IEEE Robotics & Automation Magazine - September 2014 - 139

infinite stiffness characteristic working in a zero-stiffness environment, while the pure force controller exhibits a zero-stiffness characteristic working in a stiff environment.
For domestic robots that often operate in unstructured
environments with humans, pure position control is incomplete because, if there is contact with an obstacle, the robot is
not expected to go through the obstacle. Similarly, a pure
force control is also inadequate as contactless tasks and
motions are difficult to implement. An alternative control
technique essential in domestic robotics is the interaction
control scheme, which deals with regulating the dynamic
behavior of the manipulator as it is interacting with the environment [90]. The core idea behind interaction control is that
manipulation is done through energy exchange, and, during
the energetic interaction, the robot and the environment
influence each other in a bidirectional signal exchange. Thus,
by adjusting the dynamics of the robot, how it interacts with
the environment during operation can be controlled.
One of the most widely used interaction control schemes is
impedance control presented in [91]. Most of the operating
environments of the robot, such as mass to be moved or rigid
obstacles in work space, can be described as admittances that
accept force inputs and output velocity during interaction.
Hence, for possible interactions in such an environment, the
manipulator should exhibit an impedance characteristic, which
can be regulated via impedance control. Consider a simplified
1-DOF robotic manipulator modeled as a mass m at position
x, which is to be moved to a desired position x d . A simple
physical controller that can achieve this is a spring connected
between the desired virtual point and the mass (Figure 4). To
avoid continuous oscillation of the resulting mass-spring system and stabilize at the equilibrium point, a damper should be
added to the system. The resulting controller is an impedance
controller that can shape the dynamic behavior of the system.
The controller resembles a conventional proportionalderivative controller and introduces a desirable compliance to
the system. A number of impedance controller designs have
addressed issues such as robustness [92], [93], adding adaptive control techniques [94], [95], extension with a learning
approach [96], dynamics of a flexible robot [78], [97], and
dexterous manipulation [77], [98], [99].
Another crucial requirement in controller design for
domestic robots is ensuring asymptotic stability even in the
presence of apparent uncertainties about the properties of the
operating environment [77]. To address this issue, several
authors have applied passivity theory to design controllers
commonly known as passivity-based controllers [78], [100],
[101]. Passive systems are a class of dynamic systems whose
total energy is less than or equal to the sum of its initial energy
and any external energy supplied to it during interaction.
Hence, passivity-based controller design ensures a bounded
energy content, and the system achieves equilibrium at its
minimum energy state. Any energetic interconnection of two
passive systems will not affect the passivity of the combined
system. As a result, an interconnection of a passivity-based
controller, a passive manipulator, and a typical unstructured

(a)

(b)

Figure 3. (a) The DLR lightweight robot arm and hand [77] and
(b) the Stanford Safety Robot [42].

operating environment that is often passive results in an overall passive system whose Lyapunov stability is always guaranteed. Passive controller designs for domestic robot manipulators have often been addressed together with interaction
control in a unified scheme to achieve a compliant, asymptotically stable, and robust manipulator [78], [102], [103].
Safety-aware control schemes that incorporate safety metrics in a controller design are also proposed in the literature.
Focusing on collision risks to a human user, these controllers
utilize a given safety metric to detect possible unsafe situations
and use the controller to ensure that the acceptable safety levels
defined in the metrics are achieved to avoid possible injuries.
Using impact potential as a safety metric, [46] proposes an
impact potential controller for a multiple-DOF manipulator.
In this hierarchical controller design approach, the resulting
safety status of a high-level motion controller torque output is
evaluated according to the metric by a protective layer controller and clipped to an acceptable level in case of a possible
unsafe condition. Using energy levels that cause failure of the
cranial and spinal bones as a safety criterion, [104] proposes an
energy regulation control that modifies the desired trajectory
of the controller to limit the overall energy of a manipulator.
After analyzing soft-tissue injuries and their relation with
robot parameters, [59] proposes a velocity shaping scheme,
which ensures that possible sharp contact with a multipleDOF rigid robot will not result in unacceptable injury to a
human user.

kc
m

Interaction

b
Plant

Reference + Controller

Figure 4. Impedance-controlled system.

september 2014

IEEE ROBOTICS & AUTOMATION MAGAZINE

139

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2014