Basics of safety interlocks
unexpected machine startups (by dissipating mechanical power and cutting electrical power) upon an operator’s entry into a hazardous machine workspace. Such systems can disconnect power supplies, stop motors, release fluid power actuators, and allow the spending of any remaining kinematic energy of the machine’s moving segments. In contrast with other standards mentioned in this article, ISO 14119 covers the required specifics of guard interlocks by: n Referencing the risk-analysis techniques of other safety standards. n Defining interlock features that prevent accidental and intentional safety defeats. ISO 14119 defines Type 1 interlocks as position switches using easily defeatable mechanical hinge or cam actuation. Actuating contact occurs between interchangeable (uncoded) halves. Benefits of Type 1 interlocks are low cost and high configurability. Type 2 interlocks (as first defined by DIN EN 1088) include less circumventable position switches based on mechanical actuation. Halves are coded (mated) tongues or (for safety guard locks) trapped keys. The latter force operators to lock all guards before controls allow machine startup … and key removal is only possible when the guards are latched. Fully integrated perimeter controls go even further
Some of the standards reference the core position switch or proximity-switch technology at the core of every interlock. They also outline the requirements of how
to force operators to use those same keys in keyed HMI start switches that hold the key captive during machine operation. ISO 14119 classifies all noncontact safety switches sans coded actuation as Type 3 interlocks. The most easily defeated are those employing optical, ultrasonic, or capacitive actuation; slightly less defeatable are induction and magnetism-based interlocks. Where defeatability is unacceptable, Type 4 interlocks which use matched or coded actuator halves in noncontact operation whether based on RFID, magnetic, or optical technology) are warranted. Comparing interlocks with safety sensors and perimeter switches Interlocks share similarities with other safety-rated feedback and sensing components based on the same core technologies. But to be clear, none of these other components are associated with machine perimeters like interlocks. In addition, today’s safety standards require that interlocks won’t greenlight resumption of action sans some corrective reversal process. Components supplied as industrial safety sensors verify (often via noncontact inductive or photoelectric means) machine element or workpiece positions
operation. That’s in contrast with emergency stops that necessitate more involved machine-restart sequences. The logic of such standards is that the use of interlocks is routine (so shouldn’t hinder every day operations) but that of e-stops is not. Core interlock technology and defeatability Automated machines must satisfy Type A, B, and sometimes C international safety requirements. The functional-safety ISO 12100-1 standard and other foundational Type A standards apply to all automation equipment. Electronic controls satisfying ISO 12100 can address situations involving any unavoidable maintenance of some energy source — namely by preventing any unexpected machine restart. For this purpose, e-stops are never acceptable solutions ... but key interlocks can be. Type B midrange standards include B1 safety-approach standards (including ISO 13849-1 and 62061) as well as specific B2 safe-system requirements (including ISO 13850 and 13851). In contrast, Type C standards are very specific to machine types, so are particularly stringent and most employed by OEMs for new equipment design. Standards specific to interlocks are ISO 14118 and 14119.
electronically actuated workcell guard sections network with equipment controls — typically to command any potentially dangerous motions to slow or even cease.
Accommodating time for machine to stop The most dependable interlocks satisfy specific axis-stop intervals — defined as the time a machine requires to slow to a safe state after issuance of a stop command. In fact, interlock systems accommodate for these stop intervals as well as the time in which it’s feasible that a machine operator could reach hazardous axes after issuance of a stop command. Optimized interlock installations: n Ensure a safe state is achieved long before an operator could ever possibly touch or approach dangerous machine axes. n Support efficient machine use by avoiding excessively long lockdown states. In fact, ISO 12100 details how interlock-guarded doors and panels can (with their closing) immediately trigger resumption of machine
Figure 2: Interlock switches can accommodate various orientations. International safety standards define the classifications of such interlock variations. (Image source: Design World)
ISO 4118 details ways to prevent
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