Modern manufacturing techniques demand that the raw materials fed to production lines be consistent. This is particularly true with rolled sheet steel where a variation in steel thickness could damage costly stamping dies and halt production. Walco Automation and Controls, a maker of steel-rolling-mill machines in Providence, R.I., meets these demands using a dedicated real-time control system from Delta Tau Data Systems Inc., Chatsworth, Calif.
Steel-rolling-mill machines by Walco process hot or cold sheet steel to specified thicknesses and tolerances — usually according to ASTM’s table of standard thicknesses. The machines must regulate the thickness of a 10-ft-wide sheet of steel that can be 45,000-ft long. The raw-coiled steel runs between two rollers that exert a force from 100 k to 4 M-lb to regulate the final thickness of the sheet.
Controlling the rolling process is a Delta Tau UMAC (Universal Motion and Automation Controller). The UMAC is a modular rack-mounted version of Delta Tau’s Turbo PMAC family of machine controllers. It consists of a set of 3U-format Euro-cards communicating with each other over a dedicated backplane in the rack.
The CPU card is said to have an extensive set of built-in machine-control algorithms. But it can also accept, store, and execute application-specific algorithms written by users. The control uses USB 2.0 to maintain communication with the supervisory computer that runs an operator-control HMI written in Visual Basic. The 480-Mbps of the USB 2.0 communication link ensures almost instant response to operator commands.
Walco embedded its own algorithms in the UMAC controller to control sheet thickness and speed by controlling roll force and position. The UMAC continuously reads encoders to determine sheet position, velocity, entry thickness, and exit thickness. The readings determine which corrections to make to keep the full process within specifications.
Because the machine can operate at sheet speeds up to 4,000 fpm, the speed of the encoders, processor, and communications are crucial for maintaining reliable tolerances from one end of the sheet to the other. The controller must operate on “hard real-time” to ensure key tasks work on a guaranteed schedule, to maintain tight tolerances everywhere.
While the UMAC system can control up to 32 axes, Walco uses only two axes for the hydraulic cylinders that control position and force of the pressure rollers, and two additional axes to control sheet position and speed. A few of the tandem mills can have up to 10 axes of control.
The system was designed to work at a maximum speed of 4,500 fpm (22.7 m/sec), so it must take and process more than 2,000 readings/sec. Thickness measurements are made every 10-mm regardless of sheet speed. Movement of the steel is tracked by 10,000-ppr quadrature encoders.
The linear position of the rollforce cylinders is monitored by 0.5-μm Sony Magnascales. The feedback signals the Moog Corp. flight-control hydraulic servovalves that control the position of the 3,500-psi hydraulic cylinders. The system typically maintains an overall tolerance under 10 μin. (2.5 μm).
Information about each axis and all of the sensors is instantly available to any task from memory or memory-mapped registers.
Rolling-mill machines from Walco Automation and Controls using the Delta Tau UMAC can operate at speeds up to 4,000 fpm and still maintain a thickness tolerance under 2.5 μm (10 μin.) on a 10-ft-wide steel sheet over 45,000-ft long.