{"id":30962,"date":"2026-11-10T14:18:35","date_gmt":"2026-11-10T06:18:35","guid":{"rendered":"https:\/\/safetyfencesystems.com\/blog\/safety-interlock-sto-gate-signal-plc-sequence\/"},"modified":"2026-11-10T14:18:35","modified_gmt":"2026-11-10T06:18:35","slug":"safety-interlock-sto-gate-signal-plc-sequence","status":"publish","type":"post","link":"https:\/\/safetyfencesystems.com\/it\/blog\/safety-interlock-sto-gate-signal-plc-sequence\/","title":{"rendered":"Safety Interlock STO: Tracing the Gate Signal From the Door to the PLC"},"content":{"rendered":"
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SAFETY INTERLOCK STO \/ PLC INTEGRATED SAFETY GATE<\/p>\n

The fence is installed, the door opens, and the machine keeps running<\/h2>\n

EHS and controls engineers meet this fault from two directions. Either the machine guarding fence is up but opening the gate does not stop the machine, or the line trips so often that production starts bypassing the interlock. Mechanical and electrical teams then blame each other because nobody has written down the chain. Safety interlock STO work starts with a sequence: the actuator key on the gate is withdrawn from the switch body fixed to the post, the safety circuit opens, the drive loses torque and the PLC stops issuing motion commands. Only after the gate is closed and the circuit is deliberately reset can the machine restart. What the guarding supplier contributes is the stable mounting datum and alignment that keep that actuator meeting its switch for years.<\/p>\n

\n \"PLC
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Send your gate type and switch model for a mounting carrier drawing<\/a><\/p>\n<\/section>\n

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The sequence from a mechanical action to a PLC instruction<\/h2>\n

A door that fails to stop the machine is rarely a broken switch. More often the actuator is misaligned, the circuit was designed with an automatic reset, or the mechanical side never gave the switch a fixed, repeatable position. Writing the chain in order assigns each step to an owner.<\/p>\n

\n\n\n\n\n\n\n\n\n
Step<\/th>\nWhat happens, and who owns it<\/th>\n<\/tr>\n<\/thead>\n
1. Mechanical action<\/th>\nThe gate leaf opens and the actuator key mounted on it withdraws from the switch body fixed to the post. A framed gate leaf and a door header beam hold both posts parallel so the key tracks the switch instead of drifting out of alignment.<\/td>\n<\/tr>\n
2. Electrical signal<\/th>\nThe safety switch contacts open the safety circuit. A safety lock mounting carrier is prepared for Omron D4NL or Pizzato FS series devices; the switch, its wiring, the safety relay or safety PLC input and the achieved safety rating are specified by the integrator.<\/td>\n<\/tr>\n
3. Drive and PLC<\/th>\nSTO removes torque from the drive and the PLC stops motion commands. STO is not an isolation or a power-off: the drive may remain energised, and vertical axes or gravity loads still need the braking arrangement designed by the machine builder.<\/td>\n<\/tr>\n
4. Restart inhibit<\/th>\nClosing the gate only re-establishes the circuit. A separate manual reset outside the danger zone, with a clear view of the cell, is required before restart. Without that deliberate action, a door closing on its own could restart the machine with a person still inside.<\/td>\n<\/tr>\n<\/tbody>\n<\/table><\/div>\n<\/section>\n
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Where the mechanical side actually makes the difference<\/h2>\n
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Alignment is a mechanical part of an electrical function<\/h3>\n

Intermittent nuisance stops are usually geometry. A gate leaf that sags pulls the actuator off the switch head; a post that twists moves the switch body; a latch that wears changes the closed position by a few millimetres. Framed mesh panels with a 20x30x1.5 mm tube border, flat full welds and burr-free joints give the gate a rigid leaf, while the header beam keeps the two posts parallel. M8 fixing rings hold the panels and four M10 expansion anchors hold each base plate to the concrete, so the reference position does not move with the floor or the frame.<\/p>\n

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A carrier, not a safety control system<\/h3>\n

The safety lock mounting carrier is a plate with prepared hole positions for a named third-party switch. It does not contain the safety logic, does not decide the performance level and does not validate the circuit. Robot cell guarding projects also bring light curtains, emergency stops and LOTO into the same safety circuit, and those are designed and signed off by the controls integrator against ISO 14120 and ISO 13849-1 as applicable. Stating that boundary on the drawing is what ends the blame loop: the mechanical contractor proves the door closes to a fixed datum, the electrical contractor proves the circuit behaves as designed.<\/p>\n

\n \"Industrial
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Commissioning checks that catch both failure modes<\/h3>\n

Record the key-to-switch alignment at closed position, then cycle the gate three to five times and re-measure. Check post verticality, panel flatness, fastener counts and the latch engagement before any electrical fault-finding begins. Then verify with the integrator that opening the gate removes torque, that closing it does not restart the machine, and that only the deliberate reset restores motion. A 1600 J impact figure quoted for the standard configuration applies only to that tested build and must be confirmed per project. This system is not intended for pure pedestrian guidance.<\/p>\n

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Cells where the door-to-PLC chain should be written down<\/h2>\n