Patent Abstract:
A method for controlling a calendering system having a first roll having a first roll speed controller and a second roll having a second roll speed controller is disclosed. An exemplary method comprises the steps of: (a) setting the first roll at a desired process speed with the first roll speed controller; (b) determining a target torque of the first roll; (c) contactingly engaging the first and second rolls; (d) determining an actual torque of the first roll; (e) comparing the target torque and the actual torque; and, (f) adjusting a speed of the second roll with the second roll speed controller to maintain the target torque of the first roll according to the comparison of the target torque and the actual torque.

Full Description:
FIELD OF THE INVENTION 
     The present invention relates to processes for controlling the torque developed between opposing rolls in a calendering operation. More particularly, the present method relates to the control of torque in a calendering system that is suitable for use with a paper making and/or converting operation. 
     BACKGROUND OF THE INVENTION 
     It is known to those of skill in the art that a calender or calender stack is a series of rolls, usually steel or cast iron, mounted horizontally and/or stacked vertically. During machine calendering in a paper processing application, the dry paper passes between the rolls under pressure, thereby improving the surface smoothness of the paper caused by, for example, imperfections in felt marks, cockle lumps, fibrils, and the like. Additionally, such a calender stack can improve the gloss and create a more uniform caliper and porosity. These improvements can make the paper better suited for printing and decrease manufacturing problems during printing and rewinding operations. As would be known to those of skill in the art, a typical loading range between opposed rolls generally varies from 0 N/cm (Gap) to 85,000 N/cm (0 lbs. per linear inch (Gap)-1,000 lbs. per linear inch). 
     Some known calendering systems are provided with a steel roll and a roll having a rubberized coating. In such systems, the steel roll is known as the king roll and it may be located in the top or bottom position of the calender. The king roll may be larger or smaller than the other rolls in the calender stack and may be crowned (i.e., has a larger or smaller diameter in the center of the roll as compared to the ends) in order to permit even pressure being applied to a substrate passing between opposing loaded roll faces. However, one of skill in the art will realize that the king roll and/or the queen roll can be crowned and/or provided with variable crown capability. A variable crown can be achieved using various methods including, a pressurized oil filled roll where the oil pressure controls the degree of crowning, internal hydraulic shoes that press against the roll shell to control the degree of crowning, or roll bending. The roll in mateable engagement with the king roll is known as the queen roll. In certain operations, the queen roll can be provided with a rubberized coating in order to increase the engagement of the surface of the queen roll with the surface of the king roll. 
     In conventional calendering operations, as the two rolls come in contact, one or both surfaces of the king roll and/or queen roll deform. In operations where the queen roll is provided with a rubberized coating, such a coating will be provided on the queen roll in about ½-inch to 1-inch (1.27 cm to 2.54 cm) in thickness. As the surface of the rubberized queen roll deforms, the rubberized coating deforms in order to pass through the nip formed between the king roll and queen roll. This cover flows to conform to the nip surface. Such conformation can result in shear forces being formed across the area of contact between the two rolls. 
     A second mechanism that can create shear forces across a nip in a calendering operation exists when one roll of the calender attempts to drive the second roll. As one roll attempts to speed up or slow down, it forces the rubberized coating deposited upon the second roll to deform in such a way as to force the second roll to speed up or slow down. In doing so, the interaction between the first and second rolls of the calender create a shear force that is transmitted through a substrate disposed therebetween. This shear force cannot be avoided in a calendering operation with only one driven roll. These forces can be generated by rolls of a calender system having steel rolls and/or rolls having no coating disposed thereon due to frictional forces caused by roll deformation. 
     When the rolls forming the calender nip are separately driven and are forced together, they are provided with the capability of transferring forces across the nip to drive each roll. If the rolls tend towards asynchronous behavior (i.e., the rolls are not surface speed matched in the nip), a net torque is developed between the rolls with associated forces across the nip, and the resulting calendering operations can become unpredictable. The nip torque imbalance creates a shear force across a material passing between the rolls of the nip that is greater than the shear forces caused by the roll deformation alone. This shear force can damage a substrate placed between the rolls of a calender system. 
     A known method for controlling the shear force developed across the nip in a calendering operation provides for an operator to manually set the torques between multiple drives to minimize the shear force transmitted through the substrate. The most common means to manually manipulate the torque division between the multiple drives are 1) through torque division to multiple motors of a common speed controller output, 2) operating one drive to control speed and one to provide a constant torque or 3) operating one speed controller as a lead, or master, speed controller and the second as a droop, or current compounded, speed controller. 
     Such systems may be suitable for use in situations where constant loading of the rolls of a calender system is utilized. However, some processes require variable calender loading as the product (such as paper) passes between the calender rolls. In variable calender loading systems where total motor torque loads can change, manual adjustments such as those used in constant loading processes, are not suitable. This is because an operator of a variable calender system would be required to provide continual (if not continuous) adjustments to the motor torques to maintain the desired minimum level of shear force in the nip. 
     Thus, it would be useful to provide for a method to control torque in a calendering system that keeps one roll torque (or current) at a desired value while a second roll (preferably rubber covered) is nipped against the first roll. Such a mechanism would effectively change the torque on the second roll to affect a change of the torque utilized by the first roll. Such a process would control the amount of shear forces developed across a substrate passing between the calender rolls. This can minimize the shear damage to the substrate and improve the tensile loss during a calender, combiner, or embosser/laminator operation. This can effectively reduce web losses through reduced substrate damage by minimizing shear forces transmitted across the substrate. 
     SUMMARY OF THE INVENTION 
     The present invention provides for a method for controlling a calendering system having a first roll having a first roll speed controller and a second roll having a second roll speed controller. The method comprises the steps of: (a) setting the first roll at a desired process speed with the first roll speed controller; (b) determining a target torque required of the first roll; (c) contactingly engaging the first and second rolls; (d) determining an actual torque of the first roll; (e) comparing the target torque and the actual torque; and, (f) adjusting a speed of the second roll with the second roll speed controller to maintain the target torque of the first roll according to the comparison of the target torque and the actual torque. 
     In an alternative embodiment of the present invention, the method comprises the steps of: (a) setting the first roll at a desired process speed with the first roll speed controller; (b) determining a target torque of the first roll; (c) contactingly engaging the first and second rolls; (d) determining an actual torque of the first roll; (e) determining a torque division between the first and second rolls by comparing the target torque and the actual torque of the first roll; and, (f) adjusting a speed of the second roll with a second roll torque controller to maintain the target torque of the first roll according to the torque division. 
     In yet another embodiment of the present invention, the method comprises the steps of: (a) setting the first roll at a desired process speed with the first roll speed controller; (b) determining a target torque of the first roll; (c) contactingly engaging the first and second rolls; (d) determining an actual torque of the first roll; (e) determining a torque set point for the second roll by comparing the target torque and the actual torque of the first roll; and, (f) adjusting a speed of the second roll with the second roll torque controller to maintain the target torque of the first roll according to the torque set point. 
     In yet still another embodiment of the present invention, the method comprises the steps of: (a) setting the first roll at a desired process speed with the first roll speed controller; (b) determining a target torque of the first roll; (c) contactingly engaging the first and second rolls; (d) determining an actual torque of the first roll; (e) determining a droop of the second roll speed controller by comparing the target torque and the actual torque of the first roll; and, (f) adjusting a speed of the second roll with the second roll speed controller to maintain the target torque of the first roll according to the droop. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an exemplary process for controlling torque (or current) in a calendering system in accordance with the present invention; 
         FIG. 2  is a block diagram of an alternative embodiment of a torque (or current) control process; 
         FIG. 2A  is a block diagram of a further embodiment of a torque (or current) control process; 
         FIG. 2B  is a block diagram of a further embodiment of a torque (or current) control process; 
         FIG. 3  is a block diagram of a further embodiment of a torque (or current) control process; 
         FIG. 4  is a block diagram of a further embodiment of a torque (or current) control process; and, 
         FIG. 5  is a block diagram of a further embodiment of a torque (or current) control process. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Provided herein are seven exemplary, but non-limiting embodiments on methods to affect the torque of the queen roll of a calendering system that, in turn, can cause a predictable change in the king roll torque of the calendering system. Six of the exemplary, but non-limiting, systems described herein utilize a process controller in concert with speed controllers and/or torque controllers in order to effectuate control of the forces generated between calendering rolls during a calendering operation. The seventh exemplary embodiment described herein does not require the use of a process controller in order to effectuate system control. However, it should be easily recognized and understood that the following systems could also be utilized in any apparatus, process, and/or situation where one roll is required to apply pressure to another. This would include processes utilizing multiple nip and/or gap combinations having at least two calendering rolls. These exemplary processes described herein could be utilized in combiner processes, embossing processes, laminating processes, processes using pressure rolls, and combinations thereof. 
     In a typical DC motor system, it should be realized that armature current draw is directly proportional to the torque produced by the motor. However, it should be realized by one of skill in the art that in AC motor systems that motor current (or total current) is not directly proportional to torque. Thus, by convention, torque is the preferred term used herein. However, one of skill in the art will understand that torque and current should be understood to be used interchangeably herein when describing exemplary DC motor systems. Additionally, some AC drives (i.e., vector-controlled AC drives, etc.), a “torque producing” component of current is proportional to torque, and is available for control. This component of such an AC drive could be treated as a DC motor current in control of motor torque. 
       FIG. 1  depicts a block diagram of an exemplary process  90  for controlling torque in a calendering system  42 . The calendering system  42  is generally provided with a first roll  12  (also referred to herein as king roll  12 ) and a second roll  14  (also referred to herein as queen roll  14 ). The first roll  12  is generally rotated by mechanical connection to first roll motor drive  18  which is operatively connected to first roll motor  16 . Similarly, the second roll  14  is generally rotated by mechanical connection to second roll motor drive  22  which is operatively connected to second roll motor  20 . 
     Generally, first roll motor  16  cooperatively associated with first roll  12  is controlled by a manipulation of the first roll  12  speed by first roll speed controller  28  and first roll torque controller  24 . This manipulation can be provided by first roll motor speed sensor  38  to provide feedback to first roll speed controller  28  and then provide a torque (or current) correction to first roll torque controller  24 . The torque correction provided by first roll torque controller  24  can either increase or decrease the torque provided by first roll motor  16  to either increase or decrease the speed of first roll  12 . 
     As with first roll motor  16 , second roll motor  20  cooperatively associated with second roll  14  is controlled by a measurement of second roll  14  speed by second roll motor speed sensor  36  that provides feedback to second roll speed controller  30  that then provides a torque, or current, correction to second roll torque controller  26 . The torque, or current, correction provided by second roll torque controller  26  can either increase or decrease the torque (current) provided by second roll motor  20  to either increase or decrease the surface speed of second roll  14 . 
     In accordance with the present invention, the motors associated with the rolls of a calendering process are preferably provided with load sharing. In other words, both motors are speed controlled all the time. However, the second roll speed controller  30  associated with the second roll  14  of the calendering system  42  can have its speed reference  44  adjusted to compensate for the reaction of second roll  14  to nip load changes between first roll  12  and second roll  14 . It was surprisingly found that cooperative coupling of first roll torque controller  24  with second roll speed controller  30  and/or second roll torque controller  26  can reduce or even prevent the development of a resultant torque between first roll  12  and second roll  14  that produces transmittable shear forces upon a web material  40  moving in a machine direction MD and disposed between first roll  12  and second roll  14 . Thus, in accordance with the present invention, it is desirable to keep the first roll  12  torque constant in order to provide for the second roll  14  torque to produce the work energy going into a rubber coating disposed upon the second roll  14  that is being deformed due to contact with the first roll  12 . In other words, the desired torque from the first roll motor drive  18  is affected by the torque applied to the second roll motor drive  22 . 
     As shown in  FIG. 1 , establishment of the correct torque from the second roll motor drive  22  can be provided by process controller  34 . When first roll  12  and second roll  14  are in non-contacting engagement (i.e., first roll  12  and second roll  14  are in an ‘un-nipped’ or ‘gapped’ state), process controller  34  is disengaged and the speed of second roll  14  is adjusted independently of first roll  12  by second roll speed controller  30  through second roll torque controller  26 . 
     The desired speed of first roll  12  can be determined by the operators to achieve process objectives, such as production rate and sheet control, from the calender system  42 . Additionally, the desired speed of first roll  12  can be determined by any downstream processing needs for web material  40 . If the web material  40  remains tight at the in-running nip and is breaking, the surface speed of the first roll  12  can be reduced by adjusting what is known to those of skill in the art as the calender draw. If the web material  40  at the in-running nip is too loose, as determined by the web material  40  sagging and weaving, the calender draw can be adjusted to speed up the first roll  12 . 
     A calender system  42  useful with the present invention can be operated with the first roll  12  and second roll  14  in non-contacting engagement or in contacting or mating engagement (i.e., providing a ‘nip’ therebetween). In any regard, the calender system  42  should be started and first roll  12  and second roll  14  accelerated to operating speed. Such start-up and acceleration can be done in either a ‘nipped’ or ‘gapped’ configuration. In a ‘nipped’ configuration, the first roll  12  sets the calender system  42  speed. Because the surface of the second roll  14  tends to deform, the second roll  14  speed should not be used as a process reference. In a ‘gapped’ mode, both the first roll  12  and second roll  14  run at the same speed to create a nip without damaging the web material  40  disposed therebetween when contact occurs between first roll  12  and second roll  14 . 
     The target first roll  12  torque (current) value is determined by providing a gap between the first roll  12  and second roll  14  and operating the calender system  42  with, or without, web material  40  disposed therebetween. The torque (current) produced by first motor  16  during this gapped condition is the torque required to maintain the first roll  12  at the necessary calendering system  42  speed. The first roll  12  in this configuration is not doing any work on its surface, on or upon any material disposed between first roll  12  and second roll  14 , or upon the surface of second roll  14 . This value provides a possible target torque for the first roll  12  that can minimize any torque transfer between the first roll  12  and second roll  14 . 
     At any time in the calendering process, the first roll  12  and second roll  14  can be matingly engaged. As is known to one of skill in the art, such mating engagement can occur by the provision of air pressure to inflate airbags or air cylinders that produce a force to load the first roll  12  and second roll  14  of calendering system  42  together. In another instance, hydraulic oil pressure can be utilized to operate hydraulic cylinders cooperatively associated to each of first roll  12  and second roll  14  of calendering system  42  to produce the force to load the first roll  12  and second roll  14  together. In yet another embodiment, a jack screw, driven either manually or with a motor, can be utilized to produce the force necessary to load the first roll  12  and second roll  14  together. In any regard, each of these processes, and others known to those of skill in the art, can give a measured degree of loading, either by actual loading pressures, weights of first roll  12  and second roll  14  and load or relief pressure levels, or by movement of the first roll  12  relative to the surface of the second roll  14 . 
     The actual first roll  12  torque is obtained from the first roll motor  16  by way of a torque sensor preferably in electrical communication with first roll torque controller  24  as a measured or calculated value. All motors are preferably provided with measures of torque that can be extracted and used by any controllers or computers external to the first roll motor  16 . 
     When first roll  12  and second roll  14  are in contacting engagement, process controller  34  dynamically compares the in situ output from first roll torque controller  24  ultimately supplied to first roll  12  through any associated gearing ratios in first roll motor drive  18  to a target torque desired by an operator of, or process requirement for, calendering system  42 . In other words, when the target torque and actual torque have been determined, the next step is to compare and determine the error as a function of target torque and actual torque. This error is then used by an algorithm associated with process controller  34  to produce an output value that is used to change the speed of second roll  14  to regulate the first roll  12  torque. The process controller  34  incorporates an integral term that is a coefficient multiplied by the time integral of the error value and adds this product to the proportional term (another coefficient multiplied by the error) to form an output of the proportional plus integral controller. For a constant error, the proportional term remains constant, and the integral term increases with time (assuming constant coefficients). This integral increases the output of the proportional plus integral controller until the calendering system  42  responds accordingly and makes the error zero. 
     As would be appreciated by one of skill in the art, the values of torque for first roll  12  and second roll  14 , in either the ‘gapped’ state or the ‘nipped’ state, can be stored as an array. These torque values may be stored with a registration value according to the acquisition frequency of the values. Compilation of the torque values for the first roll  12  and second roll  14  values can be used to develop a torque profile. This profile may then be used together with the profiles of similar web material  40  to determine a typical torque profile for the particular type of web material  40  involved in the analysis. Any of these profiles may be used to alter the control scheme to adjust the torque profile applied by calendering system  42  to subsequent web material  40 . The profiles can be used to predict when changes in the web material  40  may occur within the web material  40  in order to allow for compensatory changes in the control algorithm. 
     The profiles may also be used as data to support the use of intelligent or model-based control schemes to affect the manufacture of web material  40 . As an example, a neural network may take as inputs the operating conditions known during the process of manufacturing web material  40  that correspond to each portion of the web material  40  and associate those known conditions with the torque(s) required by the same portion of the web material  40  provided by the web material  40  history. The neural network may then predict changes necessary to the manufacturing and calendering conditions to yield a desired torque profile for web material  40 . The neural network may then control the manufacturing and calendering processes to dynamically implement the predicted torque changes. The neural network may associate known manufacturing and calendering conditions with the torque values these conditions produced, as provided by the torque history. These associations may form the basis for predictions by the neural network of the operating conditions that will yield a desired torque profile in subsequent web material  40 . 
     Referring again to  FIG. 1 , an exemplary, but non-limiting, process to influence the torque in the first roll  12  can use a process controller  34  to manipulate the second roll speed controller speed reference  44  through a subtractor  46  (a subtractor  46  may also be known in the art as a summer having appropriate polarity). This can dynamically change the speed of the second roll  14  through the second roll speed controller  30 . As shown, second roll speed controller  30  can be influenced by the output of a process controller  34 , operating as a proportional plus integral controller, through the second roll speed controller speed reference  44  to the speed controller  30  The proportional plus integral controller operates as described supra. Process controller  34  can monitor (either continuously or by sampling) the output of actual torque signal of the first roll torque controller  24  and send a correction to the second roll speed controller speed reference  44 . 
     In a gapped condition, both speed control systems for first roll  12  and second roll  14  preferably operate independently and the process controller  34  is turned off. When the calender system  42  operates in a “nipped” condition, the process controller  34  is turned on in order to provide a load share control for exemplary process  90 . This can be accomplished by setting the initial output value for the process controller  34 . 
     The first value the process controller  34  sends to the second roll speed controller speed reference  44  is zero, in order to keep the same target speed for the second roll speed controller  30 . At the same time, the process controller  34  minimum and maximum output limits are set at the initial value of zero and can increase steadily (i.e., ramp) to their final values. 
     When the calender system  42  changes from a “nipped” condition to a “gapped” condition, the process controller  34  is turned off with its limits set to the initial values. The transition from “gapped” condition to “nipped” condition and back to “gapped” condition can be accomplished by a switching mechanism  93 . An exemplary switching mechanism  93  can utilize a physical switch that senses the distance, loading pressure, and/or force necessary to contact the first roll  12  and second roll  14 . Alternatively, an exemplary switching mechanism  93  can provide for a measurement of the distance moved compared to an operator entered point of contact of first roll  12  with second roll  14 . 
     Speed Controller Droop 
     When a typical DC motor is operated with a constant armature voltage, the speed of the motor changes as the load is increased. This speed/load characteristic of a motor is known to those of skill in the art as droop. A positive droop indicates a decrease in motor speed. A negative droop indicates an increase in motor speed. A similar function can be duplicated in a speed controller by feeding a portion of the output from the speed controller to the input of the speed controller in a feedback loop. This is known to those of skill in the art as droop or current compounding. 
     As used herein, a controller can consist of operations consisting of input, comparison, processing algorithms, output functions, and combinations thereof. In operation, a controller can utilize any or all of these functions to define an output. A droop controller can be as simple as a single input, multiplier algorithm, or an output. 
       FIG. 2  depicts a block diagram of an alternate embodiment of an exemplary process  10  for controlling torque in a calendering system  42 . Here the torque in the first roll  12  is influenced by use of a droop controller  32  to control droop (i.e., current compounding) to either dynamically increase or decrease the output of second roll speed controller  30 . As shown, second roll speed controller  30  can be influenced by the output of the process controller  34  operating as a proportional plus integral controller through the droop controller  32  as described supra. 
     Droop controller  32  monitors (either continuously or by sampling) the output signal of the second roll speed controller  30  and sends a small portion of this output back to the input of second roll speed controller  30  to supplement the speed signal feedback input to second roll speed controller  30 . This process can effectively reduce the effect of the integral term output from process controller  34  and provide for the second roll speed controller  30  to allow a small error in the speed signal feedback. As would be realized by one of skill in the art, increasing the droop of the second roll speed controller  30  can effectively “soften” the second roll speed controller  30  and allow for the first roll motor  16  to increase its torque output to first roll  12 . Decreasing droop causes the second roll speed controller  30  to provide more torque to the second roll  14  by second roll motor  20  thereby decreasing the torque supplied by the first roll motor  16  to the first roll  12 . It should be understood that one of skill in the art could use both positive and negative feedback to create the range of droop suitable for use with the present invention. 
     In a gapped condition, both speed control systems for first roll  12  and second roll  14  operate independently and the process controller  34  is turned off. The droop controller  32  is provided with a manually entered value at this time. When the calender system  42  operates in a “nipped” condition, the process controller  34  is turned on in order to provide a load share control for exemplary process  10 . This can be accomplished by setting the initial torque value for the process controller  34 . 
     The first value the process controller  34  sends to the droop controller  32  is the same value as the manually entered droop value used during the “gapped” condition prior to going to a “nipped” condition. At the same time, the process controller  34  minimum and maximum output limits are set at the initial value and can increase steadily (i.e., ramp) to their final values. The resulting droop value is then sent to the droop controller  32  that has an input supplied by process controller  34  when a nipped condition is sensed. 
     When the calender system  42  changes from a “nipped” condition to a “gapped” condition, the process controller  34  is turned off with its limits set to the initial values. In other words, the original operator entered manual droop value is used in the droop controller  32 . The transition from “gapped” condition to “nipped” condition and back to “gapped” condition can be accomplished by the use of switching mechanism  93  as described supra. 
     As described (i.e., separate controllers and power supplies for each motor, regardless of whether AC or DC current is utilized for each motor), the two speed controllers act as described supra. This is because each roll motor speed controller  28 ,  30  can act on the total power applied to each roll motor  16 ,  20  independently from the other roll motor speed controller  28 ,  30 . 
     Second Roll Motor Field Adjustment 
       FIG. 2A  depicts a block diagram of an alternative exemplary process  10 A for controlling torque in a calendering system  42  (i.e., to the speed controller droop system described supra). In this alternative process, another type of drive, known to those of skill in the art as a common power supply DC drive, one motor (usually the first roll motor  16 ) of a calendering system  42  is driven and controlled from a main power supply and/or a field current controller. The second motor (usually the second roll motor  20 ) is driven from the main power supply but controlled by the field current supplied from a field current controller  50  to second roll motor  20 . Increasing field current causes the second roll motor  20  to slow down. Decreasing the field current causes the second roll motor  20  to speed up. Alternatively, both first roll motor  16  and second roll motor  20  can be controlled by their respective fields. 
     A second roll speed controller  30  based on a process of adjusting the field current to second roll motor  20  can be arranged so that the increasing output from second roll speed controller  30  subtracts from a constant value of field current and reduces the field current of second roll motor  20 , causing the second roll motor  20  to speed up in order to minimize the error feedback provided to second roll speed controller  30 . Droop controller  32  acts as previously described for  FIG. 1  supra, when the second roll speed controller  30  changes the field current to affect a change in speed of second roll motor  20  and second roll  14 . While nipped, if the second roll speed controller  30  seeks to increase the speed of second roll motor  20 , the output of second roll speed controller  30  is increased and the corresponding droop value from droop controller  32  feeds some of the signal back to the input of the second roll speed controller  30  to reduce its effect. The controller action can change a direct acting controller (i.e., the output of second roll speed controller  30  increases for an increased set point) into a reverse acting controller (i.e., field current reference  48  decreases for an increase of the set point for second roll speed controller  30 ). One of skill in the art should understand that such a reverse acting controller that provides an input to second roll speed controller  30  to the field current reference  48  can be used herein with appropriately selected limits, initial values, and droop polarity. 
     In a gapped condition (first roll  12 /second roll  14  separated), both speed control systems for first roll  12  and second roll  14  operate independently and the process controller  34  is turned off. The droop controller  32  is provided with a manually entered value at this time. When the calender system  42  operates in a “nipped” condition (first roll  12 /second roll  14  contacting), the process controller  34  is turned on in order to provide a load share control for process  10 A. This can be accomplished by setting the initial value of the process controller  34 . 
     The first value the process controller  34  sends to the droop controller  32  is the same value as the manually entered droop value used during the “gapped” condition prior to going to a “nipped” condition. At the same time, the process controller  34  minimum and maximum output limits are set at the initial value and can increase steadily (i.e., ramp) to the final values as discussed supra. The resulting droop value is then applied to the droop controller  32  that also has an input supplied by the output of process controller  34  when a nipped condition is sensed. 
     When the calender system  42  changes from a “nipped” condition to a “gapped” condition, the process controller  34  is turned off with its limits set to their initial values. In other words, the original operator entered manual droop value is used in the droop controller  32 . The transition from a “gapped” condition to a “nipped” condition and back to a “gapped” condition can be accomplished by the use of a switching mechanism  93  as described supra. 
     Speed Reference Manipulation on Speed Controller with Droop 
       FIG. 2B  depicts a block diagram of an exemplary but non-limiting alternative embodiment of a process  10 B for controlling torque in a calendering system  42 . In this process  10 B, process controller  34  is capable of manipulating the second roll speed controller speed reference  44  through a subtractor  46 . Additionally, the output from subtractor  46  that becomes the input to second roll speed controller  30  can then be further compensated with the use of a manually manipulated droop controller  32  as described supra. This alternative process can provide for the recognized benefits inured with both the speed reference control scheme as described with respect to  FIG. 1  with the benefits of a speed controller droop control scheme as described in association with  FIG. 2 . The gapped to nipped to gapped transitions of calendering system  42  can be identical to those as described supra. Additionally, the droop value manually entered into droop controller  32  can be determined by the operator to benefit the process of web material  40  by calender system  42  while the calender system  42  transitions from gap to nip to gap. 
     Similarly, it should be evident to one of skill in the art that the features of the speed reference manipulation of a drooped speed controller as described with regard to  FIG. 2B  can also be applied to the second roll motor field adjustment process as described with reference to  FIG. 2A . Such an exemplary system would provide a combination of the benefits realized from each of the systems if utilized individually. In any regard, one of skill in the art would understand that the various embodiments of the calender control processes described herein can be combined in virtually any manner to provide the control scheme required for the particular calendering process utilized and to realize any combined benefits cooperatively associated thereto. 
     Torque (Current) Division Between the First Roll and Second Roll 
       FIG. 3  depicts a block diagram of an alternative embodiment of an exemplary, but non-limiting, process  60  for controlling torque in a calendering system  42 . In this method of control for calendering system  42 , when a gapped condition exists between first roll  12  and second roll  14 , the first roll speed controller  28  manipulates the first roll torque controller  24  and the second roll speed controller  30  manipulates the second roll torque controller  26  independently. However, when a nipped condition exists between first roll  12  and second roll  14 , the first roll speed controller  28  manipulates both the first roll torque controller  24  and the second roll torque controller  26 . In this process  60 , the output torque signal of the first roll speed controller  28  is preferably divided and scaled between the first roll motor torque controller  24  and second roll motor torque controller  26  by a function  66  that collectively adds up to 100% through torque division multipliers  62 ,  64 . By way of non-limiting example, the output of first roll speed controller  28  provides a portion of its output therefrom to one motor (e.g., X percentage of the output from first roll speed controller  28  to the first roll motor  16  from first roll torque (current) division multiplier  64 ) and the remainder to the other motor (e.g., 100% minus X percentage of the output from first roll speed controller  28  to the second roll motor  20  from second roll torque (current) division multiplier  62 ). It should be clear to those of skill in the art that in a gapped condition, both portions of the function can equal the same number, typically operator-entered. 
     To implement such an exemplary controller system, one of skill in the art will understand that the output of the process controller  34  can be used to adjust the first roll load share multiplier  64 . If the torque supplied to first roll motor  16  driving first roll  12  must be increased, the output of first roll load share multiplier  64  should be increased and the corresponding output of the second roll load share multiplier  62  should be decreased. However, if the torque supplied to first roll motor  16  driving first roll  12  must be decreased, then the output of first roll load share multiplier  64  should be decreased and the corresponding output of the second roll load share multiplier  62  should be increased. 
     In a gapped condition (first roll  12 /second roll  14  separated), both speed control systems for first roll  12  and second roll  14  operate independently and the process controller  34  is turned off. The torque (current) division multipliers  62 ,  64  can be provided with manually entered values. When the calender system  42  operates in a “nipped” condition (first roll  12 /second roll  14  contacting), the process controller  34  is turned on in order to provide a load share control for exemplary process  60 . This can be accomplished by setting the initial value of the process controller  34 . 
     The first value the process controller  34  sends to the torque (current) division multipliers  62 ,  64  is the same value as the manually entered torque (current) division multiplier  62 ,  64  values used during the “gapped” condition prior to going to a “nipped” condition. At the same time, the process controller  34  minimum and maximum output limits are set at the initial value and can increase steadily (i.e., ramp) to their final values. Concurrently, the output of the first roll speed controller  28  to the input of second roll torque division multiplier  62  should preferably be increased by the difference in the outputs of the second roll speed controller  30  and the properly scaled output of the first roll speed controller  28  at the time of transition from nip to gap to account for potential differences in load torques for the two different rolls. 
     When the calender system  42  changes from a “nipped” condition to a “gapped” condition, the process controller  34  is turned off with its limits set to their initial values. Next, the second roll speed controller  30  is turned on with its initial value set to a value that will maintain the input of second roll torque controller  26  through the second roll torque division multiplier  62  at the transition. Additionally, the original operator entered current division values are used in the torque (current) division multipliers  62 ,  64 . In the nipped condition and immediately prior to the gapped condition, the first roll speed controller torque command may not be fast enough to provide the proper torque signal to the first and second roll torque controllers  24 ,  26 . A feed-forward control that relates torque-to-nip conditions (i.e., a nip force—the amount of loading pressure or nip width) can be useful to prevent too much torque from being applied to the nip and the over-speeding of either roll motor  16 ,  20  when the calender achieves a gap condition between the rolls  12 ,  14 . First roll speed controller  28  proportional gain scheduling based upon the first roll torque division multiplier  64  may be desirable in order to keep the speed response of the first roll motor  16  constant over the range of operation and improve response to fast changing calender system  42  load conditions. A transition from a “gapped” condition to a “nipped” condition and back to a “gapped” condition can be controlled by the use of a switching mechanism  93  as described supra. 
     It should be understood by those of skill in the art that the implementation of the torque division multipliers  62 ,  64  can be based on percent, per unit, or any other desired base multiplier. Further, it should be clear that a variation of this embodiment may require no particular change in the first roll torque division multiplier  64 . If this is the case, the output of the first roll torque division multiplier  64  can remain constant, and all control can be accomplished by the process controller  34  by properly adjusting the second roll torque division multiplier  62  to accomplish the desired torque control. The method described herein does not create a base for percent, per unit, or any fixed ratio for calculations. 
     Torque Target Set Point for Queen Roll Drive 
       FIG. 4  depicts a block diagram of an alternative, but non-limiting, embodiment of a process  70  for controlling torque in a calendering system  42 . As shown, a first roll speed controller  28  controls the torque controller  24  for first roll motor  16 . Second roll motor  20  is controlled by second roll torque controller  26  when a nipped condition exists between first roll  12  and second roll  14 . The first roll speed controller  28  produces the torque necessary to control the speed of first roll motor  16  thereby controlling the speed of first roll  12 . The second roll torque controller  26  produces the torque required to accommodate the set point torque for the second roll motor  20 . In nipped configuration the output of process controller  34  provides the torque set point for the second roll torque controller  26 . If the signal from first roll motor torque controller  24  indicates that the torque from the first roll motor  16  should be increased, the second roll motor torque controller  26  set point is decreased by process controller  34 . However, if the first roll motor  16  torque needs to be decreased, the second roll motor torque controller  26  set point is increased by process controller  34 . This can be accomplished by the process controller  34  output subtracting from a constant value to provide the appropriate signal change to the second roll motor  20  torque loop. The controller action can change a direct acting controller (i.e., the output of process controller  34  increases for an increased set point) into a reverse acting controller (i.e., the set point for torque controller  26  decreases for an increase of the set point for process controller  34 ). One of skill in the art should understand that such a reverse acting controller can be used herein with appropriately selected limits and initial values. In a gapped condition, preferably both speed controllers independently control their respective motors. 
     As described supra, in a gapped condition (first roll  12 /second roll  14  separated), both speed control systems for first roll  12  and second roll  14  operate independently and the process controller  34  is turned off. In this embodiment, second roll speed controller  30  provides the set-point for the second roll torque controller  26 . When the exemplary process  70  for controlling calender system  42  operates in a “nipped” condition (first roll  12 /second roll  14  contacting), the process controller  34  is turned on in order to provide load share control. This can be accomplished by setting the torque initial value of the process controller  34 . 
     After the calender system  42  switches to a “nipped” condition, the process controller  34  outputs a first value so that the set-point to the second roll torque controller  26  is the same value as the recent average value from the second roll speed controller  30  during the “gapped” condition prior to going to a “nipped” condition. This initial value is the difference of the maximum torque minus the recent average value from the second roll speed controller  30 . At the same time, the process controller  34  minimum and maximum output limits are set at their initial values and can increase steadily (i.e., ramp) to their final values. Additionally, the second roll speed controller  30  is turned off. 
     When the calender system  42  changes from a “nipped” condition to a “gapped” condition, the process controller  34  is turned off. The second roll speed controller  30  is turned on with its initial value set at the same value as the recent average output from the process controller  34  subtracted from the maximum torque. This is also known to those of skill in the art as a ‘bumpless’ transfer. The transition from a “gapped” condition to a “nipped” condition and back to a “gapped” condition can be accomplished by the use of a switching mechanism as described supra. 
     Torque Target Set Point for King Roll Drive 
       FIG. 5  depicts a block diagram of an alternative embodiment of a process  80  for controlling torque in a calendering system  42 . In this exemplary, but non-limiting process, when the first roll  12  and second roll  14  are nipped, the second roll motor speed controller  30  controls the second roll motor torque controller  26  for the second roll motor  20 . Similarly, first roll  12  is controlled by a separate first roll torque controller  24 . Here, the second roll motor speed controller  30  could produce the torque required to control the speed of first roll  12  through second roll  14 . The first roll motor torque controller  24  for the first roll motor  16  produces the target torque required by the set point. 
     It was surprisingly found in this exemplary embodiment that no process controller is required. Since the first roll motor  16  maintains a constant torque set at the target torque level, the second roll torque controller  26  produces the torque the second roll motor  20  requires to drive the entire calender  42  at the desired process speed. In order to use the second roll motor speed controller  30  during nip conditions, the speed feedback from the first roll motor  16  is used as the second motor speed controller  30  feedback. During gap conditions, each roll motor  16 ,  20  will utilize its respective speed controller  28 ,  30  and its respective roll motor speed sensor  38 ,  36 . 
     Similar to the exemplary processes described supra, the exemplary process  80  for controlling calender system  42  can operate in both a “gapped” and “nipped” configuration. However, the process  80  was found through simulation to minimize the shear forces disposed across a web substrate  40  in a calender system  42  without the need for a process controller. In a gapped condition, both speed control systems for first roll  12  and second roll  14  operate independently. In this configuration, the first roll speed controller  28  provides the set-point for the first roll torque controller  24  and the second roll speed controller  30  provides the set-point for the second roll torque controller  26 . 
     When the process operates in a “nipped” condition, the first roll speed controller  28  is turned off and the first roll torque controller  24  receives its set-point from a manually entered set-point determined by the process operators. The set-point can be based on minimum torque for minimum shear or related to any other process requirements (including, but not limited to, a torque table, and the like). Concurrently, the second roll speed controller  30  switches its feedback from the second roll speed sensor  36  to the first roll speed sensor  38 . 
     This transition of the second roll speed controller  30  feedback from the second roll motor speed sensor  36  to the first roll motor speed sensor  38  can be accomplished by the use of a transition controller  82 . In a preferred embodiment, the transition controller  82  is provided with a transition control algorithm. The transition control algorithm preferably conditions the transition controller  82  input and output signals to create a smooth transition from second roll motor speed sensor  36  to first roll speed sensor  38 . The transition control algorithm can include averaging functions, filtering functions, ramp functions, scaling functions, switch functions, and combinations thereof as required in order to switch the scaled feedbacks from one source to another. Scaling, conditioning, and switching both the speed feedbacks and references may be necessary for some installations depending on how the speed reference is scaled. When the calender system  42  changes from a “nipped” condition to “gapped” condition, the first roll speed controller  28  then is turned on and the second roll speed controller  30  is switched to operate from the second roll speed sensor  36  signal. The same signal conditioning algorithms may need to be applied to both the speed reference and any controller feedbacks to create a smooth transition to “gap” operation. 
     In a gapped condition, the first roll speed controller  28  transition to “on” is preferably accomplished by setting the first roll speed controller  28  initial value to the target torque set-point value for first roll  12 . The limits for first roll speed controller  28  start at this initial value and are steadily increased (i.e., ramped) to the final maximum and minimum values. However, it would also be possible to provide only the final maximum, only the final minimum, or even provide no limits to the first roll speed controller  28  depending upon any process parameters required for the system during a transition. The second motor speed controller  30  is also transitioned from the first motor speed sensor  38  signal to the second motor speed sensor  36  signal during the time the first roll speed controller  28  is turned “on”. This transition can be accomplished by the use of a transition controller  82  that smoothly transitions the first motor speed sensor  38  and the second motor speed sensor  36  scaled values during the “nipped” condition and transitions the second roll speed controller  30  from the first motor speed sensor  38  to the second motor speed sensor  36  after a “gapped” condition is sensed. 
     The transitions of the feedback from one motor to the other should be performed on the properly scaled values considering motor operating speeds in rpm, roll diameters and gear ratios. It should be readily realized that a smooth transition requires such properly scaled values. Additionally, transitions from a “gapped” condition to a “nipped” condition and back to a “gapped” condition can be determined by a switching mechanism as described supra. 
     In all embodiments described above, the implementation control strategy should account for acceleration, known load disturbance torques, and motor power and torque limits to adjust the target torque set-points. Additionally, one of skill in the art should easily recognize that any system for controlling the torque in a calendering system  42  should be tuned in order to control interactions between the first roll and second roll of any of the exemplary processes described herein. Further, the control methodologies and techniques described herein can be coupled with, and/or be included into, control schemes, including known ‘position’ controller processes, to produce the result desired. 
     All documents cited in the Detailed Description of the Invention are, in relevant part, incorporated herein by reference; the citation of any document is not to be construed as an admission that it is prior art with respect to the present invention. To the extent that any meaning or definition of a term in this written document conflicts with any meaning or definition of the term in a document incorporated by reference, the meaning or definition assigned to the term in this written document shall govern. 
     Any dimensions and/or calculated values disclosed herein are not to be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each such dimension and/or value is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as “40 mm” is intended to mean “about 40 mm”. 
     While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.

Technology Classification (CPC): 3