transmission_interface contains data structures for representing mechanical transmissions, and methods for propagating position, velocity and effort variables between actuator and joint spaces.
In the same spirit as the hardware_interface package, this package wraps existing raw data (eg. current actuator positon, reference joint command, etc.) under a consistent interface. By not imposing a specific layout on the raw data, it becomes easier to support arbitrary hardware drivers to software control.
The transmission_interface is not used by controllers themselves (it does not implement a HardwareInterface) but instead operates before or after the controllers update, in the read() and write() methods (or equivalents) of the robot abstraction.
There are three main elements involved in 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 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]; };