Basics of safety interlocks
for controllers to command responses suitable for the reported conditions. In contrast, industrial safety switches turn power supplies off and on upon detection of machine element or workpiece positions. Upon verification of the trigger positions, such switches either prompt disconnection or resumption of power to the related machine section. No more is it sufficient to use an ordinary proximity switch as an interlock. Demanding IEC 60947 requirements now demand that components used as interlocks have very specific safety-related features to prevent defeats and other failures. Also found in safety systems are relays that directly make or break electrical contacts — in the most common arrangements, essentially communicating a small command voltage onward to ultimately spur a larger current through the power contacts it commands. Consider two common functions that interlocks render interdependent: the opening of a guard door and a motor-driven spindle on a machine
Figure 4: Some doubly capable interlocks have actuators to function as guard locks. These are position switches with deadbolts or electromagnetic assemblies that can keep doors barred until the guarded robotic arm or machine ceases its hazardous motion. Unfortunately, some engineers mistakenly believe that all interlocks are of the guard-lock variety. (Image source: Omron)
Figure 5: Innovative connectivity options have only increased the reliability of multi-guard installations in recent years. Here, an interface module is connected via T-adapter networks to other safety components. (Image source: Banner Engineering)
Conclusion
total). Exceeding that number of sensors can degrade a design’s performance level (PL) and increase the likelihood of fault masking. Safety interlocks employing one spring-actuated NC switch (whether position or limit) typically deliver positive breaking — so that opening the guard presses against the interlock’s spring to spread its electrical contacts apart. In contrast, more reliable twin-switch interlocks use one switch to actuate upon guard opening and another switch with electrical contacts spread apart upon guard closing. Electronic self-reporting of shorts (usually by monitoring the potential difference between two input channels) is a complementary feature to detect severing of wires due to shearing, corrosion, or overheating.
The reliability of plunger-and- coil solenoid operation renders solenoid-based safety components suitable for critical interlock applications. Electrical input typically causes linear plunger output (with a spring-set return upon power off). When integrated into guarding and deadbolting interlocks, solenoids are the input source for the latching mechanisms. Other such solenoid-based designs can also ensure correct mechanical operation — for example, to ensure consistent conveyor travel even when tending equipment handle or process workpieces riding its belt. Solenoid-based redundancies (with series-wired and double-pole switches for verifying position) can minimize faulty interlock signaling.
tool. Mutuality between these renders the milling station unlikely to damage its own subsystems or injure the operator. In this regard, interlocks function as switches in an operational sequence. Most rare are mechanically cammed interlocks with arms that pivot on an axis to lock dangerous machine axes. Far more common are electromechanical and electronic interlocks employing circuits and microprocessors for cost-effective reliability and even Figure 3: The mere closing of guard interlocks doesn’t trigger the restart hazardous machine processes; instead, those honors go to a separate double- duty control interlock or start switch like the soft-touch capacitive finger switch shown here. (Image source: Getty Images)
reconfigurability. For example, electromechanical hinged interlocks on perimeter doors include a mechanical elbow or lever arm that opens with the hinged guard; beyond the set switching angle, it triggers commands to stop the perimetered machine. Upon door reclosing, the door force ultimately prompts the interlock’s solenoid to reclose the circuit. Typical wiring and solenoid types in interlocks Interlocks are most commonly wired for normally closed or NC logic to only let machines run if the circuit is closed. Most safety standards require that safety-circuit components wire in series for maximally reliable error and event detection (up to an allowable sensor
Interlocks render machine perimeter status interdependent with safety controls. In fact, today’s interlock feedback to such controllers can spur exceptionally sophisticated machine responses to various machine-gating positions. The most advanced interlocks can assume failsafe edge computing, IIoT, and reliability tasks beyond the capabilities of traditional industrial switches and sensors. The main caveat is that guard interlocks mustn’t be cumbersome to machine operators. Automatic functions and conditional unlocking on the most frequently accessed guard doors can improve functionality with minimizing undetected faults.
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