2 posts tagged with "embedded"

This is the second part of the "ROS2 Control with the JetBot" series, where I show you how to get a JetBot working with ROS2 Control! This is a sequel to the part 1 blog post, where I showed how to drive the JetBot's motors using I²C and PWM with code written in C++.

In this post, I show the next step in making ROS2 Control work with the WaveShare JetBot - wrapping the motor control code in a System. I'll walk through some concepts, show the example repository for ROS2 Control implementations, and then show how to implement the System for JetBot and see it running.

This post is also available in video form - check the video link below if you want to follow along!

ROS2 Control Concepts​

First, before talking about any of these concepts, there's an important distinction to make: ROS Control and ROS2 Control are different frameworks, and are not compatible with one another. This post is focused on ROS2 Control - or as their documentation calls it, ros2_control.

ros2_control's purpose is to simplify integrating new hardware into ROS2. The central idea is to separate controllers from systems, actuators, and sensors. A controller is responsible for controlling the movement of a robot; an actuator is responsible for moving a particular joint, like a motor moving a wheel. There's a good reason for this separation: it allows us to write a controller for a wheel configuration, without knowing which specific motors are used to move the wheels.

Let's take an example: the Turtlebot and the JetBot are both driven using one wheel on each side and casters to keep the robots level. These are known as differential drive robots.

As the motor configuration is the same, the mathematics for controlling them is also the same, which means we can write one controller to control either robot - assuming we can abstract away the code to move the motors.

In fact, this is exactly what's provided by the ros2_controllers library. This library contains several standard controllers, including our differential drive controller. We could build a JetBot and a Turtlebot by setting up this standard controller to be able to move their motors - all we need to do is write the code for moving the motors when commanded to by the controller.

ros2_control also provides the controller manager, which is used to manage resources and activate/deactivate controllers, to allow for advanced functionality like switching between controllers. Our use case is simple, so we will only use it to activate the controller. This architecture is explained well in the ros2_control documentation - see the architecture page for more information.

This post shows how to perform this process for the JetBot. We're going to use the I²C and motor classes from the previous post in the series to define a ros2_control system that will work with the differential drive controller. We use a System rather than an Actuator because we want to define one class that can control both motors in one write call, instead of having two separate Actuators.

ROS2 Control Demos Repository​

To help us with our ros2_control system implementation, the ros2_control framework has helpfully provided us with a set of examples. One of these examples is exactly what we want - building a differential drive robot (or diffbot, in the examples) with a custom System for driving the motors.

The repository has a great many examples available. If you're here to learn about ros2_control, but not to build a diffbot, there are examples of building simulations, building URDF files representing robots, externally connected sensors, and many more.

We will be using example 2 from this demo repository as a basis, but stripping out anything we don't require right now, like supporting simulation; we can return these parts in later iterations as we come to understand them.

JetBot System Implementation​

In this section, I'll take you through the key parts of my JetBot System implementation for ros2_control. The code is available on Github - remember that this repository will be updated over time, so select the tag jetbot-motors-pt2 to get the same code version as in this article!

Components are libraries, not nodes​

ros2_control uses a different method of communication from the standard ROS2 publish/subscribe messaging. Instead, the controller will load the code for the motors as a plugin library, and directly call functions inside it. This is the reason we had to rewrite the motor driver in C++ - it has to be a library that can be loaded by ros2_control, which is written in C++.

Previously, we wrote an example node that span the wheels using the motor driver; now we are replacing this executable by a library that can be loaded by ros2_control. In CMakeLists.txt, we can see:

add_library(${PROJECT_NAME}
  SHARED
  hardware/src/jetbot_system.cpp
  hardware/src/i2c_device.cpp
  hardware/src/motor.cpp
)

...

pluginlib_export_plugin_description_file(hardware_interface jetbot_control.xml)

These are the lines that build the JetBot code as a library instead of a system, and export definitions that show it is a valid plugin library to be loaded by ros2_control. A new file, jetbot_control.xml, tells ros2_control more information about this library to allow it to be loaded - in this case, the library name and ros2_control plugin type (SystemInterface - we'll discuss this more in the Describing the JetBot section).

Code Deep Dive​

For all of the concepts in ros2_control, the actual implementation of a System is quite simple. Our JetBotSystemHardware class extends the SystemInterface class:

class JetBotSystemHardware : public hardware_interface::SystemInterface {

In the private fields of the class, we create the fields that we will need during execution. This includes the I2CDevice and two Motor classes from the previous post, along with two vectors for the hardware commands and hardware velocities:

 private:
  std::vector<MotorPins> motor_pin_sets_;
  std::vector<Motor> motors_;
  std::shared_ptr<I2CDevice> i2c_device_;
  std::vector<double> hw_commands_;
  std::vector<double> hw_velocities_;

Then, a number of methods need to be overridden from the base class. Take a look at the full header file to see them, but essentially it boils down to three concepts:

1. export_state_interfaces/export_command_interfaces: report the state and command interfaces supported by this system class. These interfaces can then be checked by the controller for compatibility.
2. on_init/on_activate/on_deactivate: lifecycle methods automatically called by the controller. Different setup stages for the System occur in these methods, including enabling the motors in the on_activate method and stopping them in on_deactivate.
3. read/write: methods called every controller update. read is for reading the velocities from the motors, and write is for writing requested speeds into the motors.

From these, we use the on_init method to:

1. Initialize the base SystemInterface class
2. Read the pin configuration used for connecting to the motors from the parameters
3. Check that the provided hardware information matches the expected information - for example, that there are two velocity command interfaces
4. Initialize the I2CDevice and Motors

This leaves the System initialized, but not yet activated. Once on_activate is called, the motors are enabled and ready to receive commands. The read and write methods are then repeatedly called for reading from and writing to the motors respectively. When it's time to shutdown, on_deactivate will stop the motors, and the destructors of the classes perform any required cleanup. There are more lifecycle states that could potentially be used for a more complex system - these are documented in the ros2 demos repository.

This System class, plus the I2CDevice and Motor classes, are compiled into the plugin library, ready to be loaded by the controller.

Describing the JetBot​

The SystemInterface then comes into play when describing the robot. The description folder from the example contains the files that define the robot, including its ros2_control configuration, simulation configuration, and materials used to represent it during simulation. As this implementation has been pared down to basics, only the ros2_control configuration with mock hardware flag have been kept in.

The jetbot.ros2_control.xacro file defines the ros2_control configuration needed to control the robot. It uses xacro files to define this configuration, where xacro is a tool that extends XML files by allowing us to define macros that can be referenced in other files:

<xacro:macro name="jetbot_ros2_control" params="name prefix use_mock_hardware">

In this case, we are defining a macro for the ros2_control part of the JetBot that can be used in the overall robot description.

We then define the ros2_control portion with type system:

<ros2_control name="${name}" type="system">

Inside this block, we give the path to the plugin library, along with the parameters needed to configure it. You may recognize the pin numbers in this section!

<hardware>
  <plugin>jetbot_control/JetBotSystemHardware</plugin>
  <param name="pin_enable_0">8</param>
  <param name="pin_pos_0">9</param>
  <param name="pin_neg_0">10</param>
  <param name="pin_enable_1">13</param>
  <param name="pin_pos_1">12</param>
  <param name="pin_neg_1">11</param>
</hardware>

This tells any controller loading our JetBot system hardware which pins are used to drive the PWM chip. But, we're not done yet - we also need to tell ros2_control the command and state interfaces available.

ros2_control Joints, Command Interfaces, and State Interfaces​

ros2_control uses joints to understand what the movable parts of a robot are. In our case, we define one joint for each motor.

Each joint then defines a number of command and state interfaces. Each command interface accepts velocity, position, or effort commands, which allows ros2_control controllers to command the joints to move as it needs. State interfaces report a measurement from the joint out of velocity, position, or effort, which allows ros2_control to monitor how much the joint has actually moved and adjust itself. In our case, each joint accepts velocity commands and reports measured velocity - although we configure the controller to ignore the velocity, because we don't actually have a sensor like an encoder in the JetBot. This means we're using open loop control, as opposed to closed loop control.

<joint name="${prefix}left_wheel_joint">
  <command_interface name="velocity"/>
  <state_interface name="velocity"/>
</joint>

Closed loop control is far more accurate than open loop control. Imagine you're trying to sprint exactly 100 metres from a starting line, but you have to do it once blindfolded, and once again without a blindfold and line markings every ten metres - which run is likely to be more accurate? In the JetBot, there's no sensor to measure how much it has moved, so the robot is effectively blindfolded and guessing how far it has travelled. This means our navigation won't be as accurate - we are limited by hardware.

JetBot Description​

With the ros2_control part of the JetBot defined, we can import and use this macro in the overall JetBot definition. As we've stripped out all other definitions, such as simulation parameters, this forms the only part of the overall JetBot definition:

<xacro:include filename="$(find jetbot_control)/ros2_control/jetbot.ros2_control.xacro" />
<xacro:jetbot_ros2_control
    name="JetBot" prefix="$(arg prefix)" use_mock_hardware="$(arg use_mock_hardware)"/>

Let's summarize what we've created so far:

1. A plugin library capable of writing commands to the JetBot motors
2. A ros2_control xacro file, describing the plugin to load and the parameters to give it
3. One joint per motor, each with a velocity command and state interface
4. An overall description file that imports the ros2_control file and calls the macro

Now when we use xacro to build the overall description file, it will import the ros2_control file macro and expand it, giving a complete robot description that we can add to later. It's now time to look at creating a controller manager and a differential drive controller.

Creating A Controller​

So far, we've defined a JetBot using description files. Now we want to be able to launch ros2_control and tell it what controller to create, how to configure it, and how load our defined JetBot. For this, we use the jetbot_controllers.yaml file.

We start with the controller_manager. This is used to load one or more controllers and swap between them. It also makes sure that resources are only used by one controller at a time and manages the change between controllers. In our case, we're only using it to load and run one controller:

controller_manager:
  ros__parameters:
    update_rate: 10  # Hz

    jetbot_base_controller:
      type: diff_drive_controller/DiffDriveController

We tell the manager to update at 10Hz and to load the diff_drive_controller/DiffDriveController controller. This is the standard differential drive controller discussed earlier. If we take a look at the information page, we can see a lot of configuration for it - we provide this configuration in the same file.

We define that the controller is open loop, as there is no feedback. We give the names of the joints for the controller to control - this is how the controller knows it can send velocities to the two wheels implemented by our system class. We also set velocity limits on both linear and angular movement:

linear.x.max_velocity: 0.016
linear.x.min_velocity: -0.016
angular.z.max_velocity: 0.25
angular.z.min_velocity: -0.25

These numbers are obtained through experimentation! ros2_control operates using target velocities specified in radians per second [source]. However, the velocity we send to motors doesn't correspond to radians per second - the range of -1 to +1 is the minimum velocity up to maximum velocity of the motors, which change with the battery level of the robot. I obtained the numbers given through experimentation - these move the robot at a reasonable pace.

Finally, we supply the wheel separation and radius, specified in metres. I measured these from my own robot. The separation is the minimum separation between wheels, and the radius is from the centre of one wheel to the very edge:

wheel_separation: 0.104
wheel_radius: 0.032

With this, we have described how to configure a controller manager with a differential drive controller to control our JetBot!

Launching the Controller​

The last step here is to provide a launch script to bring everything up. The example again provides us with the launch script, including a field that allows us to launch with mock hardware if we want - this is great for testing that everything loads correctly on a system that doesn't have the right hardware.

The launch script goes through a few steps to get to the full ros2_control system, starting with loading the robot description. We specify the path to the description file relative to the package, and use the xacro tool to generate the full XML for us:

# Get URDF via xacro
robot_description_content = Command(
    [
        PathJoinSubstitution([FindExecutable(name="xacro")]),
        " ",
        PathJoinSubstitution(
            [FindPackageShare("jetbot_control"), "urdf", "jetbot.urdf.xacro"]
        ),
        " ",
        "use_mock_hardware:=",
        use_mock_hardware,
    ]
)
robot_description = {"robot_description": robot_description_content}

Following this, we load the jetbot controller configuration:

robot_controllers = PathJoinSubstitution(
    [
        FindPackageShare("jetbot_control"),
        "config",
        "jetbot_controllers.yaml",
    ]
)

With the robot description and the robot controller configuration loaded, we can pass these to the controller manager:

control_node = Node(
    package="controller_manager",
    executable="ros2_control_node",
    parameters=[robot_description, robot_controllers],
    output="both",
)

Finally, we ask the launched controller manager to start up the jetbot_base_controller:

robot_controller_spawner = Node(
    package="controller_manager",
    executable="spawner",
    arguments=[
        "jetbot_base_controller",
        "--controller-manager",
        "/controller_manager",
    ],
)

All that remains is to build the package and launch the new launch file!

ros2_control Launch Execution​

This article has been written from the bottom up, but now we have the full story, we can look from the top down:

1. We launch the JetBot launch file defined in the package
2. The launch file spawns the controller manager, which is used to load controllers and manage resources
3. The launch file requests that the controller manager launches the differential drive controller
4. The differential drive controller loads the JetBot System as a plugin library
5. The System connects to the I2C bus, and hence, the motors
6. The controller can then command the System to move the motors as requested by ROS2 messaging

Running on the JetBot​

To try the package out, we first need a working JetBot. If you're not sure how to do the initial setup, I've created a video on exactly that:

With the JetBot working, we can create a workspace and clone the code into it. Use VSCode over SSH to execute the following commands:

mkdir ~/dev_ws
cd ~/dev_ws
git clone https://github.com/mikelikesrobots/jetbot-ros-control -b jetbot-motors-pt2
cp -r ./jetbot-ros-control/.devcontainer .

Then use the Dev Containers plugin to rebuild and reload the container. This will take a few minutes, but the step is crucial to allow us to run ROS2 Humble on the JetBot, which uses an older version of Ubuntu. Once complete, we can build the workspace, source it, and launch the controller:

source /opt/ros/humble/setup.bash
colcon build --symlink-install
source install/setup.bash
ros2 launch jetbot_control jetbot.launch.py

This should launch the controller and allow it to connect to the motors successfully. Now we can use teleop_twist_keyboard to test it - but with a couple of changes.

First, we now expect messages to go to /jetbot_base_controller/cmd_vel topic instead of the previous /cmd_vel topic. We can fix that by asking teleop_twist_keyboard to remap the topic it normally publishes to.

Secondly, we normally expect /cmd_vel to accept Twist messages, but the controller expects TwistStamped messages. There is a parameter for teleop_twist_keyboard that turns its messages into TwistStamped messages, but while trying it out I found that the node ignored that parameter. Checking it out from source fixed it for me, so in order to run the keyboard test, I recommend building and running from source:

git clone https://github.com/ros2/teleop_twist_keyboard
colcon build --symlink-install
source install/setup.bash
ros2 run teleop_twist_keyboard teleop_twist_keyboard \
    --ros-args \
    -p stamped:=true \
    -r /cmd_vel:=/jetbot_base_controller/cmd_vel

Once running, you should be able to use the standard keyboard controls written on screen to move the robot around. Cool!

Let's do one more experiment, to see how the configuration works. Go into the jetbot_controllers.yaml file and play with the maximum velocity and acceleration fields, to see how the robot reacts. Relaunch after every configuration change to see the result. You can also tune these parameters to match what you expect more closely.

That's all for this stage - we have successfully integrated our JetBot's motors into a ros2_control System interface!

Next Steps​

Having this setup gives us a couple of options going forwards.

First, we stripped out a lot of configuration that supported simulation - we could add this back in to support Gazebo simulation, where the robot in the simulation should act nearly identically to the real life robot. This allows us to start developing robotics applications purely in simulation, which is likely to be faster due to the reset speed of the simulation, lack of hardware requirements, and so on.

Second, we could start running a navigation stack that can move the robot for us; for example, we could request that the robot reaches an end point, and the navigation system will plan a path to take the robot to that point, and even face the right direction.

Stay tuned for more posts in this series, where we will explore one or both of these options, now that we have the robot integrated into ROS2 using ros2_control.

Welcome to a new series - setting up the JetBot to work with ROS2 Control interfaces! Previously, I showed how to set up the JetBot to work from ROS commands, but that was a very basic motor control method. It didn't need to be advanced because a human was remote controlling it. However, if we want autonomous control, we need to be able to travel a specific distance or follow a defined path, like a spline. A better way of moving a robot using ROS is by using the ROS Control interfaces; if done right, this means your robot can autonomously follow a path sent by the ROS navigation stack. That's our goal for this series: move the JetBot using RViz!

The first step towards this goal is giving ourselves the ability to control the motors using C++. That's because the controllers in ROS Control requires extending C++ classes. Unfortunately, the existing drivers are in Python, meaning we'll need to rewrite them in C++ - which is a good opportunity to learn how the serial control works. We use I²C to talk to the motor controller chip, an AdaFruit DC Motor + Stepper FeatherWing, which sets the PWM duty cycle that makes the motors move. I'll refer to this chip as the FeatherWing for the rest of this article.

First, we'll look at how I²C works in general. We don't need to know this, but it helps to understand how the serial communication works so we can understand the function calls in the code better.

Once we've seen how I²C works, we'll look at the commands sent to set up and control the motors. This will help us understand how to translate the ROS commands into something our motors will understand.

The stage after this will be in another article in this series, so stay tuned!

This post is also available in video form - check the video link below if you want to follow along!

Inter-Integrated Circuit (IIC/I²C)​

I²C can get complicated! If you want to really dive deep into the timings and circuitry needed to make it work, this article has great diagrams and explanations. The image I use here is from the same site.

SDA and SCLK​

I²C is a serial protocol, meaning that it sends bits one at a time. It uses two wires called SDA and SCLK; together, these form the I²C bus. Multiple devices can be attached to these lines and take it in turns to send data. We can see the bus in the image below:

Data is sent on the SDA line, and a clock signal is sent on the SCLK line. The clock helps the devices know when to send the next bit. This is a very helpful part of I²C - the speed doesn't need to be known beforehand! Compare this with UART communication, which has two lines between every pair of devices: one to send data from A to B, and one to send data from B to A. Both devices must know in advance how fast to send their data so the other side can understand it. If they don't agree on timing, or even if one side's timing is off, the communication fails. By using a line for the clock in I²C, all devices are given the timing to send data - no prior knowledge required!

The downside of this is that there's only one line to send data on: SDA. The devices must take it in turns to send data. I²C solves this by designating a master device and one or more slave devices on the bus. The master device is responsible for sending the clock signal and telling the slave devices when to send data. In our case, the master device is the Jetson Nano, and the slave device is the FeatherWing. We could add extra FeatherWing boards to the bus, each with extra motors, and I²C would allow the Jetson to communicate with all of them - but this brings a new problem: how would each device know when it is the one meant to respond to a request?

Addressing​

The answer is simple. Each slave device on the bus has a unique address. In our case, the FeatherWing has a default address of 0x60, which is hex notation for the number 96. In fact, if we look at the Python version of the JetBot motor code, we can see the following:

if 96 in addresses:

Aha! So when we check what devices are available on the bus, we see device 96 - the FeatherWing.

When the Jetson wants to talk to a specific device, it starts by selecting the address. It sends the address it wants on the SDA line before making a request, and each device on the bus can check that address with the address it is expecting. If it's the wrong address, it ignores the request. For example, if the FeatherWing has a device of 0x61, and the Jetson sends the address 0x60, the FeatherWing should ignore that request - it's for a different address.

But, how do we assign an address to each device?

The answer comes by looking at the documentation for the FeatherWing:

By soldering different pins on the board together, we can tell the board a new address to take when it starts up. That way, we can have multiple FeatherWing boards, each with a different address, all controllable from the Jetson. Cool!

Pulse Width Modulation (PWM)​

With that, we have a basic understanding of how the Jetson controls I²C devices connected to it, including our FeatherWing board. Next we want to understand how the FeatherWing controls the motors, so we can program the Jetson to issue corresponding commands to the FeatherWing.

The first step of this is PWM itself - how does the board run a motor at a particular speed? The following step is the I²C commands needed to make the FeatherWing do that. I'll start with the speed.

Motor Wires​

Each JetBot motor is a DC motor with two wires. By making one wire a high voltage with the other wire at 0V, the motor will run at full speed in one direction; if we flip which wire is the high voltage, the motor will turn in the opposite direction. We will say that the wire that makes the motor move forwards is the positive terminal, and the backwards wire is the negative terminal.

We can see the positive (red) wire and the negative (black) wire from the product information page:

That means we know how to move the motor at full speed:

1. Forwards - red wire has voltage, black wire is 0V
2. Backwards - black wire has voltage, red wire is 0V

There are another couple of modes that we should know about:

1. Motor off - both wires are 0V
2. Motor brakes - both wires have voltage

Which gives us full speed forwards, full speed backwards, brake, and off. How do we move the motor at a particular speed? Say, half speed forwards?

Controlling Motor Speed​

The answer is PWM. Essentially, instead of having a wire constantly have high voltage, we turn the voltage on and off. For half speed forwards, we have the wire on for 50% of the time, and off for 50% of the time. By switching between them really fast, we effectively make the motor move at half speed because it can't switch on and off fast enough to match the wire - the average voltage is half the full voltage!

That, in essence, is PWM: switch the voltage on the wire very fast from high to low and back again. The proportion of time spent high determines how much of the time the motor is on.

We can formalize this a bit more with some language. The frequency is how quickly the signal changes from high to low and back. The duty cycle is the proportion of time the wire is on. We can see this in the following diagram from Cadence:

We can use this to set a slower motor speed. We choose a high enough frequency, which in our case is 1.6 kHz. This means the PWM signal goes through a high-low cycle 1600 times per second. Then if we want to go forwards at 25% speed, we cane set the duty cycle of the positive wire to our desired speed - 25% speed means 25% duty cycle. We can go backwards at 60% speed by setting 60% duty cycle on the negative wire.

Producing this signal sounds very manual, which is why the FeatherWing comes with a dedicated PWM chip. We can use I²C commands from the Jetson to set the PWM frequency and duty cycle for a motor, and it handles generating the signal for us, driving the motor. Excellent!

Controlling Motors through the FeatherWing​

Now we know how to move a particular motor at a particular speed, forwards or backwards, we need to understand how to command the FeatherWing to do so. I struggled with this part! I couldn't find information on the product page about how to do this, which I would ordinarily use to set up an embedded system like this. This is because AdaFruit provides libraries to use the FeatherWing without needing any of this I²C or PWM knowledge.

Thankfully, the AdaFruit MotorKit library and its dependencies had all of the code I needed to write a basic driver in C++ - thank you, AdaFruit! The following is the list of links I used for a reference on controlling the FeatherWing:

1. Adafruit_CircuitPython_MotorKit
2. Adafruit_CircuitPython_PCA9685
3. Adafruit_CircuitPython_Motor
4. Adafruit_CircuitPython_BusDevice
5. Adafruit_CircuitPython_Register

Thanks to those links, I was able to put together a basic C++ driver, available on here on Github.

FeatherWing Initial Setup​

To get the PWM chip running, we first do some chip setup - resetting, and setting the clock value. This is done in the I2CDevice constructor by opening the I²C bus with the Linux I²C driver; selecting the FeatherWing device by address; setting its mode to reset it; and setting its clock using a few reads and writes of the mode and prescaler registers. Once this is done, the chip is ready to move the motors.

I2CDevice::I2CDevice() {
  i2c_fd_ = open("/dev/i2c-1", O_RDWR);
  if (!i2c_fd_) {
    std::__throw_runtime_error("Failed to open I2C interface!");
  }

  // Select the PWM device
  if (!trySelectDevice())
    std::__throw_runtime_error("Failed to select PWM device!");

  // Reset the PWM device
  if (!tryReset()) std::__throw_runtime_error("Failed to reset PWM device!");

  // Set the PWM device clock
  if (!trySetClock()) std::__throw_runtime_error("Failed to set PWM clock!");
}

Let's break this down into its separate steps. First, we use the Linux driver to open the I²C device.

// Headers needed for Linux I2C driver
#include <fcntl.h>
#include <linux/i2c-dev.h>
#include <linux/i2c.h>
#include <linux/types.h>
#include <sys/ioctl.h>
#include <unistd.h>

// ...

// Open the I2C bus
i2c_fd_ = open("/dev/i2c-1", O_RDWR);
// Check that the open was successful
if (!i2c_fd_) {
  std::__throw_runtime_error("Failed to open I2C interface!");
}

We can now use this i2c_fd_ with Linux system calls to select the device address and read/write data. To select the device by address:

bool I2CDevice::trySelectDevice() {
  return ioctl(i2c_fd_, I2C_SLAVE, kDefaultDeviceAddress) >= 0;
}

This uses the ioctl function to select a slave device by the default device address 0x60. It checks the return code to see if it was successful. Assuming it was, we can proceed to reset:

bool I2CDevice::tryReset() { return tryWriteReg(kMode1Reg, 0x00); }

The reset is done by writing a 0 into the Mode1 register of the device. Finally, we can set the clock. I'll omit the code for brevity, but you can take a look at the source code for yourself. It involves setting the mode register to accept a new clock value, then setting the clock, then setting the mode to accept the new value and waiting 5ms. After this, the PWM should run at 1.6 kHz.

Once the setup is complete, the I2CDevice exposes two methods: one to enable the motors, and one to set a duty cycle. The motor enable sets a particular pin on, so I'll skip that function. The duty cycle setter has more logic:

buf_[0] = kPwmReg + 4 * pin;

if (duty_cycle == 0xFFFF) {
  // Special case - fully on
  buf_[1] = 0x00;
  buf_[2] = 0x10;
  buf_[3] = 0x00;
  buf_[4] = 0x00;
} else if (duty_cycle < 0x0010) {
  // Special case - fully off
  buf_[1] = 0x00;
  buf_[2] = 0x00;
  buf_[3] = 0x00;
  buf_[4] = 0x10;
} else {
  // Shift by 4 to fit 12-bit register
  uint16_t value = duty_cycle >> 4;
  buf_[1] = 0x00;
  buf_[2] = 0x00;
  buf_[3] = value & 0xFF;
  buf_[4] = (value >> 8) & 0xFF;
}

Here we can see that the function checks the requested duty cycle. If at the maximum, it sets the motor PWM signal at its maximum - 0x1000. If below a minimum, it sets the duty cycle to its minimum. Anywhere in between will shift the value by 4 bits to match the size of the register in the PWM chip, then transmit that. Between the three if blocks, the I2CDevice has the ability to set any duty cycle for a particular pin. It's then up to the Motor class to decide which pins should be set, and the duty cycle to set them to.

Initialize Motors​

Following the setup of I2CDevice, each Motor gets a reference to the I2CDevice to allow it to talk to the PWM chip, as well as a set of pins that correspond to the motor. The pins are as follows:

The I2CDevice and Motors are constructed in the JetBot control node. Note that the pins are passed inline:

device_ptr_ = std::make_shared<JetBotControl::I2CDevice>();
motor_1_ = JetBotControl::Motor(device_ptr_, std::make_tuple(8, 9, 10), 1);
motor_2_ =
    JetBotControl::Motor(device_ptr_, std::make_tuple(13, 11, 12), 2);

Each motor can then request the chip enables the motor on the enable pin, which again is done in the constructor:

Motor::Motor(I2CDevicePtr i2c, MotorPins pins, uint32_t motor_number)
    : i2c_{i2c}, pins_{pins}, motor_number_{motor_number} {
  u8 enable_pin = std::get<0>(pins_);
  if (!i2c_->tryEnableMotor(enable_pin)) {
    std::string error =
        "Failed to enable motor " + std::to_string(motor_number) + "!";
    std::__throw_runtime_error(error.c_str());
  }
}

Once each motor has enabled itself, it is ready to send the command to spin forwards or backwards, brake, or turn off. This example only allows the motor to set itself to spinning or not spinning. The command is sent by the control node once more:

motor_1_.trySetSpinning(spinning_);
motor_2_.trySetSpinning(spinning_);

To turn on, the positive pin is set to fully on, or 0xFFFF, while the negative pin is set to off:

if (!i2c_->trySetDutyCycle(pos_pin, 0xFFFF)) {
    return false;
}
if (!i2c_->trySetDutyCycle(neg_pin, 0)) {
    return false;
}

To turn off, both positive and negative pins are set to fully on, or 0xFFFF:

if (!i2c_->trySetDutyCycle(pos_pin, 0xFFFF)) {
    return false;
}
if (!i2c_->trySetDutyCycle(neg_pin, 0xFFFF)) {
    return false;
}

Finally, when the node is stopped, the motors make sure they stop spinning. This is done in the destructor of the Motor class:

Motor::~Motor() {
  trySetSpinning(false);
}

While the Motor class only has the ability to turn fully on or fully off, it has the code to set a duty cycle anywhere between 0 and 0xFFFF - which means we can set any speed in the direction we want the motors to spin!

Trying it out​

If you want to give it a try, and you have a JetBot to do it with, you can follow my setup guide in this video:

Once this is done, follow the instructions in the README to get set up. This means cloning the code onto the JetBot, opening the folder inside the dev container, and then running:

source /opt/ros/humble/setup.bash
colcon build
source install/setup.bash
ros2 run jetbot_control jetbot_control

This will set the motors spinning for half a second, then off for half a second.