Three different ways to hold something
The most common gripper on a factory floor is the parallel-jaw gripper: two rigid fingers that close toward each other along a straight line, usually driven by a single actuator through a linkage or lead screw. It is mechanically simple, repeatable, and easy to control, which is exactly why it dominates industrial assembly lines picking rigid, regularly shaped parts like bolts, blocks, or housings. Its weakness is equally simple: two flat or lightly contoured surfaces closing on a straight line can only accommodate a limited range of shapes. A part that is round, oddly contoured, or larger than the jaw stroke can slip, rock, or simply not fit between the fingers at all.
Vacuum grippers take a completely different approach: instead of squeezing an object between surfaces, they press a suction cup or array of cups against it and pull air out, letting atmospheric pressure hold the object in place. This works extremely well on flat, smooth, non-porous surfaces, cardboard boxes, glass panes, plastic sheets, painted metal, where a cup can form an airtight seal. It fails just as predictably on porous materials like fabric or unsealed wood, on heavily textured or perforated surfaces where air leaks around the seal, and on strongly curved or irregular surfaces where the cup cannot conform enough to seal at all. Soft or compliant grippers, built from flexible materials such as silicone or actuated by pneumatic bellows, take a third approach: rather than relying on rigid precision or an airtight seal, their fingers or membranes physically deform to wrap around whatever shape they encounter, trading peak grip force and positional precision for tolerance to shape variation that neither parallel-jaw nor vacuum designs offer.