Presence pauses, perimeter faults
What you'll learn
Split the two protections: the light curtain HOLDs the cycle and lets it resume; a perimeter breach drops the enable and FAULTS the cell.
Two safeguards protect the same cell — and they answer a breach in two completely different ways, on purpose. Mixing them up is how cells end up either dangerous or unusable.
Presence: the curtain pauses
The light curtain guards the load window — the opening an operator reaches through every single cycle. That's not an emergency; that's the job. So the print's answer is proportionate:
r3 ──[LC1]──( RB1.HOLD )
Blocked beam → HOLD goes high → motion freezes with the cycle's remaining time preserved. Clear the beam and the cycle picks up exactly where it stopped. Nothing to reset, nothing lost — the machine treated the reach-in as a normal part of production, because it is.
Perimeter: the gate faults
The fence gate is a different story. Nobody opens the perimeter as part of the cycle — a gate opening means a person is entering the robot's envelope. The chain from last lesson answers accordingly: CR1 drops, EN drops mid-cycle, and the robot declares a FAULT.
Look at what that costs, deliberately:
| HOLD (curtain) | FAULT (gate, e-stop) | |
|---|---|---|
| what triggers it | beam blocked | enable lost mid-cycle |
| the running cycle | frozen, time preserved | aborted, time gone |
| when it clears | beam clears — resumes alone | never by itself |
| what it takes | nothing | re-arm chain + deliberate reset |
The fault latches. Close the gate, press RESET, re-arm the whole chain — the robot stays faulted until someone explicitly clears it. The machine is making you finish the story: someone was inside; a human decides when that's over.
Which device gets which answer?
In this cell: curtain pauses, perimeter kills. In yours? That call comes from the risk assessment — a curtain can absolutely be wired to drop the enable where reach-in exposure is severe, and some gates request a controlled stop before isolating. What stays constant is the vocabulary you just learned: presence-sensing wants a resumable pause; a perimeter breach wants de-energization that stays down until a person answers for it.
Run it
Open the safety-chain circuit, close the gate (GS1 and GS2 on), press RESET, and start a cycle:
- Mid-cycle, flip LC1 — status reads HELD, the countdown freezes. Flip it back: the cycle finishes from where it stood.
- Start another cycle and open GS1 instead. FAULT — and note the countdown didn't freeze, it died.
- Now close the gate and press RESET. Chain healthy, CELL SAFE lit — and the robot is still faulted. That missing rung is next.
Check your understanding
Question 1 of 2
An operator blocks the light curtain mid-cycle, then steps back. What happened to the cycle?