The
design goal for Aerotech's new small footprint AGS1500 gantry is to optimize
mechanical stiffness of the gantry for precise contouring accuracy in 2-D
space.
Designed
for a smaller footprint and payload capacity than the company's AGS15000 gantry
system, the new gantry maintains the same design goals in a smaller format. It
optimizes the mechanical stiffness in the structure of the gantry, reducing
angular errors to improve contouring accuracy. The system is designed for
travel distances up to 500 x 500 mm, loads up to 15 kg and provides velocity to
3 m/s, and accelerations up to 5 g using brushless linear servomotors.
Target applications include ultra-precise,
high-dynamic contouring, precision micromachining, stencil cutting, fuel cell
manufacturing, solder-ball placement, printed electronics manufacturing,
high-speed pick-and-place, automated assembly, vision inspection, dispensing
stations and high-accuracy inspection.
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"The
key with contouring applications is maintaining precise control of curved
motions while also maintaining high speeds to maximize throughput," says Jim
Johnston, product manager for Aerotech's Automation Systems Group. "For many
mechanical systems, the throughput versus tolerance trade-off is the key
challenge. Especially with a gantry-type system, there is a lot of inertial
energy to deal with to optimize performance when machining small features or
maintaining small contour accuracies."
As a system executes small contoured
motions, overcoming the inertia of the system during those moves usually
results in position errors. With micromachining a square corner, for example,
it's impossible to instantaneously decelerate one axis and instantaneously
accelerate the other axis, so the system can easily overshoot and produce
square corners that are distorted. The planar design of the AGS1500 optimizes
the motion to reliably machine the sharp corners, or any other contour that may
be required.
"Innovations in the mechanical design include
the gantry's planar design, which helps to reduce both the center of gravity of
the system and angular errors," says Johnston. "From a software standpoint,
advanced control algorithms optimize gantry operation. An Enhanced Throughput
Module measures the energy that the gantry system is inducing into the base
plate, and allows the controller to feed forward or inject that energy signal
back into the gantry control."
Gain Scheduling is another feature that is
part of the controller's dynamic toolbox. It reduces settling times and
provides a mechanism within the controller to dynamically change servo loop
gains as it completes the move. The result is that an application can move into
position and settle more quickly by using higher servo gains.
Johnston
says there are two trends with this type of precision machinery. One trend is
to produce components with ever-increasing tolerances as technology improves. The
other trend is to maintain a given tolerance while increasing throughput to
expand capacity by making parts faster. Normally as you increase tolerances,
throughput decreases. The AGS1500 design keeps both in mind so that higher
tolerances can be maintained with higher throughput.
Another goal is optimizing the system for
thermal expansion to ensure consistent operation during temperature variations.
These variations occur both from changes in the surrounding environment, as
well as differences in application demands such as speed and accelerations from
different part programs.
Temperature variations cause components to
expand and exert different stresses in different areas over travel. As such,
lower servo loop gains must be used, which limit the capabilities and bandwidth
of the system. Elements designed into the AGS1500 gantry enable it to operate
at full capacity over a wide temperature range, allowing it to use a higher set
of gains without causing instabilities
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