Chapter 2. Prehension

412 A ppe It dic e s
are active, while the other two are passive. The thumb and fingers are
opened using a worm nut, which has the advantage that it cannot be
driven backwards; thus, it retains its exact position unless the motor is
activated. When the load goes above a maximum pinch force, the
thumb releases. Steeper includes a battery saver mechanism in the
controller, which senses the maximum opening mechanically and a
preset pinch force electrically.
The Otto Bock Svstem Electric Hand (Minneapolis, Minn.) has
an automatic gear that ensures that capturing an object takes place at
the maximum speed. When a maximum grip force is obtained, the gear
changes to a higher gear reduction, and the grip force increases at a
lower speed. This allows the user to increase the grip force until the
required force is achieved. In order to reduce its weight, it has a
removable battery that can be clipped to the patient’s clothing.
The Syste mteknik mvoelectric hand, from Systemteknik AB of
Stockholm (distributed by Liberty Mutual), comes in two sizes: one
for children 2-6 years of age, and a larger hand for children 5-9 years
of age. A clutch permits slippage when a strong opening force is
detected. The user stops the motor when the desired pinch force is
reached, since the hand will maintain a force indefinitely until it is
opened voluntarily.
D.2.5 Commercially available specialized devices: neither
split hook nor hand
Specialized devices have been developed for activites relating to
sports, recreation, and work. In addition, general purpose devices
have been designed to take the place of the hand and hook. Passive
devices include a knife, fork, spoon, hooks, hammer, typewriter key
push, automobile steering wheel appliance, kitchen appliances, nail
brush holder, garden tool holders, and fishing rod holders (Law,
198 1). Active devices include include pliers, tweezers, tool holders
and general devices, such as the CAPP 2 and the Otto Bock Greifer
(see Table D.4). General non-standard devices are more cosmetically
pleasing than split hooks and are usually more functional than hands.
However, as with most other types of devices available, they have
only one degree of freedom.