Sensors in Robotics

By Marta Kargol, Ph.D.Jan 10 2019

Shutterstock.com/ FeelGoodLuck

Robots are programmable machines which interact  with the physical  world via sensors and actuators, and can be made to work autonomously or semi-autonomously. Sensors measure a physical  parameter, e.g. temperature, speed, and convert it into a signal which can be measured electrically.

Why Robots need Sensors

During the design of a robot it is important to choose the accurate sensors to enable it to be aware of its environment and perform required tasks. Robots without sensors are simply constant automation doing the same repetitive task.

Robotic sensing gives the robots sensing capabilities so that the robots are more human-like. Thus, the robots can see, touch, hear and move. Some common robotic sensors and their functions are given in the table below.

Table 1. Common sensors used in robots [1].

Sensors are devices that can sense and measure physical properties of the environment, e.g., temperature, size, weight, luminance, resistance to touch, etc. The term sensor differs from the term transducer. Transducers convert one type of energy into another, whereas sensors convert any type of energy into electrical [2].

A sensor is part of a larger system that contains other detectors, memory devices, data recorders, actuators, signal conditioners and signal processors. The general classification of sensors is as follows:

• Internal/Proprioceptive sensors (for robot control) that include position sensors, velocity sensors, torque and acceleration sensors
• External/Exteroceptive sensors (for robot guidance, obstacle identification, monitoring) that include tactile sensors, force and torque sensors, proximity sensors, range sensors, vision sensors and others

Sensors can be classified in many ways, they can be:

• active or passive (non-active)
• contact or non-contact
• visual (or eyes) or non-visual
• absolute or relative

All sensors can be characterized by various properties describing their capabilities, such as sensitivity, linearity, measurement/dynamic range, response time, accuracy, repeatability, resolution or bandwidth.

Sensors Used in Robotics

Proximity sensors – detect objects that are closely placed without any point of contact. These sensors can detect the presence or absence of the object by using light, sound or electromagnetic field. Proximity sensors include:

•            Infrared sensors
•            Ultrasonic sensors
•            Photoresistor

Range sensors – measure the distance between objects in the operation area. Range sensors are used for robot navigation and for avoiding obstacles.

Tactile sensors – specify the contact between an object and sensor. These sensors are divided in two types, namely:

• Touch Sensor
• Force Sensor

Light sensors – detect light density and they are capable to convert light into either voltage or current. They include:

• Photoresistors
• Photovoltaic cells

Sound sensors – detect sound and return a voltage proportional to the sound level.

Temperature sensors – provide voltage difference for a change in temperature.

Contact sensors – require physical contact against other objects to trigger; mostly used for obstacle avoidance robots.

Voltage sensors – convert lower voltages to higher voltages or vice versa.

Current sensors – electronic circuits which monitor the current flow in a circuit and output either a proportional voltage or a current.

Sensors are highly applicable in robotics in safety monitoring, interlocking in work cell control, the quality control in work part inspection and data collection of the objects in the robot work cell.

For example, sensors are used in industrial robotics for monitoring the safety conditions in the robot cell layout, and to avoid physical injuries. Sensor technologies and components applied in robotics are currently under increasing development and are under the growing focus of researchers and investors.

According to Market and Forecasts Report on Sensors in Robotics [3], by 2027 the market for robotic sensing will reach over $16.1 billion; only vision systems will reach a market of $5.7 billion, force sensing will reach over $6.9 billion while multiple sensors in domestic robots will attain $3.6 billion.

Sources

[1] J. Fraden, Handbook of Modern Sensors: Physics, Designs and Applications, Third Edition, 2004.

[2] H. R. Everett, Sensors for Mobile Robots: Theory and Application, A. K. Peters/CRC Press; First edition, 1995.

[3] K. Ghaffarzadeh, J. Harrop, Sensors for Robotics: Technologies, Markets and Forecasts 2017-2027, Research reports, www.idtechex.com.

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