The Servo That Tripped on Its Own Cable
We used a servo drive (article 132) with a motor cable (power) and an encoder cable (signal). On the floor, the drive faulted (it showed a “encoder error”). The cables were bundled together (power + encoder). The motor power cable (high current, switching) emitted noise. The encoder cable (signal) picked up the noise (even shielded). The drive lost the position feedback (encoder error). The problem: power and signal were bundled (article 166). We separated the cables (power in one tray, encoder in another, 10 cm apart). The noise stopped. The drive worked. The mistake was bundling the power and encoder cables.
Servo cable shielding separates the power from the signal. This article covers the layout.
The Servo Cables
A servo system has two cables:
- Motor power (U/V/W): High current (the motor). Switching (PWM). Emits noise.
- Encoder signal: Low voltage (the position feedback). Sensitive to noise.
Both are shielded (article 166). But the power cable (high current) emits noise. The encoder cable (signal) picks it up (even shielded, if bundled).
Step 1: Separate (Article 166)
Route the power cable (motor) and the encoder cable (signal) separately. Don’t bundle them (in the same cable tray). Keep them 10 cm apart (or cross at 90°, not parallel). The power cable’s noise doesn’t reach the encoder.
The servo cable rule: Separate the motor power from the encoder signal (10 cm apart). The drive that faulted had bundled power + encoder. Separate them. The shield on the encoder helps (article 182), but separation is better.
Step 2: The Shield (Article 182)
The encoder cable’s shield is grounded at one end (the drive, article 182). The motor power cable’s shield is grounded at both ends (the drive and the motor) — for the power cable, both ends is right (it’s a high-current cable, the ground loop is less of an issue, and the shielding needs both ends). Don’t mix: encoder = one end, power = both ends.
Step 3: The Cable Tray (Article 166)
Use separate trays (or a divider). The power tray (motor) and the signal tray (encoder). If they share a tray, use a metal divider (between power and signal). The metal divider shields the signal from the power.
Step 4: The Motor Cable Length
A long motor cable (over 10 m) radiates more noise (and the voltage drops). For a long run, use a servo drive that’s rated for the length (or add a choke). Keep the motor cable short (under 5 m, if possible).
| Cable | Shield Ground |
|---|---|
| Encoder signal | One end (drive, article 182) |
| Motor power (U/V/W) | Both ends (drive + motor) |
| Separation | 10 cm (or 90° cross) |
A Servo Cable Checklist
- Is the encoder cable shielded? (Article 166?)
- Is the motor cable shielded?
- Is the encoder shield grounded at one end? (Article 182?)
- Is the power shield grounded at both ends?
- Are they separated? (10 cm? Article 166?)
- Is there a divider? (In the tray?)
- Is the motor cable short? (Under 5 m?)
- Does the drive fault? (Encoder error?)
- Is the noise gone? (Test?)
- Are the glands EMV? (Article 189?)
The Bottom Line
Servo cable shielding separates power from signal. The drive that faulted had bundled them. Separate (10 cm). Ground the encoder at one end (article 182), the power at both ends. The drive that ran clean wasn’t the best servo — it had separated cables.
