I. Introduction to Photoelectric Encoders
An optical encoder is a digital detection device integrating optics, mechanics, and electronics. It is a sensor that converts mechanical and geometric displacement input to a shaft into pulses or digital signals through photoelectric conversion. It is mainly used for speed or position (angle) detection. It has significant advantages such as high accuracy, fast response, strong anti-interference ability, and stable and reliable performance. According to its structure, it can be divided into two types: linear encoders and rotary encoders.
A rotary encoder mainly consists of a grating, a light source, a reader, a signal conversion circuit, and a mechanical transmission. The grating surface has radially spaced light-transmitting slits with equal pitch, and the interval between two adjacent slits represents one increment cycle; two grating surfaces are used for photosensitive operation. Because the two grating surfaces have a 90° phase difference, this output can be input into a digital adder/subtractor to represent angles using graduation values. Based on the type of output signal, photoelectric encoders can be divided into two main categories: incremental and absolute.
When a rotary incremental encoder rotates, it outputs pulses, which are used by a counting device to determine its position. When the encoder is stationary or when power is off, the position is remembered by the internal memory of the counting device. Therefore, the encoder must not 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.
An absolute encoder has many etched lines on its code disk, arranged sequentially with 2, 4, 8, 16 lines, and so on. 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. This is called a bit absolute encoder. The position of this encoder is determined by the mechanical position of the code disk, and it is unaffected by power outages or interference. The uniqueness of each position determined by the mechanical position of an absolute encoder means it requires no memory, no reference point, and no continuous counting; the position is read only when needed. This greatly improves the encoder's anti-interference characteristics and data reliability. Because absolute encoders are significantly superior to incremental encoders in positioning, they are increasingly used in industrial control positioning.
Encoder signal outputs include parallel output, serial output, bus-type output, and integrated transmitter output. Serial output involves data being output sequentially according to a pre-defined timeframe; spatially, all bits of data are transmitted sequentially over a single cable. This pre-defined timeframe is called a "communication protocol," and physical connection methods include RS232, RS422 (TTL), and RS485. Serial output requires fewer cables, allows for longer transmission distances, and significantly improves reliability, but its transmission speed is slower than parallel output. For absolute encoders, parallel output means data is transmitted simultaneously in time; spatially, each bit of data occupies its own cable. For low-bit absolute encoders, this method is commonly used to directly output digital data, which can be directly connected to the I/O interface of a PLC or host computer. This method offers instantaneous output and simple connection. However, for high-bit absolute encoders, the use of multi-core cables introduces engineering complexity, inconvenience, and reduces reliability. Therefore, for high-bit absolute encoders, parallel output is generally not used; instead, serial or bus-type output is preferred.
II. Classification of Photoelectric Encoders
Classification by measurement method:
• Rotary encoder
• Ruler encoder
Classification by encoding method:
• Absolute encoder
• Incremental encoder
• Hybrid encoder
III. Applications of Photoelectric Encoders
In the past decade or so, photoelectric encoders have developed into a mature series of high-performance industrial products with multiple specifications, and have been widely used in many fields such as CNC machine tools, robots, radar, photoelectric theodolites, ground command instruments, high-precision closed-loop speed control systems, and servo systems.
