Lab 2: Services and Custom Messages

Objective

Learn to create ROS2 services and define custom message types for robot control.

Learning Outcomes

✅ Create custom message definitions
✅ Implement service servers
✅ Build service clients
✅ Handle request-response patterns

Estimated Time

⏱️ 3 hours

Part 1: Create Custom Messages (45 min)

Create Interface Package

cd ~/ros2_ws/src
ros2 pkg create --build-type ament_cmake robot_interfaces
cd robot_interfaces
mkdir msg srv

Define Custom Message

Create msg/BatteryStatus.msg:

float32 voltage
float32 current
float32 percentage
bool is_charging
string status

Define Service

Create srv/SetSpeed.srv:

# Request
float32 linear_speed
float32 angular_speed
---
# Response
bool success
string message

Configure CMakeLists.txt

find_package(rosidl_default_generators REQUIRED)

rosidl_generate_interfaces(${PROJECT_NAME}
  "msg/BatteryStatus.msg"
  "srv/SetSpeed.srv"
)

Build

cd ~/ros2_ws
colcon build --packages-select robot_interfaces
source install/setup.bash

Part 2: Service Server (60 min)

Create robot_control/speed_service.py:

import rclpy
from rclpy.node import Node
from robot_interfaces.srv import SetSpeed

class SpeedService(Node):
    def __init__(self):
        super().__init__('speed_service')
        self.srv = self.create_service(
            SetSpeed,
            'set_speed',
            self.set_speed_callback
        )
        self.current_linear = 0.0
        self.current_angular = 0.0
        self.get_logger().info('Speed service ready')
    
    def set_speed_callback(self, request, response):
        # Validate speeds
        if abs(request.linear_speed) > 2.0:
            response.success = False
            response.message = 'Linear speed too high (max 2.0 m/s)'
            return response
        
        if abs(request.angular_speed) > 3.0:
            response.success = False
            response.message = 'Angular speed too high (max 3.0 rad/s)'
            return response
        
        # Set speeds
        self.current_linear = request.linear_speed
        self.current_angular = request.angular_speed
        
        response.success = True
        response.message = f'Speed set: linear={request.linear_speed}, angular={request.angular_speed}'
        
        self.get_logger().info(response.message)
        return response

Part 3: Service Client (60 min)

Create robot_control/speed_client.py:

import rclpy
from rclpy.node import Node
from robot_interfaces.srv import SetSpeed

class SpeedClient(Node):
    def __init__(self):
        super().__init__('speed_client')
        self.client = self.create_client(SetSpeed, 'set_speed')
        
        while not self.client.wait_for_service(timeout_sec=1.0):
            self.get_logger().info('Waiting for service...')
    
    def send_request(self, linear, angular):
        request = SetSpeed.Request()
        request.linear_speed = linear
        request.angular_speed = angular
        
        future = self.client.call_async(request)
        rclpy.spin_until_future_complete(self, future)
        
        return future.result()

def main():
    rclpy.init()
    client = SpeedClient()
    
    # Test different speeds
    result = client.send_request(1.0, 0.5)
    print(f'Result: {result.success}, {result.message}')
    
    result = client.send_request(5.0, 0.0)  # Should fail
    print(f'Result: {result.success}, {result.message}')
    
    client.destroy_node()
    rclpy.shutdown()

Deliverables

Custom message and service definitions
Working service server
Working service client
Test results showing validation

Time: 3 hours

Lab 2 - Services and Custom Messages

Lab Repository

Quick Start