1. Dept. of Electrical and Computer Engineering
ECE5320 Mechatronics Literature Survey on Actuators Topic: Micro Electro Mechanical Systems: Comb-Drives Actuators
Prepared by:
Ricardo Estevez
Dept. of Electrical and Computer Engineering
Utah State University E:
T: (435)
3/7/2008

2. Outline Reference list To probe further Definition Major Applications
Basic working principle
Sample configuration
Limitations
Comb Driver Construction

3. Reference List

4. To Explore Future
For more information about MEMS: Comb Drives and its applications please read the following links:
MEMS tunable capacitor based on angular vertical comb drives:
Failure Analysis of Thermal Actuators, Comb Drives,and Other Microelectromechanical Elements:
Comb-drive actuators for large displacements:
A Micromachined Comb-Drive Tuning Fork Rate Gyroscope:

5. Definition
Comb-Drives Actuators are linear motors that utilize electrostatic forces that act between two metal combs.
The force developed by the motor is the force between the two combs (which increases with voltage difference, the number of comb teeth, and the length of the teeth, and decrease as the combs are further apart.
The combs are arranged so that they never touch (because then there would be no voltage difference). Typically the teeth are arranged so that they can slide past one another until each tooth occupies the slot in the opposite comb.

6. Major Applications
Electrostatic comb-drive actuators have been used in numerous MEMS applications for advantages in extended ranges-of movement, stable and reliable operation, and design flexibility
Restoring springs, levers, and crankshafts.
MEMS tunable capacitors.
Micro-deformable mirrors for adaptive optics that can be used for vision science applications.
Telecommunications networks such as an optical switch, variable optical attenuator, tunable filter and scanning micro mirror
Microaccelerometers make high use of comb-drive.

7. Basic working principle
A bias voltage VB and a harmonic voltage having VA amplitude and Ωdrive frequency are applied to the stators of the comb-drive. The harmonic voltage is applied in counter-phase to the two-stator parts of the comb-drive:
This figure shows a scheme of the comb fingers highlighting the stator and rotor parts, the capacitances C1 and C2 and the applied voltages V1 and V2.

8. Basic working principle
Assuming that the capacitance C1 of the left part of the comb-drive is equal to the capacitance C2 of the right part when no voltage is applied and that the capacitances on the two sides of the comb-drive vary linearly with the rotor displacement, we obtain:
C0 being the capacitance when no voltage is applied, x the rotor displacement (hence the mass displacement) along drive direction and C′ the derivative of the capacitance with respect to the x-coordinate. It is assumed that C′ is independent of the rotor displacement along drive direction. This assumption is correct if boundary effects are neglected.

9. Basic working principle
The excitation force on the C-shaped and square-shaped masses is therefore equal to:
where N is the number of actuation comb-drives.
In theory, comb-drive would require higher voltage than straight actuation because of the electric-field direction being different than displacement one. But most of the time, theses structures are geometrically optimized, so the gap between electrodes and the distance to travel are also shorter.

10. Basic working principle
The blue part is fixed and anchored to the substrate. The red part is free to move, except at the end where it is anchored to the substrate, so that the actuator can come back to its initial position thanks to the spring force.

11. Basic working principle
In this configuration, the force is equally and symmetrically applied on both side of beams, and the mobile part moves along the beams direction.
The total displacement is rather shorter than what is possible in straight actuation, but the force is constant, meaning an easier control of the displacement. This kind of system offers a very high precision level, and a simpler electronic control.
In a comb-drive, there is no pull-in effect, unless a design error make electrodes reach each others. The displacement is linear, contrary to straight actuation. But the design is limited by the needs to place electrodes beams opposite to each other. Designer must also take care of the end stop to avoid contact between mobile part and fixed part, and to keep forces quite symmetric so that the direction of the displacement is kept as planned.

12. Typical sample configurations
An electrostatic comb drive actuator, consisting of interdigitated capacitors, is one of the most important of MEMS devices. The use of comb drive actuators has made it possible to achieve a very large tuning ratio
In a typical comb drive, the gap between the fixed and moving fingers is uniform, resulting in an electrostatic driving force that is independent of the position of the moving fingers except at the ends of the range of travel.

13. Sample configurations
While lateral comb drives do not suffer from pull-in, the capacitance tuning relies on the lateral motion of the movable fingers
Hence, the tuning ratio is limited by the maximum separation of these fingers and their overall lengths

14. Sample configurations
The vertical electrostatic comb drive actuator has the advantages of low driving voltage with large displacement, high motion speed and absence of pull-in phenomenon.
It also allows the use of stiffer springs for higher resonant frequencies without excessively high operating voltages.
The principle of vertical comb drive actuation is illustrated in the following figure. Each comb finger is composed of an upper electrode layer, an insulation layer and a lower electrode layer. The fixed and moving comb fingers are separated upper and lower electrodes insulated by an insulation layer.

15. Typical sample configurations
When a voltage is applied between electrodes V1 and V2, the imbalance of the electric field distribution will result in a vertical induced force. Consequently, comb fingers will move in an upward motion. On the other hand, when the voltage is applied between electrodes V3 and V4, the comb fingers will move in a downward motion.

16. Limitations
Electrostatic force depends largely on the size of the structures and the distance between electrode. For large electrode surface compared to distance to travel, electrostatic actuation has a large advantage. But the equation of the force gives a dependency to the square of the distance.
This means that the longer the distance is, the higher the actuation voltage is. This is one of the main problem with this physics principle: actuation voltage are often quite high, easily reaching tens, and even hundreds of volts to be used.

17. Limitations
Another consideration to take care of is the electric field itself: the nature of the material between electrodes: water, for example, is conductive at low frequencies, so electrostatic actuation cannot be used in these conditions. Void and neutral gases are the best environments.
Finally, the hysteresis behaviour of straight actuators can be as well an advantage and a problem, depending on the application. It reduces sensitivity of devices to electrical noise, but it also means larger voltage variation for a complete actuation cycle when pull-in/pull-out is desired

18. Comb Driver Construction
Selective stiction has been successfully applied to the batch fabrication of angular vertical comb actuators made of single-crystal silicon with self-aligned comb sets.
The fabrication process with unique designs of mechanical springs enables the stiction of microstructures in a controlled manner and significantly reduces unwanted compliances on the actuators, preventing unwarranted motion and providing stable operations.

19. Comb Driver Construction
The figure to the right shows schematically how, in our process, selective stiction is applied to construct self-aligned vertical comb-sets.
The mechanical springs attached to the stiction-plate could allow only vertical motion, and the original locations of the stiction-plates are held to maintain self-alignment between the rotor and the stator comb sets
The stiction-plate springs must be flexible in the out-ofs ubstrate direction to allow stiction to occur but rigid in the in-plane direction without affecting the self-alignment of stator and rotor comb sets.
UC-Berkeley Micro fabrication Laboratory

20. Major specifications
Detailed dimensions of a comb driver actuator