Abstract:
A non-contact system for sensing a property of a passing web of material includes first and second sensors positioned on opposite sides of the web. The opposing sensors are repelled from the web and each other by gas bearings formed between the sensors and the web and attracted to each other by one or more polarized magnets embedded in the sensors. The resultant net force properly spaces the sensors from the web to ensure reliable and accurate sensing. The repelling force enables the sensors to react substantially instantaneously to changes in the pass line of the web. An automatic actuator/retractor can be provided to withdraw the sensors extend the sensors to or retract the sensors from adjacent the feed path.

Description:
TECHNICAL FIELD  
         [0001]    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  
         [0002]    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.  
           [0003]    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.  
           [0004]    In the &#39;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.  
           [0005]    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.  
           [0006]    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.  
           [0007]    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.  
           [0008]    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 acceptable measurement accuracy in typical paper industry applications.  
           [0009]    On-line measurement of optical surface properties, like gloss or smoothness, on one or both sides of paper webs can have measurement errors introduced by sheet flutter, non-flat sheets, sensor head deflections or vibrations. Examples of prior art gloss measurement are disclosed in U.S. Pat. Nos. 6,233,053 (Preston); 6,031,620 (Typpo); and 4,830,504 (Frohardt). An example of prior art on-line smoothness measurement is disclosed in U.S. Pat. No. 5,654,799 (Chase, et al.). The main unresolved problem in this prior art is the inability to simultaneously control a moving web relative to measurement heads on each side of the web, since each head is subject to deflection errors and sheet flutter. Another problem is introduced by the large free gap necessary for a safe non-contacting web passage. These problems makes it difficult to construct a physically small sensor with a narrow measurement area, since there is a general scaling rule that dictates the size of the optical system in relation to the maximum free distance to the web. Such constraints have limited the accuracy and practicality of on-line measurement of gloss and smoothness as well as additional sheet properties including formation, brightness, opacity, color, basis weight and moisture.  
           [0010]    Accordingly, a need exists for a web material property sensing platform in which opposed first and second non-contact sensors remain aligned in the web plane and evenly spaced in close proximity to a web of passing material at all times, even when the pass line or sheet curl changes rapidly, and a method for achieving accurate measurement of thickness, optical and other sheet properties by means of integrating magnetic, optical or other sensing elements in this platform with a close proximity to, but not contacting, the process surface.  
         SUMMARY OF THE INVENTION  
         [0011]    This need is met by the invention of the present application wherein a non-contact, sheet sensing system employs first and second sensors positioned on opposite sides of a passing web with the sensors being simultaneously repelled from the passing web and attracted to each other. The resultant net force keeps the sensors closely and evenly spaced from the passing web to ensure that reliable and precise sensing or detecting functions are provided. Since the sensors are repelled from the web, they react substantially instantaneously to any changes in the pass line or curling of the web as it moves along the feed path while the attractive force keeps any relative tilting or movement of the sensors in check. In one embodiment of the invention, an automatic actuator/retractor retracts the sensors to ensure that the sensors do not contact each other when the web is absent from the feed path, and may also be used to withdraw the sensors from adjacent the web as necessary. The sensing system results in a great improvement over prior art efforts to provide non-contact sensors for a passing sheet or web of material, especially in terms of operational reliability and accuracy.  
           [0012]    Thus, the sensors of the present application enable non-contact sensing of surface properties on one or both sides of the web, as well as reflective/transmissive properties of the web by sensing elements located within the sensors in close and controlled proximity to the web surfaces. Web properties that can be measured by sensing elements in the sensors may include, but are not restricted to, caliper, surface smoothness, gloss, brightness, opacity and formation.  
           [0013]    In accordance with a first aspect of the present invention, a non-contact system for sensing or measuring a property or characteristic of a sheet or web of material moving along a feed path comprises a first sensor positioned adjacent a first side of the feed path. The first sensor includes a passage for receiving pressurized gas and directing the gas toward the web when it is present in the feed path. A second sensor is positioned adjacent a second side of the feed path opposing the first sensor. The second sensor also includes a passage for receiving pressurized gas and directing the gas toward the web in the feed path. At least one magnet is mounted in each of the first and second sensors, with the magnet of the first sensor being aligned with and attracted to the magnet of the second sensor and forming a magnetic coupling. This magnetic coupling urges the sensors toward the corresponding side of the web and one another, with the gas directed toward the web from the first and second sensors forming gas bearings that simultaneously urge the sensors away from the web. As a result, a net force generated by the magnets and the gas bearings keeps the sensors substantially evenly spaced from the web passing along the feed path, thereby ensuring accurate sensing and measuring of the desired properties or characteristics of the web.  
           [0014]    To permit relative movement of the sensors in the vertical plane, a first flexible mount is provided for the first sensor and a second flexible mount is provided for the second sensor. This relative movement capability ensures that the first and second sensors may remain substantially parallel to the passing web of material at all times, even during changes in the pass line and the process itself. At least one of the first and second flexible mounts may also permit movement of the associated sensor in the horizontal plane. However, as should be appreciated, the associated sensor is prevented from moving any significant distance in this plane relative to the other sensor when the first and second sensors are magnetically coupled to one another.  
           [0015]    The magnet of the first sensor may comprise a first plurality of magnets that align with and correspond to a second plurality of magnets comprising the magnet in the second sensor, with north and south poles facing each other to create an attraction force.  
           [0016]    The sensing system may also include an actuator associated with at least one of the first and second sensors. In operation, each actuator enables or urges the associated sensor to move toward the feed path and the opposite sensor. Preferably, first and second actuators are associated with the first and second sensors, respectively, with the first and second actuators together enabling or urging the first and second sensors to move toward one another.  
           [0017]    In one possible embodiment, each actuator comprises a pneumatic cylinder including a plunger having a first head for engaging the corresponding sensor. When each of the cylinders is pressurized, the corresponding plunger moves the sensor associated therewith toward the feed path and the opposite sensor. Each plunger further includes a second head disposed in the pneumatic cylinder for engaging a spring held therein. Upon de-pressurizing the pneumatic cylinders, the springs bias the corresponding plungers away from the web and thereby retract the corresponding sensors. Thus, each sensor “floats” on the first head of the corresponding plunger upon actuation as a result of the net force created by the combination of the attractive force supplied by the magnetic coupling and the repelling force created by the gas bearings, as well as the flexible mountings that permit the sensors to move up and down in the vertical plane and to tilt. Preferably, each pneumatic cylinder is in communication with and activated by the pressurized gas also used to form the gas bearings. Accordingly, the first and second sensors are automatically retracted upon a loss of pressure in the pressurized gas.  
           [0018]    In accordance with a second aspect of the present invention, a non-contact system for sensing or measuring a property or characteristic of a web of material moving along a feed path is provided. The system comprises first and second sensors positioned on opposite sides of the feed path. Each of the first and second sensors includes at least one magnet, an inlet for receiving a pressurized gas, and at least one outlet for directing pressurized gas toward the feed path to create first and second gas bearings for the web of material when present. The at least one magnet of the first sensor is aligned with and attracted to the at least one magnet of the second sensor by magnetic coupling, which in turn aligns and urges the first and second sensors toward one another and the web. However, the gas directed from each of the first and second sensors to form the first and second gas bearings simultaneously urges the sensors away from the web and one another so that the net force keeps the sensors closely and substantially evenly spaced from the web moving along the feed path.  
           [0019]    In one embodiment, the sensor system also comprises a first retractor for engaging and moving a first sensor away from a second sensor and the feed path defined between the first and second sensors, and a second retractor for engaging and moving the second sensor away from the first sensor and the feed path. Each of the first and second retractors comprises a cylinder including a pressure-activated plunger having a first head for engaging the corresponding sensor and a second head disposed in the cylinder for engaging a spring held therein. Thus, upon relieving the pressure in each cylinder, the spring biases the second head of the plunger such that the first head engages and moves the corresponding sensor away from the feed path. Preferably, the cylinders of the first and second retractors comprise pneumatic cylinders, but the use of other equivalent arrangements is of course possible.  
           [0020]    In accordance with a third aspect of the present invention, first and second sensors are provided for use in an overall system for measuring or sensing a property or characteristic of a web of material moving along a feed path. Each sensor comprises a sensor head including at least one magnet, an inlet for receiving pressurized gas, and at least one outlet for issuing the pressurized gas towards a passing web of material to form a gas bearing. The first and second opposed sensors are simultaneously attracted and aligned by the magnets and repelled by the gas bearings to keep the first and second opposed sensors evenly spaced from the passing web.  
           [0021]    In accordance with a fourth aspect of the present invention, a method of sensing or measuring a property or characteristic of a web of material moving along a feed path is provided. The method comprises positioning first and second opposed sensors adjacent to and on opposite sides of the feed path. Each of the sensors includes a magnet. Together, these magnets form a magnetic coupling that serves to align and draw the sensors toward one another and the web of material passing through the feed path. The method also includes the step of pressurizing passages in the sensors to form a fluid bearing on either side of the web so that net magnetic and bearing forces keep the sensors closely and substantially evenly spaced from the passing web. The method may also include the step of retracting the sensors when the web of material is not to be sensed.  
           [0022]    In accordance with a fifth aspect of the present invention, a method of sensing or measuring a property or characteristic of a web of material moving along a feed path comprises positioning a first sensor on one side of a web of material in the feed path and positioning a second sensor on a second side of the web of material. The first and second sensors are drawn toward one another and the web of material with a magnetizing force, and repelled from one another and the web with a repelling force. The magnetizing force and repelling force are controlled to maintain the first and second sensors closely and evenly spaced from the passing web.  
           [0023]    In one embodiment, repelling the first and second sensors from one another and the web comprises forming gas bearings in the first and second sensors and applying pressurized gas to the gas bearings. Also, drawing the first and second sensors toward one another and the web of material comprises mounting at least one magnet on the first sensor and mounting at least one magnet on the second sensor, wherein the magnet on the first sensor and the magnet on the second sensor are polarized to attract one another. Preferably, the first and second sensors have a substantially flat surface facing the feed path, and the at least one magnet of the first sensor is recessed below the substantially flat surface of the first sensor, while the at least one magnet of the second sensor is recessed below the substantially flat surface of the second sensor.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 a  is a partially sectioned schematic side view of one possible embodiment of the sensing system of the present invention;  
         [0025]    [0025]FIG. 1  b  is a partially sectioned schematic side view of the opposing sensors of the system of FIG. 1 a  on an enlarged scale;  
         [0026]    [0026]FIG. 1 c  is a perspective view of one sensor used in the embodiment of FIG. 1 a  and the corresponding mount that allows it to move in the vertical plane and to tilt;  
         [0027]    [0027]FIG. 2 a  is an enlarged perspective view of an alternate embodiment of the sensor of FIG. 1 a  wherein the magnet is comprised of a plurality of magnets, and each one of these magnets are oriented similar to the arrangement in FIGS. 1 a - 1   c ;  
         [0028]    [0028]FIG. 2 b  is a partially sectioned schematic front view of the opposing pair of sensors of the alternate embodiment shown in FIG. 2 a ;  
         [0029]    [0029]FIG. 2 c  is an enlarged perspective view of yet another alternate embodiment of the sensor of FIG. 1 a  wherein the magnet is comprised of a plurality of magnets oriented with their magnetic axes parallel to the face of the sensor;  
         [0030]    [0030]FIG. 2 d  is a partially sectioned schematic side view of the opposing pair of sensors of the alternate embodiment shown in FIG. 2 c ;  
         [0031]    [0031]FIG. 3 is a partially sectioned schematic side view of another embodiment of the sensing system of the present invention, including actuators/retractors for selectively moving the sensors away from the web and each other;  
         [0032]    [0032]FIG. 4 is a view similar to FIG. 3 but showing the sensors moved into sensing position adjacent the web by the actuators/retractors;  
         [0033]    [0033]FIG. 5 a  is a perspective view of a set of opposing sensors similar to those of FIG. 2 a  but including magnetic distance measuring devices in the two sensors in order to measure the sheet thickness;  
         [0034]    [0034]FIG. 5 b  is a partially sectioned frontal view of the opposing pair of sensors of the alternate embodiment shown in FIG. 5 a ;  
         [0035]    [0035]FIG. 6 a  is a perspective view of a set of opposing sensors similar to those of FIG. 2 a  but including a magnetic distance measuring system and an optical system for measuring the height of the gas bearings in the two sensors in order to more accurately measure the sheet thickness;  
         [0036]    [0036]FIG. 6 b  is a partially sectioned front view of the opposing pair of sensors of the alternate embodiment shown in FIG. 6 a ;  
         [0037]    [0037]FIG. 7 a  is a perspective view of a set of opposing sensors similar to those of FIG. 2 a  but including an optical measurement system in each head to measure gloss of the sheet surfaces; and  
         [0038]    [0038]FIG. 7 b  is a partially sectioned front view of the opposing pair of sensors of the alternate embodiment shown in FIG. 7 a .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0039]    Reference is now made to FIG. 1 a , which illustrates one possible embodiment of the sensing system  10  of the present invention. The system  10  includes first and second sensors  14 ,  16  in the form of sensor heads or shoes. The first and second sensors  14 ,  16  are positioned in an opposing relationship along a feed path of a moving sheet or web of material, such as a web of paper W or the like. As described in commonly assigned U.S. Pat. No. 5,479,720, the sensors  14 ,  16  usually are mounted on transverse beams of a scanner (not shown) and are driven to and from across the width of the passing web W in substantial alignment with each other in the horizontal plane. As is well known in the art, the web of paper W is initially formed in an upstream portion of a paper making machine (not shown) and progresses to downstream processing equipment including calendaring rolls and the like for further processing. The direction of web travel is shown by arrow A in FIG. 1 a.    
         [0040]    In this embodiment, each of the first and second sensors  14 ,  16  comprises a cylindrical body or housing including a substantially flat face  14   a ,  16   a  for facing the web W, an annular sidewall  14   b ,  16   b , and a connecting wall  14   c ,  16   c  opposing the face (see FIG. 1 b ). Together, these walls define a first chamber C 1  for, among other things, housing the instrumentation (not shown) used to detect or sense desired properties or characteristics of the web W, such as for example, magnetic sensors or optical sensors.  
         [0041]    The housing for each of the first and second sensors  14 ,  16  also includes a passage  18  for receiving pressurized gas, such as air, and introducing the pressurized gas into the chamber C 1 . Preferably, the passage  18  is formed in the connecting wall  14   c ,  16   c  of the first and second sensors  14 ,  16 . The pressurized gas for the first and second sensors  14 ,  16  may be supplied by separate gas sources G 1 , G 2 , as illustrated schematically in FIG. 1 a , or by a single source (see FIG. 3). The pressurized gas is conveyed by a tube that can be any appropriate form of gas conduit or line L that is sufficiently flexible so that it does not interfere with the relative movement of the corresponding sensors  14 ,  16 .  
         [0042]    At least one outlet is formed in the substantially flat face  14   a ,  16   a  of each of the first and second sensors  14 ,  16 , with a plurality of outlets  20  being formed in the illustrated embodiments. The plurality of outlets  20  in each of the sensors  14 ,  16  are arranged in a circular pattern in the embodiment illustrated in FIG. 2 a . The outlets  20  allow pressurized gas supplied to the chamber C 1  in each of the sensors  14 ,  16  to issue toward the passing web W when present to form relatively narrow first and second gas bearings  22 ,  24  (see FIG. 1 b ). As should be appreciated, the gas bearings  22 ,  24  create a repelling force against the faces  14   a ,  16   a  of the sensors  14 ,  16  that prevents the faces  14   a ,  16   a  from contacting the passing web W. Of course, since gas bearings  22 ,  24  are employed, only negligible friction or resistance is created on or against the passing web W.  
         [0043]    Each of the first and second sensors  14 ,  16  also carries at least one magnet  26 . It is currently preferred to make each magnet  26  a rare earth magnet recessed in the corresponding substantially flat face  14   a ,  16   a  of each sensor  14 ,  16  (see FIG. 1 b ). Recessing the magnets  26  advantageously reduces the effects of the highly non-linear force created when a very small or no gap is present between the opposing first and second sensors  14 ,  16 . In the embodiment of FIGS. 1 a  and  1   b , the opposing magnets  26  are polarized to attract each other (that is, the corresponding ends of the magnets  26  have opposite polarities) and are positioned in alignment along a vertical center axis of magnetism such that both attractive and aligning forces are created therebetween. Thus, when the sensors  14 ,  16  are brought into close proximity to each other and adjacent to the corresponding side of the web W, a magnetic coupling is formed between the magnets  26 . This magnetic coupling serves to not only draw the sensors  14 ,  16  toward one another and the passing web W, but the attractive force also causes the sensors to substantially align with one another in the horizontal plane due to the negligible friction of the gas bearings.  
         [0044]    Although a single magnet  26  is illustrated for each sensor  14 ,  16  in FIGS. 1 a ,  1   b , and  1   c , it should be appreciated that other magnet arrangements are possible. For example, instead of placing a single magnet  26  in the center of each of the sensors  14 ,  16 , one or more pairs of magnets may be provided and spaced symmetrically from the center of the sensors  14 ,  16 . An example illustrating two pairs of magnets  25   a - 25   d  paired respectively with  27   a - 27   d  is illustrated in FIGS. 2 a  and  2   b . By symmetrically spacing one or more pairs of attracting magnets, space is created within the chambers C 1  of the sensors  14 ,  16  for receiving instrumentation placed at or near the center of the sensors  14 ,  16 . Flat magnets polarized parallel to the sensor flat faces  14   a ,  16   a  may also be employed to improve magnetic efficiency and to increase the overall space available in the chambers C 1  in the first and second sensors  14 ,  16  for instrumentation or the like. While one or more pairs of flat magnets may be used in the present invention, two pairs of flat magnets  25   a′ ,  27   a′ ;  25   b′ ,  27   b′ are illustrated in FIGS. 2 c  and  2   d . Other magnet arrangements will be suggested to those of skill in the art from the disclosure of the present application. The magnets can consist of permanent magnets or electromagnets. Permanent super magnets made from rare earth elements or similar compounds are advantageous in order to provide a high force to weight ratio of the magnets and to enable small dimensions.  
         [0045]    In operation, the repelling force resulting from the gas bearings  22 ,  24  formed by the pressurized gas issuing from the outlets  20  counterbalances the attractive force created by the magnets  25 - 27  in each of the first and second sensors  14 ,  16  so that the sensors  14 ,  16  are kept from contacting the respective side of the passing web W. The system is controlled so that the resultant net force generated by the attracting force of the magnets  26  and the repelling force created by the gas bearings  22 ,  24  keeps the sensors  14 ,  16  substantially evenly spaced from the web W as it passes along the feed path. This even spacing assists in ensuring that accurate sensing and measuring of the desired properties or characteristics of the web W is achieved. Of course, the gaps between the faces  14   a ,  16   a  of the sensors  14 ,  16  and the corresponding sides of the web W can be adjusted by increasing or decreasing the attractive force created by the magnets  26  or the repelling force created by the gas bearings  22 ,  24 , as necessary or desired for a particular type of sensor.  
         [0046]    Often, the pass line of the web W changes rapidly and unpredictably as the result of events occurring upstream or downstream from the location where the sensors  14 ,  16  are positioned, or because of variations in the thickness of the web W. As should be appreciated, since the gas bearings  22 ,  24  are formed between the opposite surfaces of the web W, the sensors  14 ,  16  respond to even small changes in the pass line. In other words, when the position of the web W in the vertical plane changes as the feed path varies, each sensor  14  or  16  automatically moves or tilts accordingly and in unison as a result of the bearing action provided by the gas bearing  22  or  24 . At the same time, the magnetic coupling between the magnets  26  keeps the sensors  14 ,  16  substantially evenly spaced from the web W, as well as aligned in the horizontal plane in cases where the sensor mounts permit horizontal movement (see below).  
         [0047]    To allow the sensors  14 ,  16  to react to changes in the vertical position of the web W, including “curling” on the lateral side edges or “tilting” along the sides or midsection, at least one flexible mount  28  is provided. As illustrated schematically in FIG. 1, and perhaps best shown in the perspective view of FIG. 1 c , the mount  28  may be a relatively thin sheet of a resilient, polymeric material, such as Kevlar® fabric. The mount  28  has a first end  28   a  that is secured to the sensor  14  or  16  and a second end  28   b  for attachment to support structure S, such as by using conventional fasteners (not shown). As a result of the resiliency of the material forming the mount  28 , the sensors  14 ,  16  are capable of moving in the vertical plane in response to similar changes in the pass line of the web W. The movement of the sensors  14 ,  16  can include tilting in the cross-machine direction as necessary as the mount  28  can twist to accommodate such tilting movement.  
         [0048]    Separate mounts  28  are provided for carrying each of the first and second sensors  14 ,  16 . These mounts may be the same as described above or, as shown schematically in FIG. 1, one or both of the mounts may include a gimbal  29  or other mechanism that permits movement of the corresponding sensor in the horizontal plane as well as in the vertical plane. Also, it is within the broadest aspects of the invention of the present application to provide a stationary or fixed mount for one of the sensors  14  or  16  in combination with a flexible mount  28  of the type shown and described or otherwise for the opposite sensor for applications where the changes in the vertical position or pass line of the web W are small or substantially non-existent.  
         [0049]    As briefly mentioned above, instead of providing a single magnet  26  in each of the first and second sensors  14 ,  16 , a plurality of magnets may be provided in each of the sensors  14 ,  16 . An example of one possible arrangement of magnets is shown in FIGS. 2 a  and  2   b . More specifically, the magnet  26  of the first sensor  14  may be supplemented or replaced by a first plurality of magnets  25   a  . . .  25   n  ( 25   a - 25   d  illustrated) in the first sensor  14  that align with and correspond to a second plurality of magnets  27   a  . . .  27   n  ( 27   a - 27   d  illustrated) in the opposing second sensor  16 . The magnets are polarized so as to attract each other. As a result of this polarization, the magnets  25   a  . . .  25   n ,  27   a  . . .  27   n  attract one another to provide or assist in providing the desired net force to keep the sensors  14 ,  16  appropriately spaced from each other. The plurality of magnets  25   a  . . .  25   n ;  27   a  . . .  27   n  in each first and second sensor  14 ,  16  are preferably arranged in a symmetrical pattern to create a magnetic force that is balanced around the sensor center axis (see the sensor  16  and magnets  27   a  . . .  27   d  in FIG. 2 a ).  
         [0050]    The magnetic orientation axes of the magnets  25   a  . . .  25   n  and  27   a  . . .  27   n  in the first and second sensors  14 ,  16  does not need to be perpendicular to the faces  14   a ,  16   a  of the sensors  14 ,  16  facing the web W to provide a suitable attractive force between the sensors  14 ,  16 . One arrangement utilizing two pairs of magnets magnetized in the plane of the flat sensor area consists of magnet bars  25   a′ ( 25   b′ , not shown) in one sensor head  14 , and magnet bars  27   a′ ,  27   b′  in the opposing sensor head  16 , as shown in FIGS. 2 c  and  2   d . The magnetic poles are reversed in the opposing magnets of the two sensor heads  14 ,  16  in order to provide an attractive force. The magnet bars  25   a′ ,  25   b′ ,  27   a′ ,  27   b′  offers the advantage of compact size. Further, magnetic efficiency is improved since stray magnetic fields are reduced due to the magnets  25   a′ ,  25   b′  and  27   a′ ,  27   b′  facing the opposing magnet with both poles.  
         [0051]    To facilitate machine start-up and off-web access to the sensors  14 ,  16 , a second embodiment of the sensor system  10  includes an actuator/retractor  30  for one or both of the sensors  14 ,  16 . As the name suggests, the actuator/retractor  30  actuates one or both of the sensors  14 ,  16  by moving the sensor(s) toward the web W or pass line when sensing is to be performed and by retracting the sensor(s) away from the web or pass line otherwise. In this way, contact between the sensors  14 ,  16  is advantageously avoided, as is the concomitant damage that may result from such contact.  
         [0052]    In one embodiment, as illustrated schematically in FIG. 3, both of the sensors  14 ,  16  are provided with an actuator/retractor  30 , sometimes also known in the art as a “liftoff” mechanism, although it is within the broadest aspects of the invention of the present application to provide an actuator/retractor  30  for only one of the sensors. Each actuator/retractor  30  includes a pressure-activated cylinder  32  housing a plunger  34 . The plunger  34  has a first oversized head  36  for engaging the corresponding sensor  14  or  16  and a second oversized head  38  positioned in the cylinder  32  to engage a biasing means carried in the cylinder  32 , such as a helical compression spring  40 . The spring force constant of each spring  40  is greater than the attractive force between the magnets  26  at zero gap of the sensors  14 ,  16 . Each cylinder  32  also includes an inlet  39  for receiving a pressurized gas, such as air. The pressurized gas acts against the second head  38  to actuate the corresponding plunger  34  and overcome the biasing force supplied by the associated spring  40 . Separate gas sources G 1 , G 2  may be used to supply the gas to each cylinder  32 , or as shown in FIG. 3, a common gas source G may be provided for both of the cylinders  32  as well as for supplying the gas forming the gas bearings  22 ,  24 , see below. Instead of a gas, it should be appreciated that the closed nature of the cylinders  32   a ,  32   b  also permits pressurized liquids to be used as well.  
         [0053]    When each of the cylinders  32  is pressurized, as shown in FIG. 4, the corresponding plunger  34  engages and moves the sensor  14  or  16  associated therewith toward the feed path, the web W, and of course the opposite sensor (see action arrows D). Thus, when the web of paper W or other material is to be sensed, the cylinder  32  of each actuator/retractor  30  is pressurized to actuate the sensors  14 ,  16 . Pressurized gas is supplied to the first and second sensor  14 ,  16  to create the gas bearings  22 ,  24  as the magnetic coupling increases with reduction of the gap between the sensors  14 ,  16 . The sensing function is then activated in accordance with the sensing equipment included with the sensors  14 ,  16 .  
         [0054]    When the sensors  14 ,  16  are to be retracted or “lifted-off” the web W, the cylinders  32  are de-pressurized. Upon de-pressurization of the cylinders  32 , the springs  40  bias the corresponding plungers  34  away from the web W. As a result of this biasing force, the sensors  14 ,  16  are retracted from the web W and away from each other such that the magnetic coupling is greatly reduced.  
         [0055]    In the illustrated embodiment of the actuators/retractors, as shown in FIGS. 3 and 4, each sensor  14 ,  16  is carried on the first head  36  of the corresponding plunger  34  by a detachable mounting. The detachable mounting allows the sensors  14 ,  16  to “float” in at least the vertical plane, and to some extent in the horizontal plane depending on the type of mount that is used to support the sensors  14 ,  16 , such as the mount  28 . More specifically, the oversized head  36  of the plunger  34  extends through an aperture  35  in each sensor  14 ,  16  which is smaller than the head  36 . As a result of this arrangement, the head  36  is captured in a cylindrical second chamber C 2  defined in the sensors  14 ,  16  adjacent to the first chamber C 1  (see FIGS. 3 and 4).  
         [0056]    In the retracted position, as shown in FIG. 3, the movement of the plungers  34  away from the web W as a result of the biasing force supplied by the springs  40  also moves the sensors  14 ,  16  away from the web W. However, when the cylinder  32  is pressurized, as shown in FIG. 4 and indicated by vertical action arrows D, the plunger  34  moves toward the web W. In the case of the sensor  14 , this motion allows it to move toward one side of the web W, while the sensor  16  is engaged by the plunger head  36  and moved against gravity toward the opposite side of the web W. The pressurized gas is supplied to the sensors  14 ,  16  to create the repelling force resulting from the gas bearings  22 ,  24  to thereby not only prevent the sensors  14 ,  16  from contacting the web W, but also to move them out of engagement with the head  36  of each plunger  34  (see action arrows E). As a result, small gaps F are created between the plunger heads  36  and the sensors  14 ,  16 , which thus “float” above the web W according to the net force resulting from the simultaneous attractive magnetic forces and the repelling bearing forces.  
         [0057]    Depending on the relative size of the apertures  35 , the sensors  14 ,  16  may also move slightly from front-to-back and side-to-side in the horizontal plane. The mounts  28  resist this movement and keep the sensors  14 ,  16  in the illustrated embodiment generally aligned over the same area of the passing web W at all times. As should be appreciated, one of the sensors may not need a mount since the magnetic coupling automatically keeps it aligned with the opposite sensor in the horizontal plane.  
         [0058]    In the illustrated embodiment, each pneumatic cylinder  32  is in communication with and activated by a single source of pressurized gas G that also supplies the gas used to form the gas bearings  22 ,  24  (see the dashed line in FIG. 3 connecting the partially cutaway supply lines with the gas source G). This arrangement is particularly advantageous, since the first and second sensors  14 ,  16  are automatically retracted upon a loss of pressure in the pressurized gas.  
         [0059]    In order to provide a non-contacting measurement of web material properties, various sensing elements are included inside the chambers C 1  of the first and second sensors  14 ,  16 . FIGS. 5 a  and  5   b  show an embodiment that provides a non-contacting web thickness measurement. FIG. 5 a  is a perspective view and FIG. 5 b  is a frontal view the first and second sensors  14 ,  16  with the addition of magnetic sensing elements comprising a ferrite C-core  41  and an inductor coil  42  the second sensor  16 , and a ferrite target  43  in the opposing first sensor  14 . These sensing elements and associated methods for thickness measurement are known from prior art, for example by U.S. Pat. No. 5,479,720. The sheet thickness t can be calculated from the distance d between the sensors  14 ,  16  provided that the height of gas bearings  22 ,  24  remain constant for a given web thickness. The embodiment shown in FIGS. 5 a  and  5   b  can be utilized for a simple and low cost non-contacting thickness measurement system, with acceptable accuracy mainly for products that are considerably thicker than the gas bearing heights, for example cardboard.  
         [0060]    In order to provide thickness measurement with improved accuracy making the measurement system acceptable for use in producing a large variety of web materials including printing paper and tissue products, additional refinements of the invention will now be described. FIG. 6 a  shows a perspective view and FIG. 6 b  shows a cross section of the sensors  14 ,  16  of FIG. 2 with added magnetic sensing devices as well as supplemental gas bearing height sensing elements for non-contacting thickness measurement of the sheet W.  
         [0061]    The magnetic sensing elements may comprise a first induction coil  46  in the sensor  14  and a second induction coil  47  in the opposing sensor  16  and with wires  48  connected to suitable electronics to measure the gap size d by means of magnetic induction coupling. This principle is well known from prior art. However, due to the self-centering properties of the sensors  14 ,  16  by the magnet pairs  25 ,  27  and the low friction gas bearings  22 ,  24 , the induction coils  46 ,  47  in the invention will always remain coaxially aligned to simplify the induction coil design and enhance measurement accuracy.  
         [0062]    The supplemental sensing elements  44 ,  45  in FIGS. 6 a  and  6   b  are included in order to measure the height h 1  of gas bearing  22  and the height h 2  of gas bearing  24 , in addition to the magnetic measurement of the total gap size d. The sheet thickness t is calculated by the wellknown expression: 
           t=d− ( h   1   +h   2 ) 
         [0063]    The distance sensing elements  44 ,  45  can be mounted inside or outside of the induction coils  46 ,  47  and may include different known sensors including fiber optics displacement sensors, laser triangulators and ultrasonic devices. However, the invention achieves a closer and controlled proximity of the sensors  14 ,  16  to the process due to the gas bearings  22 ,  24  riding on the web W compared to prior art arrangements that use a wide, free sensor gap. This enables improved measurement accuracy of the supplemental sensing devices  44 ,  45  to determine the distance to the sheet surface.  
         [0064]    In another embodiment of the invention, improved optical measurement of other sheet properties, for instance gloss or smoothness, is accomplished by embedding appropriate devices in the sensors  14 ,  16 . FIGS. 7 a  and  7   b , respectively, show a perspective view and a schematic cross section of the sensors  14 ,  16  of FIG. 2 adapted to measure gloss or smoothness on both sides of the web W. Angularly oriented apertures  50  are formed in the sensors  14 , 16  to permit measurement of gloss or smoothness of each side of the web W by optical systems included in the sensors  14 ,  16 . The optical systems each comprise a commercially available light source  51  and light detector  55 , or may optionally use remote sensing via fiber optics in a manner similar to the supplemental sensing elements  44 ,  45  of FIGS. 6 a  and  6   b . Light  52  from the light source  51  is directed toward the web W by a first reflective surface  53 . Light  54  reflected from the web W is directed toward the detector  55  by a second reflective surface  53 ′ so that the reflected light  54  is received by the detector  55 . The controlled and small gap between each of the sensor  14 ,  16  and the opposite surfaces of the web W, together with the assured parallelism between the sensors  14 ,  16  and the web W enable the use of miniaturized and narrowly focused optics, due to dimensional scaling rules, to provide improved measurement of gloss and/or smoothness.  
         [0065]    A method of sensing or measuring at least one property or characteristic of a web of material moving along a feed path is also disclosed. The method comprises positioning a first sensor  14  on one side of a web W of material moving along a feed path, while a second sensor  16  is positioned in an opposing relationship. The two opposing sensors  14 ,  16  are then drawn together toward the web W of material by polarized magnets carried by each of the sensors  14 ,  16  as described above. The first and second sensors  14 ,  16  are also repelled from one another by a repelling force created by gas bearings  22 ,  24  formed between the sensors  14 ,  16  and the adjacent surfaces of the web W. The magnetizing force and repelling force are controlled to maintain the first and second sensors  14 ,  16  evenly spaced from the passing web W.  
         [0066]    Preferably, the first and second sensors  14 ,  16  have a substantially flat surface  14   a ,  16   a  facing the feed path, and the at least one magnet  26  of the first sensor  14  is recessed below the substantially flat surface  14   a  thereof, while the at least one magnet  26  of the second sensor  16  is recessed below the substantially flat surface  16   a  of the second sensor  16 .  
         [0067]    The foregoing description of the invention of the present application has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.