2. Motion Control Course

Basic Control Course

2.1 Gait Description

Gait Concept

Robot gait refers to a periodic phenomenon describing animal locomotion. It also refers to the movement process of each leg of a robot following a specific sequence and trajectory, which is a critical factor in ensuring the stable operation of walking mechanisms. Research on gaits is primarily aimed at achieving stable periodic motion in robots.

In simple terms, it describes how animals walk. Common gait characteristics of hexapods include the tripod gait and the ripple gait.

Tripod Gait: Three alternating legs support the body. Specifically, when the left front leg touches the ground, the right rear leg and left rear leg touch the ground simultaneously, followed by the right front leg. This gait ensures stability during movement and reduces energy consumption.

Ripple Gait: The six legs of the animal touch the ground simultaneously according to a specific rhythm. Specifically, when the left front leg touches the ground, the other five legs will also touch the ground simultaneously. This gait improves movement speed and coordination.

The following table lists some common terms used in gait descriptions:

Term	Description
Phase	The most direct understanding is the angle, representing the position within a periodic motion.
Phase difference	The difference in lead or lag of motion between different legs.
Swing phase	The leg is lifted and in an airborne state.
Stance phase	The leg is in contact with the ground.
Cycle	During locomotion, the entire process from one heel strike to the next heel strike on the same side is one cycle.
Gait frequency	The number of gait cycles completed per unit of time.
Step length	The distance moved by the foot end from lifting to landing within one cycle.
Stride length	The distance the body moves within one cycle.
Duty cycle	The ratio of the time a single leg is in the stance phase to the entire gait cycle.

2.2 Tripod Gait

2.2.1 Tripod Gait Description

Tripod Gait is a typical gait for hexapod walking robots to achieve locomotion. When insects of the class Hexapoda walk, the six legs generally do not move forward in a straight line simultaneously. Instead, the three pairs of legs are divided into two groups, alternating forward in a triangular support structure. Simply put, it is an alternating up-and-down movement in groups of three.

Under this triangular support structure, the body remains in a statically stable state.

In the tripod gait, each leg swings in a specific sequence. Generally speaking, in one step, three legs swing forward while the other three legs swing backward. This gait helps maintain balance and allows walking on uneven terrain.

The advantages of the hexapod robot tripod gait include:

High stability: Since the six legs can touch the ground simultaneously, the robot is less likely to tip over or lose balance. This allows the hexapod robot to perform better on unstable terrain.
Strong adaptability: The tripod gait allows the robot to better adapt to terrain and environments. Because the motion trajectory resembles a triangle, it can more easily cross obstacles or climb slopes.
Good flexibility: The tripod gait enables easy changes in direction and speed. This allows for better performance when rapid responses are required, such as avoiding obstacles or tracking targets in narrow spaces.

The following uses the diagrams below to analyze and explain the tripod gait:

As shown in Figure A, legs 1, 3, and 5 of the hexapod robot are lifted and swing forward, while legs 2, 4, and 6 are used to support the body, ensuring the center of gravity is located at the intersection of the diagonals. At this time, legs 1, 3, and 5 are in the swing phase, and legs 2, 4, and 6 are in the stance phase.

As shown in Figure B, all six legs touch the ground simultaneously. The positions of legs 2, 4, and 6 remain unchanged, legs 1, 3, and 5 move forward, and all legs are in the stance phase.

As shown in Figure C, legs 2, 4, and 6 are lifted and swing forward, while legs 1, 3, and 5 are used to support the body, ensuring the center of gravity is located at the intersection of the diagonals. At this time, legs 2, 4, and 6 are in the swing phase, and legs 1, 3, and 5 are in the stance phase.

As shown in Figure D, all six legs touch the ground simultaneously. The positions of legs 1, 3, and 5 remain unchanged, legs 2, 4, and 6 move forward, and all legs are in the stance phase.

When these four sets of actions in Figure A, B, C, and D are completed, the hexapod robot completes one full cycle of motion.

2.1.2 Operation Steps

Note

Commands are strictly case-sensitive, and the Tab key can be used to auto-complete keywords.

Power on the robot and connect it to the remote control software NoMachine.
Click the icon on the system desktop to open the command line terminal.
Enter the command and press Enter to close the auto-start program.

~/.stop_ros.sh

Enter the command and press Enter to navigate to the directory where the source code file is located.

cd ros2_ws/src/example/example/body_control/include

Enter the command and press Enter to open the source code file.

vim tripod_gait.py

Locate the code shown in the following block.

def main(args=None):
    rclpy.init(args=args)
    node = TripodGait()

    node.move(gait=2)   # Go straight
    node.get_logger().info('\033[1;32m%s\033[0m' % 'forward')
    time.sleep(5)  # Wait for 5 seconds
    node.move(gait=0)     # Stop
    node.get_logger().info('\033[1;32m%s\033[0m' % 'stop')

    node.controller.destroy_node()  # Clean up the node
    rclpy.shutdown()  # Shutdown ROS 2

gait represents the gait mode. A value of 0 indicates stationary, a value of 1 indicates the ripple gait, and a value of 2 indicates the tripod gait. Ensure that gait=2 is set to execute the tripod gait operation.

After the modification is complete, press the Esc key, enter :wq and press Enter to save and exit.

:wq

Enter the command and press Enter to run the program.

ros2 launch example tripod_gait.launch.py

To close the operation, use the shortcut key Ctrl + C.

2.1.3 Program Outcome

After the operation is initiated, the hexapod robot will advance for 5 seconds under the tripod gait, with the three pairs of legs divided into two groups alternating forward, and then come to a stop.

2.1.4 Program Analysis

The program source code is located at: /home/ubuntu/ros2_ws/src/example/example/body_control/include/tripod_gait.py

class TripodGait(Node):
    def __init__(self):
        super().__init__('tripod_gait')
        self.controller = ControllerClient()
        self.client = self.create_client(Trigger, '/controller_manager/init_finish')
        self.client.wait_for_service()

    def move(
        self,
        gait=1,              # Gait type, 0 for stop, 1 for ripple gait, 2 for tripod gait
        stride=40.0,         # Stride length (mm), default 40mm, range 0~65
        height=15.0,         # Step height (mm), default 15mm, range 0~50
        direction=0,         # Movement direction (degrees), range 0°~360°, 0° is forward, increases counterclockwise
        rotation=0.0,        # Rotation angle, default no rotation, positive value for counterclockwise rotation, negative value for clockwise rotation
        time=1,              # Single step duration (seconds), default 1 second
        steps=0,             # Number of steps, 0 indicates continuous movement
        interrupt=True,      # Whether to allow interruption, default allowed
        relative_height=False # Whether the height is a relative value, default absolute height
    ):
        """Method to control robot movement

        Args:
            gait (int): Gait type (e.g., 1=ripple gait)
            stride (float): Stride size (millimeters)
            height (float): Leg lifting height (millimeters)
            direction (float): Movement direction (0=forward, 180=backward)
            rotation (float): Rotation angle (positive right, negative left)
            time (float): Single step execution time (seconds)
            steps (int): Number of moving steps (0=continuous)
            interrupt (bool): Whether to allow interruption by new commands
            relative_height (bool): Whether the height is a relative value
        """
        self.controller.traveling(
            gait=gait,
            stride=stride,
            height=height,
            direction=direction,
            rotation=rotation,
            time=time,
            steps=steps,
            interrupt=interrupt,
            relative_height=relative_height
        )

The program controls movement primarily by setting the parameters within the move function. The specific parameters are as follows:

The first parameter gait is the gait mode. A value of “0” represents stationary, a value of “1” represents the ripple gait, and a value of “2” represents the tripod gait.

The second parameter stride is the stride length in millimeters, with a default range of 0 to 65. Adjusting the value beyond this range may cause the robot to get stuck or trip.

The third parameter height is the step height in millimeters, with a default range of 0 to 50. Adjusting the value beyond this range may cause the robot to get stuck or trip.

The fourth parameter direction is the forward direction in degrees, with a value range of 0-360. When the value is 0-180, the hexapod robot moves forward. When the value is 180-360, the hexapod robot moves backward.

The fifth parameter rotation is the turning rate in radians per second. When the value is positive, the hexapod robot rotates counterclockwise. When the value is negative, the hexapod robot rotates clockwise.

The sixth parameter time is the interval duration for each step moved, in seconds.

The seventh parameter steps is the number of movement steps, and a value of “0” indicates continuous movement.

The eighth parameter interrupt is used to set whether execution can be interrupted.

The ninth parameter relative_height is used to set whether the step height height is relative to the ground. A value of False adopts relative height.

2.3 Ripple Gait

2.3.1 Ripple Gait Description

Ripple Gait is also known as overlapping wave gait or wave-shaped gait. In the following text, it is uniformly referred to as the ripple gait.

The hexapod robot ripple gait refers to a specific swinging pattern of the six legs during walking, forming a trajectory similar to a ripple. This gait design allows the robot to walk stably on various terrains and environments while improving walking efficiency and flexibility.

In the ripple gait, each leg of the robot swings according to a specific rhythm and sequence. In one step, each front leg steps forward while each rear leg pushes backward, forming a dynamic resembling a wave. This gait helps the robot maintain balance and walk on undulating terrain.

The advantages of the hexapod robot ripple gait include:

High stability: Since the six legs can touch the ground simultaneously, the robot is less likely to tip over or lose balance. This allows the hexapod robot to perform better on unstable terrain.
High efficiency: The ripple gait allows the robot to better adapt to terrain and environments, reducing energy consumption. In addition, because the motion trajectory resembles a wave, it can more easily cross obstacles or climb slopes.
Good flexibility: The ripple gait enables easy changes in direction and speed. This allows for better performance when rapid responses are required, such as avoiding obstacles or tracking targets in narrow spaces.

When the hexapod robot is in the ripple gait, each pair of opposing legs operates alternately. The following uses the diagrams above to analyze and explain the ripple gait:

As shown in Figure A, legs 2 and 4 of the hexapod robot are lifted and swing forward, while legs 1, 3, 5, and 6 are used to support the body, ensuring the center of gravity is located at the intersection of the diagonals. At this time, legs 2 and 4 are in the swing phase, and legs 1, 3, 5, and 6 are in the stance phase.

As shown in Figure B, legs 3 and 6 of the hexapod robot are lifted and swing forward, while legs 1, 2, 4, and 5 are used to support the body, ensuring the center of gravity is located at the intersection of the diagonals. At this time, legs 3 and 6 are in the swing phase, and legs 1, 2, 4, and 5 are in the stance phase.

As shown in Figure C, legs 1 and 5 of the hexapod robot are lifted and swing forward, while legs 2, 3, 4, and 6 are used to support the body, ensuring the center of gravity is located at the intersection of the diagonals. At this time, legs 1 and 5 are in the swing phase, and legs 2, 3, 4, and 6 are in the stance phase.

As shown in Figure D, legs 2 and 6 of the hexapod robot are lifted and swing forward, while legs 1, 3, 4, and 5 are used to support the body, ensuring the center of gravity is located at the intersection of the diagonals. At this time, legs 2 and 6 are in the swing phase, and legs 1, 3, 4, and 5 are in the stance phase.

2.3.2 Operation Steps

Power on the robot and connect it to the remote control software NoMachine.
Click the icon on the system desktop to open the command line terminal.
Enter the command and press Enter to close the auto-start program.

~/.stop_ros.sh

Enter the command and press Enter to navigate to the directory where the source code file is located.

cd ros2_ws/src/example/example/body_control/include

Enter the command and press Enter to open the source code file.

vim tripod_gait.py

Locate the code shown in the following block.

def main(args=None):
    rclpy.init(args=args)
    node = TripodGait()

    node.move(gait=1)   # Go straight
    node.get_logger().info('\033[1;32m%s\033[0m' % 'forward')
    time.sleep(5)  # Wait for 5 seconds
    node.move(gait=0)     # Stop
    node.get_logger().info('\033[1;32m%s\033[0m' % 'stop')

    node.controller.destroy_node()  # Clean up the node
    rclpy.shutdown()  # Shutdown ROS 2

gait represents the gait mode. A value of 0 indicates stationary, a value of 1 indicates the ripple gait, and a value of 2 indicates the tripod gait. Ensure that gait=1 is set to execute the ripple gait operation.

After the modification is complete, press the Esc key, enter :wq, and press Enter to save and exit.

:wq

Enter the command and press Enter to run the program.

ros2 launch example tripod_gait.launch.py

To close the operation, use the shortcut key Ctrl + C.

2.3.3 Program Outcome

After the operation is initiated, the hexapod robot will advance for 5 seconds under the ripple gait and then come to a stop.

2.3.4 Program Analysis

The program source code is located at: /home/ubuntu/ros2_ws/src/example/example/body_control/include/tripod_gait.py

class TripodGait(Node):
    def __init__(self):
        super().__init__('tripod_gait')
        self.controller = ControllerClient()
        self.client = self.create_client(Trigger, '/controller_manager/init_finish')
        self.client.wait_for_service()

    def move(
        self,
        gait=1,              # Gait type, 0 for stop, 1 for ripple gait, 2 for tripod gait
        stride=40.0,         # Stride length (mm), default 40mm, range 0~65
        height=15.0,         # Step height (mm), default 15mm, range 0~50
        direction=0,         # Movement direction (degrees), range 0°~360°, 0° is forward, increases counterclockwise
        rotation=0.0,        # Rotation angle, default no rotation, positive value for counterclockwise rotation, negative value for clockwise rotation
        time=1,              # Single step duration (seconds), default 1 second
        steps=0,             # Number of steps, 0 indicates continuous movement
        interrupt=True,      # Whether to allow interruption, default allowed
        relative_height=False # Whether the height is a relative value, default absolute height
    ):
        """Method to control robot movement

        Args:
            gait (int): Gait type (e.g., 1=ripple gait)
            stride (float): Stride size (millimeters)
            height (float): Leg lifting height (millimeters)
            direction (float): Movement direction (0=forward, 180=backward)
            rotation (float): Rotation angle (positive right, negative left)
            time (float): Single step execution time (seconds)
            steps (int): Number of moving steps (0=continuous)
            interrupt (bool): Whether to allow interruption by new commands
            relative_height (bool): Whether the height is a relative value
        """
        self.controller.traveling(
            gait=gait,
            stride=stride,
            height=height,
            direction=direction,
            rotation=rotation,
            time=time,
            steps=steps,
            interrupt=interrupt,
            relative_height=relative_height
        )