how to publish pitch, roll, yaw to robot_pose_ekf

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I have accurate pitch, roll, and yaw values from an IMU, computed on an arduino. I have good serial comm and am already sending odometry data from the encoders and publishing that on /odom. I know that robot_pose_ekf can take in imu_data through sensor_msgs/imu but cannot figure out how to fill in the pitch, roll, yaw data into:

geometry_msgs/Quaternion orientation

float64[9] orientation_covariance

// Row major about x, y, z axes.

Also, do I need to supply values for angular velocity and linear acceleration? I see that those are part of sensor_msgs/imu, but don't want to send serial data at the rate required to accurately keep up with these velocities and it seems that /odom is doing fine with translation.

geometry_msgs/Vector3 angular_velocity

float64[9] angular_velocity_covariance

//Row major about x, y, z axes

geometry_msgs/Vector3

linear_acceleration float64[9] linear_acceleration_covariance

Row major x, y z

I have gone through the nav tutorials, looked (a lot) at turtlebot code, but just cannot decipher the details of computing the quaternion and properly filling in the covariances.

I did use this in /odom for yaw:

//since all odometry is 6DOF we'll need a quaternion created from yaw
geometry_msgs::Quaternion> odom_quat => tf::createQuaternionMsgFromYaw(th);

but how do you do that for pitch, roll, and yaw in the context of /imu?

Ultimately, I hope to figure out how to publish TwistWithCovariance and PoseWithCovariance, but for now I'll settle for just getting the imu data to robot_pose_ekf.

Thanks for the help!