Background

XBotInterface

XBot::Joint

The most atomic component is XBot::Joint, which collects all joint states and commands, and provides the corresponding getters and setters. Joints are organized into kinematic chains, which are implemented by the XBot::KinematicChain class.

XBot::KinematicChain

Each kinematic chain of the robot (as described by the SRDF file) is parsed into an object of this class. It stores internally the vector of joints belonging to the chain, and provides chain-wise getters and setters for the robot state, so that is easy for the user to command or read a single chain of the robot. Moreover, the KinematicChain provides the ability of getting sensors attached to it. The set of all kinematic chains is organized into the main object of the library, the XBot::XBotInterface.

XBot::XBotInterface

XBotInterface provides a view of the robot as a set of kinematic chains. Internally, it stores the vector of all chains that make up the robot. It provides robot-wise getters and setters for the robot state, and for all sensors attached to it.

XBot::RobotInterface

The XBot::XBotInterface library acts as a simple, organized container for the robot state. The XBot::RobotInterface inherits from such a class, and adds a few method that allow for communication from/to a robot:

• sense() acquires data from the robot sensors.

• move() sends commands to the robot actuators.

Depending on the actual implementation of the RobotInterface being used, these methods will perform different operations, e.g.

• reading/publishing to suitable ROS topics

• reading/writing to suitable YARP ports.

• reading/writing to suitable OROCOS ports.

• reading/writing directly from the robot hardware abstraction layer as in the case of the XBotRT implementation.

XBot::ModelInterface

XBot::ModelInterface adds to XBot::XBotInterface methods for computing kinematics/dynamics quantities corresponding to the robot state described by the underlying XBotInterface object. These include:

• forward kinematics (link pose, orientation, twists, jacobians, jdotqdot terms, centre-of-mass, centroidal momentum and centroidal momentum matrix, …).

• inverse dynamics (gravity compensation torques, inertia matrix, …).

• forward dynamics.

• …

Before computing any of these quantities, an update() method must be called. T his has the effect of updating the underlying back-end with the joint states stored inside the XBot::ModelInterface.

Moreover, since robot and model objects derive from a common base class, it is very simple to exchange information between them. More specifically:

• a model can be synchronized to the actual state of the robot by calling a syncFrom() method on the model, and providing the robot as argument.

• a model state can be used as desired state for the robot, by calling a setReferenceFrom() method on the robot and providing a model as argument. Both these methods work also in the case that the model only represents a subset of the robot kinematic tree (e.g. just the upperbody of a humanoid robot).

The ModelInterface uses external library to compute kinematics/dynamics quantities such as RBDL.

Floating Base Robot

Some of the most interesting dynamical systems come from the borderline science fiction robots that walk around on two or four legs. These robots are what we call floating-based robots. The base isn’t bolted to the floor. It instead floats above the ground and moves from place to place with the robot. At any moment in time, the robot must choose carefully torques to apply that will induce the perfect reaction forces from the environment at its points of contact so that those reaction forces correctly manipulate its free floating base.

We also now have an underactuated system. Since the base of the robot is free to move around, we can no longer fully control all degrees of freedom. We often choose the floating-base of the robot to be situated somewhere around the robot’s torso near the robot’s center of mass. These floating base dimensions consist of three positional coordinates and three rotational components totaling a full six degrees of freedom that we can’t directly control. The robot doesn’t have a collection of jet engines attached to its torso that can directly induce forces and torques on the floating base. All forces and torques on that base must be generated by some contact with the environment. Physically this means that in order to fully control these unactuated degrees of freedom, the robot needs to reason about how it can generate the requisite reaction forces from the environment around it:

XBotInterface API’s Translated

XBotInterface

Methods and Class description

Method	Input	Output
getEnabledJointId	NONE	int Vector of joints enabled
getJointPosition	NONE	double Vector of joints position
getMotorPosition	NONE	double Vector of motors position
getJointVelocity	NONE	double Vector of joints velocity
getMotorVelocity	NONE	double Vector of motors velocity
getJointAcceleration	NONE	double Vector of joints acceleration
getJointEffort	NONE	double Vector of joints torque
getTemperature	NONE	double Vector of joints temperature
getStiffness	NONE	double Vector of joints stiffnes
getDamping	NONE	double Vector of joints damping
getPositionReference	NONE	double Vector of joints position reference
getVelocityReference	NONE	double Vector of joints velocity reference
getEffortReference	NONE	double Vector of joints torque reference
setPositionReference	double Vector of joints position reference	NONE
setJointPosition	double Vector of joints position reference	NONE
setMotorPosition	double Vector of motors position reference	NONE
setVelocityReference	double Vector of joints velocity reference	NONE
setJointVelocity	double Vector of joints velocity reference	NONE
setMotorVelocity	double Vector of motors velocity reference	NONE
setJointAcceleration	double Vector of joints acceleration reference	NONE
setEffortReference	double Vector of joints torque reference	NONE
setJointEffort	double Vector of joints torque reference	NONE
setTemperature	double Vector of joints temperature reference	NONE
setStiffness	double Vector of joints stiffness reference	NONE
setDamping	double Vector of joints damping reference	NONE
enforceJointLimits	double Vector of joints position reference	NONE
enforceVelocityLimit	double Vector of joints velocity reference	NONE
enforceEffortLimit	double Vector of joints torque reference	NONE
getJointLimits	NONE	double Vectors of mechanical position limits
getVelocityLimits	NONE	double Vector of mechanical velocity limits
getEffortLimits	NONE	double Vector of mechanical torque limits
checkJointLimits	BOOL value if position limits are satisfied	double Vector of joints position reference
checkVelocityLimits	BOOL value if velocity limits are satisfied	double Vector of joints velocity reference
checkEffortLimits	BOOL value if torque limits are satisfied	double Vector of joints torque reference
getForceTorque	NONE	ForceTorqueSensor::ConstPtr MAP
getForceTorque	Force Torque Sensor ID	ForceTorqueSensor::ConstPtr
getImu	NONE	ImuSensor::ConstPtr MAP
getImu	IMU Sensor ID	ImuSensor::ConstPtr

RobotInterface

Methods and Class description

Method	Input	Output
sense	NONE	BOOL if the sense is successful
move	NONE	BOOL if the move is successful
setReferenceFrom	ModelInterface	BOOL if the synchronization is allowed

ModelInterface

Methods and Class description

Method	Input	Output
update	NONE	BOOL if the update is successful
syncFrom	XBotInterface	BOOL if the synchronization is allowed
getFloatingBasePose	NONE	BOOL if the the model is floating base
getFloatingBaseTwist	NONE	BOOl if the the model is floating base
setFloatingBasePose	Homogeneous transformation	BOOL if the floating_base_pose frame is valid
setFloatingBaseState	Homogeneous transformation and Twist vector	BOOL if the the model is floating base
setFloatingBaseOrientation	Rotation matrix	BOOL if the the model is floating base
setFloatingBaseTwist	Twist vector	BOOL if the the model is floating base
setFloatingBaseAngularVelocity	Angulator velocity	BOOL if the the model is floating base
getGravity	NONE	Gravity vector
getGravity	Reference frame	Gravity vector, BOOL if the reference frame is valid
setGravity	Gravity vector	NONE
setGravity	Reference frame, Gravity vector	BOOL if the reference frame is a valid link name
getPose	Source and Target frame	Homogeneous transformation, BOOL if the source/target frame are valid
getPose	Source frame	Homogeneous transformation, BOOL if the source frame is valid
getOrientation	Source and Target frame	Rotation matrix, BOOL if the source/target frame are valid
getOrientation	Source frame	Rotation matrix, BOOL if the source frame is valid
getPointPosition	Source, Target frame and source point vector	Point vector, BOOL if the source/target frame are valid
getPointPosition	Source frame and source point vector	Point vector, BOOL if the source frame is valid
getPointAcceleration	Link Name and point vector	Acceleration vector, BOOL if the link name is valid
getCOM	NONE	COM Point vector
getCOM	Reference frame	COM Point vector, BOOL if the reference frame is valid
getCOMVelocity	NONE	COM Velocity vector
getCOMAcceleration	NONE	COM Acceleration vector
getCentroidalMomentum	NONE	Centroidal Momentum
getCentroidalMomentumMatrix	NONE	Centroidal Momentum Matrix
getCentroidalMomentumMatrix	NONE	Centroidal Momentum Matrix, The d(CMM)/dt * qdot term
getVelocityTwist	Link Name	Twist velocity vector, BOOL if the link name is valid
getAccelerationTwist	Link Name	Twist acceleration vector, BOOL if the link name is valid
getRelativeVelocityTwist	Link and Base Link Name	Twist velocity vector, BOOL if the link and base link names are valid
getRelativeAccelerationTwist	Link and Base Link Name	Twist acceleration vector, BOOL if the link and base link names are valid
getJacobian	Link Name	Jacobina Matrix, BOOL if the link name is valid
getJacobian	Link Name and reference point	Jacobina Matrix, BOOL if the link name is valid
getJacobian	Link Name and Target Frame	Jacobina Matrix, BOOL if the link name and target frame are valid
getRelativeJacobian	Target and Base Link Name	Jacobina Matrix, BOOL if the target and base link name are valid
getCOMJacobian	NONE	COM Jacobian Matrix
getCOMJacobian	NONE	COM Jacobian Matrix The d(Jcom)/dt * qdot term
getPosturalJacobian	NONE	Postural Jacobian Matrix
computeJdotQdot	Link Name and point vector	Jacobian time derivative by the joint velocity BOOL if link name is valid
computeRelativeJdotQdot	Target and Base Link Name	Jacobian time derivative by the joint velocity BOOL if target and base link name are valid
getMass	NONE	Mass of the robot
getInertiaMatrix	NONE	Inertia matrix
getInertiaInverse	NONE	Inverse Inertia matrix
computeNonlinearTerm	NONE	The non-linear torques vector
computeGravityCompensation	NONE	The gravity compensation torque vector
computeInverseDynamics	NONE	The inverse dynamics torque vector
computeConstrainedInverseDynamics	Jacobian and weight vector	The inverse dynamics torque vector

ForceTorqueSensor

Methods and Class description

Method	Input	Output
getForce	NONE	Force vector
getToque	NONE	Torque vector
getWrench	NONE	Wrench vector

IMUSensor

Methods and Class description

Method	Input	Output
getOrientation	NONE	Orientation matrix
getOrientation	NONE	Quaternion
getLinearAcceleration	NONE	Linear Acceleration vector
getAngularVelocity	NONE	Angular Velocity vector