produced by device 20 be g. Thus when first setting up and during standby, the input voltage to the microprocessor is Vd.g.

When the copier is running, the attenuator 20 is switched out, and thus the input voltage to the microprocessor is in effect Vd attenuated by the overlying film of toner, or Vp (although Vd never appears, because the light reaching the photodetector has already been attenuated by the toner).

The normal control point for the apparatus is when g.Vd=Vp.

A 'dark patch' signal is produced when Vp69.Vd A 'light patch' signal is produced when Vp > g.Vd Thus, with reference to Fig. 7, the 'Density Drop 0.5du?' decision is made by injecting a voltage K1 from the microprocessor of value equivalent to 0.5 density units (du). Thus the decision is 'yes' (Y) if VP > g.Vd+K1.

In this case the copier moves onto its mandatory (non-interruptible) tone-up mode. In this mode the introduction of the black patch generator both before (B) and after (A) the next copy cycle gives rise to voltage VpB and VpA.

In decision gate 'Density Increased?' the answer y is given if VpA < VpB-Vn, where Vn is a noise threshold voltage.

If the density has not increased, the question of whether or not the 'Density Drop 0.3du?' is answered by relationship VpA > VpB+K2, where K2 > Vn and is equivalent to 0.3du, allowing the addition of toner to cause an exit from the strategy by further pressing of the 'start print' button. Similarly to K1 voltage K2 is injected by the microprocessor.

When a persistent fault indication is generated on the automatic density control side of the machine, such as by the bar mentioned above remaining illuminated, the copier is able to be set into a diagnostics mode, making available to the service engineer a library of programs. When that pertaining to ADC is selected, the copier displays 'Diagnostics' ppp BBB' where 'ppp' is a number proportional to Vp, and 'BBB' to g.Vd. These can be equalised by adjusting attenuator 20 to vary 'BBB'. If, after the copier has left the diagnostics mode, the bar remains illuminated after 30s, this indicates that the ADC is inoperative.

Claims (4)

Claims

1. A plain paper copier including a toner hopper having means for delivering toner to a developer housing at at least two different rates; a photoreceptor; means for charging a path 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 initiating a 'low-toner' strategy when the intensity of light reflected from the path is higher than a chosen threshold value, and for initiating an 'out-of-toner' strategy when a chosen number of successive copies have been made without the patch signal having fallen below the said threshold value.

2. A copier as claimed in claim 1, in which during the 'low-toner' strategy, the occurrence of less than N successive 'light patch' signals causes toner to replenished at a lower (nominal) rate, and N or more light patch signals to a higher (maximum) rate.

3. A copier as claimed in claim 2, in which the occurence of M ( N) successive light patch signals causes the copier to go through a chosen small number of interruptible tone-up cycles of known duration at the higher toner replenishment rate.

4. A copier as claimed in claim 3, in which the continued arrival of light patch signals causes the copier to go through an indefinite number of noninterruptible tone-up cycles.