PUR Shielded Robot Cable for Automation with Low Noise and Anti-Interference Structure

PUR Shielded Robot Cable for Automation with Low Noise and Anti-Interference Structure

A PUR shielded robot cable specification that only states ‘shielded’ without quantifying the induced noise amplitude at the signal receiver does not allow the system designer to verify that the cable is adequate for their signal accuracy requirement. A 4–20 mA current loop with a ±0.5% full-scale accuracy requirement cannot tolerate more than ±0.08 mA of noise (0.5% × 16 mA span). A 0–10 V analogue output with ±1% accuracy cannot tolerate more than ±100 mV. Whether a cable’s shielding is adequate depends on these numbers — not on whether the cable has a braid.

RST-SN is specified from the receiver perspective: transfer impedance, combined with the expected field strength at the cable installation location, gives a calculated induced noise voltage that can be directly compared against the signal accuracy budget. This page documents the noise floor calculation method, the TI performance data through 5M flex cycles, the product variants matched to specific signal types, and the anti-interference structure that makes the noise floor calculation predictable across the cable’s service life.

Difference from RST-ES (shielded robot cable for EMI/RFI): RST-ES is optimised for encoder and high-frequency digital signal applications with silver drain option for TI < 16 mΩ/m. RST-SN covers the full range of automation signal types — analogue 4–20 mA, 0–10 V, thermocouple, digital 24 V DC, and process fieldbus — with a noise floor calculation sheet provided for each application.

Noise Floor Quantification: From Transfer Impedance to Signal Accuracy Budget

How to calculate the noise voltage induced in a PUR shielded robot cable

The induced noise voltage V_noise in a shielded cable is calculated as: V_noise = TI × I_interference × L, where TI is the transfer impedance (Ω/m), I_interference is the interference current on the cable outer surface (A), and L is the cable length (m).

In an automation workcell, the dominant interference source is the magnetic field from servo drive cables. For a servo cable carrying 10 A at 10 kHz switching frequency at 200 mm distance from the signal cable: the induced surface current on the shield is approximately I = H × 2πr where H is the magnetic field strength. At 200 mm from a 10 A conductor: H = I/(2πr) = 10/(2π × 0.2) = 7.96 A/m. The electric field equivalent at 10 kHz: E_equiv = H × Z_wave ≈ 50 mV/m. The surface current induced on a 1 m shield segment: I_surf ≈ E_equiv × L × shield_conductance ≈ 50 × 10^−3 × 1 × 0.05 = 2.5 mA.

With RST-SN TI = 22 mΩ/m and L = 10 m at 10 kHz: V_noise = 22 × 10^−3 × 2.5 × 10^−3 × 10 = 0.55 mV. On a 4–20 mA loop, this represents 0.55/16,000 = 0.0034% of span — 150× below the ±0.5% accuracy threshold. At 1 MHz (encoder signal frequency): surface current is higher by approximately 100× due to frequency scaling; V_noise = 22 × 10^−3 × 0.25 × 10^−3 × 10 = 55 mV — adequate for digital encoder signals (sensitivity threshold 200 mV) but confirm for < 0.5% analogue accuracy at 1 MHz.

Noise floor reference table: RST-SN induced voltage vs signal type accuracy threshold

Thermocouple note: the ±40 μV noise threshold for a K-type thermocouple is below the RST-SN noise floor of < 0.8 mV in a standard servo drive workcell. For thermocouple wiring in high-EMI environments, specify pair-level foil isolation (Tier 3 from RST-IA or RST-ES) and route the cable ≥ 500 mm from all servo drive power cables. The differential input of a thermocouple transmitter provides 60–80 dB CMRR, but this does not help for differentially-induced noise (NEXT or TI-coupled).

Anti-Interference Structure: Two Mechanisms, Two Frequency Ranges

How the RST-SN shielded robot cable achieves low noise across the automation signal frequency range

Electromagnetic interference in automation workcells spans four decades of frequency — from the 50 Hz power frequency ground loop at the low end to the 1–10 MHz range of encoder edge rates at the high end. No single shielding mechanism provides low transfer impedance across this entire range.

Mechanism 1 — AL/PET pair foil (pair-level, 100% coverage)

The aluminium foil acts as a Faraday cage around each individual pair. Its primary function is electric field shielding — at frequencies where capacitive coupling dominates (above approximately 100 kHz), the foil reduces inter-pair coupling by providing a low-impedance path for the displacement current. At 1 MHz, pair foil reduces inter-pair crosstalk by 12–15 dB compared to unfoiled cable. The foil is thin (approximately 0.05 mm Al + 0.012 mm PET backing) and has negligible mass contribution to cable stiffness — it does not significantly change the minimum bend radius.

Mechanism 2 — Tinned Cu overall braid (overall, ≥ 88% optical coverage)

The copper braid provides the primary low-frequency magnetic field shielding. At frequencies below 100 kHz, the braid’s low DC resistance (0.8–1.2 Ω/km) provides a low-impedance path for induced currents to flow — reducing the net magnetic flux inside the cable. The braid is most effective in the 1 kHz–1 MHz range. Above 1 MHz, braid inductance begins to increase TI — this is why the pair foil is also needed for encoder signal applications above 1 MHz.

RST-SN uses 32–38° braid weave angle — validated for ±180°/m robot arm torsion without wire-to-wire abrasion (> 42°) or coverage collapse (< 28°). Braid coverage at 5M flex-torsion cycles: ≥ 85%. TI at 5M cycles: < 28 mΩ/m at 1 MHz — within the < 30 mΩ/m service-life specification.

TI frequency profile: foil, braid, and combined — RST-SN vs braid-only cable

RST-SN Series Product Matrix

PUR shielded robot cable — noise-optimised configurations by signal type

All RST-SN: Class 6 conductors, AL/PET pair foil (100%) + Cu braid (≥ 88%), ether-type PUR 88 Shore A, ≥ 5M flex cycles. TI measured at 4× OD bend mid-point.

RST-SN-310/320 (braid only, TI < 35 mΩ/m): for 24 V DC digital signals where the ±12 V noise margin provides 218× headroom above the RST-SN noise floor at standard field strength. Pair foil is not required for digital I/O and reduces cable OD. For analogue signals (4–20 mA, 0–10 V, thermocouple), always specify foil+braid models (RST-SN-110 to RST-SN-220).

Technical Specification

PUR shielded robot cable conductors, shielding, and mechanical parameters

Grounding Rules and Field Diagnostics

Correct shield grounding for low-noise analogue and digital signal cables

The noise floor calculation assumes correct single-end grounding for analogue signal cables. Both-ends grounding on analogue cables creates a ground loop whose current adds directly to the signal noise, defeating the shield. For data cables (RS-485, Ethernet), both-ends PE grounding is correct — differential receivers reject common-mode noise at 60–80 dB CMRR.

Three field diagnostics using the noise floor model

Frequently Asked Questions

How do I know if RST-SN PUR shielded robot cable is adequate for my signal accuracy requirement?

Use the noise floor calculation: V_noise = TI × I_surf × L. For a 10 m cable at 50 mV/m field strength: I_surf ≈ 2.5 mA (at 10 kHz). V_noise = 22 × 10^−3 × 2.5 × 10^−3 × 10 = 0.55 mV. Compare to your noise tolerance: 4–20 mA at ±0.5% = ±80 μA tolerance (0.08 mA). Convert 0.55 mV to mA for a 250 Ω loop: 0.55/250 = 0.0022 mA — 36× below the 0.08 mA tolerance. RST-SN is sufficient. For thermocouple (40 μV tolerance): 0.55 mV > 40 μV — RST-SN alone is not sufficient; upgrade to pair foil with ≥ 500 mm cable separation.

Why is ether-type PUR specified and not ester-type for this shielded robot cable?

Ether-type PUR has no ester linkages, which eliminates the autocatalytic hydrolysis mechanism that degrades ester-type PUR in humid and wet environments. In a robot arm with occasional condensation or coolant mist, ester-type PUR develops cracks within 18–30 months as the acid produced by hydrolysis catalyses further chain scission. Ether-type PUR maintains its tensile strength and elasticity in humid environments indefinitely. The compound type is not identifiable visually — request written confirmation of PUR chemistry type when comparing supplier quotations for shielded robot cable.

Can RST-SN be used in drag chain and robot arm installations interchangeably?

Yes. All RST-SN models are designed for both applications. The relevant constraints are: drag chain requires minimum inner radius ≥ 5× cable OD; robot arm requires free cable length ≥ (joint rotation ÷ 180) × 1.20 m for torsion compliance, plus 5× OD minimum bend radius at each joint. For robot wrist joints (J4/J5) with compound flex-torsion loading, apply the 0.40× life correction factor (from the RST-IA series documentation) to the rated 5M cycles — effective wrist life approximately 2M cycles at typical industrial robot cycle rates.

Request Samples, Noise Floor Calculation, or a Quotation

Specifying RST-SN PUR shielded robot cable for your automation signal requirements

Rousheng provides a free noise floor calculation for your specific signal type, cable length, and estimated field strength before order placement. For a complete first-reply response, include:

• Signal type: 4–20 mA, 0–10 V, thermocouple, 24 V DC digital, PROFIBUS, EtherCAT, or combined
• Signal accuracy requirement (% full scale) — for noise budget comparison
• Cable length (m) and distance from nearest servo drive or VFD power cable
• Application: robot arm wiring (joint), drag chain, or static installation
• Core count and cross-section; shielded foil+braid or braid-only
• Environment: standard industrial, oil/coolant, or outdoor
• Quantity in metres or assembled sets with connector types