New motion control for big tube bending machines

Timax turned to tube bending specialists Arena to develop a new control system for its Vector Pipeline tube bending machines, first installed in the mid-1970s

There are only a handful of Vector Pipeline tube bending machines in the world capable of creating a tube from steel strip material and delivering a finished vehicle exhaust pipe at the other end at rates of up to one product every 6 seconds! Three such machines are installed at the Timax tube bending plant at Cheadle, near Stoke on Trent.

But the machines, first installed in the mid-1970s had suffered from increasingly unreliable controls, and Timax turned to tube bending specialists Arena to develop a new control system.

Arena designed and installed a new system using Delta Tau's MACRO (Motion And Control Ring Optical) coupled with a Delta Tau Turbo PMAC2 motion controller.

Arena has long experience using Delta Tau's PMAC motion control products for its own CNC tube bending machines, but the company had never exploited the potential of MACRO before.

In spite of this, the company was able to quickly develop a system and once the design was complete, the new control solution was installed in less than two weeks! The machine has immense capability.

Flat steel strip enters at one end from a coil.

It is formed into a round tube profile and welded in line using a high frequency welding head.

The completed tube section is played out and into a huge arc rising up and over the machine itself.

The tube then feeds through a sizing and straightening module that is hydraulically powered, before progressing down into the bending, forming and cutting head.

The finished, trimmed, exhaust pipes are then dropped into a waiting collection bin where they are transported directly to the trucks that deliver them to the exhaust centres serving the vehicle aftermarket.

The previous control system relied on three separate PCs to manage each key element in the manufacturing cycle.

These computers communicated with each other across standard RS232 serial comms.

There have been upgrades to the system over the years, but there was inherent unreliability in the communications and the speed was restricted.

Arena's Delta Tau solution has managed to close the servo loops between the sizing/straightening module and the bending head at near real time.

This was an important control feature since the output of the machine and the feed rate of the welded tube to the straightening/sizing module is dictated by the rate at which the bending head can perform its given tasks.

A MACRO station is installed in the bending unit and a further MACRO station in the same control panel as the single PC that is now required.

The PC is fitted with the Turbo PMAC2 control card.

Each MACRO station, which handles all the machine interface signals, is housed in a compact 3U high rack.

While the control task is apparently undemanding - there are only four servo hydraulic axes - there are also over 60 distinct I/O including inputs from rotary encoders and so forth.

Opto 22 boards are used for this level of communication, but these are just dumb boards and the system also takes advantage of some of the native I/O capability of the Turbo PMAC2 controller.

There are also a number of mitigating factors that posed potential problems for the Delta Tau system.

Notably, the straightening module uses an unusual rotating coupling, made by Litton Precision in the USA, that is subjected to huge vibration and G-forces.

This coupling is very unusual in that it uses a pick-up construction made up of strands and even Delta Tau's experts had never encountered an application similar.

Running at 50 rev/min the coupling has eight channels - five of which are used for RJ45 connections to the MACRO ring and two for powering the MACRO station and its boards.

The coupling, which sits on top of the hydraulic head that uses 2000psi and consumes 4-5A at 10V, has the MACRO station installed within it and the RJ45 cable used for the ring topology of the system has to pass through the coupling.

It was a challenge! A measure of the arduous nature of the installation is that the previous PC had all its critical components cemented into place with epoxy resin! To compound the communications issue, this MACRO station is positioned almost immediately above the electrical cabinet of the high frequency welder, while the master PC with its sister MACRO station is positioned immediately next to the same cabinet.

Roger Eads, senior engineer of Arena, explained, "we have been using Delta Tau's PMAC for about 10 years and have got machines in the field over eight years old with the original controls still functioning well.

Occasionally we bring in a machine for refurbishment and might add a standard EPROM to gain all the latest PMAC functionality without redundancy or obsolescence".

The Delta Tau MACRO/PMAC2 solution has proved unhindered by the harshness of the environment or the arduous duty cycles of the machine.

Cycle times vary between six seconds and 36 seconds being the longest cycle time.

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