Miniature Aerial Vehicle. Lecture 4: MEMS. Design Build & Fly MIT Lecture 4 MEMS. IIT Bombay

Transcription

1 Lecture 4 MEMS

2 MEMS Micro Electrical Mechanical Systems Practice of making and combining miniaturized mechanical and electrical components Micromachines in Japan Microsystems Technology in Europe

3 MEMS Fabrication Electronics industry : doubles the transistors in a microchip every 18 months Using similar techniques : able to manufacture beams, diaphragm, motors, pumps etc on micro scale Microfabrication methods ¾ Bulk Silicon Micromachining ¾ Surface Micromachining ¾ LIGA for Deep Structures

4 Cantilever Beam

5 Gears

6 Diaphragm

7 Comb Drives

8 Electrostatic Motors

9 Moving Parts

10 Moving Parts

11 Moving Parts

12 MEMS Market Growth

13 Conventional Accelerometer Piezoelectric material - sensing element

14 MEMS based Single Chip Sensor Sensor forms Differential Capacitor

15 Sensor Operation Folded tethers have more consistent spring constants, leading to better part to part consistency

16 Self Test Operation Extra fixed outer plates may be added which when excited, force the proof mass to move. So you can electronically test the accelerometer

17 Interesting Facts 0.1µgrams Proof Mass 0.1pF per side for the Differential Capacitor 20aF (10-18 f) least detectable Capacitance change Total Capacitance change for Full Scale is 10fF 1.3µm gaps between Capacitor Plates 0.2A minimum detectable beam deflection 1.6µm between suspended beam and substrate 10 to 22 khz resonant frequency of beam

18 Advantages Low cost (can even be made disposable ) FFTs can be used to increase the performance Will work for many machine health applications Onboard signal conditioning. No charge amplifiers required.

19 Disadvantages Performance still below that of more expensive sensors May not be available in industrial hardened packages

20 Summary New accelerometers open the door for new applications in tilt, inertial and vibration ¾ Low cost ¾ High level of integration: Multiple sensors, signal conditioning Clever design can allow use of a less accurate but less expensive sensor ¾ using microcontrollers for calibration and algorithms ¾ Using signal analysis to improve noise levels ¾ Taking new approaches to traditional problems

21 Specs for MEMS accelerometers Range : ±2 to ± 500g Sensitivity : 10% FS output v/g (+5v supply) Noise level : 0.1 % FS output mg (+5v supply) Non-linearity : 0.2% FS Bandwidth : DC - 10kHz Current : 5-15 ma Temperature range : C Weight : 2-15 grams Axis : Single, tri-axial

22 Angular Rate Sensor Coriolis forces ¾ generated when a moving mass is rotated about an axis at right angles ¾ are along the third orthogonal axis ¾ are proportional to the amplitude of the moving mass

23 Angular Rate Sensor Mechanical - Spinning mass

24 Angular Rate Sensor Two masses supported by torsional springs Electrical excitation to oscillate in rotating mode around x axis If rotated about z axis, coriolis force about y axis Motion of beam mass sensed capacitively in four air gaps Structure enclosed in low pressure chamber and remaining gas in air gaps cause damping

25 Angular Rate Sensor Piezoelectric Ceramic disc mounted at centre Excited at resonant frequency by applying voltage at specific drive electrode points Angular rate to be determined by voltage generated on output electrodes

26 Gyroscopes - Comparison Conventional ¾ Bulky ¾ Power hungry ¾ Mechanical wear and tear ¾ Highly accurate ¾ Expensive MEMS based ¾ Miniaturised ¾ Low power consumption ¾ Less moving parts ¾ Accuracy not up to inertial class ¾ Less weight ¾ Low cost

27 Rate Gyro Specs. Range : ±50 to ± 500 /sec Bandwidth : DC - 50 Hz Sensitivity : 1-10 mv / /sec Linearity : 1% FS Current : 5-15 ma Weight : 2-15 grams Axis : Single, Dual axis Power on time : few milliseconds

28 What can MEMS Offer Miniaturization : micromachines (sensors and actuators) can handle microobjects and move freely in small spaces Multiplicity: cooperative work from many small micromachines may be best way to perform a large task inexpensive to make many machines in parallel Microelectronics: integrate microelectronic control devices with sensors and actuators

29 MEMS Features Low interference with environment Accurate, Compact, Shock resistant Inexpensive - based on IC batch fabrication Use in previously unfeasible domains Employ large numbers of devices Redundancy Large sampling size, greater data certainty

30 Lecture 4 MEMS