There are several types but I know mechanical. These are machines designed to automatically pull out the onion from the ground. They use various mechanism like rotating belts and mechanical fingers to separate onion from the soil. Some have adjustable blades to cut the foliage and separate it from the bulb.

Then we have top onion harvester where onions are cut from leaves leaving the roots in the ground and this is more used in kenya as a method of harvesting onions especially in Mount Elgon