The Secure Disable feature on the Unidrive  AC allows
the drive output to be disabled so that the drive cannot
generate torque in the motor. Secure Disable also provides a
normal “enable” input to the drive, and can be used in the
same way.
A +24V logic level must be applied in order to enable the
drive power functions. However the internal circuitry has been
designed using in-depth fail safe approved techniques and
principles so that no single fault or component failure can cause
a disabled drive to produce motor shaft torque. Only in
extremely unlikely combinations of faults could this function be
defeated. Secure Disable can therefore be used in safety-related
applications to prevent unintended operation of the motor.
Figure 1:The Secure Disable function can take the place of control
gear, saving components and space for the same level of security
The reliability of the Secure Disable function is superior to that
offered by virtually any single channel electromechanical device
such as a contactor. It is like having a special highly reliable
contactor built in to the drive output, but there are no moving
parts, no extra cost, and no special requirements for
preventing the contactor from opening on load. It offers the
possibility of eliminating contactors, including special safety
contactors, from systems where the prevention of movement
is important to prevent safety hazards or expensive damage.
Special note on emergency stop functions
To avoid misunderstandings it is important to make clear the
purpose of the Secure Disable function. It has the ability on
request to prevent unintended operation of the motor, with a
very high level of integrity, i.e. despite the occurrence of faulty
components.This is intended to be used in the normal range
of functions of a machine, to prevent unexpected movement
when this might cause a hazard. For example, a “safeguard” or
“interlock” system might be provided so that when a guard is
open and an operator might be within the area protected by
the guard, operation of the drives within that area is inhibited.
This could be a normal and frequent condition of the machine,
so that a failure to inhibit the drive could be quite likely to
result in serious injury.
In general, Secure Disable is not intended to provide an
emergency stop function.The requirement for an emergency
stop is different from a safeguard.The emergency stop is not
specifically a very high integrity function, because it is not
intended to be used routinely to ensure personal safety. It is
intended to provide a simple over-riding method for
completely removing power to all actuators in the event of an
unforeseen hazardous situation, regardless of the state of the
machine. It is used when the intended, designed safety
provisions and operating rules of the machines have failed, and
an operator realises that a hazardous situation has occurred.
According to standards such as EN 60204-1 (Europe), IEC
60204-1 (International) and NFPA 79 (U.S.A.), the final
removal of power for emergency stop must be carried out by
hard-wired electromechanical components.The use of Secure
Disable alone will not fulfil this requirement.
Important Warning
The design of safety-related systems requires specialist
knowledge.To ensure that a complete system is safe
requires an overall risk assessment.The use of Secure
Disable and other equipment intended for safety-related
applications does not of itself ensure safety.They must be
correctly incorporated into the complete design. |
| |
The information in this publication gives guidance
on the use of Unidrive Secure Disable, and also some
general background material on the design of safety-related
systems.This information is believed to be correct and to
reflect accepted practice at the time of writing. However it
is the responsibilty of the designer of the end product or
application to ensure that it is safe and in compliance with
relevant regulations. |
|
