1. Overview
Photoelectric encoders are mainly used in CNC machine tools, gantry machining centers, control switches and other fields. They have a novel design, are easy to install, have a strong load capacity, and adopt an oil-proof sealing design, making them energy-saving and environmentally friendly. Those familiar with this mechanical equipment should know that it is also known as a manual pulse generator, or simply a handwheel. It is a sensor that converts the mechanical geometric displacement of the output shaft into pulse or digital quantities through photoelectric conversion, and it is currently the most widely used type of sensor.
2. Classification
It mainly consists of four parts: a light source, a code disk, an optical system, and a circuit. Below, we will explain the different types of photoelectric encoders.
Figure 1
I. Incremental Encoder
It converts displacement into periodic electrical signals, then into counting pulses. A rotary incremental encoder outputs these pulses during rotation, and a counting device determines its position. The position is determined by the number of pulses counted from the zero mark. When power is off, the encoder cannot move. Upon power-up, there must be no interference causing pulse loss during output; otherwise, the zero point stored in the counting device will shift. Errors are difficult to detect, and without an accurate reference point, position accuracy cannot be guaranteed. Therefore, a reference point must be found before each operation. However, this type of encoder is unaffected by power outages or interference.
Figure 2
Incremental encoders utilize photoelectric conversion to output three sets of square wave pulses: A, B, and Z. The A and B pulses are 90 degrees out of phase, allowing the motor's rotation direction to be determined. The Z phase outputs one pulse per revolution, used for reference point positioning. This encoder has a simple structure, good mechanical properties, a lifespan of tens of thousands of hours, strong anti-interference capabilities, and high reliability. However, it cannot output the absolute position information of the shaft rotation.
II. Absolute Encoder
An absolute encoder assigns a unique digital code to each position, so its reading depends only on the start and end positions of the measurement, and not on the intermediate steps. The position is determined by the reading of the output code. When power is off, the absolute encoder does not disconnect from the actual position. Upon power-up, the position reading remains the same.
Figure 3
Absolute encoders can directly output large amounts of digital data. The code disk has several code tracks, the number of which corresponds to the number of binary bits. Each code track consists of transparent and opaque sector areas, and signals are collected using photoelectric sensors. Light sources and photosensitive elements are located on both sides of the code disk. The photosensitive elements can switch levels based on whether they receive a light signal, outputting a binary number. Different digital codes are output at different positions, thus allowing for absolute position detection. However, the resolution is determined by the number of binary bits; that is, the accuracy depends on the number of bits. Advantages: Absolute angular coordinates can be read directly, there is no accumulated error, and position information is not lost after power is cut off. The encoder's anti-interference characteristics and data reliability are greatly improved.
III. Hybrid Absolute Encoder
The hybrid absolute encoder outputs two sets of information: one set is used to detect the magnetic pole position and has absolute information function; the other set is exactly the same as the output information of the incremental encoder.
IV. Rotary Transformer
A resolver, or simply resolver, is constructed from high-performance silicon steel laminations and enameled wire with a special electromagnetic design. Compared to encoders that use photoelectric technology, it is more adaptable to harsh working environments, including those requiring heat resistance, vibration resistance, shock resistance, oil resistance, and even corrosion resistance. A single-pole (single-speed) resolver can be considered a single-turn absolute feedback system and is the most widely used.
Figure 4
V. Sine and Cosine Servo Motor Encoders
High-precision microstepping can be achieved in servo drives without the need for high-frequency communication, which reduces hardware requirements. At the same time, the presence of a single-turn angle signal allows for smooth start-up of the servo motor and provides a large starting torque.
3. Working principle of photoelectric encoder
A photoelectric encoder mainly consists of a grating disk and a photoelectric detection device. In a servo system, the grating disk is coaxial with the motor, causing the motor to rotate and drive the grating disk to rotate. The photoelectric detection device then outputs several pulse signals, and the current motor speed can be calculated based on the number of pulses per second of the signal.
The code disk of the photoelectric encoder outputs two optical codes with a phase difference of 90 degrees. The rotation direction of the motor can be determined by the change in the state of the dual-channel output optical codes.
4. Precautions for using photoelectric encoders
Whether a photoelectric encoder can produce the desired conversion effect depends not only on its quality but also on whether it is used correctly. The following are precautions for using photoelectric encoders:
1. How to control resolution
These signal conversion devices generally have varying resolutions during use. Simply put, this refers to the number of pulses generated per revolution. Different encoder models have different pulse counts, typically ranging from 6 to over 5000 revolutions. Those familiar with encoders know that a higher pulse count results in higher resolution. However, poor resolution control during encoder use can affect signal conversion. Therefore, to reduce errors caused by poor resolution control, users can deepen their understanding of resolution by referring to the product manual.
2: PLC Data Acquisition
Accurate data measurement does not guarantee that the final recorded data will be accurate. To ensure the accuracy of the recorded data, PLC data acquisition is required. Generally, a high-speed counting module is used. With the help of the high-speed counting board, data is simultaneously acquired by a microcontroller. This data acquisition method is also known as computer acquisition.
Three: Line Connection
The wiring connection is mainly based on the power supply connection. It is important to know that there are three common types of power supplies for photoelectric encoders. Each type of encoder requires a different PLC operating mode. For example, an encoder with an 11V power supply requires a 24V PLC for use.
5. Summary
Human beings have never stopped trying and innovating in the industrial field. It is the emergence of machines such as photoelectric encoders that has greatly facilitated human production and life. Because photoelectric encoders are small in size, highly practical, have a long service life, and are stable in use, they are widely used in the manufacture of machinery and play an important role in the industrial production field. Photoelectric encoders are an invention of human beings in industry and technology, which greatly facilitates the smooth operation of machines.
