Page 80 - Handbook of Biomechatronics

Sensors and Transducers 75

Fig. 16 Load cell.

one or more simple sensors, supporting hardware for signal amplification,
filtering, and power management, and mechanical structure support.

4.1 Load Cell/Force Plates
A load cell is used for measuring force (Fig. 16). A load cell consists of a
spring with a known spring constant and a way to sense the displacement
of the spring. Given the spring constant the displacement, Hook’s law
can be applied to find the force (Wang, 2014). A force plate is a load cell
that is configured for measuring ground reaction forces.
The spring used is often either a coil-type spring or a cantilever spring.
Many different simple sensors can be used to sense the displacement. For
instance, some load cells have strain gauges mounted on the deflection
arm of the cantilever spring. Others have a magnet and Hall effect config-
uration. Some load cells have a potentiometer which measures spring dis-
placement. Another option is to measure the change in capacitance
between to conductors, where one conductor is at the end of the spring.
Recall the strain gauge mounted on the pylon of Jacob’s prosthetic
system. To enhance the usefulness of the strain gauge signal, you add some
support electronics to reduce the noise, amplify the signal, and deal with
the drift of the signal overtime. Also, you apply various loads to the end
of the pylon and record the filtered and amplified strain gauge output to
calibrate the sensor. The work accomplished results in a reliable load cell.