Overview
This is one of the most common questions operators ask: how tall does my edge protection (toe kick, guardrail, or barrier) need to be for the FieldPrinter to print accurately right up to a slab edge?
The short version: edge protection that is at least 2" tall keeps the robot from driving off the edge, but you need at least 7" tall to get clean, accurate printing right up to that edge. Those are two different numbers doing two different jobs, and the difference is worth understanding.
This article explains the two heights, why 7" is the number that matters for print quality, and what to do when you can't get there.
The Short Answer
| Edge protection height | What it does | Result for printing near the edge |
|---|---|---|
| Less than 2" | The robot may not reliably detect the edge protection as an obstacle. | Treat as an unprotected edge — the robot keeps its full safety buffer and prints short of the edge. |
| At least 2" | The robot detects the edge protection and recognizes it as something occupying that space, which helps keep it from driving off. | The robot is protected from falling, but it can still back away as it nears the edge, which makes printing right up to the line inconsistent. |
| At least 7" | The edge protection fully blocks the robot's downward-looking sensors from "seeing over" the edge. | Best case. The robot prints up to the edge without backing off. This is the height to ask for when edge layout matters. |
Recommendation: specify 7" minimum edge protection wherever the layout has to land accurately against a slab edge.
The Two Sensor Systems
The reason for the two numbers is that the FieldPrinter uses two separate sensor systems near an edge. They do different jobs and respond to different heights.
Obstacle detection sensors look outward and have a wide field of view. They detect edge protection (a toe kick, guardrail, or barrier) once it is at least 2" tall and treat that space as occupied — so they recognize the edge protection from a distance and help keep the robot from driving off.
Cliff sensors look down at the floor right in front of the robot to catch real drop-offs. They will detect a cliff when the toe kick is shorter than 7" — once the robot gets close enough, the cliff sensors see past the short barrier, read the drop-off, and the robot backs away from the edge.
The key interaction: when obstacle detection sensors are on, they detect a 2"+ toe kick before the cliff sensors detect the edge. Because obstacle detection has the wider field of view, it picks up the toe kick while the robot is still farther away — well before the robot gets close enough for the cliff sensors to trigger. That earlier detection is what keeps the robot safely on the slab.
So with a 2"–7" toe kick, the robot is safe — obstacle detection stops it from driving off — but printing can be inconsistent at the perimeter, because once the robot moves in close, the cliff sensors can still trigger and pull it back. Only edge protection of 7" or taller blocks the cliff sensors entirely.
Why 7" Specifically
It comes down to where the sensors sit on the robot. Both the cameras and the cliff sensors are roughly 7" above the ground. Edge protection only blocks a sensor's line of sight once it reaches the sensor's height. Below 7", the cliff sensors can still catch a sliver of the drop-off as the robot approaches — enough to trigger cliff detection and make the robot retreat. At 7" and above, the barrier fully blocks that downward sightline, so the robot stays confident right up to the edge.
What Happens at a 2" Toe Kick
If you only have a 2" toe kick, here's the practical sequence:
The obstacle detection sensors spot the toe kick first, from farther away, because of their wider field of view — and that detection is what keeps the robot from falling off the edge. The cliff sensors only come into play when the robot gets very close; if they then trigger, the robot backs up. So a 2" toe kick is safe, but it isn't ideal for printing tight to the edge.
Key Takeaways
- 2" is the minimum height for the robot to detect edge protection and stay on the slab.
- 7" is the minimum height for the robot to print accurately right up to the edge, because that's where the cameras and cliff sensors sit.
- When edge layout matters, ask the GC or trade for 7" minimum edge protection along that slab edge.