Page 167 - MEMS Mechanical Sensors

156 Force and Torque Sensors

A silicon piezoresistive force sensor has been used in a tonometric transducer
[10]. A plunger is positioned with silicon gel-like glue to press onto the force sensor.
The other end of the plunger has a disposable protecting latex cap to touch the eye-
ball cornea.
The simultaneous use of silicon bulk-machined components and miniaturized
high precision mechanical structures in a hybrid configuration can solve industrial
measurement problems elegantly. As one example, a micro-torque sensor based on
differential force for use in the watch industry has been developed [11] with a resolu-
tion better than 0.5 µNm over the range –200 to 200 µNm; it has a volume 3×3×1
cm. The torque sensor is schematically represented in Figure 7.4. It consists of two
piezoresistive force sensors. A 100-µm-thick spring blade made of copper beryllium
and mounted perpendicular to the torque axis converts the torque to a force acting
on the two force sensors. The force sensors are micromachined silicon cantilevers. A
perpendicular bar mounted on the torque axis acts on the spring blade by way of
two adjustable screws. The spring blade acts through two points on the two cantile-
ver force sensors. A torque applied on the axis will increase the pressure on one force
sensor and decrease the pressure on the other.
Load cells are force sensors that are used in weighing equipment [3]. In most
conventional load cells the spring element is made from steel or aluminum, and
metal resistance strain gauges are used as the sensor elements. Silicon does not suffer
from hysteresis and creep, and therefore, a load cell made from silicon might be a
good alternative to traditional load cells made from steel. Bending beam structures
may be used for loads up to 150 kg, but for high loads, certainly above 1,000 kg, a
load cell has to be based on the compression of silicon as shown in Figure 7.5 [7].
This sensor consists of two bonded silicon wafers. The edge of the sensor chip is
compressed under the load, and the amount of compression can be measured by
measuring the change in capacitance between two capacitor plates located in the
center. An improved design to apply the load homogeneously will be discussed in
Section 7.5. Another design of silicon load cell for loads up to 1,000 kg has been
reported [12]. Besides large forces/torques, very small quantities can be sensed; a
micro-torque sensor based on differential force measurement was reported more
than 10 years ago [11].

Adjust screws

bar
Torque axis

Spring blade

Force sensor chip

Figure 7.4 Schematic representation of the micro-torque sensor. (From: [11]. © 2003 IEEE.
Reprinted with permission.)