Kshitij Tiwari Ph.D.

Introduction To Mobile Robotics

9 min read

In robotics, robots can either move from one place to another or, stay fixed in place. Mobile robotics is the study of robots that fall under the former category and has been my main focal theme of research over the years. If you have read my 5 step guide to get started with robotics you also are likely interested in this sub-niche and this post will give an introduction to mobile robotics and its applications.

What is a mobile robot?

Before we even begin to talk about mobile robots, a sub-niche of robotics, we first need to clearly define what we mean by a robot.

A robot is an automatically operated machine meant to replace mundane human efforts utilizing computer programs.

Under this definition, gadgets such as a washing machine, a microwave or a mobile phone would not qualify as robots. Now that we have this out of the way, let us start looking deeper into mobile robots and the various types of such robots.

Mobile robotics involves the study of robots which are small devices that move around in an environment.  They are used for many purposes, including search and rescue, surveillance, exploration, entertainment, and military operations.

Their level of intelligence governs the level of autonomy they have when it comes to their mobility in the environment. Also, they can use different mechanisms to move from one place to another such as fly, crawl, roll, slip, slide, or a combination of one or more such modes.

Types of mobile robots by locomotion

When it comes to mobile robots, as an early stage researcher, you might be wondering, how many types of mobile robots are there. This can be answered in a couple different ways based on how the robots are classified.

For instance, if they are classified based on the environments in which they operate, they could be aerial, ground or marine robots. Similarly, they could be classified based on design inspirations, such as bioinspired, biomimetic etc., but in this article, we will look at the various types of mobile robots classified based on their locomotion modes.

  • Wheeled: Wheeled robots can have anywhere from one wheel like a unicycle to multiple wheels depending on the mass and structure of the robot platform and their intended use. For instance, a segway like robot could be used for a Segway tour of a city or a smaller scaled version of it could be used to learn basic control principles. On the other hand, for operations over an uneven terrain such as rubble, you might need multiple wheeled robot with independent suspension. When dealing with wheeled robots, another aspect to consider is the drive train which governs how they will be steered to get to the destination.

  • Tracked: Just like wheeled ground robots, another contender is the tracked robot. This is similar to a tank’s tracks only much smaller and lighter allowing it to conquer various adverse terrain conditions.

  • Legged: These robots have some form of legs to move around. If the overall robot form is a Humanoid, then these would likely be human-like legs as is the case for bipedal humanoid robots like the Asimo robot by Honda. But there can be a whole lot of variations such as Cassie the ostrich like bipedal robot whose legs bend opposite to human knee joints. And, why only stop at bipedal walking robots when you can have a lot more legs such as a hexapod with six legs.

  • Propellers: Another mode of locomotion could be propellers. This is especially useful for unmanned aerial vehicles such as drones with multiple propellers allowing them to control the altitude and attitude in flight. Not only the aerial robots, this is useful even for the marine robots.

  • [Caveat] Manipulators: Robots that resemble an arm used to manipulate objects are unfortunately a misfit in the class of mobile robots. The reason being, although the end effector (tip of the arm) and it’s joints can move around, the base itself is fixed. So, this does not fit the same level of mobility as the robots discussed above.

    However, there is a hybrid class of robot- the mobile manipulator which involves mounting a robotic manipulator on top of a mobile base. Typically these used to be based on unmanned ground rovers given the large form factor of the manipulator. But recently, there is also a push towards exploring aerial manipulation wherein the mobile base is a drone.

At this point, you might wonder, if a robot can have only one mode of locomotion. The answer to this is, not necessarily. Recent advances have pushed the boundaries of locomotion abilities for mobile robots towards hybrid modes.

For instance, a humanoid robot called LEONARDO developed by researchers at Caltech, USA can perform bipedal locomotion but also has propellers to fly such as over staircases.

Thus, the possibilities of robots employing hybrid locomotion modes for harsh environmental conditions is endless and yet to be fully explored.

Autonomy in mobile robots

Mobile robots are often endowed with some computational power meaning they are able to “think” to some extent. The extent to which they are able to do so governs their level of autonomy which can be broadly classified as follows:

  • Fully autonomous: such robots are able to handle situations almost on their own. such as the self-driving cars. However, such level of autonomy also attracts widespread criticism in case a robot makes an error. Whose liability is it if the robot makes an error that results in damages? Although, this problem might not be so pronounced for smaller robots such as hobby drones but is an ongoing discussion. This is why there is also a research focus on Explainable Artificial Intelligence (XAI) in robotics so that the decisions being made by the robot using the AI models can be understood by humans.

  • Semi-autonomous: Think of semi-autonomy as a human in-the-loop setting. For instance, when you use Maps on your mobile device, you just enter the destination and the app needs to plan the route for you. Similarly, a human operator can tell the robot ”where” to go next and leave the low level plan of ”how” to get there on the robot.

  • Fully teleoperated: This basically refers to a robot that needs to be given all the commands on where to go and how to go there. Think of this as a joystick controlled robot like the avatar in computer games which a gamer has to control. 

During deployment, just like the hybrid locomotion modes, there is a possibility to employ hybrid autonomy too. This would mean that the robot can be say, fully autonomous until a certain condition is met or a human overrides the actions. When the trigger happens, then the behavior could be changed to semi-autonomy or fully-teleoperated depending on situation.

Sensing in robots

Robots use sensors to detect objects and then move towards them or avoid them as needed.  Sensors can detect things such as temperature, light, sound, pressure, and motion among other environmental characteristics. These sensors help the robot gather the raw data which then needs to be processed either on-board or shipped off to a base-station so the robot can perceive the environment. While we will look into sensing and perception in an upcoming article, just know that sensing abilities can be broadly classified as follows:

  • Interoceptive sensing: in this case, the robot only has limited sensor data available primarily from the vicinity of its current location. This could be the case when sensing discrete signals.

  • Exteroceptive sensing: in this case, the robot has a broader sensor area coverage such as when using camera or lidars etc.

Planning the path to the next destination might be slightly challenging for a robot capable only of interoceptive sensing as compared to exteroceptive sensing which allows for more sensor coverage.

Challenges to real world deployment

If you have never really had the chance to deploy a mobile robot in the real world, you may not be aware of the numerous challenges that come with it. While you may be fascinated with the videos and media coverage of robots doing agile maneuvers, you should also know that the robots fail as well. Not only do they tumble and fall, they also run out of battery, loose communication with base station, damage sensors or motors and a whole lot more. Simulations are a great way to test things out with relatively low liability but not even the high end simulators can prepare you for all the real world challenges out there. 

Key takeaways

Robots are used in many different industries like manufacturing, construction, agriculture, military, search and rescue, and more.  A lot of the robot’s ability to perform a certain is primarily governed by factors like- autonomy, sensing, and locomotion modalities available. Additionally, there is scope of programming various components of a robot in order to automate tasks and decision  making. Hopefully, this article gave you some insights into the various facets of mobile robots and can help you pick your niche moving forward.