sensors for transducers - yazd
TRANSCRIPT
� 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.