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Incremental encoder and absolute encoder
The rotary incremental encoder outputs pulses as it rotates, allowing a counting device to determine its position. When the encoder is stationary or powered off, it relies on the internal memory of the counting device to remember its position. Thus, when power is restored, the encoder must not have moved, and during the pulse output process, there must be no interference that could cause pulse loss; otherwise, the zero point remembered by the counting device will shift, and the amount of this shift is unknown until erroneous production results occur.
The solution is to add reference points. Each time the encoder passes a reference point, it corrects the reference position into the memory of the counting device. Before reaching the reference point, the accuracy of the position cannot be guaranteed. Therefore, in industrial control, methods such as finding reference points before each operation and zeroing upon startup are employed.
For example, the positioning of a printer scanner uses the principle of incremental encoders. Each time it starts up, we can hear a series of clicks as it finds the reference zero point before it begins to work.
This method can be quite troublesome for some industrial control projects, and sometimes it is not allowed to find zero upon startup (the exact position must be known immediately after powering on), which led to the emergence of absolute encoders.
Absolute rotary photoelectric encoders have a unique position that is absolutely distinct, resistant to interference, and do not require power-off memory. They are increasingly used in various industrial systems for angle, length measurement, and positioning control.
Absolute encoders have many engraved lines on their optical code discs, arranged in sequences of 2 lines, 4 lines, 8 lines, 16 lines, etc. Thus, at each position of the encoder, by reading the on and off states of each engraved line, a unique binary code (Gray code) ranging from 2 to the power of n-1 is obtained, which is referred to as an n-bit absolute encoder. This type of encoder is determined by the mechanical position of the code disc and is not affected by power outages or interference.
The uniqueness of each position determined by the mechanical position of the absolute encoder means it does not require memory, does not need to find reference points, and does not need continuous counting. Whenever the position needs to be known, it can be read at that moment. This greatly enhances the encoder's resistance to interference and the reliability of the data.
Due to the clear advantages of absolute encoders in positioning over incremental encoders, they are increasingly used in industrial control positioning. Absolute encoders offer high precision, a greater number of output bits, and diverse output signals.
From single-turn absolute encoders to multi-turn absolute encoders, the rotary single-turn absolute encoder measures the engraved lines on the optical code disc during rotation to obtain a unique code. When the rotation exceeds 360 degrees, the code returns to the origin, which does not comply with the principle of absolute coding uniqueness. Such encoders can only be used for measurements within a rotation range of 360 degrees, known as single-turn absolute encoders.
If measurements exceeding 360 degrees of rotation are required, multi-turn absolute encoders must be used.
We apply the principle of clock gear mechanics. When the central code disc rotates, it drives another set of code discs (or multiple sets of gears and code discs) through gears. Based on single-turn encoding, we can increase the number of turns to 16,384 to expand the measurement range of the encoder. Such absolute encoders are called multi-turn absolute encoders, which are also determined by mechanical positions, with each position having a unique and non-repeating code without the need for memory.
Another advantage of multi-turn encoders is that due to their large measurement range, they often have ample capacity in practical use. Therefore, during installation, it is not necessary to struggle to find the zero point; any intermediate position can be used as the starting point, greatly simplifying the difficulty of installation and debugging.
The advantages of multi-turn absolute encoders in length positioning are evident, and they are increasingly used in industrial control positioning.
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