ABB Motion control products 1 www.abb.com/motion This application note describes how ABB servo products may be used to achieve web tension control. Particular detail is applied to the winding of web materials. Product films often have to be wound and unwound many times during their manufacture. It is important that this is done correctly or there is a chance that the product may become damaged during transport or when loaded onto a new machine for further processing. Introduction Introduction Introduction Introduction A brief background is given of the common mechanical setups for web tension control. Problems associated with winding or unwinding of web materials are described. Finally, an example shows how to code a servo application for taper tension control of a re-winder. Tension zones and Nip Induced Tension Tension zones and Nip Induced Tension Tension zones and Nip Induced Tension Tension zones and Nip Induced Tension Different processes require different tensions. For example consistent tension profiles are necessary for printing. To load the material onto the machine from an un-winder may need a smaller tension than for the machines mid section. Web guide systems will not work without tension. Many other processes will not work unless the material is under tension. A tension zone will exist between any two rolls that are either driven or braked. An example of a two zone machine would be as follows. A driven master roller draws the web material through the machine and sets the machine speed. An unwind brake at the start pulls against the master roller to create the first tension zone. A re-winder at the end of the machine operates in torque mode. The re-winder pulls against master roller creating tension in zone 2. It is much easier to fault find problems with web transport if a machine is divided into different tension zones. All materials will elongate or strain under the effects of tension or stress. The material tension will be in proportion to its elongation by the coefficient of elasticity for the material, up until the point of failure. If the material strain goes above its elastic limit the material will be permanently deformed. If the material is under strain it will try to retract to its original length like a spring. For a wound roll this Motion Control Products Application note Tension control AN00212-002
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ABB Motion control products 1
www.abb.com/motion
This application note describes how ABB servo products may
be used to achieve web tension control. Particular detail is
applied to the winding of web materials.
Product films often have to be wound and unwound many times
during their manufacture. It is important that this is done
correctly or there is a chance that the product may become
damaged during transport or when loaded onto a new machine
for further processing.
IntroductionIntroductionIntroductionIntroduction
A brief background is given of the common mechanical setups for web
tension control. Problems associated with winding or unwinding of web materials are described. Finally, an example shows how to
code a servo application for taper tension control of a re-winder.
Tension zones and Nip Induced TensionTension zones and Nip Induced TensionTension zones and Nip Induced TensionTension zones and Nip Induced Tension
Different processes require different tensions. For example consistent tension profiles are necessary for printing. To load the material
onto the machine from an un-winder may need a smaller tension than for the machines mid section. Web guide systems will not
work without tension. Many other processes will not work unless the material is under tension.
A tension zone will exist between any two rolls that are either driven or braked. An example of a two zone machine would be as
follows. A driven master roller draws the web material through the machine and sets the machine speed. An unwind brake at the
start pulls against the master roller to create the first tension zone. A re-winder at the end of the machine operates in torque mode.
The re-winder pulls against master roller creating tension in zone 2. It is much easier to fault find problems with web transport if a
machine is divided into different tension zones.
All materials will elongate or strain under the effects of tension or stress. The material tension will be in proportion to its elongation
by the coefficient of elasticity for the material, up until the point of failure. If the material strain goes above its elastic limit the material
will be permanently deformed. If the material is under strain it will try to retract to its original length like a spring. For a wound roll this
Motion Control Products
Application note
Tension control
AN00212-002
Application note Tension control AN00212-002
ABB Motion control products 2
www.abb.com/motion
will create compressive forces on the inside layers. As a guide the maximum tension should never exceed 25% of the materials
elastic limit but it will depend on the material.
There are two basic methods of controlling the tension.
The web runs between two nip rolls and a force is applied between the two nips. This
force generates a frictional tension along the material proportional to the force and the
coefficient of friction between the material and the nip surface. By controlling the force
the tension can be controlled. The tension before and after the nip is different so the
nip has the effect of isolating different sections of the machine with different tensions.
T = T = T = T = µ * F T = Friction induced tensionµ * F T = Friction induced tensionµ * F T = Friction induced tensionµ * F T = Friction induced tension
µ = Coefficient of frictionµ = Coefficient of frictionµ = Coefficient of frictionµ = Coefficient of friction
F = Perpendicular force on materialF = Perpendicular force on materialF = Perpendicular force on materialF = Perpendicular force on material
TensionTensionTensionTension induced from a driven rollerinduced from a driven rollerinduced from a driven rollerinduced from a driven roller If the nip roll is driven from a servo system we have the option of adding or subtracting
tension from the web. We could apply a positive or negative torque. For a constant
diameter roller the tension is proportional to the torque. The torque operates like a
lever in reverse. In a lever we apply a force at a distance to achieve a torque and the
greater the distance the smaller the force required to achieve the same torque.
Because we are driving the roller from the center however we need more torque to
achieve the same tension as the diameter of the roller increases.
Tq = T * RTq = T * RTq = T * RTq = T * R Tq = TorqueTq = TorqueTq = TorqueTq = Torque
T = TensionT = TensionT = TensionT = Tension
R = RadiusR = RadiusR = RadiusR = Radius
The servo system gives us the additional option of changing the tension by driving the
roller via speed control faster or slower than the web. We would control the tension by
controlling the amount of elongation in the material. The formula opposite describes
this relationship. V2 is our controlling function. Suppose we want to increase the
speed of the material from 100m/min to 102 m/min and EA is 200N. For simplicity let
T1 = Tension before rollerT1 = Tension before rollerT1 = Tension before rollerT1 = Tension before roller
T2 = Tension after rollerT2 = Tension after rollerT2 = Tension after rollerT2 = Tension after roller
V1 = VelociV1 = VelociV1 = VelociV1 = Velocity of web before rollerty of web before rollerty of web before rollerty of web before roller
V2 = Velocity of web after rollerV2 = Velocity of web after rollerV2 = Velocity of web after rollerV2 = Velocity of web after roller
E = Coefficient of elasticity of materialE = Coefficient of elasticity of materialE = Coefficient of elasticity of materialE = Coefficient of elasticity of material
A = Cross sectional area of materialA = Cross sectional area of materialA = Cross sectional area of materialA = Cross sectional area of material
Using servo systems on an unUsing servo systems on an unUsing servo systems on an unUsing servo systems on an un----winderwinderwinderwinder
When material is taken off a reel into a machine it should ideally be at constant tension. In many cases a brake or clutch may be
used to provide a fixed counter torque to the centre of the reel. As the reel diameter decreases the tension will increase because:
In reality you can often get away with a constant torque applied to the centre of the reel if the brake or clutch is sized for the tension
required between the minimum and maximum reel diameter. If different widths and types of material are used then the range of
Application note Tension Control AN00212-002
ABB Motion control products 3
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tensions is likely to exceed the available settings on the brake. A servo drive may be used in these circumstances to set a varying
torque to achieve constant tension as the reel diameter changes.
The code example at the end of this application note is for re-winders but it can be used for un-winders as well. Remember though
that the “Taper Tension” option should not be used with un-winders.
An un-winder will generate energy which would be dissipated as heat if a brake is used. A servo system offers the option of
recovering this energy if the drive shares a DC link with others on the machine. Since brakes are normally quite large to cope with
the heat dissipation, servo systems offer a low inertia which allows much better tension control, particularly at low diameters.
If the web is accelerating during machine start-up, the un-winder may need to be driven to accelerate the web onto the machine.
Why is rollWhy is rollWhy is rollWhy is roll winding tensionwinding tensionwinding tensionwinding tension importantimportantimportantimportant
If the tension in a wound roll is two high you will create excessive residual strain in the roll. This can cause a number of problems
briefly described below:
− Web breaks can occur in the machine when the reel is unwound.
− Rolls can burst during storage.
− Tension wound into the material can create circumferential forces within the reel. If these become too high the material could
wrinkle or the core could collapse.
− Rolls can “block” i.e. some of the layers may fuse together.
Web materials are not perfectly uniform and the thickness could vary across the width of the material. Thicker layers will build up on
top of each other forming a mound. If the reel is wound too hard then the material will stretch and deform over the mound but not in
other areas. This causes a defect called corrugation or rope marks in the rolls.
If the tension in the roll is too low this can lead to telescoping of the reel during winding or unwinding. It is particularly important for
the core of the roll to be tighter as this must take the load of the outer layers of the roll.
If the web is a laminate of two or more materials then the web tension could distort the materials differently and cause the laminate
to curl or de-laminate.
If multiple print patterns are applied to a roll then the tension at each printing zone must match. Otherwise the web elongation will
be different and the print patterns will not line-up with each other.
The best way to choose the correct winding tension for a roll is from experience. It is helpful
to wind some test rolls. A good test is to do the following: Wind the material onto an empty
core up to say 100mm. Draw a straight line on the side of the reel from the edge of the core
to the edge of the reel. Wind on another 50mm thickness of material and then extend this line
to the edge of the reel. Continue this process in 50mm steps up to about 200mm and then in
100mm steps to the max reel diameter.
If the reel is wound too soft the layers will slip past one another as shown here and the
straight line will become a curved ‘J’ line. The reel should be wound until the line remains
straight.
Methods of Methods of Methods of Methods of wwwwinding mateinding mateinding mateinding materialsrialsrialsrials
There are three basic winding geometries:
− Centre winding
− Surface winding
− Centre surface winding
There are many variations and improvements on these basic methods. Some of these variations are described below to provide an
overview of the basic principles of rewinding.
Application note Tension control AN00212-002
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Centre Centre Centre Centre wwwwindingindingindinginding The core is driven from the centre by a motor and the material is wound
around the surface of the core. The main challenge with this geometry is
that air can get in between the layers of material during winding. Too
much air will produce a weak roll that will slip sideways.
This can be alleviated by pre tensioning the web before winding or by
applying a torque to the motor. Too much pre-tension could cause the
material to stretch.
Applying a central torque is also problematic in that the central layers of
the roll need to be tighter so that they can transmit the load through to
the outer layers. This limits the size of reel that can be wound using this
method.
The amount of centre wound tension required may be reduced by using a
lay-on roll. The lay-on roll applies nip induced tension to remove air. The
effect may be improved by winding the web around the lay-on roller first.
This is particularly useful for wide rolls because the web comes off the flat
surface of the lay-on roller onto the reel. This reduces the possibility of
wrinkles in the material.
Applying a central torque is a way of tensioning up the material. As the winding diameter changes, the torque will apply a different
tension to the outside material. Thus some method must be employed to keep track of the roll diameter.
Single roll surface windSingle roll surface windSingle roll surface windSingle roll surface windinginginging The basic concept behind surface winding is to force the reel into
contact with a roller which is rotating at a fixed speed. The reels outer
surface will run at the same speed as the driven rollers surface speed.
The contact force removes air between the layers and will increase the
density or hardness of the roll. The contact force applies a frictional
force to the material and thus induces tension.
A material must elongate to produce tension. The material must
therefore slip parallel to the reels surface when it stretches after the
nip. This means that adhesive materials cannot be tensioned in this
way.
With a single rewind roller there is no direct means of controlling web tension on the reel. This makes tension control difficult. By
adjusting the contact force of the roller against the reel, the tension can be controlled to some extent.
The effect can be enhanced if the winding roller is driven by a servo system. If the winding roller is driven at a speed faster than the
incoming web material it will pre tension it.
Application note Tension Control AN00212-002
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Centre surface windingCentre surface windingCentre surface windingCentre surface winding As the name would suggest, this is basically a combination of
centre and surface winding. An adjustable force applies the reel’s
surface into contact with a roller running at constant speed. The
reel is also driven from the centre. A motor applies a torque
through the reel to apply a counteracting tension in the web
material at the surface.
In the diagram the reel is shown suspended onto the winding
roller from a swing arm. In this configuration the weight of the reel
will contribute to the frictional force on the winding roller. The
force needs to be adjusted as the reel diameter increases so the
reel density must be known. This problem may be eliminated by
applying the reel to the winding roller from the side with a slider
mechanism.
Centre surface winding has an advantage over centre winding in that the wound in tension may be applied to the reel from a surface
roller and doesn’t all need to come from central torque. This means that the web tension upstream of the winder may be lower than
the wound in tension in the reel. Centre surface winding provides more options and so allows larger and denser rolls to be wound.
Twin roller surface windingTwin roller surface windingTwin roller surface windingTwin roller surface winding Twin roller surface winding is typically used on thick materials such as paper or
textiles and is not suitable for thin plastics.
The reel surface rests between two driven rollers. A force may be applied from
a rider roller from the top. As the diameter increases the reel weight will
increase so the applied force can be gradually reduced.
Shown here are two variations for loading the web material onto the reel. In the
top diagram the web material is brought between both surface rollers and the
reel. In the bottom diagram the web is brought around only the rear roller and
then onto the reel.
In the bottom diagram the rear roller drives the top layer of material onto the
roller and the front roller drives the layer underneath. The front roller is driven at
a higher speed to generate a frictional force between the layers of material. By
controlling this over-speed the amount of tension wound into the roll can be
controlled
The top configuration is ideal for slitting the material as it achieves the minimum
distance between slitter and reel. Both rolls may be driven at a similar speed
although you can get a drooping of material between the rolls.
Application note Tension control AN00212-002
ABB Motion control products 6
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Tension control on a tTension control on a tTension control on a tTension control on a twin roller surface win roller surface win roller surface win roller surface rrrreeee----winderwinderwinderwinder
Tw = Web tensionTw = Web tensionTw = Web tensionTw = Web tension
Vw = Velocity of WebVw = Velocity of WebVw = Velocity of WebVw = Velocity of Web
Tr = Tension in outer surface of reelTr = Tension in outer surface of reelTr = Tension in outer surface of reelTr = Tension in outer surface of reel
Vr = Tangential velocity of reel surface layerVr = Tangential velocity of reel surface layerVr = Tangential velocity of reel surface layerVr = Tangential velocity of reel surface layer
Vf = Tangential velocity of front rollerVf = Tangential velocity of front rollerVf = Tangential velocity of front rollerVf = Tangential velocity of front roller
Ff = Force created by the reel on the front rollerFf = Force created by the reel on the front rollerFf = Force created by the reel on the front rollerFf = Force created by the reel on the front roller
Fr = Force createdFr = Force createdFr = Force createdFr = Force created by the reel on the rear rollerby the reel on the rear rollerby the reel on the rear rollerby the reel on the rear roller
µµµµ = Coefficient of friction between rewind roller and material= Coefficient of friction between rewind roller and material= Coefficient of friction between rewind roller and material= Coefficient of friction between rewind roller and material
DrDrDrDr = Diameter of rear roller.= Diameter of rear roller.= Diameter of rear roller.= Diameter of rear roller.
If the reel tension Tr needs to be lower than the web tension Tf then Vr would have to be less than Vw. Vr will have to be less than
Vf. Also because Vr < Vw slippage would occur between the reel and the incoming web on the front roller. The rear roller motor will
have to be sized to provide enough torque to restrain the reel against the friction force on the web material. The required torque that
would have to be applied to the rear roller is:
Tr = Tr = Tr = Tr = µµµµ x Ffx Ffx Ffx Ff x x x x (Dr / 2)(Dr / 2)(Dr / 2)(Dr / 2)
The wound in tension in the reel is heavily dependent on the weight of the reel. When the reel is started there will be minimum
weight so the front rewind roller may be driven overspeed to wind in more tension into the reel. This over speed then needs to be
gradually reduced as the reel diameter increases. Beyond a certain level of overspeed the material will droop between the front a
and rear winding rollers. If the wound in tension setting is correct there will probably be some droop so this can act as a visual
indicator of the wound in tension. The correct level of tension needs to be found from experience.
Coding a surface wound applicationCoding a surface wound applicationCoding a surface wound applicationCoding a surface wound application
In the twin roll surface rewind application described above the front and rear winding rollers can be given a velocity reference. The
front roller is given a higher VELREF to overspeed it. This is easily done during steady state conditions. For stopping and starting
however, the front roller needs to have an offset speed with respect to the rear roller. We can’t just set the rear roller to follow the
front roller at a lower speed ratio because the offset will not be fixed during acceleration and deceleration.
The solution is to use FLY move profiles to stagger the
acceleration of the front and rear rollers. The resultant velocity
diagram is shown on the left.
See application note AN00116 for more information on FLY
Centre winding tension controlCentre winding tension controlCentre winding tension controlCentre winding tension control
In a basic centre winder all the wound in tension is achieved by applying a torque at the centre of the reel. The straightforward
formula for tension is:
TTTTorqueorqueorqueorque ==== TTTTensionensionensionension x Radiusx Radiusx Radiusx Radius
There is a spectrum of tension profiles to choose from varying from constant torque to constant tension. Some materials are
forgiving and we could simply get away with a constant torque. This would give us a rapidly decreasing tension profile. An
improvement can be gained by trying to achieve a constant tension. To do this we need to re-calculate the applied torque at the
centre as the radius changes. The roll will end up with a constant density profile throughout. If we want a roll that is harder towards
the centre then we want to choose a taper tension profile.
To apply a taper-tension profile we need to be able to measure the reel radius at all times or be able to estimate it accurately. We
could read the radius by wiring the output of an ultrasonic sensor or follower arm into the analogue input of the servo controller.
If we want to avoid using a sensor then the radius can be calculated if we know the material thickness. The reel axis has a position
counter that can be used to measure the amount of material that has been fed onto the reel. The position counter would have to be
reset when the empty core is loaded. To calculate the radius we use a formula derived from the fact that the material would look like
a rectangle in cross section if it were tolled out flat. It would have the same cross sectional area it does on the reel. The formula is:
Material Length x Material thickness = (Reel radius ^ 2 Material Length x Material thickness = (Reel radius ^ 2 Material Length x Material thickness = (Reel radius ^ 2 Material Length x Material thickness = (Reel radius ^ 2 –––– Core radius ^ 2) x Core radius ^ 2) x Core radius ^ 2) x Core radius ^ 2) x π
A disadvantage with this approach is that the material thickness is a guesstimate and must take into account the amount of air
included between layers. Under taper tension control the amount of air is lower towards the centre of the reel.
An alternative approach for measuring radius is to wind the web around a roller fitted with an encoder and read the web velocity. If
another axis on the machine is driven from a servo we can use its encoder feedback for this purpose. If we have a nip roller axis
with a constant radius we can say that the surface velocity of the nip roller will equal the web velocity which will equal the surface
velocity of the reel. We can then use the following relationship to measure the reel radius.
Surface velocity of nip roller = Angular velocity ofSurface velocity of nip roller = Angular velocity ofSurface velocity of nip roller = Angular velocity ofSurface velocity of nip roller = Angular velocity of nip x Radius of nipnip x Radius of nipnip x Radius of nipnip x Radius of nip = Angular velocity of reel x Radius of reel= Angular velocity of reel x Radius of reel= Angular velocity of reel x Radius of reel= Angular velocity of reel x Radius of reel
This is fine when the machine is running but it can’t be used in this form when at zero velocity. We need to use a starting value for
the reel radius such as the core diameter. The operator should be able to adjust the diameter and it is much easier to measure than
material thickness. Once the reel speed goes above a minimum threshold, the calculated radius can be gradually adjusted in steps.
This method is shown in more detail in the following section on coding an application.
If surface winding and a taper tension profile is required we cannot use this method of calculating the reel radius. We need an
external diameter measurement.
Application note Tension control AN00212-002
ABB Motion control products 8
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Coding a centerCoding a centerCoding a centerCoding a center wound applicationwound applicationwound applicationwound application open loopopen loopopen loopopen loop
The following section shows how a centre wound application may be coded. This is classed as open loop because, although the
applied torque and radius of the reel are monitored, there is no measurement of the resultant web tension. There are two axes: a
master nip and a rewind. The rewind axis will operate in torque mode. Constants for the axes are defined at the top of the program:
Const _axMASTER As Integer = 3
Const _axREWIND As Integer = 4
Each axis in mint has a SCALEFACTOR so that you can use user units like mm rather than encoder counts. In this example it is
convenient to scale the master axis to mm but the rewind axis cannot be given a linear scale as the number of encoder counts per
mm of web changes with the rewind radius. In this example the SCALEFACTOR for both axes is set to 1 and scaling is done using
constants. The rewind is scaled to revs/sec for convenience.
Const _fMasterScale = 70000 '7:1 Gearbox
Const _fRewindScale = 30000 '3:1 Gearbox
A number of user settings are used in the code. They are controller comms locations which allow for adjustment from a HMI. They
are defined at the top of the program thus:
Define hmiMACHINE_SPEED = COMMS(11) 'Machine set speed in m/min
To estimate the static torque go online to the re-winder drive in workbench. With the re-winder empty and not webbed up JOG the
axis at 5-10 % of its maximum speed. In the spy window or command line read CURRENTMEAS(0) for the drive current. The
percentage static frictional torque can be found by the formula:
Static friction = CURRENTMEAS(0) * Motor torque constant * 100 / Motor continuous torque
To estimateTo estimateTo estimateTo estimate the dynamic frictionthe dynamic frictionthe dynamic frictionthe dynamic friction,,,, JOG JOG JOG JOG thethethethe rererere----winder at full speed. Repeat the above procedure to calculate the total winder at full speed. Repeat the above procedure to calculate the total winder at full speed. Repeat the above procedure to calculate the total winder at full speed. Repeat the above procedure to calculate the total
frictional tfrictional tfrictional tfrictional torque. Take away the static friction from this to get the dynamic friction. orque. Take away the static friction from this to get the dynamic friction. orque. Take away the static friction from this to get the dynamic friction. orque. Take away the static friction from this to get the dynamic friction.
Application note Tension Control AN00212-002
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Center winder closed loop control
Open loop rewind control can’t control torque well enough if the re-
winder has to be the master driver in a one tension zone machine.
In this circumstance and others where accurate tension control is
needed we need to move to closed loop control.
Closed loop control would use a dancer arm or load cell to measure
the tension in the web. Dancer arms have the advantage that they
store some length of web and can smooth out tension fluctuations.
Load cells do not have this smoothing function.
With tension feedback a web break can be detected when the
measured tension drops below a threshold.
The target tension is applied using the method described in the previous section. For closed loop control we place this code inside
a control loop that monitors the tension and applies corrections to the target tension. We could use a PID loop to calculate the
target tension. In most applications it will not be necessary to use all the gain terms in a PID controller but they can be included if
needed.
Refer to application note AN00208 for information about coding PID loops using the Mint programming language.