IEEE Robotics & Automation Magazine - March 2016 - 26

LL
Robot
Pushed Down
0

0.1
0.075
0.05
0.025
0

fFoot, i /(migm)

fHand, i /(migm)

Dxz (m)

0.02
0.01
0
- 0.01
- 0.02

0.6
0.55
0.5
0.45
0.4

8
10
Time t (s)
(a)

Right

Disturbance
Estimators

Robot
Pulled Up
12

Left

Multibody
Dynamics

es.Dt

Lower Limit

Vest
Force
Prop

(a)
Vest

8
10
Time t (s)
(b)

Right

Left

Hip Torque

Knee Torque
0

8
10
Time t (s)
(c)

Design of the DEC Controller
The DEC controller describes human balancing of biped
stance during external disturbances, being used here in a
simpliﬁed form. Originally, it focused on sagittal plane balancing around the ankle joints, which allows simplifying the
human biomechanics to a single-inverted pendulum (SIP)
[16], [20]. The basic structure of the DEC concept is shown in
Figure 4(a). The three loops are 1) passive loop, 2) SL joint
angle proprioceptive feedback loop, and 3) LL disturbance
compensation loop. Combined, the passive and SL loops form
a servo mechanism. With appropriately adjusted parameters
and in the absence of external disturbances, it ensures that the
actual joint position matches the desired joint position given
by a set point signal. The SL and LL loops use a neural PD
controller [box neural controller (NC)] to provide the motor
command for producing active joint torque. In this article, the
neural time delays of SL and LL are represented in onelumped time delay (box e sDt) . The LL loop commands the
servo to compensate external disturbances estimated on the
basis of sensory signals. Four types of external disturbances
can be distinguished: 1) field forces, such as gravity, 2) contact
forces, such as a push against the body, 3) support surface tilt,
and 4) support surface translational acceleration. The DEC
controller allows superimposing a disturbance on other disturbances and on voluntary movements.
IEEE ROBOTICS & AUTOMATION MAGAZINE

march 2016

Hip Prop

Knee Prop

negative contact forces at the hands by a lower limit threshold that avoids losing the contact.

Passive

Figure 3. The exploitation of the redundancy in a multicontact
scenario where the contact forces are normalized with the weight
of the robot.

Set Point

Ankle
Torque

Ankle Prop

Hip Module
Sensory
Signals
Knee Module
Sensory
Signals
Ankle Module

(b)
Figure 4. The DEC controller with (a) a schematic model of the
DEC concept and (b) a modular control architecture used in the
experiments.

Human data on ankle joint balancing have successfully
been reproduced in computer simulations using the DEC
concept and through its implementation in a special purpose
SIP robot, called Posturob I [16], [17]. The concept was further developed as a modular control system, where the complexity of the control structure scales linearly with the
number of degrees of freedom (DoF) [21]. The control structure originally described for the ankle joint control is applied
to each of the controlled joints in the form of DEC control
modules, where each module controls 1 DoF. The
reconstruction of the external disturbances affecting a joint is
performed through an exchange of sensory signals between
adjacent modules. The information coming from the vestibular system is down channeled through the modules and
fused with the corresponding proprioceptive signals. Thus,
the control reconstructs, for example, the orientation of the
foot to compensate support surface tilts, providing robustness with respect to uneven and compliant ground. The sensory interaction between modules creates an internal model
of the robot's kinematics. Depending on the behavioral task,
the module can be set to control the COM position of all the
links above the controlled link, the joint angle, or the orientation in space of the supported link. The modular concept
was tested in a DEC controller that included ankle and hip

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - March 2016