Tachometer: Problems and Fixes

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The laser tachometer is the most temperamental and most fragile part of the kit. The good news: nearly all its problems are fixed in minutes once you know where to look.

Symptom → cause → fix

“Tachometer error” in the program, although the display shows RPM

1. In F4 settings set Tacho unevenness = 200 (the default is a small value, so any mark unevenness raises the error).

   

   

2. Didn’t help — check the contact: unplug/re-plug the tachometer connector (X3) several times.
3. In newer software versions the tachometer check is stricter by default — re-check this setting after updating.

RPM reads 2–4× too high / jumps around

The tachometer is seeing extra “marks”:

• shiny rotor areas act like reflective tape. Diagnostic: divide the readings. Real case: the device showed 8695 at a true 2856 rpm — 8695/2856 = 3, i.e. three shiny spots. Fix: sand/dull the shiny areas or cover them with matte tape;
• pulley holes: two holes = two marks; the RPM doubles (4000 instead of 2000). Stick the mark on a solid surface, cover the holes in the beam’s path;
  

  

• blades: a mark on a blade plus shiny neighbouring blades = the tachometer “counts blades” (a real case — it counted 4 blades as marks). Stick the mark on the hub or shaft;
  

  

• weld seams and edges glint too — shift the aiming point.

Sun: the tachometer “goes blind” outdoors

A field-work classic. Bright light floods the photocell.

1. Shade the tachometer and the mark: a piece of cardboard, a chair with a rag over it, a tarp — anything.
   

   

2. Print a 3D shade (hood): models ver2.stl / ver3.stl are on vibromera.eu (wp-content/ver3.stl, forum thread “Tachometer sunshade light shield 3D print model”).

   

   

3. Vary the angle and distance to the mark — a slight angle reduces glare.
4. The mark should be 10–15 mm and firmly stuck.

The tachometer display shows RPM but the program gets nothing

• Check connector X3 (re-plug it).
• Update the software and firmware (old versions had transfer bugs).
• If the indicator LED on the unit’s board goes out when you wiggle the USB/tachometer cable — a bad contact in the connector or inside the unit: that’s a warranty case, write to support.

The tachometer works for 10 seconds, then “freezes” on one value

A known hardware defect — the tachometer gets replaced (warranty).

Doesn’t count at all after a drop

The tachometer is fragile. After a fall it usually needs replacement. No point opening the housing — power comes through the cable, there are no user-adjustable parts inside.

Rules for the mark

1. The mark is a piece of the kit’s reflective tape, ~10–15 mm.
2. Stick it on the hub or shaft, on a clean dry surface, not on a blade.
3. Bir mark on the rotor. Make sure no other reflective objects sit in the beam’s path.
4. Do not re-stick the mark or move the tachometer between runs of a series — the phase zero shifts and the whole series is wasted.
5. The tachometer always aims at the same point of the circumference.
6. Out of tape — any reflective tape works temporarily (from a safety vest included), but the stock tape is more reliable.

Bench-testing the tachometer

If in doubt, test the tachometer on a bench grinder/drill with a mark stuck on, at a known RPM — not on a machine with blades: on blades, dropouts and extra triggers mask the true picture.

Interference from a variable-frequency drive

An ungrounded VFD can induce noise into the tachometer and sensor cables: jumping RPM, “noisy” signal. Fixes: ground the VFD, put ferrite rings on the cables, use filters, route the cables away from power lines.