How encoders work in Robotics

Imagine trying to touch a specific point in a dark room without any feedback – you’d quickly become disoriented and ineffective. Similarly, a robotic system needs continuous, accurate feedback about the state of its various joints, wheels, or actuators. This feedback loop is essential for:

• Position Control: Ensuring a robot arm moves to and stops precisely at a desired coordinate.
• Speed Control: Maintaining a constant velocity for tasks like welding or painting.
• Trajectory Planning: Allowing the robot to follow a pre-programmed path smoothly and accurately.
• Synchronization: Coordinating multiple joints or mechanisms for complex, multi-axis movements.
• Safety: Preventing collisions by knowing the exact location of all moving parts.

This indispensable feedback is primarily provided by encoders.

An encoder is an electromechanical device that converts angular or linear motion into an electrical signal. In essence, it translates physical movement into data that a robot’s control system can understand and use. This data typically represents position, but by tracking changes in position over time, it can also derive velocity and acceleration.

There are two primary categories of encoders used in robotics:

1. Rotary Encoders: Measure angular displacement and are commonly found on motor shafts, robotic joints, and wheels.
2. Linear Encoders: Measure displacement along a straight line and are used in applications requiring precise linear motion, such as CNC machines or linear actuators.

While both types are critical, rotary encoders are far more prevalent in general robotics, given the widespread use of rotary motors and articulated joints.

The fundamental distinction among encoders lies in how they report position:

1. Incremental Encoders: Counting Pulses

Incremental encoders provide information about change in position, rather than absolute position. They generate a continuous stream of pulses as they rotate or move.

How they work: An incremental encoder typically consists of a disc with a series of transparent and opaque lines (or slots) for optical encoders, or teeth for magnetic encoders. A light source (for optical) or a magnetic field (for magnetic) passes through/over this pattern to a sensor. As the disc rotates, the sensor detects the alternating light/dark (or magnetic flux changes) patterns, generating a series of digital pulses.

• Output Channels: Most incremental encoders have two output channels, often labeled A and B, which are offset by 90 degrees (quadrature encoding). This phase shift allows the control system to determine both the direction of rotation and to effectively quadruple the resolution of the encoder (by detecting transitions on the rising and falling edges of both A and B signals). A third channel, Z or Index pulse, provides a single pulse per revolution, serving as a reference or home position.
• Position Tracking: To determine absolute position, the robot’s control system must continuously count these pulses from a known starting point (e.g., after homing the robot, which often involves moving it to a limit switch or detecting the Z pulse).
• Pros: Generally simpler, more robust, and less expensive. High resolution possible.
• Cons: Lose position information if power is removed (unless a battery-backed counter is used). Requires a homing sequence after power-up. Susceptible to noise accumulating errors over long periods if not periodically re-homed.

2. Absolute Encoders: Knowing Where You Are, Always

Unlike incremental encoders, absolute encoders provide a unique digital code for each distinct position. They immediately know their exact angular or linear position upon power-up, without any need for homing.

How they work: Absolute encoders use multiple concentric tracks (for rotary) or parallel tracks (for linear), each with a unique pattern. For optical absolute encoders, each track has different light/dark patterns that, when read simultaneously by multiple sensors, generate a unique binary or Gray code word for every specific angular position.

• Output Formats: Common output formats include parallel binary or Gray code, serial synchronous interface (SSI), BiSS Interface, and various fieldbus protocols (e.g., EtherCAT, PROFINET).
• Types:
  • Single-Turn Absolute Encoders: Provide absolute position within a single 360-degree rotation.
  • Multi-Turn Absolute Encoders: Combine a single-turn absolute encoder with a geared system or a battery-backed counting mechanism to track the number of full revolutions. This allows them to provide absolute position over a much larger range, which is critical for multi-axis robots.
• Pros: Maintain position information even after power loss. No homing required. Higher reliability in critical applications where motion must resume immediately.
• Cons: More complex, generally more expensive. Larger footprint for multi-turn versions.

The data from encoders isn’t just raw numbers; it’s the lifeblood of the robot’s control loop.

1. Sensor Input: Encoder signals (pulses for incremental, digital codes for absolute) are fed into the robot’s controller (often a real-time embedded system or industrial PC).
2. Position Calculation (for Incremental): For incremental encoders, the controller’s software continuously counts the A and B pulses. Using the 90-degree phase shift, it determines direction and updates the current position.
3. Velocity and Acceleration Derivation: By tracking the rate of change of position, the controller calculates the current velocity and acceleration of the joint or wheel.
4. Feedback Loop: This real-time position, velocity, and acceleration data is compared against the desired, pre-programmed values from the robot’s trajectory planner.
5. Error Calculation: Any discrepancy between the actual and desired values generates an “error signal.”
6. Actuator Command: The controller then uses this error signal (often via a PID — Proportional-Integral-Derivative — control algorithm) to adjust the command sent to the motors or actuators. If the robot is behind schedule, the motor might get more power; if it’s overshooting, it might get less or be reversed slightly.
7. Continuous Cycle: This process occurs continuously, thousands of times per second, ensuring the robot precisely follows its intended path and achieves its target positions.

When selecting an encoder for a robotic application, several parameters are crucial:

• Resolution: The smallest incremental change in position that the encoder can detect. For incremental encoders, this is often expressed in pulses per revolution (PPR). For absolute encoders, it’s the number of unique positions per revolution (e.g., 16-bit resolution gives 65,536 unique positions). Higher resolution means greater precision.
• Accuracy: How close the reported position is to the true physical position. This can be influenced by mechanical errors, sensor limitations, and coupling.
• Repeatability: The ability of the encoder to provide the same output for the same physical position under repeated measurements.
• Frequency Response: How quickly the encoder can update its position output, directly impacting the maximum speed it can track.
• Environmental Robustness: Resistance to dust, moisture, vibration, shock, and temperature variations. Industrial robots often operate in harsh environments, requiring ruggedized encoders.
• Interface: The electrical protocol used to communicate the position data to the controller (e.g., SSI, EtherCAT, analog, parallel).

As robotics advances, so do encoder technologies:

• Miniaturization: Smaller, lighter encoders are critical for compact robots and drones.
• Integrated Intelligence: “Smart” encoders are incorporating more processing power, performing local diagnostics, data filtering, and even some control functions, reducing the load on the main controller.
• Redundancy and Safety: For collaborative robots and safety-critical applications, encoders with redundant sensing elements are becoming more common to ensure fault tolerance.
• EtherCAT and Other Fieldbus Integration: Direct integration with real-time Ethernet-based fieldbuses simplifies wiring, improves data rates, and enables distributed control architectures.
• Non-Contact Sensing: Technologies like magnetic encoders are becoming more prevalent due to their robustness against dust, dirt, and challenging environments compared to traditional optical encoders.

In conclusion, encoders are the unsung heroes of robotic precision. They are the senses that allow robots to “see” and “feel” their position in the world, transforming complex physical motion into usable digital data. Understanding how they work is fundamental to appreciating the intricate dance of modern robotics and the engineering marvels they represent. Without their continuous, accurate feedback, the sophisticated movements of today’s robots would be impossible.