Feedback interface for CNC axes: encoder interface modules that interpolate 1 Vpp signals and read EnDat or SSI positions for the drive

01

Interpolating 1 Vpp sine/cosine signals: amplitude window, cutoff frequency and steps per period

HEIDENHAIN's interface brochure defines the 1 Vpp signals the interface has to accept: two nearly sinusoidal signals A and B, 90° ±10° electrical apart, with an amplitude of 0.6 to 1.2 Vpp, 1 Vpp (typ.), measured differentially across a 120 Ω terminating resistor, plus a reference mark signal with a usable component of at least 0.2 V. The amplitude falls as frequency rises; the brochure's signal parameters hold for motion up to 20% of the encoder's –3 dB cutoff frequency, where 70% of the amplitude remains. [1]

The interface does not count the sine periods only; it divides each one. HEIDENHAIN states that for speed control interpolation factors above 1000 are used, so that usable data remain available at low shaft or traverse speeds. [1] Beckhoff's EL5021 EtherCAT terminal shows what that looks like in a product: it accepts one 1 Vpp encoder at input frequencies up to 250 kHz, supplies the encoder with 5 V, and outputs a 32-bit count made of up to 24 bits of whole periods and up to 13 bits of position within one sine period. [2] Editorial calculation: 13 bits is 2^13 = 8,192 steps per signal period.

The input frequency limit translates into an axis speed limit. Editorial calculation, using an assumed 2,048-period rotary encoder: 250,000 periods per second ÷ 2,048 periods per revolution ≈ 122 rev/s, about 7,300 rpm, before the EL5021's 250 kHz limit is reached. [2] Siemens' SME20 sensor module for 1 Vpp encoders evaluates up to 500 kHz, which on the same assumed encoder is about 244 rev/s. [4]

HEIDENHAIN's reference input circuit for the receiving side uses an operational amplifier with a gain of 3.48 and a –3 dB cutoff of about 450 kHz, or about 50 kHz with larger capacitors, a variant it says gives up bandwidth for better interference immunity. [1]

The image shows an interface box with circular encoder connectors, terminal blocks and a D-sub port on a bench, a rotary encoder beside it and two sine traces on an oscilloscope. Give the interface supplier the encoder's signal period or line count and the axis top speed, and ask at what input frequency the stated interpolation still holds.

02

Watching the signal: amplitude thresholds and error flags

HEIDENHAIN recommends that the downstream electronics monitor the 1 Vpp amplitude with a lower threshold of 0.30 Vpp and an upper threshold of 1.35 Vpp; on an oscilloscope in XY mode, A and B draw a Lissajous circle whose diameter is the amplitude. [1]

The EL5021 terminal reports amplitude errors and frequency errors of its input signals as diagnostics, and can latch or reset its counter on the encoder's reference mark (C track). [2]

HEIDENHAIN also warns that next to the reference mark the output can drop by up to 1.7 V to a quiescent value, and the receiving electronics must not be overdriven by this. [1]

03

Serial EnDat and SSI encoders: clock rate against cable length

With absolute encoders the interface acts as the master that clocks the position out. HEIDENHAIN gives the EnDat clock as variable between 100 kHz and 2 MHz depending on cable length, up to 150 m; with propagation-time compensation in the receiving electronics, clock frequencies up to 16 MHz or cables up to 100 m are possible, and the adapter cable at the encoder must not exceed 20 m. [1]

The Siemens manual (2008 edition) shows the split between module types: cabinet-mounted SMC10 for resolvers, SMC20 for 1 Vpp and EnDat 2.1, SMC30 for TTL/HTL, and external SME modules in IP67 for mounting outside the cabinet, each converting one encoder system to the DRIVE-CLiQ drive bus. [4] The EL5032 can synchronise its position read with other processes through EtherCAT distributed clocks. [3]

Cable length is limited differently by signal type: HEIDENHAIN allows up to 150 m for 1 Vpp cables with a signal propagation time of 6 ns/m, while Siemens limits the SME encoder cable to 10 m and ties the length to the encoder's current draw and the supply-conductor cross-section. [1] [4] Editorial calculation: 150 m × 6 ns/m = 900 ns of one-way delay, the kind of delay propagation-time compensation exists to correct at high EnDat clock rates. [1]

04

What to tell the interface supplier about each axis

05

Axis and encoder data for a feedback interface

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Axes: [number; linear or rotary; motor or direct measuring]
Encoder: [maker and model; 1 Vpp / TTL / HTL / EnDat / SSI / resolver; signal period or line count]
Speed: [maximum traverse or shaft speed]
Cable: [length encoder to module; adapter and extension]
Interface: [cabinet or IP67; supply voltage; bus to drive or CNC]
Diagnostics: [amplitude and frequency error reporting; reference mark handling]

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