Biomedical Engineering Reference
In-Depth Information
Fig. 6.5 The FTA sensor
(A) is mounted to the Adept
robot's end effector (B). The
communication channels
(C) are passed through the
robots internal connections at
the fourth link (D). The force-
torque sensor is integrated in
a casing which houses the
circuit board with the IMU.
The casing allows for easy
mounting to the robot end
effector
for the x- and y-directed forces and 2,000 N for the z-directed force. For torques, the
sensing range is up to 20 Nm for all axes.
We mount the FTA sensor to the Adept robot as shown in Fig.
6.5
. The
communication, emergency stop and power supply cable are passed through
the robot's internal user communication interface. In this way, intertwining of the
cable with the tool or articulated arm is avoided. For data transmission, we choose
a serial communication. Due to the robot noise, a USB connection via the robot's
internal user communication interface is not possible.
6.1.4 Calibration of IMU to FT Sensor
As IMU and FT sensor are located in the same casing, a coarse knowledge of their
coordinate systems exists. However, for our application, an accurate transforma-
tion is required. Thus, a calibration of IMU to FT sensor is mandatory.
Once the FTA sensor is installed on the robot, we use a full circular motion in
joint 4 of the articulated arm to perform calibration. For the circular motion, the
angle values are used with the measured acceleration and Joint 5 is set to 45
to
allow for non-zero measurements in all spatial axes. For calibration, we mount a
weight to the FT sensor.
For each spatial axis and for each modality (force, torque, acceleration), we
calculate a cosine fit using:
a
l
cos
ð
c
þ
b
l
Þþ
c
l
;
c
2½
p
;
p
;
ð
6
:
5
Þ
with l
¼
F
x
;
F
y
;
F
z
;
M
x
;
M
y
;
M
z
;
A
x
;
A
y
;
A
z
. In this case, the parameter c
l
describes
the offset for forces, torques and accelerations. By comparison of the phase angle
b
l
between forces F and accelerations A, we can compute the transform
FT
T
IMU
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