Detailed Description
Pure Feedback vs Composite — When One Cable Is Better Than All-in-One
The composite servo cable (power + feedback in one cable) is the convenient single-cable solution. But there are applications where a separate pure feedback cable — encoder signals only, no power conductors — is the correct specification:
1. Retrofit and Existing Installations
Many servo motors in the field have separate power and feedback connectors. Replacing the motor with a single-cable motor is an expensive retrofit. A pure feedback cable replaces the existing feedback cable without changing the power cable, connectors, or motor.
2. Cable Routing Constraints
In some machine designs, the power cable and feedback cable follow different paths. The power cable may route through a cable tray on one side of the machine, while the feedback cable routes through the drag chain on the other side — to maximize separation and minimize EMI. A composite cable forces both power and feedback into the same path, which may not be the optimal routing.
3. Feedback-Only Applications
Linear encoders, rotary encoders on non-motor shafts, and external position measurement systems require a feedback signal cable but no motor power. Specifying a composite cable for these applications wastes the power cores — adding cost, weight, and cable OD for conductors that will not be terminated.
4. Flexible Configuration
Separate power and feedback cables allow independent replacement. If the feedback cable is damaged (the more fragile of the two), only the feedback cable needs replacement — not the entire composite cable. In a production environment where downtime costs $500–5 000 per hour, the ability to replace only the failed cable can be significant.
Pair Allocation by Encoder Type
The number of pairs required depends on the encoder type:
| Encoder Type | Pairs Required | Signal Allocation |
|---|---|---|
| Incremental (RS-422) | 3–4 pairs | A/A\, B/B\, Z/Z\, +1 pair for motor temp |
| Incremental + commutation | 5–6 pairs | +U/U\, V/V\, W/W\ (or Hall sensors) |
| Absolute — EnDat 2.1/2.2 | 2–3 pairs | Data+/Data–, Clock+/Clock–, +1 pair for power |
| Absolute — Hiperface | 3–4 pairs | Data+/Data–, + SIN/COS, +1 pair for power |
| Absolute — BiSS C | 2–3 pairs | Data+/Data–, Clock+/Clock–, +1 pair for power |
| Absolute — SSI | 2–3 pairs | Data+/Data–, Clock+/Clock–, +1 pair for power |
| Resolver | 3 pairs | Excitation+, SIN/SIN\, COS/COS\ |
| Dual encoder (motor + load side) | 6–8 pairs | Two encoder sets + temp sensors |
| Full-featured: encoder + resolver + temp + vibration | 8–12 pairs | All feedback signals in one cable |
For motors with a single incremental encoder and PTC temperature sensor, 4 pairs is the standard configuration. For high-end absolute encoder motors with dual feedback and condition monitoring sensors, 10–12 pairs may be required.
Yichi Pure Feedback Cable Manufacturing
- Pair capacitance matched to ±5%: All pairs in a cable are capacitance-tested. The difference between the highest-capacitance pair and lowest-capacitance pair must be ≤5% of the average. Matched capacitance means matched signal propagation delay — critical for multi-channel encoder interfaces where timing skew between channels causes position error
- No power core contamination: Pure feedback cables are manufactured on dedicated production lines that do not process power conductors. This eliminates the risk of a stray power core strand migrating into a signal pair during stranding — a defect that would create an inter-conductor short that is invisible to routine production testing
- Small OD advantage: Without power cores, the pure feedback cable OD is 20–40% smaller than the equivalent composite cable for the same encoder type — smaller OD means tighter bend radius capability and less drag chain space required
- Labeled with encoder type compatibility: Cable jacket is printed with the encoder interface type(s) the cable is designed for (e.g., "ENCODER CABLE — EnDat 2.2 / BiSS C — 4×2×0.22 mm² + 2×0.50 mm² — PUR") — immediately identifiable during installation and maintenance