Page 77 - Handbook of Biomechatronics

72 Jeff Christenson

conditions, multiple strain gauges are used, with the gauges placed at various
orientations with respect to the direction of desired strain sensing. Temper-
ature effects can be measured and removed from the signal by placing a strain
gauge orthogonal to the direction of desired measured strain. The orthog-
onal strain gauge will expand and contract with temperature, but only
minimally respond to the orthogonal strain.
Strain gauges are often wired in a Wheatstone bridge configuration
(Fig. 12B). The Wheatstone bridge provides temperature compensation
since all the resistors in the bridge experience the same environmental tem-
peratures. Each leg of the bridge can have either a sensor or a dummy resistor
of similar resistance (Omega—Straingages, n.d.).
The application of strain gauges is difficult. Special care must be taken for
material surface preparation, orientation of the gauges, gauge to material
attachment, and postattachment sensor handling.
On the pylon, or tibial section of Jacob’s prosthetic system, you mount a
strain gauge to sense the load being born by the prosthesis. To do so, you
carefully prepare the surface of the pylon, select a strain gauge which is rated
for the expected strain range, determine the appropriate orientation to
mount the gauge, and decide to mount several gauges at various angles to
be able to sense various directions of strain. Once glued in place, you test
your gauges to verify adhesion and performance.

3.6 Thermistor

A thermistor, or temperature-dependent resistor, is used to sense a change in
temperature. A thermistor consists of a material which is highly responsive to
temperature. The resistance of the material is proportional to the expansion
and contraction of the material, which can be calibrated to the temperature
(Fig. 13)(Omega—Straingages, n.d.).
In Jacob’s knee, you select a temperature-dependent resistor for the
processor board which controls the knee motor. You will use this sensor
to monitor the temperature of the motor. If the resistor gets too hot, the
processor can limit motor power to keep the system within specified
temperature parameters.

3.7 Current Sensor
Often in electrical systems, it is useful to know the current being used by the
system. There are two ways to measure current, either directly or indirectly
(Fig. 14)(Omega—Thermistor, n.d.).