Spanda

Lesson 2 — Sensors and safety

Goal: Add perception, physical units, and safety rules that run before every motion command.

Example: examples/basics/02_sensors_and_safety.sd

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Adding a sensor

sensor lidar: Lidar on "/scan";

Sensors are first-class declarations, not opaque driver handles. The type (Lidar) tells the compiler what methods exist (read(), nearest_distance, etc.). The on "/scan" clause binds a transport topic (ROS2-style in production; simulated in spanda run).

Read sensor data inside a behavior:

let scan = lidar.read();
if scan.nearest_distance < 1.0 m {
  wheels.stop();
}

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Physical units

Spanda enforces unit algebra at compile time:

Unit	Example	Meaning
m/s	0.4 m/s	Linear speed
rad/s	0.2 rad/s	Angular speed
m	1.0 m	Distance
ms	100ms	Time interval

You cannot accidentally add meters to seconds — the type checker rejects it.

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The safety block

Safety rules are evaluated before every motion command reaches an actuator:

safety {
  max_speed = 1.0 m/s;
  stop_if lidar.nearest_distance < 0.5 m;
}

Rule	Effect
max_speed	Clamps drive velocity
stop_if	Emergency stop when condition is true

Safety is not a comment or a TODO — it is part of the language and checked by the compiler and runtime.

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Periodic behaviors

Use loop every for fixed-rate control loops:

behavior patrol() {
  loop every 100ms {
    let scan = lidar.read();
    // react to scan...
  }
}

This maps to deterministic scheduling in simulation and (eventually) real-time targets on hardware.

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Try it

spanda check examples/basics/02_sensors_and_safety.sd
spanda run examples/basics/02_sensors_and_safety.sd
spanda sim examples/basics/02_sensors_and_safety.sd

sim adds verbose simulation output — useful when debugging reactive logic.

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Exercise

Starting from Lesson 1’s robot, add:

1. A Lidar sensor on "/scan"
2. max_speed = 0.5 m/s and stop_if lidar.nearest_distance < 0.3 m
3. A patrol() behavior that drives forward at 0.2 m/s when clear, stops when close

Verify with spanda check then spanda run.

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Next: Lesson 3 — Control flow and loops

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