Kinematical analysis of an upper limb prosthesis.
Menyhardt, Karoly ; Nagy, Ramona ; Luca, Gheorghe 等
1. INTRODUCTION
The main purpose of this study is to compare the kinematical
parameters from the mechanical model (figure 1a), the 3D simulation
(SolidWorks/CosmosMotion) and the physical data gathered, with a
professional APAS measurement system, from a prosthesis designed (figure
1b) and developed at CMPICSU Research Center from
"Politehnica" University of Timisoara by the first author.
The kinematic study was initially done in order to compare the
motion of the human upper limb and the prosthesis. Further studies of
the mechanical model presented in the current paper helped us understand
the motion of the prosthesis.
This study was developed from the necessity of quantifying the
basic movements of the prosthesis in order to make it run smoother and
more natural, like the human upper limb (Pravin, 2006).
Throughout the literature there are few papers that present the
mechanical model and compare it with a physical prosthesis, realized by
the authors (Duraisamy et al., 2006), (Jadhav & Krovi, 2003). In
most cases, the kinematical analysis is made on an idealized anatomical
model where the muscles are replaced by given forces. In the current
study the analysis is done on a real mechanical model that is analogue
for the developed prosthesis.
A concrete and in detail analysis will permit to specify the
conditions for the working regime (Lloyd et al., 2000). In addition,
graphics that evaluate the phenomena are traced putting in evidence the
validity of the model.
[FIGURE 1 OMITTED]
2. MECHANICAL APPROACH
The stability and motion of the prosthesis is influenced by lots of
factors, sometimes even the least significant factor could affect the
outcome of the task (Raikova, 1992), (Veeraraghavan et al., 2004).
Because of the difficulties in calculus, the prosthesis was modeled
as a two bar mechanism (figure 1a) where OA (the arm) executes a
rotation around joint O, due to change in the prosthesis' center of
gravity. The AB segment (forearm with hand) executes a planar motion,
driven by the connected arm and elbow motor. The angular velocity
[[??].sub.2] was considered constant throughout its range of motion (the
prosthesis is able to do a 150 degrees flexion in the elbow).
Considering the position for the center of gravity being:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
[[theta].sub.2] was determined as a function of [[theta].sub.2] (2)
Trough various calculus and restraint conditions, e.g. the center
of gravity remains ahvays on ms Oy axis, results that its velocity is
mat from equation (3) and its curve Fig.2.
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)
The acceleration for the center of gravity for the prosthesis'
mechanical model is shown in equation (4) and figure Fig.3.
prosthesis' mechanical model.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
All the other linear and angular kinematical parameters were
determined for each point of the mechanism. These parameters helped in
establishing the trajectory and the workspace for the prosthesis in
order to assess the possible movements. The kinematical analysis of the
movement refers at the quantitative description of body segments without
taking into account the forces that generate the movement.
3. RESULTS OF THE ANALYSES
In this paragraph are presented the results of the kinematical
analysis from different points of view/interest. In all the studied
cases the time needed to perform the prosthesis movement was slightly
longer, the artificial "muscles" (DC steppers) being slower
than the human muscles. The trajectories and exercises were the same in
all cases, generated by computer and by the upper limb prosthesis.
For the time interval [10, 27] seconds the curve from the
theoretical model almost overlaps with the curve from the physical
model. The differences in the amplitude are due to the simplification of
the model (uneven distribution of the masses in the physical model) and
the measurement uncertainty. The curves are presented in figures Fig.4
and Fig.5.
The resulted data from the analysis for a movement cycle of the
upper limb/prosthesis or for a succession of cycles can be used to
analyze the movement pattern (Karamanidis et al., 2003). By evaluating
this movement a joint, motor or coordination deficit can be outlined or
quantified and efficient strategies of compensation can be made based on
these values (Murray & Johnston, 1998). By determining the kinematic
parameters (linear and angular acceleration), is possible to know the
inertial forces for further dynamical studies.
However, long-term cyclic testing is necessary to ensure
reliability and a valid assessment of the efficacy of the prosthesis can
only occur by clinical trial.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
4. CONCLUSION
This performed mobility study is useful in outlining the
differences between the movement of a human arm and a prosthesis. The
gathered results have been filtered out and processed in order to
correct and enhance the prosthesis's unnatural movement. Every
movement scheme has different solutions based on the forces that appear
and the prosthesis' orientation and position. Determining the
orientation and position in real time without the use of external
equipment brings forward new obstacles that need to be surmounted.
The results validate the physical prosthesis and its behavior in
normal load conditions.
Comparing the results from the mathematical approach, mechanical
simulation and the measured data from prosthesis and human upper limb,
could help us improve the control of the prosthesis' motor by
feeding adequate input parameters to the microcontrollers.
The next steps in the research will include sensorial feedback and
adaptation of the microcontrollers to these inputs.
5. REFERENCES
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