Welcome to Morpheus Chair, the series where you learn how to build and control real robots like the Open Manipulator that we are going to use today.

In this post, we are going to learn the first steps to use the servos used to control a robot arm, so that you can build whatever you want.

We are going to see how to use move the arm with python scripts, which will be the first steps to get a bit close to MoveIt, that great library that we are going to use in the next posts.

If you are new to ROS, I highly recommend you taking the ROS Basics in 5 Days (Python) or ROS Basics in 5 Days (C++) depending on which programming language you prefer.

What we will learn

In short, in this post we will learn :

• How to start a position control for Dynamixel Servos
• How to move the joints publishing in Topics and Services of ROS
• How to capture complex movements to after reproducing them.

Special Thanks to Clarkson University and especially James Carrol and its team for lending us the real robot used in this post.

Let’s get started, shall we?

Where to find what we need

Before we start, is worth mentioning that we need to be careful when working with the robot, because the servo motors are a bit strong. If you get your fingers or face in the trajectory of the arm you can get hurt. Just be careful.

All the code used in this post is available in this ROSject: http://www.rosject.io/l/b368f5c/

You can download the ROSject and run all the commands in your own computer, but if you prefer, you can also run some of the commands online by using ROS Development Studio (ROSDS).

By clicking on http://www.rosject.io/l/b368f5c/ you will have a copy of the ROSject. You should see something like the image below:

You can now open the ROSject by clicking on the Open button or you can download it by clicking on the download button, the ones pointed below by the red and green arrows respectively.

 

The ROSJect mentioned here basically contains the following git repositories on it:

• https://bitbucket.org/theconstructcore/open_manipulator_tc/src/unification/
• https://bitbucket.org/theconstructcore/openmanipulator_morpheus_chair/src/master/
• https://bitbucket.org/theconstructcore/openmanipulator_morpheus_chair_notebooks/src/master/

Open the Notebook

When you open a ROSject, by default you have the Jupyter Notebook automatically open, but if that doesn’t happen, you can manually open it by clicking on Tools -> Jupyter Notebook as shown in the image below:

 

Now on the Jupyter Notebook window let’s click openmanipulator_morpheus_chair_notebooks, then click on Ep2_Dynamixel_Control.ipynb to open it. That notebook contains the instructions we are going to follow in this post.

Once again, be careful when working with real robots because the servo motors used today, for example, they don’t have force feedback, so if somehow you get a finger stuck there it can be really harmful.

Connecting our robot to our local computer

We are assuming you already have the robot. Now let’s connect it to our computer:

 

 

 

 

 

 

 

 

 

once the USB cable is connected, let’s see that our device is connected. If we run the command below:

and disconnect our USB cable we will see a message saying so.

If we now connect our robot again we should see a message saying confirming that.

Note that our device name is called ttyUSB0 according to the logs. The name is important because we have to use it when setting up our program.

Another way of getting more information about the connected device (robot) is with the command below:

which will give us the following output when our robot is not connected.

If we now connect the robot again, we can see it now:

By comparing the messages when our device is connected with the messages when it is not connected, we can clearly see that our robot is the Future Technology Devices International, Ltd FT232H Single HS USB-UART/FIFO IC entry.

Running the controllers

If you downloaded the ROSject and extracted on your home folder (/home/your_username), you should have a simulation_ws folder there. In that folder, we will find a package called dynamixel_workbench_controllers, more precisely on the ~/simulation_ws/src/open_manipulator_tc/dynamixel-workbench/dynamixel_workbench_co
ntrollers path.

On that package, you can find a launch file called position_control.launch, which contains the following content.

Note that on the second line we have the name of our device /dev/ttyUSB0. If in your computer the device has a different name, please update that line with the correct value. You can use the command dmesg -wH to see what is your device’s name.

which in our case showed that our speed baud is 3000000.
Now with everything in place, let’s run the controllers with the commands below:
If everything goes ok, the robot now should be stiff. You shouldn’t be able to move it.

If after running the roslaunch command you have an error like the one below, that happens because you may have a different baud rate. If that is the case, please update the device_name and baud_rate variables on the launch file with the correct values.

If you open a new bash terminal and run rostopic list you should see at least the topics below:

We can also check the services we have. With rosservice list we should have at least the services below:

if for some reason the commands above don’t work, make sure you have ROS_MASTER_URI variable correctly set before running then:

Connecting to the real robot from ROSDS

We mentioned at the beginning of the post that you can run all commands directly on your own computer, but some of them you can also run remotely.

For that, we can just follow the instructions on this post: https://www.theconstructsim.com/use-real-robot-connection-rosdevelopementstudio/

If you followed the instructions correctly, running the commands rostopic list and rosservice list on ROSDS would give you the same output of when we ran on our own computer in which the robot is connected to.

Diving deeper

With rostopic list we can see that we have basically three topics:

There are the basic topics that should always exist with any robot if you use the controller we are using here.

If we want to know more information about a specific topic, like /dynamixel_state for example,  we can just run rostopic info /dynamixel_state. In this command we will see the list of publishers, subscribers and the message type, which for this topic is:

With rostopic echo /dynamixel_state -n1 we can see the model_name of each servo motor.

Note that we are using two different models, which are XM540-W270 and XM430-W350. The former is more powerful.

You can get more information about those models directly on the vendor’s page:

• http://support.robotis.com/en/product/actuator/dynamixel_x/xm_series/xm430-w350.htm
• http://support.robotis.com/en/product/actuator/dynamixel_x/xm_series/xm540-w270.htm

 

With rostopic echo /dynamixel_state -n1 we saw the present_position of each joint in a raw way. By printing the messages published on the /joint_states topic we can see the positions in radians:

Let’s also check the topic /goal_dynamixel_position.

We can see that it has only subscribers, not publishers. That happens because the robot is listening to that topic and nobody is telling the robot to move yet. Let’s publish our first message there so that our arm can move.

Moving the arm

In order to move the arm, we have to publish a message on the /goal_dynamixel_position topic. For that we can use the command below:

We will fill the name, position, velocity, and effort with the same data we got from rostopic echo /joint_states -n1. We are just going to change the value of the first joint to 0.7. At the end our message will be something like:

After pressing enter your robot arm should have moved.

We moved the arm by publishing on a topic. We can also do it by calling a ROS Service.

If you remember, we have some services that can be listed with rosservice list, which are:

The one we are interested in is /joint_command. Let’s move it with the command below:

let’s then fill the parameters:

Bean in mind that unit can be ‘rad‘, ‘raw’ or nothing. If we select raw or nothing, the effect is the same.

Creating a python ROS Node to move the servos

To move the arm with python code we have two options:

• We move it with a Service Client ( /joint_command ): We can only move one joint at a time, but it waits until movement finished.
• We move it publishing in a Topic ( /goal_dynamixel_position ): Moves everything at the same time, but there is no wait.

We are going to create a python class that allows us to use both. If you have the ROSJect, the ROS Package is already created for you at ~/simulation_ws/src/openmanipulator_morpheus_chair/openmanipulator_morpheus_chair_tutorials. We created that package with the commands below:

On the move_openmanipulator.py file we have the following content:

Now in order to move the arm using our script, we can simply:

If everything went as expected, your arm should have moved to different places.

With the command above we called the servers responsible for moving the arm. Remember that we launched our controller with roslaunch dynamixel_workbench_controllers position_control.launch, which basically runs the code defined on ~/simulation_ws/src/open_manipulator_tc/dynamixel-workbench/dynamixel_workbench_controllers/src/position_control.cpp. Please have a look at that file if you want to understand how the Service Server and the Topic Subscriber move the arms.

Manually extracting the servo positions

At this point, you might be wondering how we know the radian value for the positions we want the arm to move to. We do that by disabling the controllers, moving the arm to the desired position and then checking the joint states, as explained here:

https://youtu.be/qEKXi5hNRdg?t=2253

So, that is the post for today. Remember that we have a video showing all the content of this post. Please have a look and leave your thoughts on the comments section of the video. Also, feel free to subscribe to our channel on YouTube because we are publishing ROS-related content every day.

Below we have the video:

In the next post, we are going to learn how to use the MoveIt! Setup Assistant, so that we can easily move our robot arm later.

Edited by Miguel Angel and Ruben Alves.