Absolute value encoder
[Core Summary] Absolute encoders directly output fixed codes based on position, maintaining their position without losing it upon power loss and not returning to zero upon power-up. This article clarifies concepts such as single-turn/multi-turn, SSI/BISS, resolution, number of turns, and Gray code through a terminology comparison table, and relates them to the product implementation of the Fenac FNC series for ease of selection and document alignment.
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1. Definition of Terms
An absolute encoder is a type of position sensor that maps each mechanical angular position of a rotating shaft to a unique digital code, ensuring that no two positions have the same code. The controller reads the code to determine the position, independently of any historical counts.
In contrast, incremental encoders only output relative displacement pulses, and the position information is reset to zero upon power loss. Upon re-energization, they must return to zero. This distinction highlights the advantage of absolute value encoders in maintaining position information during power loss and eliminating the need for return-to-zero. Absolute value encoders are commonly chosen for applications such as elevators, lifting, and winding scenarios. In variable frequency drives and long-stroke conditions, the ability to maintain position information during power loss directly determines the efficiency of equipment restart, making absolute value encoders one of the mainstream choices for elevators, cranes, and new energy equipment. The Fenac FNC series falls into this category, offering both single-turn and multi-turn models.
II. Single-lap and Multi-lap
The single-turn absolute encoder covers positions within 0°–360°, and upon exceeding one turn, it resets to the starting point for counting. It is suitable for mechanisms that do not exceed one turn in a single journey.
In addition to single-turn encoding, multi-turn absolute encoders feature additional turn counting (either mechanical counting via gear sets or electronic counting), allowing for the recording of cumulative positions from tens to thousands of turns, suitable for long-range lifting and winding applications. The Fenac FNC series offers both single-turn and multi-turn models, allowing for selection based on the required travel range.
III. Interface and Encoding Format
The FNC series supports four types of interfaces: SSI, BISS, Profibus, and CANopen. SSI stands for Simplified Synchronous Serial, while BISS boasts a higher transmission rate and incorporates CRC (Cyclic Redundancy Check) verification. Profibus and CANopen utilize fieldbus technology, facilitating multi-axis networking.
The position code is commonly output in Gray code - with only one bit change between adjacent positions, it avoids instantaneous transitions and misreadings during parallel reading, thereby enhancing transmission reliability.
IV. Resolution and Turn Count Parameters
Resolution represents the number of distinguishable positions within a single revolution, measured in bits (e.g., 13 bits corresponds to 8192 positions). The number of revolutions indicates the range of rotations that can be recorded across multiple revolutions, also denoted in bits or actual revolutions.
Total addressable positions = 2^(resolution bits + number of turns bits). When selecting a model, deduce the required bits based on the lower limit of the mechanism's displacement to avoid paying for unnecessary high specifications. It should be noted that higher resolution and number of turns bits are not necessarily more suitable; they should be combined with the controller's bit width and mechanical transmission ratio to avoid incurring unnecessary costs due to high-order invalid bits. For specific bits of the FNC series, refer to the parameter table [please verify].
[Parameter Table] Glossary of Terms
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