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A Comparison of Encoder Technologies (and Selection Tips)
The Importance of Encoder Feedback in Closed-Loop Motion Control
Encoder feedback plays a crucial role in closed-loop motion control systems, significantly enhancing the accuracy, reliability, and even efficiency of machinery and robotic applications. In this article, we'll explore the inner workings of different types of encoders—optical, magnetic, and capacitive—and offer some practical tips for selecting the right encoder for your needs.
Understanding Encoder Functionality
Encoders are typically installed on the rear shaft of a motor, sensing its rotation as it moves. While both optical and magnetic encoders are widely used, this article will primarily focus on rotary encoders. Despite their differences in tracking mechanisms, all encoders produce a pulse train signal as the motor shaft rotates. By analyzing the number of pulses, you can determine the motor shaft's position and how much it has turned from its home position. For instance, if an encoder has a resolution of 200 pulses per revolution and generates 200 pulses, you know the motor shaft has completed one full rotation. The frequency of the pulses (measured in Hz) indicates the rotational speed of the motor shaft, while the direction is determined by monitoring which channel of pulses (A or B-phase) leads the other. This basic explanation outlines the core functionality of encoders.
Beyond the Basics: Practical Applications
While understanding the fundamental principles is essential, knowing how to utilize the encoder's output effectively is equally important. Take, for example, a diverter conveyor application that employs a stepper motor. This video demonstrates how an encoder can be integrated into such a system to optimize performance.
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Now, let’s delve deeper into the specifics of the various encoder types and their operational mechanisms.
Optical Encoders: The Industry Standard
Optical encoders are the most commonly used type of encoder due to their superior precision, accuracy, and resolution. Powered by an electric light source (such as an LED), they require a continuous power supply to function.
In a traditional transmissive optical encoder, typically mounted on the rear shaft of stepper motors, brushless motors, or servo motors, the primary components include a light emitter (LED), a code wheel, a light receiver (photodetector), a power circuit, and an output circuit. These three elements work together to sense motion.
As the code wheel, featuring slits along its outer edge, rotates with the motor shaft, the light from the stationary LED either passes through the slits or is blocked by the wheel. From the photodetector's perspective, this creates a series of binary on/off "pulses." The output circuit then sends an ON signal when light shines through and an OFF signal when it is blocked. A motor driver, PLC, or HMI typically interprets this pulse train and translates it into meaningful data, such as steps, degrees, inches, or millimeters, providing real-time insights into the motor shaft's position and movement. Additional adjustments, like position correction, can be made dynamically.
There are several classifications of optical encoders:
Transmissive vs. Reflective
Traditionally, optical encoders are transmissive, meaning the LED light must pass through a transmissive disk (code wheel) and reach the photodetector. In recent years, the industry has shifted toward reflective disks to save space and reduce complexity.
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Incremental vs. Absolute
Incremental encoders detect and output pulse signals but can only track changes relative to a home position. This is because the code wheel in an incremental encoder lacks unique position values; each position is treated the same. With a third Z or index channel, however, there is one absolute position that can serve as a reference point for a home starting position or to count revolutions.
Looking at the code wheel of an incremental encoder, every slit appears identical. If power is lost, the position information is also lost, necessitating a homing operation to reset the system. Incremental encoders are generally more cost-effective and suitable for applications where absolute position feedback is not critical, such as speed control systems where velocity and acceleration tracking are more important.
Absolute encoders, on the other hand, feature a code wheel with a unique pattern for each position, providing multiple bits of information. This enables them to offer a unique position value over a full rotation or stroke. As the motor shaft rotates, sensors read the coded pattern to determine the exact position. This capability ensures that the controller always knows the precise position, even after a power loss. Absolute encoders are vital for applications demanding high position accuracy and repeatability, such as robotics, CNC machines, and precision automation. Naturally, absolute encoders are more expensive than incremental encoders. Many absolute encoders provide position accuracy within ±1 degree or better over a full rotation, with some linear encoders achieving micron-level accuracy over the entire stroke.
Magnetic Encoders: A Robust Alternative
Unlike optical encoders, magnetic encoders do not rely on a light emitter or receiver but still incorporate a code wheel and a sensor. Instead of slits, the code wheel features alternating north and south pole magnets along its outer edge. The magnetic sensor detects changes in magnetic polarity as the poles pass by. The end result is the same: the output circuit generates pulses that can be interpreted by a PLC or HMI. Since there's no need to power a light emitter and receiver, magnetic encoders consume less power than optical encoders.
Magnetic encoders are more resilient than optical encoders, excelling in humid, dusty, or dirty environments. However, they may struggle in settings with significant magnetic interference. Magnetic encoders are available in both rotary and linear forms, as well as incremental and absolute types. To track absolute positions, multiple magnetic disks are required.
Capacitive Encoders: The Latest Innovation
Capacitive encoders represent a cutting-edge technology, offering the same environmental advantages as magnetic encoders. According to US Digital, this type of encoder detects changes in capacitance using a high-frequency reference signal and then converts the signal into pulses. The structure consists of a transmitter, a rotor, and a receiver. The rotor usually has a pattern etched into it or features a uniquely shaped design. As the rotor moves between the transmitter and receiver, the pattern modulates the high-frequency signal generated by the transmitter. The receiver reads the modulated signal and translates it into a pulse signal.
Similar to optical encoders, capacitive encoders are vulnerable to noise and electrical interference, necessitating additional preventive measures in certain cases. They also have a low current draw. Capacitive encoders come in both rotary and linear varieties, as well as incremental and absolute types. To enable a capacitive encoder to track absolute positions, multiple capacitive disks are necessary.
In conclusion, choosing the right encoder depends on the specific requirements of your application. Understanding the trade-offs between precision, cost, and environmental resilience will help you make an informed decision. Whether you opt for optical, magnetic, or capacitive encoders, each type brings unique strengths to the table, ensuring optimal performance in diverse industrial scenarios.