Detection method of TDK toner level sensors
TDK has developed and currently supplies toner level sensors that detect magnetic and non-magnetic developer with the piezoelectric vibration method, using the conversion between electric energy, the basic feature of piezoelectric vibrators and mechanical energy. The toner level sensors in piezoelectric vibration can provide higher accuracy in the detection of particles, each of which weighs fewer than 10 milligrams to tens of milligrams, than in light transmission. It is also superior in the stability of detection accuracy with a structure that is highly resistant to becoming dirty.
Operation mechanism and structure
The basic operation mechanism and structure of the piezoelectric vibration-type sensor is the same as those of piezoelectric sounding bodies. It has a unimolf vibrator with a disc-shaped piezoelectric ceramics applied to a thin metallic board.
Operation mechanism of piezoelectric unimolf
With the piezoelectric ceramics polarized in the direction of depth in advance, the whole element increases and decreases in thickness, expanding or contracting in the polarization direction in response to voltage applied from outside in the polarization direction. In this event, the volume stays unchanged, with its diameter contracted when its thickness is expanded and vice versa, as shown in the model below.
However, the piezoelectric ceramics are firmly adhered to the metallic board. This means that the expansion and contraction of the diameter bends the entire unimolf oscillator as shown in the model below.
This is how the unimolf vibrates with the same mechanism as piezoelectric sound bodies when alternating current voltage is applied.
Application to piezoelectric type sensors
The single component developer (toner) is always in the state of cloudy, stirred and spongy, with a particle diameter of approximately 10μm, and an apparent specific gravity (or “loosened density” in JIS standards) of some 0.3 to 0.35g/cm³. As mentioned later, the presence of these particles can be detected using phenomena in which phase characteristics are changed, depending on the degree of contact of particles to the vibrating surface of the unimolf.
Therefore, piezoelectric type sensors must have a structure in which the unimolf, which provides the surface for detection, is placed at the very front, and in which the surface is flat so that it can be wiped clean during regular service. To meet these requirements, the unimolf and the case are attached in the peripheral support structure, in which the periphery, not the sections, of the unimolf is supported (Refer to the model below).
In addition, elastic silicon resins are used for adhesion to provide high-level uniformity in adhesion width and thickness, given that uneven strength in peripheral support could severely impact the detection properties of the sensors.
Driving scheme of piezoelectric unimorph
As mentioned earlier, 2-terminal piezoelectric unimorph is driven in the separately-excited oscillating method where alternating current signals are externally impressed on the whole-surface electrodes on the both sides of piezoelectric ceramics. The phase characteristic change of unimorph is used as the signal which determines if load exists on the sensor surface.
It shows the equivalent circuit and impedance frequency characteristics of 2-terminal piezoelectric unimorph.
Cd denotes capacitance, L0 is equivalent weight, C0 is the inverse number of equivalent stiffness, and R0 is equivalent mechanical resistance. The smallest impedance point of frequency characteristics is the series resonance point of equivalent circuits L0, C0, and R0.
In this event, unimolf demonstrates its inductivity near the resonance point in no-load status, and its capacitivity else-where. As the load on the sensor surface increases, the phase properties gradually change and turn capacitive at all frequencies when the load exceeds a certain level.
This enables us to know the load status from the phase of the unimolf near the resonance point. If it is inductive the sensing surface has no load, whereas it is capacitive it has some load. This is how it detects the presence or absence of particles.
The driving circuit of piezoelectric vibration-type level sensor
TDK’s toner level sensor is mounted with our custom IC, which integrates a sweep vibration circuit, a waveform boosting/shaping circuit, a phase detection circuit, a digital processing circuit, etc. to provide stable operations and detection.
This IC sweeps the spectrum of 4 to 8kHz around the unimolf resonance frequency of 6kHz to see whether the input signal from the unimolf is inductive or capacitive. It gives out two values, at a high level and at a low level respectively, to show the no-load status when inductivity is detected, or to show the status with some load when capacitivity is detected, in a sweep.
This first output signal allows detection of the toner status, but it is sent out as final signal of the sensor through the counter circuit to ease the output chattering (repeatedly conducting judgment of the toner status) that occurs at the initial stage of detection and to provide highly accurate and stable detection.
Compatibility with finer toner
In the future, remarkable progress is expected in the pursuit of higher picture quality from photocopiers and laser printers. In this context, the toner particle size will be smaller, the volume of toner as a proportion of developer will be lower, and the permeability variation corresponding with changes in toner concentration will be infinitesimal. This could result in lower output changes (sensitivity) of permeability sensors. We therefore need to develop a new way of high sensitivity detection.
And finer toner is expected to vary markedly from existing toner in weight, bulk density and electrification. It is inevitable that the detection accuracy of existing piezoelectric vibration-type toner level sensors will be revised.
It is a known fact that oscillation stops when the energy amount — the summation of detected powder’s momentum and acoustic load — and unimorph oscillator’s momentum become equal. We will need to revise the specifications of the basic properties of piezoelectric vibration-type sensors, including vibration frequency, based on a full analysis of the acoustic effects of toner, and develop a new detection mechanism so that we will be able to respond to the trends towards reduced toner particle size.
Compliance with full color processing process
It is obvious that high accuracy toner concentration control is needed for full color processing, given the striking advances in the quest for higher resolution of color photocopiers and color laser printers.
Even though uneven toner concentration affects the shading of pictures generated in single color processing, it is acceptable in practical applications to a certain extent unless pictures are extremely dark or extremely light. But full color processing layers four colors, namely yellow, magenta, cyan and black.
Unevenness in the toner concentration of each color could severely impact the color tone of final pictures. This is a decisive factor that limits the reproduction of objective colors. For that reason, the establishment of a technology to substantially prevent erratic detection sensitivity is most needed.
For toner level sensors, with the four-color toners differing in many cases in particle size, specific gravity, etc., sensors optimized for each of the colors are required so that they can make highly accurate level detection.
Currently, a toner particle diameter of around 9μm is the mainstream size at the propagation stage, at which performance of prices and features is pursued. Further progress in toner production techniques will lower toner production costs, and toners with a particle size of as little as approximately 5μm may soon find practical application. Here in the field of color printing as well, we will need efforts to make both concentration detection and level detection support the super fine toner.
Response to needs for smaller and thinner sensors
With the rapid improvement in lower profile machines, color machines almost as large as monochrome machines have already appeared on the market. Developing components are now a quarter or less in size of those of monochrome machines, and it is likely that we will have even lower profile machines in future. Naturally, these require smaller and thinner toner concentration or level sensors to be mounted in the development component.
We will not be content with the technologies and expertise that we have already established. Rather, we will aggressively strive to go beyond the current limitations and develop new-generation toner sensors that respond to these more advanced needs, by studying each of the concepts that we have acquired from an analysis of new events.