encoder

What it is:

A sensor attached to each joint that measures and reports how much that joint has rotated. It's the robot's "position awareness system"—providing real-time feedback about where each joint actually is.

How it works:

1. As a motor rotates a joint, the encoder rotates with it
2. The encoder counts the rotation and converts it to an angle measurement
3. This data is sent back to the control system continuously
4. Result: The robot knows its actual joint angles at every moment

Types of encoders:

1. Absolute encoders — Remember their position even when powered off (like knowing the clock time)
2. Relative encoders — Only track changes from a starting point (like counting steps from home)

Why it's essential:

Feedback loop:

1. Motor receives a command: "Rotate to 45°"
2. Encoder measures actual rotation in real-time
3. Control system compares commanded angle vs. actual angle
4. If there's a mismatch, it adjusts motor power to correct it
5. This is how PID controllers work—they need encoder feedback to function

Without encoders:

1. The robot would have no idea if joints moved as commanded
2. No way to correct errors (motor slip, external forces, etc.)
3. Tasks requiring precision would be impossible
4. The robot would be "blind" to its own position

Practical example:

You command a robot to pick up a bolt at a specific location. The encoders tell you: "Joint 1 is at 45°, Joint 2 is at 90°." This data feeds into forward kinematics to calculate the exact hand position—confirming the gripper is where it needs to be.