ERGONOMICS OF MANUAL MATERIALS HANDLING - Environmentally Conscious Materials Handling - page 57

foot will slip, with the potential for a fall and/or loss of control of the load.
Ensuring a high coefficient of friction for all floor surfaces upon which pushing
and pulling is done is an important safety consideration for the use on materials
handling assist devices.

3.2.3 Device Control
Materials handling assist devices should be designed in a manner such that it is
clear to the user of the device how a manipulation of the device controls will
result in movement. This is true both when the device is manipulated by hand
(though pushing and pulling) and when it is controlled though a motor and a
control system. In designing an assist device, there is a tendency to overcome
the physical constraints to movement through the addition of more complex
segments or links. Particularly for hoists, this can result in systems where there
are more degrees of freedom than needed. In such a system, the device can be
positioned to a location in space with more than one configuration. Such systems
are inherently hard for people to use and should be avoided. For any device, the
user should be able to predict with certainty what direction the device will move
when any component is physically pushed or pulled.
With respect to automatic controls, care should be taken to consider device
control-response compatibility. If an operator is holding a push button controller,
the expectation will be that the right button moves the device to the right; the
top moves the device upwards, and so on. This is a challenge if the controller
is separate from the device, as is often the case for overhead hoist devices. For
these devices, the orientation of the worker to the device (front, back, right side,
left side) will determine the control-response relationship. What works appro-
priately on one side of the device will likely work inappropriately on the other
side. Designers should try to anticipate where workers will stand when operating
the device and make sure actuation is appropriate for that position. Redundant
labels for the keys of these devices with coding relative to the room up-down,
north-south is often a good idea to help users orient in unusual work positions. In
addition to orientation, controls should be designed with moderate system gain or
responsiveness. As a rule, human operators have difficulty controlling automated
devices that have both a long control delay and a high system gain. Material
handling assist devices often have a considerable delay in their control system,
making attention to gain an important design consideration.

3.3 Device Design Summary

Material handling assist devices can be an effective way to reduce the risk to
workers associated with manual materials handling. When installing such devices,
engineers should be cognizant that some devices address the lifting component
of the task and others address the carry component. Effective design will often