Page 37 - MetalForming February 2016

Outer Loop PID
Cylinder 1, Inner Loop PID
Cylinder 2, Inner Loop PID
Cylinder 3, Inner Loop PID
Cylinder 4, Inner Loop PID
Cylinder 1
Cylinder 2
Cylinder 3
Cylinder 4
Custom Feedback Calculations
Fig. 2—The motion con- troller on Missouri Metals’ new hot-form- ing press can control as many as eight motion axes simultaneously, allowing independent operation of each side of the press that enables simultaneous part production.
modes. The controller also needed to be capable of syn- chronizing the motion of multiple axes, which must move simultaneously. To fulfill these requirements, JHF recom- mended using a RMC151 motion controller available with eight axes of control (Fig. 2), supplied by Delta Computer Sys- tems, Inc., Battle Ground, WA.
Titanium Work Demands Precise Control of Multiple Forming Stages
The hydraulic press operates at 1700 F, with its platens con- taining heaters to soften the titanium for forming. The parts must be pressed in stages, and should the forming proceed too quickly, part material can crack. The solution: Close the press so that the platens just touch the part material, and ini- tiate a preheat stage to make the titanium pliable. The press ram then moves a few inches with that position held for a cer- tain time period, after which the process repeats. During the final forming stage, a prescribed force is applied, which can be as much as 400 tons.
“We can perform as many as 10 press operations before we reach tonnage in the final stage,” says Moore, “with the posi- tion hold times varying between 1 and 5 min.”
The four cylinders in the new press mount beneath the lower platen and push vertically to close the press, which is capable of operating in two modes—full-platen mode to press a single, very long part, or split-platen mode to inde- pendently press two parts. Setups for both modes employ fea- tures of Delta Computer Systems’ RMCTools development software, which drives the RMC151 motion controller.
One of these programming features, custom feedback, enables the motion controller to perform mathematical or logical operations on a feedback value, or combine the inputs from multiple feedback sources. The result is then incorporated as a single feedback value into the closed- loop control algorithm.
In Missouri Metals’ press system, cylinder positions must remain level during press operation. To accomplish this, custom feedback averages position inputs from the four position transducers, which provides a target position for all cylinders to hit during the next control cycle. In addition, indi- vidual force values derived from each cylinder’s differential- pressure transducers are summed to calculate the total sys- tem tonnage value, which then is compared with the force target value in the controller for use in the controller’s own closed-loop force calculations.
Fig. 3—As this diagram shows, one motion-controller feature, a method called cascade control, uses the output of one control loop to provide the target for another loop. With Missouri Metals’ press design, the inner loop run by each of the four individual cylinder axes is responsible for ensuring that its cylinder extends to the required position and delivers the set amount of force. The outer loops are responsible for determining the total response of the entire press system.
Another feature, a method called cascade control, uses the output of one control loop to provide the target for another loop. With Missouri Metals’ press design (Fig. 3.), the inner loop run by each of the four individual cylinder axes is responsible for ensuring that its cylinder extends to the required position and delivers the set amount of force. The outer loops are responsible for determining the total response of the entire press system. As described above, the target gen- erated for the outer position loop, using custom feedback, is used to keep the cylinders perfectly level, while the target gen- erated for the outer force loop ensures that forces exerted by each of the four cylinders add up to the predefined force set point. The inner and outer control loops run simultaneous- ly, yet are independent of each other. This results in a perfectly level press platen, delivering the set amount of force with no mechanical guides.
Setting up cascade control involves dedicating two of the controller’s motion axes to each cylinder, the inner loop set as a regular and faster closed-loop control axis and gen- erating an output to the cylinder’s proportional valve, with the slower outer loop set as a ‘cascade’ axis, whose control out- put is used internally by the motion controller to provide a motion target to the inner loops. The overall system is con- trolled by writing commands to the outer-loop axis. All of this functionality is set up via simple write operations to registers within the RMC151, so no complex motion programming is required.
The press uses feedback from linear magnetostrictive displacement transducers attached to the sides of the cylin- ders to measure the position of each cylinder, and pressure transducers mounted in the cylinders, one on each side of the piston, as feedback to measure the force being applied by each cylinder. The outputs of the motion controller connect
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