The Encoder Cable That Picked Up Noise
We used an encoder cable (article 166) on a servo motor. The encoder signal (position feedback) was noisy — the servo (article 132) moved in small steps (jitter). The cable was shielded (article 166). But the shield was grounded at both ends (the motor and the drive). That created a ground loop (article 169) — a current flowed in the shield, which induced noise in the signal. The jitter was the noise. We grounded the shield at one end (the drive end, the panel). The ground loop broke. The jitter stopped. The mistake was grounding the shield at both ends (ground loop).
Shielded cable grounding — one end, not both. This article revisits article 169 (encoder/servo detail).
Why Both Ends = Noise
A shield grounded at both ends (the motor and the panel) creates a loop. The two grounds (motor frame, panel ground bar) are at slightly different potentials (a few volts). The voltage difference drives a current through the shield (the loop). That current induces noise in the signal wires (inside the shield). The servo (article 132) sees the noise (jitter).
One End (Panel)
Ground the shield at one end (the panel, the drive end). The sensor/motor end (in the field) floats (not grounded). No loop (no current). The shield blocks the external noise (from the motor cable, article 186) without a ground loop.
The shield rule: Ground the shield at one end (the panel). The encoder cable that jittered had both ends grounded (ground loop). Ground one end (the drive). No loop, no noise.
Step 1: The 360° Ground (Not a Pigtail)
Don’t ground the shield with a pigtail (a short wire). A pigtail has inductance (it doesn’t block high-frequency noise). Use a 360° ground (the shield contacts the connector all around, a metal connector). The metal connector (the cable gland, article 189) clamps the shield to the panel (360°). The high-frequency noise drains (through the 360° contact, not the pigtail).
Step 2: The Cable Gland (Article 189)
The cable gland (article 189) is the shield’s exit. A shielded gland (EMV gland) clamps the shield (360°) to the panel. A non-shielded gland (a plastic one) doesn’t ground the shield (it just holds the cable). Use a shielded (EMV) gland for the encoder/servo cable.
Step 3: Separate Power and Signal (Article 166)
Route the encoder cable (signal) away from the motor power cable (high current). Don’t bundle them (article 166). The power cable emits noise (which the shielded encoder cable picks up, even shielded). Separate them (10 cm apart, or cross at 90°).
| Grounding | Result |
|---|---|
| Both ends (ground loop) | Noise (jitter) |
| One end (panel) | Clean (no loop) |
| Pigtail (short wire) | High-freq noise |
| 360° gland (EMV) | Clean (high-freq drained) |
A Shield Grounding Checklist
- Is the shield grounded? (One end?)
- Is it both ends? (Ground loop? Fix?)
- Is the ground a pigtail? (Use 360°?)
- Is the gland shielded (EMV)? (Article 189?)
- Is it separated from power? (Article 166?)
- Does the servo jitter? (Noise?)
- Is the encoder cable shielded? (Article 166?)
- Is the panel ground bar clean? (Article 169?)
- Is the connector metal? (360°?)
- Is the noise measured? (Oscilloscope?)
The Bottom Line
Shielded cable grounding is one end (panel), 360° (gland), not both. The encoder cable that jittered had both ends (ground loop). Ground one end. Use an EMV gland (360°). Separate from power (article 166). The servo that was smooth wasn’t the best encoder — it was grounded right.
