Encoders are mechanical to electrical transducers whose output is derived by “reading” a coded pattern on 

a rotating disk or a moving scale. Encoders are classified by the 

• method used to read the coded element: contact or non-contact 

• type of output: absolute digital word or series of incremental pulses 

• physical phenomenon employed to produce the output: electrical conduction, magnetic, optical, 

capacitive 

CONTACT ENCODERS 

Contact encoders are those which employ mechanical contact between a brush or pin sensor and the coded disk. The disk contains a series of concentric rings or tracks which are thin metallic strips joined at their base as shown in Figure 3-1. The four tracks shown in Figure 3-1 represent a binary code consisting of 0, 2, 4, 8. The associated contact sensors are identified at B0, B1, B2, B3, and encode the numerals 0 through 15. As the disk rotates, the sensors alternately contact conductive strips and adjacent insulators, producing a series of square wave patternsUniform and non-uniform disc patterns can be utilized depending on the application. Virtually any pattern which can be produced photographically can be imaged on an encoder disc. The typical application is measurement of shaft position which utilizes a uniform pattern. Any non-uniformity in the disc is a source of error. Non-uniform segment spacing produces position error and eccentricity causes an error which is a sinusoidal function of the shaft angle. 

Performance specifications are limited for factors such as, practical segmenting limitations on discs, bridging of disc segments, and wear of contacts.