Download Position, Force, Motion Transducers

PHYS 352
Position, Displacement, Pressure, Force
and Motion Transducers
Position, Force, Motion Transducers
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there exists a huge variety of transducers to make
position measurements
pressures and forces cause strain (e.g. bending) and
can also be sensed by measuring position or
displacement
velocity can be measured by differentiating
(measuring positions and taking differences)
accelerometers work basically via
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F=ma, F=kx…and measure the x to find a
when designing measurement systems involving
position or motion transducers (or if designing the
transducers themselves) be creative!
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Position Measurements Techniques
mechanical linkage
‡ optical
‡ capacitive
‡ piezoresistive (strain gauge)
‡ piezoelectric
‡ magnetic
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Mechanical Linkage
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position/motion you
want to sense is
mechanically attached
to a potentiometer
(i.e. variable resistor)
joystick has two potentiometers
one for each axis
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Optical Readout of Motion
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optomechanical mouse
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ball turns wheels (one for
each axis)
rotary encoder counts
pulses
can use rotary encoder
to measure linear
motion also
Q: how can you determine direction
of rotation with rotary pulse counter?
A: use two encoders out of phase!
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B transitions first before A if
rotating one way; A transitions
first if rotating the other way
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Magnetic Rotary Encoders
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magnet rotates
past Hall effect
sensor; encoder
counts the pulses
Optical Mouse –
Image Processing to Sense Motion
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…as opposed to optomechanical
red LED illuminates the area under a
sensor (~30x30 pixel photodiodes
readout with CMOS array)
imaging array records 1500 or more
images a second!
on-mouse digital signal processing
(DSP) compares subsequent images,
and computes how much the mouse
has moved.
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Other Optical Techniques for
Position/Motion Sensing
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interferometry
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very precise distance measurement
align on interference fringes
fraction of wavelength displacement
is detectable
range finder
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laser, ultrasonic, radar
time of flight determines distance (if speed is known)
can use Doppler shift to determine target velocity (toward or
away from detector)
Capacitive Sensors
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either change the relative position of the two conductor
or change the dielectric and the capacitance is changed
four techniques considered:
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1) can charge plates with DC voltage, and measure charge
flow to determine changes in position
e.g. condenser microphone
C=
εA
d
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Capacitive Sensors cont’d
2) apply AC signal (known frequency) to measure the absolute
capacitance (sometimes AC Wheatstone bridge is used)
e.g. non-contact position sensors
guard ring
3) changing level of a dielectric changes
capacitance (AC measurement of the
capacitance is used also)
e.g. capacitive liquid level probe
Capacitive Sensors cont’d
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4) proximity distorts electric field in a capacitive
geometry and the changing capacitance is sensed
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Strain Gauge (like in Lab #3)
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piezoresistive effect
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R=
ρl
metal or semiconductor film
A
resistance changes due to application of mechanical
stress
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strain (stretching) makes wire longer and narrower
ƒ metals in strain gauges respond this way
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some materials change resistivity under stress
ƒ silicon!
Strain Gauge Configurations
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http://www.rdpe.com/ex/hiw-sgpt.htm
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utilizes 4 in a full Wheatstone bridge in a
bending beam configuration
another load cell configuration S-type
silicon pressure sensor
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Piezoelectric
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some materials have asymmetric
distribution of charge in lattice
intrinsic polarization exists
deforming these crystals (or ceramic)
materials reorients
charges/polarization
detectable, sizable voltage change
used in: acoustic sensors, to detect
vibrations, in accelerometers,…
no DC response
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requires change in stress/strain in piezo to
get a signal
advantages:
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large signals generated
does not require power supply
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a common piezoelectric crystal:
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a common piezoelectric ceramic:
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quartz
lead zirconate titanate (PZT)
Magnetic
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motion in a magnetic field – electromagnetic
induction
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e.g. linear variable differential transformers
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e.g. eddy current proximity sensors
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for non-contact measurement
e.g. tachometer (basically a generator)
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Accelerometers
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old school Pendulating
Integrating Gyroscopic
Accelerometer used for inertial
navigation (guiding missiles,
aircraft)
modern accelerometers use
MEMS technology
(microelectromechanical
systems)
Summary
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we looked at many different ways to sense
position, displacement, motion and turn it into
electrical signals
many different configurations and designs
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too many out there to look at them all!
most employ one of the basic physical concepts
(or variation/combination of them) that we
outlined above
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