LF RFID Ground Point: The AGV Landmark Tag Explained

Ask an AGV fleet how it knows where it is, and the honest answer is often "approximately." Inertial odometry drifts, magnetic tape gets damaged and misread, and optical codes fail the moment dust, oil or tyre wear enters the picture. The LF RFID ground point — a small passive transponder fixed in or on the floor, read by a sensor on the vehicle's underside — is the cheapest reliable way to reset that accumulated error to zero, again and again, at every node on the route.

What a "ground point" actually is

A ground point (known in the industry as a landmark tag, site tag, or 地标标签) is a passive LF transponder installed at a physically meaningful location on the vehicle's path: a docking station, a conveyor interface, a turning corner, a junction, a charging bay, a lift or a door threshold. Each tag carries a unique ID that the control system has mapped to a coordinate and, optionally, to an instruction — stop, turn left, slow down, lift the mast, call the MES for the next task.

The reader is mounted underneath the AGV or RGV chassis, typically 30–80 mm above the floor. As the vehicle passes over the tag, the reader energises it through inductive coupling, decodes the ID, and pushes it to the onboard PLC over RS-232/RS-485 or modbus RTU/TCP. The entire exchange happens in milliseconds at travel speeds of 2–3 m/s, which is why LF landmark reading remains viable on fast production logistics lines.

Why low frequency wins this job

It is tempting to reach for UHF, since UHF gives metres of range. But range is not the goal here — positional certainty is. Three properties make 125 kHz / 134.2 kHz the default for ground points:

1. A short, hard-edged read zone. LF systems are inductively coupled near-field devices; field strength falls off roughly with the cube of distance. Practically, that means read distances in the 0–21 cm band, with a sharply defined detection window and no drift over years of service. A UHF tag with a 3-metre range cannot tell you whether the AGV is at the station or 1.5 m short of it.

2. Immunity to metal, water and electrical noise. Plant floors are steel plate, rebar, cable trays and inverter drives. LF penetrates water, concrete, oil film and non-metallic debris with little attenuation and is far less susceptible to the humidity, vibration and EMI that destabilise UHF reads. Well-designed landmark tags are also offered in on-metal versions, using a ferrite-isolated ABS housing that keeps reliable read distance even when screwed directly onto a steel mezzanine plate.

3. Zero line-of-sight requirement. The tag can be buried in concrete, capped with epoxy, or covered in years of grime, and it still reads. That is exactly the failure mode that kills QR-code and camera-based floor navigation in foundries, plating shops and food plants.

What the tag is made of

A typical industrial ground point is deliberately unglamorous:

The physical form follows the floor. Coin Tags suit smooth epoxy or polished concrete with screw or adhesive fixing; glass-tube and nail-style tags are epoxied into a drilled hole so that a scrubber or a forklift wheel cannot shear them off; PPS variants survive washdown and higher temperatures.

Deployment: the part projects get wrong

Ground points are cheap; designing the tag map is where the Engineering lives.

• Place a tag where a decision is made, not where it is convenient. Every branch point, speed-change point and stop point needs its own ID. A tag placed 40 cm before a turn leaves the AGV reacting late.

• Add correction tags between stations. On long straight runs, intermediate tags reset odometry drift before it accumulates into a docking miss.

• Set the air gap deliberately. Read distance is specified for a defined gap; mounting the reader too high, or on a chassis section that flexes, silently halves the working window.

• Respect metal. On steel plate or near heavy rebar, use an on-metal tag and verify read distance on the actual floor, not in air. Expect a meaningful reduction versus the free-air figure.

• Commission the ID map rigorously. Every tag ID must be recorded with its real coordinate and command at install time. Most "mysterious" AGV misbehaviour traces back to a mapping error, not a hardware failure.

Beyond AGVs: buried markers as permanent ground points

The same physics serves a second, much slower-moving application: underground Asset marking. Larger LF markers — HDPE-encapsulated discs or cylinders, IP68, rated for decades of burial — are placed above pipes, cables, valves and joints. A handheld locator then reads them through up to roughly a metre or more of soil and backfill, retrieving the unique ID that links to depth, pipe material, diameter, installation date and maintenance history in the GIS. This is the classic answer to "where exactly is that gas line," in the same way the AGV tag is the answer to "where exactly is that vehicle."

Honest limitations

LF is not universally superior, and specifying it without acknowledging the trade-offs leads to disappointment. LF tags have low data rates, so they carry an ID and a small payload, not a process record. They are read one at a time, with no UHF-style dense bulk reading — irrelevant for a single vehicle over a single tag, but a constraint if you wanted to inventory a whole floor at once. And the short read range means dense deployment: long routes need dozens or hundreds of tags, so installation labour, not tag cost, dominates the budget.

The practical resolution used in most modern plants is hybrid: SLAM or laser navigation for free-path travel, and LF ground points as the absolute position reference at every point that actually matters. The laser gets the vehicle close; the ground point makes "close" exact.

The bottom line

A well-executed LF RFID ground point network gives an AGV fleet something that no purely relative navigation method can: a repeatable, drift-free, centimetre-level position fix at every critical point on the route, unaffected by dust, light, water, paint or the steel around it. At a few dollars per tag and a passive, battery-free design rated for a decade or more of service, it remains one of the highest-return investments in industrial mobile robotics.