Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet
Reexamination Certificate
2002-07-03
2004-07-06
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
With circuit for evaluating a web, strand, strip, or sheet
C250S559290
Reexamination Certificate
active
06759671
ABSTRACT:
The invention relates to a method of measuring movement of a material sheet and a sheet sensor relative to each other along at least one measuring axis, which method comprises the steps of:
illuminating the sheet with a measuring laser beam for each measuring axis, and
converting a selected portion of the measuring beam radiation reflected by the sheet into an electrical signal, which is representative of the movement along said measuring axis.
The invention also relates to a sheet sensor for performing this method and to sheet processing apparatus comprising such a sensor.
Such a method and sheet sensor can be used in an apparatus wherein a material sheet, for example a sheet of paper, is fed through the apparatus in order to undergo processing, for example printing of information or scanning of information printed on a sheet. Such an apparatus may be, for example a (colour) printer, a copier, a document scanner or a facsimile apparatus. A sheet sensor is understood to mean a device by means of which, for example, the velocity of movement of a sheet or the position of a specific area of this sheet can be measured, or the quality of a print can be monitored. In printers, copiers and facsimile apparatus one or more rollers are use to move a paper through the apparatus. Currently, paper movement control, also called paper feed control, in such apparatus is carried out by controlling the movement of a roller. Thereby it is assumed that the movement of the roller exactly determines movement of the paper sheet. However, in practice slippage between the sheet and the roller can not be excluded so that it is not assured the movement of the roller always represents the movement of the sheet. Moreover a fine control of the roller movement, needed for a fine control of the paper feed, requires a very precise control of the motor, which drives the roller. Such a precise control is expensive, which is prohibitive for mass applications. For example, for high-resolution printing such as in a next generation colour printer, the movement of the paper sheet has to be controlled and thus measured with a precision of about 10 &mgr;m. Even a very precise mechanical paper feed does not allow such fine control.
The paper feed control could be made more accurate and the requirements for the roller motor control could be lessened considerably by measuring the sheet movement by a beam of optical radiation, which is a contact-less measuring. Devices for optical measurements known so far are intended for small-scale applications and are quite expensive.
It is an object of the invention to provide a method as described in the opening paragraph, which can be carried out with simple means and shows a very high accuracy and liability. This method is characterized in that measuring beam radiation reflected back along the measuring beam and re-entering the laser cavity, which emits the measuring beam, is selected and in that changes in operation of the laser cavity, which are due to interference of the re-entering radiation and the optical wave in the laser cavity and are representative of the movement, are measured.
The new method uses the so-called self-mixing effect in a diode laser. This is the phenomenon that radiation emitted by a diode laser and re-entering the cavity of the diode laser induces a variation in the gain of the laser and thus in the radiation emitted by the laser. The sheet to be measured and the sheet sensor are arranged relative to each other such that the direction of movement has a component in the direction of the laser beam. Upon movement of the sheet relative to the sheet sensor, the radiation reflected and scattered by the sheet gets a frequency different from the frequency of the radiation illuminating the sheet, because of the Doppler effect. Part of the scattered light is focused on the diode laser by the same lens that focuses the illumination beam on the object. Because some of the scattered radiation enters the laser cavity through the laser mirror, interference of light takes place in the laser. This gives rise to fundamental changes in the properties of the laser and the emitted radiation. Parameters, which change due to the self-coupling effect, are the power, the frequency and the line width of the laser radiation and the laser threshold gain. The result of the interference in the laser cavity is a fluctuation of the values of these parameters with a frequency that is equal to the difference of the two radiation frequencies. This difference is proportional to the velocity of the sheet. Thus the velocity of the sheet and, by integrating over time, the displacement of the sheet can be determined by measuring the value of one of said parameters. This method can be carried out with only a few and simple components and does not require accurate alignment of these components.
The use of the self-mixing effect for measuring velocities of objects, or in general solids and fluids, is known per se. By way of example, reference is made to the article: “Small laser Doppler velocimeter based on the self-mixing effect in a diode laser” in Applied Optics, Vol. 27, No. 2, Jan. 15, 1988, pages 379-385, and the article: “Laser Doppler velocimeter based on the self-mixing effect in a fibre-coupled semiconductor laser: theory” in Applied Optics, Vol. 31, No. 8, Jun. 20, 1992, pages 3401-3408. However, up to now, use of the self-mixing effect for measuring the movement of a sheet in a direction at an acute angle with the laser beam has not been suggested. This new application is based on the recognition that a sheet sensor using the self-coupling effect can be made so small and cheap that it can be installed easily and without much additional cost in existing devices and apparatus.
In order to detect the direction of movement, i.e. to detect whether the sheet moves forward or backward along a measuring axis, the method may be characterized in that the direction of movement along said at least one measuring axes is detected by determining the shape of the signal which represents the variation in operation of the laser cavity.
This signal is an asymmetric signal and the asymmetry for a forward movement is different from the asymmetry for a backward movement.
Under circumstances, where it is difficult to determine the asymmetry of the self-mixing signal, preferably another method is used. This method is characterized in that the direction of movement along said at least one measuring axis is determined by supplying the laser cavity with a periodically varying electric current and comparing first and second measuring signals with each other, which first and second measuring signals are generated during alternating first half periods and second half periods, respectively.
The wavelength of the radiation emitted by a diode laser increases, and thus the frequency of this radiation decreases, with increasing temperature, thus with increasing current through the diode laser. A periodically varying current through the diode laser in combination with radiation from the sheet re-entering the laser cavity results in a number of radiation pulses per half period and thus in a corresponding number of pulses in the measured signal. If there is no relative movement of the sheet and the movement sensor, the number of signal pulses is the same in each half period. If the device and the object move relative to each other, the number of pulses in one half period is larger or smaller than the number of pulses in the next half period, depending on the direction of movement. By comparing the signal measured during one half period with the signal measured during the next half period, not only the velocity of the movement but also the direction of the movement can be determined.
This method may be further characterized in that the first and second measuring signals are subtracted from each other.
The changes in the operation of the laser cavity can be determined in several ways.
A first embodiment of the method is characterized in that the impedance of the diode laser cavity is measured.
The impedance of the laser di
Liess Martin Dieter
Mimnagh-Kelleher Gillian Antoinette
Koninklijke Philips Electronics , N.V.
Le Que T.
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