Servos bring gentle handling

Sept. 1, 2004
On this four-axis tray loader, an integrated controller teams up with intelligent servo drives on all four servo motors.

In its previous design, this tray loader was also a four-axis machine, but it was all mechanically driven. We redesigned it and incorporated servos and an integrated motion system to reduce cost and maintenance and to improve reliability.”

That’s how product manager Jeff Jackson of Bosch Packaging Technology sums up the main rationale behind the firm’s controls package now used on its tray loading system for loading liquid-filled pharmaceutical vials into trays. Running at speeds to 400/min—up 25% from the previous, non-servo design—the machine now relies on four MKD digital servo motors from Bosch Rexroth for all four axes of motion. Infeed of a row of trays is one axis, the vertical and horizontal motions of the vial pick-and-place device are the second and third axes, and the indexing of the vial tray is axis number four.

It’s crucial that all these motions be handled with the utmost delicacy, too. If the liquid contents of a vial splashes up the sides, the Quality Control department at most pharma companies will deem that vial a reject. The gentleness of the acceleration and deceleration provided by the servo-driven system helps greatly in this regard, says Jackson.

A Bosch Rexroth PPC controller handles complete machine control, including logic, motion, and I/O. The controller gets a lot of help from four Bosch Rexroth Ecodrive intelligent servo drives, one for each servo motor.

“Because the drives are intelligent, you don’t need to burden the PPC controller with the task of telling the servo motors all the things they must do,” says Jackson. “They get the information they need from their drives. That frees up processing time and overall capability in the controller, because it has fewer algorithms and calculations to perform.”

The servo motor that handles the vertical axis of the pick-and-place robot uses two Bosch Rexroth ball screws to give it the gear ratio it requires. But the other three use Bosch Rexroth planetary gear boxes. “With the gear box attached to the motor,” says Jackson, “if you require motion that’s slow, the motor can still run at a speed it’s comfortable with while the gear box delivers the gear ratio required for the slower speed.”

Jackson says the integrated PPC controller—logic, motion, I/O—makes the machine less costly to build because there’s no need for separate controllers. The use of a Pro-Face pendant-style HMI screen, supplied by Xycom Automation, is another cost saver.

“The previous model had an HMI screen, too, but it was typically placed wherever the customer wanted it,” says Jackson. “That meant custom engineering, extra time, and extra cost. With this pendant approach, we can build the same standard machine for everybody.”

The Pro-Face HMI screen is linked with the Bosch Rexroth PPC controller through serial data communication. The other communications protocol deployed here is the industry-standard SERCOS, used as a motion control network.