Types of encoders

Some of the encoders include absolute and incremental encoders, optical encoders, incremental optical encoders, absolute optical encoders and resolver encoders.

Absolute and incremental encoders 

Incremental encoders when rotated generate pulses that are counted to provide position information relative to a known point, whereas absolute encoders provide a unique value at each position and retain actual shaft position even if power fails and the shaft moves. 

Where incremental encoders are less complex and have fewer outputs (2 or 3), absolute encoders typically have 12 outputs and are generally more expensive. Incremental encoder applications typically require a reset input to zero out the count and start a fresh cycle while absolute encoders do not need a reset input as the output is always unique and absolute.

Optical Encoders

Optical encoders typically consist of a rotating and a stationary member. The rotor is usually a metal or glass disc mounted on its shaft with the disc featuring an optical pattern. The stator has an LED block and phototransistors arranged so that when the LED light shines through the transparent sections of the rotor disc, it is received by phototransistors on the other side.

Incremental Optical Encoders

Incremental optical encoders use a simple disc pattern with the slotted rotor disc alternately interrupting the light beam between the LED and phototransistor to produce a pulse output. The number of pulses depends on the number of slots on the disc, and is counted to give position information. The pulse rate indicates shaft speed with an additional phototransistor also determining the direction of rotation.

Absolute Optical Encoders

The disc used in absolute optical encoders is more complex than the simple disc in incremental encoders. Since the absolute encoder needs to encode a unique value for the shaft position, the number of tracks on the disc and corresponding phototransistors depend on the resolution sought and number of bits used. 
 
The size, complexity and cost of absolute optical encoders increase exponentially with resolution as the pattern gets increasingly complex with the increased number of bits.

Resolver Encoders

Resolver encoders or resolvers are by far the most rugged position transducers available. The resolver is essentially a rotary transformer, having one rotor winding, and two stator windings located 90º apart. The rotor or the stator winding can be used as primary. Typically, the rotor winding is driven by a reference voltage at a frequency ranging from 400 Hz to several KHz.

26.07.2012