Critical Sensors for Robotics

Why This Matters

In robotics and bioinspired systems, sensors are the bridge between a machine and its environment—they're how robots "see," "feel," and "know" where they are in space. You're being tested on more than just what each sensor does; exam questions focus on why a particular sensor type is chosen for a specific application, how different sensors complement each other, and the trade-offs between accuracy, range, cost, and environmental limitations.

Understanding critical sensors means grasping the underlying physics—electromagnetic waves, acoustic propagation, inertial mechanics, and mechanical deformation—that make each technology work. When you encounter an FRQ asking you to design a sensing system for a mobile robot or explain why autonomous vehicles use sensor fusion, you need to connect hardware choices to functional requirements. Don't just memorize sensor names—know what physical principle each exploits and what problems it solves best.

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Proprioceptive Sensors: Internal State Awareness

These sensors measure what's happening inside the robot—its own motion, position, and joint states. They answer the question: "What is my body doing right now?" Proprioception in biology refers to the body's sense of self-movement and position, and these sensors provide the robotic equivalent.

Inertial Measurement Units (IMUs)

• Combines accelerometers and gyroscopes—measures linear acceleration and angular velocity simultaneously for complete motion tracking
• Provides real-time orientation data through sensor fusion algorithms that integrate measurements over time
• Essential for stabilization in drones, legged robots, and any system requiring balance or attitude control

Encoders (Rotary and Linear)

• Rotary encoders track shaft rotation—output angular position and velocity for motor control and odometry calculations
• Linear encoders measure displacement along a straight path, critical for CNC machines and precision positioning systems
• Enable closed-loop control by providing the feedback signal necessary for accurate trajectory following

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Exteroceptive Range Sensors: Measuring Distance

These sensors detect how far away objects are—critical for navigation, obstacle avoidance, and mapping. Each uses a different physical principle (electromagnetic radiation, acoustic waves, or structured light) with distinct trade-offs in range, resolution, and environmental robustness.

LiDAR Sensors

• Emits laser pulses and measures return time—calculates distance using $d = \frac{c \cdot t}{2}$ where $c$ is the speed of light
• Creates high-resolution 3D point clouds with centimeter-level accuracy over distances up to hundreds of meters
• Dominant in autonomous vehicles because performance is consistent regardless of lighting conditions

Ultrasonic Sensors

• Uses sound waves at frequencies above human hearing (typically 40 kHz) to measure distance via echo timing
• Low cost and reliable for short range—effective from a few centimeters to several meters
• Struggles with soft or angled surfaces that absorb or deflect sound waves away from the sensor

Infrared Sensors

• Detects reflected IR radiation to estimate proximity or distance, often using triangulation or intensity measurement
• Compact and inexpensive for short-range applications like line following and edge detection
• Susceptible to interference from sunlight and variations in surface reflectivity

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Vision Sensors: Rich Environmental Perception

Cameras provide the densest information about the environment—color, texture, shape, and spatial relationships. Vision processing is computationally intensive but enables capabilities no other sensor type can match, like reading signs or recognizing faces.

Cameras (RGB and Depth)

• RGB cameras capture 2D color images—enable object recognition, visual odometry, and scene understanding through computer vision algorithms
• Depth cameras add range information using stereo disparity, structured light, or time-of-flight principles to create $2.5D$ representations
• Critical for manipulation tasks where robots must identify objects and plan grasps based on shape and position

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Localization Sensors: Global Position Awareness

These sensors tell the robot where it is in the world, not just relative to nearby objects. Global localization is essential for outdoor navigation and multi-robot coordination.

GPS Sensors

• Triangulates position from satellite signals—provides absolute coordinates (latitude, longitude, altitude) anywhere on Earth with sky visibility
• Accuracy varies from meters to centimeters depending on whether standard GPS, DGPS, or RTK corrections are used
• Fails indoors and in urban canyons—signal blockage and multipath reflections from buildings degrade performance significantly

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Contact and Force Sensors: Physical Interaction

These sensors measure forces and contact events when robots physically interact with objects or environments. Bioinspired designs often mimic the mechanoreceptors in human skin and joints.

Force and Torque Sensors

• Measures forces and moments in up to six degrees of freedom ($F_x, F_y, F_z, \tau_x, \tau_y, \tau_z$) at joints or end effectors
• Enables compliant control where robots adjust motion based on contact forces rather than just position commands
• Critical for safe human-robot interaction—allows robots to detect collisions and limit applied forces

Tactile Sensors

• Mimics biological mechanoreceptors—detects pressure distribution, texture, slip, and vibration across a contact surface
• Enables dexterous manipulation by providing the feedback needed to grasp fragile objects without crushing them
• Technologies include resistive, capacitive, and piezoelectric arrays—each with different spatial resolution and sensitivity characteristics

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Proximity Sensors: Non-Contact Detection

These sensors detect nearby objects without measuring precise distance—they answer "is something there?" rather than "how far is it?" Simple binary or threshold-based detection is often sufficient for safety systems.

Proximity Sensors

• Detects object presence without contact—uses capacitive, inductive, or photoelectric principles depending on target material
• Inductive types sense metal only—ideal for detecting machine parts but useless for organic materials
• Essential for safety interlocks that stop robot motion when humans enter the workspace

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Quick Reference Table

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Self-Check Questions

1. Which two sensor types would you combine for robust outdoor mobile robot localization, and what weakness does each compensate for in the other?

2. A robot must navigate a warehouse with glass shelving and metal racks. Compare the limitations of LiDAR and ultrasonic sensors for this environment—which objects challenge each technology?

3. Explain the difference between proprioceptive and exteroceptive sensors, and classify IMUs, cameras, and encoders accordingly.

4. An FRQ asks you to design a sensing system for a robot that must gently grasp eggs without breaking them. Which sensors would you include, and what specific feedback would each provide?

5. Compare how GPS and encoders each contribute to a mobile robot's position estimate—what error sources affect each, and why is neither sufficient alone?