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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.
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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