Page 76 - Handbook of Biomechatronics

Sensors and Transducers 71

Proportional Hall effect sensors return a voltage proportional to the position
of the magnet within a certain range, generally at a fixed orientation. Rotary
Hall effect sensors are designed to sense the change in orientation of the
magnet, generally at a fixed position. For both proportional and rotary Hall
effect sensors, the orientation or position, respectively, do not have to be
fixed, but doing so often helps for calibration of the signal, minimizing
the number of setup variables.
Hall effect sensors do tend to drift and can be affected by the environ-
ment, such as when other magnets or magnetic materials are close to the
sensor. The orientation and position of the magnet being used with the Hall
effect sensor is critical to the function of the sensor.
You decide to use an on/off Hall effect sensor to determine when the
knee is at the limits of its range. To do so, you install two magnets in the
rotating joint of the knee and mount the Hall effect sensor on the stationary
side of the joint. You position the magnets such that when at the limits, the
Hall effect sensor senses the magnets, but not before.

3.5 Strain Gauge

A strain gauge (Fig. 12) is a sensor which responds to the expansion or con-
traction of a material, or the strain. A strain gauge consists of a long thin piece
of metal which folds back on itself, or zig zags across the sensor. As the mate-
rial expands or contracts, the long thin piece of metal gets longer or shorter
with the material, changing the resistance of the metal. The voltage output
of the sensor corresponds to the change in resistance (Omega—Straingages,
n.d.). Strain gauges work well for most metals but are seldom successful in
use with plastic.
A single strain gauge is highly susceptible to environment temperature
changes and placement on the material. To normalize environmental

Fig. 12 (A) Schematic representation of a strain gauge and (B) circuit diagram of a strain
gauge in a Wheatstone bridge.