When discussing motors, control is an unavoidable topic. Broadly speaking, control can be classified into two types: open-loop control and closed-loop control. The characteristic of open-loop control is that the system's output does not affect the system's control action. Closed-loop control, on the other hand, is a control method that directly or indirectly feeds the output back to the input to form a closed loop, thus participating in the control process.
Previously, applying variable frequency voltage to an inverter-driven motor using pulse width modulation (PWM) easily enabled open-loop speed control. In many lower-performance applications, open-loop speed control was common for motor drives, eliminating the need for encoders. However, as motors evolve towards higher efficiency, lower energy consumption, and more precise control, encoders are becoming increasingly integrated with motors.
Closed-loop motor control and position encoder
Open-loop control without encoders has several significant weaknesses. Due to the lack of feedback, the speed accuracy achievable by the motor is very limited; because current control cannot be optimized, it's difficult to achieve high motor efficiency; and transient response must be strictly limited, otherwise the motor will lose steps. Therefore, many motor applications no longer use open-loop control. For example, stepper motors, which previously used extensively in open-loop control, can now also be controlled in a closed-loop manner.
Closed-loop motor control improves the efficiency of both the motor and terminal equipment, enhancing motor performance, improving quality and synchronization in demanding applications, and saving significant energy. The entire closed-loop motor control feedback system works in concert, with power inverters, high-performance position detection, and current/voltage closed-loop feedback working together to improve motor performance and efficiency.
As one of the most critical components in a servo system, the encoder has always played a crucial role in determining the system's upper limits. Encoders provide closed-loop feedback signals by tracking the speed and position of a rotating shaft, with both optical and magnetic encoder technologies being widely used. In general-purpose servo drives, encoders are used to measure shaft position, and the drive's rotational speed can be derived from the data provided by the encoder.
An optical encoder consists of a code track with finely etched grooves and a code disk. When light passes through or is reflected from the disk, a photodiode sensor detects the change in light. The analog output of the photodiode is amplified and digitized before being fed back to the controller. A magnetic encoder, on the other hand, uses a magnetic sensor mounted on the motor shaft. This sensor provides sine and cosine analog outputs, which are amplified and digitized for control purposes.
Key performance indicators of encoders
Regardless of whether photoelectric or magnetic encoding is used to detect rotational or linear displacement, encoders can be distinguished as incremental or absolute. Their structures and output signals are completely different. In general, incremental signals do not indicate a specific position, but only that the position has changed, while absolute signals can both indicate a change in position and provide an absolute position indication.
Whether absolute or incremental, there are several very important performance indicators. The first is resolution. Encoder resolution refers to the number of positions that can be distinguished when the motor shaft rotates 360°. Currently, the highest resolution encoders require the use of optical technology, while medium-to-high resolution encoders can use either magnetic or optical technologies. Medium-to-low resolution encoders use resolvers or Hall sensors. The higher the encoder resolution, the more suitable it is for high-precision closed-loop control.
This assessment only considers the encoder's resolution and the correlation between different levels of closed-loop control precision. It doesn't take into account other control hardware, algorithms, or other factors. Currently, many applications can achieve high-precision closed-loop control without using optical encoders.
Furthermore, encoders can be used for position and speed feedback, and these two reference encoder accuracies are different. For position control, the focus is on absolute accuracy, ensuring that the unique signal output at each position on each revolution deviates from the actual position. Speed control, on the other hand, relies more on differential accuracy.
Another rarely mentioned metric is repeatability, which refers to the consistency with which the encoder returns to the same command position. In many closed-loop control applications, the equipment needs to perform a large number of repetitive tasks, and whether the encoder will deviate after multiple repetitions is also a very important metric.
summary
The choice of encoder depends on the application and type of motion. Given a matching resolution, the appropriate accuracy should be selected for position and speed control. Modern encoders, especially magnetic encoder chips, offer excellent compatibility with ASIC-level integrated solutions and dedicated sensing and decoding chips, significantly enhancing motor closed-loop control.
