Robot Sensors – All concepts you need to know and 5+ Important FAQs

Robot Sensors

Image Credits: “SEER: Simulative Emotional Expression Ro” (CC BY-NC-ND 2.0) by Ars Electronica

The subject of discussion: Robot Sensors and their characteristics

What are Robot Sensors? | Sensor guided Robotics

Robot Sensors are the sensors that a robot uses to communicate with its environment by calculating physical quantities around it. Sensors operate on the theory of transduction, which involves the conversion of energy from one form to another and sensed data is processed by a controller, which allows the robot to take action. Robot Sensors also keep track of a robot’s condition along with its surrounding situation.

robot sensors
Different Types of Robot Sensors; Image Credits: Open Source Robotics Foundation, Роботизовані датчикиCC BY 3.0

As mentioned earlier, Robot Sensors are used to assess a robot’s condition and its surroundings. To allow acceptable behavior, these signals are transferred to a controller. Robotic sensors are modeled after the roles of human sensory organs. In order to work appropriately, robots need a lot of knowledge about their surroundings.

Why are sensors important to robots ?

Robot Sensors may be mechanical, chemical, or electrical in nature and each of the sensor’s operation is based on the transduction principle, which transfers energy from one type to another. The robot sensors allow the robot to respond to its surroundings in a flexible manner. Robots can see and feel with the aid of sensors, which will enable them to perform more complex tasks.

Robot Sensors track robots’ health and their surroundings, sending electronic signals to the robots’ controllers. Sensors are needed for robots to monitor themselves. Robots need knowledge about the location and movement of their bodies and parts in order to monitor their behavior.

Characteristics of sensors in robotics

The characteristics of robot sensors help us in determining the appropriate sensor for the robot in various situations. Some of the essential attributes of robot sensors are described below:

Accuracy

The accuracy of a sensor refers to the closeness of the recorded sensor value is to the actual value. This is often articulated as a range of values. For instance, +/-1mm. By looking at the calibration section below, we can often increase the accuracy of robot sensors. Hence accuracy is the difference between the sensorʼs output and the actual value, i.e., error = measured value – true value.

Calibration

The robot sensors’ accuracy and resolution can also be enhanced by calibrating it. A separate portion has been dedicated to resolution. Calibration is the method of comparing the sensor performance with some known quantities, which can be performed by a vendor or by you and this info can be further utilized to create an equation that connects this two.

This equation will produce better results than the default values when processing sensor data. You must also understand when the sensor becomes overloaded (when you go above or below the limit of what it can measure) and the data becomes less reliable or meaningless.

Resolution

It’s crucial to know how tiny of an increment the robot sensors can detect until we know how accurate it is. A temperature sensor with a resolution of 5 degrees, for example, cannot distinguish between 30 and 32 degrees. As a result, resolution refers to the smallest amount of change in the input that the sensor can detect and reliably indicate., e.g., What is the resolution of a regular ruler or that of Vernier Calipers?

Linearity

Whether the robot sensors’ performance is linear or not, that information becomes useful when feeding the sensor’s output to a low-level computer that can’t do much computation and designing the calibration equation. The calibration curve determines linearity. Under the static condition, the fixed reference curve plots the o/p amplitude versus the i/p amplitude and resemblance to a straight line or linearity.

Repeatability

A critical feature of the robot sensors is that it should give the same result every time you measure the same conditions. This provides the repeatability of the sensors.

Dead Band and Hysteresis

In mechanical systems like the robots, some slop in the gears always causes a different value depending on the direction of motion (hysteresis) or a dead band when the robot sensors do not detect some motion.

Drift

The calculation of specific robot sensors has an intrinsic drift. This is particularly true for rate gyroscopes. You’ll want a model with low drift (the lower the drift, the more it costs), as well as the ability to filter the output of the sensors. For example, the robot is stationary; it is understood that the sensor is not rotating, so you can ignore the gyro and do fancy things like ignore the sensor and/or determine drift rates and apply them to increase sensor performance.

Temperature

The temperature has two components that account for the robot sensors’ characteristics. The first is the ability to maintain temperature control. A common issue with many sensors is whether the sensor value drifts/changes as the temperature changes or not. There are two sections of the second temperature specification as well:

  1. Usable Temperature – What are the sensor’s minimum and maximum temperature range?
  2. Storage Temperature – What is the lowest/highest temperature that the sensors can be until they are damaged?
Robot Sensors: Temperature Sensor LM 35© Nevit Dilmen, LM35 temperature sensor semiconductor thermometer 1480374 5 6 HDR enhancerCC BY-SA 3.0

Field of View (FOV)

The FOV (Field of View) is a critical specification that indicates what area (usually angular) the robot sensors can see. Horizontal (hFOV) and vertical (vFOV) components are frequently mentioned. For instance, 70×30 degrees represents hFOV x vFOV respectively.

Spot Size

This is mainly about lasers, but knowing how big the spot size is at a given distance is significant (the spot gets larger with space). This spot size is critical for deciding the size of items that can be seen through. For seeing through dust, rain, and snow, small spot size is essential. Both a horizontal and vertical spot scale can be used to express this. It is common for robot sensors’ manufacturer only to publish one of the two values because the other is larger.

Output Form

One must understand the sensor’s output form. For analog output, for example, you may want to know what the voltage or resistance range is. If the production is at a higher stage, make sure you have the appropriate input. 4-20mA, voltage, USB, ethernet, serial, and CAN are all common output types. Keep in mind that gigabit ethernet cameras often use a jumbo packet (large MTU) that is incompatible with 802.11 wireless standards and requires a wired link.

Power

One must know how much power the robot sensors consume and what voltage range it can accept in order to power the system correctly. Some robot sensors will have a wide range, while others will only need one DC-DC for a tightly controlled input voltage.

Reliability

Reliability is a complex parameter to evaluate for robot sensors. Several factors influence reliability. Is the program well-developed and robust? Is the sensor robust in terms of physical strength? Is it well-constructed? Is there any electrical safety (protection diodes, fuses, etc.)? Are the connectors in good shape? Are the connectors going to fall out? Is it water-resistant? Does it have a dust-proof seal? The list of such questions can never fall short.

How are the sensors classified? | Types of Robot Sensors

Proprioceptive and Exteroceptive Sensors in Robotics

The primary classification of the robot sensors is done based on the location of the stimuli-

Proprioceptive Sensors | Internal Sensors in Robotics

Proprioceptive (PC) sensors provide the robot with a sense of self. They calculate internal values to the robot system, such as joint angle, wheel position, battery level, and so on.

Exteroceptive Sensors | External Sensors in Robotics

Sensors that provide knowledge about the external state, such as observations of the environment and its objects, are known as exteroceptive (EC).

Active and Passive Sensors in Robotics

Another set of classification is based on the way of energy dissipation-

Active Sensors in Robotics

Active sensors, such as radar-based sensors, operate by emitting radiation (A).

Passive Sensors in Robotics

Passive sensors are sensors that passively obtain energy, such as a camera (P).

Different types of sensors used in Robots | Robot sensors and transducers

Some of the standard robot sensors can be classified into proprioceptive and exteroceptive, and active and passive, respectively, as given in the following table:

Sensor TypeSensor SystemPC/ECA/P
Tactile Sensor (Physical contact detection)Contact SwitchesECP
Optical BarriersA
Non-contact Proximity SensorsA
Wheel and Motion Sensor (Speed and Position detection)Brush EncoderPCP
Optical EncoderA
Synchros, RevolverA
PotentiometerP
Inductive EncoderA
Capacitive EncoderA
Magnetic EncoderA
Heading Sensor (Robot orientation concerning reference frame)GyroscopePCP
CompassECP
InclinometerECP/A
Ground-based Beacon (Localization in fixed reference frame)GPSECA
Reflective Beacon
Active Ultrasonic Beacon
Active Optical / RF Beacon
Active RangingUltrasonic SensorsECA
Reflectivity Sensors
Laser Rangefinder
Motion/Speed SensorDoppler RadarECA
Doppler Sound
Vision-based SensorObject Tracking PackageECP
Visual Ranging Package

How do robots use sensors? | What problems can robots solve using sensors ?

Applications of sensors in Robotics

Robots, unlike humans and animals, lack naturally occurring senses. Engineers will have to develop them as sensors for robots. Robots use sensors to build a view of the world they are in. LIDAR is an example of a sensor used in some robots (Light Detection And Ranging).

LiDAR is a distance measurement device that employs a laser. Lasers shine a light on objects in the atmosphere and then mirror them back. The robot uses these reflections to build a map of its surroundings. LiDAR tells robots what is going on in their environment and where it is.

LiDAR Robot Sensor ; Image Credits: “Garmin LIDAR-Lite Optical Distant Sensor” (CC BY-NC-SA 2.0) by adafruit

What sensors are used in robots?

Types of Vision Sensors used in Robotics | Visual Sensors Robotics

Vision sensors use images to assess the presence, orientation, and accuracy of nearby objects. Image acquisition and processing are combined in vision sensors, and multi-point inspections can be performed with only one sensor. Data is also exchanged between the video camera and the computer processing unit through vision sensors. The monochrome and color vision sensors are the two forms of vision sensors.

Cameras are necessary for robots to traverse the environment and avoid colliding with nearby objects because they are sensors that capture and analyze data. 2D imaging, 3D sensing, ultrasonic, and infrared are all examples of camera technology.

2D Imaging

Digital cameras resemble film cameras in appearance, but they are based on very different scientific concepts. A digital camera, unlike a television, which creates images by pixels, captures photon and converts them into electrical signal, that can be processed as number. The CCD and CMOS are the two types of  two dimensional  digital cameras.

3D Sensing

3D sensing is an effective tool for robot navigation since it provides data on an object’s volume, shape, location, orientation, and distance. Different processes, such as stereo vision, organized light, and laser triangulation, can produce 3D data.

Ultrasonic

Ultrasonic cameras, also known as sonar cameras, calculate the time lapse between the transmission and detection of sound waves to determine the distance between the camera and an object. Other ultrasonic sensors or ultrasonic sensor-bearing robots may also be detected using ultrasonic cameras.

Ultrasonic Robot Sensor; Image Credits: Image by beear from Pixabay

Infrared Sensor Robot

Infrared robot sensors detect the infrared (IR) rays emitted by an object. They may also use IR light to project toward a target object and receive the reflected light to determine its distance or proximity. Infrared sensors are cost-effective and can track infrared light over a wide area. They also work in real-time. They’re better than ultrasonic sensors at describing an object’s edge and distinguishing one thing from another.

IR sensor working in line follower robot

In the vision based or optical navigation, a computer vision algorithm and optical robot sensors, such as laser-based range finders and photo metric cameras with CCD arrays, are utilized for extracting the visual features required for localization in the surrounding world, though there are other varieties of vision based navigation system and localization techniques. The following are the critical components of each method:

  • Representations of the natural world
  • Models for sensing
  • Algorithms for localization

The simplest way to get a robot to go to a specific location is to direct it simply. This can be accomplished in various ways, including burying an inductive loop or magnets in the floor, drawing lines on the floor, or inserting beacons, markers, or bar codes in the environment. In industrial scenarios, such Automated Guided Vehicles (AGVs) are used for transportation tasks. Robots can navigate indoors using IMU-based indoor positioning systems.

Sonar-based navigation systems have also been developed. Robots can also use radio navigation to assess their location. The onboard flight controller utilized GPS for navigation and stabilization, and satellite-based augmentation systems (SBAS) and altitude sensors such as barometric pressure sensors are often used for the measurements and Inertial sensors are used in some airborne robot navigation systems. Underwater acoustic positioning systems can direct autonomous underwater vehicles.

Force Sensors in Robotics

Robot Arm Force Sensor

Force sensors are used to sense forces between the sensor’s base and the sensing layer. FT Sensors, or Force-Torque Sensors, sense both forces and torques. They’re typically mounted just before the end-effector on a robot’s arm. Sensors can be used in a wide range of applications and there are inexpensive analog pressure sensors all the way up to the most popular 6-axis FT sensors.

Since they are not tactile sensors, they cannot be used to detect slipping forces. However, they can be used to detect power. Given the variety of force sensors available as given below, deciding which one you need can be difficult.

  • Simple Pressure sensor.
  • Piezo-electric sensor.
  • Strain Gauge based sensor.
  • Capacitive FT-Sensors.
  • Capacitive and Resistive Flexible Force Sensors.

Temperature Sensor in Robotics

Temperature sensors are used to detect changes in ambient temperature and it is based on the idea that a change in voltage would have the same temperature value as the surrounding for a temperature change. TMP35, TMP37, LM34, LM35, and others are some of the most commonly used temperature sensors ICs.

What Sensors does an Industrial Robot have?

2D visual sensors, 3D visual sensors, force or torque sensor and collision-detection sensors are the most widely used sensors for industrial robots. Some of these explained as follows:

2D vision sensor

A two-dimensional vision sensor is a camera that can track moving objects and locate pieces on a conveyor belt, among other things. This capable to detect and assist the robot in determining their location, and the robot can then change its motion accordingly based on the information obtained.

3D vision sensor

To sense the object’s third dimension, the 3D vision device must have two cameras or laser scanners at different angles. Part picking and placement, for example, requires the use of 3D vision technology to identify objects and produce 3D images, as well as the analysis and selection of the best picking process.

Force Sensor | Torque sensors in Robotics

If the visual sensor provides the robot with eyes, the force/torque sensor provides the robot with a sense of touch. The end effector force is sensed by the robot using force/torque sensors. In most situations, the force/torque sensor is placed between the robot and the fixture, allowing the robot to track all forces fed back to the fixture.

Collision Detection sensor

This sensor is available in a variety of shapes and sizes, and its primary purpose is to provide a secure working environment for operators, which collaborative robots need the most.

What Sensors do Assistive Robots have?

An assistive robot is a computer that can feel, process, and perform acts in the everyday lives of people with disabilities and older adults. The most popular use of robot sensors is the use of ultrasonic sonar to assist blind people. For years, robots have used ultrasonic sonars as a ranging system.

Range Sensor in Robotics | Proximity Sensor Robot

The range sensors are used to determine the distance between the object and the robot side. Its operational range is limited. Visual processing is used to calculate the distance. Robots use range sensors to navigate and avoid obstacles in their way. Special applications for the range sensors are to determine the position and general shape characteristics of the component in the robot’s work envelope. The illumination source in these situations may be a light source, a laser beam, or ultrasonic.

Position Sensors in Robotics

Any sensor that measures the location of an object for use in control applications is referred to as a position sensor. They have a wide range of sensors and have a variety of applications, ranging from robotics to MRI machines. This is one of the most powerful sensors, and it’s used in almost every autonomous vehicle that moves.

Both rotary and linear motion can be measured using position sensors. They can be used to calculate absolute or relative location. Brushless motors are controlled by rotary sensors, which often keep track of the angular areas of various mechanical devices in the system. Motor encoders are sensors used in robotics to keep track of location on a circular disc, converting position into electrical pulses that a controlling entity can use.

Robot Arm Position Control

Six individual servo motors are capable of moving each joint in a typical six-axis robot. A motor encoder is used on the back of these servo motors to maintain position. To keep track of the rotations produced by the servo motor, a motor encoder uses cutouts in a disc. Light pulses are produced by these cutouts, which are then converted into electrical pulses.

Optical Position Sensor in Robotics

Optical sensors are used to track, count, and position parts without using any touch. Internal or external optical sensors are available. Internal sensors are typically used to measure bends and other subtle changes in direction, whereas external sensors collect and relay a specified amount of light.

Velocity Sensors in Robotics

A velocity or speed sensor takes several position measurements at regular intervals and calculates the rate of change in position values over time. One of the essential velocity sensors used in robotics is the tachometer.

Tachometer

The tachometer is one of the most critical devices for providing velocity feedback. It’s also used as a revolution counter and an rpm gauge. In a motor, a tachometer is used to measure the rotational speed of a shaft. In an analog unit, the output is shown as RPM (revolutions per minute).

Acceleration Sensors in Robotics

Acceleration and tilt are measured using an acceleration sensor,a device that measures acceleration is known as an accelerometer. Static Force and Dynamic Force are the two types of forces that influence an accelerometer.

  • Static Force- It’s the frictional force that exists between two things. We can calculate how much the robot is tilting by calculating the gravitational force. This calculation helps balance the robot or decide whether it is moving uphill or on a flat surface.
  • Dynamic Force- It refers to the amount of force needed to move an object. The velocity/speed of a robot can be determined by measuring dynamic force with an accelerometer.

Robot Vacuum Cleaner Sensors

Robot Vacuum Cleaners use various sensors to detect obstacles and monitor their progress and discover new areas to explore and these robot vacuum cleaner sensors trigger programmed responses, which decide how the robot should reacts if it face some obstacles.

Obstacle Sensors

To work in a home environment, there  are different obstacles such as chair and table legs, sofas, other home appliances stands and stray-toys etc. In the Vacuum’s sensors installed in shock-absorbing bumpers allow it to navigate these obstacles efficiently with out slowing down and this sensor will be activated when the bumper collides with a barrier, and the robot is auto instructed to turn and walk away.

Cliff Sensors

Stairs are perhaps the most dangerous obstacle for robot vacuums; a fall could damage the vacuum as well as anything in its way. As a result, all robot vacuums must have cliff sensors as a safety feature. They use infrared signals to calculate the distance to the floor’s surface continuously.

Wall Sensors

They actually assist them in detecting walls using infrared light so that they can follow them. This enables them to clean the edges of the wall where it meets the floor. The best part is that they can do so without scuffing the wall, as we sometimes do with standup vacuums.

Wheel Sensors

The wheel rotation of a robot vacuum is measured using light sensors. It will estimate how far it has traveled using this number and the wheel circumference.

What is Touch Sensor in Robotics?

The touch sensor used in robotics is also termed a Tactile Sensor. To learn further about it, click here.

Sensors used in Robotic Welding

To learn about robot sensors used in robotic welding click here.

Sensor Fusion Robotics

Many industries and environments are seeing an increase in demand for rigid, multipurpose robots that are simple to set up. Sensors for contextual awareness and intuitive interfaces for ease of use are now needed for robots. Some applications, for example, may use gesture recognition to manipulate a physical device.

At the same time, IoT protection, low power consumption, safety, and reliability are all strict requirements. This often entails using sensors to monitor electrical current, temperature, and other variables to ensure that a system operates efficiently and safely. In the near future, robotics will increase the number of motors and the versatility of the environment, and more collaborative robots will be seen worldwide. The number of sensors used by robots will increase as more control systems and settings are designed.

About Esha Chakraborty

I have a background in Aerospace Engineering, currently working towards the application of Robotics in the Defense and the Space Science Industry. I am a continuous learner and my passion for creative arts keeps me inclined towards designing novel engineering concepts.
With robots substituting almost all human actions in the future, I like to bring to my readers the foundational aspects of the subject in an easy yet informative manner. I also like to keep updated with the advancements in the aerospace industry simultaneously.

Connect with me with LinkedIn - http://linkedin.com/in/eshachakraborty93