WO1998046826A1

WO1998046826A1 - Method, apparatus and the use thereof for the condition monitoring of a roll in a paper machine

Info

Publication number: WO1998046826A1
Application number: PCT/FI1998/000299
Authority: WO
Inventors: Markku HÄKKINEN; Harri Kaarta; Kimmo Simomaa
Original assignee: Enso Oy
Priority date: 1997-04-11
Filing date: 1998-04-06
Publication date: 1998-10-22
Also published as: AU6833598A; FI971518A; FI971518A0; FI111176B

Abstract

The invention relates to a method, apparatus and the use thereof for monitoring the condition of a roll (1) in a paper machine by virtue of monitoring the surface (2) of the roll (1) with the help of at least one electronic camera (6, 7). The invention is characterized in that the electronic camera (6, 7), which is controlled by a micro-computer, stores in its memory a linear-scan multipixel image of the varied pattern of the surface (2) of the roll (1) over the axial direction of the roll and, in the case of a disturbance situation, the control processor of the camera (6, 7) triggers an alarm (16).

Description

Method, apparatus and the use thereof for the condition monitoring of a roil in a paper machine

The present invention relates to a method according to the preamble of 5 claim 1 and an apparatus based thereon for monitoring the condition of a roll in a paper machine.

Since the beginning of the 1 970's, calendering in paper machines has been carried out to an increasing extent using soft-surfaced calender rolls ι o called "soft" calenders. They are capable of rendering the paper web a higher surface quality than that achievable by means a pair of conventional metal-metal calender rolls. The softness of the calender roll is accomplished by means of a coating made from a polymer such as polyurethane, hard-vulcanized rubber or an epoxy compound.

1 5

A soft-surfaced calender roll is, however, more vulnerable to damage than a conventional hard-surfaced roll. The linear pressure in the calender roll nip is very high and the surface temperature of the rolls becomes very high during operation. To avoid damage to the roils, their condition

20 is monitored during running by noncontacting means such as, e.g., an infrared thermometer capable of detecting overheated hot spots on the calender roll surface. Overheating can cause damage to the delicate coating of the roll. However, temperature measurement based on the intensity of the infra-red radiation emitted by the hot surface can reveal a

25 process disturbance only at a relatively late phase of the emerging problem, whereby roll damage is possibly already beyond control. Roll repair is a costly operation during which the paper machine is entirely out of production.

30 It is an object of the present invention to overcome the above-described drawbacks. The characterizing properties of the method and apparatus according to the invention for roll condition monitoring in a paper machine are specified in the appended claims. In the embodiment according to the invention, the roll surface is monitored by a high-speed electronic camera operating essentially within the wavelength range of visible light. In the invention, the detected wavelength range is typically about

400-900 nm. When required, the wavelength range can be limited to cover a specific spectral band if, for instance, the absorptive or reflective qualities of the potential source of malfunction are known to be concentrated on a certain wavelength. The goal of the invention is achieved by virtue of a system in which an electronic camera controlled by a microcomputer records into the camera system memory a linear-scan multi- pixel image of the varied surface pattern of the roll over the axial direction of the roll and, in the case of a disturbance-indicating deviation from the recorded image, triggers a malfunction alarm.

In the following the invention will be examined in greater detail with the help of exemplifying embodiments by making reference to the appended drawings, in which

Figure 1 shows a preferred technique of implementing the method according to the invention;

Figure 2 shows a visualization of a typical line-scan brightness profile along the axial direction of the roll surface; and

Figure 3 shows a side view of the preferred technique of mounting the line-scan camera and locating the illuminating lamp with regard to the camera, illustrated from the axial direction of the roll.

Referring to Fig. 1 , therein is illustrated a preferred technique of illuminating the surface of a roll ( 1 ) by means of lamps (3,4, 5) so that the electronic cameras (6,7), within their conical field of view (8), can record the surface (2) of the roll ( 1 ), advantageously in a diffuse illumination.

Each camera (6,7) is provided with an objective (20) which projects the brightness profile of the roll surface onto the focal plane of an electronic image sensor (21 ) mounted inside the camera housing. Advantageously, the sensor is, e.g., a linear-array detector of the CCD (Charge-Coupled Device) type. Such an array may comprise 2000 pixels, for instance, but depending on the system design the number of detector elements in the array may vary in the range 1 28-1 5,000 pixels. Thus, the image sensor can record the brightness profile of the roll surface in the axial direction of the roll along the line which is focused from the roll surface onto the focal plane of camera sensor.

Now referring to Fig. 2, the curve plotted therein visualizes the axial line scan of a possible brightness profile along the roll surface. The vertical coordinate (1 9) of the graph is proportional to the roll surface brightness and the horizontal coordinate ( 1 8) gives the axial position of the scan on the roll surface. The plot of the brightness curve ( 1 7) is shown to have brighter (22) and darker (23) spots. In the illustrated case, the plot comprises 2048 successive points of the line scan. Assuming that each camera is adjusted to scan, e.g., a length of 1 m along the roll surface, the mutual distance of the imaged points on the object plane, which is the roll surface, will be 0.5 mm. As the roll surface revolves about the roll axis, the variations in the apparent brightness profile will be reduced and the contrast between the different points of the recorded linear-scan brightness curve minimized if the each exposure cycle is arranged to integrate light at least over one full revolution of the roll onto the detector elements of the camera. For example, if the perimeter length of the roll is 1 m and the paper web runs at a speed of 1 0 m/s, each exposure should last at least 1 00 ms in order to cover each peripheral point of the roll surface at least once during each exposure.

In Fig. 3 is shown a side view, particularly as seen from the axial direction of the roll (1 ), of a preferred technique of mounting the camera (6) with respect to the calender roll ( 1 ), its backing roll (32) and the running paper web (33) . The lamp (3) is advantageously adapted so as to illuminate the imaged points by a diffuse light (31 ) rather than permitting a gloss illumination (30) by directly reflected rays.

The function of the cameral system is initiated so that, under the control of the operating personnel, the camera is first used to record a "signature" line-scan brightness profile of the virgin roll surface, which inherently is somewhat randomly patterned. Such an acceptable line-scan brightness profile ( 1 7) of the roll surface is stored in the camera system memory which may be of the conventional RAM, EEPROM or FLASH memory type. The camera control processor may be arranged to update the memory-stored brightness profile with long-term changes caused by such factors as dust accumulation on the surfaces of the camera optics as well as contamination and/or aging of the lamp (3) .

During a disturbance situation, dirt may suddenly adhere to the roll surface thus causing a local increase of the linear nip pressure at that point. The dirt may stem from a disturbance in coater operation permitting the occurrence of a coat stripe or similar defect. While most frequently such a defect will be located to the area of the web edge, it may also occur more centrally on the roll surface. The adhering dirt is immediately detected as a change in the real-time brightness profile ( 1 7) recorded by the camera system. As can be seen from Fig. 2, a clump of light dirt causes an upward intensity peak (24) and a clump of dark dirt, respectively, causes a downward intensity dip (25) . The program-controlled processor of the camera system identifies immediately disturbances of this type in the camera output signal and can then issue an alarm ( 1 6) via a relay contact closure or other equivalent manner.

As shown in Fig. 1 , the entire surface of the roll is monitored using a system advantageously comprising a plurality of cameras (6,7) connected via signal lines (9, 1 0) to a separate signal concentrator ( 1 1 ) adapted to issue a centralized alarm ( 1 5) when required. The signal concentrator ( 1 1 ) may also be connected via signal lines ( 1 2) to a control-room computer ( 1 3) and its alarm control system ( 1 4) . The signal transmission may be based on galvanic lines (using RS-485, RS-232 or a similar signal transfer similar protocol), optical fibers, infrared links or the like techniques. According to accumulated practical experiences, the operating personnel has a competence to adjust the disturbance alarm limits (26,27) for the detected deviations. The magnitude of the detected disturbance is identified as a positive or negative deviation from the accepted roll brightness profile ( 1 7) . Alternatively, the alarm criteria can be based on the number of adjacent detector elements (28,29) detecting the disturbance. With optimum sensitivity adjustments, the system is capable of detecting a roll surface defect as narrow as 0.5 mm within a line-scan length of 1 m. As the performance figures cited above are given by reference only, it is obvious that the system can be adapted to monitor a narrower or wider linear length of the roll surface. For system commissioning and tuning, provision is made for the control-room computer to view separately any of the brightness profiles recorded by each camera.

The invention further concerns the use of at least one electronic camera (6,7) operating essentially in the visible light range for condition monitoring of a roll ( 1 ) in a paper machine, whereby the camera system is adapted to monitor the surface (2) of the roll ( 1 ), particularly the surface of a soft-calender roll. As taught above, the invention is characterized in that each camera of the electronic camera system records in its memory and automatically learns to identify the details of the varied brightness pattern imaged by the camera from its dedicated area of the calender roll surface. Thus, the camera system is capable of issuing to the operating personnel an immediate alarm, that is, in less than one second, of a sudden disturbance caused by, e.g. , a coat lump or other dirt adhering in the calender roll nip. At such a disturbance alarm, the calender roll nip can be opened immediately and the roll surfaces cleaned before any damage has been caused to the rolls.

As required, the wavelength range employed in the invention may be adapted and modified to meet given needs, e.g., by limiting the sensitivity of the camera to a specific spectral range. Moreover, while the appa- ratus according to the invention is capable of accommodating to the contamination of the illuminating lamp or camera system optics with a desired time constant (e.g. , 4 h), it is capable of immediately reacting to sudden brightness profile changes. The updating of the system operating point, that is, the brightness profile set value is stopped at each issuance of an alarm. The alarm is cancelled when the cause of the disturbance is cleared. The system is designed to perform self-compensation for contamination and other drift factors, however, not without warning the operator from such changes. The cause of disturbance alarm can be identified from the intensity magnitude or linear width of the detected brightness profile deviation.

The apparatus according to the invention can utilize readily available fluorescent lamps or similar luminaires as the diffuse light source. When required, the operating personnel can readily check the flawless function of the apparatus by hanging an obstacle such as a piece of thread into the linear field imaged by the camera on the roll surface. Such an object should trigger an alarm depending on the disturbance detection limit settings of the camera system.

The control processor of the camera system is provided with a watchdog function: the processor program is reset automatically when the execution of the control program is disturbed by a malfunction such as a supply voltage outage, for instance. Also this occurrence is reported to the operator. Stopping the rotation of the calender roll always triggers an alarm, because the line-scanned brightness profile of a standing roll has greater deviations than the running roll brightness profile which is averaged over more than one revolution.

To those versed in the art it is obvious that the invention is not limited by the exemplifying embodiments described above, but rather, may be varied within the scope and spirit of the appended claims.

Claims

Claims:

1. A method of monitoring the condition of a roll (1) in a paper machine, c h a r a cte r i z e d in that the surface (2) of the roll (1), particularly the surface of a soft-calender roll, is monitored with the help of at least one electronic camera (6,7), advantageously provided with automatic functions, operating essentially within the wavelength range of visible light.

2. A method as defined in claim 1, c h a r a cte r i ze d in that an electronic camera (6,7) of the linear-array type, controlled by a microcomputer, is used so that said camera stores in its memory a linear-scan multipixel image of the varied pattern of the surface (2) of the roll (1) over the axial direction of the roll and, in the case of a malfunction situation or similar disturbance such a sudden change in the roll surface

(2) or factors related thereto, the control processor of the camera (6,7) triggers an alarm (16).

3. A method as defined in claim 1 or 2, c h a ra cte r i ze d in that the surface (2) of the roll (1) coated with a polymer coating or similar is illuminated by means of at least one lamp (3,4,5) so as to permit the electronic camera (6,7), within the conical field of view (8) of the camera, to record the surface (2) of the roll (1), preferably in an essentially diffuse illumination (31).

4. A method as defined in any of foregoing claims 1-3, c h a r a cte r i z e d in that the camera (6,7) is provided with an objective (20) which projects the surface brightness profile of the roll (1) onto the focal plane of an electronic image sensor (21) of the camera (6,7).

5. A method as defined in any of foregoing claims 1-4, c h a r a cte ri z e d in that the camera (6,7) is adapted to update long-term changes of the stored line-scan brightness profile in the camera system memory such as a RAM, EEPROM or FLASH memory, provided that no alarms have been triggered during the monitoring period.

6. An apparatus for condition monitoring of a roll (1) in a paper machine, c h a r a cte r i z e d in that, for the monitoring of the surface (2) of the roll (1), particularly the surface of a soft-calender roll, there is adapted at least one electronic camera (6,7), advantageously provided with automatic functions, operating essentially within the wavelength range of visible light, so as to perform an axial-direction scan along the surface of the roll (1).

7. An apparatus as defined in claim 6, c h a racte ri zed in that for the illumination of the surface (2) of the roll (1) coated with a polymer coating or similar is adapted at least one lamp (3,4,5) so as to permit said electronic camera (6,7), within the conical field of view (8) of the camera, to record the surface (2) of the roll (1), preferably in an essentially diffuse illumination (31).

8. An apparatus as defined in claim 6 or 7, c h a r a cte ri ze d in that said apparatus is comprised of a linear-array-type electronic camera (6,7) having an electronic image sensor (21) and being controlled by a microcomputer, an alarm system (16) of the camera outfit, a lamp

(3,4,5), a signal concentrator (11) having connections to a control-room computer (13) and an alarm system (14) thereof as required, and a centralized alarm system (15).

9. Use of at least one electronic camera (6,7), operating essentially within the wavelength range of visible light and adapted to monitor the surface (2) of a roll ( 1 ), particularly a soft-calender roll, for the condition monitoring of a roll ( 1 ) in a paper machine.

1 0. Use of a camera (6,7) as defined in claim 9 for the condition monitoring of a roll ( 1 ), particularly by virtue of detecting a sudden change in the roll surface (2) or factors related thereto from the magnitude and/or width deviation in the brightness profile of the roll, for issuing an alarm from such a disturbance condition.