Learn: Sense

To enable robots to act upon and process their environment meaningfully, a robot needs to have sensors to read things about the world around it. Sensors are also widely used to measure attributes of the robot itself, which can be useful to protect a robot from damage (e.g. shutting down when overheating or getting wet), to facilitate control (e.g. an encoder measurement is fed into a control algorithm trying to maintain the position of a motor), to orient other sensors (e.g. a tilt sensor is referenced by distance sensors mapping a room), or anything else the robot designer can think of. When choosing sensors, the balance is typically made between applicability to environment, accuracy & precision, and cost. We hope to provide some exposé on major categories of sensors and how they fit into each of these metrics for selection; there are so many sensors on the market that we can only scrape the surface, but we hope to catch the more important and common sensors in robotics today.

As with all of our LEARN pages right now, we’re still feverishly producing content. By the end of the summer, all of these titles should have articles associated with them. Keep checking back in to see links appear!

Idiothetic Sensors: Sensing your own state

Spiritual Idiothetics: A young Buddhist practices sensing his own state.

Idiothetic sensing is the category of sensing involving sensors measuring physical states and quantities local to and with the reference frame of the platform being considered. For example, a tilt sensor mounted on a robot falls into the domain of idiothetic sensing; an ultrasonic rangefinder mounted on a robot to determine the range of other things in the environment does not. In the context of navigation, "idiothetic" is usually associated with the use of internal clues to navigation, and the terms "path integration" and "dead reckoning." Most often heard in the nautical and aviation contexts, "dead reckoning" describes what sailors and pilots do in order to guide their craft without being able to see their environment, due to darkness, cloud, fog, or very dirty windshields. To imagine what this is like, try closing your eyes before getting up and walking to the nearest window (or your other favorite nearby destination), avoiding touching things as much as possible.

Now let's think about what's happening in your body as you're walking blind. Your body has many different types of sensors. Most famous of these are touch, smell, taste, hearing, and sight, the last of which you've just disabled for your walk. Let's also imagine that you put in earplugs and there aren't any freshly baked cookies to lead you by your nose. What we're trying to accomplish here is ignoring all of your non-idiothetic (allothetic) "sensors." So what's left? It's fairly common knowledge that we have senses besides the Big 5 that help us balance and keep track of where our body parts are; these are formally known as equilibrioception and proprioception. There are of course more human senses (though the strict definition of a sense is debated), especially ones useful for sensing the status of various internal organs; some animals also possess non-human senses like echolocation, electroreception, and magnetoception.

While you walk, your internal senses track how far you believe you've moved each muscle, and your brain has made your muscles move you quite a few times, so it's not bad at estimating the effect of what it affected your muscles. Additionally, you can estimate approximately how far you've rotated in space (and even more approximately how far you've translated) using your sense of balance. Because you are limited to using these internal senses for this thought experiment, you also have no reference point to guess your position from besides your starting position and anything you remember of your environment before you closed your eyes. All of these estimations of your position are filled with error and only relative to an arbitrary place where you decided to begin guessing your position with your eyes closed, but it's still incredible to realize what these less famous senses can accomplish. But how exactly does your body get its sense of balance? Could a robot with similar sensors do an even better job at navigating "blind"?

The vestibular system and a close-up on a hair that acts as a force sensor "cell" in the system.

Your sense of equilibrioception originates in the vestibular system, which is a major part of the inner ear. Fluid in the ear canals and otolith organs (other parts of the inner ear) moves around according to the net proper force (see proper acceleration) acting on your body. There are actually five "sensors" in each of your ears. Three of these are semicircular canals, which because of their shape and orientation allow for sensing rotational acceleration in three independent axes (and less accurately, linear). Additionally, there are two organs (the utricle and saccule) which allow sensing vertical and horizontal linear acceleration. These organs all measure acceleration by means of nerves attached to hairs with minuscule masses on their ends (often originating outside the body, such as sand!). 

Closeup of a MEMS accelerometer, courtesy of Princeton University.

This is remarkably similar to the operational method of high-tech, modern accelerometers!

Examples of Idiothetic Sensors:

• Accelerometer
• GPS
• Compass / Magnetometer
• Tilt
• Gyro
• Encoder
• Temperature

Allothetic Sensors: Sensing the world around you

• Bump / Touch
• Rangefinding
• IR
• Sonar
• Ultrasonic
• LiDAR
• Kinect
• Radar
• Camera
• Microphone
• Temperature
• Fire (IR)
• Humidity
• Pressure / touchpad
• Motion
• Tripwire / Breakbeam
• Chemical
• Signal detectors (directional)
• RADAR detector
• Airflow
• Capacitance/V/I/R/L
• Hall effect

How to Choose the Right Sensors for Your Robot

Sensor Drivers: The basic thinking part of sensing

Filtering: Thinking smartly about sensing