Add cart-pole demo

[christophfroehlich]

Similar to ros-controls/gazebo_ros2_control#172

The difference to gazebo_ros2_control is that it also works with position command interface of the cart. gz_ros2_control does not directly set the position but implements a position controller with velocity output:

gz_ros2_control/gz_ros2_control/src/gz_system.cpp

Lines 632 to 651 in 81dca71

	} else if (this->dataPtr->joints_[i].joint_control_method & POSITION) {
	// Get error in position
	double error;
	error = (this->dataPtr->joints_[i].joint_position -
	this->dataPtr->joints_[i].joint_position_cmd) * *this->dataPtr->update_rate;
	// Calculate target velcity
	double target_vel = -this->dataPtr->position_proportional_gain_ * error;
	auto vel =
	this->dataPtr->ecm->Component<sim::components::JointVelocityCmd>(
	this->dataPtr->joints_[i].sim_joint);
	if (vel == nullptr) {
	this->dataPtr->ecm->CreateComponent(
	this->dataPtr->joints_[i].sim_joint,
	sim::components::JointVelocityCmd({target_vel}));
	} else if (!vel->Data().empty()) {
	vel->Data()[0] = target_vel;
	}

[ahcorde]

Thank you @christophfroehlich for the examples, do you want to backport this ?

[christophfroehlich]

yes please!

[christophfroehlich]

@Mergifyio backport humble iron

[mergify]

backport humble iron

✅ Backports have been created

• #290 Add cart-pole demo (backport #289) has been created for branch humble but encountered conflicts
• #291 Add cart-pole demo (backport #289) has been created for branch iron