Introduction to robotics - module 3, Slides of Introduction to Robotics

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MODULE-

Sensors in Robotics

Introduction

• Human-beings collect information of the surroundings

using their sensors, namely eyes, ears, nose, skin etc.,

in order to perform various tasks.

• A sensor is used to take measurement of physical

variable.

• Sensors in robotics are devices that detect and measure

physical properties or environmental conditions,

providing crucial input for decision-making and control

within robotic systems.

Characteristics of sensors

Response Time: Response time refers to the time it takes for a sensor to respond to changes in the measured quantity. A sensor with a fast response time can quickly detect and react to changes in the input signal. Repeatability: Repeatability measures the sensor's ability to provide consistent measurements when exposed to the same input signal multiple times under similar conditions. A sensor with high repeatability produces consistent results with minimal variation. Stability: Stability refers to the sensor's ability to maintain its calibration and performance characteristics over time. A stable sensor retains its accuracy and reliability over extended periods of use. Noise: Noise refers to unwanted variations in the sensor's output that are not caused by changes in the measured quantity. A sensor with low noise produces a clean and reliable output signal, free from interference. Durability: Durability indicates the sensor's ability to withstand environmental factors such as temperature, humidity, vibration, and shock without degradation in performance. A durable sensor is suitable for use in harsh or demanding conditions.

Different types of sensors

Active Sensors : Require an external power source to operate and emit a signal,

such as ultrasound or radar sensors.

Examples of active sensors:

Electrical Sensors : Utilize electrical properties for sensing, such as voltage, current, or resistance, like thermistors or photovoltaic cells. Optical Sensors : Utilize light for sensing, including photodiodes, phototransistors, or light sensors. Mechanical Sensors : Operate based on mechanical properties such as displacement, strain, or pressure, including strain gauges, accelerometers, or pressure sensors. Chemical Sensors : Detect chemical properties or changes, such as gas sensors or pH sensors. Biological Sensors : Sense biological phenomena, such as biosensors used in healthcare or environmental monitoring.

Passive Sensors : Do not require an external power source and rely on ambient

energy sources, such as thermocouples or photodiodes.

Classification of sensors used in robots

Internal Sensors (Also known as proprioceptive sensors):

• (^) These sensors are integrated into the body or the structure of a robot system which sense the

information's related to the internal parameters of a robot.

• (^) Proprioceptive sensors are like the "sensory organs" of robots. They help robots understand where

their own parts are and how they are moving.

• (^) Just like how humans know where their arms and legs are moving without looking at them,

proprioceptive sensors give robots a sense of their own body position and movement.

• (^) These sensors provide feedback to the robot's control system, helping it adjust its actions

accurately and safely.

• (^) They mainly include position, velocity, and acceleration sensors.

Classification of sensors used in robots

External Sensors (Also known as exteroceptive sensors):

• (^) These are the sensors that interact with the environment or the surroundings.
• (^) Exteroceptive sensors are like the "eyes" and "ears" of a robot. They help robots

perceive and understand the external world around them. These sensors detect things

outside of the robot's body, such as objects, obstacles, or environmental conditions.

They provide information about the robot's surroundings, like distances to objects,

colors, shapes, sounds, or temperatures. This data helps robots navigate, avoid

obstacles, interact with objects, and make decisions based on what they "see" and

"hear" in their environment.

• (^) Examples of exteroceptive sensors include cameras, ultrasonic sensors, infrared

sensors, and microphones.

• (^) They may include touch sensors, proximity sensors, force/torque sensors, vision sensors, and many

others.

Position sensors Potentiometer

• (^) It is a position sensor, that generates voltage as a form of output across the resistor corresponding to the change in the position.
• (^) The potentiometer can be of linear or angular type. It works on the principle of conversion of mechanical displacement into an electrical signal.
• (^) The sensor has a resistive element and a sliding contact (wiper). The slider moves along this conductive body, acting as a movable electric contact.
• (^) As the position changes, the location of the wiper on the potentiometer also changes, based on which effective resistance also changes.

Position sensors

Incremental optical encoder:

• (^) An incremental encoder consist of a disc which is rigidly mounted on

the shaft whose angular displacement is to be measured and two

photo-detectors.

• (^) The coded disc has light and dark zones.
• (^) When the light is projected on the disc from one of the side, the plots

corresponding to the light and dark zones are obtained using two

photodetectors.

• (^) Now, by counting the number of light and dark zones, angular

displacement of the shaft can be measured with respect to its known

starting position (that is, reference).

• (^) It can also determine the direction of rotation of the shaft. It is

construction-wise simpler, less accurate and less expensive compared

to absolute optical encoder.

Position sensors

LVDT (Linear Variable Differential Transducer)

Velocity sensor

• (^) Velocity can be calculated by monitoring the position changes in a fixed time

interval.

• (^) Hence, all position sensors can be utilized for calculating velocity also.
• (^) While using encoders, the number of pulses, which gives the measure of

displacement when divided by the time taken, will give the velocity measure.

Tachometer

• (^) It directly measures the velocity or rotations of an element in unit time.
• (^) It works based on Fleming's rule, which states that the voltage produced is

directly proportional to the magnetic flux linkage.

• (^) The conducting coil develops a voltage as the shaft rotates in a magnetic field

produced by the permanent magnet (stator), and the voltage developed in the

coil is proportional to the speed of rotation of the shaft.

• (^) The arrangement can also be reversed by keeping a magnet on the rotating

shaft and making the magnet as the rotor and coil on the stator.

• (^) In both cases, the speed of rotation of shaft can be measured using the voltage

developed in the coil as they are proportional to each other

Force sensors (Contact Sensor)

• (^) Force sensors are used for calculating the force applied or experienced are

termed as ford

Strain gauge:

• (^) Strain gauges are popularly used in robotics for calculating the force

applied at the end-effector and the wrist of a robot.

• (^) In a few applications they are also employed for measuring the loads on

the joints and links of the robot.

• (^) A strain gauge consists of a conducting material of fine wire or foil that is

cemented on a surface where strains are to be measured.

• (^) The resistance changes due to the change in length of conducting wire

due the strain.

• (^) The resistance of a conductor is directly proportional to its length and

inversely proportional to the cross-sectional area.

• (^) When a conducting wire is stretched or compressed, its resistance varies

depending on the amount of stress/force applied on it.