Page 81 - Handbook of Biomechatronics

76 Jeff Christenson

4.2 Pressure Sensors
Pressure sensors are used for measuring pressure. There are two types of
pressure that is of interest, either pressure from a gas or pressure from a touch
(Bolton, 2003c).
Gas pressure sensors work by measuring the displacement or strain of
a diaphragm over a given area using simple sensor components. Pressure
is calculated by dividing the force by the displacement.
A common way of measuring tactile pressure is to use force-sensitive
resistors (FSR) (Fig. 17). A FSR consists of a grid of small thin wires. When
the grid is touched and pressure applied, the wires become longer which
changes the output voltage (Bolton, 2003d).
In order to provide a sleeker design, you switch the knee resistance
adjustment method from potentiometers to two FSRs. One FSR is used
for increasing the resistance, the other is for decreasing the resistance. As
Jacob taps the FSR, the resistance increments by a set amount defined in
the microprocessor code.

4.3 Accelerometer
Accelerometers measure the acceleration of an object. A MEMS accelerom-
eter consists of a mass suspended in an elastic support system (Fig. 18). When
the sensor is moved, the elastic support responds and the force is measured

Fig. 17 Example of a force-sensitive resistor.

Fig. 18 Schematic representation of MEMS accelerometer. As the mass moves, the
capacitive sensor measures the deflection of the spring supporting the mass. Based
on that deflection, the force can be calculated.