ECE5320 Mechatronics Assignment#01: Literature Survey on Sensors and Actuators Topic: Comb Drive

1. ECE5320 MechatronicsAssignment#01: Literature Survey on Sensors and Actuators Topic: Comb Drive Prepared by: Michael Tait Dept. of Electrical and Computer Engineering Utah State University 3/9/2012

2. Outline • Reference list • To probe further • Definition • Operation • Equations • Structure • 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://en.wikipedia.org/wiki/Comb_drive • http://ecourses.ou.edu/cgi-bin/ebook.cgi?doc=&topic=mz&chap_sec=05.0 • http://mems.sandia.gov/about/actuators.html ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

4. To explore further (survival pointers of web references etc) • http://en.wikipedia.org/wiki/Comb_drive • Many scholarly articles are available on the web, however, they are very in depth (search Comb Drive on Google and view the PDFs) ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

5. Definition • Comb drives are linear motors that utilize electrostatic forces that act between two metal combs www.globalspec.com ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

6. Definition • Comb drives built on normal sized “human” scales are extremely inefficient • They are built very small to maintain efficiency, on microscopic or nano-scale devices ecourses.ou.edu ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

7. Definition • As the size of a comb drive is very small, they are manufactured generally in two ways: • bulk micromachining • surface micromachining a silicon wafer substrate www.crossfiber.com ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

8. Operation • Uses electrostatic forces • A voltage is applied between the static and moving combs causing them to be drawn together • The combs must be arranged so that the teeth never touch (otherwise the voltage potential between them would be zero, see the image on the next slide) ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

9. Operation The electric potential on the surface of a comb drive www.comsol.no ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

10. Operation • The force developed by the actuator is proportional to the change in capacitance between the two combs • The force also depends on the number of comb teeth and the gap between the teeth • Typically the teeth are arranged so that they can slide past one another until each tooth occupies the slot in the opposite comb ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

11. Useful Equations • The energy in a capacitor: E = ½*C*V^2 • The force derived by differentiating the energy in the direction of the force: F = ½*(∂C/∂dxdrive)*V^2 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

12. Useful Equations • Parallel plate capacitance: C = ε*A/d • Force equation: F = ½*(n*t*ε0*εr*V^2)/d ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

13. Equation Definitions F = ½*(n*t*ε0*εr*V^2)/d • V = applied electric potential • εr = relative permittivity of dielectric • ε0 = permittivity of free space • n = number of pairs of electrodes • t = thickness in the out of plane direction of electrodes • d = gap between electrodes ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

14. Structure • Rows of interlocking teeth • Teeth half fixed, half moveable assembly • Electrically isolated • Electrostatic attraction/repulsion • Typically 10 μm long ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

15. Application • Comb drive use locations: • Gyroscopes • Microengines • Resonators Typical Comb Drive Resonator mems.sandia.gov ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

16. Application • The force output for a comb drive is typically less than 50 μN • Remains a popular actuator because: • Linear response • Highly predictive behavior ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

17. Limitations • As mentioned above, a comb drive can only be used in very small implementations as the efficiency is very poor in large arrangements • The force created by a comb drive is very small and may be too small for some implementations • The drive is very sensitive to surrounding electrostatic potential and must be shielded against unwanted potentials ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

18. Cost and Availability • As a comb drive is very small and must be manufactured on silicon wafers, I was unable to find one for sale commercially • It appears that the comb drive is relatively new as it is a nanotechnology and there isn’t a lot of information available regarding the actuator ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

19. Cost and Availability • Many scholarly articles refer to comb drives (especially at Sandia National Laboratories) leading one to believe they are still very much in development ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators