ROS and Node-RED Part 2: Publishing drive commands

In this second tutorial, we are going to learn how to publish drive commands the a ROS robot from Node-RED. To do so, we need to use the publish node from the node-red-contrib-ros nodes. Unfortunately, the original is not supporting drive messages at the moment, I forked that repo and modified the code on some lines. In order to use my forked repo, we need to reinstall node-red-contrib-ros from my Github space. I assume you have installed the software as shown in the preceding tutorial at this point. Do update the software as follows:

cd ~/.node-red
npm uninstall node-red-contrib-ros
npm install git+https://github.com/marvinferber/node-red-contrib-ros

Using the publish node

Now lets move back to the flow and set up the ROS publish node. If not done already, start Node-RED. We first need a new publish node and connect it to the ROS master that was configured in the preceding tutorial. As you might know as a ROS user, drive commands are published to /cmd_vel_mux/input/teleop using a data object of type geometry_msgs/Twist. Such a message consists of velocity commands for linear and radial velocities.

geometry_msgs/Vector3 linear
  float64 x
  float64 y
  float64 z
geometry_msgs/Vector3 angular
  float64 x
  float64 y
  float64 z

In Node-RED, we need to prepare a message with exactly the properties shown above using a function node for instance. Setting this message object of type geometry_msgs/Twist as msg.payload, we are now able to publish drive commands.

Publish messages in a loop

The last hurdle to make the robot actually move is to publish the drive commands in a constant loop at around 10-20Hz. There are many ways to implement a loop that publishes drive commands in Node-RED. You can implement a while loop following this blog post by a while pattern based on simple nodes only. Using simple nodes only, you can simply insert a delay node in your loop to slow down processing to a specific rate while keeping the event loop in Node-RED unstressed.

Another approach would be to use a function node and publish messages from within a Javascript loop. In this case, be care not to block the event loop using any type of busy waiting. See this post for an explanation. I used a recursive program to achieve a non-blocking loop as shown below.

Simulator users welcome

Again, this tutorial works perfectly fine for a simulated robot too. Be careful using drive commands on a real robot. We did not set up any safety mechanisms in this tutorial such as bumper stop or cliff detection.You can import the following flow into your Node-RED and try out the different possibilities of publishing drive commands. Start the process by injecting a single message or by starting the for loop to publish a sequence of messages.

[
    {
        "id": "b3b3ae01.82c708",
        "type": "tab",
        "label": "Flow 1"
    },
    {
        "id": "1edf948e.06392b",
        "type": "ros-publish",
        "z": "b3b3ae01.82c708",
        "server": "f50f6311.80b15",
        "topicname": "/cmd_vel_mux/input/teleop",
        "msgtype": "geometry_msgs/Twist",
        "x": 758.5,
        "y": 218,
        "wires": []
    },
    {
        "id": "1469c2ad.5ecf8d",
        "type": "function",
        "z": "b3b3ae01.82c708",
        "name": "set geometry_msg/Twist",
        "func": "\nvar twist = {\nlinear : {\n  x : 0.1,\n  y : 0.0,\n  z : 0.0\n},\nangular : {\n  x : 0.0,\n  y : 0.0,\n  z : 0.0\n}\n};\nmsg.payload = twist;\nreturn msg;",
        "outputs": 1,
        "noerr": 0,
        "x": 508.5,
        "y": 219,
        "wires": [
            [
                "1edf948e.06392b"
            ]
        ]
    },
    {
        "id": "fc72bc5c.ccea3",
        "type": "inject",
        "z": "b3b3ae01.82c708",
        "name": "inject a message",
        "topic": "",
        "payload": "",
        "payloadType": "date",
        "repeat": "",
        "crontab": "",
        "once": false,
        "x": 247.5,
        "y": 219,
        "wires": [
            [
                "1469c2ad.5ecf8d"
            ]
        ]
    },
    {
        "id": "f7e9cf01.9776a",
        "type": "inject",
        "z": "b3b3ae01.82c708",
        "name": "inject a sequence",
        "topic": "",
        "payload": "",
        "payloadType": "date",
        "repeat": "",
        "crontab": "",
        "once": false,
        "x": 258.5,
        "y": 118,
        "wires": [
            [
                "138cb7e5.f57e2"
            ]
        ]
    },
    {
        "id": "138cb7e5.f57e2",
        "type": "function",
        "z": "b3b3ae01.82c708",
        "name": "for loop",
        "func": "var i = 0;\nvar threshold = 20;\nvar delay = 100;\n\nfunction recursive(){\n    if (i > threshold - 1)return;\n    else{\n        i++;\n        msg.payload = i;\n        node.send(msg);\n        setTimeout(recursive, delay);\n    }\n}\nsetTimeout(recursive, delay);\n\n\n",
        "outputs": 1,
        "noerr": 0,
        "x": 446.5,
        "y": 118,
        "wires": [
            [
                "1469c2ad.5ecf8d"
            ]
        ]
    },
    {
        "id": "f50f6311.80b15",
        "type": "ros-server",
        "z": "",
        "url": "ws://127.0.0.1:9090/"
    }
]

That’s it! Happy hacking 😉