Lecture 4 MEMS
MEMS Micro Electrical Mechanical Systems Practice of making and combining miniaturized mechanical and electrical components Micromachines in Japan Microsystems Technology in Europe
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
Cantilever Beam
Gears
Diaphragm
Comb Drives
Electrostatic Motors
Moving Parts
Moving Parts
Moving Parts
MEMS Market Growth
Conventional Accelerometer Piezoelectric material - sensing element
MEMS based Single Chip Sensor Sensor forms Differential Capacitor
Sensor Operation Folded tethers have more consistent spring constants, leading to better part to part consistency
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
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
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.
Disadvantages Performance still below that of more expensive sensors May not be available in industrial hardened packages
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
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 : 0-70 0 C Weight : 2-15 grams Axis : Single, tri-axial
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
Angular Rate Sensor Mechanical - Spinning mass
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
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
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
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
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
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
Lecture 4 MEMS