Detailed Description
The Servo Feedback Signal Chain — Why Cable Quality Matters
A servo motor's encoder generates position and speed data that the servo drive uses to control the motor — typically at update rates of 2–16 kHz for standard drives and up to 62.5 µs (16 kHz) for high-performance systems. The feedback cable carries these signals from the motor-mounted encoder to the drive's feedback interface.
The signals are electrically fragile:
- Incremental encoder A/B: 5 V differential (RS-422), up to 500 kHz pulse frequency. At 500 kHz, a single pulse is 2 µs wide. Cable capacitance rounds the pulse edges — if the rise time exceeds 0.5 µs, the drive's decoder may misread the pulse count, causing position error
- Absolute encoder digital (EnDat, BiSS, SSI): Clock frequencies from 100 kHz to 16 MHz. At 16 MHz, the clock period is 62.5 ns. Cable capacitance and inductance must be controlled to maintain signal integrity — the cable is effectively a transmission line
- Resolver SIN/COS: Analog signals at 4–10 kHz excitation frequency, mV-level amplitude. Noise from adjacent motor power cables directly corrupts the angular position calculation — every 1 mV of noise translates to a measurable angular error
Capacitance — The Critical Parameter
For digital encoder signals at multi-MHz frequencies, cable capacitance is the dominant signal degradation mechanism. Capacitance per meter (pF/m) is determined by:
- Insulation material: PE and PP have dielectric constants of 2.3–2.4. PVC has a dielectric constant of 4–6. The cable's capacitance is directly proportional to the dielectric constant — PE/PP cable has approximately half the capacitance of PVC cable of the same geometry. This is why servo feedback cables use PE/PP insulation: every picofarad of capacitance per meter rounds the digital pulse edges and limits the maximum cable length
- Insulation thickness: Thinner insulation = higher capacitance (conductors closer together). But thinner insulation = lower voltage rating and less mechanical robustness. The servo cable insulation wall thickness is a balance — typically 0.2–0.4 mm for 0.14–0.34 mm² signal conductors
- Pair twist lay length: Tighter twist (shorter lay length) = better common-mode noise rejection but slightly higher capacitance per meter. Servo cables optimize for the mid-range — lay length of 30–50 mm per twist
Cable Length Limits for Digital Encoder Interfaces
The maximum cable length for a digital encoder interface depends on the clock/data frequency and the cable's capacitance per meter:
| Encoder Interface | Clock / Data Rate | Max Cable Length | Limiting Factor |
|---|---|---|---|
| Incremental (RS-422) | 500 kHz pulse | 50 m | Capacitance (pulse edge rounding) |
| EnDat 2.1 | 2 MHz clock | 50 m | Capacitance + timing |
| EnDat 2.2 | 16 MHz clock | 10 m | Transmission line effects |
| Hiperface DSL | Digital, 100 Mbps | 10 m | Signal attenuation |
| BiSS C | 10 MHz clock | 25 m | Capacitance + crosstalk |
| SSI | 1 MHz clock | 50 m | Capacitance |
| Resolver (analog) | 4–10 kHz excitation | 100 m | Cable resistance (voltage drop) |
For cable lengths exceeding these limits, an active line driver/repeater at the motor end is required — or the drive must support the longer length with reduced clock frequency.
Yichi Servo Cable Manufacturing
- Capacitance measured on every production length: Core-to-core and core-to-shield capacitance at 800 Hz is measured as a routine production test. Values exceeding the specification trigger rejection — capacitance is the single most critical electrical parameter for a servo feedback cable
- Pair twist lay length verified: Optical measurement of pair lay length on production samples. Inconsistent lay length causes pair-to-pair capacitance variation and unpredictable signal timing skew
- Shield coverage optically measured: Overall braid coverage ≥85% verified by image analysis. Braid angle 30–45° from cable axis — balanced for coverage vs flexibility
- Drain wire continuity tested: Each individually shielded pair's drain wire is continuity-tested. An open drain wire means the pair's foil shield is electrically floating — providing zero capacitive noise protection
- Green jacket (RAL 6018): Standard servo cable color per industry convention — visually identifies the cable as a servo feedback cable in the control cabinet and on the machine