Designing for robotic assembly

01 February 2008

Mitsubishi’s Himeji Works assembles a wide range of automotive alternators and other products via a production line that is so automated, it requires a team of of only eleven employees to assemble thousands of alternators every day.

Segmented core stator assembly
Segmented core stator assembly

For automation to be effective the products have to be designed with automation in mind. ‘Design for manufacturability’ concepts have made designers think about how their creations are to be manufactured right from the start.

Himeji Works practices a further refinement that might be called ‘design for robotic assembly.’ Constraints for automated assemblers are quite different—and more restrictive— than those for humans. For example, it’s quite natural for a human to pick up a subassembly and turn it over to reach the bit that needs to be worked on. Picking a subassembly up off a conveyor, then turning it over requires quite a complex machine, however.

Coil winding on free-standing forms was one of the first manufacturing operations to be automated at the dawn of the electronics industry. Adding the copper coils to the magnetic poles of an alternator stator, on the other hand, has always been difficult, even for human assemblers. The traditional method uses a rigid ring-shaped laminated-steel core with a dozen poles. This pattern forces the assembler to stick his or her hands (or robotic grippers) into the small space inside the core to wrap copper wire over each pole.

Mitsubishi engineers realised that winding the coils would be much easier if the stator were inside out, so they designed a flexible alternator stator made up of hinged segments, one segment for each pole, like a bicycle chain (top illustration). A robotic handler presents each of the segments in turn to a coil winding machine, which adds the copper wire as if it were on a free-standing form. After winding one coil, the handler indexes to present the next segment in the same place for the winding machine. After all coils have been wound, the stator goes to a machine that bends it around a form into a circle and welds the hinged joints to keep the stator’s shape.

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