The different types of weighted keyboards:-
Digital pianos, acoustic pianos, organs and electronic keyboards can be weighted in a number of ways:- 1. Not weighted, 2. Semi-weighted, 3. hammer action or 4. graded hammer action. This, combined with the different types of touch sensitivity available, can contribute to the overall feel of "realism" when playing a keyboard as compared to an upright or grand piano.
1. Not weighted
Keyboards that are not weighted include most home electronic organs and pipe organs commonly found in churches and recital venues. Other instruments not weighted are accordians and entry-level keyboards - usually aimed at the beginner. Nowadays, most professional keyboards claim to be weighted in some way. (A decade or so ago synthesizers and most electronic keyboards were not weighted).
2. Semi-weighted keyboards
This action is common amongst budget-end keyboards and keyboards designed for portability. A sprung-action key with more resistance than normally found in budget-end keyboards.
3. Hammer action keys
A digital piano or stage electronic piano claiming to have "hammer action" keys mean than the key mechanism is in some way replicating the same action found on acoustic pianos. Here the resistance comes from a small hammer located and attached by a lever system near the key, rather than a spring. Manufacturers offer differing types of hammer action simulation (and usually claim their's to be the superior one!), and seek to add more realism on higher-end models. For example, Roland boast a realistic 'escapement' effect on some of their models, emulating the feel of key when played at pianissimo with a slight 'clicking' feeling.
4. Graded hammer weighting
On acoustic pianos, there is more resistance encountered in playing keys in the lower range than the upper range. Keyboards with "graded hammer weighting" or "progressive hammer-action" will exhibit heavier touch on the low notes of the instrument and lighter touch on the higher notes. Some manufacturers of high-end models make the keys out of wood rather than plastic to further enhance the feel of a real piano.