The difference between absolute and incremental servo motor encoder

An encoder is a device that compiles and converts a signal (such as a bit stream) 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 wheel and the latter is called a code ruler. Encoders can be divided into contact type and non-contact type according to the read-out mode; according to the working principle, encoders can be divided into two types: incremental and absolute. The incremental encoder converts the displacement into a periodic electric signal, and then converts this electric signal into a counting pulse, and the number of pulses is used to indicate the magnitude of the displacement. Each position of the absolute encoder corresponds to a certain digital code, so its indication is only related to the start and end positions of the measurement, and has nothing to do with the middle process of the measurement.

Introduction of incremental servo motor encoder

In addition to the ABZ signal of ordinary encoders, incremental encoders also have UVW signals. Domestic and early imported servos mostly adopt this form, with more lines.

Incremental encoders output pulses when they rotate, and their position is known by the counting device. When the encoder does not move or the power is cut off, it relies on the internal memory of the counting device to remember the position. In this way, when the power is off, the encoder cannot move. When the power is on, the encoder cannot lose the pulse due to interference during the pulse output. Otherwise, the zero point of the counting device will shift, and this deviation There is no way to know the amount of shift, only after the wrong production result appears

The solution is to increase the reference point. Every time the encoder passes the reference point, the reference position is corrected into the memory position of the counting device. Before the reference point, the accuracy of the position cannot be guaranteed. For this reason, in industrial control, there are methods such as first finding the reference point for each operation, turning on the machine and making change.

For example, the positioning of the printer scanner is based on the principle of incremental encoder. Every time we turn it on, we can hear a crackling noise. It is looking for the reference zero point before it works.

Introduction of absolute servo motor encoder

Absolute type rotary photoelectric encoder, because of its absolute unique position, anti-interference, no power-down memory, has been more and more widely used in angle, length measurement and positioning control in various industrial systems.

There are many engraved lines on the code disk of absolute encoder, and each engraved line is arranged in 2 lines, 4 lines, 8 lines, 16 lines...in this way, at each position of the encoder, by reading each engraved line To obtain a set of unique binary codes (Gray codes) from the zero power of 2 to the n-1 power of 2 (Gray code), this is called an n-bit absolute encoder. Such an encoder is determined by the mechanical position of the encoder, and it is not affected by power failure or interference.

The absolute encoder has the uniqueness of each position determined by the mechanical position. It does not need to be memorized, does not need to find a reference point, and does not need to be counted all the time. When it needs to know the position, when to read its position. In this way, the anti-interference characteristics of the encoder and the reliability of the data are greatly improved.

Since absolute encoders are significantly better than incremental encoders in positioning, they have been increasingly used in servo motors. Because of 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 working conditions, it must be isolated and the number of connecting cables is large. It brings a lot of inconvenience and reduces reliability. Therefore, the absolute encoder in the multi-digit output type generally uses serial output or bus type output. The most commonly used serial output of the absolute encoder produced in Germany is SSI (synchronous serial Line output).

From single-turn absolute encoder to multi-turn absolute encoder, the single-turn absolute encoder is rotated to measure each engraved line of the optical code disc during rotation to obtain a unique code. When the rotation exceeds 360 degrees, the code returns To the origin, this does not comply with the principle of absolute coding uniqueness. Such an encoder can only be used for measurement within a rotation range of 360 degrees, and is called a single-turn absolute encoder. If you want to measure rotation beyond 360 degrees, you must use a multi-turn absolute encoder.

The encoder manufacturer uses the principle of clock gear mechanism. When the center code wheel rotates, another set of code discs (or multiple sets of gears, multiple sets of code discs) are driven by gears, and the number of turns is added to the single-turn encoding. Encoding, in order to expand the measuring range of the encoder, such an absolute encoder is called a multi-turn absolute encoder, it is also determined by the mechanical position of the code, each position code is unique and does not repeat, without the need to remember.

Another advantage of the multi-turn encoder is that due to the large measurement range, the actual use is often richer, so it is not necessary to find a change point during installation, and a certain intermediate position can be used as the starting point, which greatly simplifies the difficulty of installation and debugging. Multi-turn absolute encoders have obvious advantages in length positioning. The new servo motors in Europe basically use multi-turn absolute encoders.

The difference between absolute and incremental servo motor encoder

Incremental and absolute refer to whether the encoder is incremental or absolute. Incremental can only remember how many steps it has taken, of course, there will be an origin. It doesn't know where its position is before it is turned on and it passes the origin for the first time. The absolute encoder can know where it is now as long as it is powered on. Absolute encoders need to engrave more lines, cost more, and perform better, so they are expensive.