An automation yarn

April 2008 PLCs, DCSs & Controllers

Mortex Knits were experiencing costly breakdowns on their knitting machines, a situation aggravated by difficulty in controlling the take-off tension causing large variances in product quality. Elonics, the KZN distributor for LS industrial systems, were requested to investigate the possible causes and propose a system solution using locally available equipment.

During evaluation the knitters were judged mechanically sound and that the problems could be resolved by implementing a control system based on the following:

* A programmable logic controller (PLC) to control sequences and alarms.

* A human machine interface (HMI) for entry of the various yarn parameters.

* Variable frequency drives (VFDs) to ensure smooth starting and speed control.

Interfacing of the prototype system with existing machinery required a dedicated interface design to overcome the problems of non-standardisation. Elonics manufactured a custom circuit for relaying fault conditions from the machines to the PLC.

Mortex says the improvement in productivity has been substantial. The smooth machine starting thanks to the VFD has resulted in less breakage due to even tension control. The HMI depicts total production, machine efficiency and desired knitting speed. The latter is communicated from the PLC to the VFD using RS485 communication. Based on the results achieved by the prototype the company has placed an order for the conversion of the rest of the factory.

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