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Curve tool eases motion programming
by John Lewis, Northeast Technical Editor -- Design News, February 4, 2002
Vancouver, WA — A new development tool simplifies motion controller programming and tuning by taking a graphical approach to spline-function programming. The curve tool feature, as it is called, in RMCWin development software from Delta Computer Systems Inc., lets engineers use mouse clicks to graphically place key coordinates of mode, acceleration, deceleration, speed, and position over time to represent the motion of an axis.
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| With the curve tool, engineers can time scale the motion simply by selecting the curves and stretching | them, changing the rate of motion while keeping the axes synchronized as shown above. |
It then automatically connects the coordinates with a smooth motion profile using a third-order cubic spline algorithm that generates a sequence of instructions, which can be downloaded to the company's RMC line of motion controllers to effect the motion. Prior to downloading, though, the curve tool allows engineers to see the curve, adjust complex motion profiles graphically, and check it over until it is just the way they want it.
In contrast, typical motion controller programming techniques involve assembling, managing, and calculating extensive sequences of discrete motion programming instructions, according to Delta President Peter Nachtwey. "Tuning for optimal performance is difficult because programs require time-consuming recompilation and downloading of steps between trials. And the complexity increases if the motion of multiple axes must be related, such as if one axis is 'geared' to the motion of another axis."
The case for smooth moves in motion control stems from the assumption that rapid changes in acceleration cause vibrations in the drive train between the prime mover, actuator, and the load, or in the machine's structure and supporting stages. These vibrations can reduce the life of the machinery, and impact the process by increasing the time it takes for the load to settle into its final position. Such long settle times tend to reduce process quality, while increasing waste and cycle time.
In addition to making the system stiffer and the load lighter, engineers can also reduce settle times by smoothly varying acceleration. From rather ordinary s-curves to more complex NURBS (non-uniform rational B-splines), "there are several different ways to smooth out motion profiles with cubic splines," Nachtwey explains. Put into action, splines smooth motion profiles by fitting a cubic equation between all the points in a profile such that acceleration varies linearly and smoothly along a spline profile.
One tradeoff with the approach, however, is that larger motors may be required to produce enough power to make the move smoothly in the same amount of time. Another is that some tweaking at the endpoints may be necessary to zero the acceleration at the start or the end of a motion segment. "This can be time consuming using traditional programmable motion controllers," says Nachtwey, "since the engineer must program five functions per point, then calculate the third-order spline equation value between each point. If multiple related axes are involved, an engineer must change each axis separately, perform all the required calculations, and then validate the calculations to make sure all axes are still synchronized."
Altec Integrated Solutions Ltd. (Coquitlam, British Columbia, Canada) recently used Delta's curve tool on a veneer lathe with 32 axes of motion. The motion of several axes on the machine must be tightly synchronized to the carriage position.
"We use the RMCWin curve tool to generate the profiles for the knife angle, roller bar gap, and two back-up roll axes which must follow curves relative to the carriage position," explains Altec President Bill Long. "With the curve tool it's easy to generate the curve. You can visualize it before you download, and make sure that it's what you want. If it doesn't look right, you just adjust it by pointing and dragging the points around. It really speeds things up."
For more information about motion controllers from Delta Computer Systems Inc.: Enter 538
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