1. PROBLEM STATEMENT
Variable Impedance Knee Mechanism
Fynn McPherson, Jacob Horowitz, Sara Malang,
Joseph Lee, Kevin Nossem, Jonathan Lopatin
Department of Biomedical Engineering, Case Western Reserve University, Cleveland, OH
INTEGRATED SOLUTION ARCHITECTURE AND ANALYSIS VERIFICATION RESULTS
CONCUSIONS & FUTURE WORK
REFERENCES
AKNOWLEDGMENT
Market Need:
In the United States, there are more than 1.5 million
people suffering from partial or complete paralysis of the
lower extremities. The number of paraplegic patients has
increased dramatically with the improvement of vehicle
safety and increased survival rate after an accident.
Functional neuromuscular stimulation (FNS) has been
developed in order to electrically stimulate the muscles
to assist paraplegic patients with regaining movement of
their limbs, and exo-skeletal bracing can provide support
and control. Variable impedance in the knee joint is
needed to provide control during stance change and
allow free-swinging motion during the swing phase. With
the combination of the functional neuromuscular
stimulation technology and the appropriate variable
impedance knee-mechanism, mobility of paraplegic
patients will be improved.
User Needs:
• Minimized device weight
• Low profile
• No louder than the ambient environment
• Physiologically comparable range-of-motion of the
knee joint
• Low passive resistance allowing joint to move freely in
passive mode
• Support joint moments exerted by a person of average
body weight including the safety factor of 1.5 at large
knee angles, such as stand-to-sit/sit-to-stand and stair
descent/ascent.
• Modulate resistive torque under loads exerted during
walking.
Iterations of the Prototypes
Department of
Veterans Affairs
Prototype 1
Benefits
• Able to sustain 100 lbs,
200 N-m (provide torque)
Areas of Improvement
• Reduce weight
• Reduce size
• Interface with Hybrid
Neural Prosthesis
• Prevent disengagement of
the rack from pinion
Prototype 2
Design Additions/Improvements
• Hydraulic system to modulate and provide torque
• Design protective covers to shield user from moving
parts
• Interface with Hybrid Neural Prosthesis
• Minimize size and weight
The next step for this design is to implement a feedback
control system into the hydraulics. The current
prototype is dependent on manual control of the
pressure valve, with no active control. The next
iteration of this device should incorporate a feedback
control system to properly operate the hydraulic
components. In addition to the hydraulic upgrade,
prototype 2 should be attached to the Hybrid Neural
Prosthesis and verified as a successfully integrated
component of the device.
0 5 10 15 20 25 30
-10
0
10
20
30
40
50
60
70
Torque(Nm)
Variable Stand-to-Sit: Torque vs. Time, 60 Nm
time (s)
0 5 10 15 20 25 30
-200
0
200
AngularVelocity(deg/s)
Testing completed on
Biodex machine which
applies precise levels of
torque to the end of the
distal upright, simulating
the force patterns that will
be experienced during
intended use of the device.
• Provide 60 Nm torque at an angular velocity of
120º/s, meets the requirements for the sit-to-
stand motion.
• Successfully tested locking capability at a constant
pressure with applied torque
FEA analysis was
completed on all
parts, and some
interfaces. The
minimum factor
of safety defining
the passing criteria
for a part was 2.
Mechanical Analysis:
Hydraulic Analysis:
Analysis included calculating the flow coefficient
based on angular velocity and passive torque
requirements. All purchased components had a factor
of safety of over 200%, and a pressure rating over
1000 psi.
Machining the Device:
The figure to the right
is a MasterCam file
for the bearing
brackets. CNC was
used because of the
complex geometry
inherent in the design
due to weight
minimizing design
choices.
SOLUTION: DETAILED DESIGN
Thanks to Jason Bradshaw, Jim Drake, Sarah Chang, Dr. Tyler, Mark
Nandor, Rudi Kobetic, Kevin Foglyano, Dr. Musa Audu
Ibid Curtis S. To, PhD;1* Rudi Kobetic, MS;1 Thomas C.
Bulea, MS;2 Musa L. Audu, PhD;2 John R. Schnellenberger,
MS;1 Gilles Pinault, MD;1 Ronald J. Triolo, PhD1–3. Stance
control knee mechanism for lower-limb support in hybrid
neuroprosthesis (p. 841).