Types of Sensors in Robotics

Introduction

Sensors in Robotics refer to a mechanical function used to calculate the condition and environment of a robot. This sensor is based on the functions of the human sensory organs. Robots receive a broad range of data about their surroundings, such as position, size, orientation, velocity, distance, temperature, weight, force, etc. This information is what allows the robot to function efficiently while interacting with its environment to perform complex tasks.

The working of robot sensors derives from the principle of energy conversion, also known as transduction. Different sensors are required by different robots to attain measures of control and respond flexibly in their environment. 

Types of Robot Sensors

There are many different types of robot sensors, discussed in the following sections:

Light Sensors

The light sensor is used to detect light and it usually generates a voltage difference. Robotic light sensors are of two types: Photovoltaic cells and photoresistors. Photovoltaic cells are applied when changing solar radiation energy to electrical. Naturally, these sensors are commonly used in the production of solar robots. 

On another hand, photoresistors are used to adjust their resistance by changing light intensities. As more light is on it, the resistance decreases. These light sensors are usually not expensive, so they are vastly used in robots.

Sound Sensor

A sound sensor detects a sound and converts it into an electrical signal. By applying this type of sensor, robots can navigate through sound, even to the point of creating a sound-controlled robot that recognizes and responds to specific sounds or series of sounds, to carry out certain tasks.

Temperature Sensor

A temperature sensor is used to detect temperature changes within the environment. This sensor mainly uses the voltage difference principle to get the temperature change, thereby generating the equivalent temperature value of the environment. There are different types of temperature sensor ICs (integrated circuits) used to detect temperature, including LM34, TMP37, TMP35, TMP36, LM35, etc. These sensors can be used in robots required to work in extreme weather conditions like an ice glacier or a desert.

Contact Sensor

Contact sensors are also known as touch sensors. They mainly function to detect a change in velocity, position, acceleration, torque, or force at the joints of the manipulator and the end-effector in robots. Physical contact is required for these sensors to efficiently direct the robot to act accordingly. The sensor is executed in different switches such as a limit switch, button switch, and tactile bumper switch.

The application of contact sensors is commonly found in obstacle avoidance robots. Upon detection of any obstacle, it transmits a signal to the robot so that it may perform various actions like reversing, turning, or simply stopping. [1]

Recommended Reading: Robotic Joint: How Robotic Solution is Boosting Efficiency.

Proximity Sensor

In robotics, a proximity sensor is used to detect objects that are close to a robot and measure the distance between a robot and particular objects without making physical contact. This is possible because the sensors use magnetic fields to sense the objects in question. Proximity sensors are typed into photoresistors, infrared transceivers, and ultrasonic sensors.

Infrared (IR) Transceivers

An infrared (IR) transceiver or sensor measures and detects infrared radiation in its environment. Infrared sensors are either active or passive. Active infrared sensors both emit and detect infrared radiation, using two parts: a light-emitting diode (LED) and a receiver.  These active transceivers act as proximity sensors, and they are commonly used in robotic obstacle detection systems.

On the other hand, passive infrared (PIR) sensors only detect infrared radiation and do not emit it from the LED. Passive sensors are mostly used in motion-based detection.

Ultrasonic Sensor

An ultrasonic sensor is a device that measures the distance of a specific object by emitting ultrasonic sound waves and converts the reflected sound into an electrical signal. Ultrasonic sensors radiate sound waves toward an object and determine its distance by detecting reflected waves. This is why they are mainly used as proximity sensors, applied in robotic obstacle detection systems and anti-collision safety systems.

Photoresistor

Photoresistors are devices that modify resistance depending on the amount of light placed over them. They are also called light-dependent resistors (LDR). Due to their sensitivity to light, they are often used to detect the presence or absence of light and measure the light intensity. With photoresistors, more light means less resistance. [2]

Distance Sensor

Distance sensors are used to define the distance of an object from another object without needing any physical contact. Distance sensors work by emitting a signal and measuring the difference when the signal returns. Depending on the technology, this signal can be infrared, LED, or ultrasonic waves, which is why distance sensors are commonly associated with ultrasonic sensors.

Ultrasonic Distance Sensors

An ultrasonic distance sensor is a tool that measures the distance to an object using high-frequency sound waves. Ultrasonic sensors work by emitting sound waves at a much higher frequency than humans can hear. They then wait for the sound to be reflected. Measuring time lapses between the sending and relay of the ultrasonic wave, and calculating against the speed of sound is how the sensor determines the distance to the target.

Robotic Ultrasonic Wave Transducer

Infrared Distance Sensor

Infrared (IR) distance sensors perceive distance by emitting infrared waves and calculating the angle of reflection.

These sensors usually contain two lenses:

• An IR LED emitter lens casts a light beam and a position-sensitive photodetector (PSD) where the reflected beam is displayed.

• IR distance sensors employ the principle of triangulation; measuring distance based on the angle of the reflected beam.

Laser Range Sensor

A laser range sensor measures the range of target objects using light waves from a laser instead of radio or sound waves. The transmitter on the sensor emits laser light at the target object, which then reflects the pulse of the laser. Distance is then calculated by using the correlation between the speed of light and the time between sending/receiving of the signal. [3]

Encoders

This is a sensing device that offers feedback in robotics. It sends feedback signals that can be employed in determining count, position, direction, or speed. They convert a motion to an electrical signal that can be interpreted by a control device of sorts in a motion control system. Such a control device then uses the information to send a command for a specific function. Encoders make use of different technology types to create signals: magnetic, mechanical, optical (most commonly – based on the interruption of light), and resistive. [4]

Stereo Camera

Stereo Camera is a technique/method of filtering noisy video signals into systematic data sets that are processed into applicable symbolic abstractions or objects. In the broader fields of machine vision and computer vision, stereo cameras are one of many approaches applied. Its application entails the use of two cameras with an established physical relationship (i.e. a common viewing field that the cameras can see, and the distance between their focal points in physical space.

The systems can be used to sense the distance of objects through triangulation. Stereoscopic image processing techniques are used in robotic control and sensing, mobile robot navigation, and off-planet terrestrial rovers. In a nutshell, sensor cameras enable your robots to perceive the environment in real-time, allowing for the implementation of vision-guided robotic applications in your process automation. [5]

Pressure Sensors 

A pressure sensor, also called a pressure transducer, is an electronic device that monitors and regulates pressure, and converts sensed physical data into an electronic signal. Pressure sensors usually employ piezoelectric technology, as piezoelectric elements emit an electric charge proportional to the stress they experience. This stress is typically caused by pressure.

Pressure sensors are used in automobiles for the detection of tire pressure or combustion pressure in engines; also used in factories to regulate steam in machinery, and in aircraft to gauge altitude and atmospheric situations.[6]

Tilt Sensors

This is used in robotics to measure slant angles with horizontal plane reference. They are used to detect inclination or orientation. They are low-powered, small, easy to use, and inexpensive. If used correctly, they will not wear out, and are sometimes referred to as ‘tilt switches’, ‘mercury switches’, or ‘rolling ball sensors’.

These sensor types are used in service robots. Robots use legs or wheels to move, and the inclination has to be measured several times every second to ensure that the robot maintains its constant position. [7]

Navigation/Positioning Sensors

These are sensors that aid the accurate and precise determination of an object’s position in space. Technologies used in positioning, range from worldwide coverage with meter accuracy to workspace coverage with sub-millimeter accuracy.

Navigation sensors provide the ability to determine desired and current positions, and apply corrections to orientation, speed, and course to reach desired positions anywhere around the world. In robotics, navigation and positioning is a major task for autonomous mobile robots. Other common navigation methods in addition to GPS include visual navigation and bionic navigation.

GPS (Global Positioning System)

The GPS is a space-based system of radio navigation. It consists of a constellation of satellites casting navigational signals and a network of satellite control stations and ground stations used for control and monitoring. It does not require the user to transmit any data and independently operates free of any internet or telephonic reception. 

In robotics, GPS is used to aid the robot in its positioning and navigation over a long distance. The robot compares its GPS data and the target’s GPS data to get the relative positioning between the two, which aids in guiding the subsequent direction of motion. This means that such robots are self-piloted vehicles that do not require an operator to accomplish their tasks and navigate.

Digital Magnetic Compass

This is a navigational device capable of showing referential directions relative to the earth’s surface and can be a valuable sensor for robots in navigating their surroundings. It is aligned with the Earth’s magnetic field and comprises a magnetized pointer that is usually marked at the North, and a magnetized needle or bar moving freely upon an axis. Digitally, the compass provides measurements based on the Earth’s magnetic field for the navigation of a robot.

The main robotics applications include motion control and detection of the heading of mobile robots. It is hence part of the vital principles of how autonomous robotic systems navigate.

Localization

This refers to the robot’s ability to identify its position in a given space and within its environment. This is considered important since there are no physical sensors to directly report and measure a robot’s pose, and depending on the application, a range of sensors is used to measure and infer the data measured.  The GPS allows for such reporting of a robot’s position.

However, the GPS is insufficient in itself. For the robot to navigate autonomously, the estimation of the robot’s position and orientation within the map, using gathered information from sensors, is needed. Robotics developers usually use a camera and related technology to create a map of the environment and use it as a reference to localize the robot. This makes it more accurate in detecting obstacles and other relevant objects within indoor and outdoor environments. [8]

Acceleration Sensor 

The acceleration sensor is used for measuring acceleration and tilt. An accelerometer is a device used for measuring static or dynamic acceleration. The sensors provide a reading based on acceleration in singular or multiple directions at a particular time. Accelerometers come in various configurations, so it is important to select one which is appropriate for the robot, taking bandwidth, sensitivity, number of axes (1-3), and output type (analog or digital) into consideration.

Static Force

This is a gravitational force and refers to the frictional force between any two objects. This force is always present, due to gravity. Through the measurement of this force, the extent of a robot's tilt can be determined. This is vital in robot balancing or in determining whether the robot is driving uphill or on a flat surface.

Dynamic Force

This refers to the amount of acceleration required for an object to move. It is the force that acts on an object, causing it to change its position, size, or direction. It is also time-dependent. In robotics, dynamic forces occur due to the motion of a mechanism. The measurement of this force via an accelerometer informs on the speed/velocity at which a robot is moving.

Gyroscope

Gyroscopes, also called "gyros" for short, are devices that measure the rate of rotation. In robotics, they are especially useful in the stabilization of driving robots or the measurement of the heading or tilt. This is done by integrating the rate measurements to obtain the total angular displacement measurement. Hence, gyros can be used in the balancing of a robot and can inform the robot if it is falling over, through programming. The three commonly used types of Gyroscopes are the single-axis, three-axis gyros, and IMUs.

IMU (Inertial Measurement Units)

This is an electronic device that measures and indicates acceleration, angular rates, orientation, and other gravitational forces. It is made up of 3 accelerometers, 3 gyroscopes, and sometimes (depending on the heading requirement) – 3 magnetometers. IMUs are used to maneuver modern vehicles such as aircraft, missiles, unmanned aerial vehicles, satellites, etc. In navigation systems, the data provided by the IMU is entered into a processor that uses it to calculate altitude, position, and velocity. [9]

Voltage Sensors 

Voltage sensors are wireless devices that can be affixed to pieces of machinery or electronic assets. They provide continuous monitoring, looking out for voltage data that might indicate an issue. Extremely high or low voltage can endanger assets or indicate a possible problem. Alerts are quickly transmitted to a centralized computer system when thresholds are breached.

Current Sensors

 This device monitors electric current that flows through a wire and produces a signal corresponding to that current. This generated signal could be a digital output or an analog voltage. The signal can then be used to control the device, be stored for further analysis in a data-collecting system, or be used to show the measured current in an ammeter. [10]

Sensor Fusion

The capacity to combine data from various radars, LiDARs, and cameras to create a single model or image of the area surrounding a vehicle is known as sensor fusion. As a result of balancing the strengths of the various sensors, the model that results is more precise. The data obtained through sensor fusion can then be used by vehicle systems to enable more intelligent behaviors. Utilizing software algorithms, sensor fusion combines data from different types of sensors to produce the most complete and accurate environmental model possible. [11]

Key Takeaways

Sensors provide vital sensory data in robotic systems. These data include position, size, orientation, velocity, distance, temperature, weight, force, and many other factors that aid robots in perceiving their environment and carrying out tasks. Sensors are increasingly relevant with the need for automation and computerized machinery.

References

1. EVSINT. Types of Sensors in Robotics. 2022. [Cited 2023 Jan 9] Available from: https://www.evsint.com/types-of-sensors-in-robotics/

2. Linquip Technews. 5 Types of Proximity Sensors (Application and Advantages) 2021. [Cited 2023 Jan 9] Available from: https://www.linquip.com/blog/types-of-proximity-sensors/?amp=1

3. DreamVu. Different Distance Sensor Types: How do I Choose One? 2022. [Cited 2023 Jan 9] Available from: https://dreamvu.com/different-distance-sensor-types-how-do-i-choose-one/

4. Electric Shocks. What is Encoder? Encoder Construction, Working, and Types Explained. 2022. [Cited 2023 Jan 10] Available from: https://electric-shocks.com/what-is-encoder/

5. ZAPT. Sensors. IMUs, Stereo Cameras, LiDAR, Ultrasonics, and Resolvers. 2021. [Cited 2023 Jan 10] Available from: https://www.zaptllc.com/post/sensors-imu-s-stereo-cameras-lidar-ultrasonics-and-resolvers

6. Thomasnet. Common Types of Pressure Sensors. 2022. [Cited 2023 Jan 10]

7. Utmel Electronic. What are Tilt Sensors? 2020. [Cited 2023 Jan 10] Available from: https://www.utmel.com/blog/categories/sensors/what-are-tilt-sensors

8. Spiceworks. What is GPS? Meaning, Types, Working, Examples, and Applications. 2022. [Cited 2023 Jan 11] Available from: https://www.spiceworks.com/tech/iot/articles/what-is-gps/amp/

9. Omega. How to Measure Acceleration. 2021. [Cited 2023 Jan 11] Available from: https://www.omega.com/en-us/resources/accelerometers

10. Thomasnet. All About Electrical Sensors. 2022. [Cited 2023 Jan 11] 

11. Appen. What is Sensor Fusion? 2021. [Cited 2023 Jan 11]