Page 41 - Handbook of Biomechatronics

Actuator Technologies 35

several sets of recently developed safety regulations for robots that interact
with humans; for example, the International Standards Organization
(ISO) has developed requirements for robots that perform surgical, rehabil-
itation, personal care, and industrial tasks (ISO, 2011, 2014, 2017a,b). Below
we introduce several quantitative measurements of safety that can be helpful
in designing biomechatronic actuators.

2.1.1 Impedance and Compliance
Mechanical impedance is the frequency-dependent relationship between
forces and motions. When you impose motion on an object, the amount
of force generated in response is determined by the object’s impedance.
Low impedance is typically desirable for actuators that interact with humans.
Compliance is the ratio of the force generated by an elastic element in
response to deformation. Compliance is the reciprocal of stiffness. One of
the most common methods of designing a low-impedance actuator is by
including compliant elements in the transmission.
Intuitively, humans are safest when the objects around them give way or
yield upon contact. When objects are very rigid or heavy, a high-speed
impact with humans can cause serious injury. Typical industrial robots are
both rigid and heavy, and are programmed to follow precise positions with
no consideration for obstacles (or humans) that may impede motion. If
humans are located in the path of such a robot moving at high speeds, they
will be subjected to dangerously large forces. One way to avoid injury is to
limit the speed and torque of the robot. Another way is to lower the
impedance.

2.1.2 Head Injury Criterion
One of the most severe risks of a biomechatronic actuator is that of a head
injury to the human operator or bystander. The potential for head injury can
be quantified using the Head Injury Criterion (HIC), which is calculated as a
function of the magnitude and time duration of head acceleration (Gao and
Wampler, 2009). The HIC metric was first developed for automotive appli-
cations, later applied to athletics, and is now often used for human-robot
interaction. The values of HIC have been correlated to the abbreviated
injury scale (AIS), which encodes the severity of injuries to all body regions.

2.1.3 Voltage, Current, and Heat
Caution must be taken to ensure that the human operators are shielded from
electrical circuits of the actuator. The potential injury from electric shock