Absolute encoder
An absolute encoder is a sensor that directly outputs digital data. Its circular code disk has several concentric code discs radially arranged. Each track consists of alternating transparent and opaque sector areas, with adjacent tracks having a double ratio of sector sizes. The number of code tracks on the disk corresponds to the number of bits in its binary code. A light source is located on one side of the code disk, and a photosensitive element is located on the other side corresponding to each code track. When the code disk is in different positions, each photosensitive element converts the light source into a corresponding voltage level signal, forming a binary number. The key feature of this type of encoder is that it does not require a counter; a fixed digital code corresponding to the position can be read at any position of the shaft. Obviously, there must be N code tracks. Currently, 16- bit absolute encoder products are available in China .
Incremental encoder
Incremental encoders convert displacement into periodic electrical signals, then convert these electrical signals into counting pulses, using the number of pulses to represent the magnitude of displacement. Based on their working principle, encoders can be divided into two categories: incremental and absolute.
When the incremental encoder shaft rotates, it outputs corresponding pulses. The direction of rotation and the increase or decrease of the pulse count are determined by the rear direction-determining circuit and counter. The counting start point can be arbitrarily set, allowing for infinite accumulation and measurement over multiple revolutions. The Z signal, which emits one pulse per revolution, can also be used as a reference mechanical zero position. The encoder shaft outputs a fixed number of pulses per revolution, the number of which is determined by the number of lines in the encoder grating. To improve resolution, the original pulse count can be multiplied using the A and B signals with a 90- degree phase difference , or a higher-resolution encoder can be used.
The difference between absolute encoders and incremental encoders
I. Different in nature
1. Incremental encoder: The displacement is converted into a periodic electrical signal, and then the electrical signal is converted into counting pulses. The number of pulses represents the magnitude of the displacement.
2. Absolute encoders: Because each position is absolutely unique, anti-interference, and does not require power-off memory, they are increasingly widely used in angle and length measurement and positioning control in various industrial systems.
II. Different Principles
1. Incremental encoder: A code disk has slits (divided into transparent and opaque sections) at equal angles along its edge. Light sources and photosensitive elements are mounted on both sides of the encoder. As the code disk rotates with the working shaft, the light and shadow change with each rotation of one slot.
After shaping and amplification, an electrical pulse output signal of a certain amplitude and power can be obtained, with the number of pulses equal to the number of rotating slots. The pulse signal is sent to a counter for counting, and the angle of rotation of the disk can be determined from the measured number.
2. Absolute encoder: Due to its high precision, the absolute encoder has a large number of output bits. If parallel output is still used, each output signal must be well connected. For more complex isolation conditions, the cable cores are more numerous, which brings many inconveniences and reduces reliability.
Therefore, absolute encoders have multiple digital output types. Generally, serial output or bus output is chosen. The most commonly used serial output for German absolute encoders is SSI (Synchronous Serial Output).
