Linear, bit, resolution, incremental, absolute
Lines: The markings on the encoder's photoelectric code disk. Incremental code disks can have 10, 100, or 2500 lines; the number can be selected arbitrarily, as long as the code disk can accommodate them. Absolute code disks, due to the Gray code arrangement, are generally composed of powers of 2 lines, such as 256, 1024, or 8192 lines. However, absolute code disks also have special Gray remainder code outputs, such as 360, 720, or 3600 lines.
Bit: 2 to the power of n. Since absolute value encoders often output lines that are powers of 2, most absolute value encoders use "bits" to represent this value. However, there are exceptions, such as 360-line, 720-line, and 3600-line (Gray Remainder codes). Incremental encoders also use bits, such as 15-bit and 17-bit encoders. These are achieved by internally subdividing the calculated line count, which is generally greater than 10,000 lines, and thus expressed in "bits".
Resolution: The angle that the encoder can distinguish. For general calculations, it is calculated in 360 degrees per line, and most currently express it directly in terms of the number of lines. However, this can lead to some conceptual confusion. For example, in incremental encoders, if a quadruple frequency multiplication of A/B phases is used, a 2500-line encoder can actually have a resolution of 360/10000. If the internal subdivision calculation uses even more "lines," reaching 15 or 17 bits, then the resolution can be achieved. Therefore, incremental encoders often use "lines" to express resolution, indicating that there has been no frequency multiplication and subdivision yet, while they use "bits," indicating that the subdivision has already been achieved.
Incremental encoders: The encoder disk has two lines: A/B and Z. The rotation angle is calculated by accumulating (incrementing) the number of lines. Some also add U, V, and W, dividing the encoder into three zones of 120 degrees to determine the position; these are called hybrid encoders. Some use internal subdivision circuits to improve the resolution of "lines" and use internal batteries for memory and "bits" to express the value; these are often confused with "absolute values," but should actually be called "pseudo-absolute values."
Absolute type: The code disk has n lines, arranged in combinations of 2, 4, 8, 16... The reading is done directly from the optical disc using the "0" and "1" encoding method, rather than being accumulated, so it is unaffected by power outages or interference.
Regarding whether incremental or absolute subdivision offers higher resolution and accuracy, for actual code disk engraving, the absolute code disk resolution "number" can be twice that of the incremental code disk. For frequency multiplication technology, the incremental code disk resolution "number" can be greater than the absolute value. However, note that I'm using "resolution number," not accuracy, because subdivision and frequency multiplication are electrical simulation techniques and do not improve accuracy. Accuracy is determined by a combination of factors, including code disk engraving, shaft mechanical installation, and electrical response. In summary, incremental subdivision can achieve higher resolution than absolute subdivision, while absolute subdivision offers higher accuracy because it is unaffected by power outages, interference, speed, and electrical response. Especially in situations requiring both high accuracy and high speed, incremental subdivision cannot meet the requirements.
In the past, absolute value encoders were prohibitively expensive, leading to the widespread use of incremental encoders. However, due to technological advancements in absolute value encoders, their prices have decreased. Servo systems in Europe and America are gradually converging towards absolute value. It is understood that in the past two years, Europe and America have invested almost nothing in R&D for incremental encoders compared to their investment in absolute value encoders. It is hoped that domestic servo manufacturers will pay attention to the application of absolute value encoders as soon as possible.
European market servo applications with absolute value multi-turn
Resolution per lap: 13-bit 8192 lines; 16-bit 65536 lines; 17-bit 131072 lines; 25-bit 33554432 lines!
Continuous measurement cycle count: Most 12-digit 4096 cycles, a few 14-digit 16384 cycles.
Total number of digits: 25-37.
Output signal:
SSI+sin/cos, 1MHz, Gray code
Biss, 2MHz, pure binary code
Hipeface+sin/cos, 2MHz, pure binary code (including checksum).
Endat, 8MHz, pure binary code, CRC (up to 25 bits per revolution, truly high precision and high speed).
In the past, SSI was more common, but now Hipeface and EnDat are the trends, especially EnDat 2.2, which has obvious potential for technological development.
