Beam Bots Pigpio servo control

BB.Servo.Pigpio

BB integration for driving RC servos via pigpio on Raspberry Pi.

This library provides an actuator module for controlling RC servos directly connected to Raspberry Pi GPIO pins using the pigpio daemon.

Installation

Add bb_servo_pigpio to your list of dependencies in mix.exs:

def deps do
  [
    {:bb_servo_pigpio, "~> 0.9.0"}
  ]
end

Requirements

• Raspberry Pi with pigpio daemon running (sudo pigpiod)
• BB framework (~> 0.25)

Usage

Define a joint with a servo actuator in your robot DSL:

defmodule MyRobot do
  use BB

  # A robot won't move until armed, and arming is a command.
  commands do
    command :arm do
      handler BB.Command.Arm
      allowed_states [:disarmed]
    end

    command :disarm do
      handler BB.Command.Disarm
      allowed_states [:idle]
    end
  end

  topology do
    link :base do
      joint :shoulder do
        type :revolute

        limit lower: ~u(-45 degree), upper: ~u(45 degree),
              velocity: ~u(60 degree_per_second), effort: ~u(1 newton_meter)

        actuator :servo, {BB.Servo.Pigpio.Actuator, pin: 17}
        sensor :feedback, {BB.Sensor.OpenLoopPositionEstimator, actuator: :servo}

        link :arm do
          # ...
        end
      end
    end
  end
end

The actuator automatically derives its configuration from the joint limits - no need to specify servo rotation range or speed separately. The sensor entry is not optional decoration: an RC servo reports nothing back, and without the estimator nothing tells BB.Robot.State where the joint is.

Sending Commands

Use the BB.Actuator module to send commands to servos. Arm the robot first — commands to a disarmed robot are refused before they reach the driver:

{:ok, cmd} = MyRobot.arm()
{:ok, :armed, _} = BB.Command.await(cmd)

BB.Actuator.set_position/4 has two deliveries, chosen with :delivery. They differ in transport, not in what the driver sees: both arrive at the actuator's handle_command/2, and neither can skip the framework's arm check or its joint-to-motor transmission.

Every function takes either the actuator's unique name or its full path through the topology.

Default Delivery (published and acknowledged)

The command is published to [:actuator | path], which is what enables logging, replay and multi-subscriber patterns, and delivered to the actuator by a call, so the caller learns whether the joint is actually moving:

case BB.Actuator.set_position(MyRobot, [:base, :shoulder, :servo], 0.5) do
  :ok -> :moving
  {:error, reason} -> handle_error(reason)
end

# With options
:ok = BB.Actuator.set_position(MyRobot, [:base, :shoulder, :servo], 0.5,
        command_id: make_ref()
      )

Direct Delivery (for time-critical control)

Casts to the actuator and publishes nothing, for when responsiveness matters more than observability. It always returns :ok, so a refusal reaches the log and [:bb, :actuator, :rejected] telemetry and nowhere else — don't write an error branch that can never run:

# Fire-and-forget
BB.Actuator.set_position(MyRobot, :servo, 0.5, delivery: :direct)

Components

Actuator

BB.Servo.Pigpio.Actuator controls servo position via PWM.

Options:

To reverse the servo relative to the joint, configure the actuator's joint transmission rather than passing an actuator option:

actuator :servo, {BB.Servo.Pigpio.Actuator, pin: 17} do
  transmission do
    reversed? true
  end
end

Behaviour:

• Maps joint position limits directly to PWM range
• Clamps commanded positions to joint limits
• Publishes BB.Message.Actuator.BeginMotion after each command
• Calculates expected arrival time based on joint velocity limit

Sensor

Use BB.Sensor.OpenLoopPositionEstimator from the BB core library for position feedback. It subscribes to actuator BeginMotion messages, interpolates position during movement, and publishes it as BB.Message.Sensor.JointState.

sensor :feedback, {BB.Sensor.OpenLoopPositionEstimator, actuator: :servo}

Give every servo-driven joint one. BB.Robot.State is written from JointState messages and from nothing else — commanding a joint doesn't move it in state — so a joint without an estimator stays at its initial configuration forever, and forward kinematics, the URDF visualisers and inverse kinematics all keep working from a robot that never moved. BB warns at compile time about a joint nothing reports on.

How It Works

Position Mapping

The actuator maps the joint's position limits to the servo's PWM range:

Joint lower limit  ->  min_pulse (500 microseconds)
Joint upper limit  ->  max_pulse (2500 microseconds)
Joint centre       ->  mid_pulse (1500 microseconds)

For a joint with limits -45 degrees to +45 degrees:

• -45 degrees maps to 500 microseconds
• 0 degrees maps to 1500 microseconds
• +45 degrees maps to 2500 microseconds

Position Feedback

Since RC servos don't provide position feedback, the open-loop position estimator estimates position based on commanded targets and expected arrival times:

1. Actuator sends command and publishes BeginMotion with expected arrival time
2. Sensor receives BeginMotion and interpolates position during movement
3. After arrival time, sensor reports the target position
4. Sensor publishes the estimate as JointState, which is what writes BB.Robot.State

That last step is why the estimator is part of the wiring rather than an extra: it is the only thing that tells the rest of the framework where an RC servo is.

Motion Lifecycle

When a position command is processed:

1. Actuator clamps position to joint limits
2. Converts angle to PWM pulse width
3. Sends PWM command to pigpiod
4. Publishes BB.Message.Actuator.BeginMotion with:
   • initial_position - where the servo was
   • target_position - where it's going
   • expected_arrival - when it should arrive (monotonic milliseconds)
   • command_id - correlation ID (if provided)
   • command_type - :position

Documentation

Full documentation is available at HexDocs.