IEEE Robotics & Automation Magazine - December 2014 - 29

Force-Based Position Control
An alternative to force control that requires a much lower
actuator bandwidth is to use position control of the Bowden
cable. By driving a Bowden cable through a specified position
trajectory, consistent forces are created in the suit assuming
repeatable force-displacement characteristics of the suit and
human (shown in Figure 4). With the suit-human force-displacement model described in the "Structured Functional
Textiles" section, we can generate the correct cable position
profile and play it back as a function of the percentage
through the gait cycle. When no force should be present in
the suit, the actuators are commanded to stay at a fixed initial
position so that the suit is slack. We used this scheme in earlier work, which resulted in consistent force profiles delivered
to the wearer [50].
As discussed in the "Structured Functional Textiles" section, the multiarticular exosuit architecture becomes stretched
when the body is in the correct pose for forces to be applied
and absorbs energy and returns it to the body even when the
actuators are in the initial offset position. The resulting passively induced force as a function of the percentage through
the gait cycle is shown in Figure 7(b) as the black line.
From a control standpoint, this passively induced force is
extremely useful. If the actuators are held at a fixed initial position, force in the exosuit means that the wearer is beginning to

Gait %

Gait %
Calculation

Suit Tension
(a)
200

Passive Active

150

200
Passive
Active
Position

150

100

0
0%

20%

40%
60%
Gait Cycle
(b)

80%

Position (mm)

Force Control
While pneumatic actuation was sufficient for our proof-ofconcept work, achieving accurate position or force control
with this type of actuation is challenging. Thus, to enable better control over the applied force profile, we switched to using
electromechanical actuation and Bowden cable transmissions
for our subsequent systems as were described previously.
To transmit biologically realistic torques to the human
joints, one option is to use a real-time force controller. We
have implemented this with our nonportable actuation system in Figure 5 using the suit tension load cell for feedback
[36]. Implementing a force controller requires an actuator
with a relatively high force bandwidth due to the compliance
of the soft exosuits and Bowden cable transmission. We have
characterized the force bandwidth of this system to be 20 Hz
when delivering a 200-N peak-to-peak force with the distal
ends of both the Bowden inner cable and sheath clamped to a
rigid plate. Through human subject experiments, we have
demonstrated that our real-time-controlled system can accurately deliver high forces to the user (up to 250 N) through
soft exosuits when walking at 1.25 m/s.

Position Control
Assistive
Trajectory
Generator

Force (N)

these actuators resulted in a smooth first-order time response
where the actuators increased to 90% of the desired maximum value over approximately 200 ms. In a pilot human
walking study, we varied the actuator turn-on time as a function of the gait cycle to determine when it would be most
beneficial. We found that an actuator turn-on time of 30% in
the gait cycle was metabolically optimal and corresponded to
the force profile extending from 35% to 62% in the gait cycle.

0
100%

Figure 7. An exosuit-human interface based on the integrated
sensor measurements. (a) The force-based position control
architecture. (b) The generated force profile with the suit in passive
mode (black line), commanded position profile to assist ankle
plantarflexion (blue line), and the resulting force profile (red line)
(photos courtesy of Harvard Biodesign Laboratory).

transition between legs, and so additional assistance would be
beneficial. As such, we monitor this passive force with the suit
tension load cell and use the measurements for control.
Figure 7(b) also shows the result of actuating the suit, with
the actuator position in blue and the resulting force in the suit
in red. When the actuator shortens the effective suit length,
the force in the suit increases substantially, imparting an extra
boost of power to the user at the correct time.
Due to their highly compliant nature, soft exosuits can
deform overtime or move relative to the body if worn for
extended periods of time while walking. In addition, changes
in gait may modify the resulting force profile and amplitude
with this position-control scheme. This presents a challenge
when commanding the suits in position control since the assistive profiles resulting from the position controller will vary
overtime for different human motions and suit alignments. To
improve upon this, we developed a controller that monitors
the key force profile features, including the peak force and the
passively generated force before actuation, and automatically
adjusts the assistive position profile to keep the desired force
consistent overtime or between users. If during a gait cycle, the
resulting peak force or the passively generated force is different
than desired, the initial offset and the maximum amplitude of
the position profile are increased or decreased so that the
forces are corrected for future steps. The maximum correction
December 2014

IEEE ROBOTICS & AUTOMATION MAGAZINE

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