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rclpy: Time

There are four separate ways that you may see time represented in ROS 2:

Here's how you can convert them.

Converting this →
to this ↓		 int float rclpy msg
int - int(t * 1e9) t.nanoseconds t.sec * 1e9 + t.nanosec
float t / 1e9 - t.nanoseconds / 1e9 t.sec + t.nanosec / 1e9
rclpy Time(nanoseconds=t) nano, sec = math.modf(t)
Time(int(sec), int(nano * 1e9))		 - Time.from_msg(t)
msg Time(nanoseconds=t).to_msg() nano, sec = math.modf(t)
builtin_interfaces.msg.Time(sec=int(sec), nanosec=int(nano * 1e9))		 t.to_msg() -

Important notes:

  • You cannot do comparisons/math between mixed types or messages
  • It is only mildly infuriating that rclpy.time.Time has the full word nanoseconds accessor and the message has nanosec.

Now

To get the equivalent of rospy.Time.now(), you now need a ROS 2 node:

import rclpy
from rclpy.node import Node

class MyNode(Node):

    def some_func(self):
        t = self.get_clock().now()
        msg.header.stamp = t.to_msg()

Converting from Duration to messages is common:

import rclpy
from rclpy.duration import Duration

msg.duration = Duration(seconds=1).to_msg()

Timers

Timers are created from the Node:

import rclpy
from rclpy.node import Node

class MyNode(Node):

    def __init__(self):
        super().__init__("my_node")

        # Create a timer that fires every quarter second
        timer_period = 0.25
        self.timer = self.create_timer(timer_period, self.callback)

    def callback(self):
        self.get_logger().info("timer has fired")

Rates

Using Rate objects in ROS 2 is a bit more complex than in ROS 1. Due to implementation details, we need to spin() or the sleep() function will block. This is most easily accomplished using a thread:

import threading

# Run spin in a thread, make thread daemon so we don't have to join it to exit
thread = threading.Thread(target=rclpy.spin, args=(node, ), daemon=True)
thread.start()

rate = node.create_rate(10)
while rclpy.ok():
    print('This prints at 10hz')
    rate.sleep()