New Stretch-Forming Presses Boast Smooth Motion Control
Bending extruded aluminum channel to wrap around an automotive window, without putting kinks or folds in the aluminum, requires keeping the extrusion under constant tension while wrapping it around a form. To do so, Johnson’s machines employ hydraulic actuators to apply and hold a specific amount of force and tension, whereas using electric motors would require complex gearing and braking schemes to keep the motors from overheating. Hydraulic systems also require a relatively small amount of energy to maintain the desired force, and they scale easily, allowing the control system to be used with different-sized actuators.
The key to smooth, precise operation of Johnson’s stretch-forming machines is simultaneous coordination of closed-loop position and pressure control of multiple hydraulic axes. Eight linear hydraulic actuators perform the required stretching, lifting, bending and twisting.
The typical operation starts with clamping the extrusion at each end. Then the workpiece is stretched, bent and wrapped around the mold while the hydraulic actuators control the show.
Controlling the hydraulics requires a motion controller that:
• Simultaneously supports smooth operation of multiple axes, managing the machine’s long lever arm that attaches to the end of the extrusion. This arm must move smoothly, or bend quality will suffer.
• Provides accuracy—the machine must perform its forming operations repetitively to tight tolerances to produce consistently high-quality product.
• Can easily be programmed and tuned, so that basic machine elements can easily be reconfigured and optimized for different applications.
• Offers Ethernet communication between the motion controller and PLC, to enable on-the-fly communications and online troubleshooting.
Johnson Equipment employs motion controllers manufactured by Delta Computer Systems of Vancouver, WA. For its most recent stretch press, Johnson used two of Delta’s RMC100 controllers, each one able to provide as many as eight axes of control. The Delta controllers obtain axis-position information directly from linear magneto-strictive displacement transducers, one mounted on each cylinder. To provide force control, pressure sensors mount in the cylinder on each side of the piston. To ensure that the actuators precisely respond to control signals, the system employs Bosch Rexroth servo solenoid valves with zero-overlap spool and sleeve design. Preferable to dual-position on/off valves, the servo valves are voltage-controlled and can be infinitely adjustable based on inputs from the motion controller. Depending on the task, the cylinders can range in capacity from 5 and 150 tons; the Delta motion controllers need not change from machine to machine.
Developing the new line of stretch presses was a team effort, including help with specifying and servicing the hydraulics from John Henry Foster Co., St. Louis, MO;, and programming the PLC-based HMI and Delta motion controller by Automation Solutions, Monet, MO.
“We were impressed with the controller’s self-tuning capability using the Tuning Wizard that comes as part of Delta’s RMC motion-controller software,” says Ken Strain, of John Henry Foster. “The setup and tuning is a lot easier than it used to be. What used to take a day and a half now takes half a day.”
Johnson Equipment, Inc.: 417/753-2888; www.johnsonequipment.net
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