KINEMATICS - Biomechanics and Motor Control of Human Movement - page 52

Figure 3.5 Schematic diagram of an accelerometer showing acceleration with normal
and tangential components. Voltage output is proportional to the normal component of
acceleration, a n .
Figure 3.6 Two movement situations where the acceleration in space is identical but
the normal components are quite different.
A typical electric circuit of a piezoresistive accelerometer is shown in
Figure 3.7. It comprises a half-bridge consisting of two equal resistors
R 1 . Within the transducer, resistors R a and R b change their resistances
proportionally to the acceleration acting against them. With no acceleration,
R a = R b = R 1 , and with the balance potentiometer properly adjusted, the
voltage at terminal 1 is the same as that at terminal 2. Thus, the output
voltage is V = 0. With the acceleration in the direction shown, R b increases
and R a decreases; thus, the voltage at terminal 1 increases. The resultant
imbalance in the bridge circuit results in voltage V , proportional to the
acceleration. Conversely, if the acceleration is upward, R b decreases and R a
increases; the bridge unbalances in the reverse direction, giving a signal of
the opposite polarity. Thus, over the dynamic range of the accelerometer, the
signal is proportional to both the magnitude and the direction of acceleration
acting along the axis of the accelerometer. However, if the balance poten-
tiometer is not properly set, we have an unbalanced bridge and we could get
a voltage-acceleration relationship like that indicated by the dashed lines.