Mechanical Microsystems. Prof. Tianhong Cui ME 8254

Transcription

1 Mechanica Microsystems Prof. Tianhong Cui ME 8254

2 Mechanica Actuators Actuation mechanisms: eectrostatic = eectrostatic attraction of charged pates therma = expansion of soids or fuids shape memory aoy = considerabe change in ength pneumatic/hydrauic = fuid pressure piezoeectric = eectricay induced strain magnetic chemica bioogica

3 Eectrostatic Actuators Based on attraction of two oppositey charged pates Typicay ow power Simpe to fabricate Couomb s aw:

4 Eectrostatic Actuation Cantiever Actuators eectrostatic force tip defection

5 Eectrostatic Actuation Fabrication poysiicon with sacrificia oxide eectropated meta with sacrificia organic ayer sputtered meta with sacrificia organic ayer Torsiona Actuators dua defection eectrodes sma defection

6 Eectrostatic Actuation Comb Drives use arge number of eectrostaticay actuated fine fingers attractive force is mainy due to fringing fieds generate arge movements

7 Eectrostatic Actuation Rotary Micromotors use freey moving centra rotor with surrounding capacitive pates up to 300,000 rpm up to 300 V Fabrication: poysiicon/oxide meta/resist

8 Eectrostatic Actuation Linear Micromotors (Scratch Drive Actuators) uses fexibe conductive pate with sma bushing at one end veocities up to > 1mm/s

9 Therma Actuation Linear Therma Expansion Voume Expansion/Phase-Change Bimorph Therma Actuators uses difference in therma coefficients of expansion heater is sandwiched between two active materias environmenta ruggedness (+) high power, ow bandwidth (-)

10 SMA Actuators Shape Memory Aoy (SMA) Actuators use aoys that exhibit considerabe changes in ength when heated heat causes materia transition from one crysta phase to another aoys: Au/Cu, In/Ti, Ni/Ti

11 Gears from Sandia

12 Random Mechanica Items

13 Micro-Grippers Source: Berkeey

14 Micro-Tweezers Source: MEMS Precision Instruments

15 Strain Gages Gage factor is defined as reative resistance change over strain Types incude: Meta foi Thin-fim meta Bar semiconductor Diffused semiconductor Impantabe strain gages Penetrating micro-strain gage probe

16 Acceerometers Sources: Anaog Devices, Lucas NovaSensor, and EG&G IC Sensors

17 Acceerometers F=ma is basic concept Force measured by defection or strain Can be reated to spring constant, F=kx Generay dispacement of proof mass is measured reative to frame Dynamic system as described previousy Strain gage type most basic Strain in beam measured as proof mass defects beam Lots of configurations

18 Acceerometers Capacitive acceerometers most commerciaized Torsion bar with assymetric pates Force-baanced capacitive used in autos Comb of capacitors measures differentia capacitance Highy sensitive Force feedback to maintain centra ocation of proof mass Force required to maintain equiibrium generates signa

19 Acceerometers Piezoeectric acceerometers Generay show no DC response Specia circuitry to create DC response Typicay use ZnO Tunneing acceerometers Highy sensitive More difficut to fabricate Requires cosed oop contro Long term drift Latching acceerometers Lock in pace if acceeration exceeded

20 Acceerometers Switch arrays Array of switches sensitive to increasing eves of acceeration Simpe to buid Optimizes range of acceerometer in use Muti-axis accerometers Ony a few exampes to date Cross-axis sensitivity probem Precise aignment and ow cost are advantages A require extensive circuitry

21 Gyroscopes Measure rotation Coupe energy from one vibrationa axis to another due to Coriois effect Two micromachined modes: Open oop vibration and Force-to-rebaance mode Vibrating prismatic beams Beam driven in one direction, defection measured in orthogona direction

22 Gyroscopes Tuning forks Large inertia mass, increased sensitivity Metaic ring structure Dua acceerometer Vibrating shes Two-axis Vibration in z direction Output in both x and y

23 Mechanica Pressure Sensors Designs Absoute Gauge Seaed Gauge Differentia

24 MEMS Exampes Pressure Sensor (conventiona) Source: Mauf

25 MEMS Exampes Pressure Sensor (utra-miniature) Source: NovaSensor

26 Piezoresistive Pressure Sensors Piezoresistivity is a materia property where buk resistivity is infuenced by mechanica stress appied to materia Common piezoresistors: Si, poy Si, SiO2, ZnO Typica design: 4 piezoresistors in a Wheatstone bridge on a diaphragm Pressure sensitivity (mv/v-bar): S = ( R/ P)(1/R)

27 Capacitive Pressure Sensors Capacitive sensors convert charge into change in capacitance Advantages: more sensitive than piezoresistive ess temperature dependent Disadvantages: gap fabrication diaphragm mechanica properties

28 Capacitive Pressure Sensors Basic concept: C = A/d Sensitivity: C/ d = - A/d2 Sma Gaps: arger capacitance easier capacitance detection pates may stick together Large Gaps: sma capacitance may require wafer bonding

29 Microphones Convert acoustic energy into eectrica energy High sensitivity pressure sensors Types: Capacitive variabe gap capacitor; most common require DC bias sensitivity: 0.2 to 25 mv/pa response: 10 Hz to 15 khz

30 Microphones (cont) Piezoresistive diaphragm with 4 pezoresistors in a Wheatsone bridge sensitivity: ~25 V/Pa response: 100 Hz to 5 khz Piezoeectric use piezoeectric materia mechanicay couped to diaphragm sensitivity: 50 to 250 V/Pa response: 10 Hz to 10 khz