IEEE Robotics & Automation Magazine - March 2015 - 39

PCB with two motor drives for the elbow and wrist joints.
Box 2 is seated inside the chassis, which consists of the mast
control computer and a PCB with drives for the turret and
shoulder joints. The cables to Box 2 include the main power
input (24 and 5 V), the communication cable connecting the
mast controller with the executive computer, the power and
CAN cables that are forwarded to Box 1, and the wiring for the
motors and sensors of the turret and shoulder joints. Regarding Box 1, apart from the aforementioned power and CAN
cables from Box 2, the wiring mainly consists of those for the
motors and sensors of the elbow and wrist joints.
Control System Design
To realize the integrated arm/mast functions, the Kapvik
robotic mast is required to be able to 1) switch between
four different conﬁgurations as shown in Figure 1, 2) lock
and unlock itself, 3) retrieve samples from unknown environments, and 4) transfer them to the sample bins. Control
of the Kapvik robotic mast is a challenging task. First, large
internal forces could be generated while pushing the guiding pin into the groove and taking it out from the locking
mechanism. If not controlled properly, the generated internal forces can expedite wearing out and even damage the
robotic mast. Second, the navigation camera cannot provide accurate three-dimensional (3-D) coordinates of the
sample in the unstructured environment where a rock lies
on or is half-buried in the sand. As a result, the robotic
mast has to be able to detect when the scoop touches the
ground during sample retrieval operations. To address the
aforementioned issues, a multiple-mode control algorithm
was developed that allows the robotic mast to work in
active mode with position control or in passive mode with
friction compensation.
Position Control in Active Mode
To satisfy the requirements of light weight, large payload
capacity, and low power consumption, the joints were
designed with high gear ratios, as shown in Table 1. The gear

box and harmonic drive introduce backlash to the developed
joint, which invalidates the conventional position control
algorithms with only motor-side encoders. To address this
issue, the joints are
equipped with both a
Light weight and low power
motor-side incremental
encoder and a link-side
consumption are two
absolute position sensor
(absolute encoder or Hall
critical requirements for
effect sensor), as detailed
in the "Electronic System
planetary rovers.
Design" section. The control system diagram for
point-to-point position
control is given in Figure 9. Inputs to the control system
include the desired position and velocity of the joint, which
may be generated by a path planner or calculated by inverse
kinematics and differential kinematics. The output of the
control system is the current command, which can be calculated as follows:
I c (t) = K C $ $ K PM $ eo M (t) + K IM $

#0 t eo M (x) dx .,

(1)

where K C is a conversion factor from the D/A scale to current in amperes, and K PM and K IM are proportional and integral gains for the inner velocity control loop. The velocity
error for the inner velocity loop eo M can be detailed by
eo M (t) = K PL $ 6q d (t) - q L (t)@ + c $ qo d (t) - qo M (t),

(2)

where q d and qo d are the desired joint position and velocity
from the supervisory control computer; K PL is the proportional gain for the outer position loop; q L is the measurement of the link-side absolute position sensor; qo M represents
estimated motor side velocity, which is derived from readings of the motor side incremental encoder; and c denotes
the gear ratio.

Electronic Box 1
El
Elbow Joint
El
(a)

(b)
Shoulder Joint
Sh

Wrist Jointt
Electronic Box
ox 2

(c)

(d)

Figure 7. The PCBs design: (a) the back side of turret/shoulder
PCB, (b) the back side of elbow/wrist PCB, (c) the front side, and
(d) the sensor PCB.

Tu
Turret Joint

Figure 8. The wiring for the Kapvik robotic mast.
march 2015

IEEE ROBOTICS & AUTOMATION MAGAZINE

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - March 2015