Page 177 - MEMS Mechanical Sensors

166 Force and Torque Sensors

drive amplitude of 1 nm, the resonant frequency of the cantilever was 20 kHz with a
Q-factor of 400 in air.
Chemical imaging as well as topographical information of solid surfaces can
now be undertaken using SFMs [64]. A micromachined integrated sensor for com-
bined AFM and near-field scanning optical microscopy (NSOM) has been reported
[65, 66]. This sensor consists of a microfabricated cantilever with an integrated
waveguide and a transparent near-field aperture tip.

7.8 Tactile Sensors

The intensified miniaturization of devices requires an appropriate handling of
microparts during fabrication and assembly. Indeed, investigation at the atomic
scale level needs more accurate sample manipulation by means of a “nanorobot”
3
having, say, resolution of 10 nm and a 1-cm working space. Micromachined grip-
pers are required, and when the gripper clamps or touches an object, force sensing
would be a great advantage. A tactile microgripper with both actuation and sensing
integrated has been developed [67]. A thermal bimorph actuator and piezoresistive
force sensor are used. A 6-µm-thick, 250-µm-wide silicon beam finger has a 300-µm
stroke and time constant of 11 ms. Gripping force is about 250 µN, and this is
sensed by diffused boron piezoresistors in a Wheatstone bridge.
A silicon micromachined piezoelectric tactile sensor has been integrated on to
the tip of an endoscopic grasper used by a surgeon to manipulate tissue [68]. The
grasper has the usual rigid tooth-like surface (Figure 7.16). It consists of upper sili-
con, a perspex substrate, and a patterned polyvinylidene fluoride (PVDF) film that is
sandwiched between the two layers. Force dynamic range is 0.1N to 2N with a reso-
lution of 0.1N and bandwidth from near dc to several megahertz.
The silicon substrate used for micromachined tactile sensors is rigid and
mechanically brittle, and therefore not capable of sustaining large deformation and
sudden impact. Recently a two-dimensional tactile sensor array based solely on
polymer (polyimide) micromachining and thin-film metal resistors has been demon-
strated [69]. A schematic diagram of a single taxel is given in Figure 7.17. The mag-
nitude of in-plane surface stresses is found to be greatest at the periphery of the
membrane. The effective gauge factor of the taxels is approximately 1.3.

Tactile sensor

Grasper jaws

Endoscopic cylinderical tube
Figure 7.16 Endoscopic grasper with integrated tooth-like tactile sensor. (From: [68]. © 2003
IEEE. Reprinted with permission.)