A Voltage Variable, MEMS Capacitor Insensitive to Acceleration
Original Publication Date: 2003-May-28
Included in the Prior Art Database: 2003-May-28
A design for a MEMS (Micro Electronic Mechanical System) voltage variable capacitor is presented. This design uses a movable electrode that moves in rotation rather than translation, which greatly reduces the effect of translational acceleration on the proof mass. This would allow the use of voltage variable capacitors in products that could be subjected to acceleration, e.g. cell phones.
A Voltage Variable, MEMS Capacitor� Insensitive to Acceleration
A design for a MEMS (Micro Electronic Mechanical System) voltage variable capacitor is presented.� � This design uses a movable electrode that moves in rotation rather than translation, which greatly reduces the effect of translational acceleration on the proof mass.� � This would allow the use of voltage variable capacitors in products that could be subjected to acceleration, e.g. cell phones.
A number of researchers (ref. 1,2,3) have used MEMS to build variable capacitors.� Many of these devices use electrostatic actuation to move an electrode mounted on a compliant suspension.�
Figure 1 shows a schematic of how such a device operates.� � A DC bias is supplied between the actuator electrode 1 and the movable electrode 2, producing an electrostatic force.� This electrostatic force translates the movable electrode 2, as it is mounted on a compliant suspension 3.� � This in turn changes the capacitance between the movable electrode 2 and sense electrode 4 to change.� � Thus this device can function as a voltage controlled adjustable (“variable”) capacitor.
This concept has a number of variations.� � The actuation can be through comb fingers moving parallel to the finger orientation� (voltage causes increase in finger overlap), comb fingers moving perpendicular to the finger orientation (voltage causes gap between fingers to decrease) or parallel plates.� � Also the capacitor between the movable electrode and sense electrode can have a number of configurations.�
Most of these designs are sensitive to acceleration, under acceleration they will respond in similar manner to MEMS accelerometers.� � The movable electrode has significant mass, and when the system is subjected to acceleration it will deflect, with capacitance changing due to acceleration.� � In a mobile application (e.g. cell phones) this could make capacitance control difficult, as device movement will change capacitance.
This problem is significant in many cases.� An example is the variable capacitor in reference 2.� � The mass of this device is 15E-9 kg, the spring constant is about 0.5 N/m, and the full-scale displacement of the movable electrode is about 20 micron.� If this device is subjected to an acceleration of 20 g, the capacitance would vary about 30% of its adjustment range.
This problem can be reduced somewhat with design adjustments, but these design changes affect system performance.� � The mass of the movable electrode can be reduced by making it smaller, but this reduces the capacitance value.� � The compliant suspension can be made more stiff, but this increases the amount of DC bias required to adjust the capacitor (the device in reference 1 requires about 8V of bias).
The goal of the proposed device is to provide a design technique that will greatly reduce the acceleration sensitivity of this type of voltage variable, MEMS, adjustable capacitors.