1.7 Commandline services

Apart from topics, nodes can also commnicate through services. There are two main differences between topics and services. For one, topics only communitcate in one direction (from one node to another), while services can comminucate in two directions (back and forth between nodes). The other difference is that services are ‘blocking’, which means that the node calling the service has to wait for the other node to respond.

Like with topics, you can also commincate with services usign the commandline. For the next steps, you need to make sure the MIRTE telemetrix node is running, and iff this is not the case start it.

First we can have a look which services are active in the ROS system:

mirte$ ros2 service list

warning

When you want to drive around with the robot, please make sure that the robot is either on the groud, or unable to drive from the table.

One of the services is called ‘/mirte/set_left_speed’, which will set the speed of the left motor (values range from -100 to 100). You can set the speed of the motor:

requester: making request: mirte_msgs.srv.SetMotorSpeed_Request(speed=50)

response:
mirte_msgs.srv.SetMotorSpeed_Response(status=True)

mirte$ ros2 service call /mirte/set_left_speed mirte_msgs/srv/SetMotorSpeed "{speed: 50}"

This will set the motor speed to 50% forward. You will probably want to set the speed back to 0 againg to stop the motor.