Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Utility Patent
1998-03-30
2001-01-02
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S664000, C324S667000
Utility Patent
active
06169407
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to printing presses and, more particularly, to a water content metering apparatus for monitoring the concentration of water in a water-ink emulsion in a printing press.
BACKGROUND OF THE INVENTION
Lithographic printing presses typically involve two basic materials: ink and dampening solution. The dampening solution usually comprises water with small amounts of additives to enhance water wetting of the nonimage area of the printing plate. Conventionally, ink and dampening solution are supplied as two separate streams.
To simplify the printing press and reduce costs, some single fluid printing presses use a premixed ink and water (or dampening solution) emulsion. For economical operation, the water-ink emulsion must be scraped off the ink train and re-circulated back to the system, since only a fraction of the emulsion is consumed by the printing process at any given time. The scraped emulsion usually contains about 10-20% less water than the original emulsion does. In order to obtain the best printing results, the emulsion must maintain a proper water concentration, which is usually between 30-50% depending upon the ink formulation. In some instances this can be done by mixing a proper proportion of fresh ink and water at the start-up. However, over a period of time as the printing progresses, the proportion of ink and water changes in the re-circulated emulsion. In the case of a continuous application of new emulsion during the printing process, it is often necessary to measure the water concentration of the emulsion on a continuous, real-time basis.
Meters are known which measure the dielectric constant of an emulsion or mixture to determine the moisture content of materials or the density of the emulsion. Some of these prior art meters use a Wheatstone bridge-type circuit to measure dielectric properties (e.g., U.S. Pat. No. 3,696,299). Other prior art meters use two oscillators to obtain a frequency differential which is related to the dielectric properties of the material being tested (e.g., U.S. Pat. No. 4,048,844). Still other prior art meters use an oscillator with peak detectors (e.g., U.S. Pat. No. 4,399,404) or a logic circuit (e.g., U.S. Pat. No. 4,130,796). U.S. Pat. No. 4,559,493 discloses another prior art meter for measuring the concentration of water in a water-ink emulsion. Although the '493 Patent indicates that a logarithmic amplifier may be excluded for concentrations of water in the 0-20% range, most presses employ water-ink emulsions having water concentrations in the 20-50% range and, thus, in the preferred embodiment described in the '493 Patent, a logarithmic amplifier is employed to linearize the output of the sensor.
In addition to requiring logarithmic amplifiers over much of their operating range, prior art meters have suffered from other disadvantages. For example, some prior art meters have been constructed to include capacitive sensors employing electrodes or plates which deform or move under pressure. Such deformation or movement has resulted in outputs which vary in response to flow rates thereby causing inaccuracies in measurements of the subject variable, namely, water concentration.
The outputs of prior art capacitive sensors have also been effected by the distribution of the components within the measured emulsion. For example, water-ink emulsions having precisely the same components will often have different dielectric constants if those components are distributed differently within the emulsion. As an extreme instance, a water-ink emulsion having a uniform distribution or consistent blend will have a different dielectric constant than a water-ink emulsion stratified into water and ink levels even if the two emulsions have precisely the same concentration of ink and water. This problem is explained in the context of oil and water mixtures in U.S. Pat. No. 4,916,490 to Mougne.
Other disadvantages present in the prior art include susceptibility to stray electromagnetic fields which can interfere with accurate measurements.
SUMMARY OF THE INVENTION
The present invention overcomes the drawbacks associated with the prior art by providing an improved water metering apparatus comprising an improved sensor which produces a substantially linear output for a wide range of water concentrations in a consistent water-ink emulsion without employing a logarithmic amplifier.
In accordance with one aspect of the invention, a water metering apparatus comprising a capacitive sensor coupled to an oscillator acting as a multi-vibrator is provided. The measured emulsion is passed between the opposed plates of the capacitive sensor such that the capacitance of the sensor changes to reflect the water content of the emulsion. The output frequency of the oscillator varies with the capacitance of the sensor. The output frequency of the oscillator is converted by a water concentration converter to a signal representative of the water concentration of the measured emulsion. The water concentration converter develops the water concentration signal in accordance with a substantially linear mapping function having a negative slope wherein, as the water concentration increases, the voltage output by the frequency to voltage converter decreases.
In accordance with a further significant aspect of the invention, a water metering apparatus of the foregoing type is provided wherein the mapping function is substantially linear for water concentrations in the 0-50% range.
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PCTInternational Search Report,Dated Apr. 12, 1998. PCT Application Serial No. PCT/US98/17719. International Filing Date Aug. 26, 1998.
Chou Shem-Mong
Niemiro Thaddeus A.
Orzechowski Thomas W.
Wang Xin Xin
Goss Graphics Systems
Kerveros James C
Marshall O'Toole Gerstein Murray & Borun
Metjahic Safet
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