Selection between incremental and absolute encoders！

Encoders provide feedback for motion in a variety of applications, such as placing the patient in a suitable position on a magnetic resonance imager or filling the beverage at a rate of 300 bottles per minute. When choosing an encoder, the user must consider some of the most important application features: Is tracking linear or rotary motion? Is optical encoders or magnetic encoders used? In addition, for the successful implementation of professional applications, users should also consider the choice of incremental or absolute encoder encoder - even with the same sensing mechanism, the performance of these two encoders are also very different. To achieve a successful application you need to understand all the relevant characteristics of the two encoders and make the right choice.

As the name suggests, absolute encoders can record the position of the encoder on an absolute coordinate system, while incremental encoders can output incremental changes in the encoder from a predefined starting position. Incremental encoders require the use of additional electronic devices (usually PLCs, counters, or frequency converters) to perform pulse counting and to convert pulse data into velocity or motion data, while absolute encoders produce numbers that can identify absolute positions signal. In summary, incremental encoders are usually more suitable for simple applications with lower performance, whereas absolute encoders are the best choice for more complex critical applications with higher speed and position control requirements . The output type depends on the application.
Incremental encoder

When the incremental encoder moves, the encoder generates a binary pulse stream that is proportional to the axis rotation speed (rotary encoder) or motion distance (linear encoder). When an optical encoder is used, a specific pattern of code or linear code strips placed between the LED light source and the optical sensor alternately conducts or blocks the light beam, thereby generating an analog signal; then an additional circuit (typically an on-board ASIC) Converts an analog signal to a square wave. Magnetic encoders can operate with a variety of mechanisms, but each rotates a magnetic field, thereby generating a voltage pulse or a change in resistance that can be converted to a pulse.

Single-channel incremental encoders can only output a single pulse stream, so only limited information is available. Depending on the resolution of the encoder, that is, the number of pulses per revolution of the rotary encoder or the movement distance (mm / in) of the linear encoder motion, the external electronic device can perform pulse counting and calculate velocity or relative to a reference coordinate Position), thereby determining the position of the encoder. The single-channel design provides an excellent solution for applications such as unidirectional conveyor systems.

The single-channel encoder is simple to operate, robust and economical, but it has an important drawback that it can not determine the direction of motion. Determining the direction of motion requires more output from the encoder (typically a dual-channel design with a phase difference of 90 ° for channel "A" and channel "B" independent outputs). Since the signal output of the dual-channel encoder has two rising edges and two falling edges, it is sometimes called quadrature encoders. The direction of motion determines which channel first goes high, which allows the processor to easily monitor the direction of motion (see Figure 1). The operator can increase the encoder resolution by triggering the leading and trailing edges of one or both channels - up to a maximum of four times.
Quadrature encoders provide a rugged solution for challenging, complex applications. In high-vibration applications, a single-channel encoder may misinterpret a pulse stream generated by an axis jitter with respect to the setpoint as a true displacement, while the quadrature encoder will recognize the change in direction and ignore the pulse flow or Which is filtered out as noise.

An incremental encoder can also contain an extra channel, the index channel (or Z channel). This channel allows a rotary encoder or a linear encoder at a specific location to generate one pulse per revolution (see Figure 2). The Z-channel can determine a specific location at startup. In high-speed applications, the use of Z-channel can easily indicate a single lap, and thus calculate the speed.
Incremental encoders are generally only suitable for simple applications, whether the control device is a counter, PLC or inverter, directly connected to the encoder and controller.

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