Page 206 - Handbook of Biomechatronics

Upper-Limb Prosthetic Devices 203

To reduce the cost of upper-limb myoelectric prostheses and address the
limitations of the Robohand, an inexpensive 3D printed prosthesis for
patients with transradial limb amputation was developed (Gretsch et al.,
2016). The prosthesis is shoulder-controlled and externally powered with
an anthropomorphic terminal device. The patient can open and close all five
fingers, and move the thumb independently at a cost of US$300. In addition,
the device is lightweight, and its size easily scalable. Limitations include low
grip strength and decreased durability compared to passive prosthetics.
It is expected that as the cost of 3D printing drops, and as materials
become stronger, complex devices such as upper-limb prostheses will ben-
efit from these techniques, leading to customizable, lightweight, easily
replaceable, and cost-effective devices (Fig. 15).

1.5.4 Actuators
The actuators are very important elements of prosthetic devices, as they
affect motion and interaction forces between the device and the environ-
ment. Candidate actuators include DC motors (brushed and brushless),
ultrasonic motors, piezoelectric motors, artificial muscles (pneumatic or
dielectric electroactive polymer based), shape memory alloys (SMAs),
and more.
A large number of factors have to be taken into account for choosing
actuators for prosthetic limbs. These include power, power density, voltage,
current, torque, torque density, speed, size, weight, precision, hysteresis,
repeatability, frequency, efficiency, noise, specific parameters depending
on technology, applicability, and cost, and apply both to prosthetics and
robotics (Hollerbach et al., 1992; Cura et al., 2003). In many studies, com-
parisons of actuators based on a number of criteria are presented; however,
to adopt some technology for upper-limb prosthesis, one has to include in
the comparison, not only the actuator but also the drivers/amplifiers needed,
the sensors, and the power source for it. This is because the use of an actuator
requires all these subsystems, and all of them have to be embedded in the
prosthesis, or carried somehow by the patient. For example, a hydraulic
actuator may look very attractive, but when one considers the power supply
and the piping needed, then its attractiveness is reduced.
DC motors. Permanent magnet DC motors produce torque due to
Lorentz forces acting on their windings. They are produced in miniature
sizes of 1W or even less, and in brushed and brushless forms. As both are
low-torque, high speed devices (up to 10–20krpm), they are used with min-
iature gearboxes, and are equipped with integrated angle sensors, usually in