The Art and Signs of a Few Good Mechanical Designs in MEMS
31
2 Simplicity of a Folded-Beam Suspension
Joints are essential components of any mechanical device with moving parts. MEMS
field got its recognition at the inception of the field when researchers showed that
hinges and sliders could be made at the micron scale with polysilicon by incorporating a sacrificial layer in microelectronic fabrication processes [13]. Electrostatic
micromotors [14], which could spin about an axis perpendicular to the silicon wafer,
had used cleverly-crafted revolute joints, i.e., hinges. Similarly, prismatic joints (i.e.,
sliders) too were realized using silicon. Sacrificial layer process was the key to
those developments. Years later, Sandia laboratory made a miniature revolute joint
with a seven-layered process called SUMMiT [15], Sandia Ultra-planar Multi-level
MEMS Technology. The geometry of the Sandia micro-hinge can be argued to be
more complex than that of a macro-scale bearing. The relative tolerances were not
stringent in those miniature revolute joints even though absolute values of tolerances
were only a few microns. If MEMS field limited itself to such kinematic joints that
merely imitate macro-scale joints, the field would not have attained the sophistication
we see today. The key was simplicity offered by compliant design [16].
In compliant design, elastic deformation is utilized instead of kinematic joints
such as hinges and sliders, to achieve relative motion. A compliant hinge can be as
simple as a short beam segment or a narrow flexure that enables a body attached
to it to rotate about a point. But this has three drawbacks: (i) the effective center of
rotation keeps shifting during the rotation; (ii) the range of motion is limited because
of stress in a deforming body; and (iii) there is inevitable resistance to motion because
some effort is needed to deform a body. So, a compliant hinge cannot come close
to a revolute joint in terms of functionality and performance. The situation is better
with a compliant slider.
A kinematic sliding joint needs a guideway in which a block moves to provide
relative translation. Such a moving block has zero resistance, barring friction, in
the direction of translation; it has infinite resistance in all other directions, namely,
two other translational directions and all three rotational directions. Resistance in
a compliant slider is due to elastic stiffness. Therefore, a compliant slider should
have as low a stiffness as possible in the intended translating direction and as high
a stiffness as possible in all other directions. A pair of beams achieves that to some
extent as can be seen in Fig. 2a.
Note that a cantilever beam has high stiffness in the axial direction and low
stiffness in the transverse directions. So, we can use a beam to translate a body. But
a block attached at the free tip of a single cantilever beam would rotate. On the other
hand, a block attached to a pair of beams does not rotate much. So, it becomes a
compliant slider. But then, we notice in Fig. 2a that the moving block experiences
slight motion in the direction perpendicular to the intended translatory motion. This
is avoided by using a folded-beam suspension [17] shown in Fig. 2b. The transverse
motion of one pair of beams is perfectly compensated by that of another pair because
of the folded-beam configuration. Figures 2c, d show the solid model and deformed
configuration of a folded-beam suspension, respectively.
31
2 Simplicity of a Folded-Beam Suspension
Joints are essential components of any mechanical device with moving parts. MEMS
field got its recognition at the inception of the field when researchers showed that
hinges and sliders could be made at the micron scale with polysilicon by incorporating a sacrificial layer in microelectronic fabrication processes [13]. Electrostatic
micromotors [14], which could spin about an axis perpendicular to the silicon wafer,
had used cleverly-crafted revolute joints, i.e., hinges. Similarly, prismatic joints (i.e.,
sliders) too were realized using silicon. Sacrificial layer process was the key to
those developments. Years later, Sandia laboratory made a miniature revolute joint
with a seven-layered process called SUMMiT [15], Sandia Ultra-planar Multi-level
MEMS Technology. The geometry of the Sandia micro-hinge can be argued to be
more complex than that of a macro-scale bearing. The relative tolerances were not
stringent in those miniature revolute joints even though absolute values of tolerances
were only a few microns. If MEMS field limited itself to such kinematic joints that
merely imitate macro-scale joints, the field would not have attained the sophistication
we see today. The key was simplicity offered by compliant design [16].
In compliant design, elastic deformation is utilized instead of kinematic joints
such as hinges and sliders, to achieve relative motion. A compliant hinge can be as
simple as a short beam segment or a narrow flexure that enables a body attached
to it to rotate about a point. But this has three drawbacks: (i) the effective center of
rotation keeps shifting during the rotation; (ii) the range of motion is limited because
of stress in a deforming body; and (iii) there is inevitable resistance to motion because
some effort is needed to deform a body. So, a compliant hinge cannot come close
to a revolute joint in terms of functionality and performance. The situation is better
with a compliant slider.
A kinematic sliding joint needs a guideway in which a block moves to provide
relative translation. Such a moving block has zero resistance, barring friction, in
the direction of translation; it has infinite resistance in all other directions, namely,
two other translational directions and all three rotational directions. Resistance in
a compliant slider is due to elastic stiffness. Therefore, a compliant slider should
have as low a stiffness as possible in the intended translating direction and as high
a stiffness as possible in all other directions. A pair of beams achieves that to some
extent as can be seen in Fig. 2a.
Note that a cantilever beam has high stiffness in the axial direction and low
stiffness in the transverse directions. So, we can use a beam to translate a body. But
a block attached at the free tip of a single cantilever beam would rotate. On the other
hand, a block attached to a pair of beams does not rotate much. So, it becomes a
compliant slider. But then, we notice in Fig. 2a that the moving block experiences
slight motion in the direction perpendicular to the intended translatory motion. This
is avoided by using a folded-beam suspension [17] shown in Fig. 2b. The transverse
motion of one pair of beams is perfectly compensated by that of another pair because
of the folded-beam configuration. Figures 2c, d show the solid model and deformed
configuration of a folded-beam suspension, respectively.