Encoders are frequently used in industrial control applications. What is an encoder? How does it work? This article will discuss the working principle of encoders and delve into incremental encoders.
An encoder is a sensor that converts distance (linear displacement) and angle (angular displacement) into electrical signals and outputs them. Encoders are commonly used in industrial motion control to measure and provide feedback on the position and state of the measured object, such as machine tools, robots, motor feedback systems, and measurement and control equipment.
Based on their working principles, encoders can be classified into optical encoders, magnetic encoders, inductive encoders, and capacitive encoders.
Among them, the most commonly used are photoelectric encoders (capacitive encoders, etc.).
An optical encoder consists of: a shaft, a code disk, a light source, an output circuit, a housing, and a connecting flange, as shown in the figure below:
The connecting shaft is connected to the code disk and the object being measured. As the object being measured (such as a motor) rotates, the code disk also rotates. The light passing through the code disk will change between bright and dark. The photosensitive element at the receiving end will detect this change and convert it into an electrical signal for output.
Depending on the structure of the code disk, encoders can be divided into incremental encoders and absolute encoders. This article mainly discusses incremental encoders.
An incremental encoder uses a code disk divided into alternating bright and dark gratings of equal size. As the code disk rotates, the receiver detects changes in light intensity (0s and 1s) and converts these into electrical pulses, which are then output. By counting these pulses, the magnitude of the displacement can be determined, as shown in the diagram below.
To distinguish between forward and reverse rotation and to detect the zero point, the actual code disk used is more complex than the one shown in the diagram above. It typically includes three parts: phase A, phase B, and phase Z. Phase A and phase B are 1/4 cycle out of phase (90 degrees), which can be used to distinguish between forward and reverse rotation. Phase Z is a single-turn pulse, generated once per revolution of the code disk, and can be used as the encoder's reference zero point, as shown in the diagram below:
The output waveform of the incremental encoder is shown in the figure below:
Because incremental encoders use pulse counting, they must find a reference zero point before measurement, so their measurement results are relative. Furthermore, incremental encoder data is lost when power is off.
To overcome the shortcomings of incremental encoders, absolute encoders were developed. We will introduce the relevant knowledge of absolute encoders in the next article.
