Future Systems: Automation and Sensing The best way to predict the future is to invent it

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Future Systems Automation and Sensing (The best
way to predict the future is to invent it!)
Attributed to Alan Kay (Xerox Palo Alto Research
Center PARC in 1971)

• Prof. Paul Wright, A. Martin Berlin Chair in
  Mechanical Engineering
• Chief Scientist of CITRIS _at_ UC Berkeley
• Co-Director of the Berkeley Wireless
  Research Center
• Co-Director of the Berkeley Manufacturing
  Institute
• ME221 - MBA 290M - INFOSYS 290.8 ---- Week 6 on
  sensing

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Key points

• Miniature, invisible electronics over the next
  5-10 years will provide a 10x reduction in cost
  (related to Moores Law).
• Sensor data combined with system models will
  provide a 10x increase in the capability of
  Products and Manufacturing Processes (over same
  period).
• Wireless smaller sensors at specific sites and
  throughout mesh-networked systems promote
  increased capability, productivity, and capacity

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Measurement systems for motes and in general

• Three basic components for measuring
• A transducer is a sensing device that converts
  physical input into an output (most often a
  voltage)
• The signal processor performs filtering,
  amplification, or other signal conditioning on
  the transducer output
• The slightly ambiguous term sensor is used for
• A) the transducer itself
• B) the combination of both transducer and signal
  processor
• So the overall measurement system might be
• A) in a robot/machine tool/ automobile/plane etc
• B) used alone for your product or some other
  field test

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Digital thermometer --- see sketch from the
webcast
Amplifier
Thermocouple
LED display
RecorderDisplay
Signal Processor
Transducer
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Most common sensing applications

• Temperature
• Position
• Acceleration\Vibration
• Strain
• Distance
• Force

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Temperature 1a Bimetallic strip

• Two or more layers of different coefficient of
  thermal expansion
• Used in standard thermostats in house industrial
• Delta fn (T, A, B)

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Temperature1b. The RTD Electrical Resistance
Thermometer or Resistance Temperature Device
(RTD)

• Metallic wire wound around a ceramic or glass
  core and hermetically sealed
• R Ro 1 a (T To)
• Where Ro and To are the ref. resistance at To
  (often ice)
• dR/dT is the sensitivity a Ro
• Platinum RTDs very stable, non-oxidizing, -220C
  to 750C

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Machine tool, RTD sensing example
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RTD sensor for placement on rotational end-mill
(1999)
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The temperature sensor was placed on the lower
face of the tool (in a previously infeasible
location)
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A mathematical model calculates, in real-time,
the temperature at the tools critical cutting
edge
Spindle torque and restricted tool geometry also
calculated, in real-time, the stress at the
tools critical cutting edge
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Hot compression dataWC12TiC7cobalt, shown at
right
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Key points in this specific example

• Miniature sensors can be close to the action
• Wireless sensors allow flexible, rapid deployment
  and opportunities to sense in rotational and/or
  hostile locations
• Combined with good models, sensors allow
  real-time prediction of temperatures and stresses
  in detailed locations for automation control
• Result Wireless miniature sensors allowed the
  cutting-tool to be operated at a speed and feed
  in a safe but productive region, automatically
  increasing parts made per hour

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Temperature1cThermistor semiconductor

• Exponential changes with temperature
• With the calibration constant or characteristic
  temperature as ß, the thermistor is accurate to
• R Ro e ß (1/T 1/To

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Two types

• If ß is negative, the resistance decreases with
  increasing temperature, and the device is called
  a negative temperature coefficient (NTC)
  thermistor.
• NTC thermistors are made from a pressed disc of
  sintered metal oxide. Raising the temperature of
  a semiconductor increases the number of electrons
  
• If ß is positive, the resistance increases with
  increasing temperature, and the device is called
  a positive temperature coefficient (PTC)
  thermistor, Posistor.
• PTCs are made of a doped polycrystalline ceramic
  containing barium titanate (BaTiO3) and other
  compounds. Used for switching

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Thermistor back-up information

• NTC thermistors are made from a pressed disc of
  sintered metal oxide. Raising the temperature of
  a semiconductor increases the number of electrons
  
• Most PTC thermistors are of the "switching" type,
  which means that their resistance rises suddenly
  at a certain critical temperature. The devices
  are made of a doped polycrystalline ceramic
  containing barium titanate (BaTiO3) and other
  compounds. The dielectric constant of this
  ferroelectric material varies with temperature.
  Below the Curie point temperature, the high
  dielectric constant prevents the formation of
  potential barriers between the crystal grains,
  leading to a low resistance. In this region the
  device has a small negative temperature
  coefficient. At the Curie point temperature, the
  dielectric constant drops sufficiently to allow
  the formation of potential barriers at the grain
  boundaries, and the resistance increases sharply.
  At even higher temperatures, the material reverts
  to NTC behaviour.
• Another type of PTC thermistor is the polymer
  PTC, which is sold under brand names such as
  "Polyfuse", "Polyswitch" and "Multiswitch". This
  consists of a slice of plastic with carbon grains
  embedded in it. When the plastic is cool, the
  carbon grains are all in contact with each other,
  forming a conductive path through the device.
  When the plastic heats up, it expands, forcing
  the carbon grains apart, and causing the
  resistance of the device to rise rapidly. Like
  the BaTiO3 thermistor, this device has a highly
  nonlinear resistance/temperature response and is
  used for switching, not for proportional
  temperature measurement.

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Temperature1dThermocouples

• Seebeck effect V proportional to T of junction
  between two dissimilar metals

Metal A
T1
T2
-

V
Metal B
Metal B
V is a function of the properties of A and B
and ?T V a (?T) where a is the Seebeck
Coefficient
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Details of thermocouple data

• E Chromel and Constantan (-100 to 1000) /0.5
  acc.
• J Iron and Constantan (0 to 760) /-0.1 acc.
• K Chromel and Alumel (0 to 1370) /- 0.7acc.
• R 87 Platinum and 13 Rhodium (0 to 1000) /0.5
  acc
• S 90 Platinum and 10 Rhodium (0 to 1750) /0.1
  acc
• T Copper and Constantan (-160 to 400) /-0.5

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2. Position measurement 2a Proximity Sensors
simple switches

• Magnetic, capacitance, inductance etc
• Photoemitter-detector
• Opposed mode (garage door type- object
  interrupts)
• Reflective mode (ditto)
• Proximity or diffuse type (the object reflects
  beam)
• Switches
• SPST
• SPDT
• Single Pole Double Throw (Hall Switch)

NC
NO
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2. Position measurement 2a Linear Variable
Differential Transformer

• Primary and two secondary windings
• Moveable iron core
• Like a transformer, when an excitation voltage is
  applied to the central core, voltages are induced
  in the two secondaries
• The output signal of the secondaries shows both
  magnitude of movement the direction of movement
  
• At midpoint there is a null reading as the two
  secondaries have the same induced voltage but
  180deg. out of phase

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http//www.rdpe.com/us/hiw-lvdt.htm

• Please see animation on the above site

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Voutput Amplitude
Linear part of LVDT range
right
left
Core displacement
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See sketches on webcast

• 1. Sketch of coils and core
• 2. Sketch of outputs of secondaries
• 3. Sketch of demodulation circuit
• 4. Sketch of low pass filter

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3. Acceleration and vibration measurement

• Bearing vibration, aerodynamic flutter, shock
  waves, blasts, impacts (from someone falling)
• Usually attached to object being measured
• In macro scale devices, the sensing elements can
  be strain gauges or piezoelectric elements. In
  MEMS devices piezoelectric elements are also used
  as well as capacitance changes (see later)
• Model with a mass-spring-damper system (mkb) and
  say a displacement transducer (LVDT) measuring
  position of vibrating mass ---

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Piezoelectricity Mostly used as a sensor for
very accurate
V
PbTixZr1-xO3
Di electric displacement Si strain Ti
stress Ei electric field d31 piezoelectric
constant e33 dielectric constant (at constant
stress) s11 elastic compliance (at constant
electric field
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(No Transcript)
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(No Transcript)
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Generic Vibration-to-Electricity Conversion Model
z spring defection y input displacement m
mass be electrical damping coefficient bm
mechanical damping coefficient k spring
coefficient
b 2mzwn
w wn
Willams and Yates, Transducers 95/Eurosensors IX,
(1995) 368-372
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Vibrational Energy ScavengingMesoscale Proof of
Concept

• Piezoelectric Material -PbZrxTi1-xO3
• High piezoelectric coeffiecient
• Large range of solid solubility
• Well characterized in the bulk as well as thin
  film form

size
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3. Acceleration measurement with MEMS
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MEMS devices

• Single monolithic IC
• Polysilicon, surface micromachined sensor and
  signal conditioning circuitry for 2 axis
  acceleration
• Outputs are analog voltages proportional to
  acceleration
• Can also be used as a tilt sensor since it
  measures gravity
• Polysilicon mass in center is held by polysilicon
  springs that create the resistance against
  acceleration forces
• Suspend the structure over the surface of Si
  wafer

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ADXL320 continued

• Deflection of the mid-structure is measured using
  a differential capacitor. This consists of
  independent fixed plates and other plates
  attached to the moving central mass
• The fixed plates are driven by 180deg
  out-of-phase square waves
• Acceleration deflects the main slab and
  unbalances the differential capacitor, resulting
  in an output square wave whose amplitude is
  proportional to acceleration.

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ADXL320 continued

• Phase sensitive demodulation techniques are then
  used to rectify the signal and determine the
  direction of acceleration
• The demodulators output is amplified and brought
  off-chip through a 32kohm resistor
• The user then sets the signal bandwidth of the
  device by adding a capacitor
• The filtering improves measurement resolution and
  helps prevent aliasing

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ADXL320 typical MEMS accelerometer

• Dual axis 5g accelerometer on a single chip
• Ultra small 4 mm x 4 mm x 1.45 mm LFCSP package
• Low power 350 µA at VS 2.4 V (typical)
• Output Type Analog
• Supply Current 0.5mA
• TypicalBand Width 2.5kHz
• Voltage Supply 2.4 to 6
• Range /- 5g
• Sensitivity174 mV/g
• of Axes 2
• Temp Range -20 to 70C

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4. Strain Traditional strain gauges

• Resistance of thin wire
• See sketch on webcast
• Constantan in grid pattern on polyimide backing
  overall the gauge is very small 5 to 15mm long
• Average strain over small area --- not usable for
  stress concentrations near a notch etc
• R ? x length / area where ? is the metals
  resistivity
• See webcast equations