Method of controlling gap in sheet manufacturing measurement process

Abstract

Sensors used in a sheet manufacturing environment may be sensitive to gaps between scanner heads. A nucleon meter measures both the sheet product and the gas in the column between the scanner heads. Depending on the sensing principle, the gap distance may affect the measurement in a linear or non-linear manner. The high frequency vibration may cause the sensor device therein to move. Techniques for controlling a sensor gap between operating surfaces on a dual scanner head during a measurement operation employ closed-loop control in which a piezoelectric actuator is used to maintain a constant gap throughout the scan by closing the gap to measure an error signal on the sensor. A system for measuring characteristics of a continuous sheet includes a dual scanner head housing a radiation emitter and a receiver. The gap measurement signal energizes an actuator that adjusts the position of one or both sensor devices to maintain the gap at a desired distance.

Description

Technical Field

[0001] The present invention generally relates to scanning measurement systems for determining parameters of continuous sheets of material during a production process, and more particularly to active measurement of sensor gap within a dual scanning head to control and maintain a constant sensor gap throughout the scanning process. Background Art

[0002] In-line measurement of properties of continuous sheet material during a manufacturing process is well known. Generally speaking, the purpose of in-line measurement is to enable monitoring of the sheet manufacturing process with the goal of improving sheet quality while reducing the amount of substandard sheet material produced before undesirable process conditions are corrected.

[0003] Sensors for continuous flat sheet production processes typically employ single or double sided packages with inline sensors that traverse or scan a traveling web of sheet material during manufacturing. Scanning is typically performed in the transverse direction, i.e., perpendicular to the direction of sheet travel. A typical transmission mode scanner system includes a housing and is mounted on opposite sides of a moving sheet (i.e., above and below the sheet) for movement across the sheet to measure selected sheet parameters. A channel or column is formed between the upper and lower housings, and the sheet can be disposed in the channel. In operation of the scanning system, the housings travel synchronously across the sheet on tracks mounted to parallel horizontal frame members.

[0004] In implementation, the housing serves as a platform for carrying sensors for detecting sheet material properties such as basis weight. Thus, for example, the upper housing may carry a radiation source such as a nuclear beta source, and the lower housing may carry a detector. In this case, the sensor may be used to perform basis weight measurements by measuring the intensity of radiation incident on the detector when the sheet material is present compared to the beta radiation incident on the detector when no sheet material is present; that is, the basis weight is measured by beta radiation attenuated by the sheet material. On-line measurements during the production of continuous sheet material are difficult to perform accurately. Scanning sensor systems are susceptible to various errors. For example, if the vertical distance separating the housings is not kept constant during scanning, the incident radiation intensity will vary independently of changes in the sheet material properties.

[0005] The reflection mode scanner system includes a sensor mounted in an upper housing facing the moving sheet. In operation, radiation from a radiation source in the upper housing is directed toward the sheet, and the reflected radiation is received by a detector in the upper housing. The distance between the sensor and the moving sheet must be maintained.

[0006] Various techniques have been used to correct for sensor position errors, such as misalignment, etc. However, current approaches do not provide for dynamic active measurement and control of the sensor gap within the dual housing of a scanning system. Summary of the invention

[0007] The present invention relates to techniques for controlling the sensor gap within the measurement channel between dual scanner heads during a transmission measurement mode of operation. Many sensors used in sheet manufacturing environments are sensitive to the gap between scanner heads. For example, nuclear based weight sensors effectively measure both the weight of the product and the weight of the air in the column between the sensor halves. This is a function of the gap distance and the air temperature. Other types of sensors use subtraction to measure the distance to the product surface and subtract that distance from the total gap to obtain a thickness measurement, such as in laser thickness sensors. By controlling the gap itself, correction for the effect can be simplified on a per sensor basis.

[0008] The present invention also relates to techniques for controlling the measurement gap between a sensor head and a reflective operating surface of a corresponding mounting head or frame in a reflective scanner system.

[0009] The present invention is based in part on the recognition that some sensors may benefit from closed loop control, wherein an actuator is used to maintain a constant sensor gap throughout a scan by closing the gap to measure an error signal on the sensor. In one aspect, the present invention is directed to a system for measuring a property of a continuous sheet of material having a first side and a second side and traveling in a downstream longitudinal direction, the system comprising:

[0010] (a) a first scanner head disposed adjacent to a first side of the sheet, the first scanner head comprising:

[0011] (i) a first sensor device having a first movable sensor base defining a first operating surface facing a first side of the sheet;

[0012] (ii) a first adjustable actuator configured to move the first sensor base along a first axis;

[0013] (b) a second scanner head disposed adjacent a second side of the sheet, the second scanner head comprising a second operating surface facing the second side of the sheet, wherein the first operating surface and the second operating surface define a sensor gap through which the continuous sheet travels;

[0014] (c) means for measuring a distance of a first sensor gap and generating a first signal representative of the measured sensor gap distance; and

[0015] (d) a control device for controlling the first adjustable actuator in response to the first signal to maintain the sensor gap distance at a desired gap distance.

[0016] In another aspect, the present invention provides a system for monitoring a characteristic of a continuous web of material having a first side and a second side and traveling in a downstream longitudinal direction, the system comprising:

[0017] (a) a first mounting head, the first mounting head being disposed adjacent to a first outer surface of the sheet, the first mounting head comprising:

[0018] (i) a first movable sensor base defining a first operating surface facing a first outer surface of the sheet;

[0019] (ii) a first actuator configured to move the first sensor base along a first axis perpendicular to the first operating surface;

[0020] (b) a second mounting head disposed adjacent a second outer portion of the sheet, the second mounting head including a second operating surface facing the second outer surface of the sheet, wherein the first operating surface and the second operating surface define a sensor gap through which the continuous sheet travels, wherein the first mounting head and the second mounting head are aligned and configured to move in a synchronized manner in a cross-machine direction, the cross-machine direction being perpendicular to the longitudinal direction, and wherein at least one sensor device is mounted in the first mounting head, the second mounting head, or both;

[0021] (c) means for measuring the distance of the sensor gap and generating a signal representative of the measured sensor gap distance; and

[0022] (d) a control device for controlling the first actuator in response to the signal to maintain the measured gap distance at a desired gap distance.

[0023] In another aspect, the invention relates to a method of controlling a system for monitoring a property of a continuously traveling web, the system comprising a sensor head having an operating surface connected to an actuator for moving the operating surface. The method regulates a sensor gap between the operating surface and the moving web, and comprises:

[0024] (a) monitoring the distance between the operating surface and the traveling web and generating a distance signal indicative of the measured sensor gap distance;

[0025] (b) comparing the measured sensor gap distance with a target sensor gap distance; and

[0026] (c) Manipulate the operating surface to adjust the sensor gap distance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a side cross-sectional view of a dual scanner head;

[0028] Figure 2 is a side cross-sectional view of a dual scanner head operating in transmission mode;

[0029] Figure 3is a side cross-sectional view of a dual scanner head operating in a reflection mode; and

[0030] Figure 4 A scanner in a web manufacturing or processing system is shown. DETAILED DESCRIPTION

[0031] Figure 1 A dual scanning system 2 is shown with scanner heads 4 and 6. The upper scanner head 4 houses a sensor device 8 supported on a substrate 10 having an operating flat surface 11. Actuators 12A and 12B attached to a lower surface 24 of the upper scanner head 4 are fixed to the substrate 10. The upper sensor head 4 also houses a sensor device 28 supported on a substrate 30. Actuators 32A and 32B attached to a lower surface 24 of the upper scanner head 4 are fixed to the substrate 30 having an operating flat surface 31. The upper scanner head 4 is equipped with upper sheet deflectors 50 and 52.

[0032] The lower scanner head 6 accommodates a sensor device 14 connected to a substrate 16 having an operating flat surface 17. Actuators 18A and 18B attached to an upper surface 26 of the lower sensor head 6 are fixed to the substrate 16. The lower scanner head 6 also accommodates a sensor device 34 connected to a substrate 36 having an operating flat surface 37. Actuators 38A and 38B attached to an upper surface 26 of the lower scanner head 6 are fixed to the substrate 36. The lower scanner head 6 is equipped with lower sheet deflectors 54 and 56.

[0033] The displacement between the operating surface 11 of the sensor device 8 and the operating surface 17 of the sensor device 14 defines a gap 60 and a distance that can be measured by a displacement measuring device. A suitable device is an inductive sensor having an RF or z-coil 20 that is positioned in the sensor device 8 and that measures the distance from the z-coil 20 to a reference surface or plate 22, which is the sensor substrate 16. Inductive sensors may be particularly suitable when the web or sheet material being monitored is paper or plastic. The distance of the gap 60 may also be measured by optical displacement, laser-based triangulation, inductive sensors, radar, capacitive sensors, magnetic field-based sensors, microwave radiation-based and acoustic-based sensors.

[0034] Similarly, the displacement between the operating surface 31 of the sensor device 28 and the operating surface 37 of the sensor device 34 defines a gap 62, and the distance can be measured by the displacement measuring device. In the case where the sheet product includes a metal substrate, the preferred displacement measuring mechanism includes an electromagnetic coil 40 located in the sensor device 28 and a first magnetic sensor 42 and a second magnetic sensor 44 located in the sensor device 34. The two magnetic sensors are preferably connected in series and aligned with the coil along the axis. The electromagnetic coil 40 is connected to a DC power supply or an AC power supply (not shown) to generate a magnetic field measured by the pair of magnetic sensors such as fluxgate sensors. A permanent magnet can be used as a magnetic field source instead of using an electromagnetic coil and an associated drive current. This magnetic displacement sensor is read through a metal foil material (such as a coated metal electrode) and is described in U.S. Patent No. 11,519,710 to Tixier et al., which is incorporated herein by reference.

[0035] Each of the actuators may include a multilayer piezoelectric actuator in the form of a piezoelectric stack; the stack may be used in conjunction with a lever action to increase the actuation distance at the expense of peak force. The piezoelectric actuator may expand or contract when actuated depending on the applied electric field. Thus, in one application, when the stack is energized, it elongates and pushes a substrate to which the stack is secured in a vertical direction perpendicular to an associated operating surface. Figure 1 As shown, each scanner head 4 and 6 may include a plurality of sensor devices, each defining a sensor gap 60, 62. The displacement distances of the gaps may be the same or different, and each gap may be controlled independently of the others.

[0036] Figure 2 A dual scanning system 72 is shown in which an upper scanner head 74 houses a sensor device 78 supported on a substrate 80 having an operating flat surface 81. Actuators 82A and 82B attached to a lower surface 94 of the upper scanner head 74 are fixed to the substrate 80. The sensor device 78 includes a radiation source 100. The lower scanner head 76 houses a sensor device 84 including a radiation detector 102 connected to a substrate 86 having an operating flat surface 87. Actuators 88A and 88B attached to an upper surface 96 of the lower sensor head 76 are fixed to the substrate 86. The displacement between the sensor device 78 and the sensor device 84 is measured with a z-coil 90 and a reference surface or plate 92.

[0037] A continuous web or sheet of material 130, such as paper, plastic, fabric, etc., travels in the machine direction (MD) through a measurement channel between an upper scanner head and a lower scanner head. In one embodiment, the radiation source 100 comprises a nuclear beta source for measuring a property of the material 130, such as basis weight, by measuring the intensity of the radiation incident on the detector 102 when the material 130 is present compared to the beta radiation incident on the detector when no material is present. If the vertical distance separating the radiation source 100 and the radiation detector 102 fluctuates, the readings are susceptible to error. Other radiation sources, such as, for example, X-rays and infrared radiation, may be employed depending on the material being monitored.

[0038] With the present invention, PID controller 104 receives signals from gap measurement devices 90 and 92 and is configured to transmit gap modulation signals to voltage sources 106, 108, 110 and 112. Increasing amplitude, decreasing amplitude and polarity of the voltage source all result in extension or contraction.

[0039] exist Figure 2 In the scanning system 72 of FIG. 7 , both the upper sensor device 78 and the lower sensor device 84 are equipped with actuators to modulate the gap distance. Obviously, the gap displacement can be modulated by configuring one or more actuators for only one of the sensor devices. However, enabling both the sensor devices 78 and 84 to be operated with actuators achieves a faster response.

[0040] It should be noted that with the present invention, it is not the vertical position of the scanner heads 74 and 76 that is adjusted by the actuators. The scanning head (or mounting head) is typically a relatively large structure that houses the sensor equipment and other components. The sensor equipment, such as the radiation source and the detector, is subject to mechanical vibrations of frequencies typically in the range of 10's to 100'Hz, and is primarily limited to 1kHz and below. The present invention corrects the adverse effects caused by vibrations by actuating a minimum mass at each sensor. The mass that must be controlled is the minimum mass that affects the sensing principle used. For example, the sensor electronics do not need to be controlled while the radiation source does, because the distance of the radiation source to the web affects the intensity and therefore the measurement, while the location of some circuits does not affect the measurement. The speed of the feedback mechanism must be able to counteract these vibrations; this can be achieved with piezoelectric actuators. For actuation (gap) distances of tens to hundreds of microns, the response time of these actuators can be up to 2 orders of magnitude faster than this (single microsecond).

[0041] Figure 3A scanning system 102 is shown in which an upper scanner (or mounting) head 104 houses a sensor device 108 supported on a substrate 110. Actuators 112A and 112B attached to the lower surface of the upper sensor head 104 are fixed to the substrate 110 having an operating flat surface 111. The actuators move the substrate 110 in a vertical direction with an axis perpendicular to the operating flat surface 111. The sensor device 108 includes a radiation emitter source 154 and a receiver detector 152.

[0042] The continuous web or sheet of material 150 moves in the MD past the upper scanner head. The displacement between the operating surface 111 and the upper surface of the material 150 is measured using a laser triangulation device 120. In one embodiment, a radiation source 154 comprises an infrared radiation source focused onto the sheet of material 150; radiation reflected from the material is captured by a radiation detector 152. Actuators 112A and 112B maintain a gap distance between the operating surface 111 and the upper surface of the material 150.

[0043] Figure 4 A dual scanner 170 is shown in a web manufacturing or processing system. The scanner 170 travels back and forth in a cross direction, which is perpendicular to the MD direction of a moving web (not shown). The scanner 170 is driven by a motorized drive mechanism (not shown). The scanner 170 includes an upper mounting head 180 and a lower mounting head 182 mounted on a carriage supported by an upper guide rail 172 and a lower guide rail 174, respectively. The movement of the two aligned mounting heads is synchronized, and the heads define a channel or gap 184 through which the web travels. Support structures 176, 178 maintain the spacing of the guide rails. As the scanner travels over at least one surface of the web, the web (not shown) passes horizontally between the support structures 176, 178. In this embodiment, the scanner 170 has sensor devices 186 and 196, which operate in a transmission mode to measure one or more characteristics of the web.

[0044] The foregoing describes the principles, preferred embodiments and modes of operation of the present invention. However, the present invention should not be construed as being limited to the specific embodiments discussed. Therefore, the above embodiments should be considered as exemplary rather than restrictive, and it should be understood that variations may be made in these embodiments by those skilled in the art without departing from the scope of the present invention as defined by the following claims.

Claims

1. A system for measuring a property of a continuous sheet of material having a first side and a second side and traveling in a downstream longitudinal direction, the system comprising: (a) a first scanner head disposed adjacent to a first side of a sheet, the first scanner head comprising: (i) a first sensor device having a first movable sensor base defining a first operating surface facing the first side of the sheet; (ii) a first adjustable actuator configured to move the first sensor base along a first axis; (b) a second scanner head disposed adjacent the second side of the sheet, the second scanner head comprising a second operating surface facing the second side of the sheet, wherein the first operating surface and the second operating surface define a sensor gap through which the continuous sheet travels; (c) means for measuring a distance of a first sensor gap and generating a first signal representative of the measured sensor gap distance; and (d) a control device for controlling the first adjustable actuator in response to the first signal to maintain the sensor gap distance at a desired gap distance.

2. The system of claim 1, wherein the first sensor device comprises (i) a source radiation transmitted toward the continuous sheet and (ii) a detector configured to receive reflected radiation.

3. The system of claim 1, wherein the first sensor device comprises a radiation source transmitted toward the continuous sheet, and the second scanner head comprises a second sensor device, the second sensor device comprising a detector and defining the second operating surface.

4. The system of claim 3 , wherein the second sensor device has a second movable sensor base and has a second adjustable actuator, the second adjustable actuator being configured to move the second sensor device along a second axis, and wherein the control device controls the first adjustable actuator and the second adjustable actuator in response to the signal to maintain the sensor gap distance at a desired gap distance. The system of claim 4 , wherein the second movable sensor base defines the second operating surface.

6. The system of claim 1, wherein the first adjustable actuator comprises a piezoelectric actuator.

7. A system for monitoring characteristics of a continuous sheet of material having a first side and a second side and traveling in a downstream longitudinal direction, the system comprising: (a) a first mounting head, the first mounting head being disposed adjacent to a first outer surface of the sheet, the first mounting head comprising: (i) a first movable sensor base defining a first operating surface facing the first outer surface of the sheet; (ii) a first actuator configured to move the first sensor base along a first axis perpendicular to the first operating surface; (b) a second mounting head disposed adjacent a second outer portion of the sheet, the second mounting head including a second operating surface facing a second outer surface of the sheet, wherein the first operating surface and the second operating surface define a sensor gap through which the continuous sheet travels, wherein the first mounting head and the second mounting head are aligned and configured to move in a synchronized manner in a lateral direction, the lateral direction being perpendicular to the longitudinal direction, and wherein at least one sensor device is mounted in the first mounting head, the second mounting head, or both; (c) means for measuring the distance of the sensor gap and generating a signal representative of the measured sensor gap distance; and (d) a control device for controlling the first actuator in response to the signal to maintain the measured gap distance at a desired gap distance.

8. A system according to claim 7, wherein the second mounting head includes a second movable sensor base and a second actuator, the second movable sensor base defining a second operating surface facing the second outer portion of the sheet, the second actuator being configured to move the second sensor base along a second axis, and wherein the control device is configured to control the first actuator and the second actuator in response to the signal to maintain the measured gap distance at the desired gap distance.

9. A method of controlling a sensor gap between an operating surface and a traveling web in a system for monitoring a property of a continuously traveling web comprising a sensor head having an operating surface connected to an actuator for moving the operating surface, the method comprising: (a) monitoring the distance between the operating surface and the traveling web and generating a distance signal indicative of the measured sensor gap distance; (b) comparing the measured sensor gap distance with a target sensor gap distance; and (c) Manipulating the operating surface to adjust the sensor gap distance.

10. The method of claim 9, wherein the sensor head comprises a movable sensor base and an actuator, the movable sensor base defining the operating surface facing the first side of the web, the actuator being configured to manipulate the sensor base along an axis perpendicular to the operating surface, wherein step (c) comprises activating the actuator to manipulate the movable sensor base.

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