Page 304 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

VIBRATION, MECHANICAL SHOCK, AND IMPACT 281

be reduced at frequencies above the resonance frequency of the mechanical system defined by the
spring, damper, and total supported mass [i.e., for r > 2 in Fig. 11.6 and Eq. (11.10)]. The
frequency weighting W k (Fig. 11.1) suggests that effective vibration isolation for humans will require
a resonance frequency of ~2 Hz or less. The low resonance frequency can be achieved by using a
soft coiled spring, or an air spring, and a viscous damper. Vehicle suspensions with these properties
are commonly employed. So-called suspension seats are commercially available, but are limited to
applications in which the vertical displacement of the seat pan that results from the spring deflection
is acceptable. A situation can be created in which the ability of a driver to control a vehicle is
impaired by the position, or motion, of the person sitting on the vibration-isolated seat relative to the
(nonisolated) controls.
Active Vibration Reduction. An active vibration control system consists of a hydraulic or electro-
dynamic actuator, vibration sensor, and electronic controller designed to maintain the seat pan
stationary irrespective of the motion of the seat support. Such a control system must be capable of
reproducing the vehicle motion at the seat support, which will commonly possess large displacement
at low frequencies, and supply a phase-inverted version to the seat pan to counteract the vehicle
motion in real time. This imposes a challenging performance requirement for the control system and
vibration actuator. Also, the control system must possess safety interlocks to ensure it does not
erroneously generate harmful vibration at the seat pan. While active control systems have been
employed commercially to adjust the static stiffness or damping of vehicle suspensions, to improve the
ride comfort on different road surfaces, there do not appear to be currently any active seat suspensions.

11.4.3 Protection against Hand-Transmitted Vibration
Vibration-Isolated Tool Handles. Vibration isolation systems have been applied to a range of
powered hand tools, often with dramatic consequences. For example, the introduction of vibration-
isolated handles to gasoline-powered chain saws has significantly reduced the incidence of HAVS
among professional saw operators. Unfortunately, such systems are not provided for the handles of
all consumer-grade chain saws. The principle is the same as that described for whole-body vibration
isolation, but in this case the angular resonance frequency can be ~350 rad/s (i.e., f ≈ 55 Hz) and
0
still effectively reduce chain-saw vibration. The higher resonance frequency results in a static deflection
of the saw tip relative to the handles that, with skill, does not impede the utility of the tool.

Tool Redesign. Some hand and power tools have been redesigned to reduce the vibration at the
handles. Many are now commercially available (Linqvist, 1986). The most effective designs
counteract the dynamic imbalance forces at the source—for example, a two-cylinder chain saw with
180° opposed cylinders and synchronous firing. A second example is a pneumatic chisel in which
the compressed air drives both a cylindrical piston into the chisel (and workpiece) and an opposing
counterbalancing piston; both are returned to their original positions by springs. A third is a rotary
grinder in which the rotational imbalance introduced by the grinding wheel and motor is removed by
a dynamic balancer. The dynamic balancer consists of a cylindrical enclosure, attached to the motor
spindle, containing small ball bearings that self-adjust with axial rotation of the cylinder to positions
on the walls that result in the least radial vibration—the desired condition.
Gloves. There have been attempts to apply the principle of vibration isolation to gloves, and so-
called antivibration gloves are commercially available. However, none has yet demonstrated a capa-
bility to reduce vibration substantially at the frequencies most commonly responsible for HAVS,
namely 200 Hz and below (an equinoxious frequency contour for HAVS is the inverse of frequency
weighting W in Fig. 11.1). Performance requirements for antivibration gloves are defined by an
h
international standard (ISO 10819, 1997). No glove has satisfied the transmissibility requirements,
namely <1 at vibration frequencies from 31.5 to 200 Hz, and <0.6 at frequencies from 200 to 1000 Hz.
An extremely soft spring is needed for the vibration isolation system because of the small
dynamic mass of the hand if the resonance frequency is to remain low [see Eq. (11.11)]. An air spring