An encoder fails quietly: a counter that drifts, a drive that trips on a feedback error, an axis that loses its position. The replacement is often found by model name, and the model name is rarely enough. These are the terms to check, with the Heidenhain ERN 420 as the example.
Incremental or absolute
An incremental encoder sends pulses as the shaft turns, and the controller counts them from a reference point. An absolute encoder reports the position itself, over a protocol such as EnDat or SSI, and knows it after a power cut. Heidenhain’s names show which is which: ERN for incremental, ECN for absolute single-turn, EQN for absolute multi-turn. One does not replace the other without changes in the controller.
Line count and reference mark
The line count is the number of signal periods per revolution. The controller’s scaling is set for it, so a different line count shifts every position by the ratio of the two unless the parameter changes too. The reference mark is one extra pulse per revolution that the controller uses to find its zero; check that the new encoder has it where the old one did.
Output signal and supply voltage
| Signal | What it is | Typical supply |
|---|---|---|
| TTL | Square wave, differential to RS-422 | 5 V |
| HTL | Square wave at supply level | 10–30 V |
| 1 Vpp | Sine and cosine, interpolated in the drive | 5 V |
The family name does not settle this. In Heidenhain’s ERN 400 series, the ERN 420 outputs TTL and runs on 5 V, the ERN 460 outputs TTL and runs on 10–30 V, and the ERN 430 outputs HTL. A TTL input fed with HTL can be damaged; an HTL input fed with TTL sees nothing.
Shaft, mounting and connector
Four mechanical details must match: hollow, blind hollow or solid shaft; the shaft diameter; how the housing is held against turning, by a stator coupling or a torque support; and the cable or connector with its pin assignment. The ERN 420 is a hollow-shaft encoder, so the shaft diameter and the coupling must suit the motor or machine it sits on.
Ordering by the ID number
Heidenhain labels carry an ID number next to the model name and the serial number. The ID fixes the line count, the shaft, the signal and the connector in one code, so it is the number to search for and to quote. Other manufacturers do the same with a full order code. If the label is gone, the drive’s parameter list and the machine’s drawings usually name the line count and the signal.
Search the full code on Sparehawk, and if nobody lists it, a wanted request lets suppliers who hold it answer. The guide to finding a replacement covers what to do when the exact encoder is no longer made.