Explain how the controller coordinates robot movement.

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## How the controller coordinates robot movement

```mermaid
flowchart TD
    A[Program instruction, e.g. MoveL target] --> B[Interpreter reads instruction]
    B --> C[Trajectory planner: path and speed profile]
    C --> D[Interpolation every few ms]
    D --> E[Inverse kinematics: joint set-points]
    E --> F[Servo loop per joint]
    F --> G[Motors move the arm]
    G --> H[Encoder feedback]
    H --> F
```

### Steps
1. **Interpretation:** the program interpreter reads the next motion command with its target, speed and zone data.
2. **Trajectory planning:** the planner builds a path with acceleration, constant-speed and deceleration phases.
3. **Interpolation:** intermediate points are calculated at a fixed cycle time (a few milliseconds).
4. **Inverse kinematics:** each Cartesian point is converted into joint angles.
5. **Joint synchronisation:** all joints start and stop together so the tool follows the planned path.
6. **Servo control:** each joint loop compares the set-point with encoder feedback and corrects the error, typically with PID control, u = Kp·e + Ki·∫e dt + Kd·de/dt.
7. **Sensor and I/O coordination:** the controller waits for or reacts to signals, for example a conveyor sensor, using interlocks.
8. **Safety monitoring:** limits, collisions and emergency stop are checked continuously.

### Exam tip
The key sequence is instruction, interpolation, inverse kinematics, servo loop.

How the controller coordinates robot movement

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