Biomedical Engineering Reference
In-Depth Information
V +
r
V o
+
DAQ
-
R
Figure 5.3
The electric circuit used for calibration and subsequent experiments
The signals from each force sensor ( V o 1 and V o2 ), together with the signal from the
potentiometer ( V P ) and the supply voltage ( V + ), were fed into a data acquisition card
(DAQ).
To examine the linearity of the FSR sensor, and to obtain its calibration curve, an
electric circuit, as shown in Figure 5.3, was used, in which the DAQ was used to analyze
and record the measured data.
To experimentally determine the force - conductance relation for the FSR, several dif-
ferent standard weights were placed on the active area of the FSR and its conductance
was measured. Experiments showed that the conductance was almost linear in relation to
the applied force.
Using the parameters obtained from the electric circuit shown in Figure 5.3, the resis-
tance of the FSR was found and its conductance C calculated from Equation 5.1 .
R
R + r V + C =
1
r
1
V 0 =
=
V +
V 0
1
(5.1)
R
where, r is the variable resistance of FSR (force-dependent), R is a known biasing resistor
(constant); V + is the known voltage of an external power supply. The symbol V o represents
the FSR voltage as seen by DAQ. The signals from the FSR force sensors at the handle
and tip of the grasper are denoted by V o 1 and V o 2 , respectively. At the calibration step,
by applying different standard forces ranged from 1 to 9 N on the FSR, nine data points,
as shown in Figure 5.4, were obtained. Using plotted force - conductance data points, the
following expression (5.2) was obtained using the least squares regression method.
1
r
C =
= a.f + b
(5.2)
where a = 0.079 and b= 0.016.
Combining Equations 5.1 and 5.2, the following voltage - force relationship
(Equation 5.3) was derived, from which the real applied force to grasped objects can be
found:
1
b
a
f =
V +
V r 1
(5.3)
R. a
 
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