transmission_interface

Setting up a transmission interface

The first example is minimal, and shows how to propagate the position of a single actuator to joint space through a reducer.

#include <transmission_interface/simple_transmission.h>
#include <transmission_interface/transmission_interface.h>

int main(int argc, char** argv)
{
  using namespace transmission_interface;

  // Raw data
  double a_pos;
  double j_pos;

  // Transmission
  SimpleTransmission trans(10.0); // 10x reducer

  // Wrap raw data
  ActuatorData a_data;
  a_data.position.push_back(&a_pos);

  JointData j_data;
  j_data.position.push_back(&j_pos);

  // Transmission interface
  ActuatorToJointPositionInterface act_to_jnt_pos;
  act_to_jnt_pos.registerHandle(ActuatorToJointPositionHandle("trans", &trans, a_data, j_data));

  // Propagate actuator position to joint space
  act_to_jnt_pos.propagate();
}

The second example is a bit more complicated, and represents a robot with three actuators and three joints:

• The first actuator/joint are coupled through a reducer.
• The last two actuators/joints are coupled through a differential.

The hardware is such that one can read current actuator position, velocity and effort, and send position commands.

#include <vector>

#include <transmission_interface/simple_transmission.h>
#include <transmission_interface/differential_transmission.h>
#include <transmission_interface/transmission_interface.h>

using std::vector;
using namespace transmission_interface;

class MyRobot
{
public:
  MyRobot()
   : sim_trans(-10.0,  // 10x reducer. Negative sign: actuator and joint spin in opposite directions
                 1.0), // joint position offset

     dif_trans(vector<double>(2, 5.0), // 5x reducer on each actuator
               vector<double>(2, 1.0)) // No reducer in joint output
  {
    // Wrapping the raw data is the most verbose part of the setup process... //////////////////////////////////////////

    // Wrap simple transmission raw data - current state
    a_state_data[0].position.push_back(&a_curr_pos[0]);
    a_state_data[0].velocity.push_back(&a_curr_vel[0]);
    a_state_data[0].effort.push_back(&a_curr_eff[0]);

    j_state_data[0].position.push_back(&j_curr_pos[0]);
    j_state_data[0].velocity.push_back(&j_curr_vel[0]);
    j_state_data[0].effort.push_back(&j_curr_eff[0]);

    // Wrap simple transmission raw data - position command
    a_cmd_data[0].position.push_back(&a_cmd_pos[0]); // Velocity and effort vectors are unused

    j_cmd_data[0].position.push_back(&j_cmd_pos[0]); // Velocity and effort vectors are unused

    // Wrap differential transmission raw data - current state
    a_state_data[1].position.push_back(&a_curr_pos[1]); a_state_data[1].position.push_back(&a_curr_pos[2]);
    a_state_data[1].velocity.push_back(&a_curr_vel[1]); a_state_data[1].velocity.push_back(&a_curr_vel[2]);
    a_state_data[1].effort.push_back(&a_curr_eff[1]);   a_state_data[1].effort.push_back(&a_curr_eff[2]);

    j_state_data[1].position.push_back(&j_curr_pos[1]); j_state_data[1].position.push_back(&j_curr_pos[2]);
    j_state_data[1].velocity.push_back(&j_curr_vel[1]); j_state_data[1].velocity.push_back(&j_curr_vel[2]);
    j_state_data[1].effort.push_back(&j_curr_eff[1]);   j_state_data[1].effort.push_back(&j_curr_eff[2]);

    // Wrap differential transmission raw data - position command
    a_cmd_data[1].position.push_back(&a_cmd_pos[1]); a_cmd_data[1].position.push_back(&a_cmd_pos[2]);

    j_cmd_data[1].position.push_back(&j_cmd_pos[1]); j_cmd_data[1].position.push_back(&j_cmd_pos[2]);

    // ...once the raw data has been wrapped, the rest is straightforward //////////////////////////////////////////////

    // Register transmissions to each interface
    act_to_jnt_state.registerHandle(ActuatorToJointStateHandle("sim_trans",
                                                               &sim_trans,
                                                               a_state_data[0],
                                                               j_state_data[0]));

    act_to_jnt_state.registerHandle(ActuatorToJointStateHandle("dif_trans",
                                                              &dif_trans,
                                                              a_state_data[1],
                                                              j_state_data[1]));

    jnt_to_act_pos.registerHandle(JointToActuatorPositionHandle("sim_trans",
                                                                &sim_trans,
                                                                a_cmd_data[0],
                                                                j_cmd_data[0]));

    jnt_to_act_pos.registerHandle(JointToActuatorPositionHandle("dif_trans",
                                                                &dif_trans,
                                                                a_cmd_data[1],
                                                                j_cmd_data[1]));

    // Names must be unique within a single transmission interface, but a same name can be used in
    // multiple interfaces, as shown above
  }

  void read()
  {
    // Read actuator state from hardware
    // ...

    // Propagate current actuator state to joints
    act_to_jnt_state.propagate();
  }

  void write()
  {
    // Porpagate joint commands to actuators
    jnt_to_act_pos.propagate();

    // Send actuator command to hardware
    // ...
  }

private:

  // Transmission interfaces
  ActuatorToJointStateInterface    act_to_jnt_state; // For propagating current actuator state to joint space
  JointToActuatorPositionInterface jnt_to_act_pos;   // For propagating joint position commands to actuator space

  // Transmissions
  SimpleTransmission       sim_trans;
  DifferentialTransmission dif_trans;

  // Actuator and joint space variables: wrappers around raw data
  ActuatorData a_state_data[2]; // Size 2: One per transmission
  ActuatorData a_cmd_data[2];

  JointData j_state_data[2];
  JointData j_cmd_data[2];

  // Actuator and joint space variables - raw data:
  // The first actuator/joint are coupled through a reducer.
  // The last two actuators/joints are coupled through a differential.
  double a_curr_pos[3]; // Size 3: One per actuator
  double a_curr_vel[3];
  double a_curr_eff[3];
  double a_cmd_pos[3];

  double j_curr_pos[3]; // Size 3: One per joint
  double j_curr_vel[3];
  double j_curr_eff[3];
  double j_cmd_pos[3];
};

Loading transmission interface from URDF

Rather than manually specifying the transmission elements, you can also load them from the URDF using the TransmissionInterfaceLoader, for which an example exists in the docs.