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					          Wireless Sensor Networks
                  “Disappearing Computer”
Low Power Radio                                 Energy Storage
                  B. Gates, Economist (2003)




                                                Renewable Power
     Sensor


                         “Picocube”

                                                     Supply


                                          CALIFORNIA ENERGY COMMISSION
    MEMS Sensors for Electric Power
           Measurement


Eli Leland, Giovanni Gonzalez, Christopher
            Sherman, Peter Minor

       Presentation to the DR-TAC
            February 19, 2008

                             CALIFORNIA ENERGY COMMISSION
   If you remember nothing else…

Passive, proximity-based electric current sensors
work at the meso-scale

MEMS-scale devices are feasible in theory, and
we’re working on the cleanroom process

Sensors for distribution cable monitoring and AC
voltage sensing are under development

                                  CALIFORNIA ENERGY COMMISSION
            Wires and magnetic fields
  Electric power is 60 Hz AC     120 V
  in the Americas, 50 Hz in
  Europe
  Voltage and current are
  sinusoidal – rated value is
  root-mean-square (rms)
  Magnetic field surrounding a
  current-carrying wire is                   16.7 ms
  circumferential (right-hand
  rule) and alternating
                                                    µ 0i
Pavg = Vrms I rms cos(φ )                    Bair =
                                                    2πr
                                         CALIFORNIA ENERGY COMMISSION
        Current sensor design concept:
  Permanent magnets and piezoelectric materials
Permanent magnets can couple to the magnetic fields
surrounding AC current carriers
Piezoelectric materials can transduce the forces on the
permanent magnet to an output voltage
Sensor device does not require power supply or wraparound
of conductor
                       output voltage
                                        piezoelectric
                                          bimorph                      permanent
                                                                        magnets
                 rigid cantilever
                    mounting


                                                                           appliance
                                                                          power cord
                                                                        (cross-section
                                                                             view)

                                                        CALIFORNIA ENERGY COMMISSION
                                        What do these fields and
                                          forces look like?
                                                                                                                  d (H y )
       Zip-cord magnetic intensity field                    Force proportional to                     Fy = Br ∫              dV
                                                            grad(H)                                                 dy
          Currents 180° out of phase
          Fields add along vertical line at                                         d (H y )    i   2dy
          center                                           grad(H) proportional              =−
          Fields cancel as distance
                                                           to current                 dy        π y2 + d 2          (         )   2


          increases                                    ...so force is linearly proportional to current!
                                                                                   Magnitude of gradient of Hy
        H-field surrounding zip-cord, 10 A current                              surrounding zip-cord, 10 A current x 104
                    10                               1000                             10
                                                                                                                         8
                                                     900
                            magnet




                                                                  y-coordinate (mm)
                                                                                                                         7
y-coordinate (mm)




                                                     800
                      5                                                                 5
                                        M                                                            M                   6
                                                     700
                                                     600                                                                 5
                      0                                                                 0
                                                     500                                                                 4

                                                     400                                                                 3

                     -5                              300                               -5                                2
                                                     200                                                                 1
                             zip-cord
                                                     100                              -10
                    -10                                                                 -10    -5     0     5       10
                      -10     -5        0   5   10     units in                                                          units in
                            x-coordinate (mm)           A/m
                                                                                              x-coordinate (mm)           A/m2
                                                                                              CALIFORNIA ENERGY COMMISSION
                   These sensors work,
              and they show linear behavior
                                        c p d 31t p a3                                                                  1
                                                                                              (                    )− ω
Voltage frequency           Voc = Fin                                                    ⋅
                                         εk 2 ma2 ω n 2 1 − c                                                                   − j (2ζ mω nω )
                                                                                                               2
                                                                                                      p d 31
response function:                                                                                                          2
                                                                                                       ε


For a constant frequency input of 60 Hz (w = 2p × 60), the voltage output is a linear
function of magnetic input force

                                                                                 Sensor output – current in heater cord excitation
                                                                               1.6


                                             Maxiumum sensor voltage out (V)
                                                                               1.4

                                                                               1.2

                                                                                1

                                                                               0.8

                                                                               0.6

                                                                               0.4                                               experiment
                                                                                                                                 theory
                                                                               0.2

                                                                                0
                                                                                     0         5                   10             15          20
                                                                                             Maximum current in heater cord (A)

                                                                                                   CALIFORNIA ENERGY COMMISSION
                                               Everybody loves PZT, but
                                            Aluminum Nitride may be better
                                                  for this application
                                                                                                                                             d 
                                                                                                                                  Vout ≈ Fin  31  K
                                                 PZT                         Aluminum Nitride                                                 ε 
                                      1 mm               500 µm                1 mm                500 µm                                             PZT                 AlN
                                    cantilever          cantilever           cantilever           cantilever
                                                                                                                                    d31              -138                   -3
             resonance                                                                                                           (pm/V)
             frequency                  281                 927                  434                 1425
                  (Hz)
                                                                                                                                          εr         1800                   9

           sensitivity                                                                                                               d31/ε          8.66 x              37.7 x
                                      0.59                 0.28                  2.4                  1.2                                            10-3                10-3
             (mV/A)
                                                                                                                                 about 4.3x greater for AlN

          • Aluminum nitride sensitivities are about 4x those of PZT
          • Many AlN devices have been fabricated successfully in the
            Microlab (UC Berkeley cleanroom)
          • Geometry can be optimized to maximize voltage output
Notes: All simulations used 1µm platinum elastic layer and 1µm piezoelectric layer (AlN or PZT). 100µm cantilever width and 100µm x 100µm x 100µm magnet size for all simulations. PZT
properties: d31 = -141 pm/V, εr = 1800, density = 7800 kg/m3, cp = 66 GPa. AlN properties: d31 = -3 pm/V, εr = 9, density = 3200 kg/m3, cp = 350 GPa. Magnet properties: density = 7500 kg/m3,
Br = 0.4 T. Pt properties: cp = 171 GPa, density = 21450 kg/m3.
                                                                                                                             CALIFORNIA ENERGY COMMISSION
                 Recipe for a MEMS
                  AC current sensor
MEMS sensor fabrication process based on well-
characterized recipe for AlN devices (G. Piazza,
et al, 2004)
Magnet can be printed and magnetized before
release step
                 dispenser-printed   Silicon dioxide
     electrode                        elastic layer
      access       micromagnet
                                           Aluminum nitride
                                          piezoelectric layer




                                                                Si substrate




                                              CALIFORNIA ENERGY COMMISSION
                    What is a dispenser-
                   printed micromagnet?
                                     SEM of SrFe-PVDF magnet on Si

We have made composite magnets                2 µm             20 µm

using the dispenser printer
(Steingart, et al)
Magnetic powder (SrFe, SmCo and
NdFeB) in a PVDF polymer matrix
They stick to steel!
They need to be charged with
magnetic intensity H = 2-3x Hci
   NdFeB: 2-3 Tesla(!!!)
   Ferrite: 0.4-1 Tesla
   What’s the best way to do this?
Curing the magnets in an external
field may enhance magnetic
properties

                                             CALIFORNIA ENERGY COMMISSION
         Magnetizing the micromagnets
                               Following a mishap with the super
                                   high-power magnetizing rig…




…we decided to take matters in to our own hands and build a
benchtop 1 Tesla electromagnet. Sufficient for the ferrite
magnets, we’ll need to send out the higher-energy samples for
magnetization.
                                            CALIFORNIA ENERGY COMMISSION
     Related project: Assessing integrity of
       high-voltage underground power
               distribution cables
Goal: To identify cables, operating at 12 kV and above, needing
replacement before their failure




Failure Mechanism: Microvoids and channels form in insulator
due to electrical forces, water seeps in and fills them, forms tree-
like conducting structure. Ultimately a high-powered brief arc
occurs producing damage pictured above.
                                             CALIFORNIA ENERGY COMMISSION
                                          Proposed cable
                                       diagnostic techniques
     Distribution Cable, Tree, and Partial Discharge
                          Charge and current flow
      Acoustic energy
                               Material property changes
                                           RF energy
   Tip dia.          II     i(t)
                                           Acoustic energy              CC
   1 micron                                                                  I

                                CC                                                J
                                                CC Central conductor
                                                CN Concentric neutral
                                                I Insulator                      CN
                              CN                J Protective jacket

      Transient surface heating, light, acoustic energy, chemical changes


              i(t)

                                     time (t)
                              1 ns


A. Near cable ends where concentric neutrals are all connected together and grounded,
   use MEMS-based current sensors to measure current in each concentric neutral (CN)
   wire. Detects open CNs (no CN current flowing). Asymmetric CN currents may
   indicate presence of potentially destructive water trees near those CNs.
B. Use concentric neutrals as transmission line to probe cable insulator. Launch test
   pulse by capacitive coupling of Electrostatic Discharge Simulator (gift from Kikusui
   Co.) to a CN wire beneath insulating jacket (J). Pulser produces up to 30 kV, 60 ns
   pulse. Use to sample cable for water trees.
                                                                                      CALIFORNIA ENERGY COMMISSION
   If you remember nothing else…

Passive, proximity-based electric current sensors
work at the meso-scale

MEMS-scale devices are feasible in theory, and
we’re working on the cleanroom process

Sensors for distribution cable monitoring and AC
voltage sensing are under development

                                         Thanks!
                                 CALIFORNIA ENERGY COMMISSION

				
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Description: Electric system (Micro-Electro-Mechanism System, MEMS), system on chip (SOC, System on Chip), wireless communications and low-power embedded technology, the rapid development of wireless sensor networks bred (Wireless Sensor Networks, WSN), and its low-power, low cost, the characteristics of distributed and self-organization has brought a revolution in information perception. Wireless sensor network is deployed in the monitoring area by the large number of low-cost micro sensor nodes, wireless communication through the formation of a multi-hop ad hoc networks.