Tension control is a basic technique in printing, laminating, slitting, sheeting, and coating machines. The mechanical tension of the material, often provided in roll size, needs to be controlled. The material could be paper, plastic film, textiles, aluminum foil, or wires/cables. Some companies offer tension controller sub-systems based on CANopen to be integrated into the machines control network. These sub-systems comprise a host-controller, motion controllers, and the necessary sensors, e.g. load-cells.
THE TENSION CONTROL SHOULD WORK during acceleration, steady machine speed, and deceleration. The speed point location in the machine must be identified. In general, one of the machine’s nip rolls is driven, setting the linear speed of the machine. This speed is the master function. The tension control is a slave function. The operator sets the machine speed and all tension control systems existing on the machine have to follow, keeping the desired tension at any speed and during all transitory speed phases. The tension control could be realized by means of a closed-loop control or by an open-loop setting. The offered CANopen-based tension controllers use mainly the closed-loop control approach. The benefits compared with open-loop solutions are the higher accuracy and the independency of mechanical quality, which may change over the time. On the other hand, there is a risk of instability (if not proper designed), the set-up is more complex, and the costs are higher.
There are three main options to sense the tension: direct measurements with load-cells optionally with CANopen interfaces, indirect measurements with dancer arms or with a free loop. The indirect measurements are often used in printing and textile machines. They absorb tension peaks well, in particular during acceleration and deceleration. Load-cells are suitable for heavy material and where space is limited. Acceleration and deceleration should not be extreme. Load-cells locations should be vibration-free. Vibrations decrease the measurement quality. An unbalanced sensing shaft creates measurement oscillations causing variations in control quality. It is recommended to use self-aligning ball bearings. The minimum wrapping angle should be larger than 240°. The web tension should be in the same direction as the weight of the shaft.
Closed-loop tension controller
The CMGZ 480 tension controller by FMS Technology (Switzerland) features PID cycle-times of 2 ms. The IP67-rated device is configurable via its CANopen interface. It can be used with force sensors from the manufacturer. The sensors feature a high accuracy and a high overload protection. They are available in different mounting arrangements, and measures 1 N to more than 100 kN. The controller is suitable for example for stranding machines. The tension force values are fed to the PID controller via an A/D converter. Signal processing and controlling tasks are performed digitally. This results in long-term temperature stability and control performance.
FMS was founded in 1993, following a buyout from the FAG Kugelfischer company. Besides steering frames and actuators, the company offers web guiding sensors and controllers. The 600 series provide also CANopen connectivity.
Tension and dancer controller
Montalvo (USA) offers the X-3400 closed-loop tension controller, the D-3400 dancer controller, and DTI-3400 dancer controller with tension indication. These controllers come with CANopen interfaces. These products can be used also for non-woven and composite materials. Such materials are employed in many unusual and exotic applications, and are produced in many different ways. In fiber extrusion technology where melted polymer is forced under pressure to make non-woven material directly, two techniques have been developed, one of which is termed ‘spun wound’ and the second is ‘melt blown.’ The webs produced by the ‘melt blown’ technique can be controlled to provide a high-density capillary structure suitable for meeting two seemingly entirely different requirements. In the manufacturing process the structure can be made so dense that it resists moisture and is used for waterproof liners, etc. Conversely the structure can be made so that it provides excellent water pick up making it ideal for kitchen surface wipes.
The DTI-3400 CANopen-connectable dancer controller by Montalvo regulates output to the unwind brake and maintains the position of the dancer arm. A signal provided by load cells and displayed on the analog meter is used for setting and monitoring actual web tension.
Some of the most recent areas in which non-woven materials and composites are becoming widely used are in the production of geo-synthetics, geo-textiles, geo-membranes, geo-grids and geo-nets. This group of products is used in a variety of civil engineering applications. Non-woven geo-textiles for example are highly desirable for sub-surface drainage and erosion control purposes.
To be a leading player in non-woven materials the producer of web fed rolled goods must not only master new production techniques, but there is also a need to master the inconsistencies and irregularities that crop up in the day-to -ay production process. One of the most effective ways is by improving web control through better tension control. The production of non-woven materials can present unique tension control problems, for instance, the production of kitchen towels, wipes and toilet tissue, some of which may be embossed and/or printed require very low tensions. Low extensibility and tight tolerances has meant that many producers and OEM’s require precision tension control systems that integrate seamlessly with the machine’s host controller (i.e. PLC/PC systems).
Validated and tested tension controllers
Last year, Schneider Electric (France) has launched its MachineStruxture initiative. The company provides validated and tested hardware and software platforms for the machine building industry. For packaging machines, the French enterprise offers digital tension control sub-systems based on CANopen. These sub-systems comprise the CANopen-connectable Modicon PLCs and the CANopen Lexium drives. According to the company, these pre-programmed sub-systems save up to 50% in system design and installation. The company provides ready-to-use function blocks for its CANopen PLCs and pre-defined CAD panel designs and wiring diagrams. The provided digital tension control solutions are suitable for horizontal and vertical bagging machines, for shrink and stretch wrapping machines as well as for labeling machines. Due to the precision of the motion drive controlled by the motion controller using the function block “control of film tension”, the tension of the film is increased by more than two. This saves about 10% of plastic film material, claims the provider. The French company also offers dancer control solutions based on its CANopen hardware platform.
Web tension amplifier
Besides complete tension control systems, some suppliers offer CANopen-connectable web tension amplifiers. These devices collect the sensor values and provide them to the host controller running the tension control application software. Because of possible non-symmetrical load distribution over the rolls it is customary to measure the forces in both bearings of the idler roll. The core of the web tension sensors is the strain gauge transducer measuring the acting forces. Normally, the measuring system consists of two sensors and one amplifier. The second sensor is not necessary if the web runs steady in the middle of the rolls and the process does not require high precision web tension measurement.
The Busbox with CANopen interface by Haehne comprises an amplifier for two tension sensors. The IP65-rated device is suitable for one roll and
Haehne (Germany) provides optionally for its amplifiers CANopen interfaces. The Busboxes are available in different enclosures including hermetically sealed compression-proofed housing. Erhardt + Leimer (Germany) also offers a CANopen-connectable measuring amplifier for tension control applications. The IP65-rated CV70 features two analog inputs. The zero point adjustment and the calibration of the output signal is performed via CANopen. The web tension actual value for each side as well as the sum signal and the difference signal are also mapped into PDOs. If the wrapping angle and mounting position of the load sensors is known, the measuring amplifier may even be calibrated without involving a test weight. The company offers also the DC-60/61 web tension controller featuring CAN connectivity.