introduction
An optical axial angle encoder (hereinafter referred to as an optical encoder) uses a high-precision measuring circular grating as the detection element. It converts the input angular position information into corresponding digital codes through photoelectric conversion and corresponding hardware processing circuits. It can be connected to computer and other control systems and display devices to realize digital measurement, control and display.
Photoelectric shaft encoders offer a high performance-price ratio and boast advantages such as compact structure, high reliability, and high precision. They are widely used in many fields, including CNC machine tools, robots, radar, photoelectric theodolites, ground control instruments, and high-precision closed-loop speed control systems. They are ideal angle and speed sensors for automated equipment and have become one of the most important means of detecting rotational angles and linear positions.
This article mainly introduces the selection principles of photoelectric encoders.
Introduction to Photoelectric Encoder Types
Photoelectric encoders are generally classified into three types according to the optical code disk pattern used: incremental, absolute, and hybrid. Incremental encoders are mainly used to measure the speed of rotating shafts, absolute encoders are mainly used to measure the spatial position of rotating shafts, and hybrid encoders are a combination of incremental and absolute encoders, possessing some characteristics of both. With the addition of a processing chip at the back end of the photoelectric encoder, all three types can measure both the speed and spatial position of rotating shafts. Specific grating patterns are shown in Figures 1-3; their principles have been described in many articles and will not be repeated here.
Selection principles of photoelectric encoders
Currently, using photoelectric encoders to measure angles and lengths is a mature technology. When selecting a photoelectric encoder, the following aspects should be considered:
Types of photoelectric encoders;
shape;
Resolution, accuracy;
Signal output format.
Photoelectric encoder type selection
Photoelectric encoders are commonly used for angle measurement, and can also be used for length measurement after being converted by a mechanical device. Therefore, photoelectric encoders are widely used in the fields of angle and length measurement.
Whether to measure length or angle; how to convert length measurement mechanically; whether the measurement range is within 360° (single turn) or exceeds 360° (multiple turns); whether the rotation direction is a cyclic rotation in one direction or a cyclic rotation in both directions; whether to measure speed or angular position—these are all things that need to be determined in advance when selecting a photoelectric encoder.
Incremental encoders are commonly used for angle speed measurement due to their advantages such as small size, low price, and the ability to accumulate measurements indefinitely. Therefore, incremental encoders remain the dominant and irreplaceable technology in speed measurement applications. Incremental encoders output pulses during rotation, which are used by a counting device to calculate speed and position. When the encoder is powered off, it relies on the internal memory of the counting device to preserve the position; therefore, the encoder must not rotate after power failure. When power is restored, there must be no pulse interference during the encoder's pulse output process; otherwise, the zero point stored in the counting device will shift, and this shift is uncertain, only becoming apparent after erroneous production results occur. The solution is to add a reference point. Each time the encoder passes a reference point, the counting value of the counting device is corrected using a reference marker. Before passing the reference point, the position accuracy cannot be guaranteed. Therefore, in industrial control, there is a procedure of finding a reference point before each operation. This operation is cumbersome for some industrial control projects, and some projects do not allow finding a reference point upon startup and have strict requirements for position and zero position. In such cases, absolute encoders should be used.
Absolute encoders output unique position codes (single or multiple turns). After processing by the subsequent control system, they can measure both position and speed. An absolute encoder's grating code disk has many lines, arranged sequentially with 2, 4, 8, 16 lines, and so on. Thus, at each position of the encoder, by reading the brightness of each line, a unique binary code (Gray code) from 20 to 2n-1 is obtained, which is called an n-bit absolute encoder. The output angle information of an absolute encoder is determined by the mechanical position of the grating code disk, therefore each position is unique. It requires no memory, no reference point, and is unaffected by power outages or interference. Therefore, the anti-interference capability and data reliability of photoelectric encoders are greatly improved. Because absolute encoders are significantly superior to incremental encoders in position positioning, they are increasingly used in industrial control positioning.
Absolute encoders are further divided into single-turn absolute encoders and multi-turn absolute encoders.
When the encoder spindle rotates more than 360°, the angle encoding returns to the origin, which does not conform to the unique principle of absolute encoding. Such an encoder can only be used for measurements within a rotation range of 360° and is called a single-turn absolute encoder.
To measure rotations exceeding 360°, a multi-turn absolute encoder is required. Utilizing the gear transmission principle of a clock, as the main grating code disk rotates, it drives another set of code disks (or multiple sets of gears and code disks) via gears. This adds more turns to the single-turn encoding, expanding the encoder's measurement range. Such an absolute encoder is called a multi-turn absolute encoder. Like other encoders, its encoding is determined by mechanical position; each position has a unique and non-repeating code, requiring no memorization. Multi-turn absolute encoders offer significant advantages in length positioning and are increasingly used in industrial control positioning.
Mechanical shape selection of photoelectric encoder
The selection of the mechanical shape of a photoelectric encoder mainly includes the following aspects:
Shaft connection types: hollow shaft (sleeve type connection), solid shaft (connected via flexible coupling);
Shaft diameter;
Position the stop and install the holes;
Cable outgoing method;
Installation space volume;
Working environment protection level. In harsh field conditions, rotary encoders with metal gratings must be used. Typically, the number of pulses per revolution of a metal grating is no more than 1000.
Photoelectric encoder resolution and accuracy selection
Photoelectric encoder type selection
Photoelectric encoders are commonly used for angle measurement, and can also be used for length measurement after being converted by a mechanical device. Therefore, photoelectric encoders are widely used in the fields of angle and length measurement.
Whether to measure length or angle; how to convert length measurement mechanically; whether the measurement range is within 360° (single turn) or exceeds 360° (multiple turns); whether the rotation direction is a cyclic rotation in one direction or a cyclic rotation in both directions; whether to measure speed or angular position—these are all things that need to be determined in advance when selecting a photoelectric encoder.
Incremental encoders are commonly used for angle speed measurement due to their advantages such as small size, low price, and the ability to accumulate measurements indefinitely. Therefore, incremental encoders remain the dominant and irreplaceable technology in speed measurement applications. Incremental encoders output pulses during rotation, which are used by a counting device to calculate speed and position. When the encoder is powered off, it relies on the internal memory of the counting device to preserve the position; therefore, the encoder must not rotate after power failure. When power is restored, there must be no pulse interference during the encoder's pulse output process; otherwise, the zero point stored in the counting device will shift, and this shift is uncertain, only becoming apparent after erroneous production results occur. The solution is to add a reference point. Each time the encoder passes a reference point, the counting value of the counting device is corrected using a reference marker. Before passing the reference point, the position accuracy cannot be guaranteed. Therefore, in industrial control, there is a procedure of finding a reference point before each operation. This operation is cumbersome for some industrial control projects, and some projects do not allow finding a reference point upon startup and have strict requirements for position and zero position. In such cases, absolute encoders should be used.
Absolute encoders output unique position codes (single or multiple turns). After processing by the subsequent control system, they can measure both position and speed. An absolute encoder's grating code disk has many lines, arranged sequentially with 2, 4, 8, 16 lines, and so on. Thus, at each position of the encoder, by reading the brightness of each line, a unique binary code (Gray code) from 20 to 2n-1 is obtained, which is called an n-bit absolute encoder. The output angle information of an absolute encoder is determined by the mechanical position of the grating code disk, therefore each position is unique. It requires no memory, no reference point, and is unaffected by power outages or interference. Therefore, the anti-interference capability and data reliability of photoelectric encoders are greatly improved. Because absolute encoders are significantly superior to incremental encoders in position positioning, they are increasingly used in industrial control positioning.
Absolute encoders are further divided into single-turn absolute encoders and multi-turn absolute encoders.
When the encoder spindle rotates more than 360°, the angle encoding returns to the origin, which does not conform to the unique principle of absolute encoding. Such an encoder can only be used for measurements within a rotation range of 360° and is called a single-turn absolute encoder.
To measure rotations exceeding 360°, a multi-turn absolute encoder is required. Utilizing the gear transmission principle of a clock, as the main grating code disk rotates, it drives another set of code disks (or multiple sets of gears and code disks) via gears. This adds more turns to the single-turn encoding, expanding the encoder's measurement range. Such an absolute encoder is called a multi-turn absolute encoder. Like other encoders, its encoding is determined by mechanical position; each position has a unique and non-repeating code, requiring no memorization. Multi-turn absolute encoders offer significant advantages in length positioning and are increasingly used in industrial control positioning.
Mechanical shape selection of photoelectric encoder
The selection of the mechanical shape of a photoelectric encoder mainly includes the following aspects:
Shaft connection types: hollow shaft (sleeve type connection), solid shaft (connected via flexible coupling);
Shaft diameter;
Position the stop and install the holes;
Cable outgoing method;
Installation space volume;
Working environment protection level. In harsh field conditions, rotary encoders with metal gratings must be used. Typically, the number of pulses per revolution of a metal grating is no more than 1000.
Photoelectric encoder resolution and accuracy selection
