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514 CAM DESIGN HANDBOOK

systems, microsurgery, microassembly, and micromanipulation of small objects such as
single biological cells, to name a few.

15.6 CAMS IN MEMS

A few specific MEMS devices consisting of cams and gears will be described in this
section. As stated earlier, cams used in MEMS do not belong to the same category of cams
used in traditional cam-follower type mechanisms because of the entirely different objec-
tive of MEMS, which so far has been primarily transduction rather than mechanical trans-
mission. When sophisticated, coordinated mechanical movement finds applications at the
microscale level and some of the practical problems of friction and wear (discussed in
Sec. 15.8) are better understood and remedied, we can expect to see more cams in MEMS.
As can be seen in the examples presented later, the scope of cams is somewhat expanded
in this presentation. Nevertheless, they all possess the essential features of cam-action in
that the motion and force transmission take place by virtue of line or point contact between
rigid bodies. In each of the devices, an association is made to a traditional cam. This may
be debatable in some cases but the connection to traditional cams is necessary to promote
the use of cams in MEMS.

15.6.1 Microindexing Motor with a Wedge Cam
The schematic in Fig. 15.6a shows the principle of operation of a mechanical stepper motor
designed and fabricated at the Sandia National Laboratories using the SUMMiT process
(Allen and Schriner, 1998). This serves as an actuator with precise indexing capability,
which is useful in positioning applications in MEMS devices. It is a multilayered struc-
ture composed of an outer ring, a reciprocating shuttle, a central post, and a linear actua-
tor for the shuttle. These components are designed in such a way that they do not interfere
with each other when they move. As shown Fig. 15.6a, as the shuttle with two indexing
teeth reciprocates from left to right, the wedge action between the two shuttle teeth and
many internal teeth on the outer ring makes the outer ring rotate. Only two internal teeth
on the outer gear are shown in Fig. 15.6a although, as indicated, many other exist. At any
given time, only one shuttle tooth interacts with a single internal tooth. The shuttle is driven
by a linear electrostatic comb drive, which is described in Sec. 15.7.2. A microscale pro-
totype in which the diameter of the ring gear is about 250mm with 63 internal teeth is
shown in Fig. 15.6b. It indexes by 2.85° per step. An operational speed of 205 steps per
second was observed. At higher speeds of operation, the indexing tooth showed a tendency
to bounce over the internal teeth. It is reported that this can be overcome by adding a cover
over the internal teeth, which can be easily accommodated in the SUMMiT process.

15.6.2 Torsional Ratcheting Microactuator
Sandia researchers have also developed a novel actuator, called an electrostatic torsional
ratcheting actuator, that is compact and easy to operate and provides accurate position
control without feedback. It consists of an angular comb drive with interdigitated electro-
static comb actuators, a pawl, a ring gear with ratcheting teeth, and an antireverse (see
Fig. 15.7a). The antireverse is attached to the fixed frame by means of a flexible beam,
and the pawl is attached to the angular elctrostatic comp drive with a flexible beam, while
the ring gear is free to rotate in the counterclockwise direction. When the angular comb