GB2096322A

GB2096322A - Analog volume sensor for electrophotographic development system

Info

Publication number: GB2096322A
Application number: GB8205425A
Authority: GB
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1981-03-20
Filing date: 1982-02-24
Publication date: 1982-10-13
Also published as: JPS57157960U; DE3202288A1

Abstract

A device and a method are described for providing an analog electrical signal corresponding to the height of a magnetically susceptible material. A variable inductor (20) is provided which serves as a sensing device, with the inductance varying as the height of the magnetically susceptible material varies. This inductance is incorporated into a resonant circuit including a capacitor (C4) thereby providing a range of resonant frequencies corresponding to the available range of inductance by driving the resonant circuit off resonance by oscillator (V1), an electrical signal is obtained which varies monotonically with the height of the magnetically susceptible material. The output signal DVS across the resonant circuit may be applied to a peak detector circuit which stores the maximum value for a short period of time and the output thereof filtered. <IMAGE>

Description

SPECIFICATION Analog volume sensor for electrophotographic development systems The present invention is concerned with improvements in or relating to an analog volume sensor for electrophotographic development systems.

In a typical electrophotographic printing operation, a xerographic plate is first sensitized by electrical charging and is then selectively exposed with light to form a latent electrostatic image.

Most commonly, this latent image is then developed by bringing a two-component developer into contact with the surface of the xerographic plate.

This two-component developer generally consists of a fine electroscopic powder known as a "toner", and a relatively coarse magnetically susceptible material known as "carrier". As the carrier and toner come into contact with each other, they tend to become triboelectrically charged. When the developer mixture is brought Into contact with the latent image bearing surface, the toner particles, being of opposite electrical polarity to the latent image as a result of triboelectric charging, are attracted imagewise to the surface and are separated from the carrier particles. These powder images are then transferred to paper or other materials and are then fixed for making a permanent reproduction.

The relative concentration of toner to carrier is generally a major factor affecting print quality.

Due to the constant depletion of toner as prints are made, the developer mixture needs continual replenishment with toner to maintain the relative concentrations at the proper levels. As a result, many types of devices have been proposed for automatically replenishing toner particles after a predetermined number of prints are made or, alternatively, after the concentration of toner particles in the developer mixture drops below a predetermined level.

In addition to the loss of toner from the developer, a small amount of carrier is also lost, since some carrier is also transferred to the xerographic plate during normal operation. Since toner addition is usually controlled in such a way as to maintain a nearly constant ratio of toner to carrier, the loss of carrier will eventually result in a reduction in the total volume of developer mixture to an unacceptable level. To compensate for this problem a volume sensor is usually used to ensure that adequate developer volume is maintained. These devices, however, are typically small magnetic sensors which yield essentially digital information, i.e., either the developer volume is at or above a particular level or it is not.

Used in this manner, such a device acts only as a volume minimum indicator so that overfilling can occur, sometimes leading to unnecessary and expensive maintenance procedures. Such problems could be overcome by adding another magnetic sensing device for indicating when the volume is at maximum. However, this adds another component to the system which in itself can contribute to maintenance probiems and still only provides limited information as to the actuai developer volume when it is between the maximum and minimum levels.

The prior art does not show any analog volume sensing devices that utilise the magnetic properties of the carrier material to determine the developer volume over a wide range of volume variations.

The present invention provides apparatus for providing an electrical signal which has a magnitude determined by the surface level of magnetically suspectible material, over a range of surface levels of said magnetically susceptible material, comprising variable inductor means for providing an inductance which varies in magnitude as the surface level of said magnetically susceptible material varies, said variable inductor means having a range of inductance corresponding to said range of surface levels; and first coupling means for coupling said variable inductor means to other electrical components.

Apparatus as set forth in the last preceding paragraph may further comprise resonant circuit means, said variable inductor means being a component thereof, and coupled thereto by said first coupling means.

Apparatus as set forth in the last preceding paragraph may further comprise oscillator means coupled to said resonant circuit means for providing an oscillating signal to said resonant circuit means, said oscillating signal having a frequency outside said range of resonant frequencies of said resonant circuit means.

Apparatus as set forth in the last preceding paragraph may further comprise output means coupled to said resonant circuit means for providing an output signal from said resonant circuit means.

In apparatus as set forth in the last preceding paragraph, it is preferred that said output means further comprises signal conditioning means for conditioning said output signal.

In apparatus as set forth in the last preceding paragraph, it is preferred that said signal conditioning means further comprises peak detector means for detecting the maximum value of said output signal of said resonant circuit means and for storing said maximum value for a short period of time.

In apparatus as set forth in the last preceding paragraph, it is preferred that said signal conditioning means further comprises low pass filter means coupled to said peak detector means for filtering out high frequency components of said maximum value of said output signal.

In apparatus as set forth in any one of the last seven immediately preceding paragraphs it is preferred that said variable inductor means further comprises second magnetically susceptible material formed into a rigid shape having a plurality of pole pieces, the pole pieces being connected one to another by said second magnetically susceptible material, each pole piece having a face, said faces being separated in space, and at least two of said faces defining a plane such that all of said second magnetically susceptible material is located on one side of said plane; and at least one of said pole pieces being wound with electrical wire and coupled to said first coupling means.

Apparatus as set forth in the last preceding paragraph is especially suitable for providing an output signal corresponding to the level of developer mixture in electrophotographic printing devices which use magnetically susceptible material as one of the component of the developer mixture.

The present invention further provides a method of providing an electrical signal which has a magnitude determined by the surface level of a magnetically susceptible material, for a range of level of said magnetically susceptible material, the method comprising the steps of positioning an inductor adjacent said magnetically susceptible material, said inductor having an inductance which varies in magnitude as the surface level of said magnetically susceptible material varies in height, and having a range of inductance determined by said range of surface levels; and providing a resonant circuit having said inductor as one component, said resonant circuit having a range of resonant frequencies determined by said range of inductance.

A method as set forth in the last preceding paragraph may further comprise the step of driving said resonant circuit at a frequency outside said range of resonant frequencies, said resonant circuit producing an output electrical signal which varies in a predictable manner as said inductance varies with the level of magnetically susceptible material.

In accordance with the illustrated preferred embodiment, the present invention provides a sensing device for sensing the level of magnetically susceptible material in its proximity.

A device according to the invention provides an LC circuit with the inductor in the circuit acting as a sensing mechanism. As magnetically susceptible material comes into contact with the inductor, a lower reluctance magnetic path is formed thereby changing the value of the inductance. Further as more magnetically susceptible material comes into increasingly greater contact with the inductor, the inductance monotonically increases.

As the inductance changes so does the resonant frequency of the LC circuit. Hence, the output voltage of the LC circuit can be made to vary with the amount of magnetically susceptible material in contact with the inductor. By driving the LC circuit at a frequency somewhat below the lowest possible resonant frequency, the output of the LC circuit can be used to monitor the level of the magnetically susceptible material. The range of volume levels is then commensurate with the available range of resonant frequencies.

A device according to the invention is particularly useful as a developer volume sensor in electrophotographic copying machines which use magnetically susceptible carrier particles in their developer mixture. As noted above, such machines require close control over the ratio of toner to carrier particles, with volume control being one of the key elements in maintaining that relative concentration.

In this specific application, a voltage corresponding to a minimum mixture level can be used as an error signal in a feedback system to indicate the need for adding mixture. Similarly, a maximum indication can be used to initiate a mixture depletion mechanism to avoid overfilling.

Furthermore mixture level indications between these two extremes are also useful in carrying out machine diagnostics.

There now follows a detailed description which is to be read with reference to the accompanying drawings of apparatus and method according to the invention; it is to be clearly understood that this method and apparatus have been selected for description to illustrate the invention by way of example and not by way of limitation.

In the accompanying drawings Figure 1 illustrates a circuit associated with the sensor portion of a device according to the invention; Figure 2 shows the typical configuration of the inductor sensing mechanism relative to the level of the developer mixture when the device is in use; Figure 3 is a graph showing the shift in resonant frequency with change in the developer mixture level; Figure 4 illustrates a signal conditioning circuit which is used as the output stage of a device according to the invetion; Figure 5 illustrates an equivalent circuit for the peak detector portion of the signal conditioning circuit shown in Figure 4; and Figure 6 illustrates another equivalent circuit for the peak detector portion of the signal conditioning circuit shown in Fig. 4.

Figure 1 provides a conceptual representation of the sensor portion of a device according to the invention showing the relationship among the various components.

A driving oscillator Uk is provided, which in this embodiment is a 555 timer suitably biased to provide a variable frequency range from approximately 3.7KHz to 7.1KHz, to cover parts tolerances. An inductor sensor 20 is fabricated from a stack of "E" laminations, e.g., of size El187 formed of AISI-M6 steel, the "E" laminations being wound with about 250 turns of wire to form a coil. The inductor sensor 20 together with a capacitor C4 and a resistor R2 form a voltage divider whose output voltage DVS, is a function of frequency, inductance of the inductor sensor 20, capacitance of the capacitor C4, and resistance of the resistor R2. In the specific example shown in Figure 2, the capacitor C4 was chosen-as approximately 0.047 microfarads and the resistor R2 was chosen as approximately 5.62K ohms.

In operation, the inductor sensor 20 is placed inside a container with the poles of the sensor against a Mylar (Registered Trade Mark) window 24 to sense mixture level. Figure 2 shows a typical configuration demonstrating the relationship of the inductor sensor 20 to its container 22, the mylar window 24, and the mixture level 26. The mixture contains iron carrier particles which, when present in front of the inductor sensor 20, increase the magnetic flux as the level of the carrier increases.

As the magnetic flux increases, the inductance of the inductor sensor 20 increases, thereby shifting downward the resonant frequency of the LC circuit, the latter occurring since the resonant frequency is proportional to the inverse of the square root of the inductance for the capacitance of the fixed capacitor C2. The effects of this shift in the resonant frequency can be seen most easily by noting that for a sinusoidal oscillator output:

where DVS is the output voltage at pin E3 of Figure 1, R' is the resistance of the resistor R2, W is the driving frequency, V is the voltage at the number 3 terminal of the driving oscillator U1, C is the capacitance of the capacitor C2, and L is the inductance of the inductor sensor 20. Of course, this equality becomes only an approximation as the driving waveform deviates from a sinusoid.

Figure 3 is a semilogarithmic plot of the above equation showing DVS as a function of frequency.

By fixing the frequency of the driving oscillator U 1 at a value WF which is less than the resonant frequency of the LC circuit when the mixture completely covers the inductor sensor 20, the output voltage, DVS, can be made to vary monotonically with the mixture level. In the above example, DVS would correspond to V1 when the mixture is below the sensor and would increase to a minimum of V2 when the sensor becomes completely covered with the mixture. Hence, monitoring of DVS provides a direct measure of mixture level when the level is within the range of the sensor.

In order to provide a more useful output, signal conditioning can be applied to the output voltage, DVS. The signal conditioning circuitry is shown in Figure 4. The first stage of the signal conditioning is provided by a peak detector with a gain of approximately 4.5. The action of the circuit depends on whether a subsequent peak voltage from the LC circuit is higher or lower than a previous peak. In the case where a higher peak occurs, the amplifier U 18 in the peak detector charges the storage capacitor C75 to a level approximately 4.5 times the incoming peak. The parallel combination of a resistor R105 and a capacitor C75, produces a stored peak having a 32 msec. time constant and requiring regular updating. This prevents a stored peak from lasting indefinitely, thereby enabling the detector to follow the output voltage of the inductor sensor 20.

The operation of this portion of the signal conditioning circuitry can be most easily seen by examining the equivalent circuits shown in Figures 5 and 6 for the peak detector portion of Figure 4. Figure 5 corresponds to the case where a peak voltage from the inductor sensor 20 is higher than the previously stored peak. Figure 6 is the equivalent circuit when the peak from the inductor sensor 20 falls below the previously stored peak. In this latter, the output of the amplifier U1 8 goes negative, CR13 becomes reversed biased and CR14 becomes forward biased. Hence, the amplifier U18 acts like a voltage follower and does not charge the capacitor C75.

The balance of the circuit shown in Figure 4 after the peak detector portion (i.e., after TP-26) provides the second stage of signal conditioning.

Here the buffered capacitor voltage of the peak detector is filtered by a low pass filter and is then outputted to whatever end-stage device is desired.

Other embodiments will be apparent to those skilled in the art. For example, instead of driving the LC circuit below its minimum resonant frequency. Then the output of the inductor sensor would be monotonically decreasing with increasing mixture level. Also, there is no need to restrict the device to use with iron particles, since the device would also be useful in detecting the levels of any substance which would substantially change the inductance of the inductor sensor 20 as its level changed. Furthermore, the shape of the inductor sensor 20 is subject to substantial variation. The only requirement is that it be configured in a manner that its inductance varies predictably as the level of the-subject material also varies.

Claims

1. Apparatus for providing an electrical signal which has a magnitude determined by the surface level of magnetically susceptible material, over a range of surface levels of said magnetically suspectible material, comprising: variable inductor means for providing an inductance which varies in magnitude as the surface level of said magnetically susceptible material varies, said variable inductor means having a range of inductance corresponding to said range of surface levels; and first coupling means for coupling said variable inductor means to other electrical components.

2. Apparatus according to claim 1 and further comprising resonant circuit means said variable inductor means being a component thereof, and coupled thereto by said first coupling means.

3. Apparatus according to claim 2 and further comprising oscillator means coupled to said resonant circuit means for providing an oscillating signal to said resonant circuit means, said oscillating signal having a frequency outside said range of resonant frequencies of said resonant circuit means.

4. Apparatus according to claim 3 and further comprising output means coupled to said resonant circuit means for providing an output signal from said resonant circuit means.

5. Apparatus according to claim 4 wherein said output means further comprises signal conditioning means for conditioning said output signal.

6. Apparatus according to claim 5 wherein said signal conditioning means further comprises peak detector means for detecting the maximum value of said output signal of said resonant circuit means and for storing said maximum value for a short period of time.

7. Apparatus according to claim 6 wherein said signal conditioning means further comprises low pass filter means coupled to said peak detector means for filtering out high frequency components of said maximum value of said output signal.

8. Apparatus according to any one of the preceding claims wherein said variable inductor means further comprises: second magnetically susceptible material formed into a rigid shape having a plurality of pole pieces, the pole pieces being connected one to another by said second magnetically susceptible material, each pole piece having a face, said faces being separated in space, and at least two of said faces defining a plane such that all of said second magnetically susceptible material is located on one side of said plane; and at least one of said pole pieces being wound with electrical wire and coupled to said first coupling means.

9. Apparatus according to claim 8 for providing an output signal corresponding to the level of developer mixture in electrophotographic printing devices which use magnetically susceptible material as one of the components of the developer mixture.

1 0. Apparatus for providing an electrical signal which has a magnitude determined by the surface level of magnetically susceptible material, over a range of surface levels of said magnetically susceptible material, substantially as hereinbefore described with reference to the accompanying drawings.

11. A method of providing an electrical signal which has a magnitude determined by the surface level of a magnetically susceptible material, for a range of levels of said magnetically susceptible material the method comprising the steps of: positioning an inductor adjacent said magnetically susceptible material, said inductor having an inductance which varies in magnitude as the surface level of said magnetically susceptible material varies in height, and having a range of inductance determined by said range of surface levels; and providing a resonant circuit having said inductor as one component, said resonant circuit having a range of resonant frequencies determined by said range of inductance.

1 2. A method according to claim 11 and further comprising the step of driving said resonant circuit at a frequency outside said range of resonant frequencies, said resonant circuit producing an output electrical signal which varies in a predictable manner as said inductance varies with the level of magnetically susceptible material.

1 3. A method of providing an electrical signal which has a magnitude determined by the surface level of a magnetically susceptible material, for a A range of levels of said magnetically susceptible material, substantially as hereinbefore described with reference to the accompanying drawings.