What's the encoder?

Encoders, as sensor devices that convert physical signals such as rotation angles and linear displacements into electrical signals such as pulses and codes, play a significant role in multiple fields including robotics, CNC machine tools, and motor control. There are various types, including incremental photoelectric encoders, magnetic encoders, etc. Each type has its unique working principle and application scenarios. When selecting a model, multiple parameters need to be considered, such as measurement accuracy, resolution, and signal output type, to ensure that the encoder can meet the specific application requirements.

Ii. Classification Methods of Encoders

There are various classification methods for encoders, including common ones such as photoelectric encoders and magnetic encoders. These classification methods are mainly based on the differences in their working principles and application scenarios. When choosing a suitable encoder, it is crucial to understand the various types of encoders and their characteristics.

01

Classification of encoders based on their disc engraving methods

A. Incremental encoder: It emits a pulse signal every time it rotates a certain Angle and is often used for angular displacement measurement.

b. Absolute encoder: Each position has a unique code and is suitable for absolute position measurement.

02

Classification by structure and working principle

a. Photoelectric encoder: It converts the disk code into an electrical signal through photoelectric components based on the photoelectric effect.

b. Magnetic-electric encoder: It uses magnetic-sensitive components to convert magnetic codes into electrical signals based on the magnetic-electric effect.

c. Capacitive encoder: It converts the code into an electrical signal through capacitive components and utilizes the principle of capacitance.

03

Classification of application scenarios

a. Linear encoder: Specifically designed for measuring linear displacement, it is commonly found in machine tools, robots and other equipment.

b. Rotary encoder: Also known as shaft encoder, it is used for measuring rotation angles and speeds and is often applied in equipment such as motors and robots.

04

Classification of reading methods

a. Contact encoder: It generates electrical signals through the contact between the brush and the code disk, which is simple and direct.

b. Non-contact encoder: Composed of the encoder body, code disk (or magnetic ring, grating, etc.) and detection element, it does not require direct contact and has advantages such as long service life, no wear and low maintenance cost.

01

The working principle of incremental encoders

Incremental encoders convert displacement signals into periodic electrical signals and further transform these electrical signals into counting pulses. The number of these counting pulses is used to indicate the specific magnitude of the displacement.

During the operation of an incremental encoder, a pulse signal is emitted whenever the encoder rotates one unit of Angle. This kind of pulse signal is usually output in the form of A phase, B phase and Z phase. Sometimes, sine and cosine signals are also emitted and subdivided to generate pulses with higher frequencies. The quantity and frequency of these pulses are closely related to the specific magnitude of the displacement.

Figure 24: ABZ Differential Incremental Signals

The pulse outputs of phase A and Phase B are delayed by a quarter of a period. This delay relationship can be used to distinguish between forward and reverse rotations. By capturing the rising and falling edges of phase A and Phase B, 2 or 4 octave processing can be carried out. The Z phase, on the other hand, outputs a single-revolution pulse, that is, one pulse is emitted for each revolution.

Incremental encoders usually do not come with internal storage devices, and thus do not have the function of data retention after power failure. In a CNC machine tool, the counting reference must be established and the actual position reset through the "return to reference point" operation.

Next, we will explore absolute value encoders. The output of this encoder directly reflects the absolute angles within a 360° range, that is, each reference Angle within one revolution corresponds to a unique binary value. Multiple positions can be recorded and measured through an external loop device.

The absolute position can be identified by the amplitude of the output signal or the physical coded scale of the grating. The former is called a resolver, while the latter is an absolute value encoder.

Next, we will delve into the selection parameters and basic principles of encoders. Firstly, measurement accuracy and resolution are the key parameters for choosing an incremental encoder. Accuracy refers to the error between the measurement result and the theoretical value, while resolution indicates the minimum positional change that the encoder can detect. Secondly, the type of signal output is also an important consideration factor, including analog signals such as sine and cosine waves, as well as digital signals such as square wave output. In addition, speed and acceleration are also factors that need to be considered when making a selection.

The rotational speed and acceleration limits of the encoder. When choosing an encoder, the speed and acceleration requirements in practical applications must be taken into consideration.

Power supply voltage requirements. Encoders typically require a DC power supply, such as 5V, 12V or 24V, depending on the model.

Operating temperature range. The working efficiency of the encoder is affected by the ambient temperature, so the appropriate working temperature range must be selected according to the application conditions.

Basic principles and reversing determination. Taking the photoelectric encoder as an example, let's briefly understand the working principle of the incremental encoder. At the edge of the code disk, the light-transmitting and non-light-transmitting parts are distributed at equal angles. A light source and a photosensitive element are respectively installed on both sides of the code disk. As the code disk rotates with the rotor, the passage of each gap will cause changes in the brightness of the light, thereby generating an electrical pulse output signal. These pulse signals are sent to the counter for counting, thereby obtaining the rotation Angle of the code disk.

Commutation determination of incremental photoelectric encoders:

To determine the rotation direction, we can adopt two sets of photoelectric conversion devices and ensure that their relative positions in space have a certain relationship. In this way, the signals they generate will differ in phase by 1/4 period. By observing the change relationship between phases A and B, we can determine the forward and reverse rotation directions of the encoder, as shown in the following figure

Magnetic encoders and photoelectric encoders are quite similar in principle, except that it replaces the light detection sensor with a Hall sensor, and the light-transmitting code disk becomes a magnetic code disk. The magnetic encoder rings produced by Tokyo Magnet & Kagami Magnetic Industry, with their high precision, provide key data such as position, speed, distance and direction for robot systems, thereby significantly optimizing the performance of robot systems. Not only that, it plays an indispensable role in multiple fields such as industrial automation, process control, medical equipment, energy, and aerospace.