What different types of encoders exist?！

The most widely used classification refers to the type of movement (linear or rotary). In both cases they can be incremental, semi-absolute or absolute.
Incremental information is obtained by simply counting the pulses. Therefore, it depends on the previous state and the value of the transition. Its biggest drawback consists in the need for defining a starting position reference: this information is lost whenever the system is powered down or is turned off.
In contrast, in absolute encoders (of angular or linear type) each position is properly referenced with a unique code, Figure 1, corresponding to a unique pattern of bits in the various tracks. So, the position is always known and it is not necessary to define a reference if the system is powered down or is turned off.

The semi-absolute encoders are typically rotary and are used when it is necessary to measure displacements that exceed the measuring range of the absolute encoder. In this case it is necessary to use an additional procedure to count the number of disc turns during the measurement.

The physical quantity conversion could be explained taking as an example the incremental rotary encoder depicted in Figures 2 and 3, available for remote use. The disc is divided into 18 equal sectors, alternately of reflective and non-reflective type. A sensing head reads the generated code. The element "R" is the receiver for detecting the light emitted by the emitter "E". In this case the emitter and the receiver are on the same side of the disc. When the light beam reaches a reflector disc sector, it will be reflected. The electronics associated to the receiver transform this information into an electrical signal.

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