IEEE Robotics & Automation Magazine - March 2015 - 42

mast was controlled to move from the upright conﬁguration
to the conﬁguration shown in Figure 10(a). The experiment
was repeated three times, and every time the laser spot fell
into the same starting location shown in Figure 10(a). The
details of the experiment procedure can be seen in the video
available in the supplemental material on IEEE Xplore.
Sample Acquisition and Transfer
To test the sample acquisition function, the robotic mast was
requested to retrieve pebbles lying on the surface of a sand
container, as shown in Figure 11. Sample identiﬁcation with
the onboard 3-D vision camera is beyond the research topic
of this article and will not be discussed in the experiments.
The X and Y coordinates of the samples were measured
manually and provided to the mast control software as known
parameters. However, the Z coordinate was not provided,
and the robotic mast needed to detect the ground-touching
event with the algorithm is presented in the "Control System
Design" section.
While receiving the sample acquisition command, the
robotic mast got ready for retrieval conﬁguration and

Joint Trajectories (°)

300

Turret

200

Shoulder
Elbow

100

Wrist
0

-100

0

50

100
Time (s)

150

stopped there for a moment so that the camera module could
take a picture of the scoop before sample retrieval, as shown
in Figure 11(a). It is within this step that the turret joint
turned the robotic mast to be in the same plane as the sample. Then, the robotic mast tuned the forearm and put the
upper arm down until it touched the ground, as shown in
Figure 11(b). Herein, the forearm was tuned according to the
inverse kinematics of the robotic mast to make sure that the
scoop follows the vertical line that passes the penetrating
point. After detecting that it was touching the ground, the
robotic mast moved into the sample retrieval conﬁguration,
as shown in Figure 11(c). The remained sample retrieval process consisted of penetrating and scoop withdrawing, as
shown in Figure 11(d) and (e), respectively. Finally, the
robotic mast moved back to the ready for retrieval
conﬁguration so that the camera module could take another
picture to compare with the one taken before the sample
retrieval, as shown in Figure 11(f). In addition to the snapshots shown in Figure 11, the sample acquisition process is
also demonstrated in the video available in the supplemental
material on IEEE Xplore.
The tests were conducted with pebbles and rocks of ellipse
and sphere shapes with outer diameters ranging from 2 to
5 cm, and the sand container was placed in three different
locations-the front-left, front, and front-right of the base of
the robotic mast. For each location, the container was placed
on the ground or on a 20-cm-high metal support. The experiments were repeated 24 times with four different samples,
three container orientations, and two sample heights. In each
case, the sample was randomly positioned within the central
area of the sand container to avoid potential collisions
between the scoop and the sand box. In these tests, the sample
retrieval operations were all successful. During these experiments, the link-side joint position and the control current for
each joint were recorded. Figure 12(a) and (b) presents the
trajectory and motor current for each joint while the robotic
mast retrieved the sample shown in Figure 11(c) from a

(a)
0.4

Motor Currents (A)

0.2
0

Turret
Shoulder

-0.2

-0.6

0

50

100
Time (s)

(c)

(d)

(e)

(f)

150

(b)
Figure 12. The (a) trajectories and (b) motor currents of the
joints during a sample retrieval experiment.

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(b)

Wrist

-0.4

42

(a)

Elbow

IEEE ROBOTICS & AUTOMATION MAGAZINE

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march 2015

Figure 13. Snapshots of sample transfer: (a)-(c) sample transfer
to the right sample bin and (d)-(f) sample transfer to the left
sample bin.
Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - March 2015
