Meta Vision Systems

The world population of spiral pipe mills has increased dramatically over the past ten years and continues to grow.  Part of thisexpansion is driven by the acceptance of spirally welded pipe for oil and gas transmission.  This application in particular imposes stringent standards on pipe quality and therefore on manufacturing processes.

To meet this need in respect of welding technology, Oxfordshire-based Meta Vision Systems has developed an integrated, networked digital control system as the enabling technology behind what it terms the digital spiral mill. It includes industrial networking, new digital SAW (submerged arc welding) power sources, the latest digital sensors and advanced digital controls.

All can be combined into a single, integrated mill control system. As sensors and controls are accessible, it is possible to improve significantly ease of operation, fault-finding, safety and maintenance. Further benefits include additional functionality, improved ease of operation through better HMIs (human-machine interfaces), integrated data logging and remote access.

New digital welding power sources facilitate a wider range of weld procedures while at the same time being easily interconnected for ease of use.  This increases efficiency in various ways while at the same time facilitating accurate parameter control and data reporting.

The latest laser vision sensors from Meta are being used to achieve performance gains in several areas in addition to their established use for weld seam tracking.  Laser sensors can now validate weld bevels, in combination with digital signal processing for gap and mismatch control on the mill itself, and inspect weld bead profiles.

Combining mill control and weld control into a single, integrated networked system brings major advantages in terms of performance, reliability, ease of use, safety and cost.  Previously, the sensing, welding and control systems were separate.  For example, control of the ID and OD weld heads was often from completely independent sub systems. This meant that the respective operators had to inch, start/stop and adjust parameters locally. With an integrated system, such actions can be handled from wherever is most suitable.

The central part of the control system is a pair of networked PLCs (programmable logic controllers).  One PLC controls the mill feed section, including coil loading, edge milling and main drives.  The second PLC controls the forming and pay-off section, including the weld controls and laser seam tracking.

The HMI to the integrated control system is via a main control console plus a pair of intelligent consoles for the ID and OD operators and the usual local operator stations at work points distributed across the mill.

Stability of operation is a key requirement in the forming section of a spiral pipe mill.  Sources of perturbations which impact negatively on stability are camber and strip width variations.  One serious problem which can result from lack of stability in the forming section is inconsistent edge milling, which causes problems with forming and welding.

To monitor this, a multi-function measurement system automatically checks edge milling by measuring the bevels.  This is becoming even more important with the widespread adoption of two-step spiral mill technology.

During bevel inspection, the actual shape of both sides of the strip is measured continuously by two laser sensors mounted in opposition on each side of the strip, just after edge milling but before pipe forming.  If

any out-of-tolerance conditions are identified, various actions can be programmed, ranging from informing the operator to stopping the mill.

There is enormous scope for using advanced sensing and control techniques in the forming and welding section of a spiral pipe mill. Gap control refers to controlling the mill dynamically to ensure good pipe formation and an absence of defects at the ID weld station, where the weld can burn through if the gap opens up too much. Mismatch control refers to the control of the difference in height across the weld. Maximum acceptable mismatch values specified by pipe mill customers are gradually getting smaller.

One example of where mismatch control is especially important is in large diameter water pipe. Controlling the mismatch is more difficult with very large pipe diameters, for example over two metres, and yet water pipe customers are imposing stringent limits on mismatch.  This has led to increased interest in automatic mismatch control to satisfy these requirements.

A laser sensor measures the gap, typically just ahead the welding point, and any deviation from target value is used to open or close the mill to maintain a constant gap. It is actually the helix angle that is being controlled, normally achieved by controlling the payoff table although controlling the angle of the feed section is an alternative method.

The sensor can be used for ID weld tracking as well as for gap measurement, although alternative approaches have also been investigated, including combined gap and mismatch control, new mechanical gap measurement sensors and positioning the laser sensor outside the pipe.

An automated mismatch control system that is separate from non-zero gap control has to be designed to cope with the fact that the measurement is being made after welding. A laser sensor makes highly accurate mismatch measurements many times per second, for example to ±0.1mm at 25Hz.  The automatic control moves the actuator by exactly the correct amount to eliminate the error.  Timing issues are also controlled and performance optimised based on the known positions of the actuator and sensor as well as the mill speed.

In cases where there is no gap and the edge of the incoming strip is pressing tightly against the edge of the newly formed pipe, trying to change the mismatch under these conditions can lead to unpredictable results.  The pipe is less constrained than the strip and tends to move more easily, but, depending on the pressure between the two edges, relative movement may not be smooth or well controlled.

When gap and mismatch control are not independent, an alternative regime is possible.  In this case, the main focus is on measuring the mismatch, since the gap is always zero, and actuation is by controlling the helix angle.

One big advantage of digital spiral mill architecture is that the welding equipment can be fully integrated with the mill control.  This is particularly true in the case of the latest digital SAW power sources, such as Lincoln Electric’s PowerWave AC DC 1000SD.  This power source can be relatively easily connected to a mill PLC via a DeviceNet network and also provides easy data logging via an Ethernet connection.

Having the welding equipment integrated with the overall control provides additional functionality and opportunities for system improvement, including improved operator interfaces, customer specific database storage, live parameter monitoring and warning, integrated data logging and remote internet access.

Integrating the weld controls and the laser sensors in the same system also provides an opportunity to use adaptive fill.  As well as seam tracking, the laser sensor measures the actual parameters of the weld joint and can adjust the deposition rates to match.

The same type of laser sensor that is used for weld seam tracking can also monitor the weld bead.  The laser sensor measures the height and

width of the bead as well as classifying the shape.  Such a laser sensor can be integrated, for example, with an online ultrasonic test system, tracking the weld bead to keep the test probes centred.  At the same time, the laser sensor measures and classifies the weld bead.  This is becoming more important as constraints on weld beads are becoming ever tighter to improve pipe coating. 

Release no:      1043(F)