IEEE Robotics & Automation Magazine - December 2015 - 81

transfemoral. The main way to regenerate a bipedal gait for a
walking amputee is by using a lower-extremity prosthesis [2].
Most existing commercial transtibial prostheses are energetically passive without actuation. Although these prostheses
have improved the quality of amputees' lives, they have several
deficiencies. Amputees using these passive prostheses exhibit
asymmetrical gait patterns [3]. In addition, without providing
net positive work, passive prostheses require amputees to consume 20-30% more metabolic energy to walk at the same
speed as able-bodied individuals [4].
To deal with these limitations, increasing efforts have
been made to improve active transtibial prostheses during
the last few decades, e.g., [5]-[11]. According to the joint actuation types, existing active transtibial prostheses can be
roughly divided into two main categories. The first is the
pneumatic actuator-based transtibial prosthesis. In 1998,
Klute et al. [6] built a transtibial prosthesis powered by an artificial pneumatic muscle (the Mckibben actuator). Versluys
et al. [7] designed another prosthesis powered by pneumatic
cylinders. Although these pneumatic actuators are capable of
providing enough force to propel amputees, their control difficulties, low position accuracy, and large size, due to the
high-pressure air pump have restrained the development of
pneumatic prostheses.
The second category is the motor-driven prosthesis. Au et
al. [5] developed a powered ankle-foot prosthesis that can reduce the amputee's metabolic consumption by 14% on average.
The prosthesis uses a combination of a spring and a motordriven series elastic actuator (a 150-W brushless Maxon dc
motor). A unidirectional spring structure placed parallel to the
ankle joint makes the ankle stiffer during dorsiflexion than
during plantar flexion. Another important example is the
SPARKy prosthesis which uses a motor-driven robotic tendon
actuator (150-W dc motor) [8]. Commercial versions of the
two prostheses have been released by BiOM [12] and Spring
Active [13], respectively. Recently, Cherelle et al. [10] proposed
a new concept for an energy efficient transtibial prosthesis,
which uses a low-power actuator that stores energy in the
springs during the early and middle stance phase and releases
the energy during the push-off phase.
Though these motor-spring mechanism-based transtibial
prostheses do improve the energy efficiency of amputees, the
prostheses are limited by their weight. Focusing on providing
sufficient positive work and a longer operating duration, these
prostheses are usually composed of a parallel spring, a highpower motor, and a large-capacity battery. Consequently, the
weight of an active prosthesis can be more than 2 kg [10]-[12].
A heavy prosthesis tends to increase the knee extension load
and cause larger interaction force between the adaptor and the
residual limb. Furthermore, in addition to the inability to provide assistive torque, the inability of a passive prosthesis to
adapt to terrain variations leads to gait asymmetry and walking instability [14], [15]. Ossur's Proprio Foot [16] is a typical
commercial attempt at terrain adaptation. This is done without
providing assistive torque beyond the energy that is stored
during dorsiflexion and then returned prior to toe-off. But it

can only adjust the ankle angle to prepare for the next stance
phase during swing without adjusting the ankle impedance
during the stance phase.
Focusing on terrain adaptation instead of providing high
joint torque, we present the design and control of a lightweight robotic transtibial prosthesis. A novel damping control
strategy based on the motor-winding short is proposed to enable the prosthesis to manipulate the ankle impedance during
stance with little power consumption. Three amputee subjects
were recruited to perform walking experiments on different
terrains, including level ground (LG), stairs (ascending and descending), and ramps (ascending and descending). Experimental results with the robotic prosthesis show improved
ankle angle trajectories, gait symmetry, and walking stability of
amputee subjects.
Robotic Transtibial Prosthesis Prototype
Instead of using high-power motors to obtain large joint
torque, we designed a lightweight motor-driven robotic transtibial prosthesis for terrain adaptation, the Peking University
Robotic Transtibial Prosthesis (PKU-RoboTPro).
The prototype of the proposed prosthesis is shown in
Figure 1(a). The current version is an integrated one that uses
all the modules in the transtibial prosthesis including mechanical structure, control
circuits, sensors, and battery. The model of the
A damping control strategy
ankle joint can be simplified as a three-bar mechais proposed to enable the
nism, which comprises
bars a, b, and c, and
prosthesis to manipulate
three hinges, A, B, and
C, as shown in Figure
the ankle impedance
1(b). To visualize the
model, a can be seen as
during stance with little
the foot, b as the shank,
and C as the ankle joint.
power consumption.
Made up of a motor-driven ball screw transmission, c is a customized bar. The screw pitch is 2 mm. The
motor system uses a 50-W dc brushless motor from Maxon
(EC 45 50 W), equipped with a 5.8:1 reduction gearbox. The
angle range of the ankle joint is from 25c dorsiflexion to 25c
plantar flexion. The total weight of the proposed prosthesis
(excluding the rechargeable Li-ion battery) is 1.3 kg.
Three kinds of sensors are installed on the prosthesis,
including one load cell, one angle sensor, and two inertial
measurement units (IMUs), as shown in Figure 1(a). The
single-axis load cell [Model LBS (Load Button Small)
Miniature Compression Load Button] has a measurement
range of 0-250 lbf and is used to detect the interaction
force between the residual limb of the amputee and the
prosthesis. The absolute angle sensor (Angtron-RE-25) is
used to measure the ankle angle with a 0-360c range and
12-b resolution. The two IMUs are used to measure the
inclination angle and other inertial information such as
DECEMBER 2015

IEEE ROBOTICS & AUTOMATION MAGAZINE

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