Tension measurement control

There are two types of tension measurement tension controls – dancer roll and load cell types. These controls are “closed loop”. They control tension based on tension in the web. They will compensate for tension changes due to roll diameter change on an unwind and rewind. They will also compensate for tension changes due to splices, bearings, mechanical losses and any of forces acting on the web in the machine direction.

Because these tension controls are “closed loop” and control drives based on actual web tension, they can be used to control the unwind, internal and rewind tension zones. Unfortunately, these controls can not sense roll diameter without external inputs, so by themselves they can not supply taper tension to rewind zone applications.

Tension measurement control - dancer

There are many different designs of dancer roll tension controls. However, all dancers operate with a common principle. They all incorporate idler rolls that are “loaded” in one direction, while the web tends to move them in the opposite direction. A sensor detects the position of the dancer and tells the drive to increase or decrease in speed or torque to add or remove material from the dancer. As long as the dancer roll remains between its physical limits (completely empty or completely full) tension is constant on the web.

Some of the most common types of dancer roll designs are pivot arm, linear and rotational. One advantage to using dancer roll tension controls that is consistent among all designs is they all have some amount (some designs more than others) of web storage. In other words, dancers actually accumulate a certain length of web in the machine direction. That means they can be designed to be mechanically stable through large roll build ratios (at the unwind and rewind) and will absorb tension fluctuations due to splices, defective bearings or other factors. Dancers not only control torque and/or speed to keep tension constant, they also absorb tension fluctuations, so web tension down stream of the dancer is kept smooth.

Pivot arm type dancer roll tension controls advantages:

• This design of dancer is one of the simplest.
• Most common design of dancer roll tension control.
• Least expensive dancer design.
• Can be designed with a large or small amount of web storage, but takes up a lot of space.

Pivot arm type dancer roll tension control disadvantages:

• Gravity must be taken into account during design.
• Unit can be bulky and difficult to fit into an existing process.
• Momentum and inertia is a concern.

Linear type dancer roll tension controls advantages:

• This design is compact.
• Large amounts of web storage are available. Especially with multiple web passes. Excellent design for accumulation needs, to allow splices on the fly.
• Small package can be easy to install in existing equipment.

Linear type dancer roll tension control disadvantages:

• Gravity must be taken into account during design.
• Can be a very complex design.
• One of the most expensive designs. Rodless cylinders and linear transducers are very expensive.
• Momentum and inertia is a concern.
• Rodless cylinders are not available in a low friction model.

Rotational type dancer roll tension controls advantages:

• This type of dancer roll design is not effected by gravity. It’s dancer rollers are counterbalanced with each other.
• Momentum and inertia is greatly decreased with the “push/pull” motion.
• Generous web storage in a small package.

Rotational type dancer roll tension controls advantages:

• This design can be expensive.
• This dancer roll design is somewhat complex.
• Web storage amount is fixed and determined by design. Increasing web storage is a new design.

Friction, gravity, momentum and inertia are all “enemies” of dancer rolls. These factors must be accounted for when designing dancer roll tension controls.

• Frictional losses greatly affect the accuracy of dancer roll tension controls. Low friction cylinders, such as glass lined or rolling diaphragm cylinders, should be used. Stay away from standard o-ring type cylinders or any type of cylinder with a high breakaway force.
• Momentum and inertia affect stability and accuracy of dancers. Remember a body in motion wants to stay in motion, so dancers will want to keep moving in the direction they are headed. This could cause the dancer to be unstable and tension spikes. Design dancers as lightweight as possible and avoid weight loading of dancers this will greatly reduce momentum and inertia problems.
• Gravity affects dancer accuracy. The weight of the dancer must be overcome by the web tension order for the dancer to operate. This weight of the dancer is in addition to the loading on the dancer when considering total web tension delivered to the web. Design dancers so gravity least affects them. Rotational type dancers are not prone to gravity problems because the dancer rollers counterbalance each other. Instead of designing a pivoting or linear dancer to move perpendicular to the floor, design it to move parallel to the floor. This will greatly reduce gravity problems.

There are many methods of loading dancer roll tension controls. Older units may be loaded with weights. If you need more or less tension you add or subtract weight. Newer dancer roll tension controls may use air cylinders for loading with pressure regulator valves. If you want more or less tension your turn the valve to allow more or less air pressure into an air cylinder. Today dancers are manufactured with electric to pneumatic converters loading low friction cylinders, such as glass lined or rolling diaphragm types. Electric to pneumatic converters are electronic circuits, which receive a signal voltage and output a proportional air pressure. The actual air pressure output is sensed with a pressure transducer and controlled with an electronic pressure regulator. This closed loop sensing circuit allows for very accurate dancer roll loading.

Older dancer position sensors are normally potentiometers. The problem associated with using a potentiometer to sense dancer position is that you have mechanical parts within the potentiometer (wiper against a resistance coil) that constantly wear against each other. This type of sensor may loose accuracy over time and may need periodic replacement. Newer dancer position sensors can be digital potentiometers, encoders or proximity sensors with analog output. These types of sensors do not have mechanically wearing parts, so their accuracy is not affected over time and the need for replacement is rare. 

Tension measurement control – load cell

Load cell tension controls utilize strain gauges and other weight measuring devices to measure the “weight” applied to an idler roll due to tension. The force exerted to the idler roll due to tension in the material is proportional to wrap angle around the roll. The example below shows 180° wrap. However, load cell tension controls work with any amount of wrap angle up to 180°. Wrap angles around the idler rolls utilizing load cells must not vary through roll diameter, that is why these controls use lead-in and lead-out idler rolls.

The machine operator sets the desired tension. Normally measurement signals are generated on both sides of the idler roll and fed into the control unit. The control accepts both signals and processes them together. Process values and set values are compared and an output is generated and sent to the drive to keep tension constant.

Load cell tension controls advantages:

• Excellent, accurate tension control at the unwind, rewind and internal zones.
• Many makes and models to choose from.
• Very accepted though out the industry for tension control.
• “High tech” controls are available with IO for PLC interface and data acquisition.
• Auto-tune units are available to eliminate manual calibration.
• Inexpensive, simple units are available.
• Very easy to install at any idler roll location.
• Minimal machine modifications are necessary to utilize this control.

Load cell tension control disadvantages:

• No mechanical web storage for web tension fluctuations.
• Web tension can be unstable. Especially in unwind and rewind zone control with large roll build ratios

It can be difficult to properly design both dancer roll and load cell tension controls to have 100% “authority” over an unwind or rewind drive. Large roll build ratios, very high and low speeds and elastic webs can make both types of closed loop controls very unstable. Combination open loop / closed loop controls are available to help with this problem.

Open loop controls are very stable because they are designed to measure distance (roll build). Unfortunately, open loop controls lack accuracy because they do not know what the tension actually is in the web.

Closed loop controls are very accurate because they actually measure tension in the web and control a drive to keep tension constant relative to a set point. Unfortunately, closed loop control can lack stability when controlling unwind and rewind tension zones because they do not know distance (roll build) and have to control large ratios in a relative small span.

Combination controls utilize both open loop and closed loop technology for unwind and rewind zones. The open loop control in the system normally controls 90% of the output to the drive and the closed loop control “trims” that output 10%. This makes unwind and rewind zone control very stable and very accurate. Theses controls utilize roll diameter information, so taper tension for rewind applications is available.

Combination closed loop / open loop tension control advantages:

• Very accurate, very stable tension control for very demanding applications.
• “High-tech” controls are available with IO and PLC interface.

Combination closed loop / open loop tension control disadvantages:

• High cost of two tension controls.
• Mechanically and electronically complex.

When tension controls are applied properly, they will decrease waste due to web defects. They will allow operators to concentrate their efforts in other critical areas of your process. Tension controls can allow you to run your process faster. They can even allow you to run different webs and processes through your machine. Tension controls are essential to increasing your profits.