BB.Servo.PCA9685 (bb_servo_pca9685 v0.5.0)

BB integration for driving RC servos via the PCA9685 PWM controller.

This library provides controller, actuator and sensor modules for controlling RC servos through a PCA9685 16-channel PWM controller connected via I2C.

Components

• BB.Servo.PCA9685.Controller - Manages the PCA9685 device connection
• BB.Servo.PCA9685.Actuator - Controls servo position via a controller channel
• BB.Servo.PCA9685.Sensor - Provides position feedback by subscribing to actuator commands

Requirements

• PCA9685 PWM controller connected via I2C
• The pca9685 library for communication with the device

Quick Start

Define a controller and joints with servo actuators in your robot DSL:

controller :pca9685, {BB.Servo.PCA9685.Controller, bus: "i2c-1", address: 0x40}

joint :shoulder, type: :revolute do
  limit lower: ~u(-45 degree), upper: ~u(45 degree), velocity: ~u(60 degree_per_second)

  actuator :servo, {BB.Servo.PCA9685.Actuator, channel: 0, controller: :pca9685}
  sensor :feedback, {BB.Servo.PCA9685.Sensor, actuator: :servo}
end

joint :elbow, type: :revolute do
  limit lower: ~u(-90 degree), upper: ~u(90 degree), velocity: ~u(45 degree_per_second)

  actuator :servo, {BB.Servo.PCA9685.Actuator, channel: 1, controller: :pca9685}
  sensor :feedback, {BB.Servo.PCA9685.Sensor, actuator: :servo}
end

The actuator automatically derives its configuration from the joint limits - no need to specify servo rotation range or speed separately.

How It Works

Controller

The controller wraps a PCA9685.Device process and provides a stable reference for actuators. Multiple actuators can share a single controller, each using a different channel (0-15). The controller:

• Manages the I2C connection to the PCA9685
• Sets the PWM frequency (default 50Hz for servos)
• Optionally controls an output enable pin

Actuator

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

• Joint lower limit → minimum pulse width (default 500µs)
• Joint upper limit → maximum pulse width (default 2500µs)
• Centre position calculated as midpoint of limits

When commanded to a position, the actuator:

1. Clamps the position to joint limits
2. Converts to PWM pulse width
3. Sends command to the controller
4. Publishes PositionCommand for sensors

Sensor

The sensor subscribes to actuator position commands and publishes JointState messages. It provides:

• Position interpolation during movement
• Configurable publish rate (default 50Hz)
• Periodic sync publishing even when idle (default every 5 seconds)

Multiple PCA9685 Boards

For robots with more than 16 servos, you can define multiple controllers:

controller :pca9685_a, {BB.Servo.PCA9685.Controller, bus: "i2c-1", address: 0x40}
controller :pca9685_b, {BB.Servo.PCA9685.Controller, bus: "i2c-1", address: 0x41}

joint :shoulder, type: :revolute do
  actuator :servo, {BB.Servo.PCA9685.Actuator, channel: 0, controller: :pca9685_a}
  # ...
end

joint :gripper, type: :revolute do
  actuator :servo, {BB.Servo.PCA9685.Actuator, channel: 0, controller: :pca9685_b}
  # ...
end