Gripper control integration in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the ROS node for gripper control.

ROS

import rospy

def main():
    rospy.[1]('gripper_control_node')

if __name__ == '__main__':
    main()

Drag options to blanks, or click blank then click option'

Acreate_node

Bstart_node

Cinit_node

Drun_node

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'start_node' instead of 'init_node' causes an AttributeError.

In ROS, rospy.init_node is used to initialize a node with a given name.

2fill in blank

medium

Complete the code to create a publisher for the gripper command topic.

ROS

pub = rospy.Publisher('gripper_command', [1], queue_size=10)

Drag options to blanks, or click blank then click option'

Asensor_msgs.msg.JointState

Bstd_msgs.msg.Int32

Cgeometry_msgs.msg.Twist

Dstd_msgs.msg.String

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'String' or 'Twist' message types will not match the gripper command format.

The gripper command often uses sensor_msgs.msg.JointState to specify joint positions.

3fill in blank

hard

Fix the error in the callback function to correctly handle gripper status messages.

ROS

def status_callback(msg):
    if msg.position[0] [1] 0.05:
        rospy.loginfo('Gripper is open')

Drag options to blanks, or click blank then click option'

A<

B=

C==

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' causes syntax error; '==' checks equality, but we want greater than.

To check if the gripper is open, we compare if the position is greater than a threshold.

4fill in blank

hard

Fill both blanks to publish a command to close the gripper with position 0.0 and velocity 0.1.

ROS

from sensor_msgs.msg import JointState

cmd = JointState()
cmd.name = ['gripper_joint']
cmd.position = [[1]]
cmd.velocity = [[2]]
pub.publish(cmd)

Drag options to blanks, or click blank then click option'

A0.0

B0.1

C1.0

D0.5

Attempts:

3 left

💡 Hint

Common Mistakes

Using position 1.0 opens the gripper instead of closing it.

Position 0.0 closes the gripper; velocity 0.1 sets the movement speed.

5fill in blank

hard

Fill all three blanks to create a subscriber that listens to 'gripper_status' topic with the correct message type and callback.

ROS

sub = rospy.Subscriber('gripper_status', [1], [2])

def [3](msg):
    rospy.loginfo(f'Gripper position: {msg.position[0]}')

Drag options to blanks, or click blank then click option'

Asensor_msgs.msg.JointState

Bstatus_callback

Dstd_msgs.msg.String

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'String' message type causes message format errors.

The subscriber needs the message type JointState and the callback function status_callback.

Practice

1. In ROS gripper control integration, what is the main purpose of sending commands like open or close to the gripper?

easy

A. To start the robot's main program

B. To control the physical or simulated gripper's movement

C. To reset the robot's sensors

D. To update the robot's software version

Solution

1. Step 1: Understand gripper commands

   Commands like open or close are instructions sent to the gripper hardware or simulation to change its state.

2. Step 2: Identify the command effect

   These commands directly control the gripper's physical or simulated movement to grasp or release objects.

3. Final Answer:

   To control the physical or simulated gripper's movement -> Option B

4. Quick Check:

   Gripper commands = control gripper movement [OK]

Hint: Gripper commands directly move the gripper hardware or simulation [OK]

Common Mistakes:

• Confusing gripper commands with robot startup commands
• Thinking commands update software or sensors
• Assuming commands reset the robot

2. Which of the following is the correct ROS 2 topic type to publish simple gripper commands like open or close?

easy

A. std_msgs/msg/String

B. sensor_msgs/msg/Image

C. geometry_msgs/msg/Twist

D. nav_msgs/msg/Odometry

Solution

1. Step 1: Identify message types for commands

   Simple text commands like open or close are best sent as strings.

2. Step 2: Match ROS message types

   std_msgs/msg/String is the standard message type for sending text strings in ROS 2 topics.

3. Final Answer:

   std_msgs/msg/String -> Option A

4. Quick Check:

   Text commands use std_msgs/msg/String [OK]

Hint: Text commands use std_msgs/msg/String topic type [OK]

Common Mistakes:

• Using sensor_msgs/msg/Image for text commands
• Confusing movement commands with Twist messages
• Using Odometry messages for gripper control

3. Given this ROS 2 Python snippet controlling a gripper:

import rclpy
from std_msgs.msg import String

def main():
    rclpy.init()
    node = rclpy.create_node('gripper_controller')
    pub = node.create_publisher(String, 'gripper_command', 10)

    msg = String()
    msg.data = 'close'
    pub.publish(msg)
    rclpy.spin_once(node, timeout_sec=1)
    node.destroy_node()
    rclpy.shutdown()

What will this code do when run?

medium

A. Publish an empty message to the gripper

B. Continuously send 'close' commands to the gripper

C. Fail to publish because of missing loop

D. Send a 'close' command to the gripper topic once

Solution

1. Step 1: Analyze publishing behavior

   The code creates a publisher and sends one message with data 'close' once.

2. Step 2: Check for loops or repeated publishing

   There is no loop; it publishes once and spins once to process callbacks.

3. Final Answer:

   Send a 'close' command to the gripper topic once -> Option D

4. Quick Check:

   Single publish call = one 'close' command sent [OK]

Hint: No loop means single publish only [OK]

Common Mistakes:

• Assuming continuous publishing without a loop
• Thinking message is empty
• Believing code will fail without a loop

4. You wrote this ROS 2 node to control the gripper:

import rclpy
from std_msgs.msg import String

def main():
    rclpy.init()
    node = rclpy.create_node('gripper_controller')
    pub = node.create_publisher(String, 'gripper_command', 10)

    msg = String()
    msg.data = 'open'
    pub.publish(msg)

    rclpy.spin(node)
    rclpy.shutdown()

But the gripper never opens. What is the likely problem?

medium

A. The node spins forever without destroying the node, blocking shutdown

B. The message data is incorrect; it should be 'close'

C. The publisher topic name is wrong; it should be 'gripper_control'

D. The message type should be Int32, not String

Solution

1. Step 1: Understand spin behavior

   rclpy.spin(node) blocks indefinitely, so the program never reaches shutdown or node destruction.

2. Step 2: Effect on gripper command

   The message is published once, but the node keeps spinning, possibly preventing proper cleanup or further commands.

3. Final Answer:

   The node spins forever without destroying the node, blocking shutdown -> Option A

4. Quick Check:

   Infinite spin blocks shutdown and may affect gripper response [OK]

Hint: Spin blocks forever; node cleanup needed after publishing [OK]

Common Mistakes:

• Changing message data unnecessarily
• Assuming topic name is wrong without checking
• Switching message type without cause

5. You want to integrate a gripper control node that automatically closes the gripper when a sensor detects an object. Which ROS 2 pattern best fits this integration?

hard

A. Use a service call to open the gripper only

B. Publish 'close' continuously regardless of sensor input

C. Subscribe to the sensor topic and publish 'close' to the gripper command topic when object detected

D. Run a timer to open and close the gripper every second

Solution

1. Step 1: Identify integration requirements

   The gripper should close only when the sensor detects an object, so sensor data must trigger the command.

2. Step 2: Choose ROS 2 communication pattern

   Subscribing to the sensor topic and publishing 'close' on detection is the correct reactive pattern.

3. Final Answer:

   Subscribe to the sensor topic and publish 'close' to the gripper command topic when object detected -> Option C

4. Quick Check:

   Reactive publish on sensor event = Subscribe to the sensor topic and publish 'close' to the gripper command topic when object detected [OK]

Hint: Use subscriber to sensor, publish gripper command on event [OK]

Common Mistakes:

• Publishing commands without sensor input
• Using service calls for continuous control
• Using timers unrelated to sensor events