Download Sensors - skillbank

Sensors
BADI Year 3
John Errington MSc
1
Sensors
• Allow a design to respond to its
environment – e.g. a line following robot
may use photosensors to detect a paint
line on the floor.
• Enable systems to determine when the
required action has been completed; for
example a trip switch will indicate that a lift
has reached the desired floor.
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Trip switches
Perhaps the most common sensor used in industry
is the trip switch or limit switch – a simple
microswitch in an industry-sealed case with a
rugged mechanical activation system such as a
lever + roller as shown above
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Strain gauges
Also extremely widely used,
strain gauges measure the
amount of extension or
compression experienced by
the material it is fastened to.
This gives a measure of the
strain, and can thus be used
to measure deformation or
applied load.
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Optical sensors
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Photodiodes and phototransistors
Integrated photoswitches
Light dependent resistors
Diode arrays (1d & 2d)
CCTV & digital cameras
Low light sensors e.g. photomultipliers
• No moving parts so inherently reliable BUT
• Susceptible to dirt and changes in ambient light
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Photodiodes
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OptoSwitches
Sensors comprising a light
source, photodiode and
amplifier
Diffuse scan – detects
reflected light
Through scan – detects
when beam is interrupted
Hall effect sensor also
shown here detects when
a ferrous metal breaks the
magnetic field
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Proximity sensors
Types:
• Mechanical
• Inductive
• Capacitive
• Ultrasonic
• Optical
Application etc.
• Contact sensing
• Ferrous only 1cm
• Metals only 1cm
• 6m range solid/liquid/powder
• Through beam or
• Reflective
• Clean environments only
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Non-contact sensors (1)
Inductive proximity sensor
Capacitive proximity sensor
C2
S
C1
P
C3
Coil inductance increases
as iron / steel object (S )
gets closer, because lines
of magnetic flux can flow
through the iron, making
the effective path shorter.
Capacitance increases as metal
object (P) gets closer because
additional capacitance paths C2 &
C3 are added and increase in value
as the separation reduces. C1 is
always present.
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Non-contact sensors (2)
Other non-contact sensors use sound
(ultrasonic) or electromagnetic waves (light,
microwaves, etc.) to gather information
about the distance between the sensor and
a surface.
Microwave sensors are reliable in dirty
industrial environments but exposure of
personnel must be prevented.
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Proximity switches
Capacitive,
Ultrasonic
Inductive
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Rotary and linear position
• Potentiometric (sliding contact)
• LVDT
• Encoders
Main issues: operating conditions, reliability,
analog or digital output
Contact or non contact - is wear important?
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Magnetic pick-ups
Often used to detect
engine timing by counting
teeth on the starter gear.
Robust, reliable noncontact
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Rotary shaft encoder
Gives a digital output indicating
the angular position of a shaft.
They often use a disk with marks
like a bar code, that are read by
optical photodetectors inside the
encoder.
Robust, reliable non-contact.
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Absolute rotary encoder
An absolute encoder has a number of binary outputs that
indicate the switch shaft's absolute rotational position
referenced to some spot on the switch's body. For example,
a three bit absolute rotary encoder will divide the rotational
position into eight sectors. Absolute encoders are available
in a variety of style with a variety of resolutions.
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Incremental rotary encoder
An incremental rotary encoder won't tell you where the shaft is positioned. It will only
tell which direction the shaft is being turned and how fast. Incremental encoders have
two outputs called phases. Each outputs a square wave. Turning the shaft one
direction causes one phase to lead the other by 90 degrees. Reversing the direction
will cause the other phase to lead. The frequency of the output is proportional to the
rotational speed of the shaft.
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Rotary position sensors
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Linear position
• Linear Variable
Differential Transformer
• Other (cheaper!) linear
position sensors use a
resistive track and slider
but this is less precise
and reliable due to wear
of the track
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Potentiometric sensors
Resistive track may be wire wound on a
former, metal film or graphite on
substrate
L
x
+
Vs
-
Vout
The voltage picked off is proportional to the position of the sliding
contact
Vout = Vs * x / L
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Open loop control
• Simple and easy to implement
• No feedback of controlled parameter
• No certainty that parameter is at desired
value
• Change in system parameters such as
load will result in a deviation from the
desired value
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Servo systems
Measure output variable
Compare to desired value
Amplify the difference
Apply to correct the error
Result: servo system can adjust for
changes in load, amplifier gain etc and still
give good compliance with required value
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