Robots with vision assemble with speed and precision.

To attain higher speeds and more accurate production of security tags for store merchandise, Sensormatic Electronics Corp. in Boca Raton, Fla., substituted robot arms for human hands and vision systems for human eyes.

Security tags are typically made by assembling all the components for hundreds of tags in the pockets of a thermoformed 12 x 18 in. HIPS sheet. Once the pockets are filled with two metallic identifying strips, the sheet is covered with a polyethylene lidstock and then an adhesive-backed PET film. Finally, the thermoformed sheet is put on a slitter where a perforation is made around each tag so that it can be torn off the sheet to be attached to merchandise.

To speed up tag production, Sensormatic constructs its tags in stages rather than all at once using robotics, vision systems, and a punch press instead of a slitter to automate the process. Sensormatic also makes its tags by the roll instead of by the sheet. The move speeds its form-and-cut operation to 2000 tags per minute.

Making 20-in.-diam. rolls of tags turned Sensormatic's die-cut sheet operation into a more continuous process. First, the HIPS sheet is formed into pockets, the pockets are filled with the metal strips, and the lidstock is heat-sealed over them - all on a single rotary table. The sheet of partially completed tags is then transported to a punch press, where it is simultaneously perforated and sealed against a continuous roll of adhesive-coated PET film. The finished tags are then wound into a roll.

ROBOT HAND/EYE COORDINATION

A critical step in the new automated process is placing the thermoformed sheet of partially completed tags into the 75-ton punch press. Accuracy is crucial since the metal in the sheet pockets can damage the tool-steel punch if the sheet is mislaid, says Joe Sciacca, manager of machine design.

To handle this sensitive task, Sensormatic installed a K10 robot from Motoman of West Carrollton, Ohio. The robot, which has a positioning repeatability of [+ or -]0.004 in., picks up the sheet from the rotary table and places it on the press. Although the robot's movements are precise, the position of the sheet when the robot picks it off of the table cannot be guaranteed. So before placing sheet in the press, the robot holds the sheet in front of a vision system from Acuity Imaging, Inc., Nashua, N.H. The vision system identifies the position of selected reference points on the sheet and then instructs the robot how to move with the sheet to the die in 0.00050-in. increments. Sensormatic says sheet placement is accurate to within 0.0030 in. After placing the sheet in the punch press, the robot removes the skeleton of sheet trim left from the previous cycle.

Sensormatic employs robots and vision systems at other points in the process. Loading the sheet onto the rotary table is done by a robot from Seiko Instruments U.S. Inc., Robotics Automation Div., Torrance, Calif. At the punch press, a vision system from Cognex Corp., Natick, Mass., checks the adhesive roll for telescoping or Other problems. Another Cognex vision system checks the alignment of the completed tags. That system controls the movement of a magnetic printing bar. A final vision system checks the registration of the printing on the tag.

Besides adding speed, automation also improves the quality of the labels. The company maintains a Cpk of at least 2, says Mark Nixon, director of label support. That translates into an expected defect rate of 3.4 parts per million.

Automation isn't cheap. The company spent $1-2 million on the tag-making line, but the system paid for itself within a year bemuse the automated line can run 24 hr a day.