faq.md

Frequently Asked Questions (FAQ)

Contents:

• Remind me of that command again...
• Troubleshooting :-(
• Control algorithm hints

Remind me of that command again...

This section contains those Frequently Asked Questions that pertain to "those commands" that are regularly used to interact with the D-FaLL-System and bring your control algorithms to life through a Crazyflie quadrotor.

How do I to get a clean version of the repository?

Step 1) Change to the directory where you have a copy of the D-FaLL-System repository under version control. On a machine setup as per the instructions this is:

cd ~/work/D-FaLL-System

Step 2) It is good practice always check the status of the git repository:

git status

Step 3) Remove all changes on the current branch since the previous pull

git checkout .

Step 4) Switch to the master branch of the repository

git checkout master

Step 5) Update to the latest version of the master branch from the remote repository

git pull

How do I launch the Student GUI?

The Student GUI can be launched from a terminal window with the following command:

roslaunch d_fall_pps Student.launch

This can be run from any directory because d_fall_pps is defined as an environment variable that points to the absolute foler location ~/work/D-FaLL-System/pps_ws/src/d_fall_pps/launch/

How do I make changes to a *.cpp file take effect?

In essence you need to re-compile the code and re-launch all ROS nodes.

Step 1) Kill all ROS processes that are running nodes from the D-FaLL-System repository, i.e., you must kill the Student GUI node but you do not need to kill an rqt plotting node. To kill a process press Ctrl+c from the terminal window that is running the process.

Step 2) In a terminal window, change to the pps_ws folder of the repository (where ws stands for workspace. On a machine setup as per the instructions this is:

cd ~work/D-FaLL-System/pps_ws/

Step 3) Compile the repository, which includes your changes, using the command:

catkin_make

This will compile all the code, or throw an error if your changes have introduced errors that prevent the compilation from completing successfully.

Step 4) Relaunch the Student GUI

Troubleshooting :-(

This section contains those Frequently Asked Questions that pertain to commonly encountered errors that render the D-FaLL-System as seemingly "broken".

The Student GUI fails to launch due to connection errors

This can happen for a variety of reasons, and generally relates to the local computer not being able to access the "ROS core" that is running on the Teacher's computer. Such an error can be fixed by one or all of the following steps:

Step 1) Check that the network cable is actually plugged in, and that the lights of the ethernet port are actually flashing (this needs to be checked both for the local computer and for the Teacher's computer that is running the "ROS core")

Step 2) Check that the local computer's operating system is actually connected to the local network. This can be checked via the network icon on the right of the panel in Ubuntu (i.e., at the top right of the screen just near the battery level indicator)

Step 3) Sometimes other network connections disrupt the connection to the local network on which the ROS system is communicating. Disable all other network connections via the network icon on the right of the panel in Ubuntu (i.e., disable WiFi)

Step 4) Check that the Teacher's computer is actually running a "ROS core".

Step 5) Close all terminal windows (and hence kill all processes running via a terminal window), then open a new terminal window and try to launch the Student GUI again.

To understand how Step 5) could actually fix the problem, consider that ~/.bashrc is run when a new terminal window is opened, and as part of installing the D-FaLL-System the following line is added to the .bashrc: source <catkin workspace>/src/d_fall_pps/launch/Config.sh And if you look at the Config.sh file in the repository you see that it defines environment variable relating to the ROS_MASTER_URI, ROS_IP, and ROS_NAMESPACE. On occassion these are not properly defined on start up or are changed, hence closing and re-launching Terminal can resolve the problem.

The Student GUI fails to launch due to Vicon related errors

The Vicon motion capture system provides position and attitude information via a piece of propriety software named the Datastream SDK. Hence, the proprietry software cannot be ditributed with this piece of Open Source software, and the error is likley cause by the software not being present on the local computer.

To check whether the Vicon Datastream software is properly added to the local computer, goto the Installation wiki page and follow the instructions under the title "Vicon Datastream SDK Installation".

The main requirements are that:

• The DataStreamClient.h header file needs to be located in ~/pps_ws/src/d_fall_pps/lib/vicon/,
• The libViconDataStreamSDK_CPP.so shared object needs to be located in ~/pps_ws/src/d_fall_pps/lib/vicon/, and
• A number of files of the form libboost_* should also be located in ~/pps_ws/src/d_fall_pps/lib/vicon/.

Control algorithm hints

This section contains those Frequently Asked Questions that pertain to common coding errors in the implementation of your control algorithm that causes the Crazyflie to behave in an undesirable manner (remember that computers always follow the instructions they are given).

Why does my crazyflie drop to the ground when I request a large setpoint change in altitude?

The per motor command that your algorithm requests from the Crazyflie via setting the response.controlOutput.motorCmd1 property in the function calculateControlOutput is subseuently cast as a uint16 type variable. Hence you should consider saturating the per motor commands computed by your algorithm before setting the value of response.controlOutput.motorCmd1.

Why does my crazyflie stop and Stuent GUI freeze when I enable my custom controller?

This is most likely caused by your code causing a segmentation fault, which is commonly caused due to a division by zero. This is often observed in relation to the calculation of sampling time from frequency. You need to think carefully about the order in which variables are instantiated, set to a value, and subsequently used in your code. Specifically, in the template controller code provided contains the control_frequency variable that is set to a value loaded from the .yaml parameter file. It is common to instantiate a new variable as t_s = 1.0 / control_frequency. However, if the t_s variable is accidentally used before it is set to this value then it is possible that you are dividing by zero. Remember also that the values from the .yaml parameter file are loaded during runtime, as they can be changed at any time during the running of the application, so those values are not present during compile time of your code.

How can we send and receive the Crazyflies position between our controllers?

In order to make information available between the laptops, you can use the publish and subscribe messaging framework availble in ROS.

In order to publish the position of your Crazyflie you need to introduce a variable of type ros::Publisher and you can use the Setpoint message type to publish the x,y,z, and yaw of your Crazyflie.

To add a publisher follow these steps:

• Add a ros::Publisher type class variable to your CustomControllerService.cpp file:

ros::Publisher my_current_xyz_yaw_publisher

• In the main() function of your CustomControllerService.cpp file initialise your publisher:

ros::NodeHandle nodeHandle("~");
my_current_xyz_yaw_publisher = nodeHandle.advertise<Setpoint>("/<ID>/my_current_xyz_yaw_topic", 1);

where <ID> should be replaced by the ID of your computer, for example the number 7, my_current_xyz_yaw_topic is the name of the message topic that will be published, and Setpoint specifies the stucture of the message as defined in the file /msg/Setpoint.msg and included with:

#include "d_fall_pps/Setpoint.h"

added at the top of your CustomControllerService.cpp file.

• In the calculateControlOutput function of your CustomControllerService.cpp file you can publish the current position of your Crazyflie by adding the following:

Setpoint temp_current_xyz_yaw;
temp_current_xyz_yaw.x   = request.ownCrazyflie.x;
temp_current_xyz_yaw.y   = request.ownCrazyflie.y;
temp_current_xyz_yaw.z   = request.ownCrazyflie.z;
temp_current_xyz_yaw.yaw = request.ownCrazyflie.yaw;
my_current_xyz_yaw_publisher.publish(temp_current_xyz_yaw);

In order to subscribe to a message that is published in the way described above from another student's laptop, you need to define a variable of type ros::Subscriber and you specify the function that should be called each time a message is receive on the topic to which you subscribe.

To subscribe to a topic follw these steps:

• In the main() function of your CustomControllerService.cpp file initialise the subscriber:

ros::NodeHandle nodeHandle("~");
ros::Subscriber xyz_yaw_to_follow_subscriber = nodeHandle.subscribe("/<ID>/my_current_xyz_yaw_topic", 1, xyz_yaw_to_follow_callback);

where <ID> should be replaced by the ID of the computer from which you wish to subscribe, for example the number 8, my_current_xyz_yaw_topic must match the name of the topic being published, and xyz_yaw_to_follow_callback is the function in your CustomControllerService.cpp file that will be called when the message is received.

• To add the callback function, first add the following function prototype to the top of your CustomControllerService.cpp file:

void xyz_yaw_to_follow_callback(const Setpoint& newSetpoint);

• Implement the xyz_yaw_to_follow_callback function in your CustomControllerService.cpp file to achieve the behaviour desired, the following example makes the setpoint of my own Crazyflie equal to the position received in the message:

void xyz_yaw_to_follow_callback(const Setpoint& newSetpoint)
{
		setpoint[0] = newSetpoint.x;
		setpoint[1] = newSetpoint.y;
		setpoint[2] = newSetpoint.z;
		setpoint[3] = newSetpoint.yaw;
}

How can my controller receive the position of another object recognised by the Vicon system?

The calculateControlOutput function of your CustomControllerService.cpp file is called everytime that the Vicon system provides new data, where this data is provided to the calculateControlOutput function in the request variable and the calculated control output to apply is returned in the response variable. In this way the calculateControlOutput function is performing in ROS what is referred to as a service. This service is advertised in the main() function of your CustomControllerService.cpp file by the line of code:

ros::ServiceServer service = nodeHandle.advertiseService("CustomController", calculateControlOutput);

This service is called on from the PPSClient.cpp file, and it is expected to adhere to the data structures described in /srv/Controller.srv:

CrazyflieData ownCrazyflie
CrazyflieData[] otherCrazyflies
---
ControlCommand controlOutput

Hence why the position and attitude information of your own Crazyflie is accessed from inside the calculateControlOutput function via request.ownCrazyflie.{x,y,z,roll,pitch,yaw}.

By default the property otherCrazyflies is left empty when the service request is constructed in the PPSClient.cpp. Thus, in order to have access to the position of another object recognised by the Vicon system you can edit the PPSClient.cpp as per the following steps:

• In the PPSClient.cpp file locate the implementation of the function viconCallback, which has the full prototype:

void viconCallback(const ViconData& viconData)

where the argument viconData has the structure defined in ViconData.msg:

CrazyflieData[] crazyflies
UnlabeledMarker[] markers

thus viconData.crazyflies is an array of all the objects that the Vicon system is attempting to recognise, where each element of the array adheres to the following data structure:

string crazyflieName
float64 x
float64 y
float64 z
float64 roll
float64 pitch
float64 yaw
float64 acquiringTime #delta t
bool occluded

• At the start of the viconCallback in the PPSClient.cpp file (i.e., before the for loop), add the following varible declaration:

CrazyflieData otherObject;

where the plan is to fill in this variable with the data about the object of interest and then pass it as part of the service request in the .otherCrazyflies property.

• In the viconCallback function of the PPSClient.cpp file, just after the variable declaration you added, add the following for loop:

for(std::vector<CrazyflieData>::const_iterator it = viconData.crazyflies.begin(); it != viconData.crazyflies.end(); ++it)
{
		CrazyflieData thisObject = *it;

		if ( thisObject.crazyflieName == "name_of_object_I_am_searching_for" )
		{
				otherObject = thisCrazyflie;
				break;
		}
}

This for loop iterates over all the objects provided to the viconCallback function, and fills in the otherObject variable with the object whose name matches the string "name_of_object_I_am_searching_for".

• Define a new object via the Vicon Tracker software, give it a meaningful name in the Vicon software, and replace the string "name_of_object_I_am_searching_for" with the exact string that you used in the Vicon saftware.

• In the viconCallback function of the PPSClient.cpp file, look for the following lines of code:

Controller controllerCall;
CrazyflieData local = global;
coordinatesToLocal(local);
controllerCall.request.ownCrazyflie = local;

and immediately after these existing lines of code, add the following new lines of code:

coordinatesToLocal(otherObject);
controllerCall.request.otherCrazyflies.push_back(otherObject);

Now the .otherCrazyflies property of the request variable that is passed to the calculateControlOutput function of your CustomControllerService.cpp file will contain the position of the otherObject as the first entry in the array, i.e., you can access the data via request.otherCrazyflies[0].{x,y,z,roll,pitch,yaw}.

I added some advertise, publish, subscribe topics but I am not getting the desired behaviour, what debugging tools are avilable?

If your code changes compile successfully and your node runs without crashing, then the command line tool rostopic is the most useful tool for debugging errors. Open a new terminal and type rostopic to read the desription and help files.

After launching your node, open a separate command window and type the command

rostopic list

this will list all the topics that are currently active, including both advertised and subscribed to topics.

Common pitfalls to watch out for are:

• If there are two topics with a very similar name, then double check the spelling of the topic is identical in both the .advertise and .subscribe lines of code.
• Even if the topic name is exactly matching, the name space may be different.
• If you advertise or subscribe to a topic from with a block of code enclosed by curly braces, for example:

ros::Subscriber my_subscriber;
if (true)
{
		ros::NodeHandle nodeHandle("~");
		my_subscriber = nodeHandle.subscribe("/fortytwo",1,fortytwoCallback);
}

then the subscriber only exists while the code between the {} is being exectuted and it is removed after the if statement has finished executing. To make the subscriber persist, then you need to declare the subscriber varaible outside of the if statement's context, i.e.,

ros::Subscriber my_subscriber;
if (true)
{
		ros::NodeHandle nodeHandle("~");
		my_subscriber = nodeHandle.subscribe("/fortytwo",1,fortytwoCallback);
}

This would work if it is in the main() function of a node, but if it is in another function then you may need to declare the subscriber as a class variable.