Page 190 - Handbook of Biomechatronics

Upper-Limb Prosthetic Devices 187

to the amputee. This is what Simpson clearly saw as the success of the system,
and this is why he coined the phrase EPP for it. It can be said that body-
powered prostheses are a subset of EPP systems and that is why they were
successful in the past. EPP control of externally powered prosthetic joints is
similar in concept to power steering of the car. The driver feels, through the
handling of the steering wheel, the state of the car’s front wheels. Further-
more, the driver cannot “beat” the response of the front wheels because the
steering wheel and the front wheels are coupled. This unbeatable coupling
provided by a mechanical linkage is the essence of EPP.
The use of tools such as hammers, pens, knives, and racquets illustrate the
simple form of EPP. We use these extensions of our body without thinking
because we extend our proprioception through these tools. A tennis player
does not watch the racquet during swing. The proprioceptive capabilities of
the wrist and other joints have been extended to include the racket. The
racket becomes a natural extension of the arm. Control of another joint
by a physiological joint through EPP is more complex than the EPP control
of a simple rigid extension. The artificial joint may be powered by the phys-
iological joint, or it can be externally powered and receiving the control
input from the physiological joint. Both can be forms of EPP or not. The
prerequisite for the EPP control of another joint is that the two joints,
the physiological joint and the artificial joint, must be mechanically inter-
connected. In that way the physiological and the artificial joint have equiv-
alent kinematics and kinetics. The force, position, and velocity of the
artificial joint is transmitted through the Bowden cable and is sensed by
the physiological joint and vice versa (Fig. 5). This is an “unbeatable”

Fig. 5 Diagram showing the “1-1” mapping of the force, position, and velocity between
the control site and the prosthesis. This mapping is provided by a rigid mechanical link-
age connecting the control site with the prosthesis. Shoulder elevation/depression is
associated with elbow flexion/extension. Shoulder protraction/retraction is associated
to wrist pronation/supination. (From Northwestern University Prosthetics Research
Laboratory (NUPRL).)