Narrow gauge steel strips are used in various industries as cost-effective semi-finished products. Typically the strips are supplied on a reel for further processing, e.g. in stamping presses. To ensure continuous production, the strip end of the finished reel is welded to the beginning strip of the new reel. Normally the weld seam cannot be processed but must be cut out. This is necessary for tool protection but also for mechanical and optical reasons. A reliable detection of the weld in automated processes is therefore absolutely necessary.
Various solutions have been devised for the detection of the weld, e.g. with inductive and optical sensors which however, have their own sensor typical disadvantages like unintended gap changes between sensor and metal surface or varying surface discolorations.
For many years the Roland Company has been solving this type of applications on the basis of magnetic flux leakage and eddy current sensor principles. This has the advantage, that not only geometrical changes but also changes of the electrical and magnetic conductivity are analyzed. This improves the process reliability greatly. The Roland Company has now developed a new weld seam detector based on the eddy current principle for these type of semi-finished steel strips. The system consists of a magnetizing unit and an eddy current encircling coil sensor that can be directly connected to the PLC. The orifice of the encircling coil is suitable for material with the dimensions 40 mm x 8 mm.
The key data of the new systems are:
| Material thickness: | Carbon steel | 0.2 mm ... 4 mm |
| Non-magnetic stainless steel | 0.5 mm ... 4 mm | |
| Aluminum | 0.5 mm ... 4 mm | |
| Strip width: | 5 mm ... 30 mm |
The transportation velocity of the strip should not exceed 5 m/s. This implies that the PLC can capture and process the signals at a rate of 5 kHz. If this is not possible, then the control unit XA100 from Roland is available to generate the required output switch signal.
The most important advantages of the new weld seam detection systems are:
- Suitability for narrow steel that strips
- Independency from optical surface discolorations
- Also suitability for start and stop processes
- Analog outputs 4 to 20mA for the connection to the PLC
- Cost-efficient control unit
Important factors for the success of reliable detection of weld seams are according to Roland the following points:
- Custom-made sensors for the application under consideration with "robust" electronic circuitry which is hardened against the EMC impacts of industrial environments.
- A sophisticated highly dynamic signal processing with application-specific signal filters.
- A complete stand-alone solution consisting of sensors, special cable and sophisticated digital signal analysis at an attractive price.
- Close cooperation with the OEM machine builder to find the optimum solution for each application.
