Geometrical instruments – Gauge – Continuous gauging
Reexamination Certificate
2001-10-10
2003-07-08
Fulton, Christopher W. (Department: 2859)
Geometrical instruments
Gauge
Continuous gauging
C033S501030, C033SDIG001, C033SDIG002
Reexamination Certificate
active
06588118
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to sensors for use with moving sheets or webs of material and, more particularly, to a non-contact sensor system for measuring or sensing properties or characteristics of a substantially continuous sheet or web of paper.
BACKGROUND OF THE INVENTION
Systems for measuring characteristics or properties of moving sheets or webs of material are well known in the art. Typically, these systems employ first and second sensors in the form of sensing heads or shoes positioned on the opposite sides of a passing web. These sensors contain sensitive electronic, radiation or optical detection systems for measuring one or more characteristics of the passing web, such as thickness, opacity, moisture, gloss, smoothness, or other properties. Commonly, the moving web travels in a free gap between two scanning sensor heads and sheet properties are measured via an arrangement with sensing devices in upper and lower heads. In order to measure certain sheet properties, including but not restricted to thickness (also known as caliper), gloss or smoothness, a controlled and close proximity of the sensing heads to the sheet surface is advantageous in order to achieve acceptable accuracy. This can be partially accomplished by pass line control from sheet guide devices attached to the sensor heads, or preferably by using flexible mounts for the system that permit relative sensor movement in at least the vertical plane. An example of such a system and, in particular, one type of sensing head for contacting and measuring characteristics of a passing web of paper, is shown and described in commonly assigned U.S. Pat. No. 5,479,720 to Hellstrom et al., the disclosure of which is incorporated herein by reference.
In order to ensure that the selected characteristics or properties of the web are accurately measured, it often is desirable to position the opposed first and second sensors as close as possible to the web without contacting the web. Also, the first and second sensors must be in close alignment with one another in the web plane to ensure that any measurements taken correspond to substantially the same area of the passing web. This alignment requires careful manual adjustment of the sensing heads as well as costly precision scanning mechanisms, but can still never be fully attained. Simultaneously meeting both requirements is complicated by the fact that the pass line of the web relative to the sensors may rapidly change as a result of events occurring upstream or downstream of the system. Accordingly, not only must the mounting arrangement be capable of securely and reliably holding the sensors in a precisely controlled, spaced, and aligned relationship adjacent to the corresponding side of the web, but it must also be capable of rapidly responding to changes in the pass line. Also, contact or engagement between the sensors must be avoided to prevent the instrumentation held therein from damage, especially when the web of passing material is absent from the feed path.
In the '720 patent, the sensors are designed to make actual physical contact with the passing web. This is possible due to specialized low-friction, wear-resistant contact surfaces formed of ceramic materials. A flexible mounting also ensures that the sensors are not only kept aligned in the web plane, but may also move as necessary in the vertical plane to ensure that the sensing heads can accommodate any rapid changes in the pass line of the web. Despite the advances offered by this solution, each sensor still directly contacts the web of passing material during sensing, which is not the most desirable for sensing or detecting properties of characteristics of certain web materials due to possible disruption or damage of the web at the location of contact.
One method to avoid web contact is to deploy a large free gap between upper and lower heads. This eliminates any sheet contact but typically reduces sensor accuracy since the sheet can flutter anywhere between heads and the sheet may not be flat, or parallel with the gap. Prior art suggests remedies with pass line control devices including rollers, air guides and vacuum plates to hold the moving sheet at a controlled position. This is difficult to accomplish on a fast moving or non-flat sheet and it solves only part of the problem. Examples of devices for sheet pass line control are disclosed in U.S. Pat. Nos. 4,877,485 (Carson); 4,449,398 (Williams); and, 5,654,799 (Chase). These methods have the common disadvantage of controlling the pass line to only one of either the upper or lower heads since the pass line cannot be controlled to both heads simultaneously due to variable head alignment.
In order to achieve non contacting thickness measurement in close proximity with the sheet, others have proposed supplying pressurized air to form a gas bearing between a single sensor or a pair of opposing sensors on one or both sides of a passing web. Usually, the air bearing sensors are supported by fixed or flexible mountings. These mountings create a measurement force against the process in order to balance the repelling force created by the air bearing(s) from moving the sensors to permit accurate measurements. However, in such a passive arrangement, the measurement force must be carefully controlled to ensure that the sensor(s) remain even approximately spaced at the desired distance from the web at all times, or a complex pneumatic or mechanical system is required. Furthermore, since the sheet may have curl, waves and draw wrinkles, it is not possible to apply a sensing force that is always at a normal to the sheet plane in such arrangements. Where two opposed sensors are provided, creating the desired spacing using air bearings with fully articulated mountings would require complex designs.
Non-contacting measurement of sheet thickness using a magnetic measurement system separated by air bearings on one or both sides of the process is known in prior art, for instance as disclosed in U.S. Pat. Nos. 5,243,849 (Williams); 4,528,507 (Williams et al.); 4,647,855 (Berglund); 5,865,059 (Alessandro); 4,292,838 (Larsen); and, 4,107,606 (Typpo). Although these designs eliminate sheet contact on one or both sides of the sheet, the accuracy of sensors using these methods has not been acceptable due to excessive web influence parameters including web flutter, waves and smoothness changes as well as measurement errors caused by head misalignment.
Other non-contacting thickness measurement methods have been suggested including distance measurement across a pair of large free gap sensing heads that measure location of the upper and lower paper surface relative to each sensor head augmented with gap measurement devices for measuring the head to head separation. Examples of methods for optical thickness measurement using this arrangement are disclosed in U.S. Pat. Nos. 5,210,593 (Kramer); 5,805,291 (Calvin, et al.); 5,355,083 (George, et al.); 4,358,960 (Porter); 6,281,679 (King) and, WO00/37885 (King, et al.). An example of ultrasonic thickness measurement using this arrangement is disclosed in U.S. Pat. No. 5,113,358 (Reber). The prior art involves sheet surface location sensing devices spaced by a certain large distance from the sheet, to maintain a safe separation for no contact with the process, in conjunction with magnetic gap measurement. The large separation distance presents a challenge to measure a large dimension accurately in order to estimate a sheet thickness value that is much smaller than this distance. Product quality requirements for fabrication of many paper products demand measurement errors no larger than one half micron (0.5×10
−6
meter) at any point across the web. This has to be fulfilled despite severe environmental conditions, a scanning device with certain mechanical errors between upper and lower heads, plus a process with variable pass line, curl and sheet surface conditions. Thus, in principle, these methods produce a non-contacting thickness measurement; but in practice, they never have achieved accep
ABB Inc.
Fulton Christopher W.
Stevens & Showalter LLP
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