Biomedical Engineering Reference

In-Depth Information

t
stop
¼
t
ð
max
ð
f
z
ÞÞ
t
ð
f
z
[ 10 N
Þ:

ð
6
:
19
Þ

Note that the rubber foam is used to have an additional elastic layer to protect

robot and sensor from damage. However, the iron plate is heavy enough to produce

forces at impact larger than the security limit.

6.2 Performance of the FTA Sensor

6.2.1 Calibration

As the performance of the FTA sensor strongly depends on the accuracy of the

calibration of IMU to FT sensor, we have systematically analyzed the accuracy of

the calibration method. Therefore, we have performed 60 calibrations with 4

different FTA sensors that we have utilized for evaluation.

6.2.1.1 Quality of the Fit

As a cosine fit is the basis for calibration of IMU to FT sensor, we have foremost

estimated the quality of the fit. The quality of the fit is expressed as the absolute

distance of the measured values to the fitted cosine.

Figure
6.9
shows the overall cosine fitting quality used for calibration as

Boxplot. The median deviations for forces are 0.14, 0.11 and 0.15 N for the three

spatial axes. For torques, the deviations are 0.0034, 0.0023 and 0.0017 Nm,

respectively. The median deviations for the accelerations are 0.016, 0.027 and

0.022 g, respectively. Due to noise, we were not able to perform a valid cosine

fitting in two recordings. Therefore, these two recordings are excluded from fur-

ther analysis.

6.2.1.2 Calibration Error

In order to estimate the calibration error, we have transferred the measured

acceleration into the FT coordinate frame and have compared the angle difference

to the measured FT recordings.

We have found that the median calibration error is 3.4
for the x-axis and 3.5

and 1.6
for the y- and z-axis, respectively. Figure
6.10
shows these results as

Boxplot. Interestingly, the median calibration error in the z-axis is essentially

smaller than for the x- and y-axis, respectively, but has some larger outliers

(around 12 N).

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