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# BB.Message.Actuator.BeginMotion (bb v0.17.0)

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Message published by actuators when beginning a motion.

Used by `BB.Sensor.OpenLoopPositionEstimator` to estimate current position during open-loop control (actuators without position feedback).

## Fields

- `initial_position` - Position before motion begins (radians or metres)
- `target_position` - Target position (radians or metres)
- `expected_arrival` - When motion should complete (monotonic milliseconds)
- `command_id` - Optional correlation ID from the originating command
- `command_type` - Optional type of command that initiated this motion
- `acceleration` - Optional acceleration magnitude (rad/s² or m/s²) used when the motion follows a trapezoidal/triangular velocity profile. `nil` indicates a rectangular profile and is the legacy behaviour.
- `peak_velocity` - Optional peak velocity magnitude (rad/s or m/s) achieved during the motion. Paired with `acceleration` to fully describe a trapezoidal or triangular profile. `nil` falls back to easing-based interpolation.

## Example

    alias BB.Message
    alias BB.Message.Actuator.BeginMotion

    expected_arrival = System.monotonic_time(:millisecond) + 500

    {:ok, msg} = Message.new(BeginMotion, :shoulder,
      initial_position: 0.0,
      target_position: 1.57,
      expected_arrival: expected_arrival
    )

## Summary

### Types

command_type()
t()

### Functions

new(frame_id, attrs)

## Types

# command_type()

    @type command_type() :: :position | :velocity | :effort | :trajectory

# t()

    @type t() :: %BB.Message.Actuator.BeginMotion{
      acceleration: float() | nil,
      command_id: reference() | nil,
      command_type: command_type() | nil,
      expected_arrival: integer(),
      initial_position: float(),
      peak_velocity: float() | nil,
      target_position: float()
    }

## Functions

# new(frame_id, attrs)

    @spec new(
      atom(),
      keyword()
    ) :: {:ok, BB.Message.t()} | {:error, term()}