Custom sensors

Ignition Sensors comes with various built-in sensor types ready to be used. Users aren't limited to those sensor types though. This tutorial will go over the process of implementing a custom sensor that leverages Ignition Sensors and can be used with downstream applications such as Ignition Gazebo.

Why custom sensors

The set of built-in sensors provided by Ignition Sensors is tied to the SDFormat specification. SDFormat is meant to be as general-purpose as possible, so adding lots of exotic sensors to the specification may become a maintenance burden and counter-productive. Therefore, sensors that are very specific to certain applications are better implemented closer to the applications that need them, than added to the general-purpose specification.

It may also be interesting for downstream projects to implement very specific versions of sensors which are not generally useful for other projects.

Finally, another reason to develop custom sensors is for projects that need to implement proprietary sensors that can't be shared publicly.

Do I need to use Ignition Sensors?

A simulated sensor consists of code that can be used to extract specific data from a running simulation. A camera extracts pixels, an IMU extracts accelerations, etc. This doesn't necessarily need to be done though Ignition Sensors. On Ignition Gazebo, for example, it's possible to implement functionality similar to a sensor just using a system plugin without any connection to Ignition Sensors.

With that in mind, it's worth going over some of the features that Ignition Sensors provides that make it easier to implement sensors, instead of writing everything from scratch:

• Standard handling of common parameters, like topic and update rate;
• Common APIs for updating the sensors in a loop;
• A standard way for loading custom sensors from an SDF <sensor> tag;
• Efficient updates for various rendering sensors using the same scene.

Let's get started!

Let's go over the creation of a custom odometer sensor which will measure the total distance travelled by a robot.

This example is located at ign-sensors/examples/custom_sensor, and the integration with Ignition Gazebo is at ign-gazebo/examples/plugins/custom_sensor_system.

Wherever "odometer" is used, that can be substituted for any other custom sensor type. Seismometer? Breathalyzer? Just adapt the logic to your needs.

SDF

Custom sensors follow these rules to be loaded from SDFormat:

• Use type="custom" inside the <sensor> tag
• Add an extra ignition:type="odometer" attribute, where odometer is a string that uniquely identifies the sensor type.
• Optionally, add an <ignition:odometer/> element with configuration specific to that sensor.

With that in mind, here's what the sensor tag would look like:

<sensor name="the_odometer" type="custom" ignition:type="odometer">
  <always_on>1</always_on>
  <update_rate>30</update_rate>
  <topic>odom</topic>
  <ignition:odometer>
    <noise type="gaussian">
      <mean>0.2</mean>
      <stddev>0.1</stddev>
    </noise>
  </ignition:odometer>
</sensor>

Sensor implementation

The sensor consists of a class, Odometer, which inherits from ignition::sensors::Sensor.

Take a look at ign-sensors/examples/custom_sensor for the full code. Here are some important pointers:

• Be sure to link your sensor to Ignition Sensors
• Implement all pure abstraction functions, like Update
• Be sure to implement at least one of the Load functions so the sensor can be initialized
• Don't forget to call Sensor::Load to load common sensor properties
• It's recommended that the sensor publishes its data using Ignition Transport

You may choose to compile your sensor as a shared library, or to embed it directly into other application code.

Loading into Ignition Gazebo

In order to use the sensor on Ignition Gazebo, one needs to write a system plugin that instantiates the sensor and updates it periodically with data from simulation.

Take a look at ign-gazebo/examples/plugins/custom_sensor_system. for the full code. Here are some important pointers:

• Check for new entities that have ignition::gazebo::components::CustomSensor during the PreUpdate callback and instantiate new sensors as they appear in simulation.
• Don't assume all CustomSensors are of the type you need, be sure to check the type using the ignition::sensors::customType function.
• During PostUpdate, update all sensors with new data coming from simulation.
• Also during PostUpdate, delete any sensors that have been removed from simulation.