How do elevators, which we use every day, precisely transport people to their designated floors? How do machine tools cut materials with precision? How do servo motors ensure accurate rotational positioning? All of this is thanks to a magical device—the encoder. But what exactly is an encoder? And how does it accurately measure the position of a motor? Let's talk about encoders today.
I. What is an encoder?
An encoder is a device that encodes and converts signals or data into a signal form that can be used for communication, transmission, and storage. An encoder converts angular displacement or linear displacement into electrical signals; the former is called a code disk, and the latter a code scale. It is a commonly used motor positioning device in industry, capable of accurately measuring the angular displacement and rotational position of a motor.
Figure 1 Encoder
II. Encoder Classification
Encoders can be classified into two categories based on their working principle: incremental and absolute. Incremental encoders convert displacement into periodic electrical signals, then convert these signals into counting pulses, using the number of pulses to represent the magnitude of the displacement. Absolute encoders, on the other hand, assign a unique digital code to each position; therefore, their reading depends only on the starting and ending positions of the measurement, and is independent of the intermediate steps.
Incremental
Incremental encoders typically have three output ports: A-phase, B-phase, and Z-phase outputs. The A-phase and B-phase outputs are pulses that are delayed by 1/4 cycle. The forward and reverse directions can be distinguished based on the delay relationship. Furthermore, the frequency can be multiplied by 2 or 4 by taking the rising and falling edges of the A-phase and B-phase outputs. The Z-phase outputs a single-turn pulse, meaning that one pulse is emitted per revolution.
The grating used in incremental measurement consists of periodic grating bars. Position information is obtained by calculating the number of increments (measurement steps) from a given point. Since an absolute reference point must be used to determine the position value, the circular grating code disk also has a reference point track.
Absolute
An absolute encoder outputs a unique binary value corresponding to each reference angle for one revolution. Multiple positions can be recorded and measured using an external clocking device.
The encoder immediately obtains its position value upon power-up, which is readily available for subsequent signal processing circuitry. There is no need to move the axis to perform a reference point homing operation. Absolute position information comes from a circular grating code disk, consisting of a series of absolute codes. Individual incremental tracking signals generate position values through subdivision, and can also generate selectable incremental signals.
A single-turn encoder repeats its absolute position value information once per revolution. A multi-turn encoder can also distinguish the position value for each revolution.
Figure 2. Circular grating of an absolute rotary encoder.
The biggest difference lies in the number of pulses counted from the zero mark in the case of an incremental encoder, while the position of an absolute encoder is determined by the reading of the output code. In one revolution, the output code reading for each position is unique, so when the power is off, an absolute encoder does not become disconnected from the actual position. If the power is restored, the position reading remains current and valid, unlike an incremental encoder which requires searching for the zero mark.
III. Encoder Working Principle
A photoelectric encoder disk with a central axis has circular light and dark markings. Photoelectric transmitters and receivers read these markings and obtain four sets of sine wave signals, which are combined to form A, B, C, and D. Each sine wave is 90 degrees out of phase (360 degrees in one cycle). The C and D signals are inverted and superimposed on the A and B phases to enhance signal stability. Additionally, a Z-phase pulse is output every revolution to represent the zero-position reference.
Since phases A and B are 90 degrees out of phase, the forward and reverse rotation of the encoder can be determined by comparing whether phase A or phase B comes first. The zero-position reference position of the encoder can be obtained through the zero-position pulse. Encoder code disks are made of glass, metal, or plastic. Glass code disks have very thin lines deposited on glass, resulting in good thermal stability and high precision. Metal code disks have lines directly engraved with both through and non-through surfaces, making them less prone to breakage. However, due to the thickness of the metal, the precision is limited, and its thermal stability is an order of magnitude worse than that of glass. Plastic code disks are economical, with low cost, but their precision, thermal stability, and lifespan are all inferior.
Resolution—The number of through or dark lines provided by an encoder per 360 degrees of rotation is called resolution, also known as resolution scale or simply the number of lines. It is generally 5 to 10,000 lines per revolution.
Figure 3 Encoder
IV. Position Measurement and Feedback Control Principle
Encoders occupy an extremely important position in elevators, machine tools, material processing, motor feedback systems, and measurement and control equipment. Encoders use gratings and infrared light sources to convert optical signals into TTL (HTL) electrical signals through a receiver. By analyzing the frequency of the TTL level and the number of high levels, the rotation angle and rotation position of the motor can be intuitively reflected.
Since angles and positions can be measured precisely, encoders and frequency converters can be combined to form a closed-loop control system, making the control more precise. This is why elevators, machine tools, and other similar devices can be used with such precision.
In summary, we understand that encoders are structurally classified into incremental and absolute types. Both convert other signals, such as optical signals, into electrical signals that can be analyzed and controlled. Common applications in our lives, such as elevators and machine tools, rely on precise motor adjustment and closed-loop control through electrical signal feedback. Therefore, encoders, in conjunction with frequency converters, naturally achieve precise control.
