Download Sensors

Sensors
Instructor: Shuvra Das
Mechanical Engineering Dept.
University of Detroit Mercy
Flowchart of Mechatronic
Systems
Sensors
• Transducers convert one form of energy
into another. Two types: Sensors and
Actuators
• Devices that permit the measurement of
physical quantities such as Force, Stress,
Temperature, velocity, pressure, flow rate
etc. (converts physical quantity to electrical
signal)
Sensor Types
Sensor Applications
Examples
Mechanical
Position, disp., velocity, Acc.,vib.,
stress/strain, pressure
Voltage, current, resist., cap.,
inductance, magnetic, radiation
Sound intensity, visc., flow rate,
ultrasound NDE, frequency
Enzymes, pH, concentration, gases,
hum.,
Intensity, wavelength, phase, vision,
interference, polarization, scattering,
etc.
Temperature, infrared radiation, etc.
Electrical
Acoustic
And Flow
Chemical and
Biological
Optical
Thermal
Some Sensors: Potentiometers
• Measures Linear or angular position
• Measures voltage between the wiper and one end
of pot
• Voltage linearly related to distance
• Simple, common
• Disadvantage: Subject to wear and tear
• Disadvantage: difficulty with displacement that
varies with time quickly (limitation for dynamic
measurements)
Potentiometers
• For linear measurement it consists of a wire
coil, a moving contact or wiper and a DC
source with constant voltage V
• V0 = (V/ymax)y
Potentiometers
• Rotary potentiometer is used for angular
measurement.
• V0 = (Vi/span in degrees)y
• Sensitivity r = ymax/N; N= number of turns/coils
Example: Throttle sensor
• Refer to doc file…pot_example.doc
Some Sensors: Optical Encoders
• Converts motion into a series of digital pulses.
• By counting a single bit or a set of bits the pulses
can be converted into relative or absolute position
measurements.
• Rotational encoders are manufactured in two basic
forms: the absolute encoder and the incremental
encoder (allows measurement of relative position
of the shaft).
Optical Encoders: Absolute
Digital signal  linear or angular position
Useful in motion control
A light source shines light on pad
Reflected/transmitted light picked up by
photodiodes
• The code increment is converted to displacement
• A n bit code corresponds to 2n combinations
representing range
•
•
•
•
Optical Encoders:Absolute
•
•
•
•
•
•
Several concentric tracks divided into sectors
for n=4 tracks divided into 2n = 24=16 sectors
Dark = 0 , light = 1 in each sector
resolution = 360/ 24=22.5 (with 4 bits)
resolution = 360/ 25 =11.25 (with 5 bits)
Can have upto 22 tracks
– resolution = 360/222=0.000086
Optical Encoders:Absolute
• The code used is not binary but modified
binary or Gray code.
• Advantage: only one window changes at a
time.
Binary to
Gray
Decimal code
Binary
Gray
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
0000
0001
0011
0010
0110
0111
0101
0100
1100
1101
1111
1110
1010
1011
1001
1000
Incremental encoder
• Simpler in design
• With relative motion a
succession of rows read
• A and B are about 1/4
cycle out of phase so with
disc rotation the actual
phase difference
determines direction
• Speed is from freq. Of
pulses.
• An index row counts
number of total revs.
Light detecting Transducers
• Light sensitive detectors, or photocells can
be categorized as either thermal detectors or
photon detectors.
• Thermal detectors include a temperature
sensitive element, which is heated by
incident light.
• The photon detectors respond directly to
absorbed photons.
Light Detecting Transducers
•
•
•
•
•
•
Some of the photon devices are:
photoemmissive\ or photomultiplier
photoresistive
photovoltaic (solar cell)
photodiode
phototransistor
Photoresistors
• Photoresistors are variable resistors that
function in ways similar to potentiometers
• The change in resistance is caused by a
change in light level.
• The active elements are made of chemicals
such as Cadmium Sulphide
• The electrical resistance is inversely
proportional to light intensity.
Physics of Light
• Light can be considered as energy carrying particles,
photons
• Also, it can be considered as an electromagnetic
wave.
• Energy of photon:
• E = hf = hc/ l
– h – Planck’s constant (6.6326 X 10-34 Ws2)
– f - Frequency
l– Wavelength (m)
– c - Speed of light (300,000 Km/s)
Photoconductive Sensors
• Photoresistor: A piece of
semiconductor material
placed between conducting
end plates forming a
sandwich
• materials used:
–
–
–
–
Cadmium sulfide (CdS)
Cadmium selenide (CdSe)
Germaium (Ge)
Silicon (Si)
Photoconductive Sensors
• Principle: Resistance
changes as luminous
intensity changes
• Kphoto - sensitivity of
photoresistor
 DI - change in
luminous intensity
 DR - change in
resistance
• Kphoto = DI/ DR
RCTime
• BS2 command that can be used to compute the value of
variable resistance.
• Syntax: RCTIME <Pin>, <State>, <Variable>
• Measure time while Pin remains in State and put the result
on Variable; usually to measure the charge/ discharge time
of resistor/ capacitor (RC) circuit.
• We will use that on our following light program, by
comparing the RCTIME of the left and the right photo
sensors (resistor on our case) and use the comparison result
to decide whether we we should turn right or left.
RCtime
RCTime
• The RC circuit can be represented as a first order
system:
• Rdq/dt+q/C = V; time constant - RC
• Through the above command in BS2 time required
to discharge (or charge) the capacitor in an RC
circuit is measured and is related to the time
constant and thus RC. From that data the variable
value of R may be found.
• See stamp manual for more details.
programming
•
' Measure RC time for left photoresistor.
– high left_pin
' Set detector to
output-high.
–
pause 3
' Pause for 3 ms.
–
rctime left_pin,1,left_photo
' Measure RC
time on left.
• ' Measure RC time for right photoresistor.
–
high right_pin
' Set detector to outputhigh.
–
pause 3
' Pause for 3 ms.
–
rctime right_pin,1,right_photo
' Measure
RC time on right.
programming
• ' Take the difference between
right_photo and left_photo, then decide
what to do.
–
if abs(left_photo-right_photo) > 2 then
check_dir
– ' Check if difference between RC times is
within the deadband, 2 in this case.
–
' If yes, then forward. If no then skip
to check_dir subroutine.
The Hall Effect
• An electrical conductor is
subjected to a magnetic
field.
• The magnetic field is at
right angles to a current
flow inside the conductor.
• The magnetic field pushes
the moving electron
towards the bottom edge of
the conductor (right hand
rule).
The Hall Effect
• The bottom edge
becomes negatively
charged while the
upper edge becomes
positively charged.
• An EMF will develop
across the width of
the slab that is in
proportion to the
amount of flux,
current, and D.
Applications of the Hall Effect:
Measuring Blood Flow
• Tiny voltage probes are
attached on either side of
the blood vessel.
• Poles of an
electromagnet are placed
perpendicular to these
probes.
• The amount of voltage
generated due to the Hall
effect created by the ions
within the blood supply is
a direct indication of a
persons blood pressure.
Applications of the Hall Effect:
Tachometer
• As the metal tooth
passes the Hall detector,
the redirected magnetic
field passes through the
Hall device
• The generated emf
indicates one rotation.
• The same system can be
adapted for a flow
monitoring system for
fluids by replacing the
gear with a rotating
turbine and its blades.