DE3202288A1 - VOLUME SENSOR FOR ELECTROPHOTOGRAPHIC DEVELOPMENT SYSTEMS - Google Patents

Description

Int. Ref .: Case 1504 January 18, 1982

Hewlett-Packard Company

VOLUME SENSOR FOR ELECTROPHOTOGRAPHIC DEVELOPMENT SYSTEMS

In a typical electrophotographic printing operation first sensitized a xerographic plate by electrical charging and then selectively exposed it, creating a latent electrostatic image is created, usually this is latent image is then developed by using a two-component developer brought into contact with the xerographic plate will.

This two-component developer generally consists of a fine electroscopic powder, for which the name "Toner" is known, as well as a relatively coarse magnetically susceptible material known as a "carrier". If carrier and toner come into contact with each other, they become triboelectrically charged. When the developer mix comes in contact with the the latent image-bearing surface is brought to the toner particles, which are due to the triboelectric charge have the opposite electrical polarity as the latent image, corresponding to the latter, are attracted to the surface and separated from the carrier particles. This powder picture then becomes transferred to paper or other material and fixed for permanent reproduction.

The relative concentration of the toner in the carrier is generally a major factor in print quality. The developer mix must correspond to the constant depletion of the toner during printing are constantly enriched with toner so that the mutual concentrations keep the correct values. Accordingly many types of devices have been proposed in which

3202283

Hewlett-Packard Company
Int. Ref .: Case 1504

automatically refilled, either after a certain number of prints or after the concentration of the toner particles Toner particles in the developer mixture has dropped below a certain value.

In addition to the loss of toner from the developer, a small amount of the carrier is also lost, as is some of the carrier material is transferred to the xerographic plate during normal operation. Since the toner replenishment usually is controlled so that a nearly constant ratio between toner and carrier is maintained, the can. loss of Carrier material may lead to a reduction in the total volume the developer mixture below an unacceptable level Worth driving. To solve this problem, a volume sensor is usually used to ensure that an adequate Developer volume is maintained. However, such sensors are usually small magnetic feelers that are essentially provide digital information, i.e. indicate whether the amount of developer exceeds a certain amount or not. A sensor used in this way only works as a minimum volume indicator, so that overfilling can occur, which sometimes leads to unnecessary and expensive maintenance. Such Problems can be eliminated by adding another magnetic sensor that indicates when the developer volume is running out reached its maximum. However, this adds another component to the system, which itself creates maintenance problems can contribute and still only provides limited information about the actual developer volume, if this moves between the maximum and minimum amount.

The invention according to claim 1 is based on the object of providing a volume sensor for electrophotographic developers, which enables an analog recording of the developer volume.

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3202283

Hewlett-Packard Company

Int. Ref .: Case 1504 - 5 -

According to a preferred embodiment of the invention, a sensor element is provided that the amount of magnetically susceptible Material detected in its vicinity. An LC circuit is provided for this, the inductance of which serves as a sensor element. When magnetically susceptible material is in contact with the inductance comes, a magnetic path with lower reluctance is formed, which changes the value of the inductance. As more magnetically susceptible material comes into increasingly closer contact with the inductor, its value increases monotonically.

The resonance frequency of the LC circuit changes with the inductance. In this way it can be achieved that the output voltage of the LC circuit increases with the amount of in contact with the inductance magnetically susceptible material located changes. Will the LC circuit operate at a frequency slightly below the operated at the lowest possible resonance frequency, the output signal of the LC circuit can be used to monitor the amount of Use magnetically susceptible material. The range of possible developer volumes then corresponds to the available range of Resonance frequencies.

The sensor according to the invention is particularly useful as a volume sensor for developers in electrophotographic copying machines where the developer mixture contains magnetically susceptible carrier particles be used. As mentioned above, such machines require precise control of the relationship between Toner and carrier particles, the volume amount being one of the Key elements in maintaining this relative focus is.

In this special application, a voltage corresponding to a minimum amount of mixture can be used as an error signal in a Feedback system can be used which indicates that mixture must be added. In a similar way, a maximum

lcwlcl l.-IMc-knrrl Company

InL. Λ / .: Cdse 1504 - 6 -

mum display can be used to create mechanisms to reduce the To control the amount of mixture, so that overfilling is avoided will. There is also a display of the amount of mix between them useful in both extremes when performing diagnostic procedures on the machine.

Further embodiments or developments of the invention are characterized in the subclaims.

The invention is illustrated below with the aid of exemplary embodiments explained in connection with the accompanying drawing »Show in drawing

Figure 1 is assigned to a volume sensor according to the invention Circuit;

Figure 2 shows the typical configuration of an inductive sensor element in operation relative to the fill level of a developer mixture; FIG. 3 is a graphic representation of the shift in the resonance frequency when changing the fill level of the developer mixture;

Figure 4 shows a signal processing circuit that is used in the invention Volume sensor is used as an output stage;

FIG. 5 is an equivalent circuit diagram for that in the signal processing circuit according to Figure 4 included peak value detector; and

FIG. 6 shows a further equivalent circuit diagram for the peak value detector contained in the signal processing circuit according to FIG.

In Figure 1, a volume sensor according to the invention is shown schematically, whereby the relationships between the individual components can be seen.

In the present exemplary embodiment, a driver oscillator U1 is a timer of the 555 type, which is suitably biased, so that it has a variable frequency range from about 3.5 kHz to

Hewlett-Packard Company

Int. Ref .: Case 1504 - 7 -

7.1 kHz supplies so that component tolerances are covered. A Inductance sensor 20 is made from a stack of E-shaped sheets manufactured, e.g. the size EI-187 from the iron type AISI-M6. Of the The middle leg of the magnetic core formed in this way is wound with about 250 turns of wire, so that a coil is formed. Of the inductive sensor 20, together with a capacitor C4 and a resistor R2, forms a voltage divider whose output voltage DVS is a function of the frequency, the inductance of the sensor element 20, the capacitance of the capacitor C4 and the resistor R2. In the special embodiment shown in FIG the capacitor C4 has a capacitance of about 0.047 yF and the resistor R2 is about 5.62 kJl.

In operation, the inductive sensor element 20 is arranged within a container, the poles of the sensor element 20 against a window made of mylar are directed so that the level of the mixture can be detected. Figure 2 shows a typical configuration, which illustrates the mutual position of inductive sensor element 20, container 22, Mylar window 24 and level 26. The mixture contains carrier particles of iron, which in the presence of Yor the inductive sensor element 20 with increasing level increase the magnetic flux.

When the magnetic flux increases, the inductance of the sensor element 20 increases, thereby increasing the resonance frequency of the LC circuit is shifted downwards, since the resonance frequency with constant capacitance C2 is inversely proportional to the square root from the inductance. The effects of this shift in resonance frequency are easily seen from the equation for the Output voltage of a sine wave oscillator:

IDVSj =

Hewlett-Packard Company

Int. Ref .: Case 1504 - 8 -

DVS is the output voltage at terminal E3, R is the resistance value of resistor R2, ω is the control frequency, V is the voltage at terminal 3 of driver oscillator 111, C is the capacitance of capacitor C2 and L is the inductance of inductive sensor element 20 is only an approximation, as the waveform of the driver signal deviates from the sinusoidal shape.

Figure 3 is a semi-log plot of the above Equation showing DVS as a function of frequency »Av Fixing the frequency of the driver oscillator Ut to one Value below the resonance frequency of the LC circuit, if the mixture completely covers the sensor element 20, it can be achieved that the output voltage DVS is monotonic changes with the level of the mixture. In the example above, DVS corresponds to a level Vl when the mixture located below the sensor element, and increases to a maximum of V2 when the sensor element is completely with the Mixture is covered. Thus the voltage DVS is a direct measure of the fill level of the mixture as long as it is within the detection area of the sensor element.

To improve the output signal, the output voltage DVS can be fed to a signal processing circuit. This circuit is shown in FIG. The first stage of the signal processing circuit is a peak detector with a gain of about 4.5. The way the circuit works depends on whether one of the following Peak voltage from the LC circuit is higher or lower than a previous peak voltage. If a higher If a peak voltage occurs, an amplifier U18 in the peak value detector charges a storage capacitor C75 to a voltage level that is approximately 4.5 times the input voltage peak. The parallel connection of a resistor R105

3202283

Hewlett-Packard Company

Int. Ref .: Case 1504 - 9 -

and capacitor C75 results in a stored peak value with a time constant of 32 ms, that of the regular Renewal is needed. This prevents a stored peak value from being retained indefinitely, and so does the detector can follow the output voltage of the inductive sensor element 20.

The operation of this part of the signal processing circuit can easily be found in the equivalent circuit diagrams according to FIG and 6 for the peak detector. The circuit of Figure 5 corresponds to the case in which a peak voltage from the sensor element 20 higher than that previously stored Peak is. Figure 6 illustrates the case in which the peak voltage from the sensor element 20 to a value below of the previously stored peak value drops »In this latter case, the output signal of the operational amplifier Ut8 negative, diode CR13 is reverse biased and diode CR14 is forward biased. This is how the operational amplifier works U18 as a voltage follower and does not change the charge of capacitor C75.

The balance part of the circuit shown in Figure 4 after the peak detector part (i.e. after TP26) represents the second Level of signal processing. Here, the buffered capacitor voltage of the peak value detector is detected by means of a low pass filtered and then output to some kind of power amplifier.

Other possible embodiments are apparent to the person skilled in the art. For example, the LC circuit could equally well be above its maximum resonance frequency instead of below its minimum resonance frequency operate. The output signal of the inductive sensor element would then become monotonous as the mixture level rises fall off. The device is also not available for use by

Hewlett-Packard Company

Int. Ref .: Case 1504 - 10 -

Iron particles are limited as they are just as useful in the Fill level detection of any other substance can be used, which with changing fill level the inductance of the Sensor element 20 changed significantly. Besides is also The shape of the inductive sensor element 20 can be changed significantly. The only requirement is that it be in is shaped in such a way that its inductance varies predictably with the level of the material to be monitored.

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Claims (9)

Int. Ref .: Case 1504 January 18, 1982 Hew! Ett-Packard Company PATENT CLAIMS M. Device for monitoring the amount of magnetically susceptible Material over a range of level heights, characterized by a variable inductance (20) which according to the level (26) of the magnetically susceptible Material changes, in an area that corresponds to the area of interest Corresponds to filling levels, as well as by a connection device (E4, E5) for connecting to other electrical components. 2. Device according to claim 1, characterized in that that it has a resonance circuit (R2, C4,? 0), to which the variable inductance (20) belongs and to which it is connected by the connecting device. 3. Apparatus according to claim 2, characterized in that it has an oscillator device (U1), which is connected to the resonance circuit (R2, C4, 20) and outputs a signal to it with a frequency which is outside the range of possible resonance frequencies of the resonance circuit lies. 4. Apparatus according to claim 3, characterized through an output circuit (Figure 4) connected to the resonance circuit (R2, C4, 20) is connected and derives an output signal therefrom. 5. Apparatus according to claim 4, characterized in that that the output circuit is a signal processing circuit (Figure 4) that conditions the output signal. Hewlett-Packard Company InI .. Λ / .: Case 1504 - 2 - 6. Apparatus according to claim 5, characterized in that that the signal processing circuit includes a peak value detector, which the maximum value of the output signal the resonance circuit is detected and stored for a short period of time. 7. Apparatus according to claim 6, characterized in that that the signal processing circuit contains a low-pass filter which is connected downstream of the peak value detector and filters out high-frequency components from its output signal. a. Device according to one of Claims 1 to 7, characterized in that the variable inductance (20) has a second magnetically susceptible material in the form of a core with a plurality of poles which are connected to one another and each have a pole face which are spaced apart from one another, at least two pole surfaces lie in one plane, on one side of which lies the entire second magnetically susceptible material, and wherein at least one of the poles is wound with electrically conductive wire which is connected to the connecting device. 9. Apparatus according to claim 8, characterized in that that it generates an output signal that the level of the developer mixture can be seen in an electrophotograph Printing apparatus in which magnetically susceptible material is used as one of the components of the developer mix will.