Abstract:
A web tensioning device utilizes a dancer arm which is pivoted by a servo motor controlled in a feedback mode. The controller for the servo motor receives an input signal based on the acceleration (positive or negative) of the dancer arm and implements a position adjustment necessary to maintain predetermined web tension.

Description:
FIELD OF INVENTION  
         [0001]    This invention relates to devices for tensioning webs, such as paper webs or textile webs, during processing.  
         BACKGROUND OF THE INVENTION  
         [0002]    Web tension control is important in paper conversion processes and the like. In high speed operations, even relatively small variations in web tension may cause web processing difficulties downstream. Moreover, in relatively high speed web lines, such as those exceeding about 500 feet per minute (about 150 meters per minute), manual adjustment of web tension is not practical.  
           [0003]    In web tensioning systems where web tension is controlled or maintained by a dancer, a limitation on effective control of web tension is the response time of the dancer inasmuch as often the dancer movement lags behind the actual changes in web tension. A primary cause of such relatively long response time is the weight of the dancer. Thus, it would be desirable to have a web tensioning device that responds readily to variations in web tension with minimum lag time. The present invention satisfies this need.  
         SUMMARY OF THE INVENTION  
         [0004]    A web tensioning device of the present invention provides an improved dancer arm and a controller therefor that responds readily to variations in web tension during web processing with minimal delay, and maintains web tension within predetermined limits without reliance on gravitational forces.  
           [0005]    The present web tensioning device includes a base, a dancer arm with a dancer and pivotably mounted to the base, and a servo motor that pivots the dancer arm in response to a control output signal which depends on web tension requirement at a given point in time. The control output signal to the servo motor is provided by a controller that is operably associated with the servo motor and is responsive to a web tension requirement communicated to the controller either by movement of the dancer arm or a web feed rate actuator.  
           [0006]    The dancer can be a dancer roller, a fixed shaft, an air bearing fixed shaft, and the like.  
           [0007]    In one particular embodiment, the present web tensioning device includes a dancer arm suitable for engaging the web to be tensioned. The dancer arm has a free end portion that carries a dancer roller mounted thereon and a fixed end portion that is pivotably mounted to a base and so as to coact with an angular position sensor that indicates the relative angular displacement of the dancer arm while tension is maintained in the web. The web can be a paper web fed to a paper converting machine, a fabric web, and the like.  
           [0008]    A servo motor is operably associated with the dancer arm for pivotally positioning the dancer arm in engagement with the web in response to a control signal from a controller. Input to the controller is provided by the angular position sensor, a position feedback device such as an encoder, that monitors the angular displacement of the dancer arm from a predetermined position as the web is fed from an unwind roll to further processing station or stations. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]    In the drawing, FIG. 1 is a schematic depiction of the web tensioning device embodying the present invention and utilizing a position feedback mode of operation. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0010]    An industrial web converting machine usually performs its processing function utilizing a continuous sheet or web of paper or other non-woven material (e.g., yarn, wire, tubing, or filament), fabric, foil, and the like. The web is pulled by one or more power driven rollers through a series of idler (or non-driven) rollers to one or more processing stations or stages where the web is folded, cut into segments, shaped, etc. The web is fed into the machine from a relatively large roll called an “unwind roll” which rotates on a roll stand.  
         [0011]    The web often travels at different speeds through different sections of the converting machine and becomes slack from time to time, either by design or otherwise. A web that is slack can become undesirably tangled or wrinkled. To control slack, a web processing system frequently utilizes a dancer which typically is one or more idler rollers or shafts on non-rotating arms that move freely against the web, usually by being mounted on a pivoting arm of some sort. The dancer controls web tension by increasing or decreasing at any point in time the distance traveled by the web. Under balanced conditions the dancer remains stationary, i.e., in a neutral position, but is positioned as needed depending on whether web tension is to be increased or decreased as a result of the operation of the web converting machine or the like.  
         [0012]    In a typical application, a dancer is mounted to a pivotably mounted arm to which a positioning force is applied by a spring, a solenoid, a pneumatic cylinder, a hydraulic cylinder, and the like force generator. The positioning force urges the dancer tightly against the web.  
         [0013]    Often a condition is encountered, e.g., with an out-of-round unwind roll, that causes cyclical web acceleration followed by web deceration at a relatively high frequency. As a result, the dancer rapidly pivots back and forth due to a force supplied by the web to move the mass of the dancer as the web tension cycles between tight and loose. Such rapidly alternating web tension necessarily leads to operating problems downstream.  
         [0014]    The present invention obviates such problems by simulating a “zero mass” dancer vis-a-vis the web in contact therewith. To that end, a servo motor is utilized in lieu of the conventional force generators that have been used to pivot the dancer arm and tension the web. Any action by the web that causes the dancer arm to shift position is monitored by a position sensor, typically an encoder. Other types of position sensors can be utilized as well for this purpose, however. An output signal from the position sensor is transmitted as input to a controller that signals the servo motor to correct the web tension so as to return the dancer arm to its predetermined “neutral” position.  
         [0015]    The term “servo motor,” as used herein and in the appended claims, means an electric, hydraulic or other type of motor that serves as the final control element of an automatic feedback control system for mechanical motion.  
         [0016]    Referring to FIG. 1, a web tensioning system  10  is shown utilizing web tensioning device  12  engaging a tensioned segment of web  14 . Web tensioning device  12  includes dancer arm  16  that carries at the free end  18  thereof a dancer, such as rotatably mounted dancer roll  20  that also serves as a web redirect roller. Fixed end  22  of dancer arm  16  is pivotably mounted to base  24  by shaft  26  which is affixed to dancer arm  16  and in turn is journaled in base  24 .  
         [0017]    Also fixedly mounted to shaft  26  is an encoder, such as incremental rotary optical encoder  30 , which serves as an angular position sensor and indicates the relative angular position of pivot shaft  26 , and thus dancer arm  16 . Information from the rotary optical encoder  30  provides an appropriate input signal to dancer control  34 . The encoder can be an incremental encoder or an absolute encoder, as desired.  
         [0018]    Also operably mounted to pivot shaft  26  is electric servo motor  36  which turns pivot shaft  26 , and thus pivots dancer arm  16 , in response to a control output signal from dancer arm controller  34 . In this manner, initial tension on the web  14  can be set to a predetermined value and thereafter a relatively small rotation of pivot shaft provides information via encoder  30  as input signal to controller  34 . This input signal is processed rapidly by dancer control  34 , and a control output signal is sent to energize servo motor  36 . The control output signal determines what torque output and the direction thereof (i.e., clockwise or counterclockwise) is to be provided by servo motor  36 . The amount of torque to be provided is substantially the same as but opposite to the dancer acceleration or deceleration force, i.e., positive or negative acceleration force, whatever the case may be. The response time of the servo motor  36  to the control output signal provided by dancer control  34  is relatively short, usually of the order of about 0.0005 seconds or less. Thus the torque supplied by servo motor  36  balances out the dancer movement forces as they occur, and web tension remains substantially constant so that the dancer has substantially no inertia to overcome. With respect to the web dynamics, the dancer appears to have “zero mass.” 
         [0019]    The aforedescribed operation of web tensioning system  10  is the so-called feedback control method. A compensating torque command component is supplied to servo motor  36  in proportion to measured acceleration or deceleration of the dancer. In any given application accuracy of the feedback control method is determined by the inertia of the moving parts of the web tensioning device, accuracy of the acceleration and deceleration measurement, and the information processing rate of the controller.  
         [0020]    Alternatively, a control method can be utilized for web tensioning that depends on web feed rate. In such a case the encoder  30  can be omitted, and input to dancer control  34  is provided by a web feed rate actuator in a control loop that includes servo motor  36  and dancer control  34 . That is, a compensating torque command component is applied in proportion to web acceleration or deceleration information obtained from the web feed rate actuator.  
         [0021]    The foregoing description and the drawing are illustrative, and are not to be taken as limiting. Still other variations and arrangements of parts within the spirit and scope of the present invention are possible and will readily present themselves to those skilled in the art.