A New Actuation Approach for Human
Friendly Robot Design
Zinn, M.
2
, Khatib, O.
1
, Roth, B.
2
, and Salisbury, J.K.
1
1
Robotics Laboratory, Department of Computer Science
2
Design Division, Department of Mechanical Engineering
Stanford University, Stanford, California 94305
Abstract. In recent years, many successful robotic manipulator designs have been
introduced. However, there remains the challenge of designing a manipulator that
possesses the inherent safety characteristics necessary for human-friendly robotics.
In this paper, we present a new actuation approach that has the requisite charac-
teristics for inherent safety while maintaining the performance expected of modern
designs. By drastically reducing the effective impedance of the manipulator we show
that uncontrolled impact loads can be reduced by an order of magnitude or more,
as compared to conventional manipulator designs. A discussion of the actuation
topology is presented along with analytical and experimental results validating the
efficacy of our approach.
1 Introduction
One of the major issues in introducing robots into human environments is
safety. Without a high degree of confidence in their inherent safety, robotic
manipulators will never be accepted for use in close proximity to humans.
However, safety alone will not guarantee the success of human friendly
robotics. These robotic manipulators must also possess a level of performance
that is expected of modern robotic manipulators.
Inherent safety is achieved through the use of multiple strategies, involving
all aspects of manipulator design including the mechanical, electrical, and
software architectures. However, the biggest danger present when working in
close proximity with robotic manipulators is the potential for large impact
loads which can result in serious injury or death. To evaluate the potential for
serious injury due to impact we can use the HIC index, an empirical formula
developed by the automotive industry to correlate head acceleration to injury
severity. A simple two degree of freedom mass-spring model can be used to
evaluate predicted head accelerations, a(t) expressed in g’s , and the resulting
HIC index(Equation 1).
HIC
∆T
= (t
2
− t
1
)
·
1
(t
2
− t
1
)
Z
t
2
t
1
a(t)dt
¸
2.5
∆T = t
2
− t
1
= 15ms (1)
2 Zinn, M. et al
For the PUMA 560, an impact at 1 m/s velocity produces a maximum HIC
index more than enough to cause injury
1
(see Figure 1)
100
200
300
400
500
600
5 10 15 20 25 30 35 40
5
10
15
20
25
Interface Stiffness [kN/m]
Arm Effective Inertia [Kg]
Hard Rubber
50% Injury
70% Injury
10% Injury
Soft Rubber
Plastics
PUMA 560
20% Injury
HIC Index
Fig. 1. Head injury criteria as a function of effective inertia and interface stiffness
The addition of a compliant covering can reduce impact loading by an
order of magnitude or more. However, the amount of compliant material
required to reduce impact loads to a safe level can be substantial
2
. Clearly,
this does not address the root cause of high impact loads - namely the large
effective inertia of most modern robotic arms.
Previous attempts to build lightweight, low inertia manipulators have
been met with limited success. Due to the flexibility of cable transmissions,
control bandwidths are limited. The non-collocated nature of the remotely lo-
cated actuators and flexible transmission limits the tasks that can be achieved
to those that require torques whose frequencies lie below the fundamental
mode, which can be as low as 5 Hz. Other approaches [1], attempt to solve
these problems with the use of high performance cable transmissions and
light, stiff structural materials. This design approach prevents interaction
with the robot control system by increasing the structural modes to frequen-
cies above the control bandwidth. The need for high performance compo-
nents and materials means that the cost of such systems is high. In addition,
the stiff connection between the manipulator link and the actuator couples
1
The HIC index is correlated with the Maximum Abbreviated Injury Scale (MAIS)
to provide a mapping from the calculated HIC values to the likelihood of an
occurrence of a specific injury severity level. In Figure 1, HIC values and the
corresponding likelihood of a concussive injury (or greater) are shown
2
For the PUMA robot, the thickness of a compliant cover required is more than
5 inches, assuming an impact velocity of 1 m/s and an allowable maximum HIC
index of 100
A New Actuation Approach for Human Friendly Robot Design 3
their inertias. The increase in the effective link inertia can be substantial
considering the N
2
amplification of the actuators inertia through the trans-
mission. The most promising manipulator designs to date have utilized the
joint torque control approach [2]. Perhaps the most successful of these has
been the new DLR lightweight arm design [3]. The implementation of joint
torque control allows for near zero low frequency impedance, which gives the
DLR arm excellent force control characteristics. However, above the control
bandwidth, joint torque control is ineffective at reducing the impedance of the
manipulator. The open loop characteristics of the manipulator and reflected
actuator inertia dominate. Thus, the magnitude of impact loads, which are
determined by the high frequency impedance of the contacting surfaces, are
not attenuated.
2 New Actuation Approach: Distributed Elastically
Coupled Macro Mini Actuation
To address this challenge, we propose a new approach that seeks to relo-
cate the major source of actuation effort from the joint to the base of the
manipulator. This can substantially reduce the reflected inertia of the over-
all manipulator. Performance is maintained with small actuators collocated
with the joints. Our approach divides the torque generation into low and high
frequency components and distributes these components to the arm location
where they are most effective. We call this approach Distributed Elastically
Coupled Macro Mini Actuation (DECMMA).
The proposed approach is analogous to the design of robotic manipulators
for use in zero gravity. Under such conditions, gravity induced torques do
not exist. Joint actuators provide torques related only to the task, such as
trajectory tracking and disturbance rejection, both of which are primarily
medium to high frequency in content. We achieve the zero gravity analogy
by compensating for low frequency torques using the low frequency actuators
located at the base of the manipulator. With the effects of gravity and low
frequency torques compensated, joint torque requirements become similar to
those encountered by a zero gravity robotic manipulator.
The efficacy of this approach can be seen clearly when one considers that
most manipulation tasks involve position or force control which are domi-
nated by low frequency trajectory tracking or DC load torques. High fre-
quency torques are almost exclusively used for disturbance rejection. Even
haptic device torque profiles, which might require rapid changes approximat-
ing a square wave input, have a torque magnitude versus frequency curve that
falls off with increasing frequency by 1/ω (see Figure 2) . This torque versus
frequency profile is ideally fit using a large output, low frequency actuator
coupled with a high frequency servomotor.
In contrast to early efforts at coupled actuation [4], the low frequency
torque actuator is located remotely from the actuated joint. This is partic-
4 Zinn, M. et al
0.0
0.2
0.4
1 10 20 30 40
Frequency [Hz]
ω
1
Magnitude ~
0.0 0.5 1.0 1.5
Time [sec]
-1.0
0.0
1.0
Normalized
Input Torque
(1.2)
Normalized Input
Torque Magnitude
Fig. 2. Torque vs frequency: 1 Hz square wave
ularly advantageous as the low frequency components of most manipulation
tasks are considerably larger in magnitude than the high frequency com-
ponents and consequently require a relatively large actuator. Locating this
large actuator at the base significantly reduces the weight and inertia of the
manipulator
The high frequency actuators are located at the manipulator joints and
connected through a stiff, low friction transmission, providing the high fre-
quency torque components that the low frequency base actuators cannot.
The high frequency torque actuator must be connected to the joint inertia
through a connection, which produces a high primary mode vibration fre-
quency. By locating the actuator at the joint and by using a low inertia
servomotor we can achieve this high bandwidth connection with a minimum
amount of weight and complexity.
2.1 DECMMA Actuation
In order for the DECMMA approach to work properly, both the high and
low frequency actuators must have zero or near zero impedance. This is due
to the fact that during power transfer the actuator torques will add non-
destructively only if their respective impedance is zero. In particular, each
actuator must not have significant impedance within the frequency range
of the opposing actuator. Only if this condition is true will the DECMMA
concept work.
For the high frequency actuation, very low impedance is achieved by us-
ing a low inertia servo motor connected to the manipulator through a low
friction, low reduction cable transmission. The reduced torque output that
results from the use of a low reduction transmission is balanced against the
reduced reflected inertia and motor friction and represents one of the design
trades of the DECMMA concept. Unfortunately, this approach can not be ap-
plied to the low frequency base actuation. The large torques required to react
gravity loads make it impossible to achieve low reflected impedance without
employing very high performance actuators, most of whose power and perfor-
mance would be under utilized. To achieve the near zero impedance required
A New Actuation Approach for Human Friendly Robot Design 5
we use a new type of actuator topology, referred to as Series Elastic Actu-
ation (SEA) [5] that was developed specifically to address the problems of
high impedance actuators. The SEA actuator topology maintains the high
power and torque density of a high ratio geared DC torque motors while
also providing the very low impedance required for the DECMMA approach.
The penalty paid in implementing the SEA approach is a significant reduc-
tion in the high frequency torque capability of the actuator. However, the
DECMMA approach does not require that the base actuator be capable of
supplying high frequency torques and thus this limitation is an acceptable
trade off. While details of SEA are contained in [5,6] a brief overview of the
concept and its implications for the DECMMA approach is given below.
2.2 Low Frequency Actuation: Series Elastic Actuation
The SEA approach seeks to mitigate the limitations of conventional gearhead
actuators, namely the high impedance associated with the reflected inertia
and friction, by placing an elastic element between the output of the actuator
and the robotic link. The elastic element limits the high frequency impedance
of the actuator to the stiffness of the elastic coupling. To limit the low fre-
quency impedance, and thus transform the actuator into an approximate
pure torque source, a linear feedback system is implemented to regulate the
output torque of the actuator-spring system. (See Figure 3).
I
motor
N
motor
∆
coupling
-
Kcoupling
τ
desired
+
D(s)
Kcoupling
I
motor
Base Low Frequency (Series Elastic) Actuator
Joint High
Frequency Actuator
I
link
Fig. 3. Distributed elastically coupled macro mini actuation topology
The main advantage of the SEA topology is that it provides low out-
put impedance across the frequency spectrum. As shown in [5,6], the SEA
topology reduces the output impedance of the SEA actuator in proportion
with the stiffness of the elastic coupling (Equation 2). At frequencies below
the closed loop bandwidth of the SEA controller, the output impedance is
reduced as a function of the control gains. Impedance reduction of 10x-100x
is common and is only limited by the maximum obtainable bandwidth. At
