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Outline – Analog
g Sensors1
•
Sensors / Transducers
– Basic Properties
– Classification
•
•
Capacitive Sensors
I d i S
Inductive
Sensors
–
–
–
–
•
•
•
Proximity sensors
Resolvers
Inductosyn
LVDTs
Magnetic Scales
Magnetorestrictive Sensors
Potentiometers
Chapter 11a
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Sensors / Transducers
• A sensor is a device that detects a physical quantity and
an transmit a corresponding signal.
signal
• A transducer is a sensor that converts one form of
energy to another although not necessarily with a direct
exchange of energy.
energy
– Most often, a transducer will represent the value of the process
being sensed in terms of an analog voltage or discrete voltage
levels.
levels
Physical Quantity
- Position/Angle
- Velocityy
- Acceleration
- Force
Chapter 11a
- Torque
- Pressure
- Temperature
- Humidity
Transducer
Transducer
ME 551
Output
- Voltage
- Current
3
Basic Concepts
• Accuracy
y is said to be the accordance of the
measured value (as indicated by the output of
the sensor)) with the actual p
physical
y
quantity.
q
y
– It is expressed in terms of the maximum error of a
sensor within its operating range.
• Resolution refers to the smallest detectable
change
g in the measured p
physical
y
quantity.
q
y
• Repeatability (or precision) indicates how
consistently the sensor provides the same
output when subject to the same input.
Chapter 11a
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Basic Concepts
p ((Cont’d))
• Linearity is the variation in the constant of
proportionality between the output signal and the
physical quantity.
– It is often times expressed in terms of a percentage
p
of the full-scale output.
• Bandwidth is a dynamic metric indicating how
fast the sensor can react to the changes in
physical quantity being measured.
– It is usually expressed in Hertz.
Hertz
– Low bandwidth indicates a slow sensor.
Chapter 11a
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Classification of Motion Sensors
Measurement
Measurement
Direct / Indirect
Direct / Indirect
Analog
Analog
Absolute
Absolute
Digital
Digital
Absolute
Absolute
Incremental
Incremental
Incremental
Incremental
Rotary
Rotary
Rotary
Rotary
Rotary
Rotary
Rotary
Rotary
Linear
Linear
Linear
Linear
Linear
Linear
Linear
Linear
Chapter 11a
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Analog vs. Digital
• The output of an analog sensor (usually a voltage) is
continuous in nature and is usually directly proportional
physical
y
quantity
q
y to be measured.
to the p
• The output of a digital sensor is quantized and can only
change by an incremental amount.
• The digital sensors are quite popular owing to the fact
that their output could be directly sampled by a control
computer.
• On the other hand, the output of an analog sensor
oftentimes needs to be processed and to be converted to
a digital information.
Chapter 11a
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Absolute vs. Incremental
• In absolute sensors,
sensors the output is always relative
to a fixed reference frame, regardless of initial
conditions ((i.e. what the output
p was when p
power
was turned on).
p is
• In incremental sensors,, the current output
referenced to its previous (one step behind)
value. Hence, one needs to keep track of all the
changes
h
i the
in
h output to determine
d
i the
h value
l off
the measured quantity in absolute sense.
– Th
The conceptt off incremental
i
t l sensing
i
i mostly
is
tl
associated with digital sensors.
Chapter 11a
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Rotaryy vs. Translational/Linear
• Rotary position sensors are used to
measure the angular position of a rotating
mechanism (e.g. motor shaft, wheel, drum,
etc.).
)
• In CNC technology, linear position sensors
are employed to measure the position of a
translational mechanism (e.g.
(e g carriage)
Chapter 11a
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Common Analog
g Sensors
• In p
precision engineering
g
g applications,
pp
, a wide
variety of sensors are utilized:
– Capacitance sensors
– Hall effect sensors
– Inductive digital on/off proximity sensors
• Inductive distance measuring sensors
–
–
–
–
–
–
Chapter 11a
Linear & rotary variable differential transformers
Resolvers
Inductosyns™
Magnetic scales
Magnetostrictive sensors
Potentiometers
ME 551
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Capacitance
p
Sensors
• Generally regarded as the most accurate type of analog
g ((10 μ
μm to 10 mm).
)
motion sensor with limitted range
Chapter 11a
ME 551
11
Operating
p
g Principle
p
The capacitance between two identical
parallel p
p
plates can be expressed
p
as
C =ε
wl
d
where ε is the permittivity of the medium
in between the plates (usually air!)
[F/m]; w is the width; l is the length; d is
the separation between the plates.
w
d
l
Note that capacitance sensors are
nonlinear in nature and that the sensor’s
resoluion is determined by the electronic
signal processing unit (i.e filter + analogto-digital converter).
Chapter 11a
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Capacitance
p
Sensors ((Cont’d))
• Typical applications include
– Position sensor for micro-positioners.
– Material thickness sensing
Chapter 11a
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Capacitance
p
Sensors ((Cont’d))
Chapter 11a
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Inductive Sensors
• Typical
T i l applications
li i
i l d
include
– Industrial limit switches.
– Coarse home position sensor for machine tools (fine
home position via encoder home pulse).
Chapter 11a
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Brushless Resolvers
• Resolvers are electromagnetic position
transducers:
– Analog measurement devices
– Absolute
Ab l t position
iti over one
revolution
– Some signal processing is
needed.
needed
• They were originally
developed for military
applications:
– More than 50 years of
continuous use and
development
– Can work reliably in very harsh
environments.
Chapter 11a
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Brushless Resolvers (Cont’d)
• The device constitutes
rotor (a.k.a. reference)
winding and stator (SIN
and COS) windings.
rings, the
• To avoid slip rings
resolver includes a rotary
transformer.
• The resolver can be used
in two different modes:
– Transmitter
– Transformer
Courtesy of AMCI Corp.
Chapter 11a
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Transmiting
g Resolver
•
•
•
Reference (rotor) winding is
excited by an AC voltage with
an amplitude of Vr.
The amplitudes of induced
voltages in the SIN and COS
windings
i di
b
become
Vrsin(θ)
i (θ) or
Vrcos(θ) respectively.
The absolute p
position of the
input shaft can be determined
via ratiometric comparison of
these voltages.
Courtesy of AMCI Corp.
Chapter 11a
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Transforming
g Resolver
• Th
The stator
t t windings
i di
are
excited by two reference
AC voltages as shown.
shown
• The induced voltage in
the rotor winding is
proportional to sin(φ-θ).
position of
• The absolute p
the input shaft can be
determined via detecting
th phase
the
h
shift
hift iin th
the
rotor voltage.
Courtesy of AMCI Corp.
Chapter 11a
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Interface Electronics for Resolvers
• A typical interface perform
severall vital
it l ttasks:
k
– Generates the reference AC
g waveform(s)
( )
voltage
– Processes the output signal(s)
– Produce the angular position
of the resolver shaft in desired
format.
• Digital (Serial / Paralel)
• Analog
• Incremental
Courtesy of AMCI Corp.
Chapter 11a
• Recently, a resolver and onboard electronics are
integrated into one housing.
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Linear Inductosyn™
• The Inductosyn
Inductosyn™ (registered trademark of
Farrand Controls, Inc.) measures linear position
directly:
– Accurate and rugged
– Well-suited to severe industrial environments
• Th
The linear
li
I d t
Inductosyn
consists
i t off two
t
magnetically coupled parts:
– The scale is fixed (e
(e.g.
g with epoxy) to one axis,
axis such
as a machine tool bed.
– The other part, the slider, moves along the scale in
conjunction with the device to be positioned (for
example, carriage).
Chapter 11a
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Structure of Linear Inductosyn™
•
The scale is constructed of a base material (St, stainless St., Al etc.)
covered by an insulating layer.
– Bonded to this is a printed-circuit
printed circuit trace, in the form of a continuous rectangular
waveform pattern.
•
The slider has two separate but identical printed circuit traces bonded to the
surface that faces the scale.
– These two traces have a waveform pattern with exactly the same cyclic pitch as
the waveform on the scale, but one trace is shifted one-quarter of a cycle relative
to the other.
•
The slider and the scale remain separated by a small air gap.
SCALE
A
Chapter 11a
B
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Operating Principles
• Inductosyn™ operation resembles that of a resolver.
• When the scale winding is excited with an AC voltage, it
magnetically couples to the two slider windings (A and
B), inducing voltages that are functions of the slider's
position within a cyclic pitch of the scale.
scale
• The position of the slide (within one cyclic pitch) can be
determined via ratiometric comparison of these voltages.
• Note that the absolute position of the slider is determined
by counting successive pitches in either direction from
an established starting point
point.
Chapter 11a
ME 551
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Linear Inductosyn™
y
Types
yp
Courtesy of Farrand Controls, Inc.
Chapter 11a
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Rotaryy Inductosyn™
y
Courtesy of Farrand Controls, Inc.
Chapter 11a
ME 551
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LVDT
• Typical applications of
li
linear
variable
i bl
differential transformer
(LVDT) iinclude
l d
– Metrology equipment.
– Small range of motion
servo controlled devices.
Chapter 11a
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Magnetic
g
Scale
• Operates
p
on same p
principle
p
that allows a disk drive to
locate stored information.
– Sliding head reads magnetically
encoded position data on the
linear scale.
• More robust than linear optical
encoders.
• Becoming more and more
popular for measuring linear
motion
ti off machine
hi ttooll axes.
Chapter 11a
ME 551
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Magnetostrictive
g
Sensors
• An interrogation
g
current p
pulse
is launched in a speciallydesigned
magnetostrictive
waveguide.
id
– An instantaneous magnetic field
along the active length of a sensor
is created.
• This magnetic field then
interacts with the magnetic field
generated
by
a
magnet
attached
to
a
moveable
machine part.
Chapter 11a
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MR Sensor (Cont’d)
(
)
• The interaction of the two magnetic
g
fields creates a
torsional strain pulse traveling at sonic speed through
the waveguide.
• The strain pulse is detected by the unit's electronics.
– The elapsed time between the interrogation pulse and the
detection of the strain pulse is used to determine the position
position.
– Resolution is a few microns while the interrogation frequency
is on the order of kHz.
• Typical applications are
– Moderate accuracy linear position sensing.
– Position sensing of hydraulic pistons (sensor can be placed
inside the cylinder).
Chapter 11a
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Potentiometers
• Slider (made from precious metals) moves on a resistive
material.
– Position is detected as a function of electrical resistance registered.
• Typical applications are
– As a sensor in a high reliability all analog servo system.
– Short range of motion servo systems.
systems
Chapter 11a
ME 551
30
References
1. A. H. Slocum, Precision Machine Design, SME Press,
1992.
• A.
A H.
H Slocum,
Sl
ME 2.075
2 0 Course
C
N
Notes,
MIT 2001.
MIT,
2001
2. R. Bishop (ed), The Mechatronics Handbook, CRC
P
Press,
NY 2002.
NY,
2002
3. D. A. Krulewich, Handbook of Actuators and Edge
Alignment Sensors,
Sensors Technical Report,
Report UCRL-IDUCRL ID
112606, LLNL, 1992.
Chapter 11a
ME 551
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