METHOD OF AND DEVICE FOR DETERMINING A MEASURE OF THE SURFACE POTENTIAL OF A MEDIUM CHARGED BY MEANS OF A CORONA CHARGING DEVICE

Abstract: A medium to be charged is conveyed past a corona charging device, so that charge emitted by the corona charging device (2) is deposited on the medium surface (1). A measuring wire (4) is disposed between the medium (1) and the corona charging device (2), near the medium (1), which measuring wire (4) collects a part of the emitted charge. A first measuring circuit (11) determines a measure i d of the charge current collected by the measuring wire (4). A second measuring cir­ cuit (14) determines a measure i f of that part of the emitted charge current that flows to the medium. In accordance with a predetermined relation a measure E of the sur­ face potential is derived from these measures i f and i d .

Description: The invention relates to a method of determining a measure E of the surface potential of a medium charged by means of a corona charging device, in which charge emitted by the corona electrode or electrodes near the charged surface to collect a part of the emitted charge, and in which the measure E is derived from the value i d of the charge current collected by the measuring wire. The invention also relates to a device for performing the said method, which device is provided with a measuring wire disposed near the surface to be charged, and a first measuring circuit for determing the value i d of the current collected by the measuring wire, the measuring circuit delivering a first signal which represent the value of the deter­ mined current. A method and device of this kind are known from US patent 3 604 925. It has been found that the known method and device for deter­ mining a measure of the surface potential are relatively inaccurate. The object of the invention is to provide a method and device with which the measure E can be accurately determined. According to the invention this object is achieved in that in a method of the kind referred to in the preamble, a measure i f of the portion of the emitted charge current that is conveyed to the medium is determined, and in that the measure E is corrected by a correction C dependent upon the measure i f , which correction is derived in accordance with a predetermined relation between the measure i f and the correction C. The invention is also based on the realization that the inac­ curacy of measurement is caused primarily by variations in the charge current i f as the result of variations, for example, in the charging characteristics of the medium to be charged and also on the realiza­ tion that the influence of i f on the measurement can be predicted. The accuracy of the measuremnet is greatly improved by determining the charge current i f and then correcting the measure E by a value dependent upon the value i f . An attractive variant of the method according to the invention is characterised in that the value i tot of the charge current emitted by the corona electrode or electrodes is determined and in that the measure i f is derived from the value i tot in accordance with a prede­ termined relation. The advantage of this variant is that a measure i f of that part of the emitted charge current which is conveyed to the medium and which is difficult to measure can be readily determined from the easily measured charge current emitted by the corona electrode or electrodes. This simplification is rendered possible by the fact that the measure i f has been found to be satisfactorily pre­ dictable by reference to the value i tot . A variant of the method according to the invention, in which the position of the measuring wire is so selected that the measuring wire collects only charge moving towards the medium surface, is very attrac­ tive because of its high sensitivity. In another variant of the invention, in which the predetermined relation is equal to C = k1.i f + K2 in which K1 and K2 are constants, the measure E can easily be approached by means of the said prede­ termined relation. In a device of the kind referred to in the preamble for per­ forming the above method, the object aimed at is achieved according to the invention in that the device comprises a second measuring cir­ cuit for determining the value i f , which delivers a second signal which represents the determined value i f , and a computer circuit to which the first signal and the second signal are fed, and which from the supplied signals generates in accordance with the predetermined equation an output signal which represents the measure E. The invention will be explained in detail with reference to two embodiments and Figs. 1 to 5 in which: Fig. 1 is a diagram of the corona charging device; Fig. 2 is a graph showing the relationship between the wire current and the photoconductor current at different surface poten­ tials of the photoconductor; Fig. 3 is a diagram of an embodiment of a first control system in which the invention is used; Fig. 4 is a diagram of an embodiment of a second control system in which the invention is used; Fig. 5 is a time diagram to explain the operation of the control system of Fig. 4. In Fig. 1, a photoconductor 1 is moved by a drive roller 5 past a corona electrode 2 at a distance therefrom. The rear screen or the like 3 is indicated diagrammatically at a distance from the corona electrode 2. A measuring wire 4 is clamped over the width of the pho­ toconductor so as to be situated at the back of the corona spray cone. The way in which a photconductor current i f and a screen current i s occur as a result of the negative potential -V o of the corona electrode is illustrated diagrammatically. The measuring wire 4 is earthed through a low-ohm resistor R d . The measuring current i d of the order of a few µA's is determined by various factors. Firstly this current is determiend by the field strength between the corona electrode and the measuring wire (zero potential) itself; with reaso­ nable accuracy this part of the current is fixed part of the photo­ conductor current i f , in dependence on the construction. A second part of the current is determined by the field strength as a result of the charge level or surface potential (ASV = apparent surface voltage) of the photoconductor 1 near the measuring wire 4. An increasing nega­ tive charge of the photoconductor will cause this part of the current flowing through the resistor R d to increase. The signal delivered across the measuring resistor R d is now approximately equal to: i d R d = K(i f ) + F(ASV) + G in which G, K and F are constants and K<1 and F<1. By measuring the factor K(i f ), and since the constant G is fixed, the rest of the signal i d R d can be used to control the high-­ voltage source of the corona electrode 2 and hence for controlling the surface potential (ASV) of the photoconductor. In Fig. 2 as an example the relationship between the photocon­ ductor current i f and the wire current i d is given for different sur­ face potentials ASV as a parameter in the case of a pin corona device. The relationship i d /i f is linear except for the foot at the origin. The region (<40 µA) is outside the working range and is also unimportant. The above-mentioned constant G indicates the shift of a given line with respect to that of ASV = 0. Apart from the dependence of the wire current i d on the photoconductor current i f and the sur­ face potential ASV, the positioning (see Fig. 1) of the measuring wire 4 plays a part. The value of the wire current increases with increasing h or decreasing d. For a sensitive detector it is desirable that di d /dASV should be large. This is obtained by making h and d small, e.g. h = about 1 mm. The i d variations as a result of ASV variations with respect to the absolute value i d , i.e. (di d /dASV)|i d |, must be large. This can be achieved by making h small. Fig. 3 diagrammatically represents an application of the inven­ tion in which application the invention is used to control the sur­ face potential. A high-voltage source 10 feeds the corona electrode 2. The current i d taken from the measuring wire 4 is converted to voltage in a measuring circuit 11 and fed to the differential element 20. The photoconductor current i f in the measuring circuit 14 is mea­ sured in a first feedback loop and converted to a voltage which is compared in the differential element 17 with a voltage corresponding to the required current value i f,ref . The result of this differential measurement is fed to the high voltage source 10 so that a stabilized photoconductor current is obtained. The voltage in the voltage divider 15 corresponding to the i f current value is divided by the factor K in a second feedback loop and is increased by the constant value G in the differential element 19. The result is compared, in the differential element 20, with the voltage of the measuring circuit 11 corresponding to the wire current i d . The output signal of the differential element 20 is the corrected measure E of the surface potential. The correcterd measure E is used as an input signal to an integrating controller 12. The sur­ face potential is so controlled by means of the second integrating feedback loop that the measure E of the surface potential becomes equal to zero. The time constant of the integration amplifier 12 is, for example, one second, so that deviations of, for example, 100 V can be corrected within 1 second, i.e. within the time for charging one image area. An ASV deviation, however, is measured as an average over the total photoconductor width so that an ASV variation as high as this over the width generally does not occur. The control system operates as follows: Immediately after switching on, the high-voltage supply is controlled for about 50 msec in a preset current mod. This means that the supply acts as a fixed current source, the current setting of which is so selected that the photoconductor charge accompanying it cannnot become exceptionally high. This obviates any damage to the photoconductor on switching on. This pre-charging interval of 50 msec corresponds to the said transit time between the corona device and the measuring wire; during that interval of time after switching on there is in fact no signal in the measuring wire. Control in this preset current mode is obtained by setting the gain of the integra­ tion amplifier 12 to a minimum, the output voltage being set to the preset value. After that interval, this feedback loop for the ASV control is closed by increasing the integration amplifier gain, e.g. to 180x. Fig. 4 diagrammatically represents a control system using the invention. The control system is basically the same as that of Fig. 3 except that the buffer amplifier 21 is included and a circuit S₁ and a capacitor C₁ to earth are included in the output circuit of the integration amplifier 12, while a switch S₂ is included in the feed­ back loop of the integration amplifier 12. The operaton of this control system will be explained by reference to Fig. 5. During image charging in the interval t opl the switch S₁ is opened so that the high-voltage source 10 is controlled by a signal corrected by a correction signal of constant value from capacitor C₁. Switch S₁ is closed only briefly at the end of the image charging interval t opl . The switch S₂ is closed during the image movement interval t b so that in that interval the integration amplifier 12 will determine the correction signal for the next charging. After this image movement interval t b , the switch S₁, as stated briefly closes so that the capacitor C₁ can take over the correction signal from the integration capacitor C₂. This correction signal from capaci­ tor C₁ is used during the pre-charging interval t d to correct the control for the high-voltage source. The substantially open condition of the two switches S₁ and S₂ is necessary in order to avoid any leakage of the capacitor charges in a standby situation. In this example, each capacitor will hold its voltage approximately 8 hours or more. This also prevents the integration amplifier 12 from coming up against the feed voltage. In order to avoid any influencing of the correction signal by short-term charging disturbances, the time constant must be made large, i.e. >5 sec. As a result, after the high-voltage source has been switched on or after a possibly long standby situation, a pre-charging interval of several seconds has to be applied before a first charging can be made. It is clear that in this second embodiment no correction takes place during charging by the control system. Control of the high-­ voltage supply is determined by the correction signal detected during the previous charging. The correction thus lags one charging behind. Also, no pre-charging path is required in a series of consecutive chargings. Against this, a pre-charging interval of about 5 seconds must be applied for a first charging or after a possibly long standby situation. The operation of the ocntrol system according to the first embo­ diment depends on the properties of the photocondutor. In the case of a uniform charging, e.g. ±20V, in which the photoconductor unifor­ mity variations are of low-frequency, i.e. there are no abrupt charge variations, the control system selected will operate without problems. The advantage is also that small charging differences within one image area can be corrected directly. When the charging differences are considerable, for example ± 50 V, and of high-­ frequency, e.g. several abrupt variations per image area, the influence on the control system is measurable and such influence may be undersirable. This problem is obviated in the second embodiment, but a correct­ tion of the charging differences in one image area is then impossible. As described hereinbefore, the invention is suitable primarily for use in control systems for controlling the surface potential of a photoconductor fed past a corona device for charging purposes. However, the invention is equally applicable to those cases in which measurement of the ASV is desirable for other purposes. In the above-described embodiments of the invention, the measure i f is obtained by determining the difference between the current i tot emitted by the corona electrode and the screen current i s . However, in practice it has been found that the ratio is approximately constant over a fairly wide area. The measure i f can accordingly if required easily be determined from the value i tot in accordance with the relation i f = K2 i tot. Measurement of the screen current i s is then superfluous. Although this determination of the current i f is less accurate than the deter­ mination of i f according to the equation i f = i tot - i s, this method is quite satisfactory in practice if the accuracy of the determination of i f is not required unduly strict requirements.

Claims: 1. A method of determining a measure E of the surface potential of a medium charged by means of a corona charging device, in which charge emitted by the corona electrode or electrodes (2) is deposited on the medium surface (1) and a measuring wire (4) is disposed near the charged suface (1) to collect a part of the emitted charge, and in which the measure E is derived from the value id of the charge current collected by the measuring wire (4), characterised in that - a measure if of the portion of the emitted charge current that is conveyed to the medium (1) is determined, and - the measure E is corrected by a correction C dependent upon the meausre if, which correction is derived in accordance with a predetermined relation between the measure if and the correction C. 2. A method according to claim 1, characterised in that the value itot of the charge current emitted by the corona electrode or electrodes is determined and in that the measure if is derived from itot in accordance with a predetermined relation. 3. A method according to claim 1, characterised in that the position of the measuring wire is so selected that the measuring wire (4) collects only charge moving towards this medium surface (1). 4. A method according to claim 1 or 2, characterised in that the predetermined relation is equal to C = K1.if + K2 in which K1 and K2 are constants. 5. A device for performing the method according to claim 1, 2 or 3, which device comprises a measuring wire (4) disposed near the surface (1) to be charged, and a first measuring circuit (11) for determining the value id of the current collected by the measuring wire (4), the measuring circuit (11) delivering a first signal which represents the value of the determined current, characterised in that the device comprises a second measuring circuit (14) for determining the value if, which delivers a second signal which represents the determined value if, and a computer circuit (15, 19, 20) to which the first signal and the second signal are fed, and which from the supplied signals generates in accordance with the predetermined rela­ tion an output signal which represnts the measure E.

Inventors: COX, JAN JACOBUS FRANCISCUS MARIE, ENSING, HENDRIK
Applicants: OCE-NEDERLAND B.V.
Application year: 1986
Main classification: G01R29
IPC codes: G03G15, G01R29, G01R19