How to control wire tension on rotating process machines

February 2017 — Whether wire processes include operations like stranding (bow, tubular, planetary cage …) or twisting (twinners etc.), the ability to monitor and control the tension of each strand during the process will have a positive impact on the end product as well as on the process stability.

Whether wire processes include operations like stranding (bow, tubular, planetary cage …) or twisting (twinners etc.), the ability to monitor and control the tension of each strand during the process will have a positive impact on the end product as well as on your process stability. When the tension in a strand is measured by sight, the touch of a finger, or with a hand-held instrument in a static condition, the introduction of error from each of these during a production run produces results that are costly in terms of product quality, throughput, and scrap. Even when those methods produce acceptable results for very specific conditions, they are not repeatable or reliable enough to address other varying process parameters. 

Only with the use of a dedicated, calibrated tension monitoring system can one obtain the critical information required for the highest possible production rates and traceable process tension data. FMS introduced the “RTM X42” Telemetry System for automated strand tension monitoring decades ago, and it has proven its benefits for both manufacturers of stranding machinery (OEMs) and for end users that produce stranded products. In the case of either a machine upgrade or a new machine, the addition of tension monitoring or tension control is critical in providing the consistent product quality, production efficiency, ease of use, and reliability that they demand.

Detection of strand tension: A force measurement sensor, based on strain-gauge technology, utilized with an integral pulley at each wire position on a lay plate, provides the means to detect force (and thus tension) changes in the strand. The sensor is designed to account for the centrifugal and Coriolis forces present during lay plate rotation, and has the ability to withstand overload forces that may occur during machine upset conditions. 

Collecting strand tension data from the machine: The electrical signal from the sensors is sent off of the rotating portion of the machine wirelessly to the control station using certified radio technology. The use of high maintenance slip rings is no longer required. See Fig. 2 Once the tension information is received off of the rotating section of the machine, it can directly be displayed. A standardized Modbus TCP protocol can be used to transfer the data to a PLC for display.

Real time closed loop tension control

The data can also be monitored with the RTM X42 Monitoring & Control Software. It features a bar graph display with alarm functions to alert the operator to take corrective action. In additional to the monitoring of each strand the information is also logged in a data file for machine performance evaluation or QC reference for a particular production run. This protocol is also used to provide quality reports for reference by the end customer. 

A typical RTM X42 system will allow for complete parameterization of the tension controller (including tension set point adjustments, etc.) and evaluation of the actual tension from FMS Control Center. In the case where product changeovers are frequent due to shorter production runs, the system includes a menu screen; once a selection is made for a particular product all pertinent parameters are loaded with the correct values.  This feature provides time savings during product run changes, and becomes increasingly important when production runs are shorter.  See Fig. 3 and Fig. 4.

Incorporating closed loop tension control is a possible next step to automatically maintain proper tension in the individual strands. In a possible scenario, the tension monitoring possibilities as noted above remain, but in addition the tension feedback signal is processed on the machine to produce a control signal to the battery powered brake actuator located at each payoff. See Fig. 5
The actual tension is compared to the desired tension (or set point), and after being processed through PID algorithms an appropriate control signal is sent wirelessly to the payoff area. The signal will produce a minor adjustment to the tension when the actual tension is close to the set point, and a larger adjustment when the actual tension is further from the set point. The control signal will act on the mechanism that is being used to provide strand tension, for instance a friction based brake with a rope or belt. See Fig. 6.

Benefits …

… for the OEM: A tension monitoring system provides you with a distinct advantage in terms of higher production rates and documentable process tension. The use of a wireless system has the added benefit of eliminating the need for maintenance intensive slip rings.

… and for the End User: Automated strand tension monitoring provides you with values for one of your most important process parameters, namely Strand Tension. “…we were blind but now we can see… the times are gone when we had to rely on the individual feel of each of our operators…” was a statement from a customer. In addition the RTM Monitoring & Control software allows for the storage of tension set points, alarms, and other production parameters in dedicated recipes for easy and quick product changeovers.  The quality reports are a standard feature, and when generated are useful as a selling tool with your customers. The use of the fully extended RTM X42 System, including the Brake Control, will provide for increased product quality, faster change overs, reduced downtime, and repeatable results.

Norbert Schilling, Force Measuring Systems 

FMS Force Measuring Systems AG
Aspstrasse 6, 8154 Oberglatt/Switzerland
Contact person is Mr. Norbert Schilling
Phone: +41 44 852 80 80

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