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Custom rug example
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Blending part program vectors and looking sufficiently far ahead through the data maximises machine productivity
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Motion controllers that produce smoother tool moves boost production and improve part quality.
Chris Williams
Systems Design Engineer
NEE Controls Inc.
Maumee, Ohio
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Motion controllers with vector blending from NEE Controls Inc. Maumee, Ohio, cut
cycle times nearly 40% for processes such as this plasma cutting operation. The controllers are suitable
for OEM's and legacy-machine retrofits.
High-volume manufacturers competing in global
markets must deftly juggle the opposing goals of product quality and shorter cycle times. Powerful
CAD/CAM software combined with modern NC mills, laser cutters and routers, help such companies reach these
objectives. But turning renderings into actual parts is largely the job of the machine's motion controller.
The controller converts CAM toolpaths into voltage signals which then modulate the output of the axes-motor
amplifiers. The motors drive ball screws or other actuators to create the desired motion. Controllers
capable of producing smooth and continuous axes motion can significantly boost feed rates, improve part quality,
and prolong machine and tool life. A new control algorithm called vector blending, along with expanded
feedforward compensation, makes these smooth moves possible. The approach offers many advantages over
traditional feedback and feedforward controllers.
Expanded Feedforward Compensation
Getting parts to match prints often requires tweaking axes feed rates. To prevent overshoot and lower
axis accelerations, feedrates are typically throttled down when cutting parts with abrupt angle changes.
For example, traditional feedback controllers encountering a programmed right angle slow one axis to
stop at a corner, allow time for that axis to settle, then accelerate the perpendicular axis. This is
because feedback controllers base the next move only on the previous one. The resultant halting motion
puts added strain on hardware and tooling, exacerbates play in axis actuators and may trigger low-frequency
resonance in the machine itself (about 20 Hz, typically). All of this negatively impacts part accuracy,
surface finishes, and consumes valuable cycle time.
Feedforward compensation boosts feed rates by
looking ahead (of the present position ) several elements or moves, typically about 2 to 10. The controller
uses this advance information to join elements of differing lengths and directions. But not peering ahead
far enough severely limits machine speed and causes halting motion similar to that produced by feedback
controllers. This is especially true when executing consecutive short moves (such as small radii) because
the controller must be ready to respond to possible upcoming-stops or direction changes.
However, a controller that looks forward about 100 moves permits higher and continuous axis speeds over
consecutive short moves. More data help the controller anticipate upcoming sharp transitions allowing finer
control over acceleration and deceleration rates. The resulting smoother motion improves both surface finish
and accuracy This expanded feedforward requires significant computing horsepower and high-speed RAM for
operation, however. Still, the advantages of the feature far outweigh the disadvantages.
VECTOR BLENDING
Even higher efficiencies result when expanded feedforward is combined with
vector blending. Vector blending removes instantaneous velocity changes, that occur during abrupt direction
changes. It does this by joining consecutive vectors with another curve type such as a parabola or arc.
By avoiding sharp velocity transitions, vector blending helps quell high acceleration rates that often
plague other control schemes.
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How vector blending works
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Compare, for example, how a conventional feedforward controller and one with vector blending plus expanded
feedforward compensation direct a plasma-cutting head at constant speed about a square trajectory.
The
no-vector-blend plots depict idealised toolpaths and velocity profiles as would be generated by the
conventional controller. This velocity profile isn't theoretically possible because of the infinite
acceleration and deceleration rates. In practice, one axis is decelerated to a stop, a delay is inserted
between moves to allow settling, then the perpendicular axis is accelerated.
In other words, the path
would require even more time to execute than shown here. The with-vector-blend plots, in contrast, are #
actual results from same process using vector blending with a 0.004-in. deviation at the corners. The
velocity profile would be trapezoidal-shaped had the square been large enough to allow axes to reach
terminal velocity. Note how one axis begins accelerating before the perpendicular one arrives at a corner.
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It works like this:
The blend time and blend distance is a function of the instantaneous velocity
change at a sharp transition. Blend time, bt(n), in seconds, is given by:
bt(n)=[V(n) - V( n + 1 )]
r
where n is the programmed data point,
V(n) is the end velocity of current vector in counts/sec, V(n+l)is the beginning velocity of the following
vector in counts/sec, and r is the acceleration or deceleration rate in counts/sec'.
The minimum
blend distance, bs(n), in inches, is:
bs(n)=V(n) * bt(n)
2
The amount of deviation or blending is adjustable by the operator. While it may seem counterproductive
to purposely introduce inaccuracy, consider how accurate and repeatable a machine or process is.
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A 16-sided polygon is to be cut from sheet metal using a plasma cutter The cutting head, operated by
a conventional feedforward controller, decelerates and stops at each point the polygon, while the one
augmented with vector blending (deviation set to 0.004-in.) maintains a minimum 130 in./min feed rate.
Moreover, acceleration rates never exceed the machine's 0.08-g limit. Bottom line: Vector
blending in this example shaves about 1 hr off production time for a 2000 piece run, a 39% improvement
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In many applications, the deviation is within part tolerance and the machine's capability Controllers with vector
blending help milling machines and the like achieve shorter cycle times. But high-speed, multiaxis machines executing complex
shapes benefit most. Such machines include plasma and laser cutters, dispensing equipment, and wood and plastic routers.
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