An encoder is a device that encodes signals (such as bitstreams) or data, converting them into a signal form that can be used for communication, transmission, and storage. Encoders convert angular or linear displacement into electrical signals; the former is called a code disk, and the latter a code scale. Based on the readout method, encoders can be divided into contact and non-contact types; based on their working principle, they can be divided into incremental and absolute types. Incremental encoders convert displacement into periodic electrical signals, then convert these signals into counting pulses, using the number of pulses to represent the magnitude of the displacement. Absolute encoders assign a unique digital code to each position; therefore, their reading depends only on the starting and ending positions of the measurement, and is independent of the intermediate steps.
Incremental Servo Motor Encoder Introduction
In addition to the ABZ signals of ordinary encoders , incremental servo encoders also have UVW signals. Domestic and early imported servo encoders mostly adopted this form, which has more lines.
Incremental encoders output pulses as they rotate, and their position is determined by a counting device. When the encoder is stationary or there is a power outage, the position is remembered by the internal memory of the counting device. Therefore, the encoder cannot move at all after a power outage, and when power is restored, there must be no interference or loss of pulses during the encoder's pulse output process; otherwise, the zero point remembered by the counting device will shift, and the amount of this shift is unknown until an erroneous production result occurs.
The solution is to add reference points. Each time the encoder passes a reference point, it corrects the reference position into the counting device's memory. Before reaching the reference point, position accuracy cannot be guaranteed. Therefore, industrial control systems employ methods such as finding a reference point before each operation and zeroing upon startup.
For example, printers and scanners use incremental encoders for positioning. Every time you turn them on, you can hear a crackling sound as they search for the reference zero point before they start working.
Introduction to Absolute Servo Motor Encoders
Absolute rotary photoelectric encoders, due to their absolute uniqueness at each position, anti-interference capabilities, and lack of power-off memory, are increasingly widely used in angle and length measurement and positioning control in various industrial systems.
An absolute encoder has many etched lines on its code disk, arranged sequentially with 2, 4, 8, 16 lines, and so on. Thus, at each position of the encoder, by reading whether each etched line is on or off, a unique binary code (Gray code) from 2^0 to 2^(n-1) is obtained. This is called an n-bit absolute encoder. Such an encoder is determined by the mechanical position of the code disk and is unaffected by power outages or interference.
An absolute encoder ensures the uniqueness of each position determined by its mechanical position. It requires no memory, no reference points, and doesn't need to constantly count; it only reads the position when needed. This significantly improves the encoder's anti-interference capabilities and data reliability.
Because absolute encoders are significantly superior to incremental encoders in positioning, they are increasingly used in servo motors. Due to their high precision and large number of output bits, absolute encoders require parallel outputs. Each output signal must be securely connected, and isolation is necessary for complex operating conditions. The large number of cores in the connecting cable leads to numerous inconveniences and reduced reliability. Therefore, absolute encoders with multi-bit outputs generally use serial or bus-type outputs. The most common serial output for absolute encoders manufactured in Germany is SSI (Synchronous Serial Output).
From single-turn absolute encoders to multi-turn absolute encoders: Single-turn absolute encoders measure the lines on the optical code disk during rotation to obtain a unique code. When the rotation exceeds 360 degrees, the code returns to the origin, which violates the principle of unique absolute coding. Such encoders can only be used for measurements within a rotation range of 360 degrees, hence the name single-turn absolute encoder. To measure rotations exceeding 360 degrees, a multi-turn absolute encoder is required.
Encoder manufacturers utilize the mechanical principle of clock gears. When the central code disk rotates, it drives another set of code disks (or multiple sets of gears and multiple sets of code disks) through gear transmission. This adds more turns of encoding on top of the single-turn encoding, thereby expanding the encoder's measurement range. Such an absolute encoder is called a multi-turn absolute encoder. It also determines the encoding by mechanical position, and each position encoding is unique and non-repeating, so there is no need to memorize it.
Another advantage of multi-turn encoders is their large measurement range, which often provides ample margin for error in practical applications. This eliminates the need for painstaking zero-point finding during installation; a midpoint can be used as the starting point, greatly simplifying installation and debugging. Multi-turn absolute encoders offer significant advantages in length positioning, and most new servo motors in Europe utilize multi-turn absolute encoders.
Difference between absolute and incremental encoders for servo motors
Incremental and absolute encoders refer to whether the encoder is incremental or absolute. An incremental encoder can only remember how many steps it has taken, and of course, it has an origin. Before powering on and passing the origin for the first time, it doesn't know its current position. An absolute encoder, on the other hand, knows its current position as soon as it's powered on. Absolute encoders require more lines to be engraved, are more expensive, and have better performance.
