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Absolute Vs Incremental Encoders

Rotary encoders fall in two major categories: Incremental and Absolute. Both encoders come in two basic construction designs, Mechanical and Optical.

Absolute Encoders are slightly more complex in design and use. They provide a position value as the shaft of the encoder is rotated. This value is transmitted to a controller via one of the many available output formats. These interfaces give the position value to the controller using codes to represent actual position. The Absolute Encoder "KNOWS" its actual postion.

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Optical Vs Mechanical Encoders

Mechanical Encoders utilize a rotating metal disc containing concentric rings which have patterned openings. Typically a stgationary row of sliding contacts is aligned with each ring. As the contacts wipe accross the surface of the metal disc, contact is either made (closed) or not made (open). By design, the pattern gives a code of 1's(ON) and 0's (OFF) in a specific format; Binary, Gray or BCD.

Optical Encoders utilize a rotating glass or plastic disc containing concentric rings which have patterns etched onto them. An LED is used to shoot light through a grid; the grid projects a pattern onto the rotating disc. A photo-diode is used on the other side of the rotating disc to read the pattern of light being passed through. The pattern on the grid, combined with the pattern on the rotating disc gives a code of 1's (ON) and 0's (OFF) in a specific format; Binary, Gray, or BCD.

Absolute Encoders Output Formats

Communication between an encoder and control system can happen using one of many different formats. These formats can be divided into 3 Major Categories; Point to Point, Fieldbus and Ethernet.

Point to Point interfaces include Parallel, Serial Synchronous (SSI), Incremental Serial Interface (ISI), Analog, Asynchronous Serial Interface and others. The majority of these interfaces require a point to point connection from encoder to controller. Most of these interfaces use the above Encoding methods to transmit their data to the controller. Analog is unique in that it provides a varying signal to the controller, typically 4-20mA.

Fieldbus interfaces include ProfiBus, DeviceNet, CANOpen and others. These protocols use a BUS type topology which requires individual addressing of each unit or node on the BUS. The encoder data is read by the controller on the same line as other devices on the BUS. This output format requires a higher level of intelligence in the controller, but adds greater flexibility with less wiring.

EtherNet interfaces include ProfiNet, EtherCAT, PowerLink, and EtherNet/IP. These protocols use a Network topology which requires individual addressing of each device on the network. The encoder data is available on the network to be accessed by the controller. Ethernet interfaces utilize the newest communication technology. They are based off the standard office network protocol, refined to meet industrial requirements.