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Two heads: better than one

Article-Two heads: better than one

Two heads: better than one

Greater speed, longer life, and superior accuracy have propelled servomotor technology into many applications where air cylinders once reigned supreme. However, as servo-driven actuators tend to be too bulky for pick-and-place (P & P) heads, pneumatics remains the most popular choice for engineers designing probe tips-the end of a P & P head where a vacuum cup or end effector typically mounts. But now engineers have another option since IntellePro Inc. (Phillipsburg, NJ) commenced volume manufacturing of a servo-driven, dual-head, five-axis, P & P probe that increases throughput in chip-media transfer and other applications.

The dual head, as it is called, relies on compact brushless API servomotors, encoders, and miniature digital drives to achieve the close center-to-center distance between probe tips that's required for picking two chips simultaneously. "While the ability to rotate and position the chips is important," explains IntellePro President Don Rich, "picking two chips in a single operation requires a compact servo that allows the probe tips to move very close together." That's a feature that increases in importance as chips shrink in size.

In addition to being compact enough for the double-pick action, the servo components must provide smooth motion for P & P, rotate, and pitch translation functions. "In our design," says Rich, "the closed-loop servo drives use encoder feedback to provide 0.004-inch linear resolution with a theta resolution of 2,048 lines including quadrature." With this kind of accuracy, Rich explains, "throughput of a typical pick and place operation could be more than doubled as a combined result of simultaneous picks, high gantry accelerations, and very high-speed, z-axis motion of less than 100 msec for a 0.9-inch stroke."

Five axis, double-headed action requires a compact servo design that allows probe tips to move very close together on 0.400 to 3.00-inch centers.

The 10-million-cycle servo.

Today, more than ever, engineers seeking a competitive advantage want the higher throughputs and longer life that servos provide over pneumatics. But servos in P & P probe tip applications have had only limited success as most servo-driven actuators tend to be too large to achieve the close tip positioning required for double pick action. Hybrid designs that combine servomotors and pneumatic technologies on a single probe have also been used. But Rich explains that pneumatics is by far the most popular technology in P & P today despite the fact that air cylinders are slower, not as easy to program, and achieve only a fraction of the life when compared to servo-driven actuators.

"While air cylinders are compact in profile, low in cost, and reliable for a couple of million cycles, our servo-driven design has been tested to 10 million cycles," says Rich. IntellePro's servo-driven, dual-head design allows gantry accelerations to 6 G's, and its compact size allows two pick and place modules to be positioned on 0.400 to 3.00-inch centers, a key to picking today's smaller chips.

Each 700-gram head module independently performs z and theta (rotate) motions with individual vacuum/blow-off lines that are internally ported through a center bore to reduce exposed air lines. Numerous linear and radial ball bearings, plus custom designed helical drives, provide precision performance over extended time periods. "And the helically-driven z-axis is inherently failsafe should a power failure occur," Rich adds.

Breakaway spindle. Incorporated into each head module, a patent-pending breakaway spindle (See DN 09/04/00 p. 60) releases on side-load impacts. The crash-resistant design limits probe tip damage when an unexpected obstacle crosses its path, or when a software glitch causes loss of machine synchronization. Such crashes frequently result in damaged equipment in the form of a bent spindle or utter destruction of the entire P & P head.

To minimize such costly damage, the breakaway spindle is actually two components, the spindle adapter and the probe tip. The spindle adapter attaches to the P & P head and has a tapered bore that receives a spring-loaded collet on the probe tip. Collar-clamp screws in the spindle adapter and a keyway in the probe tip maintain rigidity in z and theta directions for accuracy and repeatability during operation, but allow it to break away upon side-load impact. After breaking away, the tapered design automatically centers and seats the probe tip when the user snaps it back into position. "The breakaway design extends life, and decreases maintenance," says Rich.

Some options include extended pitch axis travel (beyond 3 inches), single heads, ganged heads, quad matrix configurations (10 axis typical in a 2x2 programmable x, y matrix) and linear actuators. Current applications span semiconductor binning, tape and reel, media transfer, and chip singulation. Rich also anticipates applications in high performance linear actuators, medical analysis pipetting systems, biopsy devices, aerospace actuators, fluid dispensers, and robotic end effectors.

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