ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

Fig. 2 Phase plane of acceleration vs velocity in the ML and AP axes.
4. ACTUATION DESIGN BASED ON PELVIC DYNAMICS
The patterns in the phase planes described in the previous section have direct
implications in the design of actuation regarding length of linear travel and lower power
limits of the actuators and the controller response, for instance if the BWSS is set to
follow the patient´s pelvic movements without applying any force, phase planes of
acceleration versus velocity, have to be the same as walking without any support. On
the other side, acceleration-velocity patterns are related to the power characteristics of
the actuators, especially for the vertical actuator, which has to deliver enough power to
compensate gravitational and inertial body loads.
4.1 Range of linear travel
To establish the range of motion for the linear translations along V, AP, and ML axes;
we applied harmonic models of pelvic displacements [5] to our anthropometric
parameters [6]. Rotational range of motion corresponding speed and acceleration
maximum values were defined according to reported data in [7], [8].
Vertical displacement Anterior-posterior displacement Medial lateral displacement
Dz= l(1-cosq) (1) DAP = gDV
2
8pSl fs (2)
DV = 6.3cm DAP = 2.5cm
DML =
0.5z0 2 h
1+ 4p fs (3)
g
DML = 4.7cm
Where l is the leg length and q is the angle between the vertical axis and the leg; g is
gravity; is the frequency of stride; S l is the length of stride; z 0 is the step width; his the
height of the body center of mass. These travel distances were estimated by assuming
the anthropometric parameters in []. However, to guarantee that patients with irregular
patterns of pelvic movement could be included in rehabilitation protocols with our
BWSS, each actuator was designed with linear travel distance of 0.1 m. To estimate
velocity and acceleration peak values, we normalized the kinematic signals and scaled
them according to peak values reported in [7], [8].
4.2 Maximum acceleration-velocity patterns
Maximum acceleration values for each velocity coordinate were computed by using first
return maps for phase planes at V, AP, and ML axes. The purpose of these curves was
to estimate the minimum power requirements for linear actuators selection at each axis.
The actuator in the V axis has more demanding requirements than AP and ML, for the