ECE5320 Mechatronics Assignment#01: Literature Survey on Sensors and Actuators Topic: MEMS Accelerometer

1. ECE5320 MechatronicsAssignment#01: Literature Survey on Sensors and Actuators Topic: MEMS Accelerometer Prepared by: Anand S Madhusoodanan Dept. of Electrical and Computer Engineering Utah State University E: anandsmadhu@cc.usu.edu ; T: (435)797-; F: (435)797-3054 (ECE Dept.) W: http:// 3/11/2005

2. Outline • Reference list • To probe further • Major applications • Basic working principle illustrated • A typical sample configuration in application (application notes) • Major specifications • Limitations • And many more relevant issues in applications (such as, how to choose, cost information, where to buy etc.) ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

3. Reference list • http://www.sensorland.com/HowPage023.html • http://www.sensorsmag.com/articles/0203/14/main.shtml • http://www.sensorsmag.com/resources/businessdigest/sbd1003.shtml • http://www.analog.com/en/cat/0,2878,764,00.html • http://www.aero.org/publications/helvajian/helvajian-3.html • http://www.aplac.hut.fi/publications/trans-1995/trans95/node2.html ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

4. To explore further (survival pointers of web references etc) • For an understanding of how accelerometers work please refer to http://www.kionix.com/Accelerometers/accelerometers.htm • For a complete technology report refer http://www.silicondesigns.com/tech.html • For a detailed list of manufacturers refer http://www.sensorsportal.com/HTML/SENSORS/Accelerometers_Manuf.htm • For articles, papers and references refer http://www.sensorsportal.com/HTML/SENSORS/Accelerometers.htm ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

5. Major applications Accelerometers find widespread applications in the following areas • Automotive industry – release of airbag is the most well known application. Others are suspension control, brake control, fuel cut off and engine knock off. • High precision internal navigation and guidance systems. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

6. Major applications (cont’d) • Vibration control • Used to sense microgravity in space laboratories. • Used to measure motion of patients with diseases such as Parkinson’s. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

7. What are Sensors? • A sensor is a device when exposed to a physical phenomenon (temperature, displacement, force etc..) produces a proportional output signal. • A sensor responds to a change in the physical phenomenon. • A sensor must not be confused with an transducer. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

8. Types of Sensors Sensors can be classified as follows • Active or Passive • Active sensors are sensors that require an external power source (e.g. Strain gauge) • Passive sensors are sensors where the power required to produce the output is derived from the sensed physical phenomenon (e.g. thermometer) • Analog or digital • Analog sensors produce continuous signals that are proportional to the sensed parameter. • Digital sensors produce digital outputs. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

9. Accelerometers • An accelerometer is a device that measures the acceleration and provides an electrical output. • The accelerometer measures acceleration but can be used to provide information on velocity and position as well. This can be done by integrating the output of the accelerometer – once to get the velocity and twice to get the position. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

10. Working of an accelerometer • Every accelerometer can be modeled as a mass – spring – damper as shown below. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

11. The second order system can be expressed as follows m¨x + c ˙x + kx = mainput x – displacement of the mass w.r.t the frame ainput – external input acceleration k – suspension stiffness c – damping coefficient ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

12. The response can be expressed in the Laplace domain as • At frequencies sufficiently lower than ωnthe sensitivity of the accelerometer becomes independent of the excitation frequency. • This operational frequency is called the accelerometer passband. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

13. Block diagram of an accelerometer ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

14. MEMS Accelerometers • Accelerometers are one of the latest application of MEMS technologies. • The underlying principles on which a MEMS accelerometers works are • Capacitive • Piezoresistive • Electromagnetic • Piezoelectric • Ferroelectric • Optical • Tunneling ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

15. The most successful MEMS accelerometers are based on capacitive transduction. This is the best for the following reasons. • Simplicity of the sensor element. • Simple material requirements. • Low power consumption • Good stability over a wide range of operating temperatures. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

16. Working of a MEMS Accelerometer • Usually, the sensing element consists of an inertial mass suspended by compliant strings. • Under acceleration, a force acts on the inertial mass causing it to deviate from its zero – acceleration position. • The restoring force balances the force due to acceleration. • The magnitude of the mass deflection is converted to a proportional electric signal and this appears at the output of the sensor. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

17. Capacitive MEMS accelerometers • The deflection of the mass is detected by the change in the capacitance of the parallel plate capacitor that is formed by the mass and a stationary electrode. • This scheme is used to make the output of the sensor linear and to compensate for drift. • By forming two variable capacitors on opposite sides of the mass a differential capacitor bridge is formed. Hence the reduction of the capacitance of one of the capacitors results in the increase in the capacitance of the other ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

18. The damping of the system is determined by the viscous gas confined between the mass and the stationary plates. • As the viscosity of the air does not change too much with temperature it offers a better frequency response. • Mechanical over – range stops are incorporated into the supporting frame to protect the suspension by preventing a large deflection. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

19. A case study - ADXL2O2E • ADXL2O2E is a MEMS accelerometer that is the world’s smallest mass produced accelerometer. • The highlights of the ADXL2O2E are as follows • Low ‘g’ • Low cost. • The ADXL2O2E has both sensor and signal conditioning on the same chip. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

20. The multi – chip approach has the following disadvantages • Large overall silicon area. • Multi – chip modules need additional assembly stages • Lower yield. • Larger packages needed to house two – chip modules. • Need larger signals to overcome stray capacitance of chip –to – chip interconnections. This causes the overall size of the structure to increase. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

21. The mechanical structure of the ASDASD is shown below ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

22. Polysilicon suspend the MEMS structure above the substrate such that the sensor can move in the X and Y axes. • Acceleration causes the deflection of the mass from the origin. • There are 32 radial fingers around the sensor and they are positioned between the plates that are fixed to the substrate. • Each pair of fingers and the fixed plate form a differential capacitor and the deflection is measured by measuring the differential capacitance. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

23. This can sense both static (gravity) and dynamic acceleration (vibration, impact etc..). ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

24. Examples of MEMS accelerometers • Endevco 7290A – 10 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

25. Analog devices ADXL210A ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

26. Silicon designs SD2012 – 10 micrograph of the two-chip unit Schematic illustration of the sensing element ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

27. Motorola M1220D micrograph of the sensing unit ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators