cdhabecker/actuator

Implements the interface to be used by all actuators

remote_actuator

This is a ROS2 package providing the base class for all actuator drivers and the interface for all modules looking to interact with actuators.

Ros2_control and hardware_interface

Most actuator drivers need to be integrated with ros2_control using hardware_interface.

This package is used by remote_hardware_interface for position or velocity control. So all drivers using remote_actuator as the base class become integrated with ros2_control automatically.

Design

The actuator drivers implement the base class provided by this package. For C++ drivers, that means that they use remote_actuator::Implementation as a parent class. Its constructor requires an instance of rclcpp::Node to read the parameters from.

Note: There can be more than one instance of `remote_actuator::Implementation` per node. However they will all be controlled by the same parameters. Though the default value of the parameter `actuator_prefix` can be passed into each individual constructor which is sufficient for common use cases.

The following methods need to be implemented by each driver:

public:
  /* has_position must return true if it supports position control */
  virtual bool has_position() override;
  /* has_velocity must return true if it supports velocity control */
  virtual bool has_velocity() override;
protected:
  /* position_set_real_ is called to control the position */
  virtual void position_set_real_(double) override;
  /* velocity_set_real_ is called to control the velocity */
  virtual void velocity_set_real_(double) override;

The users can chose one of three ways to interact with child classes of remote_actuator::Implementation:

• Link with the driver directly to make native API calls:

  auto actuator = \
    std::make_shared<driver_package::DriverClass>(node, "/motor1");
  actuator->set_position(0.0);
  actuator->set_veloicty(1.0);

  In this case the driver is running within the same process and it is destroyed when the actuator object is destroyed.

• Link with remote_actuator to make ROS2 interface (DDS) calls (locally or over a network):

  auto actuator = \
    std::make_shared<remote_actuator::RemoteInterface>(node, "/motor1");
  actuator->set_position(0.0);
  actuator->set_veloicty(1.0);

  In this case the driver can be elsewhere within DDS' reach (same process or another side of the globe).

• Let the runtime make the choice between the above two options:

  auto actuator = \
    remote_actuator::Factory::New(node, "/motor1");
  // or
  auto actuator = \
    driver_package::Factory::New(node, "/motor1");

  In this case the boolean value of the parameter actuator_is_remote determines whether the driver is instantiated locally or if a remote interface is used to reach the driver instantiated elsewhere. Please, note, the trivial class driver_package::Factory (similar to remote_actuator::Factory) has to be written to support this use case.

No matter which option is chosen, the driver will expose the following ROS2 interfaces.

Parameters

• actuator_prefix: the prefix to ROS2 interfaces exposed by this driver
• Minimum and maximum values for filtering input as well as to support logic in other related modules (e.g. for PWM control):
  • actuator_position_min and actuator_position_max for position controlled actuators
  • actuator_velocity_min and actuator_velocity_max for position controlled actuators
• actuator_is_remote: instructs the factory class (if any) whether to instantiate the driver locally or to connect to a remote instance

Topics

Subscribers

• <namespace>/<actuator_prefix>/set_position: set the position
• <namespace>/<actuator_prefix>/set_velocity: set the velocity

Publishers

• <namespace>/<actuator_prefix>/position: the last position
• <namespace>/<actuator_prefix>/velocity: the last velocity

Services

• <namespace>/<actuator_prefix>/set_position: set the position
• <namespace>/<actuator_prefix>/set_velocity: set the velocity

Examples

Em2rs stepper driver series

See em2rs_driver for an example of a driver that implements remote_actuator.