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Compact Linear Motor Gantry

Compact Linear Motor Gantry

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.

Compact Linear Motor Gantry

"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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