Each - Draper Laboratory

stray capacitance, which mitigates the need for on-chip
circuitry. (With silicon wafers, bond pads are isolated from
the conducting substrates by thin dielectric layers so that
high stray capacitance limits performance. With on-chip
circuits, bond pads and stray capacitance are avoided.)
Sense
Plate
Torsion
Beam
Sense
Electrode
Base Beam Anchor
(a)
SPO SPO
Drive
Beam
Proof
Mass
Anchor Base Beam
LM LS MPO
RS RM
Figure 1. The Draper/Honeywell TFG mechanism. In 1(b)
and 1(c), silver is metal, diagonal lines indicate
silicon attached to glass, and white indicates
suspended silicon. Electrical contact pads are
right motor drive (RM), right sense electrode
(RS), motor pickoff (MPO), left sense electrode
(LS), left motor drive (LM), and sense pick off
(SPO).
On either side of each proof mass are interdigitated
combs. [33] The outer combs (left and right motor in Figure
1) are used for electrostatically driving the proof masses
antiparallel to the substrate in the x direction. The inner
combs (motor pickoff in Figure 1) sense the drive motion
and are typically biased to 5 Vdc through an op amp that
senses charge traversing the comb gap. As described in “The
Fundamental Dynamics of Oscillating Coriolis Sensors”
44 Error Sources in In-Plane Silicon Tuning-Fork MEMS Gyroscopes
(b)
Motor Pick Off
Substrate
W I Input Rate
(c)
y
x
Mass Motor
Sense Electrode
Proof
Mass
section, rotation about the in-plane z-axis induces Coriolis
acceleration, which deflects the proof masses in opposite
directions perpendicular to the substrate. Beneath the
plates are deposited metal electrodes that are excited with
dc voltages of opposite polarities. The right sense plate (RS
in Figure 1) is typically excited with 5 V and the left sense
plate (LS) with -5 V. Differential proof mass motion induces
electrical currents in the structure that flow through the
suspensions and sense pickoff (SPO, Figure 1) into a
preamplifier whose input contains the input angular rate
modulated by the drive frequency.
With drive resonant frequencies from 10 to 20 kHz, these
gyroscopes are relatively stiff with suspension stiffnesses
greater than 100 N/m. With 3-µm gaps, mechanical spring
force of 300 µN is available to overcome sticking; nevertheless,
care in etching and release, in electronic excitation,
and mechanical handling is required.
As detailed below, the challenge is to obtain excellent
performance in a device where the sensitivity to angular
rate is small. Obstacles include manufacturing tolerances
and the relatively large magnitudes of non-Coriolis forces
and electrical drive and excitation signals.
Mode Ordering
A first challenge is designing the angular rate sensor’s
dynamic eigenfrequencies. If one considers the TFG proof
masses (Figure 1) rigid and the suspension beams without
mass, 3 rotations and 3 translations times 2 masses imply at
least 12 dynamic modes. For advanced designs, proof mass
compliance and suspension modes add further considerations.
The TFG is designed so that the lowest frequency
modes are generally: 1) drive or tuning fork, 2) translation,
3) sense, and 4) out-of-plane. In the tuning-fork mode, the
proof masses move antiparallel to the substrate. One usually
attempts to excite this mode through the electrostatic motor
drive. Similar proof mass amplitudes are the design goal.
The drive frequency is designed for 10-20 kHz to reduce
vibration and acoustic effects. For the translation mode,
the proof masses move parallel to the substrate. Because
the drive combs are controlled to apply forces in opposite
directions, translation should not be excited by electrostatic
drive; however, translation is excited by linear acceleration.
To ensure tuning-fork operation despite beam width tolerances,
the in-plane translation frequency is usually set 10-
15% or more away from the drive frequency.
The sense mode has the two proof masses moving away
or toward the substrate in opposite directions. This could
also be a rotation about their common center. For good
gain, the sense eigenfrequency is set 5-15% away from the
drive. While higher gain can be achieved at smaller separation,
small variations in the resonant frequencies result in
larger fractional changes of scale factor. When the out-ofplane
mode is excited, the two proof masses move together
perpendicular to the substrate. It is important that the lowest
modes do not fall close to one another and that higher order
modes are not integral multiples of the basic four.