Servomotor Encoders All You Need to Know

Introduction to Servomotor Encoders

1. What is a Servomotor Encoder?
   Servomotor encoders are sensors installed on servomotors to measure the magnetic pole position and the servomotor's rotation angle and speed. Based on the different physical media used, servomotor encoders can be divided into optical encoders and magnetic encoders. Rotary transformers are also considered a special type of servomotor encoder. Currently, the most commonly used encoders are optical encoders, but magnetic encoders, as a newcomer, have the advantages of being reliable, inexpensive, and resistant to contamination, and have the potential to surpass optical encoders.

There are many types of encoders, with the most commonly used being absolute encoders, incremental encoders, and rotary transformers, as well as some higher-level communication encoders. For servos, in order to achieve high performance and precision, the encoder's resolution must be increased. Common servomotor encoders have 2000-2500 lines (pulses/revolution), but the higher the line count, the more expensive the encoder, so it is necessary to understand the requirements of the control system to choose the most suitable encoder.

For incremental encoders, which are commonly used, the biggest problem is the loss of position when there is a power failure. Therefore, to maintain the position during a power failure, absolute encoders can be used. If mechanical vibration is large, optical encoders are not suitable, and rotary transformers or magnetic encoders must be used.

2. Relationship between Servomotors and Encoders
   Servomotor drivers and encoders are two necessary components of a servo system. The control part of the servomotor driver reads the encoder to obtain information such as the rotor speed, rotor position, and mechanical position. This information can be used to complete:

Speed control of servomotors

Torque control of servomotors

Synchronized tracking of mechanical positions (multiple transmission points)

Point-to-point positioning

3. Features and Application Points of Optical Encoders
   Optical encoders have always been a popular choice in the motion control application market. They consist of an LED light source (usually infrared) and an optical detector, which are located on opposite sides of the encoder disc. The disc is made of plastic or glass and has a series of spaced transparent and non-transparent lines or slots. The rotation of the disc interrupts the LED light path, producing two typical orthogonal square wave pulses A and B, which can be used to determine the axis rotation and speed.

Optical encoders rely on the rotation of the encoder disc and the optical receiver to work together. They are very close to each other but cannot touch. However, when there is vibration and the gap between the structure increases, the disc will collide with the optical receiver. When the optical encoder's moving parts collide, their positions change, causing a reduction in accuracy.

Optical encoders must be produced in a dust-free environment. Any dust on the disc will cause the optical encoder to fail. Strict sealing is usually required, especially at the bearing, shell, and wiring points. However, sealing is very susceptible to temperature changes. Due to the heat generated by the environment and the encoder itself, the air and water vapor inside the encoder are expelled when the temperature is high, and the air and water vapor from the outside are drawn in when the temperature is low. This causes condensation on the disc, which directly leads to the failure of the optical encoder.

4. Features and Suitable Scenarios of Magnetic Encoders
   The structure of magnetic encoders is similar to that of optical encoders, but they rely on magnetic fields instead of light beams. Magnetic encoders use magnetic discs instead of slotted optical discs. The magnetic disc has spaced magnetic poles arranged on it, and a row of Hall effect sensors or magnetic resistance sensors rotate above it. Any rotation of the disc will produce a response from these sensors, and the generated signal will be transmitted to the signal conditioning front-end circuit to determine the position of the axis. Compared to optical encoders, magnetic encoders have the advantages of being more durable, resistant to vibration and shock, and unaffected by contamination such as dust, dirt, and oil.

However, magnetic encoders are greatly affected by electromagnetic interference generated by motors (especially stepper motors), and temperature changes will also cause position drift. In addition, the resolution and accuracy of magnetic encoders are relatively low.

Magnetic encoders also have slower response speeds and cannot handle high-speed motion loads for position feedback. Therefore, magnetic encoders are suitable for the following application scenarios:

Point-to-point reciprocation positioning with low accuracy requirements, such as material handling, material sorting, large equipment positioning control, general robot positioning.

Low speed variation in continuous running, such as AGV wheels, conveyor belt transmission, inverter motor feedback.

High-speed running in one direction, such as electric spindles.

Motion control scenarios with small load inertia.

Feedback for stepper motors and brushless motors.

Note: Please double check the translation for any errors or omissions.