� Touch sensors send a signal when physical contact has been made.

� Simplest form of a touch sensor: a micro-switch

� A tactile sensor is an array of touch sensors.

� It can provide additional information about the object such as shape,

size, or type of material.

Different sensors can be used for this purpose.

Touch and Tactile Sensors

� Different sensors can be used for this purpose.

Touch sensor

Light sensorLED

Plunger

When touch happens, the plunger

moves in and crosses the light beam

Proximity Sensors

� Used to determine that an object is close to another

object before contact is made.

� Magnetic:

� Activated when they are close to a magnet.

� Optical:

� Receiver can only see the signal when within a range.

Emitter

Receiver

In range

Emitter

Receiver

Out of range, too far

Emitter

Receiver

Out of range, too close

Proximity Sensors: Cont.

� Ultrasonic:

� The emitter emits frequent bursts of high frequency sound

waves.

� Opposed mode and echo (diffused) mode.

� Cannot be used with surfaces that do not reflect the sound � Cannot be used with surfaces that do not reflect the sound

waves.

Emitter/

Receiver Emitter Receiver

Echo Mode

Object

Opposed Mode

Proximity Sensors: Cont.

� Inductive:� Used to detect metal surfaces.

� An oscillator sends an alternating current to a coil with a ferrite core, that produces a magnetic field.

� When, a metallic object enters into the magnetic field, it changes the magnetic permeability of the environment and causes more energy loss in the (core + target).and causes more energy loss in the (core + target).

� This results in reduction on the amplitude of (voltage) oscillations.

� This reduction is detected by a detector circuit and activates a solid state switch.

� These types of sensors are widely used for measuring the rotation of gears, rotors, etc.

Proximity Sensors: Cont.

� Capacitive:� Reacts to the presence of objects with a dielectric constant more than

air dielectric constant.

� Can detect non-metal materials such as wood, liquids, and chemicals.

� It works similar to the inductive type; here capacitance is changed instead of permeability.

+-

Range-finders

� Used to find larger distances, to detect obstacles, and to map the

surfaces of objects or environment.

� Meant to provide advance information to the system.

� Generally based on light (visible light, infrared light, or laser) and

ultrasonic. ultrasonic.

� Two common methods of measurement:

� Triangulation

� Time of Flight

� The range or depth is calculated from the triangle formed between

the receiver, the light source, and the spot on the object.

� This only occurs at one particular instant.

� The emitter is rotated (through a rotating mirror) until the spot is

seen.

Range-finders: Triangulation

seen.

� The angle α is sensed.

� Since β and L are known, the distance d can be computed from the

value of α :

dl1

Rotating

Emitter

Dl1

Rotating

Emitter

(a) (b)

Receiver Receiver

l2

L L

l2α

βtan tan

tan tan

Ld

α β

α β=

+

� Ranging consists of sending a signal from a transmitter that bounces

back from an object and is received by a receiver.

� The distance between the object and the sensor is half the distance

traveled by the signal.

� Range is calculated by measuring the time of flight and knowing its

Range-finders: Time of Flight or Lapsed Time

� Range is calculated by measuring the time of flight and knowing its

speed of travel.

Range Finders: Ultrasonics

� Rugged, simple, inexpensive, and low powered.

� Readily used in cameras, for motion detection and navigation.

� Limited resolution due to wavelength of the sound and variations of

temperature and velocity in the medium.

� Most measure the distance using the time-of-flight technique.� Most measure the distance using the time-of-flight technique.

� Maximum range is limited by the absorption of ultrasound energy in

the medium.

� Typical frequency range of 20 kHz to above 2 MHz:

� Higher frequencies yield a better resolution, but attenuate much faster.

� Lower frequency transducers have wider beam angles, therefore

inaccurate.

� Beam angle: angle between the two planes of light where the intensity

is at least 50% of the maximum intensity at center beam.).

Range Finders: Light-Based

� Different Frequencies:

� Normal Light

� Laser

� Infra-red

� Based on the following methods:

� Direct time delay measurement,

� Triangulation.

� Stereo imaging

Range Finders: GPS

� Includes 29 satellites orbiting the Earth.

� Each satellite sends signals at precise intervals with information

about the time the signal was sent and location of the satellite.

� The GPS unit must read signals from at least 4 satellites to be able

to compute its own position.to compute its own position.

� Using the differences between current time and the time when each

signal was sent to calculate the distance to the satellite.

Range Finders: GPS

� Each distance forms a sphere centered at each satellite.

� The intersection of these 4 spheres is the location of the

GPS unit.