In a sheet metal fabrication process, an unfastened part rides on a conveyor belt moving continuously at an unpredictable velocity. Two-spot-welds are to be performed on each part, 4 inches apart, with the first weld 2 inches from the leading edge of the part. A weld takes one second.

Machine Objectives:

• Standalone operation
• Position welder according to position and velocity of each individual part
• Welding and positioning performed without stopping the conveyor
• Welding process must take 1 second to complete

Motion Control Requirements:

• Programmable I/O; sequence storage
• Position-based Following
• Motion profiling; complex following
• High linear acceleration and speed

Application Solution:

This application requires a controller that can perform following or motion profiling based on a primary encoder position. The controller in this case will receive velocity and position data from an incremental encoder mounted to a roller on the conveyor belt carrying the unfastened parts. This conveyor is the primary drive system and the secondary motor/drive system receives instructions from the controller, based on the information supplied by the primary system encoder. The weld head is carried by the linear motor forcer and is mounted on an overhead platform in line with the conveyor.

Application Solution

This application requires a controller that can perform following or motion profiling based on a primary encoder position. In this application, the controller will receive velocity and position data from an incremental encoder mounted to a roller on the conveyor belt carrying the unfastened parts. The conveyor is considered the primary drive system. The secondary motor/drive system receives instructions from the controller, based on a ratio of the velocity and position information supplied by the primary system encoder. The linear motor forcer carries the weld head and is mounted on an overhead platform in line with the conveyor.

Linear motor technology was chosen to carry the weld head because of the length of travel. The linear step motor is not subject to the same linear velocity and acceleration limitations inherent in systems converting rotary to linear motion. For example, in a leadscrew system, the inertia of the leadscrew frequently exceeds the inertia of the load and as the length of the screw increases, so does the inertia. With linear motors, all the force generated by the motor in efficiently applied directly to the load; thus length has no effect on system inertia. This application requires a 54-inch platen to enable following of conveyor speeds over 20 in/sec.

Application Solution

1. A sensor mounted on the weld head detects the leading edge of a moving part and sends a trigger pulse to the controller.
2. The controller receives the trigger signal and commands the linear motor/drive to ramp up to twice the speed of the conveyor. This provides an acceleration such that 2 inches of the part passes by the weld head by the time the weld head reaches 100% of the conveyor velocity.
3. The controller changes the speed ratio to 1:1, so that the weld head maintains the speed of the conveyor for the first weld. The weld takes 1 second.
4. The following ratio is set to zero, and the welder decelerates to zero velocity over 2 inches.
5. The controller commands the linear forcer to repeat the same acceleration ramp as in step 1 above. This causes the weld head to position itself at an equal velocity with the conveyor, 4 inches behind the first weld.
6. Step 3 is repeated to make the second weld.
7. Once the second weld is finished, the controller commands the linear forcer to return the weld head to the starting position to wait for the next part to arrive.

Product Solutions:

• Indexer/Drive - ZETA6104
• Motor - L20 Forcer and PO-L20-P54 Platen