Terry Hayward discusses man-machine safety and the options available to
the plant design engineer
The area around manufacturing robots, automated handling machines and
machine tools will always be hazardous and present a potential threat to
operator safety. To reduce the risk of accidents, plant designers must
assess the safety implications and specify appropriate protection
measures. Traditionally, a mechanical fence with switches fitted to
opening doors and gates has been the solution, but it is expensive, takes
up a great deal of space and reduces operational flexibility.
Access to the various zones in modern processes or machines cannot be
categorised as simply 'allowed' or 'disallowed'. the designer needs to
allow for variables, dependent on the stage the machine or process has
reached in its operating cycle when access is attempted. to accommodate
these variables, there are other safety solutions that can be considered
as alternatives to mechanical switches, including a whole range of
opto-electrical devices.
The main advantage of most optical devices is that they can be programmed
to allow access to machinery at specific times during the process cycle
and in specific areas. This important facility means that the safety
system can be tailored to particular hazards while allowing the process
to operate at maximum capacity. Opto-electrical safety devices fall into
three main categories: laser scanners, light barriers and light curtains.
Similar in operation to a radar system, the safety laser scanner is a
flexible and powerful device that is able to scan a 180° detection zone
continuously, and up to a maximum radius of 60m, using a perfectly
harmless beam of laser light. The safety zones and warning zones can be
user-defined, such that when an operator enters a safety zone the machine
is immediately stopped, or a beacon/sounder activated should he or she
enter the warning zone. The laser scanner can also be used to detect
objects, or as the sensor for an automatic transport system.
Safety light barriers can be used as entry controls or movement
detectors. They are ideal for guarding such things as powered windows,
doors and gates or danger zones surrounding many types of machine and
process. Any interruption of the safety light barrier produces a signal,
which stops dangerous machine movements. Light barriers can be used as
perimeter guards or as direct guards for smaller danger zones on a single
machine. They have the advantage of being small and compact, allowing
installation almost anywhere on a machine without affecting its
operation. light barriers comply with safety requirements up to Category
4 of EN 954-1 and their light beams may be infrared or visible. Using a
master/slave configuration, a series of cascading safety light curtains
can be created to protect operators from moving machinery. Light curtains
also comply with safety requirements up to Category 4 of EN 954-1.
Terry Haward is general manager of Schmersal UK.