A DC servo motor encoder is a rotary sensor that converts rotary displacement into a series of digital pulse signals. These pulses can be used to control angular displacement. If the encoder is combined with a rack and pinion or a lead screw, it can also be used to measure linear displacement.
A DC servo motor encoder is a device that encodes and converts signals (such as bit streams) or data into a signal form that can be used for communication, transmission, and storage. The encoder converts angular displacement or linear displacement into electrical signals; the former is called a code disk, and the latter a code scale. Encoders can be classified into contact and non-contact types according to their readout method; and into incremental and absolute types according to their working principle.
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.
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 not on the intermediate steps. Incremental servo motor encoders, in addition to the ABZ signals of ordinary encoders, also use UVW signals. This type of encoding is commonly used in domestically produced and early imported servo motors, and involves more lines.
An introduction to absolute DC servo motor controller 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 various industrial systems for angle and length measurement and positioning control. An absolute encoder code disk has many etched lines, each arranged sequentially with 2, 4, 8, 16 lines… Thus, at each position of the encoder, by reading the on/off state of each etched line, a unique binary code (Gray code) from 2^0 to 2^(n-1) is obtained, which 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.
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, positional 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. Each time they are powered on, we hear a series of clicking sounds as they search for the reference zero point before commencing operation.
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; the position is read only 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 being used in DC servo motors.
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.
Absolute encoders, due to their high precision and large number of output bits, require a strong connection for each output signal if parallel output is still used. For more complex operating conditions, isolation is also necessary, and the number of cores in the connecting cable is large, which brings many inconveniences and reduces reliability. Therefore, absolute encoders with multi-bit output generally use serial output or bus output. The serial output of absolute encoders produced in Germany commonly uses SSI (Synchronous Serial Output).
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.
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.
Before powering on and passing the origin for the first time, an absolute encoder doesn't know its position. An absolute encoder, however, knows its current position as soon as it's powered on. Absolute encoders require more lines to be engraved, making them more expensive due to their higher cost and better performance. The difference between absolute and incremental encoders for DC servo motors refers to whether the encoder is incremental or absolute. Incremental encoders can only remember how many steps they've taken, and of course, they still have an origin.
