However, applying an accurate bead can be difficult for robotic adhesive dispensers because of dimensional irregularities in the components to be glued. The adhesive bead can only be positioned exactly if the distance between the component and glue nozzle is kept constant. This applies to the height of the nozzle above the component, as well as the path of the nozzle along the seam.

Assemblers can use two different processes to position the adhesive bead precisely. One way would be to fix the components in place with clamps. The position of the adhesive bead is then worked out using the robot's teach pendant. But this method only tolerates minor deviations in the component's geometry. Larger deviations in component geometry can only be partly compensated for by clamping. If the distance between the nozzle and the component varies too much, it can cause an unidentified swirling of the bead. Or worse, it can lead to a collision between the nozzle and workpiece, causing damage and downtime.

Alternately, the dispense head can be equipped with a sensor that constantly monitors the position of the nozzle above the workpiece. The fixtures that hold the components can be simply designed because it is not necessary to hold their position to such small tolerances.

The position and path of the dispensing nozzle only needs to be programmed roughly for it to come into the sensor's measuring range. This takes little time. This combination of distance control and simple clamping also means greater tolerance variations in component geometry are acceptable. Downtime due to collisions can be avoided.

Just such a dispensing system is now being used at DaimlerChrysler's Warren Truck Assembly Plant (Warren, MI) and St. Louis North Assembly Plant (Fenton, MO). Manufactured by Precitec Inc. (New Hudson, MI), the system consists of a dispenser, an inductive position sensor, a control unit, a preamplifier and a linear drive. These are mounted on a pedestal stand. A robot moves the workpiece under the linear drive.

Mounted at the tip of the nozzle, the sensor continuously monitors the gap between the nozzle and workpiece, and provides feedback to the motor controller. The system then automatically adjusts the position of the nozzle to maintain the right gap. The typical gap for critical adhesive applications is 2 to 4 millimeters, and the system can control that distance to within ± 0.2 millimeters. A second sensor can be added to the system to ensure that the nozzle does not stray horizontally from the path.

The system's greatest benefit is ensuring a uniform bead line. If the glue nozzle is too far away from the workpiece, a swirling line can result. If the nozzle is too close, a flat line can occur. Neither of these is desirable.

Other benefits include a reduction of wasted glue or sealer and a reduction of robot programming time. Engineers only need to program the path of the robot from point A to point B, and the height of the nozzle above the workpiece. The Precitec system corrects for height differences between the points. It can correct for height differences of ±15 millimeters.

For more information on adhesive dispensing systems, call 248-446-8100 or visit www.precitec.com.