Abstract:
A passive tensioning system is disclosed for composite material that is dispensed by a composite placement machine. A spool is mounted on a spool shaft and material on the spool is pulled from the spool and applied to a surface. The tensioning system has a drag brake on the spool shaft and a drag brake control for the drag brake. A dancer roll is mounted on a linear slide having a spring force and a slide control is provided for the linear slide. A control system continually varies the drag brake control and the slide control to control the tension of the composite material based on the instantaneous operating characteristics of the composite placement machine.

Description:
FIELD 
       [0001]    The device relates to a system for passively controlling the tension of tow as it is paid out from a creel to a composite placement head. 
       BACKGROUND 
       [0002]    It is necessary to control the tension of tow material as it is paid out from a creel to a composite placement head. A tensioning system is active if it can provide back tension and has the ability to reverse the tow payout direction. Prior art active tensioning systems use bi- directional electronic servoed tensioners having a servo motor for each lane of tow to supply back tension and the ability to reverse the tow if required to take out slack during machine movements that decrease the distance between the head and the creel. The servoed tensioners add cost and complexity to the tow handling mechanism. A tensioning system is passive if it can only provide back tension and has no ability to reverse the tow payout direction. For pressure vessel and rocket motor case construction, simple passive tensioning systems are widely used for filament winding machine applications. Filament winding is a continuous process of pulling a tensioned fiber band from a plurality of spools in a creel. Filament winding machines wrap a wet resin fiber band or prepreg fiber band onto a smooth body of rotation having only convex surfaces so that no fiber bridging occurs. Such tensioning systems may use a simple spring on a dancer arm, and a braking mechanism on the spool. Filament winding tensioners provide tension levels of several pounds of force or higher since the shapes to be wound are usually symmetric, and are tightly wrapped in a continuous manner without stoppage. With the high tension levels, the effects of spool inertial loads are a manageable component for the passive spring design. Spring based tensioners for filament winding size material spools are not required to operate at a tension level of less than several pounds of force. 
         [0003]    In the fiber placement process, it is necessary to place prepreg fiber bands in a discontinuous manner on lay-up tools that have concave areas and near net shape perimeters. The fiber placement process requires frequent starts and stops of the fiber band application. Due to these requirements it is necessary for the fiber placement tensioners to be able to provide low tensions in the magnitude of less than 1 pound, and with a quarter-pound tolerance. This is necessary so the fiber band will not bridge (stretch across the valleys) as it is laminated by a compaction roller onto concave areas of the tool surface. It is not possible at the low tensions used in the fiber placement process to control passive units sufficiently to buffer rapid tow acceleration and deceleration changes and resulting spool inertial loads with fine enough resolution for the fiber placement process. The amount of tension spike buffering from spring loaded dancer rolls on passive tensioners is not adequate for the fiber placement process. Prior art passive tensioning systems cannot adapt to changing spool diameters and changing accelerations and decelerations needed for feeding the tow and stopping the tow during fiber placement operations. When the material spools and tensioners are integrated into the head, it becomes necessary for tensioner and its dancer roll motion to be immune to changing gravity vectors as the head changes its position in the operating zone. Prior art mechanical spring based tensioners are affected by operating orientation. This can result in tension variations from gravity loads affecting the force response of the dancer roll spring. 
       SUMMARY 
       [0004]    A tensioning system for composite material delivered from a creel for composite material layup is passive in that it can only provide back tension, and has no ability to reverse the tow feed direction. The passive system uses a dynamically controlled drag brake on the material spool shaft, and a dancer roll mounted on a dynamically controlled linear slide. The force exerted on both the brake and the dancer roll slide can be continuously varied by a control system. The material spool shaft has an encoder mounted to it to provide angular position feedback control for the spool, and the dancer roll slide has a linear position feedback device to measure its position. The device uses pneumatic based force output devices. The feedback signals are interpreted by a controller to output control signals to electro pneumatic regulators that vary pressure force on a drag brake and liner slide cylinders. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a side view of a creel showing the composite material spools, dancer rolls, and interleaver rollers mounted on the outside walls of the creel, and a composite placement head mounted on the bottom of the creel. 
           [0006]      FIG. 2  is a detail view of one of the creel walls shown in  FIG. 1  showing the path of composite material payout from the spools. 
           [0007]      FIG. 3  shows the opposite side of the creel wall of  FIG. 2 . 
           [0008]      FIG. 4  is a detail view of section  4  of the tensioning mechanisms shown in  FIG. 3 . 
           [0009]      FIG. 5  is a block diagram of the control system for the tensioning system. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0010]    Turning now to  FIG. 1 , a creel with a head mounted thereon is generally designated by the reference numeral  10 . The creel  11  has a generally rectangular shape and has generally rectangular side walls  12 - 14  (only 3 walls are shown) that provide support for a number of spools  15  of composite material or tow  18 . A composite placement head  16  is attached to the underside or bottom wall  17  of the creel, and a compaction roller or shoe  20  is provided at the bottom of the head  16  to press the composite material  18  onto an application surface  22 . The composite placement head  16  has a head centerline  19  which passes through the center of the creel  11  and the compaction roller or shoe  20 . The creel  11  may be attached to the end of a positioning mechanism  23  which maneuvers the head  16  and the compaction roller  20  to various positions and locations so that the composite material may be applied in a desired location and pattern on the application surface  22 . As used herein, the term composite material is used to designate resin impregnated fiber, tow, slit tape, prepreg materials, and other similar materials, all of which are well known to those skilled in the art, and all of which terms are used interchangeably in this application. 
         [0011]    As shown in  FIG. 2 , the outside wall  12  of the creel  11  supports four spindles  26 , each of which supports a spool  15  of composite material. Adjacent to each spindle  26  is an interleaver take-up roll  27  and a dancer roll  30 . The interleaver take-up roll  27  winds up the paper separator strip from the spool  15  of composite material  18  for eventual disposal. The dancer roll  30  is mounted on a dancer roll support bracket  33  (best seen in  FIG. 4 ) for sliding motion on a linear slide or way  32 . Each dancer roll  30  has an axial length approximately equal to the axial length of the adjacent spool  15  of composite material  18  to allow a helical unwinding of the material  18  from the spool  15  without side loads being created on the material by the dancer roll  30 . 
         [0012]      FIG. 2  shows the path of the composite material  18  from the spool  15  to the interleaver take-up roll  27 , then around the interleaver take-up roll  27  to the dancer roll  30 , and around the dancer roll  30  to an edge roller  35  that directs the composite material  18  to the head  16 . Each side of the remaining three adjacent sides of the creel  11  may have a similar outside wall with a similar pattern of spindles  26  and dancer rolls  30 , or the outside walls of the creel  11  may have different numbers of spindles and dancer rolls, or may have a different pattern of spindles and dancer rolls depending on packaging and performance requirements. 
         [0013]      FIG. 3  shows the back surface  36  of the outside creel wall  12  of  FIG. 2 , and  FIG. 4  is a detail view of the one of the tensioning mechanisms shown in  FIG. 3 . The shaft  37  from each of the spindles  26  on the front of the outside wall  12  extends through the wall  12  and has a brake rotor  38  attached to it. One or more drag brake cylinders  42 , each having a brake pad (not shown) may coupled to the wall  36  in proximity to the surface of the brake rotor  38 . If two drag brake cylinders  42  are used, they may be coupled to the wall  36  on opposite sides of the brake rotor  38  in a mirror image position relative to one another. The drag brake cylinders  42  are coupled by a drag brake control line  45  to a control system  46 . The drag brake cylinders  42  may be electrical, hydraulic or pneumatically operated. In the preferred embodiment, pneumatic brakes were used. A locking brake  48  having a brake mechanism that engages one or both sides of the brake rotor  38  may be attached to the wall  36 . The locking brake  48  may have a locking brake control line  49  that is coupled to the control system  46 . The locking brake  48  may be electrical, hydraulic or pneumatically operated. In the preferred embodiment, a pneumatic brake was used. A rotary encoder  50  may be attached to the end of the shaft  37  from the spindle  26 , and the output signals from the rotary encoder  50  may be coupled by an encoder cable  51  to the control system  46 . 
         [0014]    A dancer roll cylinder  54  may be provided for each of the dancer rolls  30  on the front of the outside wall  12 . The dancer roll cylinder  54  may be coupled to the control system  46  by a dancer roll cylinder line  56 . Each dancer roll cylinder  54  may have a cylinder rod  57  connected to a piston  58  inside of the cylinder  54 . The cylinder rod  57  may be coupled to the dancer roll support bracket  33  that extends through the linear slide or way  32  on wall of the creel and may be coupled to a dancer roll  30  on the front of the outside wall  12 . Each dancer roll support bracket  33  may be coupled to a connecting rod  59  that is coupled to an input slide  61  on a linear position feedback sensor such as linear variable displacement transformer (LVDT)  62  that is mounted on the surface  36 . The LVDT  62  may generate signals representing the movement of the dancer roll cylinder  30 , and sends the signals via a LVDT cable  63  to the control system  46 . The dancer roll cylinder  30  may be electrical, hydraulic or pneumatically operated. In the preferred embodiment, a pneumatic cylinder was used. 
         [0015]      FIG. 5  is a block diagram of the control system  46  for the passive tensioning system described in connection with  FIGS. 1-4 . The composite placement system uses a CNC control  68  which determines the movement of the head  16  relative to the application surface  22  and the placement of composite material on the application surface  22  by the head  16 . A value for the tension of the composite material may be set by an operator by means of a tension set point control  65  contained in the CNC control  68 . The tension set point is then processed by the tension control software  67 , and the tension control software  67  operates together with the CNC control  68  to meter the payout of composite material  15  from the head  16 . The tension control software  67  couples commands to an input/output device  66  such as an industrial IO system. The input/output device  66  generates a drag brake pressure analog output signal  70  that is coupled to a drag brake electro-pneumatic regulator  72 . The regulator  72  is coupled to the drag brake cylinders  42  which act on the rotor  38  that is coupled to the spool of material  15 . The rotation of the spool of material  15  is sensed by the encoder  50  which generates an encoder input  51  that is coupled to the input/output device  66 . 
         [0016]    The input output device  66  also generates a dancer arm pressure analog output signal  75  that is coupled to a dancer arm electro-pneumatic regulator  76  the output of which is coupled to the dancer roll cylinder  54 . Changing the pressure in the dancer roll cylinder  54  changes the response characteristics of the dancer roll  30 . The position of the dancer roll  30  is sensed by the linear position feedback cylinder  62 . The linear position feedback cylinder  62  generates a signal that is coupled by the LVDT cable  63  to the output device  66 . 
         [0017]    In operation, the tensioner system adjusts dynamically to the specific operations that are being performed by the composite placement system. The force of the dancer roll  30  is adjusted by the electro-pneumatic regulator  76  and the dancer roll cylinder  54  so that the dancer roll  30  acts like an adjustable spring. The dancer roll cylinder  54  may be a double acting cylinder that may have a constant back pressure on the end carrying the cylinder rod  57  and an adjustable pressure on the end which is coupled to the dancer roll cylinder cable  56  to produce a smooth, varying, spring force movement. Other cylinder designs may be used. Increasing the pressure in the dancer roll cylinder  54  increases the force necessary to depress the dancer roll  30  from the top of its travel in the linear slide or way  32  to the bottom. The force of the drag brakes  42  on the brake rotor  38  and the spring force on the dancer roll  30  vary with the signal from the tension set point control  65  that may be set into the control system  68  by the operator. The tension set point control is typically set to that the tension on the tow is less than one pound. Signals from the rotary encoder  50  may be used to determine the diameter of the composite material on the spool  15 , the inertia of the spool  15 , the speed of rotation of the spool  15 , and the acceleration of the spool  15 . Using this data, the dancer roll spring force may be adjusted dynamically by the tension control software  67  based on the diameter of the spool  15 , the inertia of the spool  15 , and the acceleration of the spool  15 . The force of the drag brakes  42  on the spool is dynamically adjusted based on the diameter of the spool  15 , the inertia of the spool  15 , and the speed and acceleration of the spool  15 . 
         [0018]    The tensioner is used on a composite placement system during the time that composite material on the spool  15  is fed out and applied to a surface  22 . At the end of a laydown path, the feed of material  18  to the head  16  may be abruptly stopped so that the material  18  may be cut as needed. These operations may be performed as fast as possible, resulting in high spool accelerations and decelerations. The CNC control  68  commands when these operations are to occur. During these occurrences, the timing and amount of the force on the drag brakes  42 , and the timing and amount of force exerted by the pneumatic cylinder  54  on the dancer roll  30  may be instantaneously adjusted in real time. 
         [0019]    In preparation for a material payout operation, the dancer roll  30  is pushed to the top of the linear slide  32  before the payout of composite material occurs. This allows the dancer roll  30  to have the maximum travel range as the tow  18  is fed out at high speed. When a tow feed command is sent from the controller  68 , the dancer roll travels  30  towards bottom of the linear slide  32  as tow  18  is fed out from a material spool  15  in order to absorb a portion of the acceleration force on the material spool  15 . This reduces the tension on the tow  18  during the feed operation which helps to maintain the tow set tension accuracy and the tow laydown position accuracy. The control of the dancer roll  30  by the pneumatic cylinder  54  allows the control system to optimize the motion of the dancer roll to help control the tension on the tow  18  during the various operations. Controlling the dancer roller  30  with the pneumatic dancer roll cylinder  54  allows the spring rate of the dancer roll  30  to change as needed to compensate for various dynamic events, such as a different size roll of material, or a different tension setpoint, or an extremely high acceleration or deceleration. The spring rate of the pneumatic cylinder  54  can also be varied as a function of the position of the dancer roll  30 . 
         [0020]    During a cutting operation, the supply of tow  18  from the spool  15  needs to stop suddenly. The drag brakes  42  are tightened against the brake rotor  38  to stop the spool  15  quickly to prevent the tow from unspooling. Additionally, the pressure in the dancer roll cylinder  54  is altered to cushion the stopping motion of the system. 
         [0021]    The creel  11  may be mounted on the end effector  23  of a composite placement machine. As the end effector  23  is maneuvered in three directions under the control of the CNC control  68 , the creel  11  is tilted. Tilting the creel  11  repositions the gravity vector relative to the head centerline  19 , and the effective force due to the component weight of the dancer roll  30  changes and may be programmed into the CNC control  68 . The pressure in the dancer roll cylinder  54  may be changed accordingly in order to compensate for the gravity vector force change seen by the dancer roll  30  due to the repositioning of the dancer roll  30  relative to gravity. 
         [0022]    The encoder  50  is mounted to the material spool shaft  37  to detect the change of angular position of the spool  15  in response to a payout command in order to compute the diameter of the spool  50  in real time. The real time diameter of the spool  50  is used to adjust the force of the drag brakes  42  on the brake rotor  38  and the effective spring force on the dancer roll  30  to control the tension on the tow  18  more effectively. 
         [0023]    The locking brake  48  is provided to lock the brake rotor  38  and thereby the spool  15  against rotation during certain machine modes such as for head servicing, head changing, or spool changing. 
         [0024]    The passive tensioner described above provides the following advantages: 
         [0025]    1. The elimination of expensive bidirectional servoed tensioner motors controlling the tension. 
         [0026]    2. The ability to operate in all orientations. 
         [0027]    3. The ability to be tightly packaged on a head that includes a creel. 
         [0028]    4. The ability to operate at low set point tensions of less than one half- pound. 
         [0029]    5. The ability to separate the effects of spool inertial loads from changing the low tension control of the tow. 
         [0030]    If the spools and passive tensioner described above reside in a creel that is separate from the head, and the head moves toward and away from the creel thus generating fiber slack, a fiber festoon may be used to take up the slack. A fiber festoon will enable real time slack elimination during machine motion to compensate for the passive tensioner&#39;s inability to reverse to remove fiber slack. Although the device described uses pneumatic based force output devices, those skilled in the art will understand that electric force output devices may be used. 
         [0031]    Having thus described the device, various modifications and alterations will occur to those skilled in the art, which modifications and alterations are intended to be within the scope of the invention as defined by the appended claims.