This article discusses different types of motor encoders in automation, specifically linear and rotary encoders and their applications.
I. What is a Motor Encoder?
A motor encoder is a device that records positional data for automation control systems or any machinery containing motors that require positional information. From robotic arms to 3D printers, they are ubiquitous. Encoders play a critical role in enabling autonomous machines to function properly. They allow for precise measurement of moving components within a system.
Motor encoders offer benefits in several areas. For instance, linear encoders are commonly used in rail applications and enable CNC machines and 3D printers to create parts with precision, while rotary encoders make robotic arms possible in manufacturing. The signals they transmit activate different outputs on controllers or PLCs at the precise moment required.
II. How Do Motor Encoders Work?
Encoders function by providing electrical information to control devices based on one of two distinct systems: rotary or linear. Several mechanisms exist within encoders to convert physical changes into electrical data: resistive, mechanical, magnetic, and optical. Optical encoders are the most common in manufacturing. They contain at least one light transmitter and one light receiver to convert physical motion into electrical signals for controller processing. Regardless of the conversion method employed, encoders are always categorized as either linear or rotary encoders.
In optical encoders, both rotary and linear types utilize "windows" cut into a solid surface, allowing light to enter the receiving unit only incrementally. Linear encoders use sensors to detect different patterns within a strip along the path length, while rotary encoders consist of a disk with slots that transmit signals back to the control system.
In optical systems, the transmitter emits a constant beam of light that is progressively interrupted as the system moves. Whenever the receiver detects light from the transmitter, it sends an electrical signal to the controller. Various disk or track configurations exist to block/receive light depending on the application. These include absolute position encoders and incremental encoders.
III. Absolute Encoders and Incremental Encoders: What's the Difference?
Absolute encoders use multiple light sensors to send binary codes to the controller. They feature distinct slots corresponding to pairs of light transmitters/receivers. For single-turn absolute encoders, these slots generate a binary code indicating the angular position within one revolution of the motor.
In applications requiring higher precision and greater range, multi-turn encoders utilize gear reducers and two encoder discs to achieve a larger range of known positions. Absolute encoders are better suited for scenarios requiring position data after power loss, most commonly in safety circuits. Incremental encoders feature uniformly spaced slots to send pulses to the controller. These encoders rely on pulses counted from a zero position, making it crucial to have a known position to restart counting if the system loses power for any reason.
When only motor speed is required, an analog signal can be sent to the controller, enabling it to process this data for useful applications. If the process requires position data, the encoder can send electrical pulses to the controller to decipher the motor's position within its boundary area.
IV. Where are linear encoders used?
Linear encoders transmit electrical pulse signals to controllers via sensors or "notches" on a scale. These pulse signals can be decoded by a PLC and converted into instructions for the device to follow.
Linear encoders are better suited for applications with sliding positioners, such as 3D printers or CNC machines. They excel in processes requiring accurate, high-speed data transmission to controllers. Certain linear encoders, if not absolute encoders, require a reference position to find zero after power loss or PLC/controller restart.
Absolute encoders use binary representation for position, while incremental encoders only send pulses counted by the controller after startup. Limit switches or sensors can provide a reference point when position data must be reset.
Absolute-code-based linear encoders can determine their position without movement or reference points. They utilize binary codes from multiple scales to establish position. This offers greater flexibility for application processes and opens more opportunities in fields requiring restart safety.
V. Applications of Rotary Encoders
Rotary encoders consist of a circular scale attached to the motor shaft. As the motor rotates, light sensors reading patterns on the scale send pulse counts or binary codes to the PLC. Rotary encoders are highly useful in applications requiring motor speed measurement or where distance is difficult to gauge without motor rotation, such as servo motors in robotic arms. Applications needing motor speed control use incremental encoders that generate pulse counts to measure motor speed.
The encoder scale has a specific number of slots, and the PLC counts these slots as the motor rotates. This count can then be converted into RPM. An example where this is useful is on a conveyor belt motor. Certain parameters may require different belt speeds, and the PLC can adjust accordingly based on the motor's RPM. They are also useful in applications where precision is critical, as they produce more accurate data than absolute rotary encoders. Despite their greater accuracy, they cannot read position without movement and may require a reference position after losing communication with the PLC.
Absolute encoders can also be used as rotary motor encoders. These are more suitable for situations requiring angular data. They also retain the ability to recall position after communication or power loss between the encoder and controller, unlike incremental rotary encoders that require movement to transmit data.