Biomedical Engineering Reference
In-Depth Information
intelligent transfemoral prostheses (Dehghani, 2010). How this is achieved is described
in the following section.
Initial swing:
An active knee prosthesis contains a sensor that monitors knee flexion and
a motor that can drive the knee at up to 300
◦
/s. This flexion removes the foot from the
ground and reduces the requirement for hip hiking, vaulting, and circumduction. Foot lift
also improves safety on uneven terrain by reducing the likelihood of snagging.
Mid-swing:
As the user continues to flex the hip to execute leg advancement, the active
prosthesis increases knee flexion up to 60
◦
to improve ground clearance still further.
Following a short pause in mid-swing, the knee starts to extend at a speed comparable
with the initial swing flexion. This transition into swing extension is required to advance
the lower leg. During this phase the motor replaces the function of the quadriceps and
hamstring to accelerate and decelerate the lower leg to produce smooth motion. The
characteristics of the forward pendulum motion maintain the correct relationship between
the leg and the center of mass of the body, which improves energy efficiency. In addition,
users become confident that the knee will reach full extension, so they become less aware
of its function. In addition, an active swing phase goes some way in overcoming the
resistance of obstacles in the path of the foot and thus reduces the likelihood of tripping.
Terminal swing:
The active prosthetic replaces the deceleration function of a nonam-
putee's hamstring, ensuring that the extension is properly controlled. The knee is posi-
tioned with a flexion of about 5
◦
that is correct for both shock absorption and stability.
Initial contact and loading response:
As the foot touches the ground, flexion angles are
allowed to increase under software control to between 5
◦
and 15
◦
to absorb the shock.
This spring-like action contributes to the user's feeling of stability without requiring the
forceful extension of the prosthetic knee joint.
Standing up from a chair:
The active prosthesis applies the same amount of force as
does the good knee to maintain symmetry and to reduce strain on the sound limb.
Climbing stairs:
With a passive prosthesis the user can only lift the mechanism to the
level of the good leg. However, with an active prosthesis, the knee identifies that the user
is climbing stairs, and it drives the knee with the same force and speed as the sound leg to
move the user up to the next step.
10.9.5.1 Control Strategies
Control strategies for intelligent passive leg prostheses involve some form of state machine
that can adjust of the damping of the knee joint during the different gait phases for different
walking speeds. Early work, starting in the 1970s, was based on “echoing” the actions of
the sound leg to control the prosthetic one. Myoelectric-based control came into vogue in
the 1980s, and, of course, fuzzy logic has been tried.
Ideally, however, leg control should be based only on parameters that can be measured
on and around the prosthetic. The C-Leg detects angle, ankle force, and torque, which it
uses to calculate the required damping for flexion and extension during the swing phase
and in addition offers damping control during stance. The system is not user adaptive, and
a trained clinician programs the various damping levels until the user is comfortable. The
