Sample Data Web And Strip Guide Control System

Abstract

Control system for guiding a traveling web or strip to facilitate the performance of work on the web or strip to provide strip positioning at a location remote from the strip actuator by employing a movable sensor adjacent the actuator modulating the actuator in response to a set point, and a stationary sensor at the control point generating a time sampled signal to the movable sensor for changing the set point in response to the position of the strip at the control point.

Description

This invention relates in general to a web or strip guide control system, and more particularly to a sample data web or strip guide control system, and still more particularly to a web or strip guide control system capable of permitting remote location of a sensor for control of the web or strip pursuant to a sample data approach.

The web or strip guide control system of the invention is primarily useful in controlling the lateral position of a traveling metal strip, although it should be appreciated that it could be applicable for a paper or plastic web or any other type of strip or web. The sample data system approach of the invention is useful where the conventional sensor location is not possible or convenient because of environment or space limitations.

Strip control systems are useful in process lines to, for example, control strip guiding into and out of accumulators, control strip position in acid and rinse sections or pickle lines, and control strip or web position in furnaces or cooling towers. Conventional control systems include a sensor and an actuator responsive to the sensor, and are located at points in the process lines where alignment is required. It is desirable to place the sensing for these control systems exactly at the point where the position alignment is required because they operate to maintain exact alignment in the sensor. Sensing, however, must be located sufficiently close to the actuator to provide adequate feedback for control stability. However, it is not always possible to place such control systems in the area where control is required.

Sample data control systems permit sensing at a point in the line where strip position accuracy is critical but where it would not be practical or convenient to install a conventional control system. Sample data control systems are used to compensate for time lags between position actuation by the actuator and the sensing of position for control by the sensor. Sample data systems provide stability in part by decreasing system gains to a level below that which would be allowed by hardware characteristics such as hysteresis, drift, etc. The sample data control system of the invention includes a primary conventional control loop having an actuator and a sensor arranged to allow sufficient gain to remove larger errors. A second control loop having only a sensor, and operating on a sampled basis, adjusts the setpoint of the primary control loop causing the strip to satisfy the position requirements remote from the actual control of the positioning of the strip.

A sample data control system used for strip tracking is known but has not always proved satisfactory because its gain is limited by the time lag between correction and sensing the effect of the correction, allowing only very small errors to be removed. In such a system, a stationary sensor is located at the control point for gating error correction to the actuator following a predetermined measurement of the traveling strip. Unless the rate of change of edge position of the strip is sufficiently small so that control can keep up with edge position error, the system performances will not be satisfactory.

The present invention is useful where the error is greater than that which could be removed by the heretofore known sample data systems. A conventional primary loop control system causes removal of errors at the actuator, while downstream a sample data control system defining a second loop is employed to shift the set point of the conventional control system to compensate against trend errors seen at the downstream sensor.

Steady state error is thereby reduced by touching up the set point on the primary loop control system. While an unwind stand is illustrated as the actuator in the present invention, it should be appreciated that any type of actuation may be used for lateral positioning of the strip. It should also be appreciated that the control system of the present invention may be employed to detect the position of one edge of a strip, or the positions of both edges as in a center guide system.

It is therefore an object of the present invention to provide a new and improved web or strip guiding control system employing in part the sample data system approach and in part a conventional control system, wherein control at places otherwise impossible to control is obtained.

Another object of this invention is in the provision of a web or strip guide control system capable of handling greater errors in strip travel than heretofore controllable by known sample data systems.

Other objects, features and advantages of the invention will be apparent from the following detailed disclosure, taken in conjunction with the accompanying sheets of drawings, wherein like reference numerals refer to like parts, in which:

FIG. 1 is a diagrammatic view and block diagram of the web and strip guide control system according to the present invention;

FIG. 2 is a diagrammatic and block diagram of a modification of the invention;

FIG. 3 is a diagrammatic and block diagram of a web and strip guide control system of the prior art; and

FIG. 4 is a chart representing the operation of the prior art embodiment of FIG. 3.

It has been generally necessary to sense strip position near the point of the actuator correction in web and strip guide control systems. However, in many installations such has not been convenient and in some cases not even possible, thereby precluding adequate strip position control. The sample data approach has been suggested to overcome the difficulty, and it will, in some cases, permit adequate strip position control where the sensor is mounted remotely from the point of corrective actuation, and where only small errors are encountered. Accordingly, when using a conventional integrating or floating control system for web or strip guide, it is necessary that the sensor be located close to the point of correcting actuation, that is, near the unwind stand, steering roll or the like. This insures control stability with sufficient gain to remove the error.

Error removal capability is proportional to system gain which, in turn, is predominantly limited by the time lag between the time the sensor calls for a correction and the time the sensor sees the effect of the correction it caused. It is important to appreciate here that the actuator in an integrating control system will continue moving to correct for strip position error until the error reduces to zero. If sufficient time lag exists, the actuator will overcorrect or overshoot and then by necessity reverse itself and overcorrect in the opposite direction. The overcorrecting process repeats itself thereby generating an oscillation condition referred to as instability.

There are instances where the ideal control point or sensor location is far removed from the point of lateral position actuation. These instances create a poor condition for control of lateral web or strip position in that the gain becomes limited to permit stability, and in some cases, perhaps to the extent that no adequate control is possible because of hardware limitations. In some instances it may be possible to provide adequate control for guiding of the strip by employing a sample data approach, which approach recognizes the existence of a long time lag between the correction actuator and the sensor. The sample data approach operates as a conventional control loop would operate, except on seeing an error it makes a correction and then stops actuator motion until the previous correction reaches the sensor. The sensor sees the effect of the previous correction before initiating another correction. It should be appreciated that as the duty cycle of the control is decreased in this approach, the system gain is also decreased. Using the sample data approach, effective system gain can be diminished well below minimum gain limitations of control hardware used in a normal manner. In any event, the control system effective gain is substantially limited by the predominant time constant, and control accuracy is limited by the effective gain.

A strip guide system employing the sample data approach heretofore known is illustrated in FIG. 3, wherein a traveling strip 10 is being delivered from a coil 11 on an unwind stand 12. The unwind stand 12 constitutes the actuator for correcting the lateral position of the strip pursuant to the set point of a stationary sensor 13 located remotely downstream from the actuator. A double acting actuator hydraulic cylinder 14 serves to jog the unwind stand 12 or actuator for changing the lateral position of the strip 10.

The sensor 13 is located at the control point which is in an area where strip lateral position accuracy is desired. As illustrated, strip 10 is fed under a measuring roll 15, over supporting idler roll 16 and 17 and under an idler roll 18. The stationary sensor 13 detects the position of the strip edge 10a, and delivers a signal to amplifier 19 where this actual position signal is compared with the set point (desired position) signal to generate an error signal. This error signal is amplified and the amplified output is delivered to a gating circuit 20 which counts a predetermined footage of the traveling strip pursuant to a signal received from a pulse generator 21 associated with the measuring roller 15. The predetermined footage counted by the gating circuit is a function of the distance through the process line between the unwind stand 12 and the sensor 13, and which may be referred to as the transport distance of the strip. Upon reaching the count of the predetermined footage, a corrective pulse, if the strip edge 10a is offset from the set point of the sensor 13, is gated to a hydraulic controller 22 that causes corresponding operation of the actuator cylinder 14 to jog the unwind stand 12. This process is repeated at every count of the predetermined footage measured as the strip travels downstream.

Under a fixed line speed for the traveling strip, and with adjustments optimized, the corrective action is illustrated by the chart of FIG. 4. The error at the sensor is first represented as a distance 23, and following an incremental jog of the unwind stand is represented by the distance 24. Subsequent repeating of the sensing and jogging process illustrates the over correction as shown by the distance 25 and thereafter a similar overcorrection as shown by the error distance 26. The square wave line 27 represents the counts of the pulse generator 21, while the line 28 represents the jogging of the unwind stand. The width of the jogging pulse T.sub.1 represents the "on time" of the controller 22, while the amplitude A of the pulse represents the correction amplitude of the jog and is proportional to the flow rate.

The system approach of FIGS. 3 and 4 may be employed where the rate of change of edge position is small so that the control can keep up with the error, this comprising a function primarily of the transport distance between the unwind stand and the stationary sensor 13. Moreover, the period of error must be several times the transport time, so that there will be in general no error reversals within the sample length or else the correction will be in the wrong direction, or out of phase with the error. This system is of use only where the incoming product is wound nearly perfectly but a slight misalignment exists in the roll within the transport distance causing a slight walking error, as the gain in the system is limited to the time lag between actuation and sensing.

The above conditions under which the prior art embodiment of FIG. 3 must be used are not realistic in those installations where winding errors are large at the unwind stand. In this situation, a two loop control system according to the present invention is necessary to provide cascaded control. This system is first illustrated in the embodiment of FIG. 1.

An unwind stand 30 is illustrated in this embodiment as the actuator, and is capable of being moved laterally by a double acting hydraulic cylinder 31, which, in turn, is controlled by a controller 32. It should be appreciated that the actuator may take any form depending upon the installation, such as a steering roll, guiding roll, or the like. Further, it should be appreciated that any suitable means may be provided for moving the actuator, other than a hydraulic cylinder such as that illustrated.

A coil or reel of strip material 33 rotatably supported on the unwind stand and from which the strip 34 is taken has the product to have work performed thereon. As illustrated, the strip is constrained below an idler roll 35, over a measuring roll 36, and an idler roll 37, and thereafter under an idler roll 38. The arrangement of the rolls may be of any desired form. In this embodiment, the work performed is intended to be done between idler roll 37 and the measuring roll 36.

Large error in strip travel is removed under control of the first sensor 39, while smaller trend errors are removed under control of the first sensor with its operating point modified by the downstream second sensor 40, wherein both sensors are associated with one edge 34a of the strip 34. A signal is delivered from the first sensor 39 at amplifier 41 which has its output connected to the hydraulic controller 32 for operating the hydraulic cylinder 31 to jog the unwind stand 30 and maintain the lateral position of the strip 34 in accordance with the set point of the sensor 39. This permits removal of large winding errors and is considered as the first loop 42 of the system.

Small error is removed by the second sensor 40 at the control point which delivers a signal to an amplifier 43. The output of the amplifier is connected to a gate or gating circuit 44 which receives counting pulses from a pulse generator 45 driven by the measuring roll 36. The gating circuit measures a predetermined length of strip passing the measuring roll 36 which is a function of this distance through the process line between the unwind stand 30 and the second sensor 40, or the transport distance 46, and depending upon what the sensor 40 sees relative to its set point, issues a signal to a controller 47 that actuates a double acting hydraulic cylinder 48 connected to the movable sensor 39 for touching up the set point of the sensor 39. It will be appreciated that the circuitry employed between the sensor 40 and the controller 47, as well as the circuitry between the sensor 39 and the controller 32 does not constitute a part of the invention other than providing the necessary means to effect the operational functions as stated. The process of delivering a signal to the controller 47 from the gate 44 is repeated to touch up the set point of the sensor 39 and correct for small errors to accurately position the traveling strip laterally at the control point. This enables more accurate work to be done at the process area and substantially increases the gain of the system relative to the prior art embodiment of FIG. 3.

Accordingly, the second loop of the system includes the sensor 40, amplifier 43, gate 44, pulse generator 45, controller 47 and hydraulic cylinder 48, and is designated 49. This system can therefore remove a much greater error by a sample data system than possible with the system of the prior art. While the sensor 39 removes the error at the unwind stand, the sensor 40 removes the error generated in the transport distance. This error must be low frequency and of small magnitude. It may be further appreciated that this system permits control of a strip or web at places where it would otherwise be impossible.

While the embodiment of the invention in FIG. 1 illustrates the use of sensors at one edge of a traveling strip, it should be appreciated that the invention could be applied to a system having sensors at both edges, such as in a centerguide strip system. To more clearly illustrate this possibility, reference is made to FIG. 2 wherein a first centerguide sensor 50 is positioned adjacent the unwind stand 51 to centerguide the strip 52 as it is removed from the reel or coil 53 on the stand, and a second centerguide sensor arrangement 54 is positioned downstream at the control point to remove small errors in lateral positioning of the strip. The strip travels from the coil on the unwind stand under an idler roll 55, over an idler roll 56, under a measuring roll 57 and through the second sensor arrangement 54. As in the embodiment of FIG. 1, the unwind stand 51 may have its position laterally changed by a double acting hydraulic cylinder 58 operated by a controller 59 which receives a signal from an amplifier 60 connected to the output of the centerguide sensor arrangement 50. Large error emanating from the coil is removed at the unwind stand by the sensor arrangement 50 in accordance with its set point.

Small error generated in the transport distance is removed at the control point by the second centerguide sensor arrangement 54. The output of the sensor arrangement 54 is connected to an amplifier 61 which delivers a signal to a gate 62 that also receives a counting signal from a pulse generator 63 connected to the measuring roll 57. As in the embodiment of FIG. 1, upon the measuring of a predetermined strip travel, and upon detection of strip edge deviation from the fixed set point of the fixed arrangement 54, the gate 62 delivers a signal to controller 64 which actuates a double acting hydraulic cylinder 65 to laterally position the sensor arrangement 50 thereby touching up its set point. Other than this embodiment being related to a centerguide system, its operation is the same as that of the embodiment of FIG. 1.

It should be appreciated that sensor arrangements may be of the usual photocell type, wherein error signal is delivered to the amplifier for ultimately applying same to the actuator control or the first sensor control so that corrective motion may be applied to the strip 52. While the embodiments of FIGS. 1 and 2 illustrate a mechanical arrangement for touching up the set point of the first sensor, it should be appreciated that the set point may be touched up through an electronic arrangement. Movement of the set point electronically would then remove the need for having a movable sensor, but would accomplish the same purpose.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

Claims

I claim:

1. An apparatus for guiding a traveling strip comprising an actuator for changing the lateral position of the strip, a first edge sensor adjacent said actuator having a movable set point and driving said actuator to guide the lateral position of the strip in response to said set point, a second edge sensor at a control point remotely located downstream from said first edge sensor having a fixed set point, means measuring the traveling strip, and means responding to the second edge sensor and the measuring means to change the position of the set point of the first sensor and correct for error in strip position at the control point.

2. An apparatus as defined in claim 1, wherein said edge sensors respond to one edge of the strip.

3. An apparatus as defined in claim 1, wherein said edge sensors respond to both edges of the strip to effectively centerguide the strip.

4. An apparatus as defined in claim 1, wherein said measuring means includes a roll driven by said strip, and a pulse generator driven by the roll to provide a pulse rate proportional to strip speed.

5. An apparatus as defined in claim 1, wherein said means responding to the second edge sensor and the measuring means includes a gating circuit.

6. An apparatus as defined in claim 4, and wherein said means responding to the second edge sensor and the measuring means includes a gating circuit.

7. An apparatus for guiding a traveling strip comprising an actuator for changing the lateral position of the strip, a movable edge sensor adjacent said actuator, means driving said actuator to guide the lateral position of the strip in response to a set point of the sensor, a stationary edge sensor at a control point remotely located downstream from the movable sensor, means measuring the traveling strip, and means responding to a set point of the stationary sensor and a signal from the measuring means pursuant to a predetermined length of traveling strip to change the position of the movable sensor and correct for error in the strip position at the control point.

8. An apparatus as defined in claim 7, wherein said edge sensors respond to one edge of the strip.

9. An apparatus as defined in claim 7, wherein said edge sensors respond to both edges of the strip to effectively centerguide the strip.

10. An apparatus as defined in claim 7, wherein said measuring means includes a roll driven by said strip, and a pulse generator driven by the roll to provide a pulse rate proportional to strip speed, and wherein said means responding to the second edge sensor and the measuring means includes a gating circuit.

11. A control system for guiding a travelling web or strip comprising, a primary control loop having an actuator for changing the lateral position of the web or strip and an edge sensor adjacent to the actuator having a movable set point and driving said actuator to guide the lateral position of the strip in response to the set point, a second control loop including an edge sensor located at a control point downstream from the primary control loop sensor, means for measuring the travelling web or strip, and means responding to said second control loop sensor and the measuring means to change the set point of the primary control loop sensor and correct for error in strip position at the control point.