Analogue Electronic Ruler Reading Jumps: Three-Step Dry Battery Isolation Method

An analogue electronic ruler whose reading jumps is usually diagnosed by swapping parts — sensor, cable, controller — until the fault moves. The dry battery method isolates the fault in three steps without swapping anything: it separates the problem into the ruler, the cable, or the controller. This article explains the method for a 4-20 mA electronic ruler, exactly as used on site.

What the dry battery does

A dry cell — 1.5 V or 9 V, whichever suits the input range — replaces the position source with a stable, known input. If the controller still jumps while the battery holds the input steady, the fault is on the controller or the cable side; if it reads cleanly, the ruler itself is at fault. It cuts the three suspects down to one in one test.

The three steps

1. Ruler vs controller — disconnect the signal wire and inject the battery through a resistor to the controller input. Steady reading → the controller is fine and the ruler is suspect. Still jumping → the controller or the input card is the problem.
Read the size of the residual jump as well: a jump as big as before points mainly at the controller, a smaller jump means the controller and the cable run are both contributing, and no jump at all points mainly at the cable run.
2. Cable vs ruler — reconnect the ruler with a known-good short cable. Reading cleans up → the original cable run is the problem (screen, routing, damage).
3. Source check — verify the 24 V supply at the ruler with a multimeter while it moves. A supply that dips when the axis moves makes an analogue output jump exactly like a fault.

Three site causes that look like a faulty ruler

If the test points at the ruler, do not swap it yet. Three things on site produce exactly the same symptom.

• Three-wire habit on a four-wire sensor. A magnetostrictive electronic ruler needs four conductors so that supply and signal stay separate. Customers who previously used three-wire rulers often carry the old habit over; tying the signal minus to the supply 0 V is what makes the reading unstable.
• Screen earthed at both ends. The screen is earthed at one end only — in practice at the controller end, left floating at the sensor. If both ends are earthed and there is any potential difference between them, current flows in the screen and the screen itself becomes the interference source.
• An unstable 0 V, not just an unstable 24 V. The accepted supply range is 20.4–28.8 V, and an analogue loop measures against 0 V as much as against 24 V. If 0 V wanders by 0.5 V, a 5 V output arrives at the controller as anything between 4.5 V and 5 V. The usual cause is one large 24 V supply feeding every device on the machine: when the high-current loads switch, they pull the 0 V rail with them. Giving the control devices their own 24 V supply is the cheapest cure.

Stroke length multiplies the same interference

An analogue output is a linear scale: the longer the stroke, the more length each volt represents, so the same interference voltage produces a bigger jump in millimetres. With a 0-10 V output and 0.005 V of induced noise on the signal, a 100 mm ruler jumps 0.05 mm, a 500 mm ruler 0.25 mm, a 1000 mm ruler 0.5 mm and a 3000 mm ruler 1.5 mm.

That is why short analogue rulers are rarely a problem and long ones are: on a medium or long stroke, moving to a digital output — SSI, CANbus, Profibus or DeviceNet — takes the analogue link out of the loop altogether. Where the budget is tight, the semi-digital Germanjet 183 series is the compromise that keeps the cost down.

Rules that make the test honest

• Do the test at rest first, then while moving — intermittent jumps while moving point to mechanics, magnet or cable flex.
• Keep the battery connection short and twisted; a floating test lead creates the jump you are trying to find.
• Never reverse polarity on a 4-20 mA input; check the terminal layout before connecting the battery.
• If the controller reads a stable value on the battery but not on the sensor, the sensor electronics or the magnet ring is the suspect — not the cable.

In one line: a jumping reading is not proof of a bad ruler. Inject a dry battery, isolate in three steps, and the guilty link shows itself.

Wiring practice that prevents many of these faults — four-wire 4-20 mA hook-up and shield earthing — is covered in displacement sensor wiring guide. For a hydraulic cylinder sensor that reads badly, see hydraulic cylinder displacement sensor fault troubleshooting.